i 

Ultra-High Molecular Weight Polyethylene

Wear Particles Effects on Bioactivity

Hsu-Wei Fang

Stephen M. Hsu

Jan V. Sengers

NIST Special Publication 1002
ii 

NIST Special Publication 1002

Ultra-High Molecular Weight Polyethylene

Wear Particle Effects on Bioactivity

Hsu-Wei Fang1,3

Stephen M. Hsu1

Jan V. Sengers2,3,4

1

Material Science and Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-8520

2

Chemical Science and Technology Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-8380

3

Department of Chemical Engineering

University of Maryland

College Park, MD 20742

4

Institute for Physical Science and Technology

University of Maryland

College Park, MD 20742

September 2003

U.S. Department of Commerce

Donald L. Evans, Secretary

Technology Administration

Phillip J. Bond, Under Secretary for Technology

National Institute of Standards and Technology

Arden L. Bement, Jr., Director
iii 

Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately. Such

identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 1002

Natl. Inst. Stand. Technol. Spec. Publ. 1002, 277 pages (September 2003)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2003

_________________________________________

For sale by the Superintendent of Documents, U.S. Government Printing Office

Internet: bookstore.gpo.gov — Phone: (202) 512-1800 — Fax: (202) 512-2250

Mail: Stop SSOP, Washington, DC 20402-0001
iii 

PREFACE 

This report is the result of a cooperative research project between the Chemical 

Engineering Department of University of Maryland and the National Institute of 

Standards and Technology. Within NIST, Dr. John Tesk of the Polymers Division has 

provided continued support and guidance. 

The project was initiated as a direct result of the NIST Orthopaedic Wear 

Consortium established by John Tesk and Stephen Hsu. One of the ongoing concerns 

of the biomedical community is how to test the bioactivity of wear particles generated 

by the artificial joints. Mr. Hsu-Wei Fang at University of Maryland chose this topic 

as his Ph.D. thesis and this report is a summary of his research in the past five years. 

Prof. Jan Sengers lent his support to this project by serving as co-advisor to 

Mr. Fang and helped guide the thesis research to a successful conclusion. In this 

report, you will find how wear particles induce bone loosening, how particles can be 

generated, the particle formation mechanisms, and theoretical models describing how 

to control the size and shape of particles using microfabricated surface textures. 

Bioactivity tests on particles were performed by the Federal Food and Drug 

Administration and the Wayne State University through collaborations. We sincerely 

thank them for their technical assistances. 

It is always a pleasure to work with someone like Dr. Hsu-Wei Fang who has 

persevered through the past five years to reach a successful goal. Cooperative research 

such as this furthers one’s educational goals while serving the programmatic goals of 

NIST. This is a model that merits encouragement. 

Stephen M. Hsu 

Leader, Nanotribology Group 

Ceramics Division 

Materials Science and Engineering Laboratory 
iv 

ABSTRACT

Ultra-high molecular weight polyethylene (UHMWPE) wear particles have 

been recognized as one of the major causes of aseptic loosening in total joint 

replacements. Macrophage phagocytosis of wear particles induces human 

biological/physiological responses which eventually lead to bone resorption and 

osteolysis. However, the dependence of these reactions on the size and shape of the 

particles has not been elucidated and is not understood. The goal of this study is to 

develop a technique for producing UHMWPE particles with specified sizes and shapes 

so as to be able to study the effects of different UHMWPE particles on bioactivity. We 

applied surface texturing techniques to generate UHMWPE particles and air pouch 

animal tests to study the biological responses induced by UHMWPE particles. 

The study describes a procedure for generating narrowly distributed 

UHMWPE particles with controlled size and shape through surface texturing by 

microfabrication. The textured surface is used to rub against the polymer pins to 

produce wear particles in water. The surface texture produces narrowly distributed 

elongated particles or equiaxed particles by design. Experimental results show that the 

cutting-edge length of the surface features is proportional to the particle length. A 

larger penetration depth, larger normal load, and smaller sliding speed lead to a 

smaller aspect ratio of the particle. With this technique, we have been able to generate 

UHMWPE wear particles with different size and shape within phagocytosable and 

non-phagocytosbale ranges for biological response studies. 
v 

A mathematical model to predict the dimensions of generated UHMWPE 

particles has been developed. A correlation model based on the empirical results is 

also presented. The models can be used to design the dimensions of the surface 

textures and the operating conditions of the wear tests for generation of the particle 

population with specified size and shape. 

Murine air pouch animal tests have been used to test the biological responses 

induced by UHMWPE particles. The results indicated elongated particles induced 

stronger immunological responses than round-like particles. UHMWPE particles that 

can be phagocytosed by the macrophage cells stimulated a higher bioactivity than the 

non-phagocytosable particles. This paves the way for future material development and 

textural designs that can minimize the most toxic wear particle generation, hence 

prolong the life of orthopaedic joint replacements. 

 
vi 

TABLE OF CONTENTS

List of Tables ................................................................................. x 

List of Figures ............................................................................... xi

Chapter 1 Introduction ................................................................ 1

Chapter 2 Background ................................................................ 6

2.1 The need to replace human joints ........................................ 6 

2.2 Total joint replacement .................................................... 7 

2.2.1 Function and components ........................................ 7 

2.2.2 Material properties of UHMWPE and their implications .... 9 

2.3 Failure of total joint replacement ......................................... 12 

2.3.1 Wear and wear particles of UHMWPE components ......... 12 

2.3.2 Wear particles induced osteolysis .............................. 16 

2.4 Wear studies of UHMWPE material .................................... 18 

2.5 Evaluation of bioactivity of UHMWPE particles ..................... 22 

2.5.1 Wear particle induced reactions & mechanisms .............. 22 

2.5.2 In-vitro cell culture model study ................................ 24 

2.5.3 In-vivo animal tests ............................................... 24 

Chapter 3 Problem definition ........................................................ 26

3.1 The need to prolong the life of joint-replacement components ...... 26 

3.2 Potential solutions if particle induced bioactivity is understood ..... 27 

3.3 The cause and effect of particle sizes and shapes ...................... 28 

3.4 Critical issues ................................................................ 29 

3.4.1 Availability of narrowly distributed particles .................. 29 

3.4.2 Characterization of particles ..................................... 31 

3.4.3 Bioactivity tests of UHMWPE particles ........................ 31 

3.5 Approaches .................................................................. 32 

3.5.1 Generation of particles by surface texturing ................... 32 

3.5.2 Modeling of particle-generation process ....................... 32 

3.5.3 Characterization of particles ..................................... 33 

3.5.4 Bioactivity tests of UHMWPE particles ....................... 33 

Chapter 4 Procedures for generation and collection of particles ............. 34

4.1 Introduction ................................................................. 34 

4.2 Generation of UHMWPE particles ....................................... 35 

4.2.1 Literature review of UHMWPE particle generation .......... 35 

4.2.2 Concepts of surface texturing .................................... 36 

4.2.3 Preparation of materials .......................................... 37 
vii 

4.2.4 Linear reciprocating wear process................................ 39 

4.3 Collection of UHMWPE particles ....................................... 41 

4.3.1 Particle-collection procedure .................................... 41 

4.3.2 Effect of pH value on particle recovery ........................ 44 

4.4 Particle characterization ................................................... 47 

4.4.1 Chemical composition of particles ..............................47 

4.4.2 Image analysis of particles ....................................... 47 

Chapter 5 Particle generation with scratched steel surfaces .................. 49 

5.1 Introduction ................................................................. 49 

5.2 Particle generation with abrasive-paper made surfaces ............... 49 

5.2.1 Preparation of steel surfaces with SiC abrasive papers ...... 49 

5.2.2 Results ............................................................... 50 

5.3 Effect of surface-texture pattern on particle shape ..................... 58 

5.4 Effect of surface-texture roughness on particle size ................... 60 

5.5 Particle generation with diamond-tip scratched surfaces ..............62 

5.5.1 Preparation of diamond-tip scratched stainless steel surfaces 

.................................................................................62 

5.5.2 Results ............................................................... 64 

5.6 Effect of surface uniformity on particle distribution .................. 68 

Chapter 6 Surface texturing by microfabrication ................................71 

6.1 Introduction .................................................................. 71 

6.2 Surface texture with wedge features ..................................... 71 

6.3 Microfabrication process .................................................. 72 

6.3.1 Substrate material ................................................. 72 

6.3.2 Masking material .................................................. 74 

6.3.3 Pattern design and transfer ....................................... 75 

6.3.4 Fabrication of surface features .................................. 76 

6.3.5 Wear-resistant coating ............................................ 79 

6.4 Measurement of the surface texture ...................................... 80 

Chapter 7 Surface-texture design and generation of narrowly distributed 

particles ..................................................................... 82 

7.1 Introduction .................................................................. 82 

7.2 Surface-texture designs ................................................... 82 

7.3 Surface texture fabricated .................................................. 84 

7.4 Results........................................................................ 86 

7.4.1 Effect of the cutting-edge length ................................ 86 

7.4.2 Effect of the cutting-edge height ................................ 91 

7.4.3 Effects of the cutting edge pattern ............................... 93 

7.4.4 Effect of pitch distance ............................................ 100 

7.4.5 Effects of mechanical conditions ................................ 103 

7.5 Discussion .................................................................... 107 
viii 

7.5.1 Effect of surface-texture uniformity on particle distribution. 107 

7.5.2 Significance for bioactivity tests ................................. 109 

7.6 Conclusions .................................................................. 111 

Chapter 8 Mechanism of UHMWPE particle generation ...................... 112 

8.1 Introduction .................................................................. 112 

8.2 Penetration of the cutting edge ........................................... 113 

8.3 Sliding of the cutting edge.................................................. 118 

8.3.1 Simple model experiments with constant load ................ 118 

8.3.2 Lateral displacement of the tip ................................... 120 

8.3.3 Reacting forces .................................................... 123 

8.3.4 Effect of normal load and cutting-edge angle .................. 125 

8.4 Shear induced molecular orientation of UHMWPE ................... 128 

8.5 Strain-hardening effect .................................................... 131 

8.5.1 Constant depth sliding model experiments ..................... 133 

8.6 Observations from the particle-generation results ..................... 141 

8.7 Conclusions .................................................................. 144 

Chapter 9 Modeling of particle generation ........................................ 145 

9.1 Introduction .................................................................. 146 

9.2 Model parameters ...........................................................147 

9.3 Contact mechanics modeling ............................................. 148 

9.3.1 Penetration of the cutting edge .................................. 148 

9.3.2 Sliding process of the cutting edge .............................. 153 

9.3.3 Validation of the model .......................................... 159 

9.4 Correlation model .......................................................... 164 

9.4.1 Effect of cutting-edge length ..................................... 164 

9.4.2 Effect of penetration depth .......................................166 

9.4.3 Effect of load ....................................................... 168 

9.4.4 Effect of sliding speed ............................................ 169 

9.4.5 Comparison with experimental results ......................... 170 

9.5 Summary ..................................................................... 170 

Chapter 10 Particle characterization by analyzing the phagocytosis of 

particles ...................................................................... 172 

10.1 Introduction ................................................................. 172 

10.2 Basic particle parameters .................................................. 173 

10.2.1 Descriptors for particle size ...................................... 173 

10.2.2 Descriptors for particle shape .................................... 175 

10.3 Osteolysis induced by UHMWPE particles ............................. 178 

10.4 Phagocytosis of particles ...................................................180 

10.4.1 Criteria for particle internalization by macrophage cells......182 

10.4.2 Concept of phagocytic capacity.................................. 183 
ix 

10.4.3 Critical threshold for phagocytosis of particles with different 

shapes................................................................ 185 

10.5 Validation against literature results ...................................... 187 

10.6 Effect of excess particles .................................................. 193 

10.7 Conclusions .................................................................. 195 

Chapter 11 Bioactivity tests of UHMWPE particles .............................. 196 

11.1 Introduction .................................................................. 196 

11.2 Murine air pouch animal test ............................................. 197 

11.3 Evaluation of biological response ........................................ 199 

11.3.1 Histology analysis ................................................. 199 

11.3.2 Measurement of cytokines ....................................... 202 

11.4 Effect of particle shape .................................................... 203 

11.5 Effect of particle size ...................................................... 207 

11.6 Effect of particle dosage ................................................... 208 

11.7 Combined effects from size, shape and dosage of particles .......... 209 

11.8 Conclusion ................................................................... 215 

Chapter 12 Summary and conclusions ............................................... 216 

12.1 Design of surface textures for particle generation ...................... 216 

12.2 Modeling of UHMWPE particle generation ............................ 217 

12.3 Bioactivity of UHMWPE particles ....................................... 219 

Chapter 13 Future work and recommendations .................................... 221 

13.1 Improvements of current techniques of particle generation .......... 221 

13.2 Mechanism study for osteolysis induced by particles ................. 222 

References .................................................................................... 223 

Appendix A Publications and awards ................................................ 241 

 

Appendix B Microfabrication of steel surface by electrochemical etching .....245 

Appendix C Surface texture and wear data ......................................... 249 

Appendix D Modeling of particle generation by correlation of model 

 Experimental data ......................................................... 251

Appendix E Matlab program for contact mechanics model of particle 

generation.................................................................... 259 
x

LIST OF TABLES

Table 2.1 Typical physical properties of HDPE and UHMWPE [21]........ 9 

Table 2.2 Summary of retrieval UHMWPE wear particle studies in the 

literature.................................................................. 14 

Table 2.3 Definition of biological terms related to osteolysis................. 17 

Table 2.4 Summary of bioactivity study of UHMWPE particles from 

literature.................................................................. 23 

Table 5.1 Results of particle generation with abrasive paper made 

surfaces.................................................................... 53 

Table 5.2 Effect of roughness of steel surfaces on particle dimensions...... 60 

Table 5.3 Results of particle generation with diamond-tip scratched 

stainless steel surfaces.................................................. 64 

Table 7.1 Value of (Ds / Hc) for effective and non-effective micro-cutting 

processes.................................................................. 99 

Table 8.1 Comparison of penetration depth for the cutting edges with 

different angles under various normal loads......................... 117 

Table 10.1 Summary of in-vitro cell culture results from Catelas et al. [79].. 188 

Table 10.2 Summary of in-vitro cell culture results from Green et al. [81]... 190 
xi

LIST OF FIGURES

Figure 2.1 Components of total hip and knee joint replacements.............. 8 

Figure 2.2 Schematic diagram of the chain orientation of UHMWPE......... 10 

Figure 2.3 Examples of worn UHMWPE components for total hip and 

knee replacements....................................................... 13 

Figure 2.4 Examples of UHMWPE particles retrieved from (a) total hip 

replacement, and (b) total hip replacement [41]..................... 15 

Figure 2.5 Osteolysis taking place around the total hip replacements which 

is detected by X-ray diagnosis......................................... 19 

Figure 2.6 An example of forces acting on the hip-joint implant during gait 

[67]........................................................................ 20 

Figure 2.7 Trajectories of contact points on the articular surface of hipjoint implant [74] and hip-joint kinematic data during gait [73].. 20 

Figure 3.1 Comparison of polyethylene powders and particles................ 30 

Figure 4.1 Weight change of pre-soaked UHMWPE pins...................... 38 

Figure 4.2 Schematic representation of linear reciprocating wear tester...... 40 

Figure 4.3 Particle-collection procedure.......................................... 43 

Figure 4.4 Effect of pH value of water on removing particles on the glass 

slide........................................................................ 45 

Figure 4.5 Effect of pH value of water on particle-recovery rate............... 46 

Figure 4.6 IR absorbance spectra of UHMWPE bulk material and of 

collected particles......................................................... 48 

Figure 5.1 Procedure of surface texturing with SiC abrasive papers........... 51 

Figure 5.2 (a) Unidirectional and (b) cross-hatched surface textures made 

on 316 steel plate......................................................... 52 
xii

Figure 5.3 Particles generated with 120 grit abrasive paper made 

unidirectional stainless steel surface. (a) SEM micrograph, (b) 

Distribution of particle length, (c) Distribution of particle aspect 

ratio......................................................................... 54 

Figure 5.4 Particles generated with 320 grit abrasive paper made 

unidirectional stainless steel surface. (a) SEM micrograph, (b) 

Distribution of particle length, (c) Distribution of particle aspect 

ratio......................................................................... 55 

Figure 5.5 Particles generated with 120 grit abrasive paper made 

crosshatched stainless steel surface. (a) SEM micrograph, (b) 

Distribution of particle length, (c) Distribution of particle aspect 

ratio......................................................................... 56 

Figure 5.6 Particles generated with 320 grit abrasive paper made 

crosshatched stainless steel surface. (a) SEM micrograph, (b) 

Distribution of particle length, (c) Distribution of particle aspect 

ratio......................................................................... 57 

Figure 5.7 Comparison of particle aspect-ratio distribution from abrasive 

paper made stainless steel surfaces with different surface-texture 

styles........................................................................ 59 

Figure 5.8 Comparison of particle-length distribution from stainless steel 

surfaces made by different grades of abrasive papers................ 61 

Figure 5.9 Procedure of surface texturing by diamond scratching.............. 63 

Figure 5.10 SEM micrographs diamond-tip scratched surfaces: (a) with 

linear scratches, (b) with a 60 μm scratched length.................. 65 

Figure 5.11 Particles generated with the diamond-tip scratched stainless steel 

surface (linear type) (a) SEM micrograph, (b) Distribution of 

particle length, (c) Distribution of particle aspect ratio.............. 66 

Figure 5.12 Particles generation with the diamond-tip scratched stainless 

steel surface (scratched distance= 60 μm) (a) SEM micrograph, 

(b) Distribution of particle length, (c) Distribution of particle 

aspect ratio............................................................... 67 

Figure 5.13 Examples of surface profiles for (a) SiC abrasive-paper made 

steel surface, and (b) Diamond-tip scratched stainless steel 

surface .................................................................... 69 
xiii

Figure 5.14 Comparison of standard deviation and mean particle length for 

different particle-size populations made with surfaces made with 

SiC abrasive papers and with diamond-tip scratching 

process..................................................................... 70 

Figure 6.1 Schematic of microfabrication process to generate wedge 

features on silicon........................................................ 73 

Figure 6.2 Schematic representation of the wedge geometry controlled by 

anisotropic and isotropic etching processes.......................... 78 

Figure 6.3 SEM micrographs of the wedge surface-features made by the 

microfabrication process................................................. 80 

Figure 6.4 Measurement of the three-dimensional geometry of the wedge 

surface textures by atomic force microscopy (AFM)................. 81 

Figure 7.1 Schematic of the surface texture with wedge features............... 

. 

83 

Figure 7.2 A schematic representation of dimensions of the surface textures 

containing wedge-shaped cutting edges................................ 85 

Figure 7.3 Micrographs of the surface texture A-1 (Lc = 55 μm, Hc = 4.2 

μm, Ds = 200 μm ) and generated UHMWPE particles (contact 

pressure = 3 MPa, average sliding speed = 57.2 mm/s)............. 88 

Figure 7.4 Micrographs of the surface texture A-2 (Lc = 15.0 μm, Hc = 3.4 

μm, Ds = 200 μm ) and generated UHMWPE particles (contact 

pressure = 3 MPa, average sliding speed = 57.2 mm/s)............. 89 

Figure 7.5 Micrographs of the surface texture A-3 (Lc = 7 μm, Hc = 3.0 μm, 

Ds = 200 μm ) and generated UHMWPE particles (contact 

pressure = 3 MPa, average sliding speed = 57.2 mm/s)............. 90 

Figure 7.6 Comparison of UHMWPE particles generated from surface 

texture A-2 with different cutting-edge heights (contact pressure 

= 3 MPa, average sliding speed = 57.2 mm/s)........................ 92 

Figure 7.7 Micrographs of the surface texture A-4 (Lc = 6.8 μm, Hc = 2.3 

μm, Ds = 10 μm ) and generated UHMWPE particles (contact 

pressure = 3 MPa, average sliding speed = 57.2 mm/s)............. 94 

Figure 7.8 Micrographs of the surface texture A-5 (Lc 3.2 μm, Hc = 0.8 μm, 
xiv

Ds = 4 μm ) and generated UHMWPE particles (contact pressure 

= 3 MPa, average sliding speed = 57.2 mm/s)........................ 95 

Figure 7.9 Micrographs of the surface texture B-1 (Lc = 6.2 μm, Hc = 1.3 

μm, Ds = 10 μm ) and generated UHMWPE particles (contact 

pressure = 3 MPa, average sliding speed = 57.2 mm/s)............. 97 

Figure 7.10 Micrographs of the surface texture B-4 (Lc = 3.8 μm, Hc = 1.2 

μm, Ds = 10 μm) and generated UHMWPE particles (contact 

pressure = 3 MPa, average sliding speed = 57.2 mm/s)............. 98 

Figure 7.11 Micrographs of the surface texture B-2 (Lc = 3.5 μm, Hc = 1.4 

μm, Ds = 20 μm ) and generated UHMWPE particles (contact 

pressure = 3 MPa, average sliding speed = 57.2 mm/s)............. 101 

Figure 7.12 Micrographs of the surface texture B-3 (Lc = 3.6 μm, Hc = 0.6 

μm, Ds = 10 μm) and generated UHMWPE particles (contact 

pressure = 3 MPa, average sliding speed = 57.2 mm/s)............. 102 

Figure 7.13 Comparison of UHMWPE particles generated under different 

normal loads (by using the surface texture A-3 with cutting edge 

length = 7 μm, average sliding speed = 57.2 mm/s)................. 104 

Figure 7.14 Comparison of micrographs of UHMWPE particles generated by 

the surface texture A-1 under 3 MPa contact pressure and sliding 

speeds of 27.6 mm/s and 57.2 mm/s respectively.................... 106 

Figure 7.15 Comparison of particle length distribution between particles 

generated from a SiC abrasive paper made surface texture and a 

microfabricated surface texture......................................... 108 

Figure 7.16 Comparison of two particle populations with distinct particlelength distribution........................................................ 110 

Figure 8.1 Load-displacement curve of nano-indentation test on UHMWPE 

with constant strain rate of 0.1 s-1....................................... 114 

Figure 8.2 Elastic modulus data for UHMWPE under different penetration 

depths from a nano-indentation test (strain rate = 0.1 s-1)........... 116 

Figure 8.3 Schematic representation of scaled-up single cutting-edge sliding 

test with fixed normal-load condition................................... 119 
xv

Figure 8.4 Video frames of a cutting edge lift-up process........................ 121 

Figure 8.5 Schematic representation of the cutting-edge sliding process over 

UHMWPE material with fixed normal load........................... 122 

Figure 8.6 Force curve of the sliding process by a 90° wedge-shaped cutting 

edge under a normal load of 66.75 N.................................. 124 

Figure 8.7 Force curve of the sliding process by a 60° wedge-shaped cutting 

edge under a normal load of 66.75 N.................................. 124 

Figure 8.8 Sliding distance between two peaks under different normal loads 

with two cutting-edge angles............................................. 125 

Figure 8.9 Schematic representation of the resolved component forces of 

the material resistant force in the vertical and horizontal 

direction during the sliding process..................................... 127 

Figure 8.10 Soft x-ray absorption spectra showing the molecular orientation 

of surface layer after 5000 unidirectional sliding cycles. E= 

electric field [27].......................................................... 129 

Figure 8.11 Soft x-ray absorption spectra showing the degree of molecular 

alignment of UHMWPE surface layer after 5000 unidirectional 

sliding cycles [26]........................................................ 130 

Figure 8.12 True stress-strain curves for UHMWPE (GUR 4150) [104]........ 132 

Figure 8.13 True stress-strain curves for UHMWPE 4150 up to 0.12 strain in 

uniaxial tension and compression and elastic-plastic model to 

describe the behavior [104].............................................. 132 

Figure 8.14 A schematic representation of the displacement controlled single 

cutting edge sliding test.................................................. 134 

Figure 8.15 Horizontal-force curve of the cutting-edge sliding process 

(cutting-edge length = 2 mm, penetration depth = 400 μm, sliding 

speed = 0.5/s)............................................................... 136 

Figure 8.16 Video frames of the cutting edge sliding process (cutting edge 

length = 2mm, penetration depth = 400 μm, sliding speed = 

0.5/s)......................................................................... 137 

Figure 8.17 Horizontal force versus sliding distance for different penetration 
xvi

depths in a displacement-controlled cutting-edge sliding 

test............................................................................ 139 

Figure 8.18 Horizontal force as a function of the sliding distance for different 

cutting-edge angles in a displacement-controlled cutting-edge 

sliding test.................................................................. 140 

Figure 8.19 SEM micrographs of surface cutting-edge feature, UHMWPE 

worn surface and generated UHMWPE particles...................... 142 

Figure 8.20 UHMWPE wear particles with irregular edges and straight line 

indent marks made by cutting edges..................................... 143 

Figure 9.1 Illustrations of significant variables for the particle-generation 

process....................................................................... 147 

Figure 9.2 Schematic of the penetration process of the cutting edges............ 148 

Figure 9.3 Comparison of the empirical data of wedge indentation in 

UHMWPE with elastic-plastic calculation results.................... 151 

Figure 9.4 Schematic representation of the sliding process of the cutting 

edge.......................................................................... 154 

Figure 9.5 A flow chart of calculation of the sliding process of the cutting 

edge.......................................................................... 156 

Figure 9.6 A schematic representation of reacting forces in a cutting-edge 

sliding process.............................................................. 

Figure 9.7 Effect of normal load from model predictions and experimental 

results....................................................................... 

157 

160 

Figure 9.8 Effect of feature height from model predictions and experimental 

results......................................................................... 162 

Figure 9.9 Effect of cutting-edge angle from model calculations................. 163 

Figure 9.10 Comparison of cutting-edge length with wear-particle length....... 164 

Figure 9.11 Plot of particle length versus cutting-edge length...................... 165 

Figure 9.12 Plot of particle thickness versus penetration depth..................... 166 
xvii

Figure 9.13 Plot of particle width versus penetration depth......................... 167 

Figure 9.14 Plot of particle volume versus load on a log-log scale................ 168 

Figure 9.15 Plot of particle volume versus sliding speed on a log-log scale..... 169 

Figure 9.16 Comparison of the correlation model with the experimental 

results........................................................................ 171 

Figure 10.1 Estimation of particle length, particle width and particle thickness 

from a SEM micrograph of UHMWPE particles....................... 173 

Figure 10.2 Schematic of the projected ellipsoid of a particle...................... 174 

Figure 10.3 Schematic of the definition of the particle length (Lp) and the 

particle width (Wp) for different shapes of particles................... 176 

Figure 10.4 Schematic of mechanism for particles-induced osteolysis............ 179 

Figure 10.5 Schematic particles-induced bioactivity from viewpoint of 

phagocytosis................................................................. 181 

Figure 10.6 Phagocytic capacity of spherical particles.............................. 185 

Figure 10.7 Phagocytic capacity of particles with different aspect ratios......... 186 

Figure 10.8 TNF-α vs. index of particle internalization from Catelas et al. 

[79]........................................................................... 188 

Figure 10.9 IL-1β vs. index of particle internalization from Green et al. [81]. 191 

Figure 10.10 TNF-α vs. Index of particle internalization from Green et al. 

[81]........................................................................... 192 

Figure 10.11 Schematic of dynamic phagocytosis model............................ 194 

Figure 11.1 Schematic of murine air-pouch model for bioactivity tests of 

UHMWPE particles........................................................ 198 

Figure 11.2 Micrographs of phagocytosis of UHMWPE particles by 

macrophage cells............................................................ 200 

Figure 11.3 Photographs of murine air pouches; (a) without particle; (b) 

reacted with round-like particles; (c) reacted with elongated 
xviii

particles..................................................................... 200 

Figure 11.4 Micrographs of sectioned air pouches................................... 201 

Figure 11.5 Comparison of pouch thickness for the murine air pouch tests 

with round-like (Group A) and elongated (Group B) UHMWPE 

particles..................................................................... 204 

Figure 11.6 IL-1β and TNF-α cytokine expression in murine air pouches with 

UHMWPE particles by real time PCR (plots A and B) and by 

ELISA (plots C and D) techniques....................................... 206 

Figure 11.7 Comparison of pouch thickness for the murine air pouch tests 

with small (Group D) and larger (Group E) elongated UHMWPE 

particles...................................................................... 208 

Figure 11.8 Comparison of pouch thickness for the murine air pouch tests 

with different number of particles (Group D, elongated particles 

within phagocytosable range)............................................. 209 

Figure 11.9 Plot of pouch thickness versus total surface area of particles....... 211 

Figure 11.10 Plot of pouch thickness versus total volume of particles............ 211 

Figure 11.11 Plots of normalized pouch thickness versus aspect ratio of 

particles; (a) pouch thickness normalized by total surface area of 

particles; (b) pouch thickness normalized by total volume of 

particles...................................................................... 214 
1

CHAPTER 1 

INTRODUCTION

 Total joint replacements have been successfully implanted clinically to replace 

damaged human joints for several decades. They have dramatically improved the life 

of patients with joint disabilities. The average life of a total hip replacement is 10 

years to 15 years and the average life of a total knee replacement is less than 10 years 

[1]. In view of the increase of life expectancy, more active daily life style, and lower 

age of arthritis patients, the useful life of total joint replacements needs to be increased 

to reduce revision surgery at old age and the associated social and human costs if the 

patient cannot be operated on. 

 A total joint replacement system comprises of a pair of material surfaces 

articulating to each other to simulate the functions of the human joints. From years of 

research and development, an artificial joint system consisting of Co-Cr alloy sliding 

on an ultra-high molecular weight polyethylene (UHMWPE) surface has been proven 

to function well because of wear resistance, biocompatibility, and durability. 

Major factors causing failure of total joint replacements include: (1) Infection 

during orthopedic surgery; (2) Fracture of the implants; (3) Fixation problem of the 

implants; (4) Wear of the implant materials; and (5) Osteolysis induced by wear 

particles. Of these factors, the rate of infections has been reduced by better surgery 

procedures. Optimizing of the material strength and minimizing its difference from the 
2

human bones have resolved most of the fracture problems [2-3]. The introduction of 

bone compatible porous surface coatings on the stem to provide another choice for 

fixation has helped to almost eliminate the fixation problem [4-6]. In the past 5 years, 

wear of UHMWPE has been dramatically reduced by using highly crosslinked 

UHMWPE [7-9]. The wear particle induced osteolysis has emerged as the critical 

failure mechanism of artificial joints. 

However, the detailed mechanism of osteolysis and bone resoprtion induced by 

wear particles leading to the loosening of the total joint replacement is still not clear. 

The purpose of this study is to determine a relationship between size and shape of 

UHMWPE particles and bioreactivity. If the relationship between UHMWPE particle 

size and shape and biological responses is understood, potential engineering solutions 

can be developed to control the production of the most toxic size and shape of 

particles and hence prolong the life of the replacement joints. 

 To conduct this investigation, the following approaches have been adopted: (1) 

Generation of narrowly-distributed UHMWPE particles with different sizes and 

shapes; (2) Characterization of these wear particles; and (3) Bioactivity tests of 

UHMWPE particles in animal studies. Research staffs from NIST concentrated on 

developing the techniques to generate UHMWPE particles. The researchers from 

University of Maryland prepared the particles for animal tests. Wayne State University 

provided funding and research staffs to conduct the animal tests. 

 Various sizes and shapes of UHMWPE wear particles are needed in order to 

evaluate their effects on biological responses. To avoid uncertainties in the 
3

experimental outcomes, the uniformity of the UHMWPE particle population should be 

tightly controlled. A novel particle generation process by rubbing the UHWMPE 

material against a dimensional controlled surface texture has been developed in this 

study. We have proved that by varying the dimensions of the surface-texture features, 

the particle size and shape can be changed. By improving the uniformity of the surface 

texture with microfabrication techniques, the particle distribution has further been 

narrowed. The protocol to collect, quantify, and package particles in a sterilized 

environment has also been developed for particle preparation for bioactivity tests. 

 To understand the relationship between the surface-texture dimensions and 

particle dimensions, a study of the mechanism of UHWMPE particle generation has 

been conducted. The goal here was to formulate a mathematical model for predicting 

particle sizes and shapes. The desired penetration depth of a cutting edge into the 

material was determined from indentation tests with consideration of viscoelasticity of 

the UHMWPE. Simple model experiments with in-situ monitoring were conducted to 

observe the cutting processes and the particle generation sequence. The relationship 

between the tip geometry and the resultant reaction forces from the UHMWPE 

material during the cutting process was obtained to explain the wear particle 

generation mechanisms. Based on the force and energy balance of the particlegeneration process, a mathematical model was developed to predict particle volume. 

The model was validated by comparison with experimental results of particle 

generation. 
4

 Murine air pouch animal tests were then carried out by collaboration between 

University of Maryland and Wayne State University to evaluate the bioactivity by 

applying UHMWPE particles with various sizes and shapes generated from our 

particle-generation processes. Related cytokine measurements and histology analyses 

of air pouches were performed to evaluate the degree of immunological responses 

induced by UHMWPE particles. Elongated wear particles were found to be much 

more toxic than round particles. The effects of particle size and shape on bioactivity 

were further analyzed from the process of phagocytosis of particles. Biophysical data 

of macrophage cells have been used to estimate the phagocytic capacity for various 

particle sizes and shapes. A critical threshold was determined and verified by our air 

pouch animal tests and by cell culture studies from the literature. 

 This study is organized in a chronological manner as follows. In Chapter 2, the 

backgrounds of total joint replacement, failure mechanism, wear of UHMWPE and 

biological responses induced by wear particles are reviewed. In Chapter 3, the current 

critical problem of joint replacements and approaches to solve the problems are 

discussed. In Chapter 4, the procedures for generating and collecting particles are 

described. In Chapter 5, UHMWPE particles generated from a SiC abrasive paper 

made steel surface are shown. The effect of surface-texture style on particle shape is 

discussed. Chapter 6 describes microfabrication techniques for surface-texture 

preparation. Chapter 7 illustrates the narrow distribution of particles generated from 

the microfabricated surface textures. In Chapter 8, the mechanism of particle 

generation from the experimental observations of worn surfaces and wear particles and 
5

from a simple model experiment of micro-cutting process is discussed. Chapter 9 

describes mathematical models to predict the particle dimensions with given surfacetexture geometry and mechanical conditions. In Chapter 10, characterization of 

particles by analyzing the phagocytosis of particles is introduced. Chapter 11 

introduces the air pouch animal tests of UHMWPE particles as well as the procedures 

for histology analysis and cytokine measurement. The effects of particle size, shape 

and dosage are compared in this chapter, also. In Chapter 12, the conclusions are 

presented. Finally, Chapter 13 mentions recommendations for future research. 
6

CHAPTER 2 

BACKGROUND

2.1 The need to replace human joints 

A human joint is formed by the ends of two or more bones which are 

connected by cartilage and tissues. For example, the hip is a ball and socket joint, 

formed by the upper end of the femur, the ball, and a part of the pelvis called the 

acetabulum, the socket. The upper end of a lower leg bone, called the tibia, and lower 

end of the thighbone, called the femur, form the knee joint. At the end of a joint, the 

bone is covered with a smooth layer of special tissue called cartilage. Normal healthy 

cartilage allows easy movement. Every joint is enclosed by a capsule with a smooth 

tissue called the synovium. The synovium produces a fluid that lubricates the joint. 

However, through a variety of causes, such as arthritis, infection and injury, articular 

cartilage can degenerate and lose its effectiveness [10]. This causes pain and stiffness 

in joint movement. A physical examination, laboratory tests and x-rays can determine 

the extent of damage. Patients suffer from chronic pain and loss of motion of the joint. 

The pain may be so severe that a person is inclined to avoid joint movement, 

weakening the muscles around the joint and making it even more difficult to move the 

joints. Medical treatment helps to relieve the pain caused by the damaged cartilage and 

helps patients to resume their normal daily life. Total joint replacement with an 
7

artificial joint will be prescribed when all other treatments cannot relieve pain and 

disability. 

2.2 Total joint replacement 

2.2.1 Function and components 

Total joint replacement surgery is one of the most common and successful 

treatments for all types of arthritis and other painful conditions of the hip or knee. 

Joint replacement replaces destroyed or worn-out surfaces with new, uniform surfaces. 

For example, in an arthritic hip, the damaged ball (the upper end of the femur) is 

replaced by a metal ball attached to a metal stem fitted into the femur, and a plastic 

socket is implanted into the pelvis, replacing the damaged socket. In an arthritic knee 

the damaged ends of the bones and cartilage are replaced with metal and plastic 

surfaces that are shaped to simulate knee movement and function. Although hip and 

knee replacements are the most common, joint replacements can be performed on 

other joints, including the ankle, foot, shoulder, elbow and fingers.

The materials used in a total joint replacement are designed to enable the joint 

to move smoothly with low friction [11, 12]. A joint implant is generally composed of 

two parts that articulate during the joint motion. In a hip, for example a metal ball is 

usually used to fit closely into a matching counter surface composed of UHMWPE. 

Several metals are used, including stainless steel, alloys of cobalt and chrome, and 

titanium. The UHMWPE is durable and wear resistant. It has been widely accepted for 

its low friction and high wear-resistance. Examples of total hip and knee replacements 
8

with Co-Cr alloy and UHMWPE parts are shown in Figure 2.1. While these materials 

are mostly used, combinations of ceramic-ceramic, metal-metal and ceramicUHMWPE total joint systems are some other alternative materials used under special 

situations [13-17]. Bone cements made by polymers are usually used to anchor the 

prosthesis into the bone [18-19]. Joint replacements also can be implanted without 

cement when a porous surface on the stems of joint replacements is proved to allow 

bone to grow into it. 

Figure 2.1 Components of total hip and knee joint replacements. 

UHMWPE

Metal Parts

Components of total hip replacements

UHMWPE

Components of total knee replacement

Metal parts 
9

2.2.2 Material properties of UHMWPE and their implications 

Ultra-high molecular weight polyethylene is widely used as the bearing 

material in total hip and knee replacements. Polyethylene has the repeating unit 

chemical structure: 

− (CH2 −CH2 )n − (2.1)

There are several kinds of polyethylene (LDPE, HDPE, UHMWPE) that are 

synthesized with different relative molecular mass and chain architectures. LDPE 

refers to low-density polyethylene with a relative molecular mass of less than 50,000 

Mr. High-density polyethylene (HDPE) is a linear polymer with a relative molecular 

mass up to 200,000 Mr. In comparison, UHMWPE has a relative molecular mass 

ranging from (2 to 6) x 106 Mr [20]. The typical modern UHMWPE used in implants 

has a relative molecular mass of (4 to 6) x 106 Mr. Table 2.1 summaries the physical 

and mechanical properties of HDPE and UHMWPE. UHMWPE has a higher ultimate 

strength and impact strength than HDPE and UHMWPE is significantly more abrasion 

and wear resistant than HDPE from a clinical perspective [21]. 

Table 2.1 Typical physical properties of HDPE and UHMWPE [21]. 

Property HDPE UHMWPE 

Relative Molecular Mass (Mr = g/mol) 0.05 to 0.25 2 to 6 

Melting Temperature (°C) 130 to 137 125 to 138 

Poisson’s Ratio 0.40 0.46 

Specific Gravity 0.952 to 0.965 0.932 to 0.945 

Tensile Modulus of Elasticity (GPa) 0.4-4.0 0.8 to 1.6 

Tensile Yield Strength (MPa) 26 to 33 21 to 28 

Tensile Ultimate Strength (MPa) 22 to 31 39 to 48 

Tensile Ultimate Elongation (%) 10 to 1200 350 to 525 

Degree of Crystallinity (%) 60 to 80 39 to 75 
10

 The molecular chain of UHMWPE can be visualized as a tangled string of 

wires. The chain is not static and its arrangement is a function of temperature. The 

molecular chain becomes mobile at elevated temperatures. When the temperature is 

lower than the melting temperature, the molecular chain of UHMWPE tends to rotate 

along the C-C bonds and create chain folds. The chain folding leads to an ordered 

arrangement of molecules so as to form a crystalline region [22]. A schematic of the 

crystalline region with folded chains and amorphous region with loose chains fused 

together is shown in Figure 2.2. The degree and orientation of the crystalline regions 

depends on its molecular weight, processing conditions and environment conditions. 

Amorphous 

Region

Crystalline 

Region

Highly cross-linked 

Figure 2.2 Schematic diagram of the chain orientation of UHMWPE.
11

To be used for medical applications, UHMWPE resin powder should meet the 

requirements specified in ASTM standard F648 [23] and ISO standard 5834-1 [24]. 

UHMWPE is produced as powder and must be consolidated under high temperature 

and pressure by various thermoplastic-processing techniques, such as compression 

molding and ram extrusion [25]. Then the material is machined into its final form. The 

UHMWPE components for total joint replacement are then packaged and sterilized 

before distribution for medical use. 

The material properties of UHMWPE are sensitive to the manufacturing 

processes, post-treatment of the material, and material-storage environments. For 

example, when a shear force is applied to the material surface, the molecules tend to 

align in the direction of shear stress [26-28], and the surface material strength may be 

strongly dependent on direction of shear motions [29]. It has been shown that the wear 

of UHMWPE is accelerated by the cross-shear that occurs [74]. 

Another factor is the sterilization process of the material. After the 

manufacturing of artificial joint components, the parts will go through a sterilization 

process before being implanted into the human body. The conventional sterilization 

technique by steam autoclaving cannot be used with polyethylene components because 

of their tendency to deform under this condition. Sterilization by gamma irradiation is 

normally used. However, this causes the polyethylene to cross-link, which in turn, 

modifies the polymer molecular weight, crystallinity, elastic modulus and other 

physical properties [30]. 
12

2.3 Failure of total joint replacement 

The long-term performance of the articulating system and materials in the total 

joint replacement is very important, in particular for younger, heavier, and more active 

patients. It has been established that the failure of total joint replacement can be 

attributed to one of the following causes: 1) Infection from surgery, 2) Fracture of 

implants, 3) Inability to obtain suitable fixation, 4) Wear of UHMWPE, and 5) 

Osteolysis induced by UHMWPE wear particles. Wear-particle induced aseptic 

loosening is recognized as a major cause leading to failure of total joint replacements 

[31-34]. In the following sections, more details are presented related to wear particles 

of UHMWPE and osteolysis induced by such wear particles. 

2.3.1 Wear and wear particles of UHMWPE components 

Ultra-high molecular weight polyethylene (UHMWPE) is identified as one the 

major components of wear for orthopedic joint implants. Figure 2.3 shows examples 

of worn UHMWPE parts observed from total hip and knee replacements. UHMWPE 

wear particles are generated around the joint region due to the articulation motion 

between the metal parts and UHMWPE components. It has been suggested that 

various wear particles are embedded among the prosthetic tissues or synovial fluid 

around the failed artificial joints. Many studies have isolated UHMWPE particles from 

the tissues retrieved from the revision surgery [35-46]. Table 2.2 provides a summary 

of retrieval UHMWPE wear particle studies in the literature. The table also lists the 
13

digestion procedure for isolating the UHMWPE particles, identification of materials 

and analysis of particle size and shape. 

Size, shape and morphology of UHMWPE wear particles have been widely 

analyzed [47-51]. It has been observed that the size range of UHMWPE wear particles 

are from sub-micrometers to millimeters. A variety of particle shapes have been 

observed: granules, beads, fibrils, shreds. The morphology of the particles is 

asymmetric and irregular. Examples of particles retrieved from total knee replacement 

(TKR) are shown in Figure 2.4. 

Worn UHMWPE component of 

total knee replacement. 

Worn UHMWPE cup 

of total hip replacement. 

Figure 2.3 Examples of worn UHMWPE components for total hip and 

knee replacements. 
14

Table 2.2 Summary of retrieval UHMWPE wear particle studies in the literature. 

Reference Sample 

source 

Particle isolation Material 

analysis 

Particle analysis 

Wolfarth et al. [35] 

JBMR, vol. 34, pp.57-61 

(1997) 

*FDA 

Synovial 

fluid, 

tissues 

(TKR) 

NaOH digestion, ultracentrifugation, water 

and ethanol 

centrifugation 

MicroRamman 

spectroscopy 

SEM 

The particles from synovial fluid 

and tissue are not much different. 

Smooth, granular shapes, thin, 

irregularly-shaped, shaving like 

particles. 

Campbell et al. [36] 

JBMR, vol. 29, pp. 127-131 

(1995) 

*LA orthopedic hospital 

Tissues 

(THR) 

NaOH digestion, 

density gradient ultracentrifugation, sucrose 

density gradient 

centrifugation 

FTIR, DSC SEM 

* Rounded (0.3 μm) 

* Elongated (1.9 μm) 

* Fibrils with rounded heads at 

one end, thin taper at the other 

Shanbhag et al. [37] 

JBJS, vol. 76-B, pp. 60-67 

(1994) 

*Rush-Presbyterian-St 

Luke’s medical center 

Tissues, 

interfacial 

membran

es (THR) 

KOH digestion, 

centrifugation, hexaneethanol fractions 

FTIR, EDX SEM 

* Spheroids: 0.1 μm to 0.2 μm 

* Fibrils: 0.2 μm to 0.3 μm wide, 

up to 10 μm long. 

* 92 % particles are smaller than 

1 μm. 

Hailey et al. [38] 

PIME, vol. 210, pp. 3-10 

*U. of Leeds, UK 

Tissue 

(THR) 

KOH digestion, 

centrifugation, 0.2 μm 

filtration 

N/A SEM 

0.3 μm up to 3 mm 

Lee et al. [39] 

JBJS, vol. 74-B, pp. 380-384 

(1992) 

*Hospital for special 

surgery, NY 

Tissues 

(THR) 

Digestion in tissue 

solubiliser, 

centrifugation 

N/A Polarized light microscopy 

Short dimension 2 μm to 4 μm, 

long dimension 8 μm to 13 μm. 

Maloney et al. [40] 

JBJS, vol. 77-A, pp. 1301-

1310 (1995) 

*Stanford U. 

Tissues 

(THR) 

Papain digestion X-ray SEM, Coulter multisizer II 

* Fixed cup: 0.4 μm 

* Bipolar: 0.7 μm 

Schmalzried et al. [41] 

JBMR, vol. 38, pp. 203-210 

(1997) 

*LA orthopedic hospital 

Tissues 

(THR, 

TKR) 

NaOH digestion, 

sucrose ultracentrifugation, 

isopropanol density 

gradient ultracentrifugation 

FTIR, DSC SEM 

* Granules: sub-micron 

* Beads: 1 μm to 2 μm 

* Fibrils: up to 5 μm 

* Shreds: 10 μm to 20 μm long, 

up to several microns wide. 

Hirakawa et al. [42] 

JBMR, vol. 31, pp. 257-263 

(1996) 

*The Cleveland clinic 

foundation 

Tissues 

(THR, 

TKR) 

70 % Nitric acid 

digestion, 

centrifugation, 0.4 μm 

filtration 

EDX SEM, Coulter multisizer II 

* <10 μm particles: 0.72 μm 

(hip), 0.74 (knee) 

* >10 μm particles: 81 μm (hip), 

283 μm (knee) 
15

Figure 2.4 Examples of UHMWPE particles retrieved from total knee

replacement. 
16

2.3.2 Wear particles induced osteolysis 

Osteolysis that causes the loosening of orthopedic joint replacements is 

recognized as the major problem leading to the failure of the implants. The region of 

osteolysis has a lower bone density and can be detected by X-ray photograph as shown 

in Figure 2.5. Macrophage phagocytosis of wear particles and the consequent 

immunological responses, including the production of bone resorptive cytokines and 

activation of osteoclast cells are the major causes of the osteolysis. Table 2.3 provides 

a list of definitions of biological terms helpful for understanding the mechanism 

mentioned below. From the articulation between metal and UHMWPE parts, wear 

particles have been generated and embedded among surrounding tissues [52-54]. 

Macrophage cells act as the first defensive line to protect the host. Macrophage cells 

are activated by wear particles as foreign substances and then engulf and digest the 

particles [55]. The process is called phagocytosis of wear particles by macrophage 

cells. It is believed that numerous of inflammatory products, including TNF-α, IL-1β, 

and IL-6 cytokines, are released during the process [56-57]. Osteoblasts are stimulated 

by those mediators to produce osteoclast promoting substances, like GM-CSF, IL-6, 

and PGE-2 [56-57]. Osteoclast cells are finally activated and these resorb bone cells 

surrounding the artificial joint insertion area. The phenomenon is called bone 

resorption and osteolysis [52-53]. 
17

Table 2.3 Definition of biological terms related to osteolysis 

Terms Definition 

Osteolysis Dissolution of bone through disease, commonly due to infection or by 

loss of blood supply to the bone. 

Necrosis The death of some of all the cells in an organ or tissue, caused by 

disease, physical or chemical injury, or interference with the blood 

supply. 

Phagocytosis The engulfment and digestion of bacteria and other foreign particles by 

a cell. 

Macrophage A large scavenger cell (a phagocyte) present in connective tissue and 

many major organs and tissues, including the bone marrow, spleen, 

lymph nodes, liver, and the central nervous system. 

Osteoblast A cell, originating in the mesoderm of the embryo that is responsible 

for the formation of bone. 

Osteoclast A large multinucleate cell that resorbs calcified bone. Osteoclasts are 

only found when bone is being resorbed and may be seen in small 

depressions on the bone surface. 

Cytokine Soluble mediators secreted by macrophages, controlling many critical 

interactions among cells of the immune system. 

IL-1β, IL-6 Cytokine, interleukins mediators 

TNF-α Cytokine, tumor necrosis factor α

GM-CSF Cytokine, colony-stimulating factors 

PGE-2 Cytokine, prostaglandins 

Figure 2.5 Osteolysis taking place around the total hip replacements

which is detected by X-ray diagnosis. 
18

2.4 Wear studies of UHMWPE material 

To simulate the wear of total joint replacement in the human body, wear 

studies of UHMWPE have been performed in bovine serum at ranges of contact 

pressure and sliding speed encountered in real situations [58-66]. Scanning electron 

microscopy results have demonstrated that surface damages of UHMWPE include 

adhesion wear, abrasion, and delamination wear. The contributions of these 

mechanisms to the overall wear rate and the generation of wear particles depend 

predominantly on the contact pressure and secondarily on the sliding speed. 

Transmission electron microscopy has provided the microstructural morphology of 

UHMWPE obtained from sliding: alignment of crystalline regions in the polyethylene 

microstructure parallel to the sliding surface was found to occur during sliding even at 

low contact pressures. The worn polymer surface and debris have been extensively 

characterized for structural changes with X-ray diffraction and differential scanning 

calorimetry. 

Friction and wear behavior depend on many parameters including contact 

geometry, types of contact materials, the quality of surface, and lubrication. It has 

been shown [59-61] that lubrication of the artificial hip joint is complicated and that 

long-term performance is governed by the combined wear, creep, and oxidation 

degradation of the articulating materials. More importantly, it has been shown that a 

tendency for heating exists during articulation in the hip joint and that elevated 

temperatures could increase the wear, creep, and oxidation degradation rate of 

UHMWPE. 
19

In order to investigate the clinical wear performance of UHMWPE, we need to 

understand the biomechanics of the total joint replacement. The force transmitted to 

the hip-joint implant during gait has been measured [67-72]. Figure 2.6 shows an 

example of the forces acting on a hip-joint implant during gait [67]. It is shown to 

exhibit a double-hump shape of the force curve. The maximum force acting on the 

joint implant is as high as 300 % body weight of a person. For a total hip replacement, 

the articulation of the artificial joints consists of three principal components: 

flexion/extension, abduction/adduction and femoral rotation. Gait analyses have 

shown that the motion of the hip joint differs from person to person. However, the 

three basic components of motion are common to most people. The hip-joint 

kinematics of a group of people during normal gait has been studied [73] and it has 

been found that the average levels of flexion/extension, abduction/adduction, and 

femoral rotation were +40°/-3°, +7.5°/-5° and +5°/-5°, respectively. The trajectories of 

contact points on the articular surface of the hip-joint implant per gait cycle were 

plotted by Wang et al. [74] as shown in Figure 2.7. It is seen that each point follows a 

different open loop. Cross-overs of the loops occur everywhere over the entire surface 

of contact. The human joint motion is a multi-directional as are the surface shear 

stresses. 
20

Figure 2.6 An example of forces acting on the hip-joint implant during gait [67].

Figure 2.7 Trajectories of contact points on the articular surface of hip-joint implant.

Contact paths on the hip joint surface 

Cross-over paths 
21

The mechanism of wear and wear particle generation of UHMWPE in a hipjoint implant can be described as follows [74]: (a) joint articulation creates a multidirectional stress field at the wear surface; (b) the principal stress component occurs in 

the direction of flexion and extension; (c) the secondary stress component occurs in 

the direction of abduction/adduction for the hip and in the direction of tibial rotation 

for the knee; (d) preferential orientation of molecules occurs in the direction of the 

principal stress component; (e) orientation leads to strain softening in the transverse 

direction which coincides with the secondary stress component; (f) transverse 

softening leads to surface rupture between the oriented molecules, and (g) surface 

rupture leads to transfer of load to the subsurface shear plan which results in shear 

rupture and the formation of UHMWPE wear particles. The size and shape of 

UHMWPE wear- particles are determined from the above processes. 

Studies have also indicated that screening wear machines based on linear 

motion do not correlate with multi-axial joint simulators and may produce 

misleadingly low wear results [75-77]. Many researchers have implemented 

multidirectional motion wear testing to evaluate the clinical performance of 

UHMWPE in a bench testing system [75-77]. The motion direction has been shown as 

a key factor for accelerated wear of UHMWPE material. 
22

2.5 Evaluation of Bioactivity of UHMWPE Particles 

2.5.1 Wear-particle induced reactions and mechanisms

Osteolysis and bone resorption is the major failure of total joint replacement. 

However, at present, there is no medical or pharmacologic treatment for this problem. 

The precise mechanism of the bone loss remains to be established. Several studies 

have indicated the breakdown of the artificial joint materials and that release of the 

wear particles may be a critical factor. Evaluation of tissue from the interface between 

implant and bone taken from the aseptically loose total joint implant has demonstrated 

the presence of macrophages phagocytosing particles in association with areas of bone 

resorption. It has been postulated that macrophage phagocytosis of wear particles may 

induce several inflammotary mediators stimulating bone resoprtion. To further 

investigate the effect of UHMWPE particles on bioactivity, cell-culture tests and 

animal tests are two major experimental approaches. Table 2.4 gives a summary of 

bioactivity tests from the literature [78-87]. Cell-culture studies and animal tests will 

be further discussed in the following sections.
23

Table 2.4 Summary of bioactivity study of UHMWPE particles from literature 

Reference Experiment PE particles Noted effect 

Allen et al. [78] 

JBJS, vol. 78-B, pp. 

32-37 (1996) 

*U. of Cambridge 

Ortho. Res. Unit 

In-vivo, rats at 

weight bearing 

bone-implant 

interface 

HDPE: round, 2.03 μm Causing chronic inflammatory response 

with numerous foreign-body giant cells 

in peri-prosthetic tissues 

Catelas et al. [79] 

JBMR, vol. 41, pp. 

600-607 (1998) 

*Ecole 

polytechnique de 

Montreal 

In-vitro, 

macrophage cell 

culture 

HDPE: round, 4.5 μm * TNF-α release 

* Cell viability decreased 

Goodman et al. [80] 

Biomaterials, vol. 

17, pp. 1943-1947 

(1996) 

* Stanford U. 

medical center 

In-vivo, rabbits 

through a drill hole 

into proximal tibia 

HDPE: round, (4.7±2.1) μm 106 to109

 particles/ml evoked 

histiocytic reaction 

Green et al. [81] 

Biomaterials, vol. 

19, pp. 2297-2302 

(1998) 

* U. of Leeds, UK 

In-vitro, 

macrophage cell 

culture 

* UHMWPE (Ceridust 3615): 

(0.21±0.069) μm, (0.49±0.11) 

μm, (4.3±1.89) μm, 

(7.2±3.15) μm 

* UHMWPE (GUR 1120): 

(88±29) μm 

* Elevated level of TNF-α and IL-1β: 

* Elevated level of IL-6: 

* Size and volume (number) are critical 

factors in macrophage activation. 

Horowitz et al. [82] 

J. Ortho. Res., vol. 

15, pp. 50-56 (1997) 

* Hospital of U. of 

Penn. 

In-vitro, 

macrophage/ 

osteoblasts coculture 

HDPE: round, (5.4±0.3) μm * TNF-α and PGE2 release 

* Bone resorption 

Howie et al. [83] 

JBJS, vol. 70-A, pp. 

257-263 (1988) 

* Royal Adelaide 

Hospital 

In-vivo, rat at 

cement-bone 

interface 

UHMWPE (Howmedica): 

irregular, 20-200 μm 

Resorption of bone occurred. 

Matinez et al. [84] 

Biomaterials, vol. 

19, pp. 183-187 

(1998) 

* Paseo de la 

Castellana, Spain 

In-vitro, humanosteoblastic cell 

culture 

* UHMWPE (Hoechst): <80 

μm 

* UHMWPE (Howmedica): 

<100 μm 

PE particles inhibit human osteoblastic 

cell growth. 

Murray et al. [85] 

JBJS, vol. 72-B, pp. 

988-992 (1990) 

* Nuffield 

orthopaedic center, 

UK 

In-vitro, 

macrophage cell 

culture 

HDPE: ≈1 μm ( 1200 grit 

silicon carbide paper 

fabricated) 

* Phagocytosis of particles 

* Release of PGE2 Æ osteoclastic bone 

resorption 

Shanbhag et al. [86] 

JBMR, vol. 28, pp. 

81-90 (1994) 

* Rush-PresbyterianSt. Luke’s medical 

center [25] 

In-vitro, 

macrophage cell 

culture 

UHMWPE (GUR 415) 

* (0.47±0.2) μm (retrieved) 

* (0.66±0.6) μm (fabricated) 

* 10x surface ratio to stimulate bone 

resorption 

* Nonosteolytic chronically tolerable 

dose: 1x surface ratio 

Voronov et al. [87] 

JBMR, vol. 39, pp. 

40-51 (1998) 

* U. of Toronto [26] 

In-vitro, 

macrophage cell 

culture 

* UHMWPE: 18 μm to 20 

μm 

* HDPE: 4 μm to 10 μm 

* Smaller particles were phagocytosed 

* Release IL-1β, IL-1α, TNF-α and 

PGE2
24

2.5.2 In-vitro cell culture study 

Many researchers have set up in-vitro cell-culture tests to study the effect of 

polyethylene particles. Catelas et al. [79] applied HDPE particles in an in-vitro 

macrophage cell-culture test and observed an increase of TNF-α release and a 

decrease of cell viability in the system. Horowitz et al. [82] set up an in-vitro 

macrophage/osteoblast co-culture system to study the stimulation response of HDPE 

particles and they observed bone resorption. Vornov et al. [87] compared commercial 

HDPE and UHMWPE particles with different sizes in a macrophage cell-culture 

study. Martinez et al. [84] obtained different UHMWPE particles with different size 

ranges and applied them in an in-vitro human-osteoblastic cell-culture test. It was 

concluded that human osteoblastic cell growth was inhibited due to the presence of 

UHMWPE particles. Green et al. [81] applied particles collected from wear tests in a 

macrophage cell-culture study; from the measurement of IL-1β and TNF-α releases, 

they concluded that the size and volume of particles are critical factors in macrophage 

activation. Shanhbag et al. [86] used particles, prepared by an attrition process, in an 

in-vitro macrophage cell-culture study and determined the critical particle to 

macrophage cell-surface ratio to stimulate bone resorption. 

2.5.3 In-vivo animal tests 

In-vivo animal tests of particles can mimic a suitable environment for 

evaluating biological response induced by polyethylene particles. Allen et al. [78] 

injected HDPE particles into rats at the weight-bearing bone-implant interface. A 
25

chronic inflammatory response with numerous foreign-body giant cells in periprosthetic tissues was observed. Goodman et al. [80] have put HDPE particles through 

a drill hole into proximal tibia in a rabbit and found a histiocytic reaction after a 

critical particle concentration was reached. Howie et al. [83] placed commercial 

UHMWPE particles at the cement-bone interface in a rat and resorption of bone 

occurred. It was found that smaller particles were phagocytosed with elevation of IL1β, TNF-α, and PGE2. 

Current understanding of the particles induced bioactivity can be summarized 

as follows: 

(1) From cell culture and animal studies, a significant elevated level of TNF-α, IL1β, and IL-6 cytokines has been measured. Most studies have indicated that 

phagocytosis of polyethylene by macrophage cells stimulated the release of 

related cytokines. 

(2) Several studies have shown the effects of the round-like UHMWPE particle 

size and concentration on bioactivity. It appears that most biologically active 

particles are within 0.3 μm to 10 μm range [81]. 

(3) From animal tests, bone resorption has been observed for intra-articular 

UHMWPE particle injection experiments. A direct proof that UHMWPE 

particles induce osteolysis has been obtained through this approach. 
26

CHAPTER 3 

PROBLEM DEFINITION

3.1 The need to prolong the life of joint-replacement components 

There are increasing numbers of total joint replacements implanted in the 

United States and around the world [1]. For total hip replacement, an estimated 

number of 170 280 implant surgeries were performed in 1998 in the United States, a 5 

% increase over those performed in 1994. For total knee replacements, the number of 

new implant surgery are 249 944 in 1998 which is a 32 % increase compared to the 

data for 1994. These numbers indicate a rapid growth of demand for total joint 

replacements. Possible reasons include the increase of life expectancy, population 

increase, more active life style, increase of arthritis disease, and younger patients. The 

average life of a total hip replacement is between 10 years to 15 years and less for a 

total knee replacement. Once the failure of a joint implant occurs, the best solution is 

to perform a revision surgery by replacing the damaged parts. The clinical data show 

that the number of revisions for total hip replacements was 29 368 in 1998, a 7 % 

increase over the revisions reported in 1994 in United States. It also shows 21 365 

revisions for total knee replacements, a 34 % increase from 1994 in the United States. 

With the current lifetime of total joint replacements, the percentage of revision 

surgery needed for patients will increase tremendously because of younger patients 
27

and longer life expectancy of the population. Thus there is an urgent need to extend 

the life of the replacement joints. 

3.2 Potential solutions if particle induced bioactivity is understood 

It is possible to enhance the life of total joint replacement by reducing the 

production of the most toxic particle populations in terms of size and shape. For this 

purpose, one needs to know the effects of UHMWPE wear particle size and shape on 

bioactivity. By changing the materials properties such as crosslinking and wear 

durability of UHMWPE, we should be able to adjust wear of the components and 

control the wear particle size. By modifying the contact geometry or surface texturing 

the joint implant surfaces or engineering the material microstructure, one may be able 

to control the size and shape of wear particles. Less toxic particles generated in-vivo 

can prevent or delay osteolysis and bone resorption induced by wear particles. 

From the viewpoint of medical treatment, if the detailed mechanism of particle 

induced bioactivity is understood, one can possibly discover medicines to block the 

pathways and inhibit the occurrence of osteolysis. One may also blend the pathwayblocking agents into the component materials to prevent resulting osteolysis. By 

studying the wear particles generated from joint replacements made by different 

manufacturers, we can possibly estimate the level of immunological response of 

patients. Close monitoring and medical treatment can be applied to help prevent the 

failure of total joint replacements. 
28

3.3 Cause and effect of particle sizes and shapes 

Uncertainty exists as to how UHMWPE wear particles stimulate the sequential 

bioactivities and as to results in clinically observed osteolysis and bone resorption. 

The following questions should be addressed in order to solve this problem. 

• Are all particles bioactive? 

• What is the effect of particle size? 

• What is the effect of particle shape? 

• What is the critical threshold concentration of a particle population 

with certain size and shape? 

In order to provide answers to the above questions, a series of issues needs to 

be addressed. Critical issues include (1) lack of narrowly distributed UHMWPE wear 

particles with controlled size and shape, (2) need of quantitative methods to describe 

the particle populations, and (3) a complete bioactivity study to determine critical 

thresholds for particle-induced biological responses. 
29

3.4 Critical issues 

3.4.1 Availability of narrowly distributed particles 

From clinical retrieval studies of wear particles, it is known that various sizes 

and shapes of UHMWPE particles are generated from total joint replacements [35-42]. 

The variety of particle sizes and shapes is an outcome from the complexity of motions 

and stress conditions on joints in humans’ daily life. Various geometrical designs for 

different joints also contribute to the size and shape of particles generated in-vivo. In 

addition, the variety of material processing and sterilization methods from orthopedic 

implant manufacturers result in different UHMWPE material properties which further 

leads to wide spectra of UHMWPE particle sizes and shapes that are generated. 

Various kinds of polyethylene particles have been prepared for biologicalresponse studies. Commercial resin powders of high-density polyethylene (HDPE) or 

UHMWPE are commonly used because of convenient availability [78-80, 82, 85]. 

HDPE resin powders have a mean particle size in the 4 μm to 10 μm range, and the 

shape of the particle is almost perfectly spherical. However, HDPE has a higher 

density and lower molecular weight than UHMWPE. Some studies have indicated the 

chemical component of particles as one critical factor for biological response. The 

smooth morphology of HDPE powders is also quite different from the rough particle 

surfaces of particles retrieved from clinical studies. Thus it is inadequate to use HDPE 

material for biological response experiments. Figure 3.1 shows a comparison of the 

particle morphology of HDPE powders, UHMWPE powders and retrieved UHMWPE 

particles. Many researchers applied UHMWPE resin particles. However, the mean size 
30

of commercial UHMWPE particles is always 40 μm or larger with a broad size 

distribution. Only round-like UHMWPE particles are available commercially and 

there is lack of elongated particles. The surface energy of commercial particles may 

also differ from wear particles due to the difference in formation mechanism. Particles 

generated from a wear process tend to carry electrostatic charges. Thus the material 

property of the polyethylene resin powders may differ from the particles generated 

from a wear process. The significance of this issue cannot be ignored for linking the 

in-vivo or in-vitro study results to clinical situations. 

To investigate the effects of size and shape of particles to biological response, 

narrowly distributed particles with controlled size and shape are needed. By utilizing 

narrowly distributed UHMWPE particles with controlled size and shape for bioactivity 

study, an understanding of direct biological responses influenced by particle size and 

shape becomes possible. 

HDPE Powders UHMWPE Powders Retrieval UHMWPE Particles

Figure 3.1 Comparison of polyethylene powders and particles. 
31

3.4.2 Characterization of particles 

 The morphology of wear particles is strongly related to the surface properties 

of material and wear mechanism under a set of operating conditions. According to 

various features of wear particles, possible wear mechanisms and failures of the 

system may be identified. It is feasible to judge the wear particles objectively by visual 

inspection as belonging to different populations. However, quantitative and systematic 

methods for determining the morphological attributes in terms of their size, shape, and 

surface texture are desirable to understand their effects on bioactivity. 

3.4.3 Bioactivity tests of UHMWPE particles 

Various in-vitro cell-culture studies and in-vivo animal tests of UHMWPE 

particles have been used to evaluate biological response induced by particles. The 

advantages of cell-culture studies are control of cell numbers, cell-incubation 

environment, in-situ observation of particles-cells reactions, and easy measurement of 

cell-released products. However, the complexity of the biological environment is 

lacking due to the simplification of the studies. In contrast, animal tests simulate a 

testing environment more similar to the real situation. However, the disadvantages 

include the variation between test animals, difficult control of testing parameters (such 

as cell numbers, temperature), and increase of uncertainty factors. 

Before applying various particle sizes and shapes for bioactivity tests, proper 

experimental design is required to understand the particle-induced osteolysis in total 

joint replacements. 
32

3.5 Approaches 

3.5.1 Generation of particles by surface texturing 

In order to generate sufficient narrowly distributed UHMWPE particles with 

controlled size and shape, we proposed to rub UHMWPE against a textured surface 

that contains numerous cutting features. The surface textures should be designed so 

that the individual feature on the surface is acting as a cutting device to UHMWPE 

material. Geometric constraints provided by the cutting edges on the surface texture 

define the volume of a particle. By changing the geometry of the surface textured 

cutting features and the mechanical conditions of the micro-cutting process, the 

particle size and shape can be changed. Thus by designing a geometrically patterned 

surface texture, particles of similar size and shape can be generated. 

3.5.2 Modeling of particle-generation process 

It is desirable to develop a model to predict the size and shape of particles 

generated with given surface texture dimensions and mechanical conditions of the 

particle-generation process. The mechanism of particle generation should be 

investigated by focusing on the particle generation by a cutting feature acting on 

UHMWPE. One approach is to perform the simple model experiments to simulate a 

micro-cutting process by observing the cutting process with the use of scaled-up 

cutting edges. In-situ observation of the cutting process is feasible by microscopic 

video- and force measurement systems. By changing the geometry of cutting edges 

and mechanical conditions of the cutting process, the critical criteria of particle 
33

generation can be obtained. With a given cutting-edge geometry and mechanical 

conditions, a contact mechanics approach may be used to calculate the plastic 

deformation of UHWMPE material. The model should be able to calculate the surfacetexture dimensions and mechanical conditions required for generation of UHMWPE 

particles with certain size and shape. 

3.5.3 Characterization of particles 

In order to quantify the effects of UHMWPE particles on induced bioactivity, a 

biophysical approach is proposed to analyze phagocytosis of particles by macrophage 

cells. By integrating the size and shape information of the particles, we hope to derive 

parameters that correlate with the level of bioactivity induced by a particle population. 

By doing so, we can further identify the critical particle size and shape for bioactivity.

3.5.4 Bioactivity tests of UHMWPE particles 

Bioactivity tests of UHMWPE particles have been performed in collaboration 

with medical laboratories. During the development of our particle-generation 

procedure, the particles were regularly examined at Wayne State University for 

possible biological contamination. Furthermore, an air pouch test performed at Wayne 

State University was adopted as the standard method to test UHMWPE particles with 

various sizes and shapes. The air pouch model of inflammation, adopted here, 

resembles the peri-prosthetic tissue encountered in aseptically loosened prosthetic 

components to evaluate the cellular response to UHMWPE particles in vivo. 
34

CHAPTER 4 

PROCEDURES FOR GENERATION AND 

COLLECTION OF PARTICLES 

4.1 Introduction 

The generation of UHMWPE particles by a wear process using surface that has 

been textured is introduced in this chapter. The goal of the particle-generation process 

is to produce sufficient amounts of UHMWPE particles with controlled size and 

shape. A procedure for the generation of such particles by a wear process is developed 

that allows precise measurement of the amounts of wear particles, eliminates possible 

particle contamination, and maintains a sterilized environment. The procedures for 

collecting the UHMWPE particles are described. These particles are needed for 

particle characterization and subsequent bioactivity testing. A high particle recovery 

rate is desirable so that the results of particle analysis can be representative and the 

production rate of particles can be increased. The sizes and shapes of the particles 

were characterized by a computer imaging analysis. The chemical composition of the 

particles was also verified by FTIR measurements. By adjusting the pH value of the 

washing solution, we found that the particle recovery rate can be raised to above 90 %. 

The detailed operation procedures are described in the sections below. 
35

4.2 Generation of UHMWPE particles 

4.2.1 Literature review of UHMWPE particle generation 

There have been several methods applied to generate UHMWPE particles invitro. To simulate the wear process, pin-on-disk unidirectional wear tests have been 

used to generate UHMWPE particles [90-93]. For that purpose an UHMWPE pin was 

applied to slide against a stainless steel flat surface. By changing the roughness of the 

steel surfaces, different average particle sizes were generated. To obtain more 

narrowly distributed particles, Green et al. [81] further divided the particles into 

different size ranges by filtration. Mean particle sizes of 0.21 μm, 0.49 μm, 4.3 μm, 

and 7.2 μm were prepared. The particles obtained from the wear process with a 

randomly polished steel surface all have a spherical-like shape. 

Shanhbag et al. [94] applied attrition of commercial UHMWPE particles under 

cryogenic conditions. The larger resin particles were filtered and the finer UHWMPE 

particles were recovered with a low yield rate (2 g of fine particles were harvested 

from 480 g starting material). The particle population contained fibrils and sphericallike particles. The particle size ranged from 0.1 μm to 33 μm in diameter, with a mean 

of 2.3 μm. It was difficult to isolate the particles with similar shapes in a particle 

population. Although sizes and shapes of particles similar to the clinically retrieved 

particles were claimed, the uniformity and control of sizes and shapes of a particle 

population was not achieved. 

Yarovoy et al. [95] fabricated micrometer and sub-micrometer sized 

UHMWPE particles from an organic solution by employing a precipitation method. 
36

This method involves the formation of an emulsion in which droplets of a solution are 

dispersed in a non-solvent medium. Since the emulsions are very sensitive to changes 

in temperature, the concentration and ratio between the components, the size of the 

droplets, and the size of the precipitated particles can be controlled. However, only 

spherical-like particles were produced. The variation of the synthesis process of 

UHMWPE may result in differences of the physical properties from the UHMWPE for 

clinical use. This method is also inadequate to produce particles for systematically 

investigating the effects of UHMWPE particle size and shape on bioactivity 

systematically.

4.2.2 Concepts of surface texturing 

We use a micro-cutting principle to generate wear particles. Surface texture 

patterns act as cutting edges against UHMWPE under rubbing conditions. Geometric 

constraints provided by the texture topography and the relative sliding determine the 

volume for the generated particles. Thus by designing a repetitive geometric pattern, 

particles of similar size and shape can indeed be generated. Dimensions of the cutting 

edges play a significant role in a micro-cutting process. Curvature or radius of the 

cutting edge is a key factor to control the local asperity contact stress. The 3-

dimensional spatial arrangement of the cutting edges controls the geometric 

constraints and results in the formation of different particles. Thus, different kinds of 

wear particles can be generated by micro-cutting processes with different surfacetexture designs. 
37

4.2.3 Preparation of materials 

GUR 415 UHMWPE1

 cylinder pins were machined to the dimensions of 6.35 

mm in diameter and 25.4 mm in length with diamond turning on both end surfaces. No 

further polishing on the material is allowed to avoid residual stresses remaining on the 

material surface. UHMWPE is hydroscopic, i.e. it absorbs water. The content of water 

absorbed will change the unit weight per volume and also change the mechanical 

property. During the wear tests, one end of the UHMWPE pin is fixed by a sample 

holder and the other end is immersed in the water. In order to maintain consistency of 

the test, the UHMWPE pin must be water saturated at the beginning of the test. It can 

prevent the pin from further absorbing water during the wear test. During the wear 

tests, the UHMWPE pin is tested in a closed space wrapped by a plastic membrane. 

The water-vapor pressure reaches equilibrium and the water content in the UHMWPE 

pin remains the same. 

To determine the time needed to reach saturation with water, six pins were 

immersed in the water and they were weighed periodically as shown in Figure 4.1. We 

have concluded that the UHMWPE must be pre-soaked in water at least 15 days 

before testing to reach water-saturation equilibrium. 

_____________________________________________________________________ 

1

GUR 415 is a brand name of UHMWPE resin produced by Hoechese-Celanese. 

Certain commercial equipment, instruments, or materials are identified in this paper to 

specify adequately the experimental procedure. Such identification does not imply 

recommendation or endorsement by the National Institute of Standards and 

Technology, nor does it imply that the materials or equipment identified are 

necessarily the best available for the purpose. 
38

Figure 4.1 Weight change of pre-soaked UHMWPE pins. 

(h) (h)

(h) (h)

(h) (h)
39

 To avoid chemical, particle, and biological contamination in the process for 

particle generation, all materials and liquids were purified and sterilized. The 

following procedure was carried out in a clean room and the operator always wore a 

hat, mask, and powder free gloves for every step of the operation. 

 The water used in this study was purified through deionization, filtration by 0.1 

μm pore-size membrane and double distillation to avoid both chemical and particle 

contamination. Then the water went through a high-temperature, high-pressure 

autoclave process to eliminate possible microbial contamination. Prior to the test, all 

parts involved in the experiments were immersed in a 70 % ethanol + 30 % purified 

water solution (volume fraction) overnight for sterilization. The ethanol also went 

through filtration by 0.1 μm pore-size membrane to avoid particle contamination. 

 Before the tests, containers, sample holders, and textured plate, made of 316 

stainless steel, must be cleaned by acetone and hexane solution (volume ratio 1:1) in 

an ultrasonic bath for 3 min. Then they were cleaned with a detergent in an ultrasonic 

bath for another 3 min. The parts were then rinsed in the purified water and blowed 

dry by filtered compressed nitrogen gas. 

4.2.4 Linear reciprocating wear process 

The linear reciprocating wear test method was used to generate UHMWPE 

wear particles. ASTM F732 is used as a guideline. The setup of the facility is shown in 

Figure 4.2. With the driving mechanism of the linear reciprocating wear tester, a 

constant rotation speed is kept while the test is performed. Thus the velocity of the 
40

linear reciprocating motion is a sinusoidal distribution depending on the stroke length 

and phase angle of the drive mechanism. 

Load (Nominal contact stress) 

Reciprocating sliding 

UHMWPE pin 

Counterpart materials 

Purified water 

 Figure 4.2 Schematic representation of linear reciprocating wear tester. 

Dead weight load

Balanced weight

Textured plate 

UHMWPE pin

Linear bearing Motor
41

Before tests of particle generation, one UHMWPE pin and three control pins 

were dried by blowing with filtered compressed nitrogen and weighed three times. 

Three control pins were soaked in water at the same water level as the wear-testing 

condition. The sample holders and the textured plate were taken from the 

(alcohol/purified water) solution and were immediately blowed dry by filtered 

compressed nitrogen. The UHMWPE pin was then mounted on the sample holder and 

the textured plate was fixed at the lower container. Ultrajet clean air was applied to the 

UHMWPE pin and the lower container and then purified water was added to the 

container. Then the UHMWPE pin with its holder and the lower container were 

wrapped by a clean plastic membrane. The contact surface between UHMWPE pin 

and textured plate was leveled and the rotation speed of the motor was checked before 

the normal load was applied. Then a dead weight was applied above the UHMWPE 

pin holder and the test was started. The rotation speed was monitored during the wear 

process. The linear reciprocating motion stopped after a specified duration. 

4.3 Collection of UHMWPE particles 

4.3.1 Particle-collection procedure 

A flow chart of the particle-collection process is shown in Figure 4.3. After the 

wear tests, the UHMWPE wear particles suspended in the water were poured into a 

sterilized glass beaker. Purified water with adjusted pH value was prepared in a Guth 

bottle that has the capability to inject water with a constant velocity. The sample, 

sample holder, and parts that came into contact with particles were repeatedly rinsed 
42

in a sterilized glass beaker by ejecting purified water from the Guth bottle. The 

particles suspended in the water went through a filtration process. To measure the 

weight of the collected UHMWPE wear particles, a polycarbonate filter membrane 

with 0.1 μm pore size (Milipore) was put into a plastic dish with the cap. The weight 

was measured with a resolution of ±0.01 mg. The filter membrane was then placed on 

the filter holder of a vacuum filtration system. Particles with water in the beaker were 

then poured into the filtration system and again the Guth bottle with purified water 

was used to wash the beaker and the upper filter holder until no particles were 

observed on the cylinder wall. After the filtration process, the UHMWPE wear 

particles were collected on the surface of the filter membrane. The filter membrane 

was then put back in the plastic dish in the clean room for three hours to evaporate the 

water absorbed by the membrane. The weight of the filter membrane with particles in 

the plastic dish with the cap was then measured and the weight of the collected wear 

particles was obtained from the difference of the two weight measurements. 
43

Preparation

Collection

Measurement

Water purification

Sterilization

Weigh 0.1 μm 

membrane filters

Pour water from 

the holder into a 

glass beaker 

- Deionization 

- 0.1 μm filtration 

-Metal parts & glassware 

immersed in ethanol/water 

solution overnight 

-Autoclave purified water

Wash sample holder 

with pH 5.5 water 

Vacuum filtration 

(0.1μm membrane) 

Dry the filter(s) in air

Weigh the filter(s) with particles

Figure 4.3 Particle-collection procedure. 
44

4.3.2 Effect of pH value on particle recovery

It turned out that UHMWPE wear particles apparently carry a weak 

electrostatic charge and have a tendency to stick to the glass beaker surface during the 

particle-collection process. By adjusting the pH value of the water, the charges on the 

particles can be controlled for a fuller recovery rate. A simple experiment was 

performed to find the optimum pH value for particle-removal process from glass. A 

known amount of UHMWPE particles was dispersed in filtered isopropyl alcohol in 

the ultrasonic bath for 3 min. 1 mL of the UHMWPE isopropyl alcohol suspension 

was dropped on a Pyrex glass slide. The number of particles in a given area was 

counted under an optical microscope. Hydrochloric acid solution was added to 

purified water (deionized, 0.1 μm pore-size filtered, and double distilled) to prepare 

washing solutions with different pH values. 1 mL of the washing solutions was used to 

rinse the glass slide with particles on it, and this procedure was repeated 5 times. Then 

the particles remaining on the glass slides were counted again under optical 

microscope. The number of particles that were removed by the rinsing process was 

obtained and the degree of particle removal can be calculated (0 for no particle 

removal, and 1 for all particles removal). Figure 4.4 shows that washing water with a 

pH value around 5 to 6 has the best ability to remove the UHMWPE particles from the 

glass slides. Several particle generation wear procedures with different total wear loss 

were performed. A comparison of particle recovery rate between the cases of particle 

collection using pH≈5.5 washing water and the cases of particle collection using no 

pH control is presented in Figure 4.5. It indicates that by adjusting the pH value of 
45

washing water to 5.5, the particle recovery rate was improved from 40 % to 60 % to 

nearly 90 %. 

Figure 4.4 Effect of pH value of water on removing particles on the glass slide. 

pH value of water sample

2 3 4 5 6 7 8 9 10 11

Degree of particles removal 

0.0

0.2

0.4

0.6

0.8

1.0
46

Total wear loss of UHMWPE pin (mg)

Percent of recovery (%)

0 1 2 3 4 5 6 7

0

20

40

60

80

100

water pH≈5.5 

no pH control 

Figure 4.5 Effect of pH value of water on particle-recovery rate. 
47

4.4 Particle characterization 

4.4.1 Chemical composition of particles 

To detect the chemical compositions of the collected wear particles, the 

microscopic Fourier transform infrared spectroscopy (FTIR) technique was applied. 

The collected particles were mixed with KBr powders to form a sample. This sample 

was then analyzed by FTIR in a transmission mode. Bulk UHMWPE raw material was 

also examined by using attenuated total-reflection Fourier-transform infrared 

spectroscopy (ATR-FTIR) in a contact mode. The IR absorbance spectra are compared 

in Figure 4.6. The peaks in the spectra represent the methyl groups (C-H stretching in 

(2800-3100) cm-1, CH2 scissors near 1450 cm-1, and CH2 rocking at 730 cm-1) that are 

the characteristic absorption for polyethylene. The noise seen from the spectrum of 

collected particles is due to small quantity of particles. The uncertainty of the 

measurement comes from the noise of the signal. A match of the IR spectrum of 

collected particles and the bulk UHMWPE material verifies the main chemical 

composition of collected wear particles is polyethylene. 

4.4.2 Image analysis of particles 

The polycarbonate membrane filters with wear particles were coated with a 

thin layer of gold. Then SEM observation of the particles on the filter was performed. 

The magnification was adjusted to clearly detect the boundaries of the particles. At 

least 200 particles were analyzed in each experiment. The photographs were scanned 

into computer image files. The Scion Image program for Windows 95 was applied to 
48

analyze the particles. Different parameters for particle size and shape can be calculated 

by incorporating an appropriate subroutine in the program. In most cases throughout 

this study, the longer axis of the particle and the minor axis of the particle represent 

the particle length and width. The aspect ratio is defined as the ratio of particle length 

to particle width. The results of the image analysis will be given for the particlegeneration processes in the following chapters.

Collected Particles 

UHMWPE

Figure 4.6 IR absorbance spectra of UHMWPE bulk material and of 

collected particles. 

Wavenumbers (cm-1)
49

CHAPTER 5 

PARTICLE GENERATION WITH 

SCRATCHED STEEL SURFACES

5.1 Introduction 

This chapter describes the first attempt at surface texturing to prove feasibility 

of concept. Stainless steel plates were roughened by SiC abrasive papers in order to 

generate surface textures. Then UHMWPE pins were rubbed against them to generate 

particles. Unidirectional and crosshatched patterns were made with SiC abrasive 

papers. Textured surfaces with different roughness were prepared by using different 

grades of abrasive papers. The scratched stainless-steel surfaces were then mounted on 

the wear test facility to generate UHMWPE particles. Effects of the feature geometry 

and roughness of the textured surfaces are discussed. To further improve the 

uniformity of textured surfaces, single diamond tips were utilized to scratch the 

stainless steel surface with specified penetration depth and length. The effect of 

uniformity of the surfaces on the particle distribution is shown in this chapter. 

5.2 Particle generation with abrasive-paper made surfaces 

5.2.1 Preparation of steel surfaces with SiC abrasive papers 

Stainless steel 316 plates (38 mm in diameter and 6.35 mm in thickness) were 

polished to a surface roughness (Ra) of less than 0.025 μm (Polishing progression with 

the sandpapers: 120grit Æ 320 grit Æ 400 grit, and then diamond suspensions: 6 μm 
50

Æ 1 μm). Texturing of the steel plate was achieved by making systematic onedirectional grooving passes using silicon carbide (SiC) abrasive papers made by Teco. 

The grooving passes were made under a nominal contact pressure of 2 kPa. Each pass 

was 150 mm long. The surface was then returned to the starting position and scratched 

again. The procedure is shown schematically in Figure 5.1. For unidirectional textures, 

twenty passes were made. To produce a crosshatched pattern, the steel plate was 

rotated 90o

 after each pass for a total of twenty passes. A single grit size of SiC 

abrasive paper was used to texture each stainless-steel plate. The textured plate was 

then cleaned ultrasonically in solvents and detergent, followed by rinsing with purified 

water and drying with nitrogen. Examples of SEM photos of unidirectional and 

crosshatched surface textures are shown in Figure 5.2. 

The steel surface made with abrasive papers was then mounted on the linear 

reciprocating wear machine. The particle generation process proceeded and the 

particles were collected and analyzed as described in Chapter 4. 

5.2.2 Results 

The wear experiments were conducted under the following conditions: normal 

load is 196 N (nominal contact pressure ≈6 MPa), stroke length is 19.05 mm, 

frequency is 1.5 Hz; average sliding velocity is 57.2 mm/s, duration is 2 h. Purified 

water was used as lubricant. Table 5.1 shows the roughness of the steel plates, wear 

loss, particle length and width, and aspect ratio from four different surface textures 
51

including unidirectional and cross-hatched surface texture features made by 120 grit 

and 320 grit SiC abrasive papers. 

Top View 

A direction 

B direction 

Load 

316 stainless steel 

SiC abrasive paper 

PROCEDURES: 

Unidirectional textures: 

1. Scratch along A direction (150mm long) 

2. Use a new paper and repeat step 1 

3. Repeat steps 1 & 2 for 20 times 

Cross-hatched textures: 

1. Scratch along A then B directions (150mm long) 

2. Use a new paper and repeat step 1 

3. Repeat steps 1 & 2 for 10 times 

 Figure 5.1 Procedure of surface texturing with SiC abrasive papers. 
52

(a) 

(b) 

Figure 5.2 (a) Unidirectional and (b) cross-hatched surface textures made 

on 316 stainless steel plate. 

UHMWPE 

pin sliding 

direction 

(top view) 

UHMWPE 

pin sliding 

direction 

(top view) 
53

Table 5.1 Results of particle generation with abrasive paper made surfaces. 

120 grit, // 120 grit, # 320 grit, // 320 grit, # 

Wear (mg) 4.81±0.08 2.66±0.03 1.26±0.04 0.30±0.06 

Mean particle length (μm) 47.0±19.0 22.7±10.6 31.6±16.7 12.4±7.0 

Mean particle width (μm) 14.8±3.2 12.7±3.0 10.1±2.8 6.4±2.4 

Mean particle aspect ratio 3.44±1.7 1.89±0.7 3.60±1.9 2.07±0.8 

Note: //: unidirectional texture; #: crosshatched texture. (mean±standard deviation) 

Wear was calculated based on the mass difference of the UHMWPE pin before 

and after the test. It has been shown in Chapter 4 that more than 90 % of the generated 

particles were recovered. The SEM micrographs, distributions of length and aspect 

ratio of the particles generated with the four different surfaces are shown in Figures 

5.3-5.6. The mean particle dimension ± standard deviation are also shown in the 

figures. Generally speaking, the particles generated from the wear process have 

irregular and rough edges similar to clinically retrieved particles. The different particle 

shapes generated by the unidirectional and crosshatched surfaces are shown in the 

figures. The mean particle size is controlled by the roughness of the surface. Effects of 

the surface roughness and surface texture styles will be discussed in detail in the 

following sections. 
54

Aspect Ratio

2 4 6 8 10

Percent (%)

0

2

4

6

8

10

12

14

16

18

Particle length (μm)

0 20 40 60 80 100

Percent (%)

0

2

4

6

8

10

12

14

Figure 5.3 Particles generated with 120 grit abrasive paper made unidirectional

stainless steel surface. (a) SEM micrograph, (b) Distribution of

particle length, (c) Distribution of particle aspect ratio. 

(a) 

(c) 

(b) 

Mean particle length 

= (47.0±19.0) μm 

Mean aspect ratio 

= 3.44±1.7 
55

(a) 

(c) 

(b) 

Aspect Ratio

2 4 6 8 10

Percent (%)

0

5

10

15

Particle length (μm)

0 20 40 60 80 100

Percent (%)

0

5

10

15

Figure 5.4 Particles generated with 320 grit abrasive paper made unidirectional

stainless steel surface. (a) SEM micrograph, (b) Distribution of

particle length, (c) Distribution of particle aspect ratio. 

Mean particle length 

= (31.6±16.7) μm 

Mean aspect ratio 

= 3.6±1.9 
56

Figure 5.5 Particles generated with 120 grit abrasive paper made crosshatched

stainless steel surface. (a) SEM micrograph, (b) Distribution of

particle length, (c) Distribution of particle aspect ratio. 

(a) 

(c) 

(b) 

Particle length (μm)

0 20 40 60 80 100

Percent (%)

0

5

10

15

20

25

30

Aspect Ratio

2 4 6 8 10

Percent (%)

0

5

10

15

20

25

30

35

40

Mean particle length 

= 22.7±10.6 μm 

Mean aspect ratio 

= 1.89±0.7 
57

Figure 5.6 Particles generated with 320 grit abrasive paper made crosshatched

stainless steel surface. (a) SEM micrograph, (b) Distribution of

particle length, (c) Distribution of particle aspect ratio. 

(a) 

(c) 

(b) 

Aspect Ratio

2 4 6 8 10

Percent (%)

0

10

20

30

Particle length (μm)

0 20 40 60 80 100

Percent (%)

0

10

20

30

40

Mean particle length 

= (12.4±7.0) μm 

Mean aspect ratio 

= 2.07±0.8 
58

5.3 Effect of surface-texture pattern on particle shape 

Particles generated from the unidirectional steel surfaces have large aspect 

ratios compared to the particles generated from the crosshatched surface. 

Unidirectional surface textures were found to generate more elongated particles 

(Figures 5.3-5.4) and crosshatched surface textures generated round-like particles 

(Figures 5.5-5.6). In Figure 5.7, we further compare the aspect-ratio distribution for the 

particles generated from unidirectional and crosshatched surfaces. Over 85 % of the 

particles in the group with elongated shapes are the ones with aspect ratio larger than 2. 

On the other hand, a portion of more than 75 % of the group with round-like shapes has 

an aspect ratio less than 2. Evidently, the different types of surface textures produce 

wear particles with two distinct aspect ratios. This result demonstrates the feasibility 

of generating UHMWPE wear particles with different shapes through surface 

texturing. 

The basic underlying concept is that the size and shape of the wear particles 

can be controlled by the cutting edges and by the geometric constraints superimposed 

on the volume defined by the groove edges. The unidirectional texture presents an 

array of parallel cutting edges on the stainless steel surface. During sliding, these 

edges cut into the UHMWPE and produce elongated particles. The crosshatched 

texture, on the other hand, has diamond-shaped edges with an internal volume. While 

the edges cut into the soft elastic UHMWPE material, the diamond-shaped volume of 

the intersection of the grooves controls the round particle size. 
59

Aspect Ratio

2 4 6 8 10

Percent (%)

0

5

10

15

20

25

30

35

40

Unidirectional, 120 grit 

Cross-hatched, 120 grit

Aspect Ratio

2 4 6 8 10

Percent (%)

0

5

10

15

20

25

30

35

40

Unidirectional, 320 grit 

Cross-hatched, 320 grit

Figure 5.7 Comparison of particle aspect-ratio distribution from abrasive paper 

made stainless steel surfaces with different surface-texture styles. 

(Uncertainty of the measurement comes from the image measurement of 

particles which is estimated to be less than 5% of the mean value)
60

5.4 Effect of surface-texture roughness on particle size 

The rougher surfaces were prepared by the abrasive papers that are bonded 

with larger SiC powders. The roughness (Ra: mean of the deviation of height to the 

average height of the surface profile) was measured perpendicular to the groove 

direction for the unidirectional steel surfaces. From Table 5.2, it is seen that for the 

unidirectional surface textures, when the surface roughness was increased from 0.223 

μm to 0.524 μm, the mean particle length increased from 31.6 μm to 47.0 μm. For the 

crosshatched surface textures, the roughness was measured in the diagonal direction to 

the grooves. When the surface roughness was increased from 0.175 μm to 0.413 μm, 

the mean particle length increased from 12.4 μm to 22.7 μm. However, the mean 

aspect ratios of the particles are similar for each kind of surface texture. Figure 5.8 

further compares the distribution of the length of the particles generated from the 

surface textures with different roughness. The surface profile is magnified while a 

rougher SiC abrasive paper is used to prepare the surface texture. The larger particles 

are generated because larger cutting features are applied in the cutting process of 

particle generation. 

Table 5.2 Effect of roughness of stainless steel surfaces on particle dimensions. 

120 grit, // 120 grit, # 320 grit, // 320 grit, # 

Surface roughness Ra (μm) 0.524±0.21 0.413±0.18 0.223±0.12 0.175±0.10 

Mean particle length (μm) 47.0±19.0 22.7±10.6 31.6±16.7 12.4±7.0 

Mean particle width (μm) 14.8±3.2 12.7±3.0 10.1±2.8 6.4±2.4 

Mean particle aspect ratio 3.44±1.7 1.89±0.7 3.60±1.9 2.07±0.8 

Note. //: unidirectional texture; #: cross-hatched texture. (mean±standard deviation) 
61

Particle length (μm)

0 20 40 60 80 100

Percent (%)

0

5

10

15

20

Unidirectional, 120 grit 

Unidirectional, 320 grit

Particle length (μm)

0 20 40 60

Percent (%)

0

5

10

15

20

25

30

35

40

Cross-hatched, 120 grit 

Cross-hatched, 320 grit

Figure 5.8 Comparison of particle-length distribution from stainless steel 

surfaces made by different grades of abrasive papers. 

(Uncertainty of the measurement comes from the image measurement 

of particles which is estimated to be less than 5% of the mean value) 
62

5.5 Particle generation with diamond-tip scratched surfaces

5.5.1 Preparation of diamond-tip scratched stainless steel surfaces 

Stainless steel 316 plates (38 mm in diameter and 6.35 mm in thickness) were 

polished to a surface roughness of less than 0.025 μm. A 90o conical diamond tip was 

mounted on a scratching tester. The tester is capable of moving in the X, Y, and Z 

directions with displacement resolution of (1±0.2) μm. The stainless-steel plate was 

fixed and leveled on the stage. The diamond tip was lowered in the Z direction to be in 

contact with the polished steel surface. The contact point was defined when the 

contact force was measured and monitored by a force-recording system. We further 

lowered the diamond tip to indent the steel surface with a 3 μm interference depth. 

Then the diamond tip was moved a pre-defined distance by a computer in the Ydirection to scratch the steel surface with a sliding speed of 1 mm/s. Purified paraffin 

oil was used here as a lubricant. After the scratching, the diamond tip was lifted. Then 

the diamond tip moved in the X and the Y directions over specified distances and the 

scratching process was repeated. A schematic representation of the scratching process 

is shown in Figure 5.9. Numerous scratches were made on the stainless-steel surface 

with various scratching lengths and depths. The textured plate was then cleaned 

ultrasonically in solvents and detergents, followed by rinsing with de-ionized water 

and drying with nitrogen. 

The diamond-tip scratched steel surfaces were then mounted on the linear 

reciprocating wear tester. The particle generation process proceeded and the particles 

were collected and analyzed. 
63

Diamond tip angle= 90°

316 Stainless steel

Programmable sliding w/ 

controlled interference depth

Polished surface

Procedure: 

• Level the steel plate 

• Detect the diamond tip-steel plate contact point by 

force sensor 

• Program the actuator with interference depth, 

travel distance and speed 

• Use pure paraffinic oil as lubricant 

• Start scratching the surface 

• Clean the surface by detergent with ultrasonic bath 

 Figure 5.9 Procedure of surface texturing by diamond scratching. 
64

5.5.2 Results 

Two diamond-tip scratched steel surfaces were prepared with an interference 

depth of 3 μm. To discuss the effect of scratched dimensions on particle dimensions, 

two scratched lengths were chosen for comparison. One had linear scratches over the 

whole steel surface. The other one had a 60 μm scratching distance with a distance of 

60 μm between two scratches along the scratching direction. The distance between 

adjacent scratches for both surfaces was 300 μm. Figure 5.10 shows the SEM 

micrographs of both diamond-tip scratched steel surfaces. 

The wear experiments were conducted under the following conditions: normal 

load is 98 N (nominal contact pressure≈ 3 MPa), stroke length is 19.05 mm, frequency 

is 1.5 Hz; average sliding velocity is 57.2 mm/s, duration is 2 h. The UHMWPE pin 

was rubbed against the diamond-tip scratched surface in the direction perpendicular to 

the scratched grooves as shown in Figure 5.10. The SEM micrographs of the 

UHMWPE particles, and the distributions of particle length and particle aspect ratio 

for the two cases are shown in Figures 5.11 and 5.12. The mean particle lengths ±

standard deviations and aspect ratios ± standard deviations for both cases are listed in 

Table 5.3. 

Table 5.3 Results of particle generation with diamond-tip scratched steel surfaces. 

 Linear scratched surface Scratched distance= 60 μm 

Mean particle-length (μm) 41.7±10.2 24.8±4.8 

Mean particle-width (μm) 14.6±3.9 13.7±3.1 

Mean particle aspect-ratio 2.85±0.62 1.81±0.32 
65

 Figure 5.10 SEM micrographs diamond-tip scratched surfaces: (a) with 

linear scratches, (b) with a 60 μm scratched length. 

UHMWPE 

pin sliding 

direction 

(top view) 

(b) 

UHMWPE 

pin sliding 

direction 

(top view) 

(a) 
66

Aspect Ratio

2 4 6 8 10

Percent (%)

0

5

10

15

20

Figure 5.11 Particles generated with the diamond-tip scratched stainless steel

surface (linear type) (a) SEM micrograph, (b) Distribution of particle

length, (c) Distribution of particle aspect ratio. 

(a) 

(c) 

(b) 

Particle length (μm)

0 20 40 60 80 100

Percent (%)

0

5

10

15

20

Mean particle length 

= (41.7±10.2) μm 

Mean aspect ratio 

= 2.85±0.62 
67

Figure 5.12 Particles generation with the diamond-tip scratched stainless steel

surface (scratched distance= 60 μm) (a) SEM micrograph, (b)

Distribution of particle length, (c) Distribution of particle aspect ratio. 

(a) 

(c) 

(b) 

Particle length(μm)

0 20 40 60 80 100

Percent (%)

0

5

10

15

20

25

30

35

Aspect Ratio

2 4 6 8 10

Percent (%)

0

5

10

15

20

25

30

35

40

Mean particle length 

= (24.8±4.8) μm 

Mean aspect ratio 

= 1.81±0.32 
68

5.6 Effect of surface uniformity on particle distribution 

When the surface textures are prepared by SiC abrasive papers, only the 

surface roughness, and surface-texture patterns can be changed by using different 

grades of abrasives and different abrading directions. This process produces a wide 

distribution of cutting edges. By applying a diamond-tip scratching technique with 

displacement control, the uniformity of the surface features is significantly improved. 

Figure 5.13 compares the surface profiles (meassured by a surface profilometer) of the 

linear-type surface features made with SiC abrasive appers and with the diamond-tip 

scratching technque, respectively. The tracing direction is perpendicular to the groove 

direction. The standard deviations of particle length generated are shown in Figure 

5.14. Results include the particle populations from the surface textures made with SiC 

abrasive papers and from the surface textures made with the diamond-scratching 

process. With similar mean particle length, diamond-scratched steel surfaces do 

produce particles with narrower distribution. However, the improvement is not 

sufficient to generate particle populations with fully separate particle distributions. 

One reason leading to the improvement is that the depth of the groove can be 

controlled by the penetration depth of diamond tip during the scratching process. 

However, positive material bumps along both sides of the grooves are produced by the 

material plastic deformation process. Thus there exists some difficulty to further 

precisely control these positive bumps which act as cutting edges. The results also 

show that when the scratched distance was reduced to 60 μm from the linear scratches, 

the particle length decreased. Ideally, the particle length should be equal to the cutting-
69

edge length defined by the scratched distance in this example. However, imperfection 

of the scratch-induced cutting edges results in smaller cutting-edge lengths in real 

situations. To further narrow the particle distribution, one needs to be able to fabricate 

cutting edges with precisely defined dimensions, especially when micrometer scaled 

features are needed. 

(a)

(b)

Figure 5.13 Examples of surface profiles for (a) SiC abrasive-paper made

stainless steel surface, and (b) Diamond-tip scratched steel

surface. 
70

Mean particle-length (μm)

10 15 20 25 30 35 40 45

Standard deviation of particle length (

μm)

0

5

10

15

20

25

Diamond scratched surface

SiC abrasive surface

Figure 5.14 Comparison of standard deviation and mean particle length for

different particle-size populations made with surfaces made

with SiC abrasive papers and with diamond-tip scratching

process. 

(Uncertainty of the particle-length measurement is denoted as 

standard deviation in the plot) 
71

CHAPTER 6 

SURFACE TEXTURING BY MICROFABRICATION

6.1 Introduction 

 The particle distribution appears to be strongly related to the uniformity of the 

textured surfaces. The uniformity of the textured surfaces needed to be further 

improved. Microfabrication techniques for semiconductor and micro-electromechanical devices (MEMs) manufacturing have been developed to fabricate features 

with a sub-micrometer resolution [96]. By preparing surface textures with 

microfabrication techniques, the dimensions of each cutting feature on the surface can 

be precisely controlled. In this chapter, we describe the procedures to texture surfaces 

by lithography and chemical etching processes. The dimensions of the surface features 

are measured by scanning electron microscopy (SEM), atomic force microscopy 

(AFM) and profilometer techniques. 

6.2 Surface texture with wedge features 

We proposed to fabricate wedge-shaped features on the surface for the microcutting process. With the controlled length and sharpness of the edge, the normal load 

during the particle-generation process can be adjusted to achieve the desired 

deformation of UHMWPE by penetrating the wedge edge into the material. By 
72

adjusting the height of the wedge feature and the distance between adjacent features, 

the confined volume can be determined. In the remaining sections of this chapter, the 

detailed microfabrication processes are described for fabricating a wedge-type surface 

texture. 

6.3 Microfabrication process 

Based on the principles of surface-texture design, microfabrication is applied 

to make effective surface-texture features for particle generation. The procedures 

include the design of surface-texture patterns on a two-dimensional photomask, 

transfer of patterns to the material surface by lithography, etching of materials to 

generate three-dimensional structures, and the surface treatments to enhance wear 

resistance. Figure 6.1 shows schematically the microfabrication process to produce the 

wedge features on the silicon material. The technical details are discussed in the 

sections below. 

6.3.1 Substrate material 

Single crystal silicon was chosen as the substrate material. Polished silicon 

wafers with type P and (100) orientation were prepared as the base material for the 

texturing process. The dimensions of a silicon wafer are 50.8 mm diameter with 0.3 

mm thickness. An additional process for microfabrication of features on stainless steel 

is described in Appendix B. 
73

Photoresist

Coating

Post-Coating 

Bake

Oxide Film Growth <100> P type

Si Wafer

UV Exposure Pattern 

development

Anisotropic

Etching

Strip SiO2 Isotropic Etching Oxide Etching Strip

Photoresist

Figure 6.1 Schematic of microfabrication process to generate wedge

features on silicon. 
74

6.3.2 Masking material 

In order to fabricate wedge features on the silicon substrate by a chemical 

process, we need to put a layer of masking material on the silicon surface. In our 

application, silicon dioxide is chosen because of its lower etching rate by the chemical 

etchant that is applied to etch silicon at the final stage. Thermal oxidation of silicon is 

easily achieved by heating the substrate to the range of 900 °C to 1200 °C. The 

atmosphere in the furnace where oxidation takes place can either contain pure oxygen 

or water vapor. Both of these molecules diffuse easily through the growing silicon 

dioxide layer at these high temperatures. Oxygen arriving at the silicon surface can 

then combine with silicon to form silicon dioxide. A wet oxidation process is applied 

in this study. Hydrogen and excess oxygen gases flow into the furnace and form the 

water vapor. The high-temperature water vapor then reacts with silicon to form silicon 

dioxide 

The relationship between the silicon dioxide thickness and the temperature, the 

time required for the process can be found in the literature [96]. A layer of silicon 

dioxide with 1 μm thickness was grown in our application under 1100 °C for 135 min. 

The thickness of the silicon dioxide film was measured by Nanospec/AFT 

(Nanometrics). The instrument uses a scanned wavelength illumination source to 

determine the thickness and refractive index of thin films. The spectrophotometerhead scans generate a corrected spectrum by computing a ratio to a bare silicon 

reference previously stored. The resulting spectral data are used to determine the exact 

film thickness corresponding to the interference pattern. 
75

6.3.3 Pattern design and transfer 

Two-dimensional patterns were designed. The dark features on the pattern 

(Figure 6.1, under UV exposure step) represent the protected area from the etching 

process. The patterns were then transferred to a chromium coated quartz photomask by 

the direct-writing process at the photomask manufacturer. The critical tolerance 

resolution of the photomask used is 1 μm. 

The silicon wafer with silicon dioxide superlayer was heated on a hot plate at 

205 °C for one min. This is the dehydration process to remove the water remaining on 

the material surface. The wafer was then spin-coated with 1,1,1,3,3-

Hexamethyldislazane (HMDS) at 5000 rpm for 30 s. The purpose of this coating is to 

enhance the adhesion between the wafer surface and the later coated photoresist. 

Shipley 1813 photoresist was spin-coated on the silicon dioxide surface at 5000 rpm 

for 30 s. The silicon wafer was then baked on a hot plate at 100 °C for 2 min to 

enhance the stability of the coated photoresist. The average thickness of the 

photoresist layer is (1.3±0.2) μm. 

 The dark-featured photomask was then placed in contact on the photoresist 

surface at a mask aligner for the following photolithography process. It was exposed 

to a UV source to decompose the surrounding polymer surface leaving the positive 

rectangular patterns that were covered by the dark features on the photomask. A postbake process was carried out at 100 °C for 90 s to accelerate and accomplish the 

decomposition of photoresist. In order to fully remove the decomposed photoresist, the 

wafer surface was rinsed by a Shipley photoresist-developer for 30 s at room 
76

temperature followed by water rinse and blow dry processes. Up to this stage, the 

patterns were transferred on the photoresist layer on the silicon dioxide film. 

 The resulting positive photoresist patterns were now acting as the protecting 

layer for the following silicon-dioxide etching process. The etchant used to etch the 

silicon dioxide contains hydrofluoric acid (HF) and nitric acid (HNO3)with a volume 

ration of 6:1. The wafer was then carefully placed on a bath of the etchant at room 

temperature for 5 min to remove the silicon dioxide areas that are not protected by the 

photoresist. At this stage, the patterns have been transferred to the masking layer of 

silicon dioxide for the following silicon etching process. 

6.3.4 Fabrication of surface features 

Up to this step, the two-dimensional patterns were made on the masking 

material (silicon dioxide). The next step was to convert the two-dimensional patterns 

into three-dimensional structures on the silicon substrate. To construct the wedge 

geometry of the surface feature, etching of the silicon in the vertical and horizontal 

directions is necessary. Etching of the silicon in the vertical direction controls the 

height of the wedge feature. In order to form the sharp linear edge of the wedge, the 

silicon material under the masking silicon dioxide should be etched horizontally from 

outside towards to the center. 

To achieve control of the edge angle, a combination of isotropic chemical 

etching and anisotropic etching in the vertical direction were used. Actually, an 

anisotropic etching process has a high ratio of the etching rate in the vertical direction 
77

to the etching rate in the horizontal direction. Anisotropic etching should be first 

carried out to create the height of the feature with limited etching in the horizontal 

direction. Then the isotropic etching process etches the silicon substrate with the same 

etching rate in any direction. We stopped the etching process when the silicon 

substrate under the masking silicon dioxide was etched away. This technique has been 

used by others to fabricate knife features for applications in telecommunication [97]. 

The final height and edge angle of the surface texture are dependent on the 

etching period of anisotropic and isotropic etching process. Figure 6.2 shows this 

process schematically. 

With a known etching rate of the isotropic etching process Ri, vertical 

anisotropic etching rate Ra,z, and the ratio of the vertical etching rate to the horizontal 

etching rate of the anisotropic etching process ca, the etched distances in the vertical 

and horizontal directions for both etching processes can be calculated. The final height 

of the surface features and the edge angles can be determined by the durations ta and ti

for the anisotropic and isotropic etching processes: 

c

1 c

c a a,z a i i

c a,z a i i

tan

( )

h

w

w c R t R t

h R t R t

− θ =

= ⋅ + ⋅

= ⋅ + ⋅

where hc is the final height of the feature, wc is the width of the feature and θ is the 

half angle of the edge. 
78

Figure 6.2 Schematic representation of the wedge geometry controlled 

by anisotropic and isotropic etching processes. 

Si 

SiO2

Anisotropic etching

wa 

ha

SiO2

Isotropic etching 

Si 

wa+ wi

ha+ hi

θ
79

The anisotropic etching process was carried out with a reactive ion etcher 

(RIE) made by Trion Corporation. SF6 gas was used to etch the silicon material. The 

process was operated under a gas pressure of 40 Pa with 100 Watt power. The etching 

rate in the vertical direction is 1μm/min, and the ratio of the vertical etching rate to the 

horizontal etching rate of the anisotropic etching process ca was about 0.1. Then the 

isotropic etching process was performed by immersing the wafer into a mixture of 

strong acid solutions containing hydrofluoric acid, nitric acid, and acetic acid (a HNA 

etchant; volume ratio HF:HNO3:CH3COOH = 8:75:17) at room temperature. The 

etching rate of silicon by HNA etchant was 0.7 μm/min. 

6.3.5 Wear-resistant coating 

The surface texture plays the role of the cutting device for the particle 

generation process. To reduce failure and wear of the surface texture, a thin film of 

chromium was coated onto the surface of the surface features. 

The wafer was mounted in a vacuum metal evaporator. The vapor pressure in 

the chamber was less than 2.67x10-3 Pa. The applied heating voltage and current was 

adjusted to reach the evaporation rate of 0.5 nm/s. A (5±0.5) nm layer of chromium 

was deposited on the surface of the features. 
80

6.4 Measurement of the surface texture 

Scanning electron microscopy (SEM) was applied to determine the 

morphology of the wedge geometry. Figure 6.3 provides an example of the SEM 

observation of the features. The length and the width of the rectangle features can be 

measured from the SEM micrographs. To measure the height and the angle of the 

wedge features, an atomic force microscope was used. The three-dimensional 

geometry and the dimensions of a wedge feature can be measured as shown in Figure 

6.4. 

Figure 6.3 SEM micrographs of the wedge surface-features made by the 

microfabrication process. 
81

 

Figure 6.4 Measurement of the three-dimensional geometry of the 

wedge surface textures by atomic force microscopy (AFM). 
82

CHAPTER 7 

SURFACE-TEXTURE DESIGN AND GENERATION OF 

NARROWLY DISTRIBUTED PARTICLES 

7.1 Introduction 

In Chapter 6, the use of microfabrication techniques to produce precisely 

controlled surface-feature dimensions is discussed. Now it is feasible to study the 

relationship between surface-texture patterns (and their dimensions) and particle size 

and shape. This chapter describes the various designs and the resultant particle sizes 

and shapes produced. 

7.2 Surface-texture designs 

To design various surface textural patterns to generate different size and shape 

of particles, one has to develop some hypothesis of how they are related. It is assumed 

here that the critical parameters of the surface texture are the feature size and shape 

and the distances between the features that constitutes the overall pattern. 

Figure 7.1 is a schematic of the surface texture used in this study. We can 

classify the parameters of the surface textures into two groups: (i) Dimensions of 

cutting edges: cutting-edge length (Lc), cutting-edge height (Hc), cutting edge width 

(Wc) and half angle of the cutting-edge (θ); (ii) Arrangement of the cutting edges: 

pitch of the surface texture in the sliding distance (Ds), distance between adjacent 
83

cutting edges (Di), distribution density (number of feature per unit area) of the cutting 

edges on the surface, and patterns of cutting-edge distributions. 

Ds

Di

Lc

Wc

Hc 

Ds

Top view of the surface texture

Lc

θ

Lc: cutting-edge length 

Hc: cutting-edge height 

Wc: cutting-edge width 

Ds: pitch in the sliding direction 

Di: distance between adjacent features 

UHMWPE pin sliding direction

Figure 7.1 Schematic of the surface texture with wedge features. 
84

7.3 Surface textures fabricated 

 Two types of surface textures were designed and microfabricated. Type A 

texture contains the wedge-shaped cutting edges lining up in a rectangular array with 

specified pitch (Ds) and distance between adjacent features (Di) as shown in Figure 

7.2. Type B texture has an offset arrangement of the features (Ds/2) as shown in 

Figure 7.2. The distance between adjacent features was designed to be equal to the 

basal length of the cutting-edge. 

 The surface pattern parameters such as Ds and Di were controlled by the 

photomask used in the microfabrication process. The density of the cutting edges on 

the surface (number of feature per unit area) was also calculated. These dimensions 

are listed in Figure 7.2 as well. Length and height of the cutting edges were measured 

after the microfabrication of the surface texture. Various cutting-edge lengths were 

designed to generate particles of different lengths. Different cutting-edge heights were 

included in an attempt to change the aspect ratio of the particles. The dimensions and 

arrangement of the cutting edges for both types of surface textures are also shown in 

Figure 7.2. These designs do not reflect a systematic statistical approach but rather a 

cumulative trial and error historical perspective. In the following sections, we will 

examine the various surface textural parameters. 
85

Surface 

texture 

Lc

(μm) 

Wc

(μm) 

Hc

(μm) 

Ds

(μm) 

Di

(μm) 

ρ

1/(μm)2

Offset 

features 

A-1 55.0 5.0 4.2 200 40 0.00005 No 

A-2 15.0 5.0 3.4; 2.8 200 80 0.00005 No 

A-3 7.0 5.0 3.0; 2.2 200 90 0.00005 No 

A-4 6.8 5.0 2.3 10 10 0.005 No 

A-5 3.2 2.0 0.8 4 4 0.3125 No 

B-1 6.2 2.0 0.9; 1.3 10 8 0.0125 Yes 

B-2 3.5 2.0 1.4 20 4 0.0125 Yes 

B-3 3.6 2.0 0.6 10 4 0.025 Yes 

B-4 3.8 1.0 1.2 10 4 0.025 Yes 

Ds

Di

Lc

Wc

Top view of the surface texture 

Lc: cutting-edge length 

Wc: cutting-edge width 

Hc: cutting-edge height 

Ds: pitch in the sliding direction 

Di: distance between adjacent features 

ρ: density of surface features (no. features per unit area) 

Figure 7.2 A schematic representation of dimensions of the surface

textures containing wedge-shaped cutting edges. 

Sliding 

direction

Ds/2

Di

Lc

Wc

Ds

Type A: 

Feature arrays 

Type B: 

Feature arrays with offset arrangement

(Uncertainty of measurement of the dimensions come from the microfabrication 

process. A relative standard uncertainty is estimated to be less than 10%) 
86

7.4 Results 

 The surface textures were fabricated and used to generate UHMWPE particles 

in linear reciprocating wear tests under a nominal contact pressure of 3MPa, a stroke 

length of 19 mm, a frequency of 1.5 Hz, and an average sliding speed of 57.2 mm/s. 

 The feature length is the most important parameter and is varied from 3.2 μm 

to 55 μm. The feature height is varied from 0.8 μm to 4.2 μm and the pitch is varied 

from (4 to 200) μm. The overall geometric design is reflected in group A and group B 

designs. 

7.4.1 Effect of the cutting-edge length 

To determine the effect of cutting-edge length, we designed the surface 

textures with different cutting-edge lengths with the same geometric parameters Ds, 

Di, and density of surface features ρs (surface textures A-1, A-2, and A-3 in Figure 

7.2). The micro-images of the surface texture, single cutting-edge feature, generated 

UHMWPE particles and particle distributions are shown in Figures 7.3-7.5. (Length 

±standard uncertainty) of cutting edges for surface textures A-1, A-2, and A-3 are 

(55±5.1) μm, (15±1.1) μm, and (7±0.5) μm respectively. The number averaged mean 

particle-length generated from these three surface textures are (43.1±7.1) μm (78 % of 

cutting edge length), (10.2±3.9) μm (68 %), and (6.2±2.1) μm (89 %). The median 

values of the particle lengths are also shown in the figures. A longer cutting-length 

leads to a larger particle-length. The results also show that the particle length is always 

smaller than the cutting-edge length about 68 % to 89 % of the cutting edge length. 
87

This may be due to particle attrition in the contact as well as viscoelastic effect at the 

two ends of the cutting edge. During the particle-generation process, the surfacefeature edge cuts into the UHMWPE material during the linear reciprocating wear 

process. Under the condition that the sliding direction is perpendicular to the cuttingedge length direction, the particle length should ideally be equal to the dimension of 

the cutting edge. Two factors may contribute to this observation. One is the 

viscoelastic behavior of UHMWPE. While the cutting edge penetrates into the 

material, the deformed length of the material is equal to the cutting-edge length. 

However, at two ends of the cutting edge, the viscoelastic behavior of the UHMWPE 

will stretch (similar to a rubber band) and eventually break. Because of the stretching, 

the final particle length will be shorter than the cutting edge length. Another possible 

explanation is the particles, once generated, can be cut again by the features before 

exiting the contact. 
88

Particle length (μm)

0 20 40 60

Percent (%)

0

5

10

15

20

25

30

Surface texture 

UHMWPE particles 

Particle-length distribution

Figure 7.3 Micrographs of the surface texture A-1 (Lc = 55 μm, Hc = 4.2 μm, 

Ds = 200 μm ) and generated UHMWPE particles (contact pressure

= 3 MPa, average sliding speed = 57.2 mm/s) 

Mean particle-length = (43.1±7.1) μm 

Median particle-length = (42.1±6.8) μm

Aspect ratio of particle = 6.7±1.1 
89

Surface texture 

UHMWPE particles 

Particle-length distribution

Figure 7.4 Micrographs of the surface texture A-2 (Lc = 15.0 μm, Hc = 3.4 μm,

Ds = 200 μm ) and generated UHMWPE particles (contact pressure

= 3 MPa, average sliding speed = 57.2 mm/s) 

Mean particle-length = (10.2±3.9) μm 

Median particle-length = (8.3±3.6) μm

Aspect ratio of particle = 2.2±0.7 

Particle length (μm)

0 2 4 6 8 10 12 14 16 18 20

Percent (%)

0

5

10

15

20

25

30

35

40
90

 Surface texture 

UHMWPE particles 

Particle-length distribution

Figure 7.5 Micrographs of the surface texture A-3 (Lc = 7 μm, Hc = 3.0 μm,

Ds = 200 μm ) and generated UHMWPE particles (contact pressure

= 3 MPa, average sliding speed = 57.2 mm/s) 

Mean particle-length = (6.2±2.1) μm 

Median particle-length = (6.2±2.0) μm

Aspect ratio of particle = 2.6±0.6 

Particle length (μm)

0 2 4 6 8 10 12 14

Percent (%)

0

5

10

15

20

25

30

35
91

7.4.2 Effect of the cutting-edge height 

To investigate the effects of cutting-edge height on the particle size and shape, 

cutting edges with different heights were microfabricated from the same pattern. 

Figure 7.6 compares the particles generated from surface pattern A-2 with different 

cutting-edge heights. The cutting-edge length is 15 μm for both features and the 

cutting-edge heights are 2.8 μm and 3.4 μm, respectively. The aspect ratios of 

particles are 3.1±0.7 and 1.8±0.3. The surface texture with smaller cutting edgeheights generated more elongated particles (larger aspect ratio of the particle), and the 

surface texture with a larger cutting-edge height generated particles with a smaller 

aspect ratio. 

From a calculation of the load applied on each cutting edge, we estimate that 

the contact pressure is sufficient to push the cutting edges to the maximum penetration 

depths. Thus a larger cutting-edge height results in a larger penetration depth of the 

cutting edge. A deeper penetration of the cutting edge induces a larger volume of shear 

deformation of UHMWPE during the sliding process of the cutting edge. With the 

similar particle length, the larger shear deformation leads to a larger width and a 

smaller aspect ratio of the particle. This suggests that we can control the aspect ratio of 

the generated particles by adjusting the height of the cutting edge. 
92

Side view of the 

cutting edge 

5.0 μm 

2.8 μm 

Side view of the 

cutting edge 

5.0 μm 

3.4 μm 

UHMWPE particles 

Aspect ratio = 3.1±0.7 

UHMWPE particles 

Aspect ratio = 1.8±0.3 

Figure 7.6 Comparison of UHMWPE particles generated from surface

texture A-2 with different cutting-edge heights (contact 

pressure = 3 MPa, average sliding speed = 57.2 mm/s). 
93

7.4.3 Effects of the cutting edge pattern 

To investigate the interaction between cutting edges along the UHMWPE pin 

sliding-direction, surface textures with different pitch (Ds) in the sliding direction and 

different distance between adjacent features (Di) were applied in the particlegeneration process. When we reduced Ds of surface texture A-3 from 200 μm to 10 

μm (surface texture A-4), many irregular particles were generated (see Figure 7.7). It 

is said that the particles were generated from a non-effective micro-cutting process. A 

non-effective micro-cutting process was also observed for the particles generated from 

surface texture A-4 which has a Ds of 4 μm as seen in Figure 7.8. When the pitch 

distance is too small in the sliding direction, overlapping of the cutting processes 

occurs by the interaction between adjacent cutting edges, and the individual cutting 

process cannot be completed. This is a two dimensional interference of the cutting 

process in the penetration and sliding directions. It ends up with the cutting edges 

wrapped in the UHMWPE. Then the repeated sliding process results in the generation 

of large and irregular particles without precise control. We suggest that there exists a 

minimum distance between the cutting features in the sliding direction in order to 

induce effective micro-cutting for generation of particles with controlled sizes and 

shapes. 
94

Surface texture 

UHMWPE particles 

Figure 7.7 Micrographs of the surface texture A-4 (Lc = 6.8 μm, Hc = 2.3 μm, Ds = 

10 μm ) and generated UHMWPE particles (contact pressure = 3 MPa, 

average sliding speed = 57.2 mm/s) 

Particle length ≈ (5 to 25) μm 

Many irregular particles were observed. 

UHMWPE particles were generated without 

precisely controlled size and shape. 
95

Figure 7.8 Micrographs of the surface texture A-5 (Lc = 3.2 μm, Hc = 0.8 μm, Ds = 

4 μm ) and generated UHMWPE particles (contact pressure = 3 MPa,

average sliding speed = 57.2 mm/s) 

Surface texture 

UHMWPE particles 

Particle length ≈ (10 to 25) μm 

Many irregular particles were observed. 

UHMWPE particles were generated without 

precisely controlled size and shape. 
96

 Particles were also generated from the surface textures with offset arranged 

features (type B). Figures 7.9-7.10 show the images of the surface texture, single 

cutting-edge feature, and UHMWPE particles generated from surface textures B-1 and 

B-4. Large and irregular particles without size and shape control were generated. 

Much larger particles were observed compared to the particles shown in Figures 7.7-

7.8 that were generated from the surface textures without offset features. It is shown 

that the offset arrangement of cutting edges further results in the interference of the 

cutting process in the direction perpendicular to the sliding direction. Materials 

between the adjacent cutting edges were involved in the non-effective cutting process. 

Thus the three dimensional interference of the micro-cutting process leads to much 

larger particles with indent marks on the surface. 

We realize that a longer sliding distance arises from a larger penetration depth 

of the cutting edge. Thus a larger cutting-edge height may need a larger pitch to 

prevent the interaction between cutting edges. We calculated the ratio of Ds to Hc for 

each case and classify the surface textures into groups of clean micro-cutting and 

interference of micro-cutting as shown in Table 7.1. We found that the surface texture 

inducing an effective micro-cutting process has a (Ds / Hc) value larger than 14. 

However, when the value of (Ds / Hc) is smaller than 9, the interference of microcutting process was observed. To make sure that micro-cutting in the particlegeneration process is effective, we should design the surface texture to meet a 

minimum (Ds / Hc) value of 14. 
97

Surface texture 

UHMWPE particles 

Particle length distribution

Figure 7.9 Micrographs of the surface texture B-1 (Lc = 6.2 μm, Hc = 1.3 μm, Ds =

10 μm ) and generated UHMWPE particles (contact pressure = 3 MPa,

average sliding speed = 57.2 mm/s) 

Many irregular and larger particles were observed. Indentation mark

by the cutting edges were observed on the larger particles.

UHMWPE particles were generated without precisely controlled size

and shape. 
98

Surface texture 

UHMWPE particles 

Figure 7.10 Micrographs of the surface texture B-4 (Lc = 3.8 μm, Hc = 1.2 μm, Ds =

10 μm) and generated UHMWPE particles (contact pressure = 3 MPa,

average sliding speed = 57.2 mm/s) 

Many irregular and larger particles were observed. Indentation mark by

the cutting edges were observed on the larger particles. UHMWPE 

particles were generated without precisely controlled size and shape. 
99

Table 7.1 Value of (Ds / Hc) for effective and non-effective micro-cutting processes.

The density of features (ρs) for each surface texture is also listed in Table 7.1. 

The surface texture with a smaller pitch has a larger density of features. The surface 

textures with offset arranged features also have larger values of the density of features. 

It is obvious that a smaller density of features can prevent the interaction between 

cutting edges on the surface. However, the experimental results did not show that the 

density of features was a critical parameter for effective micro-cutting process. On the 

other hand, the UHMWPE surface has a higher possibility to rub against the flat 

surface area for a smaller density of features. Adhesive wear may occur and tiny 

particles (submicrometer or smaller) without size and shape control may be generated. 

One should also optimize the density of features to induce an effective micro-cutting 

process and to avoid the particles generated from the non cutting-edge area. 

Surface texture Ds / Hc ρs [1/(μm)2

] Micro-cutting process 

A-1 47.6±2.0 0.00005 Clean cut 

A-2 58.8±2.2 0.00005 Clean cut 

A-3 66.7±2.5 0.00005 Clean cut 

B-2 14.3±0.4 0.0125 Clean cut 

B-3 16.7±0.6 0.0125 Clean cut 

A-4 4.3±0.3 0.005 2D interference 

A-5 5.0±0.2 0.3125 2D interference 

B-1 7.7±0.2 0.125 3D interference 

B-4 8.3±0.3 0.025 3D interference 
100

7.4.4 Effect of pitch distance 

Figures 7.11 and 7.12 compare the UHMWPE particles generated from surface 

textures with Ds = 20 μm and 10 μm. The dimensions of the cutting edges for both 

surface textures are similar. It is seen that the surface texture with Ds = 10 μm leads to 

an average aspect ratio of the particles equal to 1.6±0.3, and the surface texture with 

Ds = 20 μm leads to an average aspect ratio of particles equal to 1.9±0.5. It indicates 

that a shorter Ds results in a larger width of the generated particles. With a shorter 

distance between cutting edges, the interaction between the two features in the sliding 

direction is stronger. The adjacent cutting-edge along the sliding direction induces a 

constraint of the flow of the shear deformation during the sliding process. A shorter 

distance between features (Ds) results in a larger shear volume by the cutting-edge 

sliding process. Thus a larger volume of material is deformed and it leads to a larger 

particle width (with the same particle length, it means a smaller aspect ratio). 
101

Surface texture 

UHMWPE particles 

Particle-length distribution

Figure 7.11 Micrographs of the surface texture B-2 (Lc = 3.5 μm, Hc = 1.4 μm, Ds = 

20 μm ) and generated UHMWPE particles (contact pressure = 3 MPa, 

average sliding speed = 57.2 mm/s) 

Mean particle-length = (4.2±0.8) μm 

Median particle-length = (4.1±0.8) μm

Aspect ratio of particle = 1.9±0.5 

Particle length (μm)

0 2 4 6 8 10

Percent (%)

0

5

10

15

20

25

30

35

40
102

Surface texture 

UHMWPE particles 

Particle-length distribution

Figure 7.12 Micrographs of the surface texture B-3 (Lc = 3.6 μm, Hc = 0.6 μm, Ds = 

10 μm) and generated UHMWPE particles (contact pressure = 3 MPa,

average sliding speed = 57.2 mm/s) 

Mean particle-length (3.9±0.7) μm 

Median particle-length = (4.1±0.8) μm

Aspect ratio of particle = 1.6±0.3 

Particle length (μm)

0 2 4 6 8 10

Percent (%)

0

10

20

30

40
103

7.4.5 Effects of mechanical conditions 

The effect of the load on the dimensions of the generated particle was studied 

by changing the normal load applied on the same surface texture. Figure 7.13 shows a 

comparison of UHMWPE particles generated form the surface texture A-3 with a 7 

μm cutting-edge length (shown in Figure 7.5) under different normal loads. The wear 

tests were run under a 1.5 Hz frequency with an average speed of 57.2 mm/s. The 

average particle width is 10.1 μm under a normal load of 192 N (6 MPa contact 

pressure) compared to an average particle width of 6.2 μm under a normal load of 96 

N (3 MPa contact pressure). A larger normal load leads to a larger particle width and 

thus a smaller aspect ratio when the same surface texture is used. 

Increase of normal load yields a larger penetration depth of the cutting edge. 

While the applied normal load is sufficient to push the cutting edge all the way into 

UHMWPE, an increase of the load will not make the cutting edge penetrate deeper. 

The excess load is acting as the resistant force on the cutting edges during the cutting 

process. Thus more material deformation occurs in order to accumulate sufficient 

resistant force to push away the cutting edge and complete a micro-cutting process. 
104

Load = 192 N 

Aspect ratio of particle 

= 1.9±0.5 

Load = 96 N 

Aspect ratio of particle 

= 2.6±0.6 

Figure 7.13 Comparison of UHMWPE particles generated under different 

normal loads (by using the surface texture A-3 with cutting edge 

length = 7 μm, average sliding speed = 57.2 mm/s). 
105

Effect of the sliding speed on the dimensions of the generated particle was 

studied for the surface texture A-1. Figure 7.14 shows a comparison of UHMWPE 

particles generated under a normal load of 96 N (3 MPa contact pressure) and with 

different sliding speeds. The results show that a higher sliding speed (57.2 mm/s) 

yields more elongated particles (larger aspect ratio of the particle) and a lower sliding 

speed (27.6 mm/s) results in particles with a smaller aspect ratio. 

The sliding speed influences the residence time of the cutting edge during the 

cutting process, especially for a viscoelastic material. During the sliding process of the 

cutting edge, a lower sliding speed allows more stress relaxation while the material 

resistant force is increasing. It leads to a longer residence time to develop sufficient 

stress to break material and form a particle. Thus a lower speed leads to a larger 

particle width and a smaller aspect ratio of the particle. 
106

 

Figure 7.14 Comparison of micrographs of UHMWPE particles generated by

the surface texture A-1 under 3 MPa contact pressure and sliding 

speeds of 27.6 mm/s and 57.2 mm/s respectively. 

Average sliding speed 

= 27.6 mm/s 

Aspect ratio of particle 

= 4.3±0.9 

Average sliding speed 

= 57.2 mm/s 

Aspect ratio of particle 

= 6.7±1.1 
107

7.5 Discussion 

7.5.1 Effect of surface-texture uniformity on particle distribution 

Figure 6.8 in Chapter 6 showed the distribution of the lengths of the particles 

from surface textures made with SiC abrasive paper. Obviously, the particle-lengthdistribution curve is shifted to the right (larger particle-length) when the larger 

particles are produced by rougher surface textures. A clear difference of the average 

particle size in the two particle populations is observed in Figure 6.8. However, if we 

look closely at the particle distribution, the particle length ranges actually from several 

micrometers up to 100 μm for both cases. Although it is narrower than the distribution 

of commercial UHWMPE particles, the particle-length distribution is still sufficiently 

broad so that some overlap between particle populations is inevitable. 

The problem of surface-texture uniformity was overcome by applying 

microfabrication techniques to prepare surface features with precisely controlled 

dimensions described in Chapter 7. The uniformity of the surface texture is improved 

by precisely controlling the dimension of each feature of the surface texture. 

We compare the particle populations with similar mean particle-length with 

two different kinds of surface textures. Particles generated from the microfabricated Si 

surface texture A-1 (mean particle length 43.1 μm) are compared with the particles 

generated from SiC abrasive paper with similar mean particle length (mean particle 

length 42.1 μm) in Figure 7.15. With similar mean particle length, 60 % of the 

particles are in the range from 37 μm to 46 μm. Also the entire particle distribution 

range has been reduced from 100 μm to 35 μm, and the 95 % confidence level from 
108

3.0 μm to 1.43 μm. Hence, we have demonstrated that by using a more uniform 

surface texture, a narrower particle distribution is obtained. 

Particle length (μm)

0 20 40 60 80 100

Percentile (%)

0

10

20

30

40

Microfabricated Si Texture

SiC Abrasive made Texture

Figure 7.15 Comparison of particle length distribution between particles

generated from a SiC abrasive paper made surface texture

and a microfabricated surface texture. 

Particles generated from 

SiC abrasive paper made texture

Particles generated from 

microfabricated surface texture 

(Uncertainty of the measurement comes from the image measurement of particles 

which is estimated to be less than a relative standard uncertainty of 5%) 
109

7.5.2 Significance for bioactivity tests

Different size ranges of UHWMPE wear particles are needed for bioactivity 

studies. When a particle is small enough to be engulfed by a macrophage cell, it is said 

that the particle is phagocytosable. For a further analysis of the effects of phagocytosis 

of wear particles on sequential immunological reactions, particle populations within 

phagocytosable and non-phagocytosable regimes are needed for bioactivity tests. The 

size of a single macrophage cell is about 10 μm. It is recognized that a particle size 

larger than 10 μm is difficult for a macrophage cell to engulf. The particle populations 

described in this chapter represent one non-phagocytosable particle population (mean 

particle length 43.1 μm), two phagocytosable particle populations (mean particle 

length 6.2 μm, 4.2 μm ), and a particle population (mean particle length 10.2 μm), 

with a particle length close to the macrophage-cell size. This represents a sufficient 

coverage of particles for investigating the macrophage-phagocytosis effect on particleinduced biological responses. The particle distributions of one non-phagocytosable 

particle population (mean particle length 43.1 μm ) and one phagocytosable particle 

population (mean particle length 6.2 μm) are compared in Figure 7.16. Both particle 

populations have very narrow particle-length distributions without overlap. The 

particles can be used for biological response studies to elucidate the basic bioactivity 

mechanisms independently and systematically. 
110

Figure 7.16 Comparison of two particle populations with distinct 

particle-length distribution. 

Comparison of Particle Distributions

Major axis length (μm)

0 10 20 30 40 50 60 70 80

Percent (%)

0

10

20

30

40

50

5 x 10 μm feature

5 x 60 μm feature

Aspect Ratio

1234567

Percent (%)

0

10

20

30

40

50

5 x 10 μm feature

5 x 60 μm feature

 Mean particle length 6.2 μm 

 Mean particle length 43.1 μm

 Particle Length (μm)

 Mean particle length 6.2 μm 

 Mean particle length 43.1 μm

(Uncertainty of the measurement comes from the image 

measurement of particles which is estimated to be less than 5%) 
111

7.6 Conclusions 

 In this chapter, the basis of the surface-texture design for particle generation 

has been illustrated. The effects of cutting-edge dimensions, arrangement of surface 

features and mechanical conditions on the particle size and shape were evaluated. 

Surface textures of different kinds have been microfabricated for the generation of 

UHMWPE particles. The results showed that the cutting-edge length controls the 

particle length. The cutting-edge height is related to the aspect ratio of the particles. 

Critical values of (Ds / Hc) were identified for effective micro-cutting. We also found 

that the applied load and the sliding speed have a strong influence on the sliding 

process of the cutting edges and have influence on the width and aspect ratio of a 

particle. 

 However, the exact mechanisms for the above phenomena are not evident. To 

achieve the production of particles with desired sizes and shapes, we need to 

understand how the particle sizes and shapes are affected by these parameters 

quantitatively. We need to know the effect of the cutting-edge geometry on the 

penetration depth. In order to describe the process quantitatively, we also need to 

measure the material resistant force of UHMWPE in the sliding process of the cutting 

edges. Especially, the influence from the viscoelastic behavior of UHMWPE should 

be analyzed. More experiments need to be designed to provide a better understanding 

of the mechanism of particle generation quantitatively. 
112

CHAPTER 8 

MECHANISM OF UHMWPE PARTICLE 

GENERATION

8.1 Introduction 

The purpose of this chapter is to mechanistically elucidate the particlegeneration mechanism in the rubbing process between UHMWPE material and 

textured surfaces. To develop a predictive model for surface texture design, we need to 

understand the particle-generation mechanism. The first step is the quantification of 

the tip penetration into UHMWPE as a function of load. A nano-indenter was used to 

probe UHMWPE to observe the viscoelastic behavior of UHMWPE. After 

penetration, the sliding process was simulated with simple model experiments by 

observing in situ single asperity cutting process of UHMWPE via a microscopic video 

camera. From the results, we deduce the relationship between cutting depth, cuttingedge angle and particle generation processes. 

We hypothesize that the particle-generation mechanism stems from: (a) 

Penetration of the tip; (b) Lateral displacement of the cutting edge; (c) Strain 

hardening of the polymer; (d) Embrittlement of the surface layer by molecular 

orientation, and (e) Detachment of the particle at the tip edge-material interface. We 

will show evidence in this chapter that these processes may actually take place. 
113

8.2 Penetration of the cutting edge 

A fixed normal load was applied in the linear reciprocating wear process for 

UHMWPE particle generation. The cutting edges penetrated into UHMWPE at the 

beginning of the process. The penetration depth is determined by the normal load 

applied, angle and height of the cutting edges. When the normal pressure and material 

reaction force are balanced, a maximum penetration depth for that load is reached. 

To examine the penetration process, a Nano Indenter XP (MTS Corporation) 

was used to examine the UHMWPE behavior1

. The nano-indenter uses a compliance 

indentation system, capable of operating at loads in the microgram range, and the 

depth resolution is in the subnanometer range. Real-time indentation depth and load 

data were recorded during the process. A Berkovich tip (three–sided diamond 

pyramid) was used. The sides of the pyramid have an angle of 65.3° normal to the 

base. Thus the indent marks appear as equilateral triangles, and the length of a side of 

a perfectly plastic indent is approximately 7.3 times its depth [98]. The indentation 

was carried out at a constant loading-unloading strain rate of 0.1 s-1. A holding time of 

10 s was used between the loading and unloading stage. Figure 8.1 shows the loaddisplacement curve of the indentation. 

_____________________________________________________________________ 

1

Technical assistance from Dr. Douglas Smith at NIST on nano-indentation tests is 

deeply appreciated.
114

Strain rate =0.1 1/s

test 3

Displacement (nm)

0 1000 2000 3000 4000 5000 6000 7000

Load (mN)

0

5

10

15

20

25

Figure 8.1 Load-displacement curve of nano-indentation test on

UHMWPE with constant strain rate of 0.1 s-1. 

O 

A B 

C C2 C1

(Uncertainty of the measurement comes from surface roughness of the UHMWPE 

samples which is estimated to be less than 10 % pf the measured value) 
115

 Figure 8.1 shows the displacement increases in the loading segment (OÆA). 

At the holding period with the constant load (AÆB), creep of the material under a 

constant stress was observed. During the unloading stage (BÆC), the displacement 

curve shows additional deviation due to viscoelastic recovery of the material. 

 The data obtained from the unloading curve provide information regarding the 

elastic, viscoelastic and plastic behavior [98]. In Figure 8.1, the displacement at point 

C1 represents the maximum displacement (6375 μm) of the indenter corresponding to 

the maximum load applied during the indentation, and the displacement at point C is 

the residual displacement after removing the indenter. The displacement at point C2 is 

the intercept of the tangent line drawn from the first part of the unloading curve, which 

describes the immediate recovery of the deformation once the stress is removed. The 

difference of the displacement between point C1 and C2 (812 μm) represents the 

elastic recovery of the material deformation. The difference of the displacement 

between point C and C2 (1563 μm) further indicates the delayed recovery of the 

material deformation by viscoelastic effect. Compared with the maximum penetration 

depth, 37.3 % of the maximum penetration depth was recovered after the load is 

removed. Also comparing the ratio of the area “BCC1” (viscoelastic recovery) to the 

area “OABC1” (equal to the total energy done during the indentation) in Figure 8.1 

shows that the effect of viscoelastic on UHWMPE deformation is dramatic. The 

viscoelastic recovery of material deformation of polymers under scratching was also 

revealed and measured by other investigators [99-103]. 
116

 Elastic modulus of UHMWPE from the loading stage of the indentation test 

was calculated based on the stress and strain at certain penetration depth. Figure 8.2 

shows the elastic modulus obtained from different penetration displacements. It shows 

that the elastic modulus near the surface is as high as 7 GPa. However, the deviation 

of the value near the surface may be up to 3 GPa because of the roughness of the 

surface. The result still indicates that the elastic modulus evaluated at near-to-surface 

layers is much higher than the bulk material. A higher elastic modulus of UHMWPE 

surface layer was also indicated by Briscoe et al. [98]. The UHMWPE samples were 

prepared by a machining process. This may be due to the higher hardness of the 

surface layer from machining process. The shear stress applied on the surface may 

induce the molecular orientation and make the material surface harder. We will further 

discuss the molecular orientation of UHMWPE in the later section. The perturbation 

of the results can also be due to the roughness of the UHMWPE surface. 

 Displacement (nm)

0 200 400 600 800 1000 1200 1400

Elastic modulus (GPa)

0

2

4

6

8

Figure 8.2 Elastic modulus data for UHMWPE under different penetration

depths from a nano-indentation test (strain rate = 0.1 s-1). 

(Uncertainty of the measurement comes from surface roughness of the UHMWPE 

samples which is estimated to be less than a relative standard uncertainty of 10 %)
117

It is seen from Figure 8.2 that the elastic modulus reaches a steady value of 

around 1 GPa when the penetration depth is larger than 500 nm. This value will be 

used later.

We also measured the penetration depths of the wedge-shaped tip with 

different angles under various normal loads. The wedge-shaped tip will be later used 

in the simple model experiments of the sliding process. Two wedge tips are 90° and 

60° with 2 mm edge length. The normal load was applied and held for 15 s. Given 

sufficient time (60 min) after the indentation tests, profiles of the indentation marks 

were measured by profilometry. The final penetration depths of the various normal 

loads and cutting-edge angles are listed in Table 8.1. Obviously, the penetration depth 

increases with increasing applied normal load. A smaller cutting-edge angle (sharper 

edge) leads to a deeper penetration under the same normal load. This result may be 

due to the larger local compression stress induced by a sharper edge. 

Table 8.1 Comparison of penetration depth for the cutting edges with different 

angles under various normal loads. 

Final penetration depth 

Normal load 

=22.25 N 

Normal load 

=44.50 N 

Normal load 

=66.75 N 

angle =60° (29.6±2.1) μm (56.8±3.5) μm (85.2±4.9) μm 

angle =90° (21.9±2.0) μm (47.7±2.9) μm (70.6±4.5) μm 
118

8.3 Sliding of the cutting edge 

To understand the mechanism of particle generation, a direct observation of the 

cutting-edge sliding process would be desirable. However, the small scale of surface 

textures makes in situ observation difficult. A scaled-up model experiment was 

developed to allow the direct observation of the particle-generation process under a 

constant load. 

8.3.1 Simple model experiments with constant load 

The experimental setup of the scaled-up single cutting-edge sliding test is 

schematically shown in Figure 8.3. A wedge-shaped tip was fabricated out of tool 

steel. Two different tips (90° and 60° with 2 mm edge length) were prepared. The tip 

was mounted on a lever arm. A UHMWPE disk (5.08 cm (2”) in diameter) was 

mounted in the lower holder attached to a horizontal slider. The slider was driven by a 

rotary motor at an average sliding speed of (57.2±3.2) mm/s (the same speed in the 

particle- generation process). 

A piezoelectrical three-dimensional force transducer was mounted above the 

tip to measure forces. The load cell sent electrical signals from the applied forces. The 

electrical signals were sent to three charge amplifiers. The output of the amplifier 

could be adjusted from 1 N/V to 1000 N/V and the voltage signal was then stored by 

the data-acquisition system. Calibration was made via prior the tests to obtain the 

curve of output voltage versus load. 
119

To observe the sliding process, a video camera (Digital 5000 by Panasonic) 

was mounted on the frame of the test rig. A pair of micro lenses, made by Volpi 

Company was used. The magnifications are 165 times and 300 times for the two 

lenses and the focus distance between the lens and the specimen is 50 mm. The video 

pictures picked up by the camera were recorded by a DV system (Sony). Later on, the 

recorded video could be converted as a digital file to the computer. Thus we can show 

the video frame by frame allowing the dynamic sliding process to be analyzed. A 

fiberglass light source was applied. The light source was arranged behind the 

specimen to give a bright and clear image of the material in front of the cutting edge. 

Wedge Tip 

3D Force 

Transducer

Microscopic UHMWPE Disk 

video system

Balance weight 

Dead weight load 

Charge 

Amplifier

Computer 

Data Acquisition

System

Figure 8.3 Schematic representation of scaled-up single cuttingedge sliding test with fixed normal-load condition. 
120

8.3.2 Lateral displacement of the tip 

Various normal loads (22.25 N, 44.50 N, and 66.75 N) were applied on the 

wedge-shaped cutting edges with two different included angles (60° and 90°) under a 

average sliding speed of 57.2 mm/s. The video observation shows that the cutting edge 

moves up and down during the sliding process. Examples of the video frames are 

shown in Figure 8.4. It can be seen that the cutting edge penetrates into the material in 

Figure 8.4(a). It is observed that the cutting edge is then pushed up away from the 

surface in the following frames (Figure 8.4(b)). Then the cutting edge penetrates into 

the material again. The phenomena were repeated during the entire sliding process. 

We hypothesize that the lift-up force comes from the material strain-hardening 

of the UHMWPE and the accumulated polymer in front of the cutting edge. After the 

cutting edge is lifted up, there is no contact between the cutting edge and the material 

surface, the material resistant force (The force acting on the cutting-edge during the 

sliding process) drops to zero. The normal load then pushes the cutting edge back into 

the UHMWPE material again. This process is repeated during the process. No wear 

particle was observed for the first cycle. After several cycles, generation of wear 

particels were observed. Based on this observation, the cutting-edge sliding process 

can be summarized and is shown in Figure 8.5. 
121

Sliding direction 

Figure 8.4 Video frames of a cutting edge lift-up process. 

(a) 

(b) 

Moving up 
122

Normal load (W) 

Normal load (W) 

Normal load (W) 

Normal load (W) 

Sliding

Sliding

Sliding 

Material resistant force 

Distance between bumps xb

(a) 

(b) 

(c) 

(d) 

Figure 8.5 Schematic representation of the cutting-edge sliding process

over UHMWPE material with fixed normal load.

Material bump 

Frxz

Frx

Frz
123

8.3.3 Reacting forces 

 Normal force and horizontal force were recorded during the cutting edge 

sliding process (Fx, Fz). Figure 8.6 shows the force curve of the sliding process by a 

wedge-shaped cutting edge with an included angle of 90° and Figure 8.7 is the force 

curve of the sliding process by a wedge-shaped cutting edge with an angle of 60°. 

When the sliding process begins, the material resistant force acting on the 

cutting edge increases with increasing deformed UHWMPE accumulated in front of 

the cutting edge. Thus the horizontal force acting on the cutting edge increases. 

The initial vertical force shown in the plot represents the applied normal load 

of the test. We define Frxz is the resultant resistant force acting on the cutting edge. Frz

and Frx are the resolved component forces in the vertical and horizontal directions, 

respectively (see Figure 8.5). Frz contributes to the lift-up force of the cutting edge. 

First Frz increases with increased accumulated volume of UHMWPE, and Frz also 

contributes to lift up the cutting edge in the sliding process. The contact area between 

the cutting edge and the material decreases during the lift-up process and thus the Frz

starts to drop. At the moment that the cutting edge is pushed to the apex of the 

material bump, Frz drops to zero. According to the fact that the vertical force shown on 

the plot presents the difference between the normal force and the material resistant 

force in the vertical direction. Thus the amount of vertical force goes back to the 

original applied normal load when the cutting edge is fully retracted. The reactingforce curves do conform to the hypothesized mechanism as we proposed after viewing 

the video recording of deformation in Figure 8.5. 
124

Sliding distance (mm)

0246

Force (N)

0

50

100

150

200

250

Horizontal force

Vertical force

Figure 8.7 Force curve of the sliding process by a 60° wedge-shaped 

cutting edge under a normal load of 66.75 N. 

2.4 mm 

Sliding distance (mm)

0246

Force (N)

0

20

40

60

80

100

120

Horizontal force

Vertical force

Figure 8.6 Force curve of the sliding process by a 90° wedge-shaped 

cutting edge under a normal load of 66.75 N. 

1.6 mm 
125

8.3.4 Effect of normal load and cutting-edge angle 

The frequency of the up and down movement of the cutting edge or the 

distance between the repeated material bumps provides information on the shear 

deformation of the material during the sliding process. Thus by understanding the 

force curve of the sliding process, the influence of the geometry of the cutting edge 

and mechanical operation conditions on the wear-particle dimensions can be 

understood. 

The sliding distances between two displaced bumps under different normal 

loads and cutting-edge angles are plotted in Figure 8.8. It shows that increasing normal 

load results in a longer sliding distance. Under the same normal load, a sharper cutting 

edge (smaller cutting edge angle) results in a longer sliding distance. A linear 

relationship between the sliding distance and normal load is observed. 

Normal load (N)

0 20 40 60 80

Sliding distance (mm)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

cutting-edge angle =60 deg

cutting-edge angle =90 deg

Figure 8.8 Sliding distance between two peaks under different normal

loads with two cutting-edge angles. 
126

Dependence of the cutting-edge lift-up process on the cutting-edge angle needs further 

examination. As illustrated in Figure 8.9, the quantity of the resolved component 

forces in vertical direction is a function of the cutting-edge angle. Assuming the same 

penetration depth during the sliding process, the resultant material resistant forces 

(Frxz) from the shear of UHMWPE material are the same. The resolved force in the 

vertical direction is Frz = Frxz sinθ as shown in Figure 8.9. A larger angle causes more 

material resistant force distribute to the vertical direction that increases the lift-up 

force. Thus the time required for the cutting edge to be pushed away is shorter. With a 

constant sliding speed in the horizontal direction, a shorter distance between two peaks 

is expected. Variation of the cutting-edge angle contributes to the change of 

penetration depth and the cutting edge lift-up speed. Both consequences have an 

influence on the sliding distance. 

To sum up, the effects of normal load and cutting-edge angle on the sliding 

process and, hence, on the particles dimensions are elucidated from the results of 

simple model experiments. However, the generation of particles was not observed 

from a single pass of sliding process. Particles were generated only after several cycles 

of sliding processes. We suspect this may be due to embrittlement of UHMWPE from 

strain hardening and molecular orientation during the repeated sliding processes. 
127

Normal load (W) 

Sliding 

Material resistant force Fr

θ

θ

Frx = Frxz cosθ

Frz = Frxz sinθ

Sharper cutting-edge 

Figure 8.9 Schematic representation of the resolved component forces

of the material resistant force in the vertical and horizontal

direction during the sliding process. 
128

8.4 Shear induced molecular orientation of UHMWPE 

UHMWPE is a semi-crystalline material and the folded-chains in the 

crystalline region have a specific molecular orientation. The chains tend to align and 

orient to a common direction when the surface is subjected to a directional force such 

as shear stress from sliding. 

X-ray absorption spectroscopy (XAS), in the form of near-edge x-rayabsorption fine-structure (NEXAFS) or x-ray-absorption near-edge structure 

(XANES), has demonstrated its ability to measure the orientation of hydrocarbon 

molecules at the surface [26]. This technique involves the resonant x-ray excitation of 

an electron to an unoccupied low-lying molecular orbital. The initial-state K-shell 

excitation gives XAS its element specificity, and the final-state unoccupied molecular 

orbitals give XAS its bonding selectivity. A measurement of the intensity of XAS 

spectral can identify chemical bonds and determine their relative population density 

within a sample. The amount of soft x-ray absorption is very sensitive to the average 

molecular chain angle with respect to the electric field vector (E) of the incident x-ray 

beam. This variation allows a measurement of the average molecular orientation of 

the alkyl chain with respect to the surface of UHMWPE. 

Soft x-ray absorption-spectroscopy was used [27-28] to measure the molecular 

orientation at UHMWPE surfaces subjected to different shear motions by rubbing a 

UHMWPE pin against a polished Co-Cr disk2

.

_____________________________________________________________________ 

2

Experiments on molecular orientation of UHMWPE were conducted by Dr. Daniel 

Fischer, and Dr. Ming Shen from NIST and Dr. Sharadha Sambasivan from Brook 

Haven National Laboratory. 
129

Figure 8.10 is the partial electron yield of a sample subjected to soft x-ray 

beams after 5000 unidirectional sliding cycles. The differences between the spectra 

quantify the degree of polymer chain alignment on UHMWPE surface. Figure 8.11 

further illustrates the difference spectra of UHMWPE after 5000 unidirectional sliding 

cycles. A larger difference between the spectra reflects a higher degree of molecular 

orientation. The results show that a higher degree of chain alignment was induced by a 

uni-directional rubbing the UHMWPE surface. 

 Linear reciprocating wear testing was conducted in our UHMWPE particlegeneration processes. We think that the UHMWPE molecules align to the direction of 

shear. The molecular orientation causes the UHMWPE to behave more brittle. This 

agrees with the nano-indentation result as shown in Figure 8.2. 

4

3

2

1

0

Normalized Partial Electron Yield (I/Io)

270 280 290 300 310 320 330

Incident Photon Energy (eV)

 

E parallel to sample

 

E perpendicular to sample C - H *

 C - C σ *

Figure 8.10 Soft x-ray absorption spectra showing the molecular orientation of

surface layer after 5000 unidirectional sliding cycles. E= electric

field [27]. 

(The uncertainty of the measurement is estimated to be within 5%) 
130

8

6

4

2

0

 E Parallel-E perpendicular (arb. units)

280 285 290 295 300 305 310

Incident Photon Energy (eV)

C-H

C-C Uni 5000 cycles

Figure 8.11 Soft x-ray absorption spectra showing the degree of molecular

alignment of UHMWPE surface layer after 5000 unidirectional

sliding cycles [26]. 

C

C--H

C--H

C

C

C

Uni-Directional

Wear Cycles

E

Parallel to the 

wear direction

Molecular Axis

(The uncertainty of the measurement is estimated to be within 5 %)
131

8.5 Strain-hardening effect 

A strain-hardening effect of UHMWPE in the plastic deformation regime has 

been identified and measured in several studies [104-107]. Kurtz et al. [104] have 

studied the yielding and plastic-flow behavior medical grade of UHMWPE (GUR 

4150) by conducting uniaxial tension and compression tests. GUR 4150 UHMWPE 

was also used in our studies. They showed that the equivalent true stress-strain 

behaviors from tension and compression tests were similar. A true stress-strain 

relationship was obtained in Figure 8.12 with a strain up to 1.5. Dramatic strainhardening was observed. Figure 8.13 shows the true stress-strain relationship of GUR 

4150 UHMWPE up to a 0.12 true strain. The yield point was identified and a strainhardening effect was observed with increasing stress needed for the increasing plastic 

strain. An elastic-plastic model was then proposed especially to describe the strain 

hardening behavior of the plastic deformation of UHMWPE [104]. 

We propose that the material is accumulated in front of the cutting edge during 

the sliding process. The material resistant force arises because of increasing volume of 

UHMWPE accumulated in front of the cutting-edge surface. The material is then 

strain hardened and increases resistant force acting on the cutting-edge surface. 

Depending on the angle of the surface, the force is then resolved into a vertical force 

and a horizontal force. The vertical force then begins to push the cutting edge up. Thus 

we revised the simple model experiment to a constant depth sliding process to measure 

this resistant force. 
132

Figure 8.12 True stress-strain curves for UHMWPE (GUR 4150) [104]. 

True strain

0.0 0.5 1.0 1.5 2.0 2.5

True stress (MPa)

0

50

100

150

200

250

Figure 8.13 True stress-strain curves for UHMWPE 4150 up to 0.12 strain in

uniaxial tension and compression and elastic-plastic model to

describe the behavior [104]. 

True strain

0.00 0.02 0.04 0.06 0.08 0.10 0.12

True stress (MPa)

0

5

10

15

20

25

Compression data 

Elastic-Plastic Material Model
133

8.5.1 Constant depth sliding model experiments

To determine the displaced volume of UHMWPE before the cutting edge is 

lifted up, the simple model experiment under constant load was modified to a process 

with constant depth of sliding. The purpose is to measure the maximum material 

resistant force by pre-defining the displaced volume and by monitoring the material 

deformation in-situ.

The experimental set-up of constant depth sliding tests is shown schematically 

in Figure 8.14. The cutting edge is held in a fixed position on a vertical slider that can 

be adjusted in the vertical direction to control the penetration depth. A UHMWPE disk 

(2” in diameter) is mounted in a lower holder on a horizontal slider in the Y direction. 

The Y direction slider is attached to another horizontal slider in the X direction. The 

sliders are driven by a micro-stepping motor. This is controlled by a computer. The 

vertical and horizontal sliders move at the precise distance, speed, and acceleration 

specified by the computer controller. One motion controller determines the motion of 

vertical slider (Z direction), and the other motion controller operates the motion of two 

horizontal sliders in the X and Y directions. We used the same force recording and 

microscopic video system as described in section 8.3.1. 
134

Vertical Slider

Horizontal Y Slider 

Horizontal X Slider 

Wedge Tip 

3D Force 

Transducer

UHMWPE Disk

Charge 

Amplifier

Microscopic 

Video 

Digital 

Recorder

Computer 

Motion 

Controller 0

Motion 

Controller 1

Stepping 

Motors

Stepping 

Motors

Data Acquisition 

System 

Waveform 

Transformation 

Figure 8.14 A schematic representation of the displacement controlled

single cutting edge sliding test.
135

 Wedge-shaped cutting edges with two cutting-edge angles (60° and 90°) were 

used. The cutting edge length is 2 mm. The contact between the cutting edge and the 

UHMWPE surface can be detected from the on-line monitoring of the forces. The 

UHMWPE sample was leveled first on the stage before the tests. The indentation of 

the cutting edge was conducted at a speed of 0.5 mm/s vertically. Various penetration 

depth (ranging from 80 μm to 400 μm) were applied in the tests. After 2 s, the cutting 

edge was slid sideways for 4 mm with a constant speed of 0.5 mm/s. 

 Figure 8.15 shows a typical horizontal-force curve for a 60° cutting edge with 

a penetration depth of 400 μm. The sliding motion starts at time t=0. The horizontal 

force acting on the cutting edge grows and approaches a steady value. The video of the 

cutting-edge sliding process was synchronized with the force curve in Figure 8.16. By 

comparing the volumes of accumulated material in the video frames, it is observed 

that a maximum accumulated volume was achieved between t=0.8 s and t=1.2 s which 

corresponds to the point A in the force cure in Figure 8.15. After t=1.2 s, the 

accumulated material volume in front of the cutting edge remains the same. Plastic 

flow of the material into the near-by region continues during the following cuttingedge sliding process. By identifying the time for reaching the maximum accumulated 

volume, the sliding distance toward this point is determined with the known sliding 

speed. The maximum displaced volume of the material during the sliding process is 

equal to the product of cutting-edge length, fixed penetration depth and the sliding 

distance. 
136

Time (s)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Fx (N)

0

10

20

30

40

50

60

70

80

A 

Figure 8.15 Horizontal-force curve of the cutting-edge sliding process

(cutting-edge length = 2 mm, penetration depth = 400 μm,

sliding speed = 0.5/s) 

B 

O 
137

Time = 0 Time = 0.4 s 

Time = 0.8 s Time = 1.2 s 

Time = 2.0 s Time = 3.0 s 

Figure 8.16 Video frames of the cutting edge sliding process (cutting edge length

= 2mm, penetration depth = 400 μm, sliding speed = 0.5/s). 
138

With these observations, we may explain the sliding process as follows. From 

point O to A in Figure 8.15, the sliding process produces shear of UHMWPE and the 

material pile-up is accumulated in front of the cutting edge. The increased deformation 

volume in front of the cutting edge leads to an increase of the material resistant force 

acting on the cutting edge. Thus an increased horizontal force is observed. The 

material accumulated in front of the cutting edge reaches a maximum volume at point 

A. This maximum displaced volume is projected to be the particle volume in the 

particle-generation process. 

From point A to B, the horizontal force keeps growing without a visible 

increase of the cumulative volume in front of the cutting edge, but an increasing sidewall of material is observed. The material displaced by the cutting edge no longer goes 

in front of the cutting edge. It is suggested that the cutting edge keeps pushing the 

UHMWPE material and the strain-hardening effect of plastic deformation contributes 

to the increase of the horizontal force from A to B. Since the depth of penetration is 

fixed in the experiment, the increased force is not able to lift up the cutting edge. 

Finally, the increased contact pressure of the cutting edge face forces a plastic flow of 

materials sideways. A steady state plastic flow is established and the horizontal force 

approaches a constant value as seen after point B in Figure 8.15. 
139

Experiments conducted at various penetration depths and with different cutting 

edge angles are discussed in this section. We compare the effect from different 

penetration depths on the sliding distance in Figure 8.17. The sliding distance toward 

the maximum displaced volume for a 160 μm penetration depth is 340 μm and for a 

80 μm penetration depth it is 162 μm. The measured values have a relative standard 

uncertainty of ± 5 %. The larger penetration depth also leads to a larger sliding 

distance toward the maximum displaced volume and also leads to a larger maximum 

displaced volume during the process. 

 

Sliding Speed = 0.5 mm/s

Sliding Distance (μm)

-200 0 200 400 600 800 1000 1200 1400

Force (N)

0

5

10

15

20

25

30

35

40

Penetration = 80 μm

Penetration = 160 μm

Cutting Edge Angle = 60 deg

Cutting Edge Length = 1 mm

Figure 8.17 Horizontal force versus sliding distance for different penetration

depths in a displacement-controlled cutting-edge sliding test. 
140

We compare the experiments with different cutting-edge angles in Figure 8.18. 

It is seen that a sharper cutting edge (60°) leads to a larger sliding distance toward the 

maximum displaced volume. The maximum resistant force for the sharper cutting 

edge (60°) is also larger. We suggest that the resistant force contributing to the sliding 

direction is larger for the cutting edge with a smaller angle. More volume space in 

front of the cutting edge (especially in the vertical direction) is reserved for the 

material deformation and accumulation. Thus a long sliding distance toward the 

maximum displaced volume is expected. 

Sliding Speed = 0.5 mm/s

Sliding Distance (μm)

-200 0 200 400 600 800 1000 1200 1400

Force (N)

0

5

10

15

20

25

30

Penetration = 120 μm

Cutting Edge Length = 1 mm

Angle = 60 deg

Angle = 90 deg

Figure 8.18 Horizontal force as a function of the sliding distance for

different cutting-edge angles in a displacement-controlled

cutting-edge sliding test. 
141

8.6 Observations from the particle-generation results 

To validate the proposed mechanism for generation of UHMWPE particles we 

proposed, we also look into the morphology of the worn surface and the wear particles 

(the results from Chapter 7, an ASTM linear reciprocating wear process for 

UHMWPE particle generation). Figure 8.19 shows an example of SEM photos of the 

UHMWPE worn surface, the single surface feature and generated particles. Shear 

deformation of UHMWPE was observed along the sliding direction. The width of the 

sliding track seen on the worn surface is consistent with the length of the cutting edge. 

According to the observation above and the analysis from the previous 

sections, we conclude that the process proceeds follows: At the beginning of the 

particle generation tests, the cutting edges penetrate into the UHMWPE material under 

applied normal load. During the linear sliding motion, the UHMWPE material is 

sheared and accumulated in front of the cutting edge. The increased volume of 

accumulated material leads to the increased material resistant force acting on the 

cutting edges. Finally the cutting edges are pushed away from the surface and a wear 

particle is generated. 

From the micrographs, it is also observed that shear deformed UHMWPE is 

about to be torn away to form a wear particle. The dimensions of the volume are 

similar to the size of the wear particles shown in Figure 8.19. Figure 8.20 shows some 

other examples of higher magnified SEM micrographs of UHMWPE wear particles. 

The straight-line indent mark seen on the photos indicates the contact between the 

cutting edge and UHMWPE material. From the irregular wear particle edges, it is 
142

suggested that UHMWPE material breaks under a tensile stress when the cutting edge 

is pushed away from the surface in the sliding process. 

Sliding direction 

Single surface feature 

UHMWPE Worn Surface 

UHMWPE Particles 

Figure 8.19 SEM micrographs of surface cutting-edge feature,

UHMWPE worn surface and generated UHMWPE particles.

Material 
143

Irregular edges Straight line indent mark 

Figure 8.20 UHMWPE wear particles with irregular edges and straight 

line indent marks made by cutting edges. 
144

8.7 Conclusions

 From the model experiments with constant load, we simulated the sliding 

process of the cutting edge in situ. Results of the video analysis and analysis of the 

reacting forces of the process provided direct evidence of the micro-cutting process of 

particle generation. Also based on the data for the reacting force, the effects of cuttingedge angle and penetration depth were studied. Results further confirm that a larger 

penetration depth leads to a longer sliding distance or a larger particle volume as 

suggested in Chapter 8. Shear-induced molecular orientation makes the UHMWPE 

surface layer more brittle under multiple passes by the cutting edges. We performed 

further model experiments with constant sliding depth to measure the displaced 

volume of UHMWPE. We also obtained the specific energy required for UHMWPE 

deformation during the sliding process. 

The mechanism of particle generation with surface textures has been examined 

and supported by the model experiments. However, the quantitative relationship 

between the cutting feature dimensions and the particle generation is still not clear. 

We will attempt to model the penetration and sliding processes from first principles to 

quantify the relationship. In this way, we may be able to predict the particle size. 

 
145

CHAPTER 9 

MODELING OF PARTICLE GENERATION

9.1 Introduction 

The mechanism of UHMWPE particle generation by a textured surface has 

been elucidated in Chapter 8. In this chapter, we will attempt to distill our 

understanding into a mathematical model to predict particle size and shape with a 

given surface-texture dimensions and mechanical loading conditions. 

We will attempt to model the particle-generation process based on first 

principles in two steps: (1) penetration of the cutting edge, and (2) lateral sliding of the 

cutting edge. The penetration depth of a wedge-shaped cutting edge is based on force 

balance. Predictions of the penetration depth will be made under purely plastic and 

elastic-plastic assumptions. We will further determine the correction factor to predict 

the penetration depth for viscoelastic UHMWPE material. A mathematical equation 

will be developed to predict the penetration depth with a given surface-feature 

geometry and applied loads. 

During the sliding of the cutting edges, displaced volume of UHMWPE leads 

to the material resistant force acting on the cutting edges. The resolved component 

force in the vertical direction lifts up the cutting edge. The penetration depth then 

decreases as the tip travels further. The balance of forces between the normal load and 

the lift-up force contributed from the material resistance is then finally reached. We 
146

are going to calculate the displaced volume of UHMWPE during the sliding process. 

The volume is projected to be the volume of generated particles. Finally a contact 

mechanics model will be developed to determine the shear volume (displaced volume) 

of UHMWPE with given surface texture geometry and mechanical loading condition. 

A correlation model based on the actual particle generation process will also be 

established based on the results of UHMWPE-particle generation with microfabricated 

surface textures. By comparing the two models, we hope to validate the first principle 

model. 

9.2 Model parameters 

The variables of the model are the dimensions of the surface texture, the 

mechanical operation parameters, and the particle dimensions as output variables. A 

schematic representation of the particle-generation process is shown in Figure 9.1. The 

important dimensions of the surface texture include the cutting-edge feature length 

(Lc), the feature width (Wc), the feature height (Hc), the distance between features 

along the sliding direction (Ds), and the distance between adjacent features (Di). Two 

mechanical operation parameters during the linear reciprocating wear process are the 

load applied on each cutting-edge feature (W), and average sliding speed (V). Based on 

the sheet-like morphology generated from this process, the particle is characterized as 

the particle length (Lp), the particle width (Wp), the particle thickness (Tp), and the 

particle volume (Vp) is estimated by the product of these three dimensions. 
147

Particle length

(Lp) 

Particle width

(Wp) 

Particle thickness 

(Tp) 

Top view of the surface texture

Ds

Di

Lc

Wc

Sliding direction

Lc Hc

Ds

Load 

UHMWPE pin 

Figure 9.1 Illustrations of significant variables for the particle-generation process. 

Lc: cutting-edge length 

Hc: cutting-edge height 

Wc: cutting-edge width 

Ds: pitch in the sliding direction 

Di: distance between adjacent features
148

9.3 Contact mechanics modeling 

9.3.1 Penetration of the cutting edge 

The penetration process of the cutting edge is discussed as a wedge indentation 

problem in this section. A schematic representation is shown in Figure 9.2. The key 

parameters shown in the figure include the cutting-edge length (Lc), half angle of the 

cutting edge (θ), average normal load on each feature (W) and the penetration depth 

(Hp). Theoretical calculations of the penetration depth of a wedge tip with purely 

plastic and elastic-plastic assumptions have been carried out previously [111-113]. We 

are going to derive the equations from a force balance. 

Load

W 

Hp

Lc

θ

Figure 9.2 Schematic of the penetration process of the cutting edges. 

UHMWPE 

material 

Surface texture 
149

As shown in Figure 9.2, the average normal load acting on each cutting edge is 

W. The contact length after the indentation process is 2·Hp·tanθ. The average contact 

pressure of applied to the material can be calculated by: 

2 tanθ c p

pen ⋅ ⋅ ⋅ = L H

W Average contact pressure p . (9.1)

With the perfect plastic deformation assumption, no deformation will occur until the 

yield stress is reached, after which plastic flow occurs. The relationship between 

average contact pressure and the yield stress has the following relationship [111]: 

ppen = Cp ⋅Y , (9.2)

where Cp is a coefficient for different geometry of the indenter. For a wedge shaped 

indenter, the Cp is a function of θ and is described as [111, 112] 

λ[2.57 0.023(90 θ)] Cp = − − , (9.3)

where λ=2/√3 by the von Mises yield criterion, and where θ is expressed in degrees. 

By balancing the pressure in the z direction, we obtain: 

C Y

L H

W = ⋅ ⋅ ⋅ ⋅ p

c p 2 tanθ . (9.4)

Thus the penetration depth (Hp,p) predicted by the purely plastic hypothesis can be 

expressed as 

2 ( ) tanθ , ⋅ ⋅ ⋅ ⋅ =

p c

p p C Y L

W H , (9.5)

where Y is the yield strength of the material. 
150

With an elastic-plastic assumption of the material property, a certain amount of 

deformation will be recovered after the applied stress is released. Johnson [113] has 

derived that H/Y (H: hardness, Y: yield strength) is a function of penetration depth and 

E/Y (E: elastic modulus). By representing hardness by the normal force and the size of 

deformation site after penetration, the predicted penetration depth with elastic-plastic 

assumption can be described as: 

2 ( ) tanθ ep c

p,ep ⋅ ⋅ ⋅ ⋅ = C Y L

W H , (9.6)

where Cep is a function of material property and penetration edge geometry. For a 

wedge geometry, Cep is derived by Johnson [113] as the following: 

θ)] 2

π tan( 3π4 [1 ln( 3

1

ep = + − Y

E C , for wedge geometry. (9.7)

For example, the Cep value of a 90° cutting-edge angle (θ=45°) wedge feature is 2.49, 

and the Cep value of a 60° cutting-edge angle (θ=30°) wedge feature is 2.80 for the 

indentation of UHMWPE material. 

To discuss the effect from viscoelasticity, we shall obtain the experimental 

penetration-depth data for UHMWPE. Wedge indenters (90° and 60° with 2 mm edge 

length) were applied to the indentation tests. The normal load was applied and holds 

for 15 s. Given sufficient time (60 min) after the indentation tests, profiles of the 

indentation marks were measured by profilometry. The various normal loads were 

tested and the measured penetration depths were compared with the elastic-plastic 

calculation in Figure 9.3. 
151

2.76e-3

Normal load W (N)

0 20 40 60 80 100 120 140

Penetration depth 

Hp (mm)

0.0

0.1

0.2

0.3

0.4

0.5

Experimental results

Linear regression of experimental results

Plastic-elastic prediction

Cutting-edge angle = 60 deg (θ=30 deg)

Cutting-edge length Lc=2 mm

1.79e-3

Normal load W (N)

0 20 40 60 80 100 120 140

Penetration depth 

Hp (mm)

0.0

0.1

0.2

0.3

0.4

Experimental results

Linear regression of experimental results

Elastic-plastic prediction

Cutting-edge angle = 90 deg (θ=45 deg)

Cutting-edge length Lc= 2 mm

Figure 9.3 Comparison of the experimental data of wedge indentation 

in UHMWPE with elastic-plastic calculation results. 
152

 We can also represent the experimental results by the following equation that is 

similar to that for the plastic and elastic-plastic calculations: 

2 ( ) tanθ v c

p ⋅ ⋅ ⋅ ⋅ = C Y L

W H , (9.8)

where Cv is the coefficient for the viscoelastic UHMWPE from the experimental 

results. By comparing the elastic-plastic prediction results with the linear regression 

curve of the experimental results, we obtain 

v 75 ep C =1. ⋅C , (9.9)

where Cep is the coefficient for the elastic-plastic model. The penetration depth can be 

then predicted from the above equation after the viscoelastic properties are taken into 

account. It means the ratio of penetration depth predicted from the elastic-plastic 

assumption to the measured penetration depth is 1.75 ± 0.12 (mean ± standard 

deviation). The ratio we obtained from the nano-indentation test in section 8.2 is 1.39. 

These results are reasonably close. 

Figure 9.3 shows that the prediction from the calculation with elastic-plastic 

assumption is larger than the experimental measurements. It is suggested that the 

difference arises from the viscoelasticity of UHMWPE for the following possible 

reasons: (1) Recovery of material deformation: After the indentation process, removal 

of the applied stress results in the gradually recovery of material deformation. The 

viscoelastic recovery has been measured from a nano-indentaion test in Chapter 8. (2) 

Creep behavior during the indentation: For a viscoelastic material, the material 

deformation is a function of the strain rate. Under a high strain rate, the time allowed 
153

for material deformation is short and thus the final material deformation is smaller 

than for the case with low strain rate under the same applied stress, and (3) Strain 

hardening of the material: During the indentation process, the plastic deformation 

brings the strain hardening of the material. It leads to an increased material resistant 

during the indentation process and a smaller penetration depth. 

The viscoelastic recovery of the material in the cutting-edge penetration 

process has a dramatic influence on the final penetration depth. We conclude that the 

final penetration depth can be estimated from equation (9.8). The final penetration 

depth during the indentation process can be obtained. 

9.3.2 Sliding process of the cutting edge 

During the sliding process, a horizontal displacement is applied to the cutting 

edge under a constant normal load and the material is ploughed. A schematic 

representation of the process is shown in Figure 9.4. The introduction of the horizontal 

sliding process unloads the left-hand face of the cutting edge. Further vertical 

deformation takes place by the cutting edge sliding further deeper in the sliding 

process. UHMWPE material displaced by the cutting edge was accumulated in front of 

the edge. The resultant force from the displaced volume acts on the cutting edge as an 

encountered force to the normal load. The net normal load is decreased and it leads to 

the decrease of the penetration depth. During the subsequent sliding process, the 

displaced volume for unit sliding distance is decreased. More volume is accumulated 

in front of the cutting edge with a decreasing rate of increase of displaced volume. It is 
154

an iterative process between increase of resistant force by increased displaced volume 

and decrease of penetration depth by the increase of material resistant force. Finally 

the cutting edge is pushed away from the UHMWPE material and the displaced 

volume of UHMWPE is projected to be the volume of the generated particle. 

Normal load

Sliding

Normal load

Sliding

Sliding

Normal load

Figure 9.4 Schematic representation of the sliding process of the cutting edge. 
155

 We are going to calculate the projected volume of the particle based on various 

dimensions of cutting edges and mechanical loading conditions. Figure 9.5 shows a 

flow chart of the calculation procedure. With known Lc, Wc, Hc, and θ, the penetration 

depth can be calculated. During the sliding process, one face of the cutting edge was 

unloaded due to the horizontal sliding motion. The supporting area of the cutting edge 

is half of the area in an indentation process. Equation (9.8) is then modified as 

equation (9.10) to calculate the penetration depth during the sliding process: 

( ) tanθ v c

pc ⋅ ⋅ ⋅ = C Y L

W H . (9.10)

If the calculated penetration depth is larger than the feature height Hc, the real 

penetration depth is equal to the feature height. The minimum load Wm needed to 

reach the penetration depth Hc is then back calculated: 

( ) tanθ m c v c W = H ⋅ C ⋅Y ⋅ L ⋅ . (9.11)

With a small increment of sliding distance, we can calculate the displaced volume by 

the product of cutting edge length, penetration depth and the increment of sliding 

distance. With known quantity of displaced volume (Vd), we can estimate the material 

resistant force. From the simple model experiment described in chapter 8, we have 

obtained the material resistant force per unit volume of UHMWPE is equal to 

m=1.2⋅10-6 N/(μm)3

 (estimated slope of the plot shown in Figure 8.15) 
156

Hpc < Hc

Calculated penetration depth (Hpc): 

(cutting edge supported by one face only) 

( ) tan θ v c

pc ⋅ ⋅ ⋅ = C Y L

W H

No 

Yes 

Initial penetration depth 

Hp= Hc 

Min. load to reach Hc 

( ) tan θ m c v c W = H ⋅ C ⋅Y ⋅ L ⋅

Initial penetration depth

Hp= Hpc 

Min. load to reach Hc 

Wm=W

Start 

Sliding 

increment 

dx 

Calculate displaced volume 

dVd= Lc⋅Hp⋅dx 

Vd=Vd+dVd 

Material resistant force in the 

vertical direction 

Frz=m⋅V⋅sinθ

W- Frz < Wm

Surface-texture 

dimensions: 

Lc, Wc, Hc, θ

Load on each 

feature: W

No 

Yes 

Update penetration depth 

( ) tan θ v c

r

p ⋅ ⋅ ⋅

− = C Y L

W F H z

Penetration depth remains

Hp = Hp

Hp < 0

Yes 

Projected volume of 

the particle 

Vp = V

End 

No 

Figure 9.5 A flow chart of calculation of the sliding process of the cutting edge. 
157

 Then we can calculate the resolved resistant force in the vertical direction from 

the displaced volume of UHMWPE: 

sinθ sinθ r r d F = F ⋅ = m⋅V ⋅ z . (9.12) 

Obviously, the cutting-edge angle plays an important role at distributing the material 

resistant force into the vertical direction as shown in Figure 9.6. With the same 

displaced volume, a larger force is distributed to the vertical direction for a larger 

cutting-edge angle. The penetration depth was then updated by the new normal load in 

the equation (9.13): 

( ) tanθ v c

r

p ⋅ ⋅ ⋅

− = C Y L

W F H z . (9.13) 

Figure 9.6 A schematic representation of reacting forces in a cuttingedge sliding process. 

Lc

Hp

θ Sliding

Frx =Fr*cosθ

Fr

Frz =Fr*sinθ
158

For the case with initial penetration depth equal to the feature height, there 

exists excess load between the cutting edge and UHMWPE material. The cutting edge 

needs to displace a larger volume of UHMWPE during the sliding process in order to 

generate sufficient resistant force to push away the cutting edge from the material 

surface. This factor is also taken into account in our model. 

 After we update the penetration depth for a small increment of sliding distance, 

the new penetration depth was then used to calculate the displaced volume in the next 

increment of sliding. Assuming incompressibility of the material, the accumulated 

displaced volume of UHMWPE was determined by adding up the displaced volume 

from each sliding increment. The iteration process was carried out until the penetration 

depth reaches zero. It indicates the cutting edge was pushed away from the UHMWPE 

surface. The total displaced volume during the process is projected to the particle 

volume. 

 The process was written in a MATLAB program as shown in Appendix D. In 

the next section, we are going to compare the experiment results with the calculation 

from this model. We are also going to discuss the effect of surface-texture dimensions 

and mechanical loading conditions on the generated particles from the model 

predictions. 
159

9.3.3 Validation of the model 

In this section, we are going to calculate the predicted particle volume with

given surface-texture dimensions and mechanical loading conditions. For the 

experimental results, the particle volume was estimated by the product of particle 

length, particle width and particle thickness from the SEM observations. The 

predictions by the model will be compared to the experimental results. The effects of 

normal load, feature height, and cutting-edge angle will also be discussed. 

We have generated UHMWPE particles with surface texture A-3 which was 

listed in Figure 7.3 with two different load conditions. The dimensions of the surface 

texture are Lc= 7.0 μm, Wc= 5.0 μm, and Hc= 3.0 μm. The tests were run under an 

average sliding speed of 57.2 mm/s. The normal loads applied were 192 N and 96 N, 

which corresponds to 0.1174 N and 0.0587 N distributed on each feature on the 

surface. Figure 9.7 shows the comparison of the model predictions and the 

experimental results. Both results show that increase of load lead to a larger particle 

volume. From our calculations, it indicated that the minimum load needed (Wm) on 

each feature to reach the penetration depth equal to the feature height is only 0.0017 

N. The loads we applied were far larger than this value. It means the cutting edge was 

pushed to the end of the surface. Most of the material resistant force developed was 

used to encounter the excess load on the surface. A linear relationship is observed 

from the prediction from the model. 

However, the model predicted particle volume is larger than the ones from 

experimental results. One possible reason is that the particle length is about 10 % to 15 
160

% smaller than the cutting-edge length. In the model, we have assumed that the 

particle length is equal to the cutting-edge length. The viscoelasticity, leading to the 

shrinkage of the particle dimension after the particle is released from the bulk 

UHMWPE, can explain some of the difference between predicted and the 

experimental results. On the other hand, the multiple surface features on the surface 

texture may re-processing the generated particles and further reduce the particle size. 

With the same feature density of the surface texture, there exists higher possibility for 

a larger particle to be re-processed in the micro-cutting process. In the model, we 

assumeed a linear relationship between reacting forces and displaced volume during 

the sliding process of the cutting edge. However, a nonlinearity was also observed 

from Figure 8.15 toward a larger displaced volume. It is probably why we observed a 

larger difference between the model prediction and the experimental results for the 

larger particles. 

Figure 9.7 Effect of normal load from model predictions and experimental results. 

Load on each feature W (N)

0.02 0.04 0.06 0.08 0.10 0.12 0.14

Particle volume (

μ

m

3) 0

20

40

60

80

Model prediction

Experimental results
161

Surface textures A-2 with different feature heights were applied in the particlegeneration process. The dimensions of the surface texture are Lc= 15.0 μm, Wc= 5.0 

μm, and two different feature heights with Hc= 3.4 μm and 2.8 μm. The tests were run 

under an average sliding speed of 57.2 m/s and a normal load of 96 N (load applied on 

each feature W= 0.0587 N). The micrographs of the particles have been shown in 

Figure 7.6. The comparison of the model prediction and the experimental results are 

shown in Figure 9.8. A larger feature height leads to a larger particle volume observed 

from the model predictions and experimental results. The applied load is also 

sufficient to push the cutting edge to the end of the surface. A larger feature height 

means a larger initial penetration depth at the beginning of the sliding process. 

Obviously a larger penetration depth leads to a longer sliding distance and a larger 

particle volume. 

 The results again show that the particle volume predicted by the model is 

larger than the experimental results. From Figure 9.8 it indicates that the difference 

between the model prediction and experimental results is smaller for a larger particle. 

The trend is opposite from the previous case. Probably it is more difficult for the 

surface texture with a larger feature height to entrap the generated particles between 

the cutting edges and UHMWPE surface. Thus less number of larger particles were reprocessed so the experimental results are closer to the model predictions which are 

based on a single cutting-edge process. 
162

There is lack of experimental data with different cutting-edge angles while 

maintaining the same feature height of the surface texture. We used the feature length 

and feature height dimensions from surface texture A-2 (Lc= 15.0 μm, Hc= 2.8 μm). 

By artificially changing the feature width in the model, we can evaluate the effect of 

cutting-edge angle. A normal load of 96 N (load applied on each feature W= 0.0587 

N) was also used in the model. Figure 9.9 shows the particle volume versus different 

cutting-edge angles from the model calculations. A sharper cutting edge (smaller 

cutting-edge angle) results in a larger particle volume. With the same penetration 

depth, the sharper cutting edge leads to a longer sliding distance because a smaller 

material resistant force is distributed to the vertical direction to push away the cutting 

Figure 9.8 Effect of feature height from model predictions and experimental 

results. 

Feature height Hc (μm)

1.0 1.5 2.0 2.5 3.0 3.5 4.0

Particle volume (

μ

m

3)

0

20

40

60

80

100

120

140

Model prediction

Experimental results
163

edge. Obviously the predictions from the model are consistent with the theoretical 

approach of the particle-generation mechanism. 

To sum up, we have successfully predicted the effects of surface textures on 

particle volume qualitatively. This achievement enables us to predict the particle 

volume from a first principle approach. This model can assist on the design of surface 

textures in order to generated UHMWPE particles with desired size and shape. The 

results of the prediction are close to the experimental results of UHMWPE particle 

generated. However, the model predicts a larger particle volume. In order to further 

quantitatively predict the particle volume precisely, the model should be further 

tightened to reflect the facts of multiple cutting edges and limited penetration depth in 

the real particle-generation process. 

Figure 9.9 Effect of cutting-edge angle from model calculations. 

Cutting-edge angle 2θ (degree)

50 60 70 80 90 100 110

Particle volume (

μ

m

3) 40

60

80

100

120

140

160
164

9.4 Correlation model 

9.4.1 Effect of cutting-edge length 

As shown in Figure 9.1, the direction of the cutting-edge length is 

perpendicular to the scratching direction. It is observed experimentally that the particle 

length is close to but a little smaller than the cutting edge. Measured values of particle 

dimensions have a relative standard uncertainty of ±8 %, and measured dimensions of 

cutting edges have a relative standard uncertainty of ±5 %. SEM photos of different 

cutting-edge lengths versus different particle lengths are compared in Figure 9.10. By 

linear regression of the plot in Figure 9.11, a linear relationship is obtained: 

p 78 c L = 0. ⋅ L . (9.14)

The correlation coefficient of the regression r

2

 is 0.997. 

Cutting Edge Length 

Surface texture Wear Particles

Sliding direction

Figure 9.10 Comparison of cutting-edge length with wear-particle length. 
165

Cutting Edge Length Lc (μm)

0 10 20 30 40 50 60

Mean Particle Length Lp (

μm)

0

10

20

30

40

50

0.78

r

2

=0.997

Figure 9.11 Plot of particle length versus cutting-edge length.
166

9.4.2 Effect of penetration depth 

The variables that affect the penetration depth include (a) geometrical 

dimensions of cutting edges: cutting edge length (Lc), cutting edge height (Hc), 

cutting-edge angle, and (b) mechanical condition: applied load for each cutting-edge 

feature (W). The penetration depth with purely plastic and elastic-plastic assumptions 

concerning the material properties can be calculated. The detailed derivation and 

calculation procedures have been presented in the previous section. However, in our 

case, the maximum penetration depth is limited by the feature height (Hc). When the 

calculated penetration depth exceeds the feature height, penetration depth (Hp) is equal 

to cutting-edge height (Hc). 

Previous research has shown the effect of the cutting penetration depth on the 

thickness of formation chips during the machining process [110]. By applying the 

same principle to the generation process here, theoretically the particle thickness 

should be proportional to penetration depth of the cutting edge. The ratio between the 

two values depends on the geometry of the cutting edges. Figure 9.12 shows a plot of 

particle thickness versus different penetration depths from our experimental results 

obeying a relationship: 

Tp∝ Hp . (9.15)

We also have plotted the particle width versus penetration depth in Figure 9.13. A 

linear relationship between them is also obtained: 

Wp∝ Hp . (9.16)
167

Penetration Depth Hp (μm)

012345

Mean Particle Thickness Hp (

μm)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Figure 9.12 Plot of particle thickness versus penetration depth. 

Penetration Depth Hp (μm)

2.0 2.5 3.0 3.5 4.0 4.5 5.0

Mean Particle Width 

Wp (

μm)

0

2

4

6

8

10

12

Figure 9.13 Plot of particle width versus penetration depth. 
168

9.4.3 Effect of load 

 Figure 9.14 is a plot of effect of load on particle volume. Tests have been done 

on the same surface texture with different applied load. The particle volume increases 

with increased applied normal load. By plotting the load W vs. the particle volume Vp

on a log-log scale, we obtain: 

0.6 Vp

∝W . (9.17)

Load W (N)

30 40 50 100 200

Particle Volume 

Vp (

μ

m

3)

20

30

40

50

100

0.6

r

2

=0.99

Figure 9.14 Plot of particle volume versus load on a log-log scale. 
169

9.4.4 Effect of sliding speed 

The same surface texture was tested under different sliding speeds. By plotting 

the particle size Vp versus average sliding speed Sp on a log-log scale in Figure 9.15, 

we obtain: 

p 0.6

1

Sp

V ∝ . (9.18)

A lower speed leads to a larger particle size. The sliding speed influences the 

residence time of the cutting edge during the cutting process, especially for a 

viscoelastic material. A lower sliding speed allows more stress relaxation in the sliding 

process. It leads to a longer sliding distance before the particle is generated. 

Average Sliding Speed Sp (μm/s)

2x107 3x107 4x107 5x107 108

Particle Volume Vp (

μ

m3

)

200

300

400

500

1000

-0.6

r

2

=0.93

Figure 9.15 Plot of particle volume versus sliding speed on a log-log scale. 
170

9.4.5 Comparison with experimental results 

The relationship between particle length, width, thickness, surface texture 

geometry, and mechanical conditions are described by equations (9.14)-(9.18). For the 

micro-cutting process, the particle volume can be estimated from the product of 

particle length, width and thickness. Thus the following relationship is obtained: 

0.6

2 0.6

c p

p Sp

L H W

V ⋅ ⋅ ∝ . (9.19)

The particle volume (Vp) is plotted against 0.6

2 0.6

c p

Sp

L ⋅ H ⋅W

in Figure 9.16. A linear 

relationship is found and the 95 % confidence level shows that the experimental data 

can be predicted well by the following equation: 

0.6

2 0.6

c p

p 39.5

Sp

L H W

V ⋅ ⋅ = (unit: μm, N, s) (9.20)

9.5 Summary 

 Two approaches have been attempted to model the UHMWPE-particle 

generation with surface textures. Contact mechanics model calculated the penetration 

depth and then calculated the displaced volume of UHMWPE iteratively from a forcebalance basis. The results of the model predictions are close to the experimental 

results of the particle generation. Correlation model correlate the variables in the 

UHMWPE-particle generation process to a mathematical equation. Both model 

indicates the same trend of effects of surface-texture geometry and mechanical 

conditions on the volume of particles. 
171

(Lc)(Hp

2

)(W0.6)/Sp0.6

0 5 10 15 20 25

Particle Volume 

Vp (

μ

m3

)

-200

0

200

400

600

800

1000

1200

(Lc)(Hp

2

)(W0.6)/Sp0.6

01234

Particle Volume 

Vp (

μ

m3

)

-50

0

50

100

150

200

Figure 9.16 Comparison of the correlation model with the experimental results. 
172

CHAPTER 10 

PARTICLE CHARACTERIZATION BY ANALYZING 

THE PHAGOCYTOSIS OF PARTICLES

10.1 Introduction 

In artificial joint systems, UHMWPE wear particles generated during the life 

of the replacement have been associated with osteolysis and prosthetic loosening. The 

morphology of the wear particles varies according to different articulating geometry, 

material pairs, and mechanical conditions. From the viewpoints of phagocytosis of 

wear particles, the mechanisms of particles induced bioactivities will vary according 

to different particle size and shape [32]. In order to identify the critical parameters of 

particles for induced osteolysis, characterization of particles from the viewpoint of 

phagocytosis of particles is necessary. 

In this chapter, we will explain how the human body attempts to digest 

UHMWPE wear particles. Phagocytosis of particles is a key phenomenon in the 

process. We will investigate the phagocytic capacity which is the quantity of particles 

that one macrophage cell can phagocytose from a biophysical point of view1

. From 

this, a foundation will be built for the subsequent chapter of bioactivity testing results 

and their relationship with particle dimensions and shapes. 

____________________________________________________________________ 

1

The phagocytosis of particles only represents one perspective of the complicated 

processes for UHMWPE particle induced osteolysis. The purpose is to analyze the 

effects of particle size and shape on bioactivity from an engineering point of view. 
173

10.2 Basic particle parameters 

10.2.1 Descriptors for particle size 

After the image analysis of the particles (described in section 4.4.2), the 

dimensions of the generated particles including particle length, width and thickness 

can be estimated from the SEM photo of the particle as shown in Figure 10.1. The 

particle size can be represented by the volume of the particle. Based on the sheet-like 

geometry of the UHMWPE particles, the volume of the particle can be calculated as 

the product of the particle length, width and thickness: 

Vp Lp Wp Tp ≈ ⋅ ⋅ . (10.1)

From the viewpoints of reaction between particles and cells, the surface area of the 

particle represents the effect of particle size on the resulting outcomes [86]. For a 

sheet-like particle as shown in Figure 10.1, the surface area (Sp) of the particle can be 

estimated from equation (10.2) when the particle thickness is much smaller than the 

particle length and width: 

Sp 2 Lp Wp ≈ ⋅ ⋅ (10.2)

Particle length 

(Lp) 

Particle width

(Wp) 

Particle thickness

(Tp) 

Figure 10.1 Estimation of particle length, particle width and particle

thickness from a SEM micrograph of UHMWPE particles. 
174

From the viewpoints of the phagocytosis of the particles by microphage cells 

in bioactivity tests, the concepts of projected particle volume and the projected particle 

surface area are proposed here. When a particle is phagocytosed by a macrophage cell, 

the particle is wrapped by the membranes of the macrophage cells. The membranes of 

macrophage cells tend to form a spherical or ellipsoid shape after engulfing the 

particles. Based on this argument, we can calculate the volume and the surface area of 

an ellipsoid which has three axis lengths equivalent to the particle length, width and 

thickness as shown in Figure 10.2. We define them as the projected particle volume 

and projected particle surface area according to the following equations: 

pp p p p 3

4 V = ⋅π⋅ L ⋅W ⋅T , (10.3)

Spp 2π Lp Wp ≈ ⋅ ⋅ (for Lp>>Tp) , (10.4) 

where Vpp is the volume of the projected ellipsoid and the Spp is the surface area of the 

projected ellipsoid. 

Particle 

Projected ellipsoid 

Figure 10.2 Schematic of the projected ellipsoid of a particle. 
175

10.2.2 Descriptors for particle shape 

 The descriptors for particle shape are used to distinguish various particle 

shapes quantitatively. Elongated particles and spherical-like particles have been 

observed for the UHMWPE wear particles from clinical studies of total joint implants. 

In this section, the discussion focuses on quantifying the degree of the elongation of a 

wear particle. 

 During the particle-collection process, as mentioned in Chapter 4, a vacuum 

filtration process was applied to collect the particles on the filter membrane surface. 

The sucking of the vacuum system forces the particles to lay on a two dimensional 

plane. Then the SEM micrographs of the particles were taken for further image 

analysis. Based on the understanding of the particle collection process, it is assumed 

that the height of the particle on the filter surface is the smallest dimension of the 

particle. The following analysis of the particle shape will be made on a twodimensional analysis of the SEM micrographs of the particles. 

 As shown in Figure 10.3, the particle length (Lp) is defined as the longest 

length along its axis and the particle width (Wp) is the average dimension 

perpendicular to the particle axis. Examples of the particle length and width for 

different model particles are also shown in Figure 10.3. From this basis, the following 

shape parameters have been defined to quantify the degree of the elongation of a 

particle: 
176

Lp

Wp

Lp = Wp for a circle 

W Lp

p

Wp

Lp

Wp

Lp

Wp

Lp

Figure 10.3 Schematic of the definition of the particle length (Lp) and

the particle width (Wp) for different shapes of particles. 
177

(1) Aspect ratio (AR): The aspect ratio is the ratio of the particle length to the particle 

width. It is a measure of the elongation of a boundary profile. The larger aspect 

ratio, the longer is the particle. 

p

p

W

L

Aspect Ratio(AR) = (10.5)

(2) Roundness: The roundness is a measure of how closely a particle resembles a 

circle based on the area of the two-dimension particle image. The roundness is 

sensitive to the elongation of a boundary profile. The roundness is equal to unity 

for a circle and is smaller than unity for any other shape. The roundness can be 

calculated from equation (10.6): 

2

p

p

( )

4

L

A

Roundness

π⋅

⋅ = , (10.6)

where Ap is the area of the two-dimension particle image. 

(3) Form factor: The form factor measures the deviation of a particle from a circle. 

The baseline is the perimeter of a circle instead of the area of a circle defined in 

the roundness. The form factor is sensitive to both the ruggedness and elongation 

of a profile. The form factor is equal to unity for a circle and is less for any other 

shape. The form factor can be calculated from equation (10.7): 

2

p

( )

4

perimeter

A

Form factor ⋅π⋅ = . (10.7)
178

10.3 Osteolysis induced by UHMWPE particles 

Aseptic loosening has been recognized as a factor causing failure of total joint 

replacements since 1960. The basic science of bone loss surrounding the artificial 

joints has been investigated [78-87]. It is now generally accepted that the osteolysis is 

an outcome of an adverse cellular response that involved the phagocytosis of wear 

particles including UHMWPE debris, metal particles, bone cement particles and 

possible bone chips. Retrieved tissues, animal tests, and cell-culture studies have 

provided the approaches to understand the complexity of cellular interactions and 

chemical mediators involved in osteolysis. Major responsible cells in the process 

include macrophage, osteoblast, and osteoclast cells. The related chemical mediators 

that are responsible for the cellular responses and effects on bone resorption include 

tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), interleukin-6 (IL-6), and 

prostaglandin E2 (PGE-2). Figure 10.4 shows a schematic of possible mechanisms for 

particles-induced osteolysis. 

Studies of retrieved wear particles have provided information of size range and 

shape of particles according to the types of joint replacement, various material 

components, and material parts undergoing different sterilization processes. Some 

studies have suggested that particles of phagocytosable sizes stimulate higher 

bioactivity and that higher doses elicit more response (often up to a saturation level) 

[81]. Variation of particles-induced bioactivity is usually established by comparison of 

the amounts of released chemical mediators or calcium concentration. Different 

particle sizes have been compared based on the same particle numbers, particle 
179

volumes, particle-surface areas and their ratio to cell numbers [81, 86]. However, an 

indicator that links the degree of phaogyctosis with the particles-induced bioactivity is 

still lacking. A theoretical discussion and calculation of the physical aspects of the 

phagocytosis process is attempted in this chapter. Reviewing various results of cellculture studies validates the calculation of the phagocytosis process. 

UHMWPE 

Particles 

Macrophage 

Cells 

Osteoblast 

Cells

Osteoclast 

Cells Cytokines:

TNF-α 

IL-1 

IL-6 PGE2

Osteolysis 

Bone Resorption

Figure 10.4 Schematic of mechanism for particles-induced osteolysis. 
180

10.4 Phagocytosis of particles 

From a biophysical viewpoint of the phagocytosis of particles, the mechanisms 

of particle-induced bioactivity can be divided into three categories: 1) Phagocytosis of 

phagocytosable particles plays the most important role in the process. A quantitative 

calculation of the phagocytosis mechanism will be introduced to determine the 

dependence of the phagocytic capacity on particle size, shape and dosage factors. 2) In 

addition, the effects of excess phagocytosable particles that cannot be internalized due 

to insufficient amounts of macrophage cells should be discussed. 3) The third category 

involves the non-phagocytosable particles that are too large for macrophage cells to 

engulf. This leads to some formation of giant cells and is not considered in this study. 

Cells tend to attach to the particle surface and wrap around the particle. Some studies 

have compared various sizes and shapes of particles in this category and have shown 

stimulated inflammatory responses as well. A schematic illustration of the three 

different categories from the viewpoint of phagocytosis is presented in Figure 10.5. 

 In the following sections, the effects of internalized particles, effects of excess 

phagocytosable particles, and effects of non-phagocytosable particles will be 

discussed. Results of biological response of particles from the literature will be 

compared. 
181

Particles

Macrophage cells 

1. Internalized particles: 

Macropahge cells engulf the particles inside 

the cells 

2. Excess phagocytosable particles: 

Particles are small enough to be internalized, 

but insufficient cells are available ) 

3. Non-phagocytosable particles: 

Macrophage cell is too small to internalize 

large particles, they tend to attach to the 

particles) 

Figure 10.5 Schematic particles-induced bioactivity from viewpoint of 

phagocytosis. 
182

10.4.1 Criteria for particle internalization by macrophage cells 

Macrophage cells have adopted a variety of mechanism to internalize particles 

including pinocytosis and phagocytosis. Pinosytosis usually refers to the uptake of the 

smaller particles (usually < 0.5 μm) with fluid during the process. Although 

macrophage cells cannot digest polyethylene, particles are taken up with extracellular 

fluid through pinocytosis without stimulating active cell response such as key 

cytokines (TNF-α, IL-1, and IL-6) [86]. For larger particles, macrophage cells are 

activated according to the following phagocytosis process. Engulfment of a particle 

involves the following steps: adhesion between cell and particle, cell activation and 

projection of membrane-bound cytoplasm around the particle, and membrane fusion 

for enclosure [114, 115]. There exists a “phagocytic capacity” of a cell that will no 

longer take up additional particles. The number of particles a cell will engulf appears 

to depend on several aspects: cell type and morphometry, cell growth environment, 

chemical composition of the particle, surface property of the particle, and size and 

shape of the particle. The particle parameters cannot be changed independently to 

analyze the particle size, shape, dosage (number of particles, surface areas, volumes) 

effects individually. We proposed the level of activation expressed by macrophage 

cells depends on the degree of phagocytic capacity of that group of cells. From this 

concept, all particle parameters can be embedded in the calculation of phagocytic 

capacity and compared with the level of biological responses. 
183

10.4.2 Concept of phagocytic capacity 

The typical macrophage cells are spherical-like in shape and consist of 

numerous of different organells. They are nucleus, endoplasmic reticulum, 

centrosome, and centrioles, Golgi apparatus, cytoskeleton, cytosol, mitochondria, 

lysosomes, and vacuole. Cytoplasmic and intracellular membranes are significant for 

phagocytosis. The membrane surface areas that are available for phagocytosis have 

been measured by microscopic techniques. Summation of surface area of intracellular 

and plasma membranes has demonstrated that in phagocytosing macrophages the total 

area of these membranes remains constant [116, 117]. The intracellular membranes 

include the compartment of vacuoles that are empty spaces inside the cell and the 

compartment of phagosomes in which the particle is internalized. It is recognized that 

for a macropahge cell with initial volume V0, the total cytoplasmic membrane area S0

is available for phagocytosis. The assumption is that during the internalization process, 

the membrane of the phagocytic vacuole is closely attached to the particle surface and 

each particle is internalized by a different vacuole. For a particle with volume Vp, and 

surface area Sp, the maximum number of particles can be phagocytosed is denoted as 

n. After the phagocytosis process, the remaining cytoplasmic membrane area is S, and 

the new cell volume is V: 

V =V0 + nVp (10.8) 

S = S0 − nSp (10.9)

Assuming that in the limit of complete plasma membrane engulfment the cell is a 

sphere with radius r, so that V = (4/3)πr

3

 and S = 4πr

2

, n has been found as the thirdorder polynomial [118]: 
184

2 1 0 0 23

3

n a + n a + na + a = , (10.10)

with coefficients 

3

0

2

0 0 a = 36πV − S , 

p

2

a1 = 72πV0Vp + 3S0 S , 

2

0 p

2

a2 = 36πVp −3S S , 

3

a3 = Sp . 

A measurement of the total membrane surface area showed that the membranes 

have 300 % to 350 % of the surface area of a sphere with a volume V0. It represents a 

maximum amount of membrane area that could be used for phagocytosis of particles. 

During the phagocytosis process, vacuolar membranes tend to shift for phagocytosis. 

However, several studies have indicated that around 35 % to 55 % of membrane areas 

remained in the vacuoles compartments that cannot be used for phagocytosis purpose. 

Bos and Souza [116] have reported a measurement of volume and surface area of 

macrophage cells and its vacuoles and phagosomes compartments. For a macrophage 

cell before phagocytosis process, the volume was 388 μm3

. The total membrane 

surface area measured was 1231 μm2

. Assuming the maximum percentage of total 

membrane surface area used for phagocytosis as 65 %, we can obtain that the 

maximum diameter of the sphere that can be engulfed is 9 μm. The membrane area 

that is available for phagocytosis S0 = 800 μm2

. We also estimate that if the minimum 

percentage of total membrane surface area used for phagocytosis is 45 %, the available 

surface area available for phagocytosis is S0 = 554 μm2

. 
185

10.4.3 Critical threshold for phagocytosis of particles with different shapes 

For a population of spherical particles with the same size, the range of 

phagocytic capacity as the number of internalized particles can be determined. The 

largest spherical particle that can be internalized has a diameter of (8.5 to 10) μm. We 

also calculated the phagocytic capacity for different sizes of spherical particles. The 

phagocytic capacity versus diameter of spherical particles is plotted in Figure 10.6. 

Figure 10.6 Phagocytic capacity of spherical particles. 

Diameter of Spherical Particle (μm)

0 1 2 3 4 5 6 7 8 9 10 11

Phagocytic Capacity (# of particles/cell)

1

2

5

10

20

50

100

200

500

65% membrane used for phagocytosis

45% membrane used for phagocytosis
186

To discuss the phagocytic capacity of elongated particles, we assume that the particle 

has an ellipsoidal shape with longer axis L and two equal shorter axes W. The aspect 

ratio of the particle (AR) is defined as L/W. The projected surface area can be 

calculated from equation 10.4. By using the same set of equations, we can determine 

the phagocytic capacity for elongated particles. Based on the assumption that 45 % 

membrane area is used for phagocytosis, phagocytic capacity versus particle volume 

for particles with different aspect ratios are plotted in Figure 10.7. The figure indicates 

that for the same volume of particle, the phagocytic capacity decreases with increasing 

aspect ratio of the particle. Obviously, with the same particle volume, a sphere has the 

minimum surface area and the largest phagocytic capacity. 

Particle Volume (μm3)

0.01 0.1 1 10 100 1000

Phagocytic Capacity (# of particles/cell)

1

2

5

10

20

50

100

200

500 AR = 1 (Spherical)

AR=3

AR=6

Increasing

Aspect Ratio of Particle

Figure 10.7 Phagocytic capacity of particles with different aspect ratios. 

Larger 

pahgocytic 

capacity 

Smaller 

pahgocytic 

capacity
187

10.5 Validation against literature results 

To investigate the relationship between phagocytic capacity and related 

biological response, two cell-culture studies of polyethylene particles are reviewed and 

analyzed. Because no elongated particles have been used for biological response tests, 

all the particles mentioned below are spherical shaped. 

Catelas et al. [79] studied different kinds of particles in the phagocytosable 

range, including HDPE powders with a mean diameter of 4.5 μm. The J774 

macrophage cell line (ATCC) was used in this vitro model because it has been 

demonstrated to be morphologically similar to macrophages at the bone-cement 

interface and to respond to particles in a similar way as human macrophages at this 

interface. Macrophages were cultured in a tube that was placed on a rotator during the 

incubation to facilitate constant contact between cells and particles. Dosage baseline 

used in this study is the ratio of number of particles to the number of macrophage 

cells. The TNF-α released after 24 h was measured and the results for HDPE particles 

is shown in Table 10.1. The phagocytic capacity of a spherical particle with 4.5 μm in 

diameter ranges from 3 (45 % membrane for phagocytosis) to 6 (65 % membrane for 

phagocytosis) based on the calculation algorithm mentioned earlier. The ratio of the 

particle concentration (no. of particles/no. of cells) to the phagocytic capacity was 

calculated. This ratio is less than unity when there are sufficient cells to phagocytose 

particles. This ratio indicates the level of cell phagocytic capacity is used. This ratio is 

larger than unity when the cells are not able to internalize all the particles and there 
188

exist excess phagocytosable particles in the system. The TNF-α specific activity 

versus index of particle internalization is plotted in Figure 10.8 

Table 10.1 Summary of in-vitro cell culture results from Catelas et al. [79] 

HDPE particles 4.5 μm Particle concentration (/cell) 

5 30 80 

TNF-α specific activity 1.6±0.2 2.6±0.18 2.75±0.5 

[Ratio of (# Particles/Cell) to (Phagocytic Capacity)]

0.0 0.5 1.0 5.0 10.0 15.0

TNF-α Specific Activity

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Figure 10.8 TNF-α vs. index of particle internalization from Catelas et al. [79]. 
189

 It is seen from Figure 10.8 that the TNF-α specific activity is smaller when the 

ratio of the particle concentration (no. of particles/no. of cells) to the phagocytic 

capacity is smaller than unity. It means the cells have not activated fully to the 

stimulated particles. When the number of particles is increased, the TNF-α specific 

activity increases as well. However, after the cells reach their phagocytic capacity, the 

TNF-α specific activity remained constant in this study. It indicates that the ability of 

cells to release TNF-α is fully activated when saturation of phagocytic capacity is 

reached and the excess particles in this case seem not to have much impact on the 

TNF-α release. The size effect of the particles was not investigated in this study. 

 Green et al. [81] took Ceridust 3615 polyethylene powders and categorized 

them into the different size ranges by filtration. They obtained spherical-like particles 

with mean particle diameters±standard deviations of (7.2 ± 3.15) μm, (4.3 ± 1.89) μm, 

(0.49 ± 0.11) μm, and (0.21 ± 0.069) μm. The macrophage cells were isolated from 

C3H/Hej mice. They fixed polyethylene particles in an agarose solution, following 

centrifugation to put particles in the superficial layer. The macrophage cells then 

cultured above the layer in order to make the contact between particles and cells. They 

controlled the particle concentration in a form of particle volume (μm3

) to macropahge 

cell number ratios of 100:1, 10:1, 1:1, and 0.1:1. Several cytokines were measured 

after the experiment. They found that the particle concentrations of 1:1 and 0.1:1 did 

not stimulate cells to release cytokines. Table 10.2 lists the results of IL-1β and TNFα release with different particle concentrations. 
190

 We further converted the particle concentration to the ratio of number of 

particles to number of macrophage cells. The ratio of the particle concentration (no. of 

particles/no. of cells) to the phagocytic capacity is again calculated for the particles 

with different size and concentrations. The IL-1β and TNF-α specific activities versus 

the ratio of the particle concentration (no. of particles/no. of cells) to the phagocytic 

capacity are plotted in Figures 10.9 and 10.10, respectively. It is found that the 

cytokine specific activity increases with increasing phagocytic capacity. When the 

phagocytic capacity is larger than unity, cytokine specific activity reaches a saturated 

value and no longer increases within the experimental conditions. When the ratio is 

between 0 and 1, the bioactivity (cytokine release) increases with increase ratio of the 

particle concentration (no. of particles/no. of cells) to the phagocytic capacity. 

Although the particle concentration is rather high (high index of particle 

internalization), the cytokine release remains low compared to the control experiment. 

It is thought that this may be that the particles are so tiny so that pinocytosis takes 

place to uptake the smaller particles with fluid during the process, and therefore, a 

severe biological response may not happen under this condition. 

Table 10.2 Summary of in-vitro cell culture results from Green et al. [81] 

IL-1β specific activity TNF-α specific activity 

Particle concentration 

(particle volume: no. of cells) 

Particle concentration 

(particle volume: no. of cells) Particle size 

10:1 100:1 10:1 100:1 

0.21 μm 0.2 ± 0.15 0.3 ± 0.1 2 ± 3 8 ± 8.2 

0.49 μm 0.6 ± 0.1 1.8 ± 0.2 38 ± 1 33 ± 5 

4.3 μm 0.5 ± 0.15 1.75 ± 0.25 15 ± 10 35 ± 2 

7.2 μm 0.1 ± 0.09 1.1 ± 0.3 1 ± 0.8 16 ± 10 
191

Figure 10.9 IL-1β vs. index of particle internalization from Green et al. [81]. 

[Ratio of (# Particles/Cell) to (Phagocytic Capacity)]

0.0 0.5 1.0 3.0 4.0 5.0

IL-1

β Specific Activity

0.0

0.5

1.0

1.5

2.0

2.5
192

 

Figure 10.10 TNF-α vs. Index of particle internalization from Green et al. [81]. 

[Ratio of (# Particles/Cell) to (Phagocytic Capacity)]

0.0 0.5 1.0 3.0 4.0 5.0

TNF-α Specific Activity

0

5

10

15

20

25

30

35

40
193

10.6 Effect of excess particles 

The case studies indicated that when ratio of the particle concentration to the 

phagocytic capacity is larger than unity, the bioactivity reaches a saturation state 

without further increase. It means the cells are fully activated and there is no capacity 

left for further phagocytosis of particles. The two studies that are referenced [79, 81] 

were conducted in cell-culture studies with the condition of the cell cultures fixed after 

the experiment has begun. Compared to the dynamic particles-cells interaction in the 

joint area, it is a static process. However, in the joint area, the dynamic interaction 

includes the generation rate of the particles and the growth period of related 

immunological cells. It has been observed in animal studies that after the macrophage 

cells internalize the particles, more macrophage cells appear from the surrounding 

tissues for further phagocytosis. Thus whether the phagocytic capacity is saturated or 

not is no longer only a issue related to particle size, number and cell numbers, but also 

a topic involving the wear-particle generation rate and the growth periods of 

macrophage cells. Figure 10.11 shows a schematic of the dynamic process of this 

concept. Further efforts should be made to control the cell and particle-generation rate 

to study these dynamic effects. 
194

Particle generation 

rate 

Rparticle

Phagocytic Capacity 

(No. of particles a macrophage can engulf)

Nparticle

Residential time for Phagocytosis 

Tphagocytosis

Macrophage cells Concentration 

Cmacrophage

Particle Accumulation Rate

Raccumulation = Rparticle – Rphagocytosis 

Particle Digestion Rate 

 Nparticle• Cmacrophage

Tphagocytosis 

Rphagocytosis = 

Figure 10.11 Schematic of dynamic phagocytosis model. 
195

10.7 Conclusions 

Methods to describe the size and shape of particles are reviewed first in this 

chapter. However, according to the positive and negative curvature of the sheet-like 

UHMWPE particles generated, there exists difficulty to achieve precisely 

measurement of the dimensions and aspect ratios of the particles. Accordingly, a 

projected area concept was proposed that reflects the size of the particle based on the 

physical aspect of phagocytosis of particle mechanism. We have analyzed the 

phagocytosis of particles from a biophysical viewpoint. The phagocytic capacity of a 

particle with a certain size and shape can be determined. The ratio of the particle 

concentration (no. of particles/no. of cells) to the phagocytic capacity is a critical value 

for the bioactivity induced by a certain particle population. This ratio integrates the 

effects from particle size, shape and dosage. The literature results of bioactivity tests 

of spherical particles have been reviewed. When the ratio is smaller than unity, 

bioactivity is proportional to the value of the ratio. When the ratio is larger than unity, 

limited change of biological response was observed. 

Elongated UHMWPE particles that are seen clinically have not been used in 

bioactivity tests due to the unavailability of the elongated particles. In the previously 

chapters, we have successfully generated elongated particles with different aspect 

ratios. We shall apply these elongated particles in the bioactivity test to investigate the 

shape effect. We also can further test the mechanism proposed in this chapter. 
196

CHAPTER 11 

BIOACTIVITY TESTS OF UHMWPE PARTICLES

11.1 Introduction 

In this chapter, the results of collaborative research with Wayne State 

University on bioactivity tests of UHMPWE particles is presented1

. They investigated 

the effects of particle size and shape on biological response, and we analyzed the 

combination of particle size, shape and dosage effects on bioactivity. 

In-vitro cell culture tests and in-vivo animal tests have been widely used to 

evaluate biomaterials induced immunological responses [78-87]. However, in-vitro 

cell-culture studies may not be able to mimic the in-vivo environment because of the 

lack of human biochemical complexity. Some researchers applied in-vivo animal 

studies by injecting the wear particles into the joint areas. However, quantitative 

analysis became impossible because of lack of control of the experimental parameters. 

The Rodent air pouch has been identified as a useful model for the evaluation of the 

response to orthopedic biomaterials, providing inflammation responses resembling the 

pseudo-synovium associated with aseptic loosening [122]. We adopted the murine air 

pouch model here to evaluate the cellular response to UHMWPE particles in vivo. 

_____________________________________________________________________ 

1

Acknowledge Professor Paul Wooley, Dr. Shang-You Yang and Dr. Weiping Ren at 

Wayne State University for conducting murine air pouch animal tests. 
197

11.2 Murine air pouch animal test 

Air pouches were generated in groups of 10-15 female BALA/c mice. An area 

of the dorsal skin (2 cm2

) was cleaned with alcohol and shaved to provide the pouch 

site. Three milliliters of air was injected at a single site with 25-gauge needle and 3 ml 

syringe. The air pouches were injected with 1 ml of air on alternate days for 5 days to 

establish a definitive fluid filled pouch [123]. On day 6, the established air pouches 

were injected with 500 μL sterilized phosphate buffer solution (PBS) containing 10 % 

fetal calf serum (FCS) and controlled amounts of UHMWPE particles. Particle-less 

control pouches were injected with 500 μL of 10 % FCS/PBS alone. The mice were 

sacrificed at defined intervals subsequent to particle introduction. The pouches were 

lavaged with 500 μL of PBS (collected as lavage fluid and rapidly frozen at -80°C for 

later evaluation) before the surgical removal of the pouch. One half of each pouch was 

fixed in 10 % buffered formalin for histology processes, while the remainder was 

stored at -80°C for molecular and immunological analyses. Figure 11.1 shows a 

schematic of the processes of the murine air pouch tests for particles.
198

Air 

Particles 

Particles 

Figure 11.1 Schematic of murine air-pouch model for bioactivity 

tests of UHMWPE particles. 
199

11.3 Evaluation of biological responses 

11.3.1 Histology analysis2

Formalin-fixed pouches were embedded in paraffin, sectioned at 5 μm with a 

consistent orientation. The sections were analyzed under a Zeiss light microscope by a 

computerized image analysis system with the software. For pouch membrane 

thickness, eight measurements from different areas of each pouch were averaged. A 

uniform 100 μm longitudinal pouch area was selected to count total cells (H&E 

stained) and macrophages (Esterase positive stained) [123], respectively, and average 

of three different areas per pouch was recorded. 

 Figure 11.2 shows a typical micrograph of the tissue reaction in an air-pouch 

membrane injected with UHMWPE particles, with particle phagocytosis by 

macrophage cells. Figure 11.3 illustrates typical macroscopic appearances of air 

pouches dissected from mice. Figure 11.4 shows the different level of immunological 

responses by quantification of the pouch thickness.

____________________________________________________________________ 

2

Histology analyses of the murine air-pouch membranes were done by the research 

staffs in the department of orthopedics at Wayne state university. 
200

Figure 11.3 Photographs of murine air pouches; (a) without particle; (b) 

reacted with round-like particles; (c) reacted with elongated 

particles.

Figure 11.2 Micrographs of phagocytosis of UHMWPE particles by

macrophage cells. 

Macropahge cells UHMWPE particles 
201

Control (no particles) 

With UHMWPE Particles 

Air pouch thickness 

Figure 11.4 Micrographs of sectioned air pouches. 
202

11.3.2 Measurement of cytokines3

Enzyme Linked Immunosorbent Assay (ELISA) was performed to examine 

cytokine production at the protein level using capture and detection monoclonal 

antibody pairs against various cytokine molecules. 50 μL of sample (pouch lavage 

fluid, supernatant of pouch homogenates, or cell culture media) was added to each 

well of a 96-well plate pre-coated with capturing antibody. The plate was incubated 

overnight at 4 °C and biotinylated detection antibody was added for 90 min at 37 °C. 

Alkaline phosphatase-conjugated streptavidin (1:500) was introduced to each well and 

incubated for 45 min at room temperature. Extensive washes with PBS were applied 

following each incubation step. The optical density of each sample was measured by a 

microplate spectrophotometer (Molecular Devices, Menlo Park, CA) at 405 nm 

wavelength, after incubating the plate with paranitrophenyl phosphate in 

diethylanolamine buffer for 15 min to 30 min in dark. The level of cytokines was 

calculated by regression analysis from a standard curve included with each assay. 

The expression of inflammatory cytokines in the air pouches was also 

quantified by a real-time PCR technique. Detailed procedures have been described in 

the literature [124]. 

____________________________________________________________________ 

3

Cytokine was measured by Dr. Shang-You Yang at Wayne state University. 
203

11.4 Effect of particle shape 

UHMWPE particle populations with different aspect ratios of particles were 

tested in the murine air pouch animal tests. Particle sizes were chosen so that the 

average surface areas of the particles are similar for two particle populations. Particle 

group (A) contained round-like particles with mean particle length (22.8±3.0) μm, 

particle width (17.5±2.5) μm, and aspect ratio of (1.3±0.2). Particle group (B) 

contained much elongated particles with mean particle length (26.9±1.1) μm, particle 

width (9.9±0.4) μm, and aspect ratio of (3.0±0.1). An average of 2.4x106

 particles 

(4.8x106

 particles/ml) were contained in the suspension for injection to the air pouch 

for both particle groups. 

Both populations of particles induced tissue inflammatory responses, including 

increases cellular infiltration and membrane proliferation. Differences were observed 

between the responses to the different particle populations, with elongated particles 

provoking more severe inflammatory responses. Figure 11.3 illustrates typical 

macroscopic appearances of air pouches dissected from mice. Pouches with elongated 

particles (Figure 11.3c) had pronounced more changes compared with other two 

groups. 

Image analysis of histological sections revealed that the pouch membranes 

stimulated with elongated particles (Group B) for 7 days averaged (0.223±0.021) mm 

in thickness, significantly thicker than pouches with round-like particles (Group A) 

((0.144±0.005) mm) and non-particle control pouches ((0.071±0.01) mm) as shown in 

Figure 11.5. 
204

Time (days)

2 days 7 days

Pouch Thickness (mm)

0.00

0.05

0.10

0.15

0.20

0.25 Control (no particle)

Round-like particles (Group A)

Elongated particles (Group B)

Figure 11.5 Comparison of pouch thickness for the murine air pouch tests with

round-like (Group A) and elongated (Group B) UHMWPE particles.
205

Cytokine measurements by PCR technique revealed that the UHMWPE 

particles significantly provoked expression of proinflammatory cytokines IL-1 and 

TNF at day 2 and day 7 in the air pouch membranes compared to non-particle controls 

as shown in Figure 11.6. However, mRNA from pouches with elongated-particles 

contained markedly more gene copies of IL-1 and TNF shown in Figure 11.6a-b, 

indicating the elongated-particles’ higher inflammatory-inducing nature. ELISA assay 

further confirmed that higher level of IL-1ß and TNFα were present in pouch fluid 

lavaged from elongated-particle treated air pouches in Figure 11.6c-d. 

In another experiment, two particle populations with smaller particle size were 

applied to compare the particles with different aspect ratios. Particle group (C) 

contains spherical particles with mean particle diameter (3.6±2.0) μm, and aspect ratio 

of (1.0±0.3). Particle group (D) contains much elongated particles with mean particle 

length (4.2±0.8) μm, particle width (2.6±0.5) μm, and aspect ratio of (1.6±0.3). An 

average of 0.7x106 particles (1.4x106 particles/ml) were contained in the suspension 

for injection to the air pouch for both particle groups. Image analysis of histological 

sections revealed that the pouch membranes stimulated with elongated-particles 

(Group D) for 7 d averaged (0.215±0.012) mm in thickness, significantly thicker than 

pouches with spherical particles (Group C) ((0.138±0.020) mm), and non-particle 

control pouches ((0.102±0.01) mm). 

Both results indicate that the stronger immunological responses were 

stimulated by the elongated particles than by the round-like or spherical particles 

while the total surface areas of particles are maintained similar. 
206

Time (days)

2 days 7 days

IL-1 gene copies expressed

0.0

5.0e+6

1.0e+7

1.5e+7

2.0e+7

2.5e+7

Control (no particle)

Round-like particles (Group A)

Elongated particles (Group B)

A

Time (days)

2 days 7 days

TNF gene copies expressed

0

2000

4000

6000

8000

B 10000

Pouch lavage homogenates

IL-1ß pg/mg protein (ELISA)

0

20

40

60

80

100

120

140

Pouch lavage homogenates

TNFα pg/mg protein (ELISA)

0

20

40

60

80

100

C D

Figure 11.6 IL-1β and TNF-α cytokine expression in murine air pouches with 

UHMWPE particles by real time PCR (plots A and B) and by 

ELISA (plots C and D) techniques. 
207

11.5 Effect of particle size 

Two populations of elongated UHMWPE particles (Aspect ratio > 1.5) were 

compared in the air pouch animal tests. Particle group (D) contained smaller particles 

with mean particle length (4.2±0.8) μm, particle width (2.6±0.5) μm, and aspect ratio 

of (1.6±0.3). Particle group (E) contained larger particles with mean particle length 

(43.1±7.5) μm, particle width (12.7±2.3) μm, and aspect ratio of (3.4±0.9). An 

average of 0.7x106

 particles (1.4x106 particles/mL) were contained in the suspension 

for injection to the air pouch for both particle groups. The smaller particle group 

(group D) represented the particles in phagocytosable range for macrophage cells, and 

the particle size for larger particle group (group E) is too large to be internalized by 

macrophage cells. 

Results of histological analysis shown in Figure 11.7 revealed that the pouch 

membranes stimulated with smaller particles (Group D) for 7 d averaged 

(0.215±0.012) mm in thickness, significantly thicker than pouches with larger particles 

(Group E) ((0.178±0.013) mm). It indicates that smaller particles in the 

phagocytosable range provoke higher response of macrophages. Green et al. [81] 

arrived at the same conclusions for spherical UHMWPE particles. 
208

11.6 Effect of particle dosage 

Particle group D (elongated particles within phagocytosable range) mentioned 

in the previous sections were used in the air pouch tests. An average of 0.7x106

particles and 1.5x106

 particles were contained in the suspension for injection to the air 

pouch to investigate the particle number effect on macrophage responses. 

Figure 11.8 shows the results of histological analysis. The pouch membrane 

stimulated with 1.5x106

 particles (3.0x106 particles/ml) for 7 d averaged 

(0.345±0.040) mm in thickness, significantly thicker than pouches with 0.7x106

particles (1.4x106

 particles/mL) (pouch thickness = (0.215±0.012) mm). It indicates 

that the large number of particles induce a stronger immunological responses by the 

macrophage activation. 

Particle length (μm)

4.2 43.1

Pouch Thickness (mm)

0.00

0.05

0.10

0.15

0.20

0.25

(Group D) (Group E)

Figure 11.7 Comparison of pouch thickness for the murine air pouch tests with 

small (Group D) and larger (Group E) elongated UHMWPE 

particles. 
209

11.7 Combined effects from size, shape and dosage of particles 

The size of a particle can be expressed by the volume or the surface area of a 

particle as discussed in Chapter 10. The aspect ratio is used to characterize the shape 

of a particle in terms of the elongation of a particle. However, the size and shape of the 

particles are not independent variables. It is not possible to keep all other variables for 

particle size and shape the same while one variable is altered. For example, the change 

of the aspect ratio of a particle also leads to the change of the surface area of the 

particle when the particle volume is kept the same. 

Number of particles

700000 1500000

Pouch Thickness (mm)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Figure 11.8 Comparison of pouch thickness for the murine air pouch tests with 

different number of particles (Group D, elongated particles within 

phagocytosable range) 
210

In the previous sections 11.4-11.6 we mentioned the effects of particle size, 

shape and dosages effect on bioactivities. To further investigate the combined effects 

from particle size, shape and dosages, we shall compare the results of the bioactivity 

tests with candidate particle variables. Bioactivity results are plotted versus various 

particle descriptors that were introduced in Chapter 10. The purpose is to investigate 

the dominating factors leading to the difference of bioactivities induced by various 

UHMWPE particle populations. It is understood that the cytokine measurement of the 

air pouch experiments is sensitive to the measurement conditions and environments. 

There may exist more system errors of measurements from different batches. In this 

section, we integrate the bioactivity results from all the experiments. To reduce the 

system errors, the pouch membrane thickness is adopted as an indicator for the 

immunological responses. 

From the viewpoint of chemical reaction, the total surface area of the particles 

is proportional to the total reaction sites available in the interaction processes between 

particles and macrophage cells. A plot of pouch membrane thickness versus the total 

surface area of the particles is shown in Figure 11.9. From the viewpoint of the 

digestion process by the macrophage cells, the volume of the particle is a candidate 

variable to affect the immunological responses. Thus Figure 11.10 shows a plot of the 

air pouch thickness versus the total volume of the particles applied in the murine air 

pouch experiments. 

 
211

Total surface area of particles (μm2)

107 108 109 1010

 Pouch thichness (

μm) 

0

100

200

300

400

500

Phagocytosable particles (Lp <10 μm)

Non-phagocytosable particles (Lp >10 μm)

Figure 11.9 Plot of pouch thickness versus total surface area of particles. 

Total volume of particles (μm3)

106 107 108 109 1010

 Pouch thichness (

μm) 

0

100

200

300

400

500

Phagocytosable particles (Lp <10 μm)

Non-phagocytosable particles (Lp >10 μm)

Figure 11.10 Plot of pouch thickness versus total volume of particles. 
212

From Figures 11.9-11.10, it is seen that for the smaller particles in the 

phagocytosable range (particle length smaller than 10 μm), the pouch thickness 

increases with increasing total surface area or volume of the particles. However, the 

pouch thickness decreases for the non-phagocytosable particles (particle length larger 

than 10 μm) even though the total surface area or volume is larger than the total 

surface area or volume of the phagocytosable particles. The pouch thickness induced 

by the non-phagocytosable particles with different total surface are or volume of 

particles did not show the significantly difference from our experimental results. 

Results suggest that for the smaller particles in the phagocytosable range for 

macrophage cells, the immunological response (measured by pouch thickness) is 

stronger with larger total surface area or volume of the particles. Whether the surface 

area or the volume of the particle is the dominating factors cannot be clearly defined 

from these experimental results. When the particle is too large to be engulfed by the 

macrophage cells, the particle-macrophage interactions results in a weaker 

immunological response. It was observed that the macrophage cells tend to attach to 

the surface when the cell is not capable of engulfing the particle. Obviously the 

mechanisms of particle-macrophage interaction vary according to the critical particle 

size for phagocytosability of macrophage cells. This result suggests that the 

phagocytosis of particles induces a stronger bioactivity. The increase of the biological 

response is related to the quantity of total surface area or the volume of the particles 

applied to the murine air pouch tests. 
213

To further discuss the effect of particle shape on bioactivity, the pouch 

thickness was normalized by the total surface area or the total volume of the particles. 

The purpose is to focus on the particle-shape effect on the mechanisms of induced 

immunological response without the interference of the particle-dosage effects (total 

surface area and total volume of particles from the variance of the particle shape. 

Figure 11.11 are the plots of the normalized pouch thickness versus the aspect ratio of 

the particles. The results indicate that the phagocytosable particles induced stronger 

biological response also. It is also shown that the larger aspect ratio of particles 

stimulated stronger imuunological responses and resulted in a thicker pouch 

membrane from the plots. This applies to both phagocytosable and nonphagocytosable UHMWPE particles. 

We think elongated particles have sharper edges. When the sharper edges get 

in contact with macrophage cells in the biological environments, higher mechanical 

stresses are applied on the cells by the particle edges. The higher mechanical stress 

might cause the damage of the membrane of the cells or further leads to the apotosis of 

the macrophage cells. The damage or death of the defense cells (macropahge cells) 

results in gathering more macrophage cells to fight the foreign materials (UHMWPE 

particles) and case a thicker pouch membrane. It also means a stronger immunological 

response is induced. 
214

Aspect ratio of particle

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

 Pouch thichness (

μm) 

Total volume of particles (

μ

m3

)

0.0

1.0e-7

2.0e-7

1.0e-5

2.0e-5

3.0e-5

Phagocytosable Particles

Non-Phagocytosable Particles

Figure 11.11 Plots of normalized pouch thickness versus aspect ratio of particles;

(a) pouch thickness normalized by total surface area of particles; (b)

pouch thickness normalized by total volume of particles. 

(a) 

(b) 

Aspect ratio of particle

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

 Pouch thichness (

μm) 

Total surface area of particles (

μ

m2

) 1.0e-7

2.0e-7

3.0e-7

5.0e-6

1.0e-5

1.5e-5

2.0e-5

Phagocytosable Particles

Non-Phagocytosable Particles
215

11.8 Conclusion 

No one has investigated the effect of elongated UHMWPE particles that are 

more clinically relevant on bioactivity (interaction between macrophage cells and 

elongated glass fibers has been studied [125]). The effect of particle shape on 

bioactivity was first investigated by the murine air pouch experiments of UHMWPE 

particles in this collaborative study. 

The results indicate that elongated particles induced stronger immunological 

responses while the particle volumes remain the same for particle populations. The 

results of particle-size effects show that the particles in the phagocytosable range 

induce larger macrophage activation than the larger particles do. These results from 

elongated particles are consistent with the conclusion we made in Chapter 10. 
216

CHAPTER 12 

SUMMARY AND CONCLUSIONS

The objective of this study was to investigate the effect of UHMWPE particles 

on bioactivity. The critical questions addressed in Chapter 3 include: 

• Are all particles bioactive? 

• What is the effect of particle size? 

• What is the effect of particle shape? 

• What is the critical threshold concentration of a particle population 

with certain size and shape? 

The following approaches were attempted: (1) Design of surface textures to 

generate narrowly distributed UHMWPE particles with controlled sizes and shapes; 

(2) Modeling of the particle-generation process and prediction of the generated 

UHMWPE particles; (3) Bioactivity tests of UHMWPE particles. Through these 

studies, we are able to provide most answers of the above questions. In the following 

sections, we will make conclusions from these approaches. 

12.1 Design of surface textures for particle generation 

Microfabrication techniques have been successfully applied to prepare the 

surface textures with controlled dimensions. A specific process has been developed to 

fabricate wedge features with defined dimensions on the silicon material. A narrowly 
217

distributed particle population was achieved when the uniformity of the surface 

textures was improved. The size of the generated UHMWPE particles covers the nonphagocytosable and phagocytosable particle populations from the viewpoint of 

phagocytosis of particles. 

Conclusions of design of surface-textures are made as follows: 

(1) Particle length is ≈0.8 of the cutting-edge length. 

(2) A minimum ratio of pitch distance to cutting-edge height (Ds / Hc) of 14 is 

required for a clean micro-cutting process for UHMWPE particle 

generation. 

(3) A larger normal load leads to a smaller aspect ratio of the particle. 

(4) A larger cutting-edge height (under sufficient normal load to push the 

UHMWPE pin to the surface) leads to a smaller aspect ratio of the particle. 

(5) A higher sliding speed results in a larger aspect ratio of the particle. 

(6) We have successfully generated narrowly distributed UHMWPE particles 

in the range of 3 μm to 43 μm. 

12.2 Modeling of UHMWPE particle generation 

Based on the observation of worn surfaces and wear particles of UHMWPE, a 

mechanism of the particle-generation process was proposed. It includes the following 

steps: (1) penetration of the cutting edge into the material, (2) accumulation of the 

deformed UHMWPW material in front of the cutting edge during the sliding process, 

(3) Lift-up of the cutting edge by the material resistant force, and (4) Formation of the 
218

particle. Model experiments were performed to investigate the movement of the 

cutting edge during the particle-generation process. The viscoelasticity, molecular 

orientation, and strain hardening of UHMWPE have also been taken into 

consideration. Following conclusions are made: 

(1) In a nano-indentation test of UHMWPE, a 37.3 % of the maximum 

penetration depth was recovered due to the viscoelasticity. Similar results 

were also obtained from a wedge-tip indentation test. 

(2) Increase of material resistant force during the sliding process comes from 

strain hardening of UHMWPE and cumulative volume in front of the 

cutting face. 

(3) Shear stress induces molecular orientation on the UHMWPE surface. 

(4) Elastic modulus of UHMWPE surface is higher than the bulk material. It 

also means the surface layer of UHMWPE is harder than the bulk material. 

(5) A smaller cutting-edge angle (a sharper edge) leads to a longer sliding 

distance of the tip. It is due to less material resistant force is distributed to 

the vertical direction to lift up the tip. 

A mathematical model to predict the dimensions of generated UHMWPE 

particles was established. The calculation of penetration and sliding processes of the 

cutting edge was carried out by contact-mechanics principles. A correlation model 

based on the empirical results was also formulated. Conclusions are made as follows: 
219

(1) Initial penetration depth of the cutting edge can be calculated by 

2 ( ) tanθ v c

p ⋅ ⋅ ⋅ ⋅ = C Y L

W H , where θ)] 2

π tan( 3π4 [1 ln( 31

ep = + − Y

E C , and 

v 75 ep C =1. ⋅C . It means the ratio of penetration depth predicted from the 

elastic-plastic assumption to the measured penetration depth is 1.75. 

(2) Iterative computing of the force balance during the sliding process can 

determine the displaced volume of UHMWPE during the sliding process. 

This displaced volume is projected to the volume of the particle generated. 

(3) Correlation model has been established from the empirical results: 

0.6

2 0.6

c p

p 39.5

V

L H W

V ⋅ ⋅ = (unit: μm, N, s) 

12.3 Bioactivity of UHMWPE particles 

Murine air pouch animal tests have been used to test the biological responses 

induced by UHMWPE particles. Elongated particles that are commonly observed 

clinically were evaluated for the first time. Following conclusions are made from 

bioactivity studies of UHMWPE particles: 

(1) Smaller particles within the phagocytosable range induce stronger 

bioactivity than large particles that cannot be engulfed by the macrophage 

cells. 

(2) Elongated particles induce larger immunological responses. 
220

(3) The number of the particles with specified size and shape a macropahge 

cell can engulf (phagocytic capacity) can be estimated from a biophysical 

approach of the phagocytosis process. 

(4) For phagocytosbale particles, when the number of the particles is less than 

phagocytic capacity, the immunological response is higher with increasing 

number of particles. 

(5) When the number of particles exceeds the phagocytic capacity, the excess 

particles can be treated as non-phagocytosable particles. Thus the induced 

bioactivity remains about the same. 

 
221

CHAPTER 13 

FUTURE WORK AND RECOMMENDATIONS

13.1 Improvements of current techniques of particle generation 

There is lack of sub-micrometer particles in this study. We need to further 

generate particles in this range for bioactivity tests. We have demonstrated the 

feasibility of generating UHMWPE wear particles with different shapes by surfacetexturing techniques. The main issue is the size of the particles. Since sub-micrometer 

round particles and micron-sized elongated particles were observed from clinical 

retrieval studies, the question is whether this technique is capable of producing 

different sizes and shapes in the sub-micrometer range. The current microfabrication 

technique is capable of producing features with a 0.1 μm resolution; hence, 

theoretically there is no barrier to produce particles down to 0.1 μm size range. We 

have attempted to produce particles in the sub-micrometer range but the rate of 

producing such particles is extremely low and the particle-size distribution is much 

wider than expected. We attribute this result to our lack of understanding of the 

viscoelastic response of UHMWPE. Theoretically, there exists a minimum 

penetration depth the cutting edge should reach in order to induce an effective microcutting process. More efforts are needed to identify the critical parameters to produce 

sub-micrometer particles with different shapes. 
222

The current technique produces particles with precisely controlled size and 

shape, but the particle generation rate is low (≈(0.8 to 1) mg/d). Efforts to increase the 

particle-generation rate are needed. The strategy may include changing different 

contact geometries, speeds, and loads. However, the trade-off might be the narrowness 

of the particle-size distribution. 

13.2 Mechanism study for osteolysis induced by particles 

More bioactivity tests for UHMWPE particles with various sizes and shapes 

are needed, especially for the particles in the submicron ranges. For a specific size and 

shape of the particle population, the experiments should be further carried out with 

controlled total surface areas, total volumes, and number of the particle populations 

independently. Then the boundaries of the biological responses induced by different 

mechanisms can be further identified. 

More efforts should be focused on the biochemical pathway of osteolysis 

induced by UHMWPE particles and the gene expression of the cells during the 

process. The availability of UHMWPE particle with various sizes and shapes has 

opened an entrance to study the mechanism. Once the detailed mechanism of 

osteolysis induced by particles is discovered, the early detection of the aseptic 

loosening becomes possible. Then the development of the medical treatment to block 

the pathway and to prevent osteolysis will be feasible. Thus the life of the total joint 

replacement can be further prolonged. 
223

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241

APPENDIX A 

PUBLICATIONS AND AWARDS

A.1 Publications and presentations 

1. H-W. Fang, S. M. Hsu, and J. V. Sengers, ”Analysis OF UHMWPE 

Particles Induced Biological Response: An Approach by Microfabrication 

of Particles and Biophysical Modeling of Phagocytosis” World Congress 

for Chinese Biomedical Engineers, held Dec. 2002, Taipei, Taiwan. 

2. H-W. Fang, S. M. Hsu, and J. V. Sengers, “ Generation of Narrowly 

Distributed UHMWPE Wear Particles with Micro-fabricated Silicon 

Surface Textures” Biomedical Engineering: Recent Developments, 

Medical and Engineering Publishers, Inc, pp. 97-98, 2002. 

3. S-Y. Yang, H-W. Fang, K. Siddiqi, S. M. Hsu, and P. H. Wooley, “Size 

and Concentration of Wear Debris UHMWPE Particles Influence Local 

Inflammation and Bone Resorption” Transactions of 29th Society for 

Biomaterials Annual Meeting, p.611, 2003. 

4. H-W. Fang, S. M. Hsu, and J. V. Sengers, “Generation of Narrowly 

Distributed Ultra-high Molecular Weight Polyethylene Particles by Surface 

Texturing Techniques” (Accepted by JBMR, Applied Biomaterials) 

5. W. Ren, S. Yang, H-W. Fang. S. M. Hsu, and P. H. Wooley, “Receptor 

Activator of NF-κB (RANK) and RANK LIGAND (RANKL) Expression 
242

in Response to Varying Morphology of UHMWPE Particles in a Murine 

Model of Inflammation” (Accepted by Biomaterials) 

6. H-W. Fang, S. M. Hsu, and J. V. Sengers, “Micro-cutting Mechanism of 

Surface Texture Controlled UHMWPE Wear Particle Generation”, 

Transactions of 28th Society for Biomaterials Annual Meeting, p.308, 2002. 

7. H-W. Fang, J. V. Sengers, and S. M. Hsu, “Controlled Size and Shape 

UHMWPE Wear Particle Generation using a Silicon Micro-fabricated 

Surface Texturing Technique”, Transactions of 27th Society for 

Biomaterials Annual Meeting, p.30, 2001. 

8. H-W. Fang, J. V. Sengers, and S. M. Hsu “Generation of Controlled 

UHMWPE Wear Particle Size and Shape by Surface Texturing: 

Relationship between Surface Feature Dimensions to Particle Size and 

Shape”, Transactions of 27th Society for Biomaterials Annual Meeting, 

p.585, 2001. 

9. H-W. Fang, H-C. Liu, and T-H. Young, “Shape Discrimination of 

UHMWPE Particles by Fourier Descriptors and Classical Shape 

Parameters”, Transactions of 6th World Biomaterials Congress, p. 478, 

2000. 

10. H-C. Liu, R-M. Chen, F-H. Lin, and H-W. Fang, “Sintered β-dicalcium 

phosphate particles induce intracellular reactive oxygen species in rat 

osteoblasts”, Biomedical Engineering Applications, Basis 

Communications, v11, n5, pp. 259-264, 1999 
243

11. H-W. Fang, M. C. Shen, U. Cho, J. Tesk, A. Christou, and S. M. Hsu, 

“Generation of Different UHMWPE Particle Shape by Wear through 

Surface Texturing”, Transactions of 25th Society for Biomaterials Annual 

Meeting, 506, 1999. 

12. H-W. Fang, K.E. Herold, H.M. Holland, and E.G. Beach, “A Novel 

Stirling Engine with an Elliptic Drive”, IECE Conference in Washington, 

96259, 1996. 

13. H-W. Fang, “ Stirling Engine Performance Modeling”, M.S. Thesis, 

University of Maryland at College Park, 1996. 

14. W. M. Lu, C. M. Lan and H-W. Fang, “Local Heat Transfer in a Stirred 

Vessel with and without Aeration”, Journal of Chemical Engineering, 

Japan, v28, n6, pp. 666-672, 1995. 
244

A.2 Awards 

1. Phi Delta Gamma Graduate Fellowship for Interdisciplinary Research, 

University of Maryland, April 2002. 

2. 1st Placement Awards (twice): Graduate Research Interaction Day- a 

presentation competition, University of Maryland at College Park, March 

2001, and April 2002.

3. Student Professional Development Award: 27th Society for Biomaterials, April 

2001.

4. Academic Grant Winner: SmallTalk 2001 Conference, a microarray, 

microfluidics, bioMEMS conference held by Association for Laboratory 

Automation, San Diego, CA, Sep. 2001.
245

APPENDIX B 

MICROFABRICATION OF STEEL SURFACE BY 

ELECTROCHEMICAL ETCHING

B.1 Patterning of stainless steel surface 

Two dimensional pattern were designed by a AutoCAD software. The dark 

features on the pattern represent the protected area from the etching process. The 

patterns were then transferred to a chromium coated quartz photomask by the directwriting process at the photomask manufacturer. 

The polished stainless surface (Ra < 0.025 μm) was heated on a hot plate at 

205oC for 1 minute. This is the dehydration process to remove the water remaining on 

the material surface. The surface was then spin-coated 1,1,1,3,3-Hexamethyldislazane 

(HMDS) at 5000 rpm for 30 seconds. The purpose of this coating is to enhance the 

adhesion between the wafer surface and the later coated photoresist. Shipley 1813 

photoresist was spin-coated on the silicon dioxide surface at 5000 rpm for 30 seconds. 

It was then baked on a hot plate at 100oC for 2 minutes to enhance the stability of the 

coated photoresist. 

 The dark-featured photomask was then placed in contact on the photoresist 

surface at a mask aligner for the following photolithography process. It was exposed 

to a UV source to decompose the surrounding polymer surface leaving the positive 

rectangular patterns which were covered by the dark features on the photomask. A 

post-bake process was carried out at 100oC for 90 seconds to accelerate and 
246

accomplish the photoresist decomposition. In order to fully remove the decomposed 

photoresist, the wafer surface was rinsed by Shipley photoresist develop 351 for 30 

seconds at room temperature followed by the water rinse and blow dry processes. Up 

to this stage, the patterns were transferred on the photoresist layer on the stainless steel 

surface. The resulting photoresist pattern was then acting as the protecting layer for 

the following electrochemical etching of steel surface. 

B.2 Electrochemical etching of steel 

 The stainless steel plate with patterned surface layer of photoresist was 

mounted on the anode end of a electrochemical cell setup as shown in Figure B.1. 

Ther other end is a plate of 316 stainless steel. Solution of 5M NaCl was prepared as 

the electrolyte in the tank. A DC power was connected to the electrochemical cell. It is 

optimized with 1.125 inch between cathode and anode plates, 5V applied voltage and 

1.8A electrical current, a smooth etched surface can be obtained. 

B.3 Etched steel surfaces 

 Examples of the etched 316 stainless steel surface features are shown in Figure 

B.2. The corner of a square feature was rounded in Figure B.2(a) and it is found 

material defect on the etched surface in Figure B.2(b). There exists the limit of the 

resolution of the surface features because of the non-uniformity of materials and the 

defects of the materials. 
247

Stainless steel with patterned surface layer of photoresist 

Teflon

Stainless steel

Anode Dissolution 

Electrolyte 

5M NaCl solution 1.125”

DC Power 

Figure B.1 A schematic of facility setup for microfabrication of stainless steel 

surface by electrochemical etching. 
248

Figure B.2 Electrochemical etched stainless steel surface features (a) negative 

square features, (b) grove features. 

(b) 

(a) 
249

APPENDIX C 

SURFACE TEXTURE AND WEAR DATA

C.1 Dimensions of the surface texture 

Lc Hc

Ds

Load 

UHMWPE pin 

Top view of the surface texture

Ds

Di

Lc

Wc

Sliding direction
250

C.2 Results of linear reciprocating wear tests 

 The following table lists the wear testing results of UHMWPE pin sliding 

against various surface textures. The stroke length is 19 mm. 

Lc 

(μm) 

Wc 

(μm) 

Hc 

(μm) 

Ds 

(μm) 

Di 

(μm)

Load 

(N) 

Frequency 

(Hz) 

Duration 

(hr) 

Wear 

(mg) 

Effective

cutting 

55 5.0 4.2 200 90 93 1.5 2 4.21 yes 

55 5.0 4.2 200 90 93 1.5 6 7.89 yes 

55 5.0 4.2 200 90 93 1.0 4 1.86 yes 

55 5.0 4.2 200 90 93 0.75 4 1.9 yes 

55 5.0 4.2 200 90 23 1.5 12 10.55 yes 

55 5.0 4.2 200 90 93 0.75 46 41.85 yes 

52.3 5.0 3.3 200 90 93 1.5 2 3.84 yes 

15.6 5.0 2.3 200 130 46 1.5 12 0.37 yes 

15.6 5.0 3.0 200 130 93 1.5 12 0.04 yes 

15.6 5.0 3.0 200 130 93 1.5 24 0.28 yes 

8.2 5.0 2.4 10 10 93 1.5 2 0.03 no 

8.2 5.0 2.4 10 10 93 1.5 8 0.04 no 

8.2 5.0 2.4 10 10 186 1.5 24 0.35 no 

8.0 5.0 2.5 200 140 93 1.5 2 0.03 yes 

8.0 5.0 2.5 200 140 11 1.5 24 0.14 yes 

8.0 5.0 2.5 200 140 186 0.75 124 5.08 yes 

8.0 5.0 2.5 200 140 186 0.75 167 2.79 yes 

7.4 2.0 0.9 10 8 93 1.5 12 0.45 no 

7.4 2.0 0.9 10 8 93 0.75 20 0.11 no 

7.4 2.0 0.9 10 8 186 1.5 12 1.63 no 

7.4 2.0 1.2 10 8 186 1.5 6 1.51 no 

5.8 2.0 1.2 100 7 93 1.5 48 0.14 no 

3.6 1.0 0.32 10 4 93 1.5 12 0.37 no 

3.6 1.0 0.32 10 4 186 1.5 12 0.42 no 

3.5 2.0 0.8 20 4 93 1.5 24 0.73 yes 

3.5 2.0 0.8 20 4 93 1.5 12 0.43 yes 

3.5 2.0 0.8 20 4 93 1.5 24 0.64 yes 

3.5 2.0 0.8 20 4 93 1.5 108 1.3 yes 

3.5 2.0 0.8 20 4 93 0.75 42 0.81 yes 

3.5 2.0 0.8 20 4 93 0.75 144 4.44 yes 

3.5 2.0 0.8 20 4 186 0.75 214 3.08 yes 

3.5 2.0 0.8 10 4 93 1.5 12 0.34 no 

3.5 2.0 0.8 10 4 93 0.75 24 0.41 no 

3.4 2.0 0.55 4 4 93 1.5 24 0.26 no 
251

APPENDIX D 

MODELING OF PARTICLE GENERATION BY 

CORRELATION OF MODEL EXPERIMENTAL DATA

D.1 Introduction 

By the constant depth sliding experiments in section 9.4.1, we obtained the 

relationship between displaced volume of UHMWPE and energy input in the process 

was obtained. By correlating the results of the model experiments model is established 

to determine the shear volume (displaced volume) of UHMWPE from penetration 

depth calculation and the geometry of the cutting edges. 

D.2 Energy balance of the displaced volume of UHMWPE 

The energy input by the cutting edge to the UHMWPE material can be 

calculated from the force curve as shown in Figure D.1. By plotting the horizontal 

force as a function of the sliding distance, the work input for the displaced volume is 

the area under the curve toward the point A where the maximum deformation volume 

was reached. The displaced volume of UHMWPE is proportional to the quantity of the 

energy input during the sliding process and can be described as 

F dx m V

F dx V

⇒ = ∆

∝∆

∫

∫

x

x

 (D.1)

where m represents a specific energy of UHMWPE deformation in the sliding process. 
252

Experiments were carried out with various penetration depths. The energy 

input and the displaced volume were calculated. Figure D.2 shows a plot of the energy 

input as a function of the displaced volume. The linear relationship shown in the plot 

verifies the relationship we proposed in equation (D.1). From a linear regression 

analysis of the results, the value of m can be obtained from the slope of the line. 

Equation (D.2) can be used to estimate the material deformation in the cutting edge 

sliding process: 

3

x

mm

mW = 57.8

= ∆ ∫

m

F dx m V

 (D.2)

Figure D.1 Determination of the energy input from the cutting-edge

sliding force curve (cutting edge length = 2mm, penetration

depth = 400 μm, sliding speed = 0.5/s) 

Distance (mm)

0.0 1.0 2.0 3.0 4.0

Horizontal force Fx (N)

0

10

20

30

40

50

60

70

80

A
253

D.3 Sliding process of the cutting edge 

In Chapter 9, we have conducted constant depth sliding experiments to 

investigate the sliding distance toward the cutting edge was pushed away by the strain 

hardening of UHMWPE. The sliding distance toward the maximum displaced volume 

can be used to estimate the volume of the generated particle from the dimensions of 

the cutting-edge length and the penetration depth. The energy input required for the 

volume displaced was determined and shown in equation D.2. In this section, we try to 

relate the resistance force and sliding distance to the penetration depth and the 

geometry of the cutting edge. 

A larger contact area between the cutting-edge face and the UHMWPE leads to 

a larger material resistant force during the sliding process. The amount of matrial 

resistant force in the horizontal direction changes according to different cutting edge 

angles as shown in Figure D.3. Thus we obtain 

Fx ∝ Lc ⋅ Hp ⋅ cosθ (D.3)

Figure D.2 Plot of energy input versus displaced volume during the cutting 

edge sliding process. 

Displacement volume (mm3)

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

Energy input (mW)

0

2

4

6

8
254

We also have to understand the effect of cutting edge geometry on the resulting 

sliding distance (x). With the constant sliding speed in the horizontal direction, the 

sliding distance depends on how soon the cutting edge is pushed away from 

UHMWPE material in the particle generation process. The cutting edge will slide 

longer when a larger penetration depth is introduced in the penetration process 

because of more time needed to lift the cutting edge to complete a cutting process. 

From another viewpoint, a deeper penetration depth results in a larger contact area 

between cutting edge and UHMWPE material. A larger volume of UHMWPE material 

was compressed before the cutting edge was pushed away by the strain hardening of 

UHMWPE. 

Figure D.3 A schematic representation of reacting forces in a cuttingedge sliding process. 

Lc

Hp

W

θ Sliding

Frx =Fr*cosθ

Fr

Frz =Fr*sinθ
255

The cutting-edge angle affects the resolved component resistant force in z 

direction as shown in equation (D.4) 

sin θ rz r F = F ⋅ (D.4)

Thus the acceleration of the cutting-edge push-away process is proportional to 

the of Frz or sinθ. The ultimate speed when the strain hardening happens is also 

proportional to the sinθ. Thus the time needed to push away the cutting edge is 

proportional to the reciprocal of sinθ. The time needed to complete the push away 

process is proportional to the sliding distance. Thus we obtain the following relation: 

sinθ

1

x ∝ (D.5)

With the same speed, a larger penetration depth leads to a longer time to complete the 

micro-cutting process. This relation can be described as: 

Hp x ∝ (D.6)

From the above assumptions, we can derive the equation to calculate the energy input 

with known cutting edge geometry: 

 

H i k L H

F x k L H H

x H

F L H

,

0

2

c p

x c p p

p

x c p

p cotθ

d

sin θ

cosθ d

d

sin θ

1

cosθ

= ⋅ ⋅ ⋅

⇒ = − ⋅ ⋅ ⋅ ⋅

∝ ⋅

∝ ⋅ ⋅

∫ ∫

 (D.7)

The constant k is a material property parameter of UHMWPE in such a wedge-shaped 

cutting-edge sliding process. To determine the constant k, we plot the energy input 
256

versus ( c p k ⋅ L ⋅cotθ⋅ H 2

) from the results of the model experiments . From the slope 

of the fitted straight line as shown in Figure D.4, we can obtain the following relation: 

2

x 3 c p d 79.1 L cot H

mm

mW F x = ⋅ ⋅ θ⋅ ∫ (D.8) 

From equation (D.2), we have obtained the energy required for the volume displaced 

in the sliding process. By combining equations (D.2) and (D.8) we can estimate the 

particle volume based on the given cutting edge geometry and the penetration depth 

estimated from equation (10.8) and obtain the following equation: 

2

p c p V = 1.37 ⋅ L ⋅ cot θ ⋅ H (D.9) 

Figure D.4 Plot of energy input versus ( c p k ⋅ L ⋅cotθ⋅ H 2). 

Lc*Hpen2*cotθ

0.00 0.02 0.04 0.06 0.08 0.10

Energy input (mW)

0

2

4

6

8
257

D.4 Comparison with experimental results 

Surface textures with different dimensions (cutting-edge length, angle and 

height) were used to generate particles. The particle volume was calculated from the 

SEM observations. Table D.1 lists some of the experimental results for comparison 

purpose. 

In Figure D.5, the experimental results are compared with the model 

predictions by equation (D.9). In the small-particle volume region, the two sets of 

results are consistent. It is found, that the measured volume of the particle is smaller 

than the predicted volume for the larger particle. One possible reason is that the 

particle length is about 10 %-15 % smaller than the cutting-edge length. In the model, 

we have assumed that the particle length is equal to the cutting-edge length. The 

viscoelasticity, leading to the shrinkage of the particle dimension after the particle is 

released from the bulk UHMWPE, can explain the difference between predicted and 

the experimental results. 

Table D.1 Examples of surface-texture dimensions and generated UHMWPE 

particle dimensions. 

Cutting edge 

length (μm) 

Cutting edge 

height (μm) 

Cutting edge 

angle 2θ

Particle length 

(μm) 

Particle width 

(μm) 

Particle 

thickness (μm) 

55 4.2 61.5° 43.1 10.0 3.8 

52.3 3.3 58.5° 40.8 9.0 3.2 

15.6 2.3 92.4° 11.5 3.8 2.2 

15.2 3.0 78.5° 11.0 5.0 2.9 

8.0 2.5 90.0° 8.0 4.8 1.7 

3.5 0.8 102.6° 3.2 2.3 0.3 
258

Lc*cotθ*Hp

2

0 500 1000 1500 2000

Particle Volume (

μ

m3

)

0

500

1000

1500

2000

2500

3000

Experimental results

Predict by model

Vp = 1.37*Lc*cotθ*Hp

2

Lc*cotθ*Hp

2

0 100 200 300 400

Particle Volume (

μ

m

3)

0

100

200

300

400

Figure D.5 Comparison of the experimental results with the prediction

from a contact mechanics model. 
259

APPENDIX E 

MATLAB PROGRAM FOR 

CONTACT MECHANICS MODEL OF PARTICLE 

GENERATION

%load on each feature (N) 

w = 0.0587; 

%hc, wc, lc 

lc = 55; 

wc = 5; 

hc = 4.2; 

tangent = 0.5*wc/hc 

side = sqrt(((0.5*wc)^2)+((hc)^2)); 

sintheta = (0.5*wc)/side 

% Calculating penetration depth 

hpc = w/0.000195/lc/tangent*2; 

if hpc > hc; 

 hp = hc 

 wr = hc*0.000195*lc*tangent/2 

else 

 hp = hpc 

 wr = w 

end 

hpo=hp; 

dx=0.1; 

v=0; 

x=0; 

while hp > (0.05*hpo) 

 dv = lc*hp*dx; 

 v = v+dv; 

 x = x+dx; 

 f = 0.0000012*v; 

 fz = f*sintheta; 

 if (w-fz) < wr 

 hp = (w-fz)/0.000195/lc/tangent*2; 

 else hp = hp; 

 end 

end 
260