NBS

PUBLICATIONS

A 11 ID 7 3T07A1

NBSIR 84-2883

Characterization of Porosity in

Porous Polymer Implant Materials

R. E. Dehl

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Dental and Medical Materials

Polymers Division

Gaithersburg, MD 20899

February 1 984

Issued September 1984

Annual Report for period April through September 1983

Prepared for: Office of Medicai Devices

Center for Devices and Radiological Health

! Drug Administration

ring, MD 20910

8 U56

34-2333

1934

c. 2

NBSIR 84-2883

CHARACTERIZATION OF POROSITY IN

POROUS POLYMER IMPLANT MATERIALS

R. E. Dehl

JfATIGlf4J BDHEAUf » STM»AADS

Gc/oo

, US'

U

noC-^

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Dental and Medical Materials

Polymers Division

Gaithersburg, MD 20899

February 1984

Issued September 1984

Annual Report for period April through September 1983

Prepared for: Office of Medical Devices

National Center for Devices and Radiological Health

Food and Drug Administration

Silver Spring, MD 20910

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

Page

ANNOUNCEMENT iv

ABSTRACT v

I. INTRODUCTION 1

II. QUANTITATIVE MICROSCOPY 2

A. Preparation of PTFE-C for Photomicrography 3

B. Results and Discussion 3

1 . Void Vol ume 5

2. Mean Intercept Length ............... 6

III. MERCURY POROSIMETRY 9

A. Instrumentation 9

B. Experimental Errors 11

C. Materials 12

D. Results 12

E. Discussion 18

IV. SUMMARY 21

REFERENCES 23

l
FIGURES

Page

Fig. 1. Photomicrograph of cross-section

of polished, resin-filled PTFE-C

composite , . . 4

Fig. 2. Macro-filling accessory with penetrometer , . . 10

Fig. 3. Outline of 16 pressure-vol ume curves

for PTFE-C in sheet form

obtained

14

Fig. 4. Outline of 6 pressure-volume curves

for PTFE-C in laminated block form

obtai ned

15

i i
TABLES

Page

Table I. Porosity Parameters from Quantitative

Microscopy 7

Table II. Four Parameters Obtained from Mercury

Intrusion Curves for 16 Samples of

PTFE-C Sheet and 6 Samples of Block 17

Table III. Correlation^of Specific Pore Volume with

P at 0.1 cnr Intruded Volume 20
ANNOUNCEMENT

Certain commercial materials and suppliers are identified

in this report in order to adequately specify experimental

procedures. In no case does such identification imply

recommendation or endorsement by the National Bureau of Standards,

nor does it imply that the identified materials or suppliers are

necessarily the best available.

iv
ABSTRACT

Our investigation of the methods of characterizing the porosity of

two porous polymeric implant materials have been concluded with the work

discussed in this report. The two materials, a porous polyethylene (PPE)

and a porous composite of polytetrafluoroethylene and carbon (PTFE-C) have

been further investigated by the method of quantitative microscopy. The mean

pore volume fractions of 30 samples each of PPE and PTFE-C were found to be

0.48 and 0.69, respectively, and are in good agreement with other measurements of this quantity. The mean intercept length for PPE was found to be

76 pm, and for PTFE-C, 67 urn. Both values are somewhat larger than the

average interconnecting pore "diameters" as measured by mercury porosimetry.

The reproducibil ity of mercury porosimetry data has been tested by examining

16 samples of PTFE-C in thin sheet form and 6 samples of laminated blocks.

The mean and standard deviation of four parameters derived from the

mercury intrusion curves were calculated. A high correlation was found

between the specific pore volume and the position of the mercury intrusion

curve along the pressure axis. A number of random errors pertaining to

mercury porosimetry were discovered and are discussed in this report.

v
..

i

[

,
I. INTRODUCTION

Our previous investigations of the characterization of porosity in

two commercial ly available porous polymeric implant materials have been

1 -3

reported in three Annual Reports to the Office of Medical Devices (FDA)

The materials are a porous polyethylene (PPE) and a porous composite of

polytetrafl uoroethyl ene and carbon (PTFE-C). Following these studies,

a proposal was written to continue the investigation of these materials,

including three tasks which we felt were necessary to bring these studies

to a successful conclusion. The specified tasks were:

(1) to continue the investigation of pore size and pore volume

in PPE by the method of quantitative microscopy, until a

statistically significant sampling was achieved, and to

perform the same number of measurements on the composite

material. The results of these studies would be compared

with those obtained by other techniques such as mercury

porosimetry.

(2) to obtain a statistically significant number of mercury

intrusion curves for one of the materials and to analyze the

data to determine several parameters related to porosity.

It was proposed that the composite material in sheet form

would be most suitable for these measurements.

(3) to attempt to determine whether a real difference exists

between the porosity of the composite material in sheet form

and in the form of laminated blocks.

1
Task 1 has been successfully completed and the results are discussed

in Section II. Tasks 2 and 3 have also been concluded, although the

number of samples examined was not as large as originally anticipated,

due to restricted available time, instrumental difficulties, and the

inability to find a current supplier of the laminated composite. Nevertheless,

it is concluded that these studies have produced a significant insight

into the various methodologies for characterizing porosity.

II. QUANTITATIVE MICROSCOPY

The theoretical background for the analysis of pore size and pore

volume by quantitative microscopy (stereology) was discussed in our last

3

Annual Report . Measurements were made by means of a 6 X 4 line transparent

test grid laid on a photomicrograph of a polished cross-section of the

porous material. The latter (PPE) was filled with a hard resin to facilitate

grinding and polishing. Two parameters were determined from the test grid

analysis, namely (1) the pore volume fraction P

p

, which is the fraction

of line intersections or "points" falling within the desired (pore) phase

region, and (2) the mean intercept length L, which is defined as the point

fraction P

p divided by N^, the number of chords traversing the desired

(pore) phase region per unit of test length. L is a measure of the pore

"size". It was chosen arbitrarily; other parameters related to pore size

could equally well have been measured, but L is probably the easiest to

measure by manual methods, and it appears to be relatively free from

operator bias.

The surface of the polished PPE was found to be quite inhomogeneous

on the dimensional scale of the areas sampled in the photomicrographs , as

evidenced by a wide scatter in the experimental results. For this reason,

2
it was decided that a relatively large number of samples should be examined,

in order to estimate the population mean and standard deviation. Ten

additional photographs of the same specimen examined previously were

analyzed, for a total of 30. Thirty photomicrographs of one specimen of the

composite material were also obtained. The results are reported below.

A. Preparation of PTFE-C for Photomicrography

Preparation of the composite material in laminated block form for

optical analysis proved to be somewhat more difficult than the preparation

4

of PPE, because the liquid resin used to fill the pores did not spontaneously

flow into the material. In order to fill a sample with resin, it was

necessary first to submerge it with a lead weight into the liquid resin +

initiator, contained in a vacuum flask attached to a pump. Pumping was

continued until no further air bubbles were seen emerging from the sample.

Admission of air forced the liquid into the pores. The sample was removed

from the liquid resin and cured at 70°C overnight (15 - 20 h). The cooled

sample was ground flat with 600 grit carborundum paper, followed by a final

polishing with 0.05 ym alumina powder. Unlike the PPE, it was not found

necessary to dye the clear resin in order to obtain adequate visual phase

contrast. A sample prepared with dyed resin looked the same under the

microscope as one prepared with clear resin. A typical photomicrograph of

this material is shown in Fig. 1. The small white spots are the highly

reflective carbon particles. The dark gray streaks are the PTFE fibers,

and the medium gray regions are the filled pores.

B. Results and Discussion

The results of thirty photomicrographic analyses for each of the porous

materials are shown in Table 1. Each set of photographs was taken on one

3
Fig. 1

PTFE-Carbon

. Photomi

PTFE-C

crograph of cross-section of polished,

composite (180 X).

resin-filled

4
sample of material, by moving the overhead light beam randomly about on

the surface. All of the pictures were used; none was discarded because

it appeared less "typical" than average.

1 . Void Volume

The void volume fraction of PPE reported in Table I is slightly larger

2

than that found by two other methods. However, the difference between

the value obtained by this method and the others is well within one

standard deviation from the mean, indicating that the difference between

them may not be statistically significant.

The pore volume fraction in PTFE-C is within the range (67 to 70%)

found by three other methods , and one might therefore conclude that there

is a high probability that the true value lies within this range.

However, a question about the density of solids in the composite material

remains unresolved. The apparent density method depends upon a knowledge

of these densities, which were measured by "quantitative" expulsion of

air in an hydraulic press, followed by a measurement of the density of

a cylindrical disc cut out of the compressed sample. The density found

in this way (1.4 g/cm ) is less than the published densities of both

5

components. The density of pyrolytic carbon is reported to be from

1.5 to 2.0 g/cm3

. The density of PTFE is reported^ to be 2.3 g/cm3

.

2

From an elemental analysis of the composite, we have determined that the

material is 62% PTFE and 38% carbon, by weight. Using the minimum

density reported for carbon, we find that the density of total solids (d )

is at least

5
d

s = 0.62 (2.3) + 0.38 (1.5)

= 2.0 g/cm^

Using this value for the density of solids in the porous material, we

find that the pore volume would be 79%. This value is only one standard

deviation from the mean optical microscopic value reported in Table I,

so that a pore volume of 79% cannot be excluded.

There is evidence that two of the methods which we have previously

o discussed for measuring pore volume (apparent density and mercury

intrusion) are higher precision measurements than the optical method

and would therefore require fewer samples to estimate the population

mean. Both mercury intrusion and the optical method are "absolute" in

the sense that they do not require assumptions about density of solids

or other variables. Mercury intrusion, however, as discussed in

Section III, is subject to a number of spurious problems which could

affect its accuracy in any given measurement. Clearly, if an accurate

estimate of pore volume is desired, one must be prepared to do a thorough

error analysis of the method selected, and preferably to cross-check

this method with at least one other.

2. Mean Intercept Length

The average pore "size" defined as the mean intercept length was

found to be slightly less for the PTFE-C composite than for PPE as shown

in Table I. The difference between the mean values, however, is well

within one standard deviation for each sample of 30, and one would therefore

not be justified in reporting a significant difference between the mean

values. In order to establish whether there is a statistically significant

6
difference between the two means, It would be necessary to examine a

great many more samples from the same lot of material. Such an effort

would not appear to be justified unless one were asking a very specific

question about the materials, e.g., whether there was a real difference

between the average porosity in two or more lots of material.

Table I. Porosity Parameters from Quantitative Microscopy

Pore Volume Fraction (P

p

)

Mean(of 30) SD SD/Mean Misti Low

PPE 0.48 0.09 0.19 0.67 0.29

PTFE-C 0.69 0.10 0.14 0.88 0.38

Mean Intercept Length (ym)

Mean(of 30) SD SD/Mean High Low

PPE 76 17 0.22 102 44

PTFE-C 67 14 0.21 115 42

It is interesting that the mean intercept lengths of both materials

are substantially greater than the "interconnecting" pore diameters

determined by mercury porosimetry (30 ym for PPE and 50 ym for PTFE-C).

This observation is at least qualitatively in accord with the expectation

that the average interconnecting pore "diameter" should be smaller

than the overall average "diameter" of all pores in the material. However,

the greatly different pore morphologies in the two materials preclude an

exact comparison of the pore "size" as measured by two or more different

7
methods. We observe that mercury porosimetry finds the average interconnecting "diameter" in PPE to be significantly less than in PTFE-C,

whereas optical microscopy finds the mean intercept lengths about the

2

same, and the hybrid method of specific surface area + pore volume

finds the PPE to have at least twice the average pore "diameter" of the

PTFE-C. Not only does the average pore "diameter" of a given material

depend upon how one measures it, but the ratio of "diameters" in two

different materials is seen to vary with the measurement technique.

A significant difference between the photomicrographic crosssections of PPE and PTFE-C is the presence of a great many fine particles

in the latter material. These particles, which contribute little to the

total volume of material, nevertheless become boundaries between phases

in the measurement of mean intercept length by quantitative microscopy.

A small particle of carbon or PTFE in a pore divides the pore in two,

for purpose of counting intercepts, if a grid line happens to intersect it.

In most cases, the pore is not actually divided by the presence of a

small particle, and this paradox will tend to make the measured average

intercept length or pore "diameter" smaller than it appears to an

intruding liquid or solid particle. This dilemma was recognized early

in the optical analysis of the composite material. However, it was

decided that any attempt to correct for it by purposely ignoring small

particles would lead to a subjective judgment about which particles to

ignore, thereby defeating the overall purpose of selecting methods of

measurement which are as free from operator bias as possible. It should

be noted also that the small particle effect discussed above does not

distort the measurement of the pore volume fraction P

8
III. MERCURY POROSIMETRY

The theoretical basis for mercury porosimetry, as well as the errors

inherent in this method and various methods for displaying the data,

2 3 have been discussed in previous Annual Reports *

. It is the purpose of the

present report to continue a discussion of random errors which can occur,

and to perform some statistical analyses of the data obtained from a larger

sampling of the material than previously reported.

A. Instrumentation

All measurements were performed using a "Macro Filling" accessory

for the mercury porosimeter, obtained from Superpressure, Inc.^ A

picture of the assembled device is shown in Fig. 2. A porous sample is

placed in the large end of the glass dilatometer ("penetrometer") and

a vacuum end seal is made by means of a circular metal plate pressing

against a ground glass flange on the end of the penetrometer. The latter

is inserted into the glass chamber containing mercury and the "cap" forces

an o-ring around the penetrometer to seal against the chamber wall,

making the entire device vacuum-tight. As indicated on the figure, a

rubber hose connects to a vacuum line consisting of a thermocouple

vacuum gauge, a 0 to 0.1 MPa (0 to 15 psia) pressure gauge, and a vacuum

pump with a cold trap. A residual air pressure of 5.3 Pa (40 ym of

mercury) or less was achieved before filling the penetrometer with mercury.

In order to fill the penetrometer, the cradle to which it is attached

is first rotated about its horizontal axis, covering the penetrometer with

mercury. The device is disconnected from the vacuum pump, and a small air

pressure (<0.005 MPa) is admitted through the glass stopcock, filling

9
LEVELING SCREW

CRADLE

PAD

RESERVOIR

SECTION

CAP

PENETROMETER

(NOT SUPPLIED)

Fig. 2. Macro-filling accessory with penetrometer.

10
the penetrometer with mercury. At this point, the cradle Is rotated back

to its original position as shown in Fig. 2, and the mercury intrusion

experiment is begun. The first admitted air pressure was typically about

1.2 psia, and increments of about 1 psia were admitted, until atmospheric

pressure was achieved. The penetrometer stem has a total calibrated

3 3

volume of 0.2 cm , and the volume can be estimated to ± 0.001 cm . The

penetrometer was removed and cleaned after each intrusion experiment.

B. Experimental Errors

Although the experiment is simple, it was found to be subject to a

number of random errors, in addition to those discussed in our last

3

Annual Report . The pressure gauge was not accurate at very low pressures,

which made it difficult to fill the penetrometer using the minimum

possible pressure to avoid filling large pores, before the instrusion

experiment was begun. Occasionally, an air bubble would appear at the

top of the sample chamber after filling the penetrometer with mercury.

In such cases, the first increment of pressure resulted in an unusually

large intruded volume. In some cases the error was recognized and the run

was discarded. However, in other cases, small hidden air bubbles may have

been present but not observed, leading to variability in the initial

penetrometer reading. The air bubbles were at first thought to be due

to a leak in the penetrometer and seal. However, a vacuum leak would be

expected to increase the bubble volume in time, and this was never observed.

Most likely, the bubbles were due to air trapped in the mercury reservoir,

and after this was recognized care was taken to evacuate the mercury

chamber each time by rocking the cradle a few times to expose all surfaces

to the vacuum line.

11
Sometimes, after filling the penetrometer with mercury and returning

the cradle to Its resting position, the Initial reading indicated a

positive intruded volume. The source of this effect was not clear; It

might have been due either to a small pressure surge from the chamber or

to the mercury thread "breaking" at some point above the zero mark when

the penetrometer was separated from the mercury pool. Fortunately,

these initial non-zero readings were usually a small fraction of the total

intruded volume, but they did create another source of random errors

difficult to treat in the data analysis.

C. Materials

All experiments were performed on the PTFE-C material. Twenty

g samples in the form of thin sheets were procured ; all were from the same

lot ( HI 4K) of material. The samples were individually packaged and all

3

were nominally 15xl5xlmm in size. The weights ranged from 0.0683 g to

0.0917 g, with a mean value of 0.0801 g. Four of the 20 mercury intrusion

runs were discarded because one or more of the problems discussed above

were clearly observed.

Six samples of the laminated block were studied; each was cut from

the same oval block (Lot #C24H), 6 mm thick. Further samples of the

block composite could not be obtained from the supplier. The samples

ranged in weight from 0.0742 g. to 0.1007 g. All six sample runs are

reported here, even though in three of the cases the initial (zero)

3

penetrometer reading equalled or exceeded 0.01 cm .

D. Results

In each mercury intrusion experiment, approximately 15 pressurevolume readings were obtained at intervals of AP= 1 psi , and a smooth
curve was drawn through them. All curves were normalized to the same end point

(V = 0.21cm^) by multiplying each measured volume increment by the

appropriate constant factor. Fig. 3 is an outline of the extreme deviations

of all 16 P-V curves, for the composite sheet samples. All of the curves

fall within these boundaries, with the majority falling in a rather narrow

band near the center of the outlined zone. Fig. 4 is the correspondi ng

outline for the six block sample curves. The pore "diameters" on the lower

2

x-axis were calculated from the Washburn equation , using the formula

232

D

= -jj-, where D and P represent the interconnecting pore diameter and

applied pressure, and the constants for the composite are gathered in

the numerical term. Fig. 3 is an outline of the extreme deviations

It was found by superimposing the two sets of curves that the steep

portions of the curves for the block samples lie to the right side of

most of the curves for the sheet samples. A shift to the right (higher

pressure) corresponds to a smaller average pore size, so that this

observation is at least qualitatively in accord with the expectation

that the interconnecting pores in block samples may, on the average, be

somewhat smaller than those in sheet samples. This question is discussed

below at more length.

In order to make some numerical comparisons of the various P-V curves,

four parameters were selected for analysis. These four were selected

arbitrarily, but all together they provide some insight into the variability

of mercury porosimetry data. It should be emphasized that there is no way

of knowing how much scatter in the data is due to sample variability and

how much to instrumental random errors , which have been discussed above.

13
Pore

Volume

Pore Diameter (pm)

Fig. 3. Outline of 16 pressure-volume curves obtained for

PTFE-C in sheet form.

14
Pore

Volume

200 100 50 25 16

Pore Diameter (jam)

Fig. 4. Outline of 6 pressure-volume curves obtained for

PTFE-C in laminated block form.

15
If It were possible to make replicate measurements on the same sample, one

might be able to separate instrumental errors from sample variations.

However, it is impossible to extrude mercury quantitatively from the porous

material, so that a second intrusion experiment would be different from

the first one in some unknown way.

The four quantities selected for analysis were:

(1) the specific pore volume (cm /g).

(2) the pressure corresponding to one selected volume of pores

filled with mercury (0.1 cm , end-point normalized curves).

(3) the fraction of pore volume corresponding to one selected

interval of pressure. The interval between P = 4 psia and

P = 8 psia was selected. This interval corresponds to about

half the total intruded pore volume and is equivalent to a

pore size range from 29 to 58 pm.

(4) the volume of pores filled with mercury at 1.2 psia pressure.

This pressure is approximately that at which the first P-V

point was recorded in each intrusion experiment. It corresponds

to a pore "diameter" of 193 urn, so that the pore volume

filled with mercury at this pressure should be that of all

interconnecting pores having a diameter greater than about

200 ym. As discussed above, low pressure data are subject to a

number of random errors, and the data recorded at 1.2 psia

reflect this scatter as well as possible variations in the

samples.

16
The results of these analyses for 16 samples of the composite sheet

and six samples of the laminated block are given in Table II. All curves were

normalized to the same end point.

Table II. Four Parameters Obtained from Mercury Intrusion Curves

For 16 Samples of PTFE-C Sheets and 6 Samples of Bloc

1 . 3 Specific Pore Volume (cm /g)

Sheet (16) Block ( 6 )

Mean 1.57 1.56

SD 0.09 0.05

SD/Mean 0.06 0.03

High-Low 1.69 - 1.41 1.63 - 1.48

2 . Pressure at 0.1

3

cm Intruded Pore Volume ( psi a

)

Mean 5.6 6.5

SD 0.4 0.2

SD/Mean 0.07 0.03

High-Low 6.4 - 4.8 6.9 - 6.2

3. Fraction of Pore Volume Filled Between P = 4 and P = 8 psia

Mean 0.51 0.43

SD 0.03 0.05

SD/Mean 0.06 0.12

High-Low 0.56 - 0.46 0.50 - 0.38

4. Volume of Pores Fill ed at P = 1.2 psi

a

(cm

3

)

Mean 0.018 0.026

SD 0.006 0.008

SD/Mean 0.33 0.31

High-Low 0.033 - 0.009 0.038 - 0.016

17
E. Discussion

Because the number of laminated block samples Is much smaller

than the number of sheets, it is difficult to make quantitative

comparisons between the two forms of the composite material. However,

certain generalities appear to be justified even from this small

sampling of what presumably is a very much larger possible number

of samples in each lot of material. There is apparently no significant

difference in the specific pore volume between block and sheet

composite, and the differences, if any, between the two forms with

respect to properties 2, 3, and 4 are small. For example, the total

range of values of property 2 for the composite in sheet form is greater

than the difference between the mean values for block and sheet, and

it includes the mean of the block samples. For property 3, the mean

value for the block samples lies just outside the total range for the

sheet samples. For property 4, the data are widely scattered for

both sets of curves. It is doubtful that this scatter truly

represents differences in the volume fraction of very large pores,

because of the instrumental effects discussed above. Mercury

porosimetry is not capable of accurately measuring the size of very

large pores.

In our second Annual Report (Ref. 2, p. 19), we presented a

graph showing the effect of sample compression on a mercury intrusion

i curve for the composite material. As a sample is compressed, the

specific pore volume decreases, and the intrusion curve is shifted

to the right. The pore size distribution obtained from such curves

18
Is also shifted toward smaller average pore sizes than for the uncompressed

material. This is hardly surprising, since we know how the samples were

treated to produce this result. We might also ask a converse question. The

data of Table II indicates a range of specific pore volumes for the "asreceived" material. We might ask whether the smaller specific pore volumes

are correlated with smaller average pore sizes, as evidenced by a shift

of the intrusion curves to the right. It is difficult to compare these

curve shifts in a quantitative way, because the various curves have somewhat

different shapes. However, a rough comparison of the curve shifts can be

made by comparing the specific pore volumes with parameter 2 in Table II,

3

the pressure corresponding to 0.1 cm intruded pore volume, which occurs on

a steep slope of the intrusion curve and is a rough measure of the overall

curve position.

In Table III, the specific pore volumes of all 16 samples of sheet

composite are arranged in ascending order. The corresponding P at 0.1 cm

intruded volume is shown in the second column. There is a high correlation

between the two parameters. For example, the lowest three specific pore volumes

correspond to the highest three values of parameter 2. Also, the lowest three

values of parameter 2 correspond to numbers 11, 13, and 15 in ascending

specific pore volume. A similar high correlation was found for the six

samples of block composite. For a given porous material, therefore, it

is apparent that the specific pore volume correlates well with the position

of the mercury intrusion curve along the pressure axis.

19
Table III. Correlation of Specific Pore Volume,

with P at 0.1 cm

3

Intruded Volume, for PTFE-C

3

Specific Pore Volume (cm /g)

(ascending order)

1.37

1.41

1.48

1.52

1.54

1.55

1.56

1.59

1.59

1.61

1.63

1.64

1.65

1.67

1.68

1.69

3 Corresponding P at 0.1 cm Intruded Volume (psia)

6.4

6.3

6.1

5.8

6.0

5.6

5.6

5.3

5.8

5.75

5.1

5.45

4.95

5.35

4.85

5.75

One may question whether the porosity at the surface of a porous

material, particularly one as soft as the PTFE-C composite, is representative

of the interior porosity. Certainly, it is possible to occlude surface pores

in the process of cutting, packaging, and handling the material. It has been

q reported that the tissue ingrowth process may be enhanced by "teasing" the

surface before implantation in order to expand the surface pores. While this

may be a useful clinical procedure, it cannot be a part of the porosity

characterization process unless some way is found to treat all samples

uniformly.

From the high-low values of each parameter given in Table I, it is

clear that one mercury intrusion experiment may not be a very good representative of a given lot of porous material. Whenever possible, several

20
replicate measurements should be made. Automated porosimeters, which are

commercially available, should eliminate operator bias and reduce the effort.

If not the time, expended In gathering the necessary data.

SUMMARY

The study of porosity characterization methods has been concluded

with the following results:

(1) Quantitative Microscopy

Ten more photomicrographs of PPE have been analyzed, bringing the total

number to 30. A sample of PTFE-C was prepared for optical analysis by

filling it with a thermosetting resin and polishing a cross-section. The

mean values of the pore volume fraction and mean intercept length in the

two materials were compared, using 30 photomicrographs of each material.

The pore volume fractions were in good agreement with the values obtained

by other methods. The mean intercept lengths in PPE and PTFE-C were found

to be, respectively, 76 ym and 67 ym. The data were widely scattered,

indicating that the surfaces were quite nonuniform on the dimensional

scale of the areas sampled.

(2) Mercury Porosimetry

The mercury intrusion studies were concluded with the examination of

16 samples of the PTFE-C composite in sheet form and 6 samples of the

laminated block. Four parameters were selected arbitrarily for statistical

analysis of these data. If was concluded that little, if any, difference

exists in the porosity of the sheet and block forms of this material,

although the data suggest that the average pore size may be slightly smaller

in the block material. A high degree of correlation was found in both

21
materials between the specific pore volume and the overall position of the

mercury Intrusion curve along the x (pressure) axis, with the smaller

specific pore volumes associated with smaller average pore "diameters".

It was concluded that one mercury intrusion curve may not represent a

given lot of material very well, and that several samples from a given lot

should be examined.

22
REFERENCES

1. Dehl , R. E., Grant, W. H., and Cassel , J. M. , "Evaluation of Methods

of Characterizing the Porosity of Porous Polymeric Implant Materials:

A Review of the Current Status of Porosity Measurements", NBSIR 81-

2212. Annual Report prepared for the Bureau of Medical Devices, Food

and Drug Administration, Silver Spring, MD (February 1981).

2. Dehl, R. E., Grant, W. H., and Cassel, J. M., "Characterization of

Porosity in Porous Polymeric Implant Materials", NBSIR 81-2459.

Annual Report prepared for the Bureau of Medical Devices, Food and

Drug Administration, Silver Spring, MD (February 1982).

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Implant Materials", NBSIR 83-2645. Annual Report prepared for the

Office of Medical Devices, National Center for Devices and Radiological

Health, Food and Drug Administration, Silver Spring, MD (February 1983).

TM

4. Castolite Resin, trademark of Buehler, Ltd.

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6. "Polymer Handbook", eds. J. Brandrup and E. H. Immergut, Second Ed.,

Wiley-Interscience (New York).

7. Superpressure, Inc., Silver Spring, MD.

8. Dow Corning Wright, Inc., Memphis, TN.

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13, 677 (1979).

23

U.s. DEFiT. OF C

- 'MM. BIBLIOGRAPHIC DATA

SHEET (See in struction s)

1. PUBLICATION OR

REPORT NO.

NBSIR 84-2883

2. Performing Organ. Report No. 3. Pub! i cation D ate September 1984

4. TITLE AND SUBTITLE

Characterization of Porosity in Porous Polymer Implant Materials

5. AUTHOR(S)

R. E. Dehl

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Annual Report for the

Period April-Sept. 1983

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Office of Medical Devices

National Center for Devices and Radiological Health

Food and Drug Administration

Silver Spring, MD

10.

SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT ( A 200 -word or less factual summary of most significant information. If document includes a si gni fi cant

bi bliography or literature survey, mention it here)

Our investigation of the methods of characterizi ng the porosity of two

porous polymeric implant materials have been concluded with the work discussed

in this report. The two materials, a porous polyethylene (PPE) and a porous

composite of polytetrafl uoroethylene and carbon (PTFE-C) have been further

investigated by the method of quantitative microscopy. The mean pore volume

fractions of 30 samples each of PPE and PTFE-C were found to be 0.48 and

0.69, respectively, and are in good agreement with other measurements of this

quantity. The mean intercept length for PPE was found to be 76 um, and for

PTFE-C, 67 urn. Both values are somewhat larger than the average interconnecting pore "diameters" as measured by mercury porosimetry. The

reproducibility of mercury porosimetry data has been tested by examining

16 samples of PTFE-C in thin sheet form and 6 samples of laminated blocks.

The mean and standard deviation of four parameters derived from the mercury

intrusion curves were calculated. A high correlation was found between the

specific pore volume and the position of the mercury intrusion curve along

the pressure axis. A number of random errors pertaining to mercury

porosimetry were discovered and are discussed in this report.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key .-. - -

, Mercury porosimetry; porous implants; porous polyethylene; pore size; pore

volume; PTFE-carbon composite; stereology.

13. AVAILABILITY 177“'

IXXj U n I i mi ted

; y For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. — - 20402. 15.

]X}(!Order From National Technical Information Service (NTIS), Springfield, VA. 22161