. V.

,]• /i /

•'

f'

' Binai sf Sta»dard» MAY 2 2

Physical Testing of Polymers

for Use in

Circulatory Assist Devices

First Annual Report

July 1978

To: Devices and Technology Branch

Division of Heart and Vascular Diseases

National Heart, Lung, and Blood Institute

National Institutes of Health

Reimbursable Agreement

YOl-HV-8-0003

Polymer Science and Standards Division

National Bureau of Standards

Washington, D. C. 20234

Robert W. Penn, Ph.D.

Principal Investigator

Gregory B. McKenna, Ph.D.

Project Leader
w
Abstract

The concept of linear additive damage has been used to relate sinusoidal

fatigue data with creep failure time data on two polymers. Deviations of

fatigue data from predictions made from creep failure time data were found

to be in opposite directions for a glassy and a semi crystal line polymer. Further

work will extend this study to elastomers and candidate biocompatible materials.

The limited quantities of candidate materials have permitted preliminary

measurements of constant strain rate and creep tests. Specimens of a urethane-silicone

copolymer were obtained from rejected intra-aortic balloons. These specimens were

characterized by Fourier transform infrared attenuated total reflection spectroscopy,

as well as by optical microscopy. Significant differences between the two durfaces

of the intra-aortic balloon were observed. The use of difference spectroscopy

has permitted a more detailed characterization of the surfaces.

Optical microscopy has revealed circular features near the surface of the

balloon wall (which we are tempted to characterize as regions of segregated

poly(dimethyl siloxane) (PDMS)) on a much grosser scale than has been previously

reported.

We have prepared carefully controlled samples of an NBS standard butyl

rubber and distributed test specimens to other investigators involved in physical

testing of polymers for use in circulatory assist devices. This material has

mechanical properties similar to candidate biocompatible materials and may be

useful as an interlaboratory standard and for test method development.

Preliminary design and construction of a biaxial fatigue test cell has

been completed. Fatigue experiments on butyl rubber using this device have

indicated that only minor modifications will be necessary before initiation

of fatigue testing on candidate biomaterials. A mul ti position test station for

these cells is being assembled.
Preface

This report presents results obtained in the past year in a research

program on Physical Testing of Polymers for use in Circulatory Assist Devices

carried out at the National Bureau of Standards with the support of the

National Heart, Lung and Blood Institute of the National Institutes of Health.

The report covers work performed in the period October 1, 1977 to June 30, 1978.

The research was somewhat hampered by the lack of samples of candidate

biocompatible materials, and the originally proposed research plan could not

be followed closely. Steps have been taken to secure biocompatible materials

from suppliers and the work plan for the second year effort is modified to

include physical testing of these materials. Our objectives were refocused

toward other aspects of the problem which were accessible.

1
List of Tables

Page

1. Failure Times for Poly(methyl methacrylate) 4

2. NBS Reference Butyl Rubber Compound 9

3. Permanent Set of Elastomers after Straining 16

4. Comparison of IR Band Frequencies of PDMS and Those Obtained 25

in the Difference Spectrum.
«
Table of Contents

Page

Title Page

Preface i

Abstract ii

Table of Contents iii

List of Tables iv

List of Figures v

1. Introduction 1

2. Theoretical Framework 2

3. Preliminary Mechanical Characterization of Elastomers 8

4. Characterization of the Urethane-Silicone Film by Infrared

Spectroscopy 21

5. Optical Microscopy Study of the Urethane-Silicone Film 27

6. Biaxial Fatigue Cell Development 32

References 38
List of Figures

Page

1 . 5

2 .

3.

4.

5.

6 .

7.

8 .

9.

10 .

11 .

12 .

13.

Failure Kinetics for PMMA and PE

Stress Strain Behavior for Elastomers

Mooney Rivlin Plot for the Urethane-Silicone Copolymer

" for Polyolefin

" for NBS Butyl Rubber

Creep of NBS Butyl Rubber in Air

" " " 0.1 5N Saline

Infrared Spectroscopy of Urethane-Si 1 icone Film

Photomicrograph of Urethane-Silicone Balloon Wall

Biaxial Test Cell

Calibration Curve for Bi

Oscilligram of Pressure

axial Test Cell

Pul ses

11

13

14

15

18

19

22

28

29

33

35

36
Introducti on

This report presents preliminary results directed toward the objectives

of developing test procedures and the evaluation of candidate materials for use

in circulatory assist devices (CAD). The test procedures should permit the use

of accelerated tests to predict material performance over long time intervals

from relatively short time tests. Such extrapolation of test results must rely

on a verified theoretical basis. We present one such basis in Section 2.

The research program was hampered by the very limited supply of candidate

materials available. A number of reject intra-aortic balloons were made available

by Dr. Nyilas of AVCO Everett Laboratori es . This material was only eight mils

thick as compared with 26 to 40 mil material desired for the CAD program. One half

square foot of Polyolefin was made available by Dr. McMillin of Monsanto Research

Corporation. This material did not have the biocompatible porous surface treatment.

No Biomer film was made available. To permit some progress in test method development, we have prepared an ASTM formulation of NBS standard butyl rubber with

mechanical properties in the same range as the candidate biocompatible materials.

This compound has a known history of good property uniformity.

Section 3 presents the results of some preliminary mechanical characterization

tests on samples of the two candidate blood compatible materials as well as on

the butyl rubber compound.

Sections 4 and 5 present the results of a microscopic and spectroscopic examination of the intra-aortic balloon material and Section 6 presents preliminary

results toward the development of a biaxial fatigue test apparatus.
2. Theoretical Framework for the Prediction of Durability

In order to predict the useful life of a mechanical component from some

set of laboratory tests we need a concept or theoretical structure by which

failure can be predicted. For many materials, the lifetime of specimens is

found to depend upon the stresses to which they are subjected. This fact,

alone, suggests that some damage mechanism occurs in the material which

eventually produces failure. The simplest test which might be used to study

the damage mechanism is creep to failure under constant stress.

Coleman, [1], for example, has studied the failure of nylon fibers and

analyzed his results in terms of absolute reaction rate theory. He has also

presented a phenomenological analysis of the time dependence of mechanical

breakdown using the statistics of extreme values, £2]. For materials which he

calls Class A for which the rate of damage accumulation depends only on the

current stress (and not on the current value of the damage) he predicts that the

coefficient of variation (the standard deviation divided by the mean) for the

breaking times will be independent of stress. For such materials whose lifetime

at constant stress is known, failure in other stress histories can be described

by what we have come to call the Bailey criterion:

This is probably the simplest expression of a general linear additive damage

law. The function Xg (a) can be obtained from observations of the lifetimes of

samples held at constant stress. For many materials this function is found

to be exponential in stress:

( 1 )

Tg = A EXP(-Ba) ( 2 )

and the logarithm of the breaking time varies linearly with the stress.

2
The integral (eq. 1) can then be evaluated for any history of stress to preilict

the failure time. For e;<ample, for a sinusoidally varying stress

a = ^

(1 + Sin wt) (3)

2

(when failure occurs after a large number of cycles), the time to fail,

where Xg is the time to break for a constant stress equal to the peak stress

and Ig is the zeroth order modified Bessel function. Thus, the failure time is

predicted to be independent of the frequency of the loading. Further, the

same statistics apply and the same coefficients of variation and skewness should be

found as for the constant stress tests.

We have attempted to test these predictions experimentally using two

materials which were readily available: poly(methyl methacrylate) (PMMA)

and poly (ethyl ene) (PE). Dumbell samples were cut from a sheet approximately

1.5 mm thick with a gage length of 5 cm and a width of 1.25 cm. The samples were tested

at controlled engineering stress in a servo hydraulic test machine with both

constant and sinusoidally varying stresses. The frequency of the oscillating

stresses was held below 0.2 Hertz to reduce the effect of hysteretic heating.

The tests were run at room temperature (24°C) at 50% RH. Since it is known that PMMA

is very sensitive to humidity, the samples were stored for at least two weeks at

these conditions before testing.

The results of these tests for PMMA are presented in Table 1 and summarized

in Figure la. For constant stress, the sample lifetimes are found to follow equation

2 quite accurately with A = 1.5 x 10^^ sec and B = 0.26 MPa The coefficient

of variation is found to be nearly constant over the range studied at approximately

0.3. For the oscillating loads at high stresses, the failure times are approximately

those predicted by Equation 4 but at low stresses the failure times are significantly

shorter than predicted. At low stresses the coefficients of variation are about

3
Table 1

Failure of Poly(Methyl methacrylate)

Constant Load

Peak

Stress

(MPa)

Number

of

Specimens

Mean Time

to Fail

(seconds

Coeffi cent

of

Vari ation

Moment Coeff

of Skewn

55.2 10 12785 .353 0.44

58.6 9 3551 .233 0.14

62.1 10 1375 .364 -0.43

65.5 10 670 .172 -0.16

69.0 13 264 .239 0.20

72.4 15 78.9 .311 -0.47

75.8 30 35.5 .456 -0.05

82.7 3 .77

Sinusoidal Load, 0.164 Hz

55.2 10 18026 .449 0.46

62.1 19 5125 .579 0.60

69.0 17 1549 .783 0.41

75.8 14 257.9 .840 1 .42

82.7 6 25.8 .868 0.82

4
Fig, la. Failure kinetics for PMMA. Time to break vs. stress for dead load and fatigue

experiments are compared with the time to break predicted for a fatigue experiment

from the linear additivity of damage and the dead load data.

5
the same as for the constant load measurements, but they increase regularly with

load to nearly 1.0 at the highest loads studied.

3/2 Similar results are found for the coefficient of skewness, m^/m^ , where

m^ and m^ are the second and third moments of the distribution respecti vely . For

the constant load case, there seems to be no significant skewness while the

sinusoidal load data appear to be positively skewed.

The few results obtained for a linear polyethylene sample are shown in Figure lb.*

Insufficient measurements have been taken for reliable statistical study, but the

indication is that the deviation from the predicted behavior is opposite to that

found for PMMA. The extrapolation of the creep failure data to fatigue of these

materials would yield a dangerously optimistic prediction for the case of PMMA

and a pessimistic prediction for the case of polyethylene. This work will be

extended to include elastomers and the candidate CAD materials.

*Lifetimes for constant stress were obtained previously by Dr. J. M. Crissman of

this laboratory.

6
STRESS, MPa

Fig. lb. Failure kinetics for polyethylene. Time to break vs. stress for dead load

and fatigue experiments are compared with the time to break predicted for a fatigue

experiment from the linear additivity of damage and the dead load data.

7
3 . Preliminary Mechanical Characterization of Elastomers

A. Introduction

We were able to obtain small quantities of two candidate CAD materials,

Avcothane 51* (urethane-silicone copolymer) and the Goodyear Polyolefin* elastomers.

These permitted us to conduct prel imi nary mechanical characterization tests. In

addition, we decided to produce our own "internal standard" elastomer using the NBS

388j Standard Reference Butyl* Resin compounded according to ASTM D-31 88-73 Formula lA,

This formulation was chosen as a material with properties in the range of those of

the candidate CAD materials and because we have a known history of good property

uniformity and reproduci bi 1 i ty , Table 2 lists the formula used.

Two types of constant deformation rate tests were run on all three materials

to provide an initial comparison of the mechanical properties. In addition,

preliminary creep data have been obtained on the NBS Butyl Rubber in air and in 0,1 5N

Sal i ne at 23 + 1 °C.

*Certain commercial materials and equipment are identified in this paper in order

to specify adequately the experimental procedure. In no case does such identification

imply recommendation or endoresement by the National Bureau of Standards, nor does

it imply necessarily the best available for the purpose.

8
Table 2

Compounding Formulation for NBS Butyl Rubber

Quantity

Inqredi ent Parts by Mass

NBS 388j Butyl Rubber 100

NBS 370e Zinc Oxide 3.00

NBS 371g Sulfur 1 .75

NBS 372h Stearic Acid 1.00

NBS 378b Oil Furnace Black 50.00

NBS 374c Tetramethylthiuram 1.00

Disulfide

9
B. Constant Rate of Deformation Tests

Two different types of constant rate of deformation test were run on the

three elastomers. First, all the materials were tested in a standard fashion by

using dumbbell specimens with a 2,54cm gage length and pulling them to failure

on a Tinius Olson* testing machine at a crosshead displacement rate of 51 cm/mi n.

The Butyl and Polyolefin materials showed no grain effect due to the milling

procedure. The urethane-silicone was tested by cutting dumbbells longitudinally

from intra-aortic balloons provided by Dr. Nyilas of Avco. Figure 2 shows the

engineering stress-elongation behavior for each of the materials. Figure 2 illustrates

some differences in the behavior of the three materials. The urethane-silicone is

quite stiff at low elongations and has a high ultimate strength (44,3 MPa) and

elongation to failure (550%). The Polyolefin rubber, on the other hand, has a

low initial stiffness which increases above that of the urethane-silicone at

intermediate elongations. However, this material is characterized by a low elongation

to failure (340%) and an ultimate strength which is considerably lower that that

of the urethane-silicone at 17.5 MPa. The MBS Butyl can be seen to have the lowest

initial modulus. It shows an intermediate ultimate strength (21.5 MPa) and the

highest elongation to failure (600%).

Our comment on these results is that they point up some differences in the

way in which the materials react under a forced deformation. Their behavior under

oscillating loads can also be expected to be somewhat different.

In order to get some additional information about the nature of the materials,

we performed another type of constant rate of deformation test. Dumbbell specimens of

each material were deformed at 0.1 cm/min to a deformation of 200-250%. The specimens

were then unloaded at the same rate. Rather than plot a conventional hysteresis loop,

we chose to present the data on a so-called Mooney-Rivl in [3] plot of reduced stress, a^,

versus 1/A where:

*Certain commercial materials and equipment are identified in this paper in order to

specify adequately the experimental procedure. In no case does such identification imply

recommendation or endorsement by the National Bureau of Standards, nor does it imply

necessarily the best available for the purpose.

10
STRESS,

MPa

Fig. 2. Stress strain behavior of the urethane-silicone, Polyolefin and NBS

Butyl Rubbers. The tests were run in air at 23°C at a deformation rate of 51 cm per

mi nute

11
°R

= P/2A(X-1a2) = + i/A |«

-O

(5)

( 6 )

L = instantaneous length of specimen

Lq = initial length of specimen

W = strain energy function

= i^*^ invariant in the principal stretches

P is the load and A is the original cross sectional area. If the material is elastic

and can be represented as a Mooney-Rivl in material, then the data would plot as a

i I straight line on this type of plot and the slope would give |y and the intercept

^11 2 rN 111 with the 1/A axis equal to 0 would give . A Neo-Hookean material, e.g. ~

would plot as a horizontal line. As can be seen in Figures 3-5, none of the materials

exhibits either Mooney-Rivl in or Neo-Hookean behavior upon loading, although the

urethane-silicone is very close to exhibiting the Mooney-Rivlin type behavior. Both the

NBS Butyl and the Polyolefin elastomers show minima in the reduced stress. The

unloading plots for the materials show a large time dependent component to the behavior

of the materials. If the materials were elastic, the load and unload curves would

reproduce. However, the unload curve in all cases is considerably different from the

loading curve. The fact that the reduced stress goes to zero before 1/A equals one

is a manifestation of the viscoelastic nature of the materials. The amount of set at

the completion of the experiments was measured and is listed in Table 3.

In summarizing this section, we note that the constant rate of strain tests conducted

on the three materials indicate that all of the materials exhibit different properties

in simple stress-strain tests. Also, Mooney-Rivlin plots of the data indicate that

the materials are not simple elastic solids • They exhibit viscoelastic behavior which is

manifested in the differences in the load and unload behavior and permanent set after

deformation.

12
3.0

2.5 -

URETHANE-SILICONE COPOLYiVlER

CO o.S

o LOAD

UNLOAD

2.0 -

CM

<

<

CM

Q.

IICC

b

1.5 h

1.0 o

0.5

0.0

n °

J L

.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

1/A

Fig. 3. Mooney-Rivlin plot for urethane-silicone copolymer. Differences between

load and unload plots are due to viscoelastic behavior of the material.

13
1.50

(D

Q.

<

<

IICC

b

1.25

1.00

0.75

0.50

0.25

0.00

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

1/A

POLYOLEFIN

o LOAD

UNLOAD

o o o

OOqO O O q

O

Dq

a 0

1 “

Fig. 4. Mooney-Rivlin plot for Polyolefin elastomer. Differences between load

and unload plots are due to the viscoelastic behavior of the material.

14
1.50

CD Q.

<

a.

It

1.25

1.00

0.75

0.5Q

0.25

0.00

1 1

~ 1 ^ i i NBS BUTYL

0 LOAD

miQkU

—

o “

L o

0

0

0 O o

o O O 0 o O ®

a

g

1 1 _J 1 1 ! 1 0.2 0.3 0.4 0.5 0.6

1/A

0.7 0.8 0.9 1.0

Fig. 5. Mooney-Rivlin plot for NBS Butyl Rubber. Differences between load and unload

plots are due to the viscoelastic behavior of the material.
Table 3

Permanent Set of Elastomers After Load/Unload Cycle at 0,1 cm/min.

Material

Urethane-si 1 i cone

Polyol efi n

NBS Butyl

Permanent Set

10 %

21 %

10 %

16
C. Creep of NBS Butyl Rubber

In order to begin the baseline characterization of the NBS Butyl Rubber, we

have begun dead load testing of dumbbell specimens in which creep and time to

fail are measured. Also, a prototype creep tank for testing in simulated physiologic

media was constructed and tests are being conducted in both air and 0.15N Saline.

All tests are being conducted at room temperature (23 + 1°C).

Figure 6 shows the creep of the NBS Butyl in air and Figure 7 shows its creeo

behavior in the .15N Saline. There appears to be no significant difference in the

creep rates in air and in Saline. At this time, however, too few specimens have failed

to allow us to construct a time to break curve. When available, the data will

be used to test the linear additivity of damage concept by comparison with fatigue

tests. This should then provide guidance in determining the form of a nonlinear

damage law which may be required to successfully describe the failure behavior of

this elastomer.

One point which needs to be discussed is the creep of the Butyl Rubber in

.15N Saline. As can be seen in Figure 7, there is considerable "jerkiness" to the

data. This is postulated to be due to the sticking and slipping of the Teflon

Dulleys used in the creep tank. This indicates either that the tank design may

need to be changed to eliminate the requirement for pulleys or that much lower

friction pulleys will need to be found. This is important because at the low load

levels being used, a load of only 0.25N can change the stress in the material by 5%.

Finally, the question arises as to what simulated physiologic solution to use

in characterizing the CAD materials. We have initially selected the .15N Saline

due to its ease of use. Should any hydrolytic degradation mechanisn be at work

in the elastomer degradation it '-should be determined by this test. However the problem

of actual blood constituents (such as lipids) playing a role in elastomer degradation

needs to be considered. We are, therefore, considering using either serum or a lipid

17
PEnCENTElOriGATION,

(L-Lo)/Lo

x

100

4000

1000

1G0

10

1

ms BUTYL RUBBER

AIR

T=296 K

+

V

13.79 MPa

• •A

• • • • A AAA

+ +

+ +

• 10 09

• 0* 5.05 A AA 3.2S

+ ++ 1.64

A A A A

D D ° o o o o

Ik * * ^

A A

0

O O * Mk

A A A A A

o O O o o

ijt * * Jjc *

AA .807

.673 .625

.485

IQ' 13' 10"^ 10 ^ 10^ 10

^

TIME.s

Fig. 6. Creep of NBS Butyl Rubber in air,

18
PERCira

r.LCfjGATiOri.

U-Lo)/Lo

xlGU

1GQ0

130 >-

lb

A

O

A

C

-2?

A

DO

ijf O

A

no

•Jtr

WBSBIYYL RUBBER

.15 n SALlf^E

T=296 K

A

O

A A

O °

A

O

A A -227 MPa

c c Q3CO °

o o O OO CO .621

vt- .483

10 10 10 10

* 10

TiiVIE.s

Fig. 7. Creep of NBS Butyl Rubber in 0.15N Saline solution.

19
solution with a bacteriostat to conduct some of our tests. We expect to use air

and saline environments for the development of the test methodology and to then

use the more difficult environment for selected tests.

D. Summary

Preliminary mechanical tests show significant differences in behavior among

the three elastomers examined. Stress-strain behavior to failure shows that the

urethane-silicone has a high initial modulus and very high ultimate strength. The

polyolefin elastomer has lower initial modulus and lower ultimate strength than the

urethane-silicone, but it exhibits higher stiffness in the intermediate strain range.

The NBS Butyl Rubber chosen as an internal standard has the lowest initial modulus,

a relatively low tensile strength and exhibits the highest elongation to failure.

Tests on the NBS Butyl indicate little difference in creep rate in air or in

.15N Saline at 23°C.

20
4 .

Character!’ zati on of the Urethane-silicone Film by Infrared Spectroscopy

Specimens for infrared (IR) spectroscopic studies were taken from an

intra-aortic circulatory assist device furnished by Dr. Emery Nyilas. Attenuated

total reflection (ATR) IR spectra were recorded using a Digilab Fourier Transform

interferometer and the transformed interferograms (transmission or absorption

spectra) were stored digitally which facilitated comparisons of ATR spectra from

different surfaces of the intra-aortic device. Infrared spectra v/ere recorded on

undeformed specimens as well as on specimens which had been extended 2!30% uniaxial ly

(in the long axis direction of the intra-aortic device). Changes in the ATR infrared

spectra were observed between the surface which was in contact with air (air-facing)

and the surface which was in contact with the substrate during fabrication. No

detectable differences were observed in the IR spectra Of the same surface before

and after deformation. Absorption spectra of the two surfaces are shown in Fig. 8a

and 8b and the difference between the two in Fig. 8c. Spectral differences between

ATR spectra in the 650-1250 cm~^ region which are evident by visual comparison are

greatly accentuated by subtraction as shown in Fig. 8c. Qualitative differences have been

previously observed between the ATR infrared spectrum of the air- facing surface and

the substrate-facing surface and the ratio of optical density at 792 and 769 cm”^

shov/n to correlate with the degree of hemocompati bi 1 i ty . Monitoring IR spectral

changes during mechanical testing may be useful as a qualitative measure of

hemocompati bi 1 i ty. The quantitative difference spectrum shown in Fig. 8c can be

interpreted as arising from an increased concentration of poly(dimethyl siloxane)

in the penetration depth of the evanescent wave for the substrate- facing surface as

compared to the air- facing surface. Band maxima in the IR spectrum of PDMS [4],

are compared to those of the difference spectrum in Table 5. All IR bands between

650 and 1275 cm ^ in the PDMS spectrum can be accounted for in the difference spectrum,

both by frequency and intensity. Small frequency shifts in the urethane-silicone spectrum

21
[ilGiLA^

'iki6.

22

Fig.

8a.

ATR

infrared

spectrum

of

the

surface

of

the

intra-aortic

balloon

which

was

in

contact

with

air

during

fabrication.
23

which

was

in

contact

with

the

substrate

during

fabrication.

Spectral

differences

between

600

and

1100

cm""'

and

at

1250

cm~'

between

Figs.

8a

and

8b

can

be

attributed

to

a

higher

concentration

of

POMS

near

the

substrate-facing

surface.
Fig. 8c, ATR infrared difference spectrum, SUBSTRATE-FACING minus AIR-FACING

spectral frequencies and intensities are in substantial agreement with the IR

data of ROMS, see Table 5.

24
NBS-1 1 4A (REV. 7-73)

U.S. DtPT. OF COMM,

j 1. 1>1)H:.|CAT10N OK Rl-R^RT NO.

BIBLIOGRAPHIC DATA

sheet I NBSIR 79-1741

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE .\i\n Sb'h ITTLi; Physical Testing of Polymers for Use in Circulatory

Assist Devices

5. Publication Date 7-78

6. Performing Organization C-ode

7. AUTHOR(S)

R. W. Penn and G. B. McKenna

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/ 'i'ork Unit No.

11. Contract/Grant No. YOl-HV-8-0003

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Devices and Technology Branch

Division of Heart and Vascular Diseases

National Heart, Lung, and Blood Institute

13. Type of Report & Period

Covered

10-77 to 8-78

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. .ABSTRACT (A 200-word or less factual summary of most signiticant information. If document includes a significant

bibliography or literature survey, mention it here.)

The concept of linear additive damage has been used to relate sinusoidal fatigue

data with creep failure time data on two polymers. Deviations of fatigue data from

predictions made from creep failure time data were found to be in opposite directions

for a glassy and a semicrystalline polymer. Further work will extend this study to

elastomers and candidate biocompatible materials. Specimens of Avcothane 51, obtained

from rejected intra-aortic balloons, reflection spectroscopy, as well as by optical

microscopy. Significant differences were observed between the two surfaces of the

intra-aortic balloon similar to those which had been previously reported.

Optical microscopy has revealed circular features near the surface of the balloon

wall which we are tempted to characterize as regions of segregated PDMS on a much

grosser scale than has been previously reported.

We have prepared carefully controlled samples of an NBS standard butyl rubber

and distributed test specimens to other investigators involved in physical testing

of polym.ers for use in circulatory assist devices. This material has mechanical

properties similar to candidate biocompatible materials and may be useful as an interlaboratory standard and for test method development.

Preliminary design and construction of a biaxial fatigue test cell has been

completed. Fatigue experiments on butyl rubber using this device have indicated that

only minor modifications will be necessary before initiation of fatigue testing on | candidate biomaterials. A multiposition test station for thp^p cpI 1 i .q being asgpmb'l . i7. KEY U'ORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper i name; separated by semicolons) Additive damage; biaxial fatigue; biomaterials; circulatory

assist devices; fatigue of polymers; mechanical testing.

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Springfield, Virginia 22151 UNCLASSIFIED

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Table 4

Comparison of IR band frequencies of ROMS and those obtained in the

difference spectrum, Fig. 8c.

Band frequencies (cm and intensities^

Di fference

ROMS [4] Spectrum Assignments

660 W 661 W

skeletal deformation

685 W 687 W

700 W 702 W C-Si-C deformation

800 S 794 S Si-C stretch

820 Sh 822 Sh Si-C stretch

862 M 873 M methyl rock

1012 S

1020 S 1025 S

1093 S 1087 W

Si -0-Si asymmetric stretch

1263 S 1252 Sh^ symmetric methyl deformation

a. S, strong absorption; M, medium absorption, W, weak absorption; Sh, shoulder.

b. From comparison of the spectra in Fig. la and b.

25
can be accounted for by interactions of PDMS with the surrounding urethane matrix.

One additional band, at 1012 cm \ is observed in the difference spectrum and

its origin is unknown at this time. The consistent agreement between the difference

spectrum peak frequencies and the band frequencies of PDMS supports the notion that

PDMS is preferentially situated near the substrate-facing surface.

26
5. Microscopic Examination of Urethane-silicone Film

The urethane-silicone film taken from a reject intra-aortic balloon was also

examined under the optical microscope.

It is of interest to note, in connection with the evidence for differences

in composition between the inner (substrate-facing) and outer (air-facing) parts

of the wall of the intra-aortic balloon, that a gross difference in morphology

was observed under the light optical microscope between the region close to

the inner part of the wall as compared with the outer part. Specifically, the

inner region exhibits sharply outlined circular or quasi -ci rcul ar features

(diameter usually in the range 5vim-20ijm) . These features are illustrated at

low magnification in Fig. 9 and at higher magnification in Fig. 10a. In both

Fig. 9 and 10a the balloon wall is lying on the microscope stage with the inner

surface facing upwards. The microscope was focused, as closely as could be judged,

on the inner surface in Fig. 9 and Fig. 10a and at slightly increasing depths

(but still close to the inner surface) in Figs. 10b, c. The overlapping between

the outlines of neighboring circles clearly shows that these features are not

coplanar. The important aspect to note is that n^ circular features such as those

just described are present near the outer part of the wall.

The origins and nature of the circular features illustrated in Figs. 9 and 10

are not known to us at this stage, nor do we know whether their occurrence is

typical of the inner (substrate- faci ng ) region of all urethane-silicone membranes

or sheets. In view of the results from the ATR experiments, it is tempting to

speculate that the circular features are regions of segregated ROMS. We note,

however, that this would represent segregation on a much grosser scale than has

been previously suggested by Nyilas and Ward, [5].

27
28

Fig.

9.

Transmitted

light

optical

micrograph

of

the

wall

of

an

intra

-aortic

balloon

(polyurethane-polydimethyl

siloxane

elastomer).

The

inner

face

("substrate

side")

of

the

wall

is

facing

upwards

and

the

microscope

is

focused

on

that

face.

Note

the

small

circular

or

nearly

circular

features

in

the

field

of

view,

their

diameter

generally

falls

in

the

range

5-20pm.

No

such

features

occur

at

or

near

the

outer

face

("air

side")

of

the

wal 1
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for

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10a.
6. Biaxial Fatigue Cell Development

In all of the proposed designs of circulatory assist devices that we have

seen, elastomeric materials are used for various purposes such as diaphragms,

bladders, valves, and tubulature. In all of these applications the materials

are required to withstand cyclic multiaxial stresses and strains. Since the bulk

moduli of the elastomers proposed for these applications are much higher than their

shear moduli, it is reasonable to treat them as incompressible so that biaxial

testing is sufficient for complete evaluation. Further, failure may be expected

to be accelerated much more by positive stresses than by compressive stresses.

We have, therefore, chosen to develop an experimentally simple disc inflation test.

In this test, a circular disc of material is clamped at its edges and subjected

to a pressure difference across its faces. Many isotropic elastomers are found

to deform to a shape closely approximating a segment of a sphere. (James, et.al. [6]).

The deformation at the pole is equi-biaxial extension (X-|=A

2

='

)

• That at

the edge of the disc is pure shear (X-|=X,X2=1 ,X2=1/X) . The most damaging deformation

is thus near the center of the disc and previous workers have found that most failures

occur there.

We have designed a test cell shown in Figure 11. It is composed of two glass

flange joints one of which is a flat flange while the other is grooved for an 0-ring.

The inner diameter of the flat flange is only slightly less than the mean diameter

of the 0-ring. The test sample is clamped between the grooved flange and the 0-ring

and it is inflated so that it deforms around the 0-ring into the larger flat flange

joint without chafing against the glass cell. The sample is approximately one mm thick.

32
Fig. 11. Biaxial

between two glass

by applying press

Fatigue Test Cell. A six cm diameter test specimen is clamped

flange joints and deformed to an approximate spherical segment

ure pulses.

33
As mentioned above, the sample deforms into a shape approximating a segment

of a sphere. The volume of this segment can be determined from measurements

of the height of the liquid in the vertical tubes of the cell. Assuming the

spherical geometry, one can calculate the extension ratio at the pole of the

sample and its radius of curvature. This permits the calculation of the stress

in the material from the pressure difference. A calibration curve is shown in

Figure 12. The extension ratio is X and the stress a = K P/tg where P is the

pressure difference across the sheet which has undeformed thickness, t^ and K

is a calibration constant.

Pressure is applied from a reservoir tank and controlled by a motor driven

stop-cock which alternately connects one side of the test cell to the reservoir

and then vents it to the atmosphere. We have been operating at a rate of three

pressure pulses per second. An oscillogram showing the pressure pulses is shown in

Figure 13.

Several problems with the apparatus have appeared. First, with the maximum

amplitudes which we anticipate using (x = 1.5), considerable splashing occurs in

the liquid in the vertical tubes. This might be best controlled by enlarging these

tubes. We have had some success by floating discs of polystyrene foam on the

surfaces of the liquid. Second, wear of the stop-cock valves causes a slow drift

of the amplitude of the pressure pulse applied. This will require regular adjustment

for compensation. The stop-cock valves have a service life of about two months

and will require replacement at about this interval. Third, the inertia of the

liquid in the apparatus contributes to the pressure amplitude experienced by the

sample. A rough calculation shows that this error is less than 1% at the highest

amplitude and that it is proportional to the square of the amplitude. Finally, the

rate of evaporation from the test cell is considerably higher than anticipated due

to the air cycling.

34
VOLUME cm^

Fi=g. 12. Calibration curve for the disc inflation test. From measurement of the

volume of the inflated circular disc one can obtain the extension ratio at the

pole and the stress factor: a = KP/tg, where P is the pressure difference and

tg the original thickness.

35

EXTENSION

RATIO
Fig. 13. Oscillogram of pressure pulses. Horizontal scale 0.1 second per division.

Vertical scale 3 psi per division. The motorized valve producing these pulses

has operated reliably for more than 5 x 10° cycles.

36
Two samples of the butyl rubber have been tested thus far. They lasted

with pressure amplitude of about 10 psi for 23 days (6 x 10 cycles) and with

C pressure amplitude of 15 psi for less than two days (0.5 x 10 cycles). Their

strain amplitudes were approximately 1.3 and 1.5 respectively. They both failed

by growth of a small crack near the center of the sample growing in a radial

direction.

37
References

1. Coleman, B. D., J. Poly. $ci

, 22, 447 (1956).

2. Coleman, B. D., J. Appl . Phys. , 29, 968 (1957).

3. Green, A. E. and Adkins, J. E., Large Elastic Deformations , Clarendon Press,

Oxford, 1970.

4. Hummel, D. C., "Infrared Spectra of Polymers in the Medium and Long Wavelength

Regions," Interscience, 1966, ’New York.

5. Nyilas, E. and Ward, R. S. , J. Biomed. Mater. Res. Symposium, No. 8, pp 69-84 (1977).

6. James, A. G., Green, A., and Simpson, G. M., J. Appl. Poly. Sci

, 19 , 2033 (1975).

7. Roberts, B. J., and Benzies, J. B., Proc. Int. Rubber Conf., Brighton, 1977.

38