Polymeric Materials and Artificial Organs - American Chemical Society

7 Characteristics of an Implantable Elastomer Finger Joint Prosthesis Application

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 23, 2016 | http://pubs.acs.org Publication Date: June 8, 1984 | doi: 10.1021/bk-1984-0256.ch007

H. B. LEE, H. QUACH, D. B. BERRY, and W. J. STITH Lord Corporation, Bioengineering Department, Erie, PA 16514

Bion elastomer has been implanted i n humans as part of the Biomeric finger j o i n t prosthesis for the past four years. C l i n i c a l trial experience of the Biomeric prosthesis (over 500 j o i n t s implanted) i n many research i n s t i t u t i o n s has indicated that Bion e l a s tomer has excellent stability, f u n c t i o n a l i t y , and biocompatibility. No adverse reactions to the material have been reported. Marketing approval for the prosthesis has been granted by the FDA. Extensive characterization of the basic polymer and its compounded elastomer, Bion, has been done to support its use as an implantable material s u i t a b l e for a v a r i e t y of medical a p p l i c a t i o n s . The material e x h i b i t s excellent b i o c o m p a t i b i l i t y , is r e s i s t a n t to o x i dation, and is stable to i r r a d i a t i o n at sterilization dose l e v e l s . Other major advantages are its high f l e x life and its low permeability by liquids. Physical properties can be t a i l o r e d by judicious s e l e c t i o n of polymer composition and elastomer formulation. ,

Since the m i d - 1 9 5 0 s , a number of f i n g e r prostheses have been d e veloped f o r r e s t o r i n g f u n c t i o n , c o r r e c t i n g d e f o r m i t i e s , and r e lieving pain. P r e s e n t l y the l e a d i n g products on the market are the Swanson and Niebauer prostheses ( F i g u r e 1 ) . In e a r l y 1960, Swanson (1) introduced the use of s i l i c o n rubber i n t h i s a p p l i c a t i o n . A cruciform bar of S i l a s t i c provided support across the j o i n t and held the raw bone ends apart as a spacer. The major reported d e f i c i e n c i e s of Swanson*s product were f r a c t u r e i n the stem and the l a c k of s t a b i l i t y i n the j o i n t cavity. The Niebauer j o i n t (2) i s made of a Dacron-reinforced s i l i c o n rubber. The stems are covered w i t h a Dacron mesh i n t o which f i b r o u s t i s s u e can grow, thus e f f e c t i v e l y l o c k i n g the stem i n p l a c e . I t s major d e f i c i e n c y i s f r a c t u r e across the h i n g e . Beckenbaugh (3) reported that the f r a c t u r e r a t e s of Swanson 0097-6156/84/0256-0099S06.00/0 © 1984 American Chemical Society

Gebelein; Polymeric Materials and Artificial Organs ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


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POLYMERIC MATERIALS AND ARTIFICIAL ORGANS

(A) Swanson j o i n t (Dow Corning)

(B) Niebauer j o i n t ( S u t t e r )

(C) Biomeric j o i n t (Lord

Corporation)

Figure 1. Finger p r o s t h e s i s .


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Finger Joint Prosthesis

and Niebauer prostheses, w i t h an average follow-up of two and one-half years i n c l i n i c a l i n v e s t i g a t i o n , were 26.2% and 38.2%, respectively. In the e a r l y 1970 s, Lord Corporation began to look toward the orthopaedic f i e l d as a n a t u r a l e x t e n s i o n of i t s e x p e r t i s e i n e l a s t o m e r i c bearings. The e l a s t o m e r i c bearing p r i n c i p l e a p p l i e d to prostheses imparts s t a b i l i t y and c o n t r o l l e d motion without i n c u r r i n g h i g h r e s t r a i n i n g f o r c e s . The use of elastomer allowed the j o i n t design to c o n s i s t of t i t a n i u m stems f o r f i x a t i o n , a p i n p o s i t i o n e d t r a n s v e r s e l y through the e l a s t o m e r i c s e c t i o n f o r l a t e r a l s t a b i l i t y , and an elastomer bridge bonded between the t i tanium stems.


f

S e l e c t i o n of Elastomer Numerous elastomers were evaluated f o r f i n g e r j o i n t p r o s t h e s i s a p p l i c a t i o n . Hexsyn showed the best c h a r a c t e r i s t i c s f o r t h i s application. For s e v e r a l y e a r s , Goodyear s u p p l i e d t h e i r compounded polymer under the name of Hexsyn to v a r i o u s research c e n t e r s ; namely, Monsanto Research Corporation ( 4 ) , Washington U n i v e r s i t y ( 5 ) , N a t i o n a l Bureau of Standards ( 6 ) , Cleveland C l i n i c ( 7 ) , and Thermoelectron Corporation ( 8 ) . These i n s t i t u t i o n s have research programs f o r p h y s i c a l t e s t i n g of polymers f o r use i n c i r c u l a t o r y a s s i s t devices and f o r the development and e v a l u a t i o n of a c a r d i a c p r o s t h e s i s funded by the NHLB-NIH. The o b j e c t i v e of the f i r s t three i n s t i t u t i o n s p r o j e c t s i s to develop short-term f a t i g u e t e s t methodologies that w i l l p r e d i c t long-term i n v i t r o performance of elastomers used i n the devices and to evaluate the f a t i g u e l i f e of candidate m a t e r i a l s f o r pot e n t i a l use i n the devices. Cleveland C l i n i c and Thermoelectron Corporation u t i l i z e t h i s elastomer f o r pumping diaphragms. 1

F l e x L i f e . K i r a l y and H i l l e g a s s (9) reported f l e x l i f e of v a r i ous polymers as shown i n Table I . T h e i r r e s u l t s show c l e a r l y that the f l e x l i f e of Hexsyn i s s u p e r i o r to that of other e l a s tomers. P o i r i e r (10) at Thermoelectron C o r p o r a t i o n i n v e s t i g a t e d seven elastomers f o r blood pump bladder a p p l i c a t i o n s . The f l e x l i f e of diaphragms from the elastomers showed that Hexsyn, P e l l e t h a n e , and Biomer were s i g n i f i c a n t l y s u p e r i o r to T e c o f l e x HR, Tecothane B, S i l a s t i c , and SRI. M c M i l l i n (11) at Monsanto i n v e s t i g a t e d u n i a x i a l f a t i g u e l i f e of v a r i o u s elastomers i n a i r , n i t r o g e n , oxygen, s a l i n e , and blood environments. His method of a c c e l e r a t i n g f a t i g u e i n d i c a t e d c u t i n i t i a t e d f a t i g u e t e s t i n g to be s i g n i f i c a n t i n p r e d i c t i n g l o n g term, low s t r a i n f a t i g u e f a i l u r e . I n t h i s r e s p e c t , Hexsyn rubber was ranked number one among the t e s t m a t e r i a l s .


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Table I . F l e x L i f e of Various Polymers ASTM D430 DeMattia Test Machine Cycles t o F a i l u r e (millions)


Polymer S i l i c o n e rubber styrene-butadiene rubber n a t u r a l rubber oxypropylene rubber ethylene-propylenediene-terpolymer Biomer Hexsyn

0.8 4 4 10 15 18 352 (no f a i l u r e )

B i o c o m p a t i b i l i t y . Primary acute t o x i c i t y screening t e s t s of the elastomer were conducted by M a t e r i a l s Science Toxicology Laborat o r i e s a t the u n i v e r s i t y of Tennessee, Johnson & Johnson Research Foundation, and North American Science A s s o c i a t i o n using standard procedures. R e s u l t s of primary acute t o x i c i t y screening t e s t s on medical grade Hexsyn elastomer are summarized i n Table I I and show e x c e l l e n t b i o c o m p a t i b i l i t y of the m a t e r i a l and i t s e x t r a c t s . Table I I . B i o c o m p a t i b i l i t y T e s t i n g of Hexsyn Tests D i r e c t l y on Sample: Tissue C u l t u r e — A g a r Overlay Intramuscular Implant (Rat) Intracutaneous Implant (Rat) Hemolysis Test

Non-cytotoxic Non-toxic Non-toxic Not s i g n i f i c a n t

Tests on E x t r a c t s : T i s s u e Culture—MEM E l u t i o n Intracutaneous Test ( R a b b i t s ) Systemic T o x i c i t y (Mice) C e l l Growth I n h i b i t i o n Ames M u t a g e n i c i t y Test

Non-cytotoxic Non-irritating No adverse e f f e c t s Not s i g n i f i c a n t Non-mutagenic

Implantable Bion Elastomer In 1979, Lord Corporation became the s o l e s u p p l i e r of Hexsyn rubber under l i c e n s e from Goodyear T i r e and Rubber Company. Minor changes were made i n the p o l y m e r i z a t i o n process of the b a s i c polymer t o compensate f o r l a r g e r s c a l e production runs, and


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the o r i g i n a l elastomer f o r m u l a t i o n was modified by s l i g h t l y r e ducing the l e v e l s of cure agents and s u l f u r . The new formula t i o n , known as B i o n , maintained p h y s i c a l p r o p e r t i e s and enhanced biocompatibility. Composition. Bion polymer i s a terpolymer of 1-hexene and 4-methyl-l,4-hexadiene and 5-methyl-l,4-hexadiene, the crossl i n k i n g agent. The polymer i s compounded w i t h carbon b l a c k and t r a d i t i o n a l v u l c a n i z a t i o n a i d e s . A standard f o r m u l a t i o n c o n t a i n s 3 mol % c r o s s l i n k i n g agent (% r e l a t i v e to 1-hexene) and 50 phr carbon black l o a d i n g . Elastomer f o r i m p l a n t a t i o n use i s ex t r a c t e d w i t h an a p p r o p r i a t e s o l v e n t i n a Soxhlet-type e x t r a c t o r to remove by-products of v u l c a n i z a t i o n and any l e a c h a b l e m a t e r i a l . Polymer P r o p e r t i e s . P h y s i c a l c h a r a c t e r i s t i c s of the b a s i c p o l y mer are summarized i n Table I I I .

Table I I I .

C h a r a c t e r i s t i c s of Bion Polymer

Molecular weight: Molecular weight d i s t r i b u t i o n : Gel content: Residual solvent: Color:

Mn = 0.6 - 1.0 χ 1 0 Mw/Mn = 1.5 - 1.8 Less than 3% Less than 2% White

6

D i l u t e s o l u t i o n v i s c o s i t y measurements were made u s i n g a Cannon-Fenske viscometer. Number average molecular weight (Mn) and weight average molecular weight (Mw) were c a l c u l a t e d from v i s c o s i t y measurements and the Mark-Houwink Constants (12). Gel con tent was determined by a m o d i f i c a t i o n of procedure ASTM D3616. Elastomer P r o p e r t i e s . Mechanical p r o p e r t i e s , w e t t a b i l i t y , and s w e l l i n g c h a r a c t e r i s t i c s of a t y p i c a l elastomer are summarized i n Table IV.

Table IV.

C h a r a c t e r i s t i c s of Bion Elastomer

F l e x l i f e (ASTM D430): Tensile strength: E l o n g a t i o n at breaking p o i n t : Tear s t r e n g t h (ASTM D624, Die C ) : Contact angle of water: S w e l l i n g i n hexane at room temperature: S w e l l i n g i n H 0 at 37°C: 2

6

over 300 χ 1 0 c y c l e s 13.1 MPa 350% 24.5 KN/M l / 2 ( 0 r e c + 9adv) = 50° 170 wt. % 0.9 wt. %


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F l e x l i f e , a measure of rubber d e t e r i o r a t i o n by dynamic f a t i g u e , was determined on a DeMattia f l e x i n g machine (ASTM D430). T e n s i l e and t e a r p r o p e r t i e s were determined on elastomer sheets (1/8 i n c h t h i c k ) using ASTM D412 and ASTM D624 r e s p e c t i v e l y . The contact angle of water on the elastomer was measured w i t h a capt i v e a i r bubble method (13). Resistance to I r r a d i a t i o n . Since medical devices are o f t e n s t e r i l i z e d by gamma-radiation, m a t e r i a l p r o p e r t i e s must be maintained a f t e r i r r a d i a t i o n . P h y s i c a l p r o p e r t i e s of the Bion elastomer were measured f o l lowing i r r a d i a t i o n of one, three, and f i v e times the standard s t e r i l i z a t i o n dose l e v e l (2.5 Mrads). T e n s i l e s t r e n g t h d i d not change s i g n i f i c a n t l y up t o 12.7 Mrads i r r a d i a t i o n . Elongation and s w e l l a b i l i t y decreased w h i l e hardness increased w i t h dosage due to increased c r o s s l i n k d e n s i t y I n the rubber. The l o s s of low molecular weight polymer by e x t r a c t i o n increased slightly (Table V ) .

Table V.

R a d i a t i o n E f f e c t s on Bion Elastomer (Tensile Properties)

Dose (Mrads)

100% Modulus (MPa)

300% Modulus (MPa)

0 2.5 7.6 12.7

1.5 1.8 2.0 2.2

8.3 9.4 9.7 10.7

Ultimate T e n s i l e (MPa)

Ultimate Elong. (%) 430 390 350 350

13.1 13.1 11.7 12.4

(Hardness, E x t r a c t a b l e s , and Dose (Mrads)

Hardness Shore A

Extractable %

0 2.5 7.6 12.7

61+1 63+1 63+1 65+1

1.0 2.0 2.4 2.9

Swelling) S w e l l i n g i n hexane Wt. Gain (%) 168+3 156+4 15CH-2 147+2

P e r m e a b i l i t y . Bion elastomer has much l e s s d i f f u s i o n of s i l i c o n o i l and water than s i l i c o n rubber under the same t e s t i n g c o n d i t i o n s (ASTM D814). Comparative permeation r a t e s are l i s t e d on Table VI. A p p l i c a t i o n s demanding low permeable m a t e r i a l s i n c l u d e i m p l a n t a t i o n of encapsulated e l e c t r o n i c devices and s i l i c o n o i l f i l l e d breast prostheses.


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Table V I .

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Permeation Rate of S i l i c o n O i l and Water

Membrane

Diffusate

Permeation Rate gm/in day

Bion S i l a s t i c (a) Bion S i l a s t i c (a)

S i l i c o n o i l (a) S i l i c o n o i l (a) H0 H0

0.08 χ 10" (b) 0.8 χ 1 0 " (b) 0.01 (c) 0.2 ( c )

2

2

2

2

2

(a) : m a t e r i a l s f o r breast implant s u p p l i e d by M e d i c a l Engineering (b) : under vacuum at 37°C (c) : under f o r c e d a i r oven at 37°C

D i s p e r s i o n . The degree of f i l l e r d i s p e r s i o n i s very important i n o b t a i n i n g r e p r o d u c i b l e and d e s i r a b l e c h a r a c t e r i s t i c s of any f i l l e d rubber. The advantage of f i n e p a r t i c l e f i l l e r s i s l o s t i f aggregates of p a r t i c l e s are not broken down and i f the p a r t i c l e s are not w e l l d i s t r i b u t e d throughout the elastomer. Therefore, a l l elastomers are evaluated f o r homogeneity of d i s p e r s i o n before being accepted f o r t e s t i n g and use. A simple q u a l i t a t i v e v i s u a l method f o r r a t i n g the d i s p e r s i o n of f i l l e r s (50phr carbon b l a c k ) i n Bion elastomer was developed and i s i l l u s t r a t e d i n F i g u r e 2 . A c r o s s s e c t i o n of cured e l a s tomer i s examined under a b i n o c u l a r microscope to check gross d i s p e r s i o n of f i l l e r s . The v i s u a l d i s p e r s i o n i s r a t e d a g a i n s t a set of standard photographs of d i s p e r s i o n s which had p r e v i o u s l y been ranked and c o r r e l a t e d w i t h c e r t a i n important p h y s i c a l prop e r t i e s . For example, i n F i g u r e 3, the f l e x l i f e of a w e l l d i s persed elastomer was over 300 m i l l i o n c y c l e s w h i l e t h a t of a p o o r l y d i s p e r s e d one was below one m i l l i o n c y c l e s . The c o r r e l a t i o n of p h y s i c a l p r o p e r t i e s to d i s p e r s i o n has been s u b s t a n t i a t e d w i t h other rubbers (14). E f f e c t of C r o s s l i n k e r Content A wide v a r i e t y of p h y s i c a l p r o p e r t i e s of Bion elastomer can be obtained through v a r i a t i o n of c r o s s l i n k e r amounts i n the raw polymer and of carbon b l a c k l e v e l s i n the compounded elastomer. F i g u r e 3 shows t y p i c a l rheometer cure time curves of three Bion elastomers w i t h d i f f e r e n t c r o s s l i n k e r l e v e l s at 50 phr c a r bon b l a c k l o a d i n g . With higher c r o s s l i n k e r content i n the p o l y mer, the torque r e q u i r e d to shear the rubber d u r i n g v u l c a n i z a t i o n increased w h i l e cure time decreased. A t y p i c a l cure time of compounded elastomer having 50 phr carbon b l a c k and 3% c r o s s l i n k e r i n the raw polymer i s 48 minutes



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Figure 2. Comparison o f f i l l e r

dispersion.

Time (min) Figure 3. E f f e c t o f c r o s s - l i n k e r l e v e l on cure c h a r a c t e r i s t i c s . (Monsanto rheometer curves - 307 °F)


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Table V I I .

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Cure Time and F l e x L i f e of Various Bion Elastomers

Crosslinker Mole %

Carbon Black (phr)

Cure Time (min.)

Flex L i f e (cycles)

1 1 1 3 3 3 3

50 65 80 35 50 80 100

64 66 68 46 48 50 50

not determined not determined not determined not determined over 300 χ 1 0 (no f a i l u r e ) over 8 χ 1 0 (no f a i l u r e ) 3 χ 10 (failure) 6

6

6

at 307°F; m a t e r i a l c o n t a i n i n g 1% c r o s s l i n k e r has a s u b s t a n t i a l l y longer cure time of 68 minutes (Table V I I ) . P h y s i c a l p r o p e r t i e s of v a r i o u s Bion elastomers w i t h v a r i a t i o n of c r o s s l i n k e r amounts i n the raw polymer and carbon b l a c k l e v e l s i n the compounded elastomer are summarized i n Table V I I I . O v e r a l l , as c r o s s l i n k e r content i n c r e a s e d , cure time substan t i a l l y decreased. As c r o s s l i n k e r content i n c r e a s e s , modulus and hardness i n c r e a s e but u l t i m a t e t e n s i l e s t r e n g t h , e l o n g a t i o n and s w e l l i n g decrease. P e r m e a b i l i t y t o water was unchanged. E f f e c t of Carbon Black Loading U n l i k e c r o s s l i n k e r content, the l e v e l of carbon b l a c k i n the elastomer d i d not s i g n i f i c a n t l y a f f e c t cure time (Table V I I ) but did have a dramatic e f f e c t upon f l e x f a t i g u e l i f e . I n order to o b t a i n h i g h f l e x l i f e , the maximum l o a d i n g of carbon b l a c k was l i m i t e d t o 80 phr. Tear r e s i s t a n c e , modulus, and hardness i n c r e a s e along w i t h the f i l l e r content. M a t e r i a l c h a r a c t e r i s t i c s can be t a i l o r e d t o s u i t a d e s i r e d application. F o r example, blood c o m p a t i b i l i t y of v a r i o u s Bion elastomers was i n v e s t i g a t e d i n the a t r i a of goats by Dr. W i l liams' group (15) i n Toronto. I n i t i a l r e s u l t s i n d i c a t e d that elastomers c o n t a i n i n g high l e v e l s of carbon b l a c k showed g r e a t e r thrombo r e s i s t a n c e than those w i t h lower amounts or no carbon black. Conclusion Bion elastomer i s an implantable m a t e r i a l s u i t a b l e f o r a v a r i e t y of medical a p p l i c a t i o n s . The m a t e r i a l e x h i b i t s e x c e l l e n t biocom p a t i b i l i t y , i s r e s i s t a n t to o x i d a t i o n , and i s s t a b l e to i r r a d i a t i o n a t s t e r i l i z a t i o n dose l e v e l s . Major advantages are i t s h i g h


Gebelein; Polymeric Materials and Artificial Organs ACS Symposium Series; American Chemical Society: Washington, DC, 1984. 612 580 496 267 364 412 353 381 187 235

Ultimate Elongation (%) 1.9 1.9 1.6 2.3 1.7 1.9 1.6 1.7 2.0 1.9

2

Water Diffusion (mg/day cm ) 53 61 63 24 54 61 69 78 29 68

Hardness (Shore A) 24 26 30 1 14 25 27 28 4 23

Tear Resist. KN/M

218 184 N/A 375 193 161 134 104 N/A 129

Swelling Hexane % Wt. Increase

NOTE: T e n s i l e , tear, hardness, and f l e x l i f e c h a r a c t e r i s t i c s were determined with ASTM D412, D2240, D624, and D430, r e s p e c t i v e l y . Cure time was determined with a Monsanto rheometer, Model R-100. Swelling c h a r a c t e r i s t i c s of the elastomer were measured by weight d i f f e r e n c e a f t e r soaking the square shape (1 χ 1 χ 0.2 cm) i n hexane f o r 28 hours at room temperature.

17.0 16.0 14.4 1.4 12.7 15.0 16.7 16.6 1.0 11.8

Ultimate Tensile (MPa)

I n i t i a l feed amount during polymerization Parts per 100 parts raw polymer Not a v a i l a b l e

0.6 7.1 8.4 N/A 9.7 10.1 12.7 13.5 N/A N/A

1.2 1.4 2.0 0.2 1.7 2.2 2.7 2.9 0.4 3.6

50 65 80 0 35 50 65 80 0 50

1 1 1 3 3 3 3 3 6 6

a: b: N/A:

300% Modulus (MPa)

100% Modulus (MPa)

Carbon Black (Phr) (b)

P h y s i c a l P r o p e r t i e s of Various Bion Elastomers

Crosslinker Mole % (a)

Table V I I I .


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f l e x l i f e and i t s low p e r m e a b i l i t y by l i q u i d s . P h y s i c a l proper t i e s can be t a i l o r e d by j u d i c i o u s s e l e c t i o n of polymer composi t i o n and elastomer f o r m u l a t i o n . B i o n elastomer has been implanted i n humans as p a r t of the Biomeric f i n g e r j o i n t p r o s t h e s i s f o r the past four y e a r s .


Literature Cited 1. Swanson, A. B., J. Bone Joint Surg., 1972, 54A, 435. 2. Niebauer, J. J., J . Bone Joint Surg., 1968, 50A, 634. 3. Beckenbaugh, R. D., Dobyns, J . H., Linscheid, R. L. and Bryan, R. S., J. Bone Joint Surg., 1976, 58A, 483. 4. McMillin, C. R., Orofino, T. A. and Sheppard, D. L., "Physi cal Testing of Polymers", Devices and Technology Branch Con tractors Meeting Program, U.S. Department of Health, Educa tion and Welfare, 1979, p. 80. 5. Kardos, J. L . , Sanson, W. M. and Clark, R. E., "Physical Testing of Polymers for Use in Circulatory Assist Devices", Devices and Technology Branch Contractors Meeting Program, U.S. Department of Health, Education and Welfare, 1979, p.81. 6. Penn, R. W. and McKenna G. Β., "Physical Testing of Polymer for Use in Circulatory Assist Devices", Devices and Technol ogy Branch Contractors Meeting Program, U.S. Department of Health, Education and Welfare, 1979, p. 83. 7. Nose, Y., et a l . , "Development and Evaluation of Cardiac Prostheses", Annual Report, NIH-NHLB NO1-HV-4-2960-5, Cleve land Clinic Foundation, Cleveland, Ohio, 1979. 8. Poirier, V., "Fabrication of Cardiovascular Devices", De vices and Technology Branch Contractors Meeting Program, U.S. Department of Health, Education and Welfare, 1979, p. 35. 9. Kiraly, R. J. and Hillegass, D. V., "Polyolefin Blood Pump Components in Synthetic Biomedical Polymers: Concept and Ap plications", Szycher, Μ., Robinson W. J., Eds., Technomic Publishing Company, Inc.: Westport, 1980, p. 59. 10. Poirier, V., "Fabrication and Testing of Flocked Blood Blad ders in Synthetic Biomedical Polymers: Concepts and Applica tions", Szycher M., Robinson, W. J., Eds., Technomic Pub lishing Company, Inc.: Westport, 1980, p. 73. 11. McMillin, C. R., "Physical Testing of Polymers for Use in Circulatory Assist Devices", Annual Report, NIH-NHLB NO1-HV7-2918-3, Monsanto Research Corporation, Dayton, Ohio, 1980. 12. Lin, F. C., Stivala, S. S. and Biesenberger, J . Α., J. Appl. Polym. Sci., 1973, 17, 1073. 13. Andrade, et al., J. Polym. Sci.: Polym. Symp., 1979, 66, 313. 14. Morton, Μ., Rubber Technology, Van Nostrand Reinhold Com pany: New York, 1973, Chapter 3. 15. Williams, W. G., Hospital for Sick Children, Toronto, per sonal communication. RECEIVED March 19, 1984


Polymeric Materials and Artificial Organs - American Chemical Society

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