ARL-RP-0552 ● SEP 2015

US Army Research Laboratory

Characterization of a Composite Material to 

Mimic Human Cranial Bone

by Thomas A Plaisted, Jared M Gardner, and Jeffrey L Gair 

Reprinted from ICCM20 [accessed 2015 Sep 28]. http://www.iccm20.org/p/. Paper presented 

at: 20th International Conference on Composite Materials; 2015 Jul 19–24; Copenhagen, 

Denmark.

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ARL-RP-0552 ● SEP 2015

US Army Research Laboratory

Characterization of a Composite Material to 

Mimic Human Cranial Bone

by Thomas A Plaisted

Weapons and Materials Research Directorate, ARL

Jared M Gardner

TKC Global Inc., Herndon, VA

Jeffrey L Gair

Oak Ridge Institute for Science and Education, Oak Ridge, TN

Reprinted from ICCM20 [accessed 2015 Sep 28]. http://www.iccm20.org/p/. Paper presented 

at: 20th International Conference on Composite Materials; 2015 Jul 19–24; Copenhagen, 

Denmark.

Approved for public release; distribution is unlimited.
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Characterization of a Composite Material to Mimic Human Cranial Bone

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Reprinted from ICCM20 [accessed 2015 Sep 28]. http://www.iccm20.org/p/. Paper presented at: 20th International 

Conference on Composite Materials; 2015 Jul 19–24; Copenhagen, Denmark.

14. ABSTRACT

We report on the development and characterization of a composite material to mimic the mechanical response of human cranial 

bones. Mimicry of the mechanical response of bone requires a material that can be formed into complex architectures while 

possessing physical and mechanical characteristics similar to that of bone. We are utilizing additive manufacturing, more 

generally known as 3-D printing, as a suitable process to reproduce the curvature, variation in thickness, and gradient in porosity 

characteristic of the human cranial bone. The simulant material consists of a photocurable polymer with a high loading of 

ceramic particulate reinforcement that is compatible with stereolithographic (SLA) additive manufacturing. Specimens 

produced by SLA printing were characterized under conditions of quasi-static tensile loading and demonstrated properties that 

fell within the experimental range of values measured for the human cranial cortical bone as previously reported in the literature. 

The simulant material demonstrated a tensile modulus of elasticity of 10.3±0.5 GPa compared to the cranial cortical bone 

property of 12.8±1.6 GPa, while the tensile strength was 78.0±9.7 GPa compared to 72.0±13.8 GPa. Tissue surrogates such as 

this enable bio-fidelic experimentation and evaluation of protection schemes without the variability and difficulties typically 

associated with using real human tissue.

15. SUBJECT TERMS

bone simulant, tissue surrogate, cranial bone, bio-mimicry, additive manufacturing, polymer composite

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Thomas A Plaisted

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The research reported in this document was performed in connection with contract/instrument W911QX-14-C-0016 with the U.S. Army Research Laboratory. The 

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purposes notwithstanding any copyright notation hereon. This research was supported in part by an appointment to the Postgraduate Research Participation 

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U.S. Department of Energy and USARL. 

CHARACTERIZATION OF A COMPOSITE MATERIAL TO MIMIC 

HUMAN CRANIAL BONE 

Thomas A. Plaisted1, Jared M. Gardner2 and Jeffrey L. Gair3

1

U.S. Army Research Laboratory 

Weapons and Materials Research Directorate 

Aberdeen Proving Ground, MD 21005-5069 

Web page: http://www.arl.army.mil

2

TKC Global Inc., Contractor to the U.S. Army Research Laboratory 

13873 Park Center Road, Suite 400 North, Herndon, VA 20171 

 3

Oak Ridge Institute for Science and Education 

P.O. Box 117, Oak Ridge, TN 37831-0117 

Keywords: bone simulant, tissue surrogate, cranial bone, bio-mimicry 

ABSTRACT 

We report on the characterization of a composite material to mimic the mechanical response of 

human cranial bones. Mimicry of the mechanical response of bone requires a material that can be 

formed into complex architectures while possessing physical and mechanical characteristics similar to 

that of bone. We are utilizing additive manufacturing, more generally known as 3-D printing, as a 

suitable process to reproduce the curvature, variation in thickness, and gradient in porosity characteristic 

of the human cranial bone. The simulant material consists of a photocurable polymer with a high loading 

of ceramic particulate reinforcement that is compatible with stereolithographic (SLA) additive 

manufacturing. Specimens produced by SLA printing were characterized under conditions of quasistatic tensile loading and demonstrated properties that fell within the experimental range of values 

measured for the human cranial cortical bone as previously reported in the literature. The simulant 

material demonstrated a tensile modulus of elasticity of 10.3±0.5 GPa compared to the cranial cortical 

bone property of 12.8±1.6 GPa, while the tensile strength was 78.0±9.7 GPa compared to 72.0±13.8 

GPa. Tissue surrogates such as this enable bio-fidelic experimentation and evaluation of protection 

schemes without the variability and difficulties typically associated with using real human tissue. 

 

1 INTRODUCTION 

Prevention of head injury is receiving growing attention in the sporting arena as well as the battlefield 

due to an increase in the frequency of diagnosed concussion and traumatic brain injury (TBI) events. 

The United States Department of Defence reports that between 2000 and 2014 there were over 313,000 

Service members diagnosed with TBI, of those 83% were diagnosed as concussions, or mild TBI events 

[1]. Helmets and head protection devices are under development to reduce the frequency and severity 

of TBI injury. Bio-fidelic representation of the human head is important for assessment of these 

protective measures. Anthropomorphic test devices (ATDs), such as those used in the automotive 

industry for assessment of vehicle crashworthiness, provide a representative shape and mass of the 

human head on which devices can be tested. However, ATD headforms are typically constructed of 

metal covered in an elastomer skin for durability and as a result they do not flex, fracture or transmit 

stress waves the same as a human cranium. Alternatively, Post Mortem Human Surrogates (PMHS) 

may be used to assess protective devices, although the tissue frequently comes from older donors and 

may not represent the mechanical characteristics of the general population, particularly that of the 
20th International Conference on Composite Materials 

Copenhagen, 19-24th July 2015 

soldier. Moreover there can be large variations in the mechanical response of the tissue depending on 

the age and health of the donor. Testing with PMHS also requires careful handling and preservation 

techniques after extensive planning to ensure availability at the time of testing. 

 

Developing a synthetic material to mimic the mechanical response of the human cranial bone 

alleviates a number of these concerns. The response of the surrogate material can be tailored to represent 

a particular age population and the geometry of the surrogate can further be tailored to a specific 

anthropometry. Developments in medical imaging technology allow us to obtain detailed, threedimensional renderings of the human cranium specific to individuals which may be used for 

representation of a general population. These file formats can be sent directly to additive manufacturing 

machines, generically referred to as 3-D printers, to replicate a human skull. In this work we utilize 

additive manufacturing to process a photocurable polymer and report on the tensile properties of the 

material and correlation to human cranial bone. 

2 HUMAN CRANIAL BONE 

Human bone is a complex hierarchical composite material with variable mechanical properties 

throughout the different regions of the body. The basic constituents of human bone remain the same 

however: collagen fibers, hydroxyapatite mineral, and water. Each of these constituents are present in 

the hard outer cortical layer as well as the porous inner trabecular region of the bones. Long bones such 

as the femur exhibit increased strength and stiffness along the primary loading direction, whereas cranial 

bones are less stiff and exhibit transverse isotropy in directions tangent to the outer layer due to the 

absence of a predominant loading direction on the cranium [2]. 

The body of literature characterizing cranial bone properties is much smaller compared to that of 

long bones and properties can vary significantly for reasons mentioned previously. An extensive study 

of the tensile properties of the parietal, temporal and frontal bones was performed by Wood [3, 4]. The 

study utilized specimens from the craniums of 30 subjects ranging from age 25 to 95 years with an 

average age of 54 years. The study characterized the mechanical response of the outer cortical regions 

(tables) independent of the inner trabecular (diploë) regions at strain rates ranging from 0.005 to 150 

sec-1. The cortical cranial bone data from Wood serves as a basis for comparison to the surrogate 

material under evaluation in this work. 

2 EXPERIMENTAL 

2.1 Materials and Fabrication 

Stereolithography (SLA) was chosen as a means to manufacture the specimen due to its ability to 

create high resolution parts. A schematic representation of the SLA process is given in Figure 1. A 

typical SLA machine cyclically raises and lowers a platform on which the parts are fabricated in a bath 

of photo-sensitive liquid resin. With each cycle, a blade is passed across the platform to create a uniform 

layer of resin. The resin layer is exposed to a ultra-violet laser that traces a cross-section of the desired 

geometry, thereby selectively curing the resin into a specific pattern. The process is repeated as the 

platform is lowered into the resin and the next layer is cured on top of the previous layer. A support 

structure, visible in Figure 2, is typically first built on the platform and serves as the foundation from 

which the part is built. 
20th International Conference on Composite Materials 

Copenhagen, 19-24th July 2015 

Figure 1. Illustration of the SLA additive manufacturing process. 

The SLA machine used in this study to manufacture specimens was a Viper Si (3D Systems, Rock Hill, 

South Carolina). In ongoing work, we are evaluating the ability of additive manufacturing such as SLA 

to reproduce the curvature, variation in layer thickness, and gradient in porosity characteristic of bone 

[5]. 

 

Figure 2. Example parts fabricated by the SLA machine. 

A photocurable polymer with a high loading of ceramic particulate reinforcement was chosen for 

its similar density and anticipated mechanical properties relative to the human cranial cortical bone [5]. 

Furthermore it was chosen for its compatibility with the SLA manufacturing process. Tensile testing 

specimens were printed in a dumbbell geometry in accordance with ASTM D638-10, Type I, with a 

nominal thickness of 3 mm [6]. The planar, non-porous specimens were manufactured in two 

orientations: flat type specimens were made such that layer-by-layer deposition coincided with the 

smallest dimension of the dumbbell tensile geometry (thickness), and edge type specimens in which the 

deposition occurred across the face of the geometry, as illustrated in Figure 3. 

Figure 3. Orientation of SLA layer deposition relative to tensile testing direction for edge and flat

specimens. 

Upon removal from the SLA machine, the support scaffold on which the parts were built was 
20th International Conference on Composite Materials 

Copenhagen, 19-24th July 2015 

removed from the specimen. A post process wash in isopropyl alcohol was performed to remove any 

excess material and the samples were abraded with 600 grit sandpaper to remove any residual scaffold 

that remained on the surface. The parts were then post cured at 160o C for two hours with a two hour 

heating ramp and two hour cooling ramp to room temperature. Upon removal from the oven it was 

noted that curvature had been induced in some of the flat printed samples. 

2.2 Testing 

Tensile testing was performed in accordance with ASTM D638-10, Standard Test Method for Tensile 

Properties of Plastics [6]. A servo-hydraulic universal testing machine (model 1331, Instron Inc.) 

equipped with 50 kN load cell was used to perform testing. Pneumatic grips were used to hold the ends 

of the specimen and No. 80 grit sandpaper was placed between the grip and specimen to reduce slippage 

and the likelihood of grip failure. Tensile testing was carried out under displacement control at strain 

rates of 0.0007 sec-1 and 0.007 sec-1. Six samples of flat and six samples of edge printed specimens were 

prepared for testing at each strain rate, however excessive curvature induced into two of the flat printed 

samples caused them to break during installation into the grips and thus were excluded. 

 

For the purpose of measuring strain by digital image correlation (DIC), a speckle pattern was applied 

to each specimen. The speckling process consisted first of applying a continuous layer of white acrylic 

spray paint, followed by a mist of larger droplets of black paint to create a pattern, as shown in Figure 

4. A digital camera (Grasshopper model, Point Grey Inc.) was used to acquire images every 0.12 

seconds during testing, and strain was calculated by digital image correlation (VIC 2-D Version 6, 

Correlated Solutions). 

Figure 4. Tensile specimen undergoing fracture during testing. 

3 RESULTS AND DISCUSSION 

The tensile elongation curves for the edge printed samples and flat printed samples tested at 

0.0007 sec-1 and 0.007 sec-1 are shown in Figure 5. Young’s modulus was calculated below 0.2% 

strain and Poisson ratio was calculated based on analysis of the DIC images between 0.1 and 0.4% 

strain. The average values for tensile strength, modulus and Poisson ratio are reported in Table 1 

along with the cranial cortical bone values from the literature. 
20th International Conference on Composite Materials 

Copenhagen, 19-24th July 2015 

 

Figure 5. Stress vs. strain response of edge (denoted E) and flat (denoted F) printed samples at strain 

rate 0.0007 sec-1(left) and 0.007 sec-1 (right). 

 

Figure 6. Young’s modulus (left) and tensile strength (right) for edge and flat printed specimens at 

0.0007 sec-1 and 0.007 sec-1. 

Specimen 

Type 

Strain 

rate 

Tensile 

strength 

Strain to 

 failure 

Tensile 

modulus 

Poisson 

ratio 

 

 

[sec-1] 

nominal 

[MPa] [%] [GPa] N/A 

Avg ± Avg ± Avg ± Avg ± 

Edge 10‐2 78.1 9.6 0.98 0.22 10.3 0.5 0.26 0.02

Flat 10‐2 64.0 5.1 0.72 0.07 10.1 0.7 0.27 0.02

Edge 10‐3 78.1 8.8 1.11 0.27 10.5 0.2 0.30 0.03

Flat 10‐3 60.4 13.2 0.73 0.21 9.5 0.1 0.24 0.03

Cranial cortical

bone [4] 10‐2 72.3 14.4 0.69 0.11 12.8 1.6 0.19‐0.48  [7]

Table 1. Properties measured for edge and flat print orientations of cranial simulant material. 

The edge and flat printed samples show a similar tensile modulus at the two relatively low strain 

rates reported here. The edge samples exhibit a higher tensile strength and strain to failure compared to 

the flat samples. This difference may be a result of thickness and reinforcement content variations in 

the initial two or three layers deposited, as revealed by inspection under a microscope. In the flat

samples the deposition of sequential layers occurs in the thickness dimension of the sample. Asymmetry 

between the upper and lower surfaces in the flat printed specimens likely created internal stresses, as 

0

10

20

30

40

50

60

70

80

90

100

0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016

Stress (MPa)

Strain (mm/mm)

E7 E8 E9a E10 E11 E12 F05 F06 F07 F08 F09

0

10

20

30

40

50

60

70

80

90

100

0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016

Stress (MPa)

Strain (mm/mm)

E1a E2 E3 E4 E5 E6 F01 F02 F10 F11 F12

8.00

9.00

10.00

11.00

12.00

Edge 0.007/s Flat 0.007/s Edge 0.0007/s Flat 0.0007/s

Tensile Modulus (GPa)

40.0

50.0

60.0

70.0

80.0

90.0

100.0

Edge 0.007/s Flat 0.007/s Edge 0.0007/s Flat 0.0007/s

Tensile Strength (MPa)
20th International Conference on Composite Materials 

Copenhagen, 19-24th July 2015 

evidenced by the curvature observed after the post-cure thermal treatment, and resulted in premature 

failure during tensile loading. In the edge samples however, this asymmetry occurs on the sides of the 

specimen geometry and had a much smaller negative effect on tensile properties. 

Considering the edge printed samples alone, the simulant material demonstrated a tensile strength of 

78.0±9.7 GPa compared to 72.0±13.8 GPa for the cranial bones as measured in the study by Wood [4]. 

While the mean strength value of the simulant is approximately 8% higher, it is well within the standard 

deviation of the bone property and represents a significant improvement in correlation compared to a 

cranial cortical simulant material reported previously in the literature (tensile strength 53±4.88 MPa) 

[8]. The tensile modulus of elasticity of the edge printed simulant was 10.3±0.5 GPa compared to the 

cranial cortical bone property of 12.8±1.6 GPa. The simulant material modulus is within approximately 

20% and exhibits over 50% reduction in coefficient of variation compared to cranial cortical bone. 

Overall the variability of the simulant material is lower than that of the bone property and we expect to 

reduce variability further through refinement of the SLA processing parameters. 

9 CONCLUSIONS 

We have evaluated a composite material consisting of a highly filled photocurable polymer to serve 

as a surrogate material for human cranial cortical bone. Tensile specimens were produced by SLA 

additive manufacturing and characterized under conditions of quasi-static tensile loading. Specimens 

manufactured such that the layer-by-layer deposition coincided with the thinnest dimension of the 

dumbbell tensile geometry (flat type) exhibited noticeable warping after post cure compared to 

specimens where deposition occurred across the face of the geometry (edge type). As such, the 

variability in tensile properties of the edge type specimens was lower and provided a closer correlation 

to the experimental range of values measured for the human cranial cortical bone. Studies are currently 

underway to evaluate the dynamic fracture response of the material in comparison to human tissue. 

Additive manufacturing coupled with medical imaging is also being utilized to fabricate specimens that 

include curvature, porosity and variation in layer thickness mimicking the human skull. 

ACKNOWLEDGEMENTS 

The authors would like to acknowledge the Advanced Design and Manufacturing Team at U.S. Army 

Edgewood Chemical and Biological Center for utilizing their equipment to manufacture the specimens 

in this study. The authors also thank Larry Holmes and Ricardo Rodriguez of the U.S. Army Research 

Laboratory for many helpful discussions on additive manufacturing. 

REFERENCES 

1. DoD Numbers for Traumatic Brain Injury, Armed Forces Health Surveillance Center, 

Editor 2015, Defense and Veterans Brain Injury Center: Silver Spring, MD. p. 3. 

2. McElhaney, James H., John L. Fogle, John W. Melvin, Russell R. Haynes, Verne L. 

Roberts, and Nabih M. Alem, Mechanical properties of cranial bone. Journal of 

Biomechanics, 1970. 3(5): p. 497-511. 

3. Wood, Jack L., Dynamic response of human cranial bone. Journal of Biomechanics, 

1971. 4(1): p. 1-12. 

4. Wood, Jack L., Mechanical properties of human cranial bone in tension, in Department 

of Biomechanics1969, University of Michigan: Ann Arbor, Michigan. p. 108. 

5. Plaisted, Thomas A., Brett Sanborn, Jared Gardner, Tusit Weerasooriya, Jeffrey Gair, 

and Michael Leadore, Development of a mechanical simulant for human cranial bone.

Journal of Biomechanics, 2015. manuscript in preparation. 
20th International Conference on Composite Materials 

Copenhagen, 19-24th July 2015 

6. International, ASTM, Standard Test Method for Tensile Properties of Plastics, in D638 

– 102010, ASTM International: West Conshohocken, PA. p. 16. 

7. Peterson, Jill and Paul C. Dechow, Material properties of the inner and outer cortical 

tables of the human parietal bone. The Anatomical Record, 2002. 268(1): p. 7-15. 

8. Roberts, Jack C, Andrew C. Merkle, Catherine A. Carneal, Liming M. Voo, Matthew 

S. Johannes, Jeff M. Paulson, Sara Tankard, and Manny Uy, Development of a Human 

Cranial Bone Surrogate for Impact Studies. Frontiers in Bioengineering and 

Biotechnology, 2013. 1. 
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