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NBSIR 81-2459

Characterization of Porosity in

Porous Polymeric Implant

Materials

i

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Dental and Medical Materials

Polymer Science and Standards Division

Washington, DC 20234

February 1 982

Annual Report for Period October 1980 - September 1981

Prepared for: Bureau of Medical Devices

Food and Drug Administration

Silver Spring, Maryland 20910

GO

NBSIR 81-2459

CHARACTERIZATION OF POROSITY IN

POROUS POLYMERIC IMPLANT

MATERIALS

R. E. Dehl, W. G. Grant and J. M. Cassel

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Dental and Medical Materials

Polymer Science and Standards Division

Washington, DC 20234

February 1 982

Annual Report for Period October 1980 - September 1981

Prepared for: Bureau of Medical Devices

Food and Drug Administration

Silver Spring, Maryland 20910

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

Page

ANNOUNCEMENTS v

ABSTRACT vi

I. INTRODUCTION 1

II. PORE VOLUME 3

A. Apparent Density Method 3

B. Compression Method 6

C. Mercury Intrusion Method 7

D. Summary 8

III. PORE SIZE 9

A. Mercury Intrusion Porosimetry 10

1 . Theory 10

2. Experimental 13

3. Porous Polyethylenes 14

4. PTFE-Carbon Composite 16

B. BET Method 21

1. Theory 23

2. Experimental Results 25

3. Assumptions in the BET Equation 26

4. Effects of Pore Shape 26

IV. ASTM DRAFT STANDARDS FOR POROSITY CHARACTERIZATION OF PTFE-CARBON

COMPOSITES 28

A. Volume Pores 29

B. Pore Size Range 29

V. SUMMARY 31

REFERENCES 33

l
FIGURES

Page

Figure 1. Mercury intrusion curve for Plastipore porous

polyethylene 15

Figure 2. Pore size distribution in two porous implant materials. 17

Figure 3. Mercury intrusion curve for porous polyethylene "small

pore size" from Glasrock Corp 18

Figure 4. Effect of bulk compression on mercury intrusion curves

of PTFE-carbon composite 19

Figure 5. Mercury intrusion curves for PTFE-carbon composite,

before and after compression at 1.2 atm 22
TABLES

Page

Table 1: Void Volume Percentage in Two Porous Implant

Materials 9

Table 2: Shape Factors for BET Pore Size Calculations 27
APPENDICES

Appendix I. Revised manuscript, "On the Characterization of

Porosity in PTFE-Carbon Composite Implant

Materials by Mercury Porosimetry. "

Appendix II. BET surface area analyses for Proplast PTFE-

. ®

carbon composite and Plastipore porous

polyethylene, obtained from Quantachrome

Corporation

Appendix III. Letter from R. E. Dehl to C. A. Homsy concerning

ASTM Draft Standard Specifications for porosity

characterization of PTFE-carbon composite implant

material s

Pa^e

35

46

49

IV
ANNOUNCEMENTS

1. Certain commercial materials, instruments, and analytical

laboratories 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, instruments, or laboratories

are necessarily the best available.

2. In our first Annual Report to the Bureau of Medical Devices,

Food and Drug Administration, entitled "Evaluation of Methods of Characterizing the Porosity of Porous Polymeric Implant Materials: A Review

of the Current Status of Porosity Measurements" (NBSIR 81-2212, February,

1980), we have inadvertently reproduced certain copyrighted materials.

Pages 11 to 14 of this report consist of information contained in two

copyrighted publications of the Richards Manufacturing Company (Refs. 18

and 20 in the Annual Report). We also acknowledge that the trade names

Plasti-Pore and TORP are registered trade names of the same company,

TM and Tilt-Top is a trademark of the company. We regret any inconvenience

to the Richards Manufacturing Company resulting from these accidental

oversights.

3. We also wish to call the reader's attention to one paragraph of

^ Richards' Technical Publication No. 4240, "Plasti-Pore Material Technical

Information." On page three of this publication, under the heading

"Biocompatibility," two statements are attributed to the National Bureau

of Standards. These are as follows:

(1) "Because of the near total inertness of ultra high molecular

weight polyethylene, the National Bureau of Standards (NBS)

v
has established it as the standard to which all other implant

and tissue contact polymeric materials are compared."

(2) "... the NBS considers, by definition, UHMWPE to be the most

inert implant material that can be used as a reference."

Neither of the above opinions (1) or (2) has ever been expressed or

implied by any employee of the National Bureau of Standards. NBS has

never performed or sponsored any research or tests which could lead to

the above conclusions.

vi
ABSTRACT

In this report, we describe the application of several methods of

porosity characterization to two commercial polymeric implant materials,

a porous polyethylene and a composite of polytetrafluoroethylene and

carbon. In exploring the use of mercury porosimetry to determine interconnecting pore size distributions in the composite, we have found that,

contrary to widespread belief, this rather soft and deformable material

was not distorted by the mercury pressures required to measure its pore

size distribution. The mercury intrusion technique indicated that the

porous polyethylene has a narrower distribution of pore sizes than the

composite. In the polyethylene and the composite, the weighted average

interconnecting pore "diameters" were found by this technique to be,

respectively, 30 pm and 50 pm. The average pore diameters were also

estimated by measurement of specific surface area and pore volume.

Assuming a spherical pore shape, the average pore diameters in polyethylene and composite were calculated to be, respectively, 60 pm and

25 pm. The accessible pore volumes of the materials were measured by

three different techniques: a simple "apparent density" method, mercury

intrusion, and bulk compression, the last for the composite only. For

the polyethylene and composite we found pore volumes of about 45 and

70 percent, respectively.

v i i

1

I. INTRODUCTION

During the first half-year of this project, which was begun in

May 1980, efforts were directed to a comprehensive search of the existing

published literature pertinent to the characterization of porosity in

porous materials, with emphasis on methods appropriate to polymeric

porous solid materials used as surgical implants'*'. The following year's

efforts, which are described in this report, involve experimental approaches

to the measurement of the volume of accessible pores in porous implant

materials, and the size and size distribution of such pores.

It is important to be aware that there is a fundamental difference

between the measurement of pore volume and pore size for the porous

implant materials, in that the pore volume is a well defined physical

property of the materials, whereas the pore "size" is not. For this

reason, it is proper to refer. to the accuracy of experimentally derived

pore volumes; however, the word "accurate" has no well-defined meaning

when applied to the measurement of a poorly defined physical property

such as size or "diameter" of objects having any shape other than circular.

In order to give physical meaning to the term "size", as it applies to

irregularly shaped objects such as the pores in manufactured porous

materials, one must in general make some arbitrary assumptions about the

shape of the pores, realizing that other equally arbitrary assumptions

may lead to different "size" parameters.

A consequence of this dilemma is that there is no reason to believe

that different methods of measuring pore "size" will produce comparable

results. Every method either requires arbitrary assumptions about pore

shape or contains no information about the shape, as in the case of

certain optical image analyses. Certainly, pore shape is expected to
have some effect on the rate of tissue ingrowth, and one feels intuitively

that it should not be ignored. However, it is not clear that any particular

shape best describes the irregular pores in the porous implant materials

discussed in this report. It is clear that measured pore "sizes" or

"diameters" of irregularly shaped pores should always be accompanied by

a statement about how the numbers were obtained and what assumptions

were made about pore shape.

The activities of the past year may be divided into six categories,

as follows:

(1) Continuation of the acquisition of reprints and other information

pertinent to the use of porous polymeric implant materials.

(2) Attendance by W. Grant and R. Dehl at a one-day seminar concerning

surface area and porosity measurements, sponsored by the

American Chemical Society and presented by the Quantachrome

Corporation. Lectures and laboratory demonstrations were

presented.

(3) Laboratory studies of three porous polymeric implant materials,

using mercury porosimetry to estimate pore size distributions

and intruded void volumes. A publication has resulted from

the results of some of this work.

(4) Surface area measurements on two porous implant materials

and estimation of the average pore diameters.

(5) Accessible void volume measurements by three different techniques.

(6) Correspondence with Dr. C. A. Homsy and others concerning our

work and its possible application to current Draft Standards

of the American Society for Testing and Materials (ASTM) for

porosity characterization of implant materials.
Items (3) to (6) are discussed in the following Report. The reader

should be reminded that the work described here is ongoing, so that

extensions or modifications of the data may be necessary in the future.

II. VOID VOLUME MEASUREMENTS

As we have indicated above, the volume of accessible pores in the

porous implant materials is a well-defined physical property and is,

therefore, subject to measurement by one or more physical methods. In

the following, we describe the application of three different void volume

* measurements to two commercial implant materials, a porous polyethylene

** and a porous composite of polytetrafluoroethylene (PTFE) and carbon .

A. Apparent Density Method

The simplest way to measure the void volume of a porous material,

if the density of its solid phase is known, is simply to weigh a sample

of well-defined geometry and calculate the total volume of the specimen

from its measured dimensions. The volume of the solid phase is calculated

from its weight and density and subtracted from the total volume. The

difference between the total and solid volumes is the void volume, which

may be expressed as a percentage of the total.

Porous Polyethylene

A sample of porous polyethylene in the form of a disc, 5.4 cm in

diameter and 1 . 0 cm thick, was used to determine the void volume by this

method. Assuming the density of polyethylene in this "high density"

3 material to be 0.94 to 0.96 g/cm , we determined the volume of pores to

be 44 to 45 percent of the total. In the product literature furnished

2 by the manufacturer of this material , the void volume is stated to be

* ® Plastipore , registered trademark of the Richards Manufacturing Company.

** @ Proplast , registered trademark of Vitek, Inc.
"approximately 35 percent." Because we had only one sample of this

material in the form of a regular geometrical shape, we can only report

one value of the density. We were told by the manufacturer of this

material that all of their porous polyethylene has the same porosity,

from which we assume that our measured void volume is representative of

the company's material, but we cannot as of this writing report a range

of sample-to-sample or lot- to- lot variability. However, the value

obtained here may be compared with our mercury porosimetry value, as

reported below, for a sample derived from a different disc of material.

PTFE-Carbon Composite

The void volume of this material was determined from a rectangular

3 block, 1.5 x 4.0 x 1.0 cm . The density of solids in this material is

complicated by the fact that there are two solid components present, and

the ratio of the two is unknown. According to handbook values, the

density of graphitic carbon is 2.25 g/cm and the density of crystalline

4

PTFE (amorphous form of PTFE never observed) is 2.27 to 2.30. From

these data, the density of solids would have to lie within the range

3

2.25 to 2.30. Using a value 2.27 g/cm for the density of the composite,

we calculate that 82 percent of the sample volume is due to the volume

of pores. This value falls approximately in the middle of the range

specified by the manufacturer (70 to 90 percent) in the package inserts

accompanying this material.

As a routine check on the assumed value of the density of solids in

the composite, we have measured this density and found it to be much less

than the assumed value. Samples of the composite in the form of 10 mm

thick block were compressed between smooth metal plates in a hydraulic

press, and regular geometrical samples (circular discs and rectangles)

were cut from the compressed sheets to measure the density directly from
5

the sample weight and dimensions. While it was, of course, expected

that the measured density would increase with increasing applied pressure,

it was found that the density of a compressed sample remained unchanged,

above the lowest pressure at which a measurable reading could be observed

on the hydraulic pressure gauge, (about 3 x 10^ Pa). In fact, the material

is quite easily compressed simply by applying finger pressure, and it is

not surprising that relatively low hydraulic pressure is required to

drive out the remaining airspace.

After several measurements of the density, we concluded that,

rounded to two significant figure accuracy, the density of solids in the

3 composite is 1.4 g/cm . Using this value for the density, we calculated

that void volume of the rectangular block of composite was 70 percent.

This figure is only 5 percent greater than the value reported by Spector

5

et al. using an optical point counting technique. It is apparent that

part of the discrepancy between measured and assumed densities can be

accounted for by the fact that the carbon used in the composite was of

the "pyrolytic" type , which according to Bokros et al . has a density

3

of 1.5 to 2.0 g/cm . However, this factor alone would not be sufficient

to account for a composite density of 1.4. There is probably some

included air trapped in the PTFE solids which lowers its apparent density

somewhat. As of this writing, we do not feel that we have entirely

accounted for the large discrepancy between measured and assumed densities.

As part of the procedure, one sample of the PTFE-carbon composite

o was sent to a commercial analytical laboratory for elemental analysis

of carbon, hydrogen, and fluorine. This analysis was intended to verify

that the material was composed of the two solid materials PTFE and

elemental carbon, and if so, to determine the weight ratio of the two

materials. The following elemental analysis was determined:
% C ~ 52.81

% H -- less than 0.01

% F — 47.10

Since 99.91 percent of the total weight is accounted for in this analysis,

the data are consistent with a PTFE-carbon composite with less than

0.1 percent of other elements present. By elementary stoichiometric

analysis, these data indicate that the weight fractions of PTFE and

carbon in the composite are, respectively, 0.62 and 0.38.

B. Compression Method

In the current ASTM Draft Standards for porosity characterization

of PTFE-Carbon composites (see Section IV) the suggested method for void

volume measurement of this material is a "confined pressurization"

technique. While the experimental details of this method are not given,

the implication is that a sample is simply confined in such a way that

it cannot spread out as it is compressed, so that the change in sample

thickness is a direct measure of the void volume originally present. We

did not confine the rectangular sample in our experiment, but we allowed

it to spread out slightly, still retaining a regular geometrical shape.

In this way, the change in sample volume could be observed directly.

For one sample, we measured a void volume of 68 percent in this way,

which is only 2 percent less than the value obtained by the similar

apparent density method described above. As we have described them,

both of these methods require the measurement of sample dimensions, and

these are necessarily somewhat inaccurate due to small irregularities in

the sample shape. Also, of course, the compression method requires that

the void can be expelled quantitatively at pressures accessible with a

hydraulic press. For materials less compressible than the PTFE-carbon
7

composite such as porous polyethylene, it would undoubtedly be necessary

to melt the material during compression in order to drive out all the

air. For this material, it may be preferable to use a liquid uptake

technique such as mercury porosimetry to measure the void volume, as we

describe below.

C. Mercury Intrusion Method

Mercury intrusion porosimetry is a well-established technique for

characterizing the porosity of porous materials (see Section I I I A )

.

Both the pore size distribution and the void volume may be measured by

this technique. The volume of mercury intruded into the sample is

measured on a graduated dilatometer stem, thus giving a direct measure

of the accessible void volume of the sample. In all of the work reported

here, the mercury intrusion pressure was varied from 0-1 atmosphere

(0 to 1.01 x 10

5 Pa).

Porous Polyethylene

From the shape of the mercury intrusion curve for this material, as

shown in Fig. 1, it is apparent that most of the void volume was intruded

at one atmosphere pressure. Two samples of the material were cut from

the same circular disc for porosimetry analysis. If we assume that the

density of solid polyethylene is 0.95, the void volumes for the two

samples were calculated to be 42.6 and 45.6 percent. The average of

these (44.1 percent) is within the range of values found above by the

apparent density method for this material. It should be noted that this

method, at least in principle, appears to be the most accurate of the

three methods discussed here for determining the specific void volume

3

(cm /g of sample), since both sample weight and void volume may be

directly measured to three significant figures. The other methods

contain unknown inaccuracies both in their total measured volumes, due
8

to sample irregularities, and in their measured or assumed densities of

the solid phase. For these reasons, the mercury intrusion method would

appear to be the method of choice for measurement of specific pore

volume. However, if one desires to express the sample void as a percentage of the total, it is necessary to measure or assume a density of the

solid material which introduces possible errors not present in the

specific void volume measurement.

PTFE-Carbon Composite

For the composite material, it is apparent from the mercury intrusion

curve (Figs. 4 and 5), that the maximum applied pressure (1 atmosphere) was

not sufficient to fill all of the void volume. Therefore, we would expect

the measured void to be less than that found by the other two techniques.

Indeed, assuming a density of 1.4 for the solids, we found an average void

volume of 67 percent for five samples of the material, as compared with

70 percent measured by the apparent density method. If both figures are

accurate, the implication is that 95 percent of the total void was

filled with mercury at one atmosphere pressure. To complete the intrusion

experiment, it will be necessary to increase the pressure above one

atmosphere. These experiments will be performed during the next phase

of our experimental work.

D. Summary

During our first year of experimental work, we have measured the

void volumes of two porous polymeric implant materials, porous polyethylene and PTFE-carbon composite, in three different ways. The simplest

method, which we have chosen to call the apparent density method, requires

only the sample weight, total volume (which is determined by direct

measurements on a regular-shaped specimen) and a knowledge of the density

of the solid phase. The compression method measures the void directly
9

by the changes in one or more linear dimensions of the solid as the

sample is hydraulically compressed. The mercury intrusion method also

measures the void volume directly by dilatometric observation of the

mercury intruding the sample void as pressure is applied. Table I

summarizes the results found by these three methods.

Table I: Void Volume Percentage in Two Porous Implant Materials

Method Solid Density Void %

PE

1

2 Composite PE Composite

Apparent Density 0.94-0.96 1.4 44-45 70

Compression — — — 68

Mercury Intrusion 0.95 1.4 44 >67

assumed

2 measured

III. PORE SIZE

As we have discussed above, "size" is not a well defined physical

property, when one speaks of irregularly shaped objects such as the

pores in porous implant materials. Nevertheless, various methods are

available for estimating the size of pores, given that this is a somewhat

vague concept. A summary of the important direct methods for measuring

9

pore size is given in a review article by Rootare . The methods include

optical microscopy, electron microscopy, gas adsorption and condensation,

mercury porosimetry, water suction pressure measurement, and small angle

x-ray scattering. Of these, gas adsorption and small angle x-ray scattering can only measure pore diameters of less than 100 nm (0.1 pm), and
are therefore inappropriate to the pore size range (> 10 pm) which

applies to porous implant materials. Electron microscopy, while providing

a good picture of the structural morphology of materials, is not appropriate for quantitative analysis of pore sizes, because it presents a plane

projection of three-dimensional features of the materials. In order to

obtain information about the size of three-dimensional objects such as

pores from a photograph, it is necessary to know that one is looking at

a flat, two-dimensional cross section of the material. This may be

accomplished by optical microscopic examination of polished, flat sections.

The water suction method depends on the fact that water suction

pressure is related to the size of pore openings. This method only

senses pores which are interconnected in such a way as to pass entirely

through the material, and it does not give a measurement of the fraction

of total pore volume having a given pore size. Therefore, it would

appear to have little value for studying the pore size distribution of

the porous implant materials. Of the above techniques, only optical

microscopy and mercury porosimetry appear to have potential for studying

the materials of interest to this project. In the following, we discuss

the application of mercury porosimetry to this problem and compare

results obtained in this way with an indirect method which depends upon

measured values of specific pore volume and surface area. Future studies

will compare both of these methods with optical microscopy measurements.

A. Mercury Intrusion Porosimetry

1 . Theory

Mercury intrusion porosimetry is a well-established technique

for measuring the pore size, size distribution, and void volume of

porous materials. A number of articles have been written about the
11

9-13

method and the precautions which one should take in interpreting the

data. The method depends upon some unique physical properties of liquid

mercury, namely its non-wetting of surfaces and its high surface tension,

which require high pressure to force the liquid into various pores. The

diameter of an intruded pore and the pressure required to force intrusion

are inversely related, and for cylindrical pores of circular cross-section

the relation between applied pressure (P) and pore diameter (D) is given

by the well known Washburn equation,

PD = -4ycos0,

where y and 9 are the surface tension of mercury, 484 dynes/cm (Appendix I,

ref. 9), and the surface contact angle on the material of interest.

For cylindrical pores having non-circular cross-sections, the

pore "diameter" is not defined, and the equation may be expressed in a

more general form involving ci rcumference and area of the cross-section.

However, when one considers applying the porosimetry technique to materials

having multitudes of irregular pore shapes, it becomes obvious that no

equation can correctly describe the entire material or even one pore.

For this reason, the usual method of treating mercury porosimetry data

is to use the simplest (Washburn) equation, realizing that there is no

obvious way to improve on it for a real system of irregular pores. The

"average" pore diameters obtained in this way nevertheless provide a

useful method for rough, semi-quantitative comparison of different

porous specimens. In fact, the experimental plot of applied pressure vs

intruded volume is itself a useful "fingerprint" of a material which can

be used to distinguish one material from another and to see variations

in pore size distribution for a given material.
12

It must also be considered that the mercury intrusion method

has a built-in bias which is usually referred to as the "ink-bottle"

pore effect. This effect is illustrated in the following diagram.

As mercury enters the narrow opening of this pore (Vp, the pressure

required is governed by that opening. The entire volume of the pore

(V 1+V

2

) is measured as though it had the narrow entrance to V.| . In a

pore pathway with many constrictions, the narrowest one governs the

pressure required to intrude all the pore void beyond that point. For

this reason, it is customary to indicate that mercury porosimetry measures

"interconnecting" pore diameters rather than the true pore size distribution. However, two factors tend to minimize the errors introduced by

this effect for the porous implant materials. First, because these

materials have a very open network structure, mercury can often find its

way into a large interior pore through several different routes, avoiding

the narrowest constrictions in the pathway. The intruding liquid always

takes the path of least resistance (largest diameter) when entering the

porous structure, and it can enter from all sides of the sample at once.

Second, remembering that the ultimate objective of this work is to be able

to correlate measured pore "diameters" with the performance of the materials

as tissue ingrowth media, it might be argued that the constrictions in the

pores measured by mercury porosimetry are more important in governing the

rate of tissue ingrowth than the larger pores between the constrictions.

ft j
In other words, it may be more meaningful to measure the interconnecting

pore size distribution rather than the "true" distribution. In any

case, it would not appear that the "ink bottle" effect should be regarded

as reducing the usefulness of mercury porosimetry.

Another source of possible error in the Washburn equation is

the choice of contact angle. Many workers use a constant value of this

angle (usually 130° or 140°), realizing that most measured values have

fallen in this range. It is possible to measure the contact angle with

a goniometer instrument, and this measurement was performed on the

composite material, as discussed below.

2. Experimental

* The mercury porosimeter used in this work was equipped with a

"macro" filling accessory, which allows the graduated penetrometer to

remain in a horizontal position during the experiment. This facilitates

reading the graduated stem and eliminates a large correction for the Hg

head pressure when the sample is held in a vertical position. The

sample chamber is first evacuated to a pressure less than 50 pm of Hg.

One or two hours was usually allowed for evacuation, even though the

manufacturer suggests a half-hour evacuation. It must be remembered

that the mean free path of gas molecules in a porous matrix can be much

less than that in a surrounding chamber, and it is essential for the gas

to diffuse out of the sample before the mercury intrusion is begun. The

filling device is then tilted to immerse the open stem of the penetrometer

into a mercury pool. A small pressure, usually about 0.5 psi (3.5 x

3

10 Pa), is then admitted to the mercury chamber, causing the mercury to

enter and fill up the penetrometer. At this point, the sample in the

* Model J5-7121-B, supplied by the American Instrument Co.
penetrometer is surrounded by mercury, but little if any has entered the

porous matrix due to the very small applied pressure. The pressure is

then increased in steps, and the volume of mercury entering the sample

3

is read on the penetrometer stem, which can be interpolated to 0.001 cm .

The total void volume which can be measured by our penetrometer is 0.2

3 cm , so that the sample specimens are typically quite small (< 0.1 g).

Data were plotted, as is the custom, on semi -log paper with intruded

volume on the ordinate and pressure on the abscissa.

The macro-filling device used in this work can only measure

pressures by admission of air, up to one atmosphere. For higher pressure

measurements, which will be needed to complete some of the mercury

intrusion curves, it will be necessary to insert the penetrometer into

the high pressure chamber and apply hydraulic pressure, which is transmitted to the sample by fluid. The volume of mercury intruded is measured

by remote control rather than direct observation. These experiments

will be performed during the next phase of our studies.

Samples of the porous materials were in most cases cut from

as-received blocks or discs of the material, using a sharp surgical

scalpel with removable blades. It was found that all of our materials

could be cut without compressing the materials.

3. Porous Polyethylenes

Two samples of the porous polyethylene implant material were

examined by mercury porosimetry, and their intrusion curves were very

similar in shape. One of these is shown in Fig. 1. This curve is also

2

very similar to one published by the manufacturer , who found that a

majority of pores lay in the range of 18 to 35 pm diameter as calculated

from the Washburn equation, using a 130° contact angle. According to
15

(r) Figure 1. Mercury intrusion curve for Plastipore porous polyethylene.
our curves, almost- 80 percent of the total pore volume consists of pores

in the range 20 to 40 pm, and we are therefore in good agreement with

the manufacturer about the pore size distribution of this material.

This distribution is plotted as a bar graph in Fig. 2, together with

that of the PTFE-carbon composite.

Figure 3 is a mercury intrusion plot for another sample of

porous polyethylene obtained from Glasrock Products, Inc. The pore size

was not specified for this material, but simply designated "small pore

size". Clearly the interconnecting pores are larger in this material

than in the one discussed above, with most of the pore volume consisting

of 50 to 100 pm pores. The porosimetry plots for the two polyethylenes

illustrate quite well how this technique can distinguish between

different porous materials, even without application of any assumed

equation relating applied pressure to pore size.

4. PTFE-Carbon Composite

In Fig. 4 are shown three curves illustrating the effects of

bulk compression of the composite material. The samples were sliced

from a 10 mm thick block. Curve A, the uncompressed composite, clearly

has a broader distribution of interconnecting pore diameters than the

porous polyethylene illustrated in Fig. 1. From Fig. 2, it is seen that

approximately half of the pore volume in the composite consists of 20 to

40 pm pores, with most of the remaining volume having larger than 40 pm

pores, as calculated from Washburn's equation, using a 146° contact

angle for mercury on the composite. The contact angle was measured on a

compressed sample of the composite, using a commercial contact angle

goniometer. The range of contact angles measured with this instrument
17

Pore Size Distribution In

Two Porous Implant Materials

Composite (PTFE + Carbon)

| | Polyethylene

Pore Size (jj. m)

Figure 2. Pore size distribution in two porous implant materials.
Figure 3. Mercury intrusion curve for porous polyethylene “small pore

size" from Glasrock Corp.
19

Figure

4.

Effect

of

bulk

compression

on

mercury

intrusion

curves

of

PTFE-carbon

composite.

Curves

normalized

to

same

sample

weight.
was approximately 5°, with 146° representing an average. Even a 10°

error in this angle (135° vs 145°, for example) only introduces a 13 percent error in the value of cose, so that it is not usually considered

necessary, given the other unknown errors in using the Washburn equation,

to make very accurate measurements of 0.

Effects of Sample Compression

Curves B and C in Fig. 4 illustrate the effect of bulk compression

of this material on the observed mercury intrusion plots. All three

curves in the figure are normalized to the same sample weight. By this

procedure, it would be easy to see that B and C represent compressed

samples, even if they were not so labeled. It is apparent that mercury

porosimetry offers a convenient method of determining whether or not a

sample of this material has been accidentally compressed. It is only

necessary to have an intrusion curve for the non-compressed composite

available. The corresponding curve for the "unknown" sample, normalized

to the same weight as the known sample, will lie below the known sample

curve if compression has occurred. From the difference between intruded

volumes of the end point (plateau) of the intrusion curves, the amount

of sample compression can be estimated. In this way, if the pore volume

of the known sample was determined, one could discover whether the pore

volume of a "compressed" block of this material falls within acceptable

limits, as stated by the manufacturer. There are, of course, other

methods of determining the accessible pore volume, as discussed above,

but mercury intrusion is the only method discussed in this report which

can determine the pore size distribution and pore volume simultaneously.

As discussed in our first Annual Report‘d, a controversy has existed

concerning the application of mercury porosimetry to the composite

material. Some workers have felt that the ‘relatively soft and deformable
material would be compressed by mercury pressure during the measurement,

thus giving lower values of pore size and pore volume than were present

in the "as-received" material. Because the composite is inelastic and

easily deformed even by finger pressure, this concern was certainly a

legitimate one. In order to determine whether sample compression occurs

during the porosimetry experiment, in which the samples discussed in

this report were only subjected to a maximum of one atmosphere pressure,

we piled weights equivalent to 1 . 2 atmospheres pressure on top of a flat

sample of the material in the form of 1 mm thick sheets. The mercury

intrusion curves for this sample and for another sample not subjected to

the compression treatment are illustrated in Fig. 5. The very small

differences in the two curves are less than the difference between the

uncompressed sheet and uncompressed block composite, (curve A of Fig. 4).

In fact, sample-to-sample variations of either block or sheet composite

are expected to be at least as great as the differences observed in

Fig. 5.

The above findings for the PTFE-carbon composite have been written

up, approved by the NBS Editorial Board (WERB), and submitted for publicati

as a Note to the Journal of Biomedical Materials Research. A copy of the

latest revised edition of this manuscript is reprinted in Appendix I.

B. BET Method

It is possible to estimate an average pore diameter in a porous

material from the measured specific surface area (S) and specific void

volume (V) of the material. If one assumes for simplicity that all the

pores are spherical in shape, the average pore diameter D is shown by

simple geometry to be equal to 6 M/S. For other regular geometrical

pore shapes, the diameter (or other linear dimension of the pore if its

cross-section is noncircular) is also proportional to V/S, but the
vui

\uii ;

P (psia)

1 - 2 3 4 5 6 7 8 910 20
proportionality constant varies with pore shape. This problem is discussed

in more detail below. We have already measured the specific volumes of

both the porous polyethylene and PTFE-carbon composite, as discussed in

Section II. The specific surface area may be determined by a technique

known as the BET method, named for its inventors, Brunauer, Emmett, and

Teller .

1 . Theory

The theoretical basis for the BET surface area measurement has

been well described in textbooks. Two examples are Refs. 10 and 15.

Only a brief summary will be given here. The method is based upon the

adsorption of an inert gas (usually nitrogen) on a porous solid material.

From the weight of the adsorbed gas and an assumed surface area covered

by one molecule of gas, one may calculate the total surface area covered.

This area, divided by the initial weight of the sample, gives the specific

surface area of the material. In practice, the problem is complicated

by the fact that the surface does not simply adsorb one monolayer of gas

molecules and stop. The amount of gas adsorbed increases as the pressure

is increased, and the BET theory uses Langmuir's kinetic arguments to

describe how the weight of the desired first monolayer of gas depends

upon the gas pressure, the total weight of gas adsorbed, and a constant

parameter which expresses the ratio of molecular escape probabilities,

related to the energies of adsorption, of the first adsorbed layer relative

to all subsequent layers. The BET equation may be expressed as the followi

1 W[(P L /P)-l] W C o

J m

C-l P_

W C P m o

( 1 )

where W is the total weight of gas adsorbed, Wm is the weight in the

first monolayer, P/P

q is the ratio of the applied gas pressure to the
24

condensation pressure P , and C is a constant which describes the difference between the adsorption energy of the first layer and the following

layers, all of which are assumed to be the same. In order to measure

the surface area, one first plots the experimentally obtained quantity

W[(P /P)-l] VS P/P

o’

obtain a straight line having the slope

and intercept

s =

C-l

W C m

l = W C m

( 2 )

( 3 )

The desired is obtained by solution of these equations, to give

W =

^ m s+i ( 4 )

Usually, three points are sufficient to give a good straight-line plot.

The sample surface area (S) is given by the expression

S = W NA

m

M

( 5 )

where N is Avogadro's number, A is the cross-sectional area of a gas

molecule, and M is the molecular weight of the gas.

In summary, the purpose of the BET plot is to extract W

m

, the

desired monolayer weight of gas, from the observed total weight of

adsorbate W. This adsorbed monolayer weight then tells us the total

surface area covered by gas molecules, each of which is assumed to cover

the same amount of surface area.
25

2. Experimental Results

One sample each of porous polyethylene and composite, the latter

1 fi in the form of 10 mm thick block, were sent to a commercial laboratory

for 3-point BET surface area measurements. The results of these analyses

are given in Appendix II. As indicated, the specific surface area of the

2 polyethylene was found to be 0.083 m /g, while the composite had a much

2 larger area, 0.39 m /g. The specific void volumes for the two materials

were discussed in Section II. Using the expression D = 6 V/S to calculate the average pore diameters, we find that the average diameter for

polyethylene is 61 pm, while the correspondi ng value for the composite

is 25 pm.

It is interesting that these relative values of average pore

diameters are reversed as compared with the average diameters measured

by mercury porosimetry (p. 16). The BET polyethylene value is higher

than the porosimetry value, and the composite BET value is lower than

the porosimetry value, each by about a factor of 2. Considering the

possible errors inherent in each method, it is perhaps only surprising

that the two techniques are in as close agreement as they are. The

errors in the mercury intrusion method have already been discussed in

Section I I I A . The errors in the specific void volume, we feel, are

small, as we discussed in Section II. The errors in the BET method are

of two types: (a) errors in the assumptions needed to validate Eq. (1)

and (b) assumption of an arbitrary "shape factor", which is necessarily

inaccurate for an irregularly shaped pore, in the equation relating pore

"diameter" to the ratio V/S. Each of these problems is discussed briefly

below.
3. Assumptions in the BET Equation

The gas molecules are adsorbed to a surface by relatively weak

dispersion forces, which are not necessarily constant over a given

surface, or from one surface to another. In the BET theory, it is

assumed that all molecules in the first layer have the same adsorption

energy, and that all molecules in subsequent layers have an adsorption

energy equal to the heat of liquefaction. Neither of these assumptions

is likely to be true for a real system, although it has generally been

found that in the range of relative gas pressure P/P

Q near the value

required for surface coverage (W/W

m =1), the measured BET C-values

usually give reasonable heats of adsorption^. There is also the problem

that lateral interaction energies between adsorbed molecules increase

with increasing surface coverage; however, this effect tends to be

cancelled by decreasing energy of attachment to the surface with increasing

coverage, assuming that higher energy surface sites are the first to be

filled.

One may only speculate about the effects of these variables upon

the experimental BET surface areas of the materials which we are studying.

Certainly the composite would be expected to have quite different gas

adsorption energies of its two component species. However, from the

above arguments it is not clear that this two-component phenomenon would

lead to greater inaccuracies in the measured surface area than in the

case of the porous polyethylene, even though the latter is a chemically

more homogeneous material.

4. Effects of Pore Shape

As in the case of mercury porosimetry pore size determinations, the

pore shape as well as size must be taken into account in calculating
27

pore sizes by the BET method. In the above calculation, we assumed for

simplicity that all pores had spherical geometry. For a real porous

material with irregularly shaped pores, such as the implant materials

under investigation, it is pointless to attempt to approximate the real

pore geometry with any specific regular pore shape. However, it is

interesting to note the effect of pore geometry on pore sizes estimated

by the BET method, because it gives one an idea of the range of pore

sizes which can be calculated from the same BET data.

The equation used above to calculate the diameter of spherical

pores by the BET method can be expressed more generally as

D = k V/S

where we define D to be either the diameter of a pore having a circular

cross-section or the side length of the cross-section of a flat-sided

pore. In the following table, we summarize the k values obtained for

certain regular pore shapes having cross-section dimension D and pore

length £.

Table II: Shape Factors for

BET Pore Size Calculations.

Pore Shape k

Sphere 6

Circular cylinder, £>>D

Circular cylinder, £=3D 4.7

Circular cylinder, £=D 6

Cube 6

Rectangular tube, square

cross-section, £=3D

4.7

Triangular prism, £=3D 7.6

Triangular prism, £=D 9

Flat box, 10D x 10D x D 2.4

Flat box, very thin, thickness D ^2

From this brief sampling of pore geometries we observe a range of k

values from 2 to 9. Our assumed spherical pore shape has a k value
approximately in the middle of this range, but there is no assurance

that this "average" value is best for any given porous material. It is

apparent from published SEM photographs of the porous polyethylene and

PTFE-carbon composite1 that their pore morphologies are quite different,

and it is, therefore, hardly to be expected that any assumed regular

pore geometry will apply equally well to both of the materials.

In comparing pore sizes as measured by mercury porosimetry and

BET methods, it should be remembered that the former method measures

"interconnecting" pore diameters, which are necessarily somewhat smaller

than the true pore diameters, whereas the latter method is not subject

to this systematic error. One might therefore expect the BET method, if

everything else were equal, to yield somewhat larger pore sizes than the

porosimetry method. This expectation was realized for the porous polyethylene, but not for the composite. The reasons for this anomaly are

not readily apparent; however, the above discussion is intended to show

that quantitative comparison of pore sizes as measured by the two methods

is a difficult matter.

IV. ASTM DRAFT STANDARDS FOR POROSITY CHARACTERIZATION

OF PTFE-CARBON COMPOSITES

The ASTM Task Force F4. 20. 04. 05 is presently drafting standards

entitled, "Standard Specifications for Porous Composites of Polytetrafl uoroethylene and Carbon for Surgical Implant Use." While the use of

trade names in this document was specifically prohibited by the Task

Force, the specifications apparently apply to one commercially available

composite (Proplast ) having the properties indicated in the title above.

The current draft standards were included in our Annual Report for 1980

1

on p. 17. In a telephone conversation with Dr. C. A. Homsy this year,
he asked for our opinions about these draft standards, and our reply is

given in Appendix III. A few additional comments may be added to this

letter, as follows:

A. 4.4."Volume Pores" - It would seem to be important to decide

whether the primary objective of this specification is to verify that

the void volume falls within the range given (70 to 85 percent) or

whether it is to suggest a method sufficiently accurate to detect small

differences between individual samples, as a quality control check on

their porosity. The compression method of measuring void volume specified

in this standard is described in Section I IB of this report. As we have

indicated, this method may be subject to measurement errors which make

it less accurate than some other methods. However, if the objective of

this standard is merely to verify that the pore volume is within 70 and

85 percent, our experience indicates that the compression method is

probably adequate to verify the specified volume range. The apparent

simplicity of the method is an appealing feature, although one might

wish for a more detailed description of the "confined pressurization"

method named in this document, in order to ensure that different laboratories using this method will obtain equivalent results.

B. 4. 4."Pore Size Range" - In addition to the optical image

analytical methods for characterizi ng pore sizes, recent work of this

laboratory as indicated in Section I I I A indicates that mercury porosimetry

is also a useful method for semi-quantitative analysis of pore sizes in

this material. As discussed above, there are certain limitations inherent

in this method, such as the fact that one measures only "interconnecting"

pore diameters rather than the true pore size. However, the method is
30

probably the most widely used one for rapid semi quantitative analysis of

pore size distribution, and we would suggest that it be considered as at

least a "fingerprinting" technique, to obtain pressure- volume plots

characteristic of the material, and to determine whether or not the

rather soft material has been significantly compressed due to accidental

mishandling.

As stated in our letter of June 18 to Dr. Homsy (Appendix III), we

feel that scanning electron micrographs (SEM's) should not be used for

optical image analysis of pore sizes. These pictures provide a good

impression of the structure and morphology of a porous material by

projecting 3-dimensional rough surfaces onto a plane. However, one does

not know exactly where to measure the pore dimensions in such a projection photograph. The standard technique for optical image analysis of

the volume and size of the various phases in a multi-phase solid material

involves measurements on smooth, planar surfaces by light microscopy^.

Because of its very narrow depth of field, light microscopy is ideally

suited to determine whether the surface is smooth and flat enough to

permit quantitative measurements of the different phases present; nonpi anar surfaces cannot be completely brought into focus. For a soft

porous material such as the PTFE-carbon composite, it is necessary first

to perfuse the sample with a nonviscous liquid, followed by cutting and

polishing to achieve a flat surface. This method was used by Spector

5

et al. in their optical point-count analysis of pore volume in the PTFEcarbon composite. To date, we have not performed optical image analyses

in this laboratory and thus cannot comment on their applicability to the

composite material. Such studies will be pursued during the following

year of this project.
31

V. SUMMARY

The activities of this project during the past year have involved

the measurement of pore volume and pore size in two porous polymeric

implant materials. Pore volume has been measured by three different

methods, namely:

(1) "apparent density," which requires a knowledge of the macroscopic

volume of the sample from its measured dimensions, and the density

of solids in the material,

(2) bulk compression of the sample, from which the pore volume is

measured directly from the ratio of sample dimensions before

and after compression of the airspace in the material,

(3) mercury intrusion, which determines the void volume of the

material by dilatometric measurement of the volume of mercury

intruding the sample under pressure.

Only methods (1) and (3) were used to determine the void volume in

porous polyethylene. Method (2) would require melting the material to

allow all the air to escape. All three methods were used to determine

the void volume in the porous composite of PTFE and carbon. For the two

materials, the void volumes were found to be about 45 percent for the

polyethylene and 70 percent for the composite.

Unlike the pore volume, pore "size" is not a well-defined physical

property of materials when the pores are of irregular shape, as is the

case for the porous implant materials. In general, it is necessary to

make some arbitrary assumptions about the pore shape in order to relate

the mercury intrusion pressure to pore size. In the case of mercury

porosimetry, the simplest and most customary assumption is that the
pores are circular cylinders. With this assumption, we have measured a

weighted average pore diameter of 30 pm for the polyethylene and 50 pm

for the composite. The pores measured by this technique are interconnecting pores, rather than the "true" distribution of pore sizes, due to

the fact that constrictions in the diameter of a long pore determine the

maximum measured diameter for that pore.

A major finding of this work was the discovery that, contrary to

some widely held beliefs, the mercury intrusion technique can be used on

the relatively soft and deformable PTFE-carbon composite without distorting

the material. This finding enabled us to use the method on the composite

material without fear that the results might be confounded by sample

compression.

Another method was used to estimate the average pore "diameter" in

the two porous materials. It also required an assumption of pore shape

in order to calculate a pore size. First, we have measured the specific

surface area (S) by the BET method, and the specific pore volume. With

these measured quantitites, and assuming spherical pore shapes, we found

the average pore diameters of the two materials to be 60 pm for the polyethylene and 25 pm for the composite. A discussion of the effects of

pore shape on the calculated pore size is included in this report.
33

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 (Available from National Technical Information Service.)

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

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

2. "Plasti-Pore Material Technical Information," Technical Publication

No. 4240, 1980, Richards Manufacturing Company, Inc. (p. 7).

3. "Handbook of Chemistry and Physics," 48th Edition, The Chemical

Rubber Company, Cleveland, OH.

4. "Polymer Handbook," Second Edition, J. Brandrup and E. H. Immergut,

Eds., J. Wiley & Sons, New York, Chapter V.

5. Spector, M. , Harmon, S. L. , and Kreutner, A., J. Biomed. Mater.

Res. 13, 677 (1979).

6. Shea, J. J. and Homsy, C. A. , Laryngoscope 84, 1835 (1974).

7. Bokros, J. C. , LaGrange, L. D. , and Schoen, G. J. , "Chemistry and

Physics of Carbon," Vol . 9, P. L. Walker, Ed., Marcel Dekker, New

York, 1972.

8. Galbraith Laboratories, Inc., Knoxville, TN.

9. Rootare, H. M. , Aminco Laboratory News 24, 4A-4H, Fall, 1968.

10. Lowell, S. , "Introduction to Powder Technology," John Wiley & Sons,

New York, (1979).

11. Frevel , L. K. and Kressley, L. J. , Anal. Chem. 35, 1492 (1963).

12. Mayer, R. P. and Stowe, R. A., J. Phys. Chem. 70, 3867 (1966).

13. Winslow, D. N., J. Coll. Interf. Sci. 67, 42 (1978).
34

14. Brunauer, S. , Emmett, P. H. , and Teller, E. , J. Am. Chem. Soc. 60,

309 (1938).

15. Adamson, A. W. , "Physical Chemistry of Surfaces," Second Edition,

Interscience, New York (1967).

16. Quantachrome Corporation, Syosset, NY.

17. Dehoff, L. T. and Rhines, F. N. , "Quantitative Microscopy," McGrawHill, New York (1968).
3b

Appendix I. Revised manuscript, "On the Characterization of Porosity in PTFECarbon Composite Implant Materials by Mercury Porosimetry .

ON THE CHARACTERIZATION OF POROSITY IN PTFE-CARBON

COMPOSITE IMPLANT MATERIALS BY MERCURY POROSIMETRY

R. E. Dehl

Polymer Science & Standards Division

National Bureau of Standards

Washington, DC 20234
1 ! II
ABSTRACT

Questions have been raised about the use of mercury intrusion

porosimetry to measure interconnecting pore sizes and void volumes in

relatively soft and flexible materials such as porous implant composites

of PTFE and carbon fibers. We have studied the effect of precompression

of one such commercial composite on the mercury intrusion curves which

cover all pore diameters greater than about 16 ym, the range of interest

for tissue ingrowth applications. Prior compression by a pressure 20%

greater than that encountered by the material during a mercury intrusion

experiment did not change the ensuing pore size distribution curve, as

compared with a noncompressed sample. Deformation of the material at

higher pressures sufficient to decrease the sample volume inelastically

by 17%, 33%, and 67% changed the shape of the mercury intrusion curves

significantly, indicating that this technique can be used to detect

prior deformation of an "unknown" sample. In the undeformed material,

less than 15% of the total void as measured by mercury porosimetry

consists of interconnecting pores > 100 ym in diameter and more than 50%

of the void volume is composed of pores < 40 ym in diameter.

Key Words: Mercury porosimetry, PTFE-carbon composite, porous implant

materials, pore size, pore volume.

1
It is important to establish reliable and reproducible procedures

for characterizing the porosity of porous implant materials for several

reasons. The manufacturers of such materials need to monitor their

porosity during and after production to assure quality control. Surgeons

who use the materials for implantation need to know that the porosity is

appropriate to the particular tissue ingrowth situation, and other

clinical researchers require that their tests be performed on materials

well characterized with respect to porosity as well as other physicochemical properties in order to interpret their experimental results

properly. Mercury intrusion porosimetry is a well established semiquantitative method of measuring both the distribution of interconnecting

pore sizes and the void volume of porous materials. However, because it

is necessary to apply pressure to intrude Hg into the pores, concern has

been expressed^ about using this technique to characterize the porosity

of relatively soft and deformable polymeric materials such as Proplast ,

a composite of poly tetrafluoroethylene and carbon fibers*. The purposes

of this investigation were (1) to determine whether significant distortion

of the PTFE-carbon composite occurs during a mercury intrusion experiment,

and (2) to determine the extent to which macroscopic deformation of the

material can be detected by the porosimetry technique. This study was

restricted to the measurement of pore sizes in the range appropriate to

2

tissue ingrowth. For soft tissue ingrowth, it has been reported that

pore diameters of at least 40-50 ym are needed, although at least one

commercially available materials for soft tissue applications, a porous

3 high density polyethylene (not tested by the authors of Ref. 2) has

most of its pores in the range 20-40 ym, as measured by mercury porosimetry.

* Certain commercial materials and instruments are identified in this report to

adequately specify the experimental procedure.. In no instance does such

identification imply recommendation or endorsement by the National Bureau of

Standards, nor does it imply that the material or instrument identified is

necessarily the best available for this purpose.

2
Bone ingrowth apparently requires pores larger than about 100 ym for

4 optimum success . It is also apparent from the published literature

that porosity is only one of several variables governing the clinical

success or failure of porous implant materials.

The usual starting point for interpretation of mercury porosimetry

data is the Washburn equation^ which is strictly applicable only to

cylindrical pores with circular cross-sections. The Hg intrusion

pressure and pore diameter are related by the equation

PD = -4ycos0,

where P is the pressure necessary to intrude Hg into a pore of diameter D,

y is the surface tension of Hg, and 9 is the contact angle of Hg on the

material of interest. For pore shapes other than circular cylinders,

the cross-section "diameters" have been shown to differ by as much as a

g factor of 2 from the value predicted by the Washburn equation . In a

real system of irregularly shaped interconnecting pores such as the

PTFE-carbon composite, this correction factor lies between 1 and 2, so

that one measures an ill-defined and somewhat inaccurate "average" pore

diameter. Regardless of this problem, however, pressure-volume plots

obtained by mercury porosimetry provide a useful semi-empirical pore

size distribution curve of a porous material. They can be used to

establish a correlation with the success or failure of a material in

implant applications and they provide a basis for comparing the relative

porosities of different samples of the same material.

The contact angle 9 of Hg on the material was found to be about

146° by the use of a commercial contact angle goniometer. This value

agrees well with a reported value of 150° for the contact angle of Hg

on PTFE^, and with another value^ (149°) for Hg on Proplast The

o ' 9 surface tension of Hg in_ vacuo at 25 was assumed to be 484 dynes/cm.

3
As calculated from these values, the pressure range from 0-1.01 x 10^ Pa

(0-1 atmosphere) is sufficient to intrude all pores larger than about

16 ym, which covers the range of interest for all types of tissue ingrowth.

The manually operated porosimeter used in this work was a Superpressure ® Model J5-7121-B supplied by American Instrument Co., equipped

with a Macro-Filling accessory which keeps the Hg penetrometer in a

horizontal position during the experiment and eliminates large corrections

for the Hg head pressure. All measurements were performed in the

pressure range from 0-1 atmosphere. Samples were cut with a sharp

scalpel from a block of the composite material 6 mm in thickness and

weighed before the intrusion experiment. Data points were taken sufficiently close together to give an accurate measurement of the shape of

the intrusion curve.

A typical Hg intrusion plot for untreated Proplast

^

is shown as

3

curve A in Fig. 1, for a pressure range from 6.9 x 10 Pa (1 psia) to

one atmosphere (14.65 psia) . (The lowest pressure at which significant

intrusion occurred was typically about 1 psia.) Pore sizes shown on

the plot were calculated from the equation above. In order to determine

whether inelastic compression of the material occurs during the Hg

intrusion experiment, one sample of Proplast ^ in the form of 1 mm thick

sheet was placed under a weight equivalent to 1.2 x 10^ Pa pressure (1.2

atmospheres) for 18 hours. After the treatment, the sample yielded an

intrusion curve superimposable on that of an uncompressed sheet, indicating that insignificant inelastic deformation of the material occurred,

due to the pressure treatment. The composite material is, in fact, quite

inelastic and can be deformed by pressures well above one atmosphere.

Curves B, C, and D were obtained by precompression of rectangular samples

4
from 6 to 5 mm thick (B) , from 6 to 4 mm (C) and from 6 to 2 mm (D) .

For quantitative comparison of the intrusion curve shapes for different

samples, it is necessary to normalize the plots either by adjusting to

the same end point, as for A and B in Fig. 1, or by adjusting all plots

to the same sample weight, which will cause the curves of compressed

samples to lie entirely below the uncompressed ones. Curves B, C, and

D show significantly smaller fractions of pore volume attributable to

large pores ( 50 /urn) , as compared with the uncompressed material.

Even the relatively small reduction in total sample volume represented

by curve B (17%) is enough to shift the curve well away from that of the

uncompressed sample. It is apparent that the Hg intrusion curve for an

"unknown" sample of this material can be used to determine whether or

not significant compression of the sample has previously occurred.

For the uncompressed sample (A) it is apparent from the slope of

the curve at the highest pressure that one atmosphere was not sufficient

to intrude all of the pores. If we assume that all the void was intruded

and use the calculated volume of solids present and the volume of Hg

intruded to account for the total sample volume, we find that 78% of the

total sample volume was intruded at one atmosphere pressure. This value

falls well within the range 70-90% specified by the manufacturer of the

material, and is only 4% less than the void percentage which we calculated

for a rectangular block of the material, using the weight of the sample,

the total volume, and the densities of PTFE and graphitic carbon. Thus,

approximately 95% of the total void was apparently intruded by Hg at one

atmosphere pressure. It is also apparent from curve A that only a small

fraction (< 15%) of the total void consists of pores greater than lOO^im

5
in "diameter". Proplast ^

in sheet form has a mercury intrusion curve

similar in shape to Curve A of Figure 1, but with a slightly higher

fraction of pores (20%) of diameter 100^um or greater. Because a small

pressure (<3.5 x 10^ Pa) was used to fill the penetrometer containing

the sample with Hg before intrusion measurements were begun, it might be

argued that very large pores 400yurn ) were already filled with Hg

before the first measurement was made. If this has occurred, the fraction

of total void represented by such pores must have been extremely small

because, as demonstrated above, essentially all of the expected void

volume has been accounted for.

In summary, sub-atmospheric Hg intrusion porosimetry appears to be

a useful method for characterizing PTFE-carbon composite implant material.

It can provide an interconnecting pore size distribution curve which is

sensitive to small macroscopic deformations of the material and therefore

gives semiquantitative information about the mechanical history of the

sample and/or deviations in the manufacturing process.

Contrary to earlier suggestions, mercury porosimetry apparently can

be used on the PTFE-carbon composite implant material without fear of

mechanically compressing it. In combination with a quantitative total

void volume measurement, such as the method used above or a liquid uptake

measurement, this method can determine the fraction of total void represented by any interconnecting pore size range of interest.

This work was supported by the Bureau of Medical Devices (FDA) under an

Interagency Agreement.
References

1. See, for example, C. A. Homsy, J. Biomed. Mater. Res. 13_, 987 (1979),

and references cited therein.

2. R. A. White, R. S. Lawrence, F. M. Hirose, R. J. Nelson, Transactions

of the 5th Annual Meeting, Society for Biomaterials, Clemson, SC,

April 1979.

3. Plasti-Pore 0 Material Technical Information, Technical Publication

No. 4240, Richards Manufacturing Co., Inc.

4. J. J. Klawitter and S. F. Hulbert, J. Biomed. Mater. Res. Symp. 2_,

161 (1971)

5. E. W. Washburn, Proc. Nat. Acad. Sci. 7_, 115 (1921) .

6. J. J. Sipe, "Pore Growth During the Initial Stages of Sintering",

Ph.D. dissertation, University of Washington, Seattle, Washington, 1971.

7. H. W. Fox and W. A. Zisman, J. Colloid Sci. S_, 514 (1950).

8. M. Spector, S. L. Harmon, and A. Kreutner, J. Biomed. Mater. Res.,

13_, 677 (1979) .

9. Handbook of Chemistry and Physics, 48th Edition, The Chemical Rubber

Company, Cleveland, OH.

7
0.15

Figure Caption

TM

Figure 1. Mercury intrusion curves for uncompressed Proplast (A) , and

O

T-

« o

to

o

( uio) 0tun|OA pepmiui

u

e

Pore

Diameter

(|im)
46

(R) Appendix II. BET surface area analyses for Proplast v ' PTFE-carbon composite

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49

Appendix III. Letter from R. E. Dehl to C. A. Homsy concerning ASTM Draft

Standard Specifications for porosity characterization of PTFE-carbon composite

implant materials.
UNITED STATES DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 20234

June 18, 1981

Dr. Charles A. Homsy

6560 Fannin, Suite 2080

The Methodist Hospital

Houston, TX. 77030

Dear Dr. Homsy:

You asked for my opinions about the ASTM draft standards for porosity characterization of Proplast. I won't repeat the standards here, but will just

comment on them.

4.4.1 " Volume Pores "- The compression method suggested for measuring void

volume is not necessarily the most accurate, but considering the wide range

of acceptable void (70-85%), it is probably quite adequate; it has the advantage of simplicity, which is always desirable for inter-laboratory data com- parison. However, I don't agree that the mercury intrusion method is not

appropriate also, and I suspect that it is somewhat more accurate than the

suggested method. I'll try both and let you know the conclusion. Perhaps

I should describe the mercury method. The evacuated sample is trapped inside a glass dilatometer under a pool of mercury. As pressure is applied,

the mercury intrudes the pores and the volume intruded is measured on a graduated stem. If one applies sufficient pressure, all the void is even- tually intruded, and it doesn't matter whether the sample compresses in the

process or not. As shown in my recent manuscript, one atmosphere pressure

is sufficient to intrude all but about 5% of the void in Proplast, and I suspect that 2 or 3 atmospheres will easily take up the rest. Our porosimeter operates up to 1000 atmospheres pressure, so I have little doubt of its

ability to intrude all the void.

4.4.2 "Pore Size Range " - I think we agree that this problem is much more

difficult than the void measurement. I looked up the "ten line intercept"

method of analyzing surface composition referred to in Ref. (30). There are several variations of this general type of optical image analysis involving

combinations of lines, grids, points, etc. The method looks quite good for

the applications described in the article. However, several things bother me about applying this method to the SEM pictures of Proplast. I see a certain

degree of arbitrariness in deciding exactly where the boundaries of a pore

are in a material with such a complex morphology. Also, I'm not sure that

analysis of porosity from a photograph is necessarily equivalent to analysis

of the material itself, as by the porosimetry technique, for example, I can illustrate this problem by supposing that we see something in the photograph

that looks like a large pore opening at the surface, but directly behind it

is a "wall" with only narrow exits. The optical analysis would say that we
Dr. Charles A. Homsy - 2 - June 18, 1981

have a 200 ym "pore", whereas tissue ingrowth is going to be constrained

by the much narrower pores behind the opening. This type of difficulty

makes me wonder whether optical measurements are directly related to the

permeability of the material, which is certainly the critical aspect of

"porosity" for tissue ingrowth.

Also, I'm not sure that SEM is preferable to ordinary light microscopy

which could be used for the relatively large (ym) pores in Proplast. SEM

"sees" 3-dimensional objects in focus, while light microscopy has a narrow

depth of field. However, the 3-D SEM pictures may not be representative

of a cross section of the material, and it seems essential to me to know that

one is looking at a flat 2-dimensional surface in order to make quantitative

measurements from the picture. The results of your independent laboratory

which found only 57% void volume by the SEM technique seem to indicate that

this method does not accurately measure pore dimensions. In this connection,

I remind you that mercury porosimetry seems to measure void volume quite well.

I have not yet used optical analysis on the porous materials, but I will let

you know if I come up with more definite conclusions.

Sincerely,

Ronald E. Dehl , Ph. D,

Dental and Medical Materials

Polymer Science and Standards Division
NBS-114A 'REV. 2 -sc) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 81-2459 January 1982

4. TITLE ANO SUBTITLE

Characterization of Porosity in Porous Polymeric Implant Materials

5. AUTHOR(S)

R. E. Dehl , W. H. Grant, and J. M. Cassel

Sc PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Con tract/ Grant No. national bureau of stanoaros

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

8.

Type of Report & Period Covered

9.

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

Bureau of Medical Devices

Food and Drug Administration

Silver Spring, MD 20910

10.

SUPPLEMENTARY NOTES

I | 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 significant

bibliography or literature survey, mention it here)

In this report, we describe the application of several methods of porosity characterization to two commercial polymeric implant materials, a porous polyethylene and a composite of PTFE and carbon. In exploring the use of mercury porosimetry to determine

interconnecting pore size distributions in the composite, we have found that, contrary

to widespread belief, this rather soft and deformable material was not distorted by

the mercury pressures required to measure its pore size distribution. The mercury

intrusion technique indicated that the porous polyethylene has a narrower distribution

of pore sizes than the composite. In the polyethylene and the composite, the weighted

average interconnecting port "diameters" were found by this technique to be,

respectively 30 urn and 50 ym. The average pore diameters were also estimated by

measurement of specific surface area and pore volume. Assuming a spherical pore

shape, the average pore diameters in polyethylene and composite were calculated to

be, respectively , 60 ym and 25 ym. The accessible pore volumes of the materials were

measured by three different techniques, a simple "apparent density" method, mercury

intrusion, and bulk compression, the last for the composite only. For the polyethylene and composite we found pore volumes of about 45 and. 70 percent, respectively

.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s)

porous implants, pore size, pore volume, PTFE-carbon composite, porous polyethylene.

13. AVAILABILITY

' y Uni imited

i I For Official Distribution. Do Not Release to NT1S

1 Order From Superintendent of Documents, U.S. Government Prjnting Office, Washington, D.C.

20402. 55 O rder From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

60

IS. Price

S9 .00

USCOmm-OC 5043-080
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