Elastomers and Rubber Elasticity - American Chemical Society

29 Characterization of Elastomers for Use in Circulatory Assist Devices C A R L R. McMILLIN

Downloaded by AUBURN UNIV on September 12, 2017 | http://pubs.acs.org Publication Date: July 19, 1982 | doi: 10.1021/bk-1982-0193.ch029

Monsanto Research Corporation, Dayton Laboratory, Dayton, O H 45418

The introduction of newer and better elastomers for use i n the rubber diaphragm of circulatory assist devices i s currently hampered by a lack of short -term fatigue tests to evaluate candidate materials. Monsanto Research Corporation (MRC) has been evalu ating a variety of accelerated fatigue tests that could be used to test candidate elastomers. A l though blood i s generally considered to be a very corrosive environment, our tests indicate that it actually extends the fatigue l i f e of elastomers compared to a i r or water environments. A combina tion of fatigue tests using both cut-initiated and uncut test specimens appears to provide the most information to predict the long-term fatigue char acteristics of elastomers for use i n circulatory assist devices. The development of cardiac assist devices and other instru mentation and components which come i n contact with the cardiovas cular system requires materials which w i l l perform i n a physiolog i c a l environment. One particularly d i f f i c u l t application i s the diaphragm for blood pumps, which requires an elastomer to undergo cyclic deformation and/or flexing while i n contact with blood. Currently, few data are available on the fatigue properties of elastomers cycled at body temperature and i n contact with blood. A r t i f i c i a l heart device applications are particularly demanding of elastomers since even a three-year implantation of a heart w i l l involve i n excess of 10 cycles. Evaluation of the fatigue properties of elastomers suitable for cardiovascular application i s currently a long, and therefore, costly proposition. This i s especially true if the fatigue tests are to be conducted i n contact with blood. 8

0097-6156/82/0193-0533$06.00/0 © 1982 American Chemical Society Mark and Lal; Elastomers and Rubber Elasticity ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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The objectives of this research are to develop short-term in v i t r o fatigue test methodologies that w i l l predict long-term in vivo performance of elastomers used i n cardiac assistance and related devices and to evaluate the fatigue l i f e of candidate materials for potential use i n these applications.

Materials Characterization Many studies, p a r t i c u l a r l y i n the l i f e sciences area, are not reproducible because small quantities of unique materials were used i n the research. A commitment was made when the present program began to characterize f u l l y the elastomers u t i l i z e d i n the development of accelerated fatigue tests. At the start of the program, a limited number of candidate materials was selected to be obtained i n quantity, characterized, and used for a l l of the developmental testing. The materials selected for evaluation included three mater i a l s currently being used i n these applications:Biomer tive of segmented ether-type polyurethanes; Avcothane-51(Avco Everett Research Laboratory, Inc., Everett, MA), a block copolymer of 10% silicone rubber and 90% polyurethane; and Hexsyn(Goodyear Tire and Rubber Company, Akron, OH), a sulfur vulcanized hydrocarbon rubber that i s essentially a polyhexene. Also selected, because of their easy a v a i l a b i l i t y , were Pellethane(Upjohn Company, North Haven, CT), an ether-type of polyurethane capable of being extruded i n sheet form, and a butyl rubber formulation, compounded and molded at the National Bureau of Standards. The material thickness varied, but the sheets were generally about 1 mm thick. The quantity of these materials needed for a l l of the i n tended fatigue testing research(by three contractors) was e s t i mated to be about 10 m each. Since this i s about two orders of magnitude greater than the amount of these specialized materials generally manufactured at any one time, a substantial manufacturing effort was required p r i o r to any fatigue testing. A l l of these materials were extensively characterized(physical/ mechanical/and b i o l o g i c a l properties) p r i o r to the i n i t i a t i o n of the fatigue tests. These tests have included tensile strength, e l a s t i c modulus, and elongation at several temperatures, different i a l scanning calorimetry, thermal gravimetric analysis i n d i f ferent atmospheres, surface tension, permeability to f l u i d s , molecular weight, c y t o t o x i c i t y , mutagenicity, thermal mechanical analysis, rheovibron testing, oxygen aging, Fourier transform infrared analysis of surface and i n t e r i o r absorbed protein, tear strength and tear energy before and after long term exposure to blood, and many other properties and characteristics. The majority of the characterization for Hexsyn and Pellethane were conducted at MRC while Biomer was characterized at the University of Washington i n St. Louis by Dr. J. Kardos and Avcothane and butyl were characterized at the National Bureau of Standards by Dr. R. Penn. A few of the characteristics of the 2

Mark and Lal; Elastomers and Rubber Elasticity ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Elastomers in Circulatory Assist Devices

Pellethane and Hexsyn are summarized here. The Pellethane 236380A tested by MRC i s commercially available. Our samples had a tensile strength of 7,830 p s i and an elongation of 380% at 37°C. Hexsyn i s a trade name of Goodyear Tire and Rubber Company, Akron, OH, for a family of materials synthesized by the copolymeri z a t i o n of a C to C α-olefin with a mixture of 4-methyl- and 5-methyl-l,4-hexadiene(1, 2). The "standard" Hexsyn i n use has been produced by Goodyear by polymerizing a mixture of 95% 1-hexene and 5% methyl hexadiene with the 4-methyl and 5-methyl i n a 2:3 ratio using a Ziegler-Natta hetrogeneous catalyst. The gum rubber has a molecular weight of Mw = 781,000 by l i g h t scattering. The gum rubber i s then vulcanized using a t r a d i t i o n a l accelerated, high crosslink e f f i c i e n c y carbon black f i l l e d mixture. The resulting rubber has a tensile strength of 1,740 p s i and elonga tion of 285% at 37°C. The glass transition temperature has been found to be -52°C by d i f f e r e n t i a l scanning calorimetry. 4


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Fatigue Tests In the l i t e r a t u r e i t i s easy to observe that different types of fatigue tests rank materials i n different orders. Thus, to predict a material's behavior f o r specific long-term end-use application, i t i s important to select the correct fatigue test. Most early a r t i f i c i a l hearts and l e f t ventricular a s s i s t devices consisted of a hemispherical diaphragm alternately i n flated and deflated by compressed a i r . The controls generally consisted of the amounts of pressure and vacuum applied to the device. In this case, the elastomeric diaphragm was exposed to an alternating pressure or stress and would d e f o r m ( o r strain) accordingly. For this situation, the most predictive type of fatigue test would be a constant stress test that applied an alternating stress or load on the test specimens. That i s , i n each cycle a load of 2 kg, for example, would be applied to each test specimen. Creep tests tend to rank the candidate materials in the same order as constant stress fatigue tests. More recently, the National Institutes of Health have been emphasizing hearts and a s s i s t devices capable of mechanical or e l e c t r i c a l operation. Most of these devices have a pusher plate that moves back and forth to displace the diaphragm a f i x e d ( b u t adjustable) distance, generally independent of the pressure of each stroke. The fatigue test which most closely mimics this con d i t i o n w i l l deform the test specimens to a fixed deformation or strain i n each cycle. This i s the type of fatigue test machine that MRC has constructed for this program. A l l of the uniaxial fatigue data reported to date have been generated using a fatigue testing machine b u i l t by MRC that i s based on the p r i n c i p a l of a wobble plate. The machine has posi tions for 30 specimens and i s capable of operation at controlled



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temperatures using a wide variety of testing media. The tester provides a sinusoidal s t r a i n to the specimens and has a variable speed, although a l l data presented here were tested at 120 cycles per minute. Standard fatigue tests normally require extended periods of time to complete regardless of the type of stress/strain used. Accelerated fatigue tests change some conditions of the test i n the hope of reaching the same end point i n less time. Potential means of accelerating the tests include changing factors such as the temperature, frequency, chemical environment, levels of stress or s t r a i n , or the addition of stress concentrators. I f the s t r a i n or stress i s to be changed, one possible method of accelerating the fatigue test involves increasing the s t e e l , polystyrene, poly(methyl methacrylate), nylon, and reinforced p l a s t i c ( 5 ) . Although some deviations have been n o t e d ( 4 , 6), this type of test has the advantages of speed, elimination of long-term run-outs, and reduction of scatter of the data near the endurance l i m i t . One potential problem with a l l methods that use a higher than end-use s t r a i n or stress i s the p o s s i b i l i t y that the f a i l u r e mechanism changes at higher s t r a i n s , which may lead to erroneous predictions. However, a l l accelerated fatigue tests suffer from t h i s potential d i f f i c u l t y ; conditions for a l l of them vary from real l i f e . An increasing s t r a i n was used to accelerate the test for a l l of the uniaxial fatigue data generated at MRC to date. Five inch ASTM "dog bone" t e n s i l e specimens were used. The s t r a i n was gene r a l l y increased 5% each day. In addition to using normal test specimens, some of these tests were conducted after a 1 mm razor blade cut was placed i n the side of the fatigue test specimen. These specimens were tested under conditions where the s t r a i n was increased 1.5% each day. The type of test conducted can be seen i n Table I where the maximum c y c l i c s t r a i n i s increasing from 1.7% on day 1 to 35% on day 20. These data were plotted as cumulative probability to f a i l u r e versus percent s t r a i n at f a i l u r e , as shown i n Figure 1. Regression lines are then f i t to the data. The strain at 50% probability of f a i l u r e has been used for a l l comparisons of data. Uniaxial Fatigue Test Results Increasing temperature decreases the modulus of e l a s t i c i t y of elastomers at the same time that accelerated chemical effects occur. In our constant s t r a i n fatigue tests i t has been found that the effect on the modulus(lower stress at the same strain) i s s i g n i f i c a n t l y more important than chemical degradation e f f e c t s , as shown i n Figure 2. Thus, f a i l u r e occurred much more rapidly at 4°C than at 37°C or 60°C. This may be one manner i n which fatigue


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TABLE I . CHRONOLOGICAL FATIGUE LIFE FOR AVCOTHANE-51, MRC LOTS 141265 AND 1412690 CUT-INITIATED SPECIMENS, CYCLED AT 120 CPM IN DISTILLED WATER AT 37°C Testing day 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Percent cyclic strain 0-1.7 0-3.5 0-5.2 0-7.0 0-8.5 0-10.0 0-11.5 0-13.0 0-15.0 0-17.0 0-19.0 0-21.0 0-22.8 0-24.6 0-26.3 0-28.0 0-29.8 0-31.5 0-33.3 0-35.0

Specimens failed

Accumulated cycles

2,332,800

* * *

2,678,400 2,851,200 3,024,000 3,369,600


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STRAIN, % Figure 1. Cumulative distribution function for a step-increasing strain fatigue test cycled at 120 cpm in distilled water at 37 °C using cut-initiated specimens.




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tests can be accelerated. Extreme care must be taken, however, to assure that the tests remain v a l i d , even at 4°C, since the glass t r a n s i t i o n temperature [Tg] of the candidate materials are d i f ferent. Extrapolating data taken close to the Tg of one material to end-use conditions may not be the equivalent of reduced temperature testing of other materials with much lower glass t r a n s i t i o n temperatures. Another set of fatigue tests evaluated the effect of oxygen on accelerated fatigue tests. These tests found no apparent effect of oxygen on the fatigue l i f e of the uncut test specimens, as shown i n Figure 3. On the other hand, fatigue tests using test specimens with a 1-mm cut i n the side showed a substantial decrease i n fatigue l i f e i n the presence of oxygen, as shown i n Figure 4. The dependence on oxygen for the c u t - i n i t i a t e d tests i s believed by MRC to be s i g n i f i c a n t . The period of testing was approximately three to s i x weeks i n both tests. One hypothesis we are investigating i s that unless accelerated fatigue tests demonstrate an oxygen dependence, they are probably representative of the wrong portion of the fatigue l i f e curve and may not properly predict long-term results. Further research on the significance of oxygen dependence on the v a l i d i t y of predictive accelerated fatigue tests w i l l be conducted by MRC. Environmental effects also need to be considered when developing fatigue testing methodology. The human body presents an unbelievably corrosive environment for foreign materials. The best stainless steels, cobalt a l l o y s , and even platinum undergo p i t t i n g , cracking, and f a i l u r e when used i n vivo. I t i s l i t t l e wonder that the physical properties of polymers are generally degraded after i n vivo implantation. MRC has evaluated the effect of fatigue testing of candidate cardiovascular materials i n human blood versus fatigue testing i n air and saline - a l l at 37°C(Figures 5 and 6). Whole human blood was chosen as one of the mediums for testing at 37°C, i n spite of the d i f f i c u l t i e s associated with this testing. We used outdated human CPD(citrate-phosphate-dextrose) blood from the Dayton Community Blood Center. At 21 days, CPD blood still retains 78% survival of the red blood c e l l s and would f a i r l y well simulate i n vivo physiological conditions. During these tests, many enzymes and proteins may denature and/or precipitate. Even after suffering that trauma, the resulting f l u i d i s more suitable for material testing than other pseudo-physiological f l u i d s , since i t still contains most of the s a l t s , l i p i d s , hormones, oligomers, nucleotides, saccharides, etc., found i n whole blood i n vivo. We have found that blood used i n our fatigue tester needs to be changed about once a week. We routinely add p e n i c i l l i n , streptomycin, and mycostatin to the blood to prevent b a c t e r i a l and


Mark and Lal; Elastomers and Rubber Elasticity ACS Symposium Series; American Chemical Society: Washington, DC, 1982. AIR

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Figure 3. Oxygen influences upon uncut specimens from step-increasing strain fatigue tests cycled at 120 cpm and at37°C.

AVCOTHANE

PELLETHANE

BIOMER

HEXSYN

BUTYL RUBBER



Figure 4. Oxygen influences upon cut-initiated specimens from step-increasing strain fatigue tests cycled at 120 cpm and at 37°C.



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Figure 5. Strain at 50 percent failure in fatigue tests conducted at 37°C on uncut specimens.


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mold growth. Also, heparin and/orethylenediaminetetraacetate(EDTA)(acalcium chel In spite of these precautions, the v i s c o s i t y of the blood begins to increase and microbiological growth begins to occur after about one week. The instrument i s therefore drained, cleaned with a mild soapy solution, and r e f i l l e d on a weekly basis. Results from our fatigue tests have shown that, i n the oneto two-month accelerated fatigue t e s t s , blood i s a less severe environment than saline or a i r . This may be due to adsorption of the blood components into the microcracks, resulting i n reduced stress concentrations. More work i s anticipated to study the effect of blood on the fatigue l i f e of elastomers. A l l of the above results were generated by tests i n which the c y c l i c s t r a i n was started low and gradually increased u n t i l a l l of the specimens broke. Longer term fatigue tests at d i f f e r e n t , discrete, s t r a i n levels are also planned for the future. Ranking of the Candidate Cardiovascular Materials Results from our accelerated fatigue tests and s t a t i c mechanical/physiological tests have confirmed our assumption that a d u r a b i l i t y ranking of the selected materials with respect to their usefulness i n cardiovascular devices i s decidely influenced by the type of testing methodology selected. Table I I i s a comparison of five candidate materials* relative rankings using five test methodologies. The asterisk denotes the material with the longest d u r a b i l i t y or highest resistance to the p a r t i c u l a r test method indicated. A l l five materials tested were each found to be superior i n one of the five tests cited i n Table I I . Thus, a l l five materials being tested could be argued to be the superior elastomer currently being tested. This i l l u s t r a t e s the d i f f i c u l t y of ranking candidate materials. Care must be taken to assure that the correct types of tests are used to rank materials for specific applications. For example, most recently designed circulatory a s s i s t devices u t i l i z e a pusher plate mechanism i n which the pusher plate i s displaced a predetermined distance. This type of device would best be modeled by a constant s t r a i n fatigue test. Materials which creep and have a high permanent set tend to f a i l more rapidly on a constant stress fatigue test but l a s t a longer time i n constant s t r a i n fatigue tests. The bending and compression of finger j o i n t prostheses or the r o l l i n g of the side material i n pusher plate c i r culatory a s s i s t devices may not be well modeled by either of the above tests.



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TABLE I I . FATIGUE RANKING IS INFLUENCED BY THE TEST SELECTED

Cut Uncut blood Uncut at 37°C growth a i r at s t r a i n at Uncut N a i r at 50% f a i l u r e at 37°C 37°C 37°C 2

Elastomer

Cut growth fixed s t r a i n cycle 1 hr. failure strain

Hexsyn

18*

190

150

125

30

Biomer

12

260*

140

150

25

Avcothane

16

-

>290*

-

-

Pellethane

6

210

205

175*

12

12

140

110

115

>45*

Butyl

The material with the longest durability or highest resistance to the particular test method indicated. A l l numbers represent percent s t r a i n at 50% f a i l u r e .



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Conclusions After extensively characterizing and fatigue testing the five candidate materials, we have some opinions on their usefulness i n cardiovascular applications: Biomer - This material performs well under a variety of experimental conditions and should be considered f o r use i n most cardiovascular applications. Particular advantages are the low permanent set, high resistance to creep, and generally good f a tigue resistance. Disadvantages include a r e l a t i v e l y high permeability to f l u i d s , a necessity to be f i l m cast i n multiple layers as the only method of fabrication, and an uncertainty as to the effect of thickness on performance(better fatigue results were obtained with the thinner films). Hexsyn - Performance varies from adequate to very good i n a variety of fatigue t e s t s , and Hexsyn should be considered for use i n most cardiovascular applications. P a r t i c u l a r advantages i n clude low permeability to f l u i d s , high resistance to cut growth i n fatigue, the a b i l i t y to be compression molded(good for high v o l ume application), r e l a t i v e l y low cost of the raw materials, and ready i n medical grade material. Disadvantages include an intermediate amount of permanent set, lower resistance to creep, lower modulus of e l a s t i c i t y ( w h i c h may be an advantage i n some applications), and low tear strength(although Hexsyn has a high r e s i s tance to cut growth i n fatigue). Avcothane - This material performs exceptionally well i n most constant strain fatigue tests and should be considered for use i n most cardiovascular applications. A p a r t i c u l a r advantage i s i t s fatigue test performance under a variety of conditions. Disadvantages include a very high permeability to f l u i d s , a very high permanent set, a very low resistance to creep, a high cost of materials, and a necessity to be f i l m cast i n multiple layers under r i g i d controls available only at the material suppliers facilities. Pellethane - This material performed s a t i s f a c t o r i l y i n some of the fatigue tests and should be considered for some cardiovascular applications, p a r t i c u l a r l y i n n o n c r i t i c a l locations. Advantages of this inexpensive, commercially available thermoplastic include i t s a b i l i t y to be formed into parts by a wide variety of methods such as extrusion, blow molding, injection molding, or f i l m casting. In fatigue situations without stress concentrators, Pellethane s performance was similar to that of Biomer. From our tests, if a thick part were required, Pellethane should be considered. Disadvantages include a rapid decrease i n fatigue r e s i s tance i n the presence of cuts or other stress concentrators. I t s permeability to f l u i d s i s similar to Biomer s, but i t has a s l i g h t l y higher permanent set and lower resistance to creep. Although Pellethane i s probably the most inexpensive material tested, i t i s not available i n a s p e c i f i c , closely controlled, medical grade. 1

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Mark and Lal; Elastomers and Rubber Elasticity ACS Symposium Series;Washington, American Chemical Washington, DC, 1982. D.C.Society: 20036


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Butyl rubber - This material generally had the least endur ance in fatigue tests, but i t may be adequate for some cardiovas cular applications. Advantages include less sensitivity to stress concentrators than Pellethane, a very low permeability to fluids, a moderate creep resistance and widespread a v a i l a b i l i t y at low cost. Disadvantages include a relatively low fatigue resistance compared to the elastomers specifically designed for these appli cations. The rubber tested was not designed for medical applica tions and had standard rubber additives and modifiers that were cytotoxic unless the material was extracted after manufacture. Further testing of these and a wide variety of other mate r i a l s i s continuing at MRC. Literature Cited 1. L a l , J.; Sandstrom, P. H. U.S. Patent 3,794,696, Febru ary 26, 1974, assigned to Goodyear Tire and Rubber Company. 2. Lal, J.; Sandstrom, P. H. U.S. Patent 3,991,262, November 9, 1976, assigned to Goodyear Tire and Rubber Company. 3. Lazar, L. S. ASTM Bulletin, 67-72, February 1957. 4. Weibull, W. "Fatigue Testing and Analysis of Results;" Pergammon Press: New York, New York, 1961. 5. Thorkildsen, R. L. In "Engineering Design for Plastics;" E. Baer, Ed., Reinhold Publishing Corporation: New York, New York, 1964. 6. Prevorsek, D. C.; Brooks, M. L. J. Appl. Poly. Sci. 1967, 11, 925-927. RECEIVED April 1,

1982.


Elastomers and Rubber Elasticity - American Chemical Society

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