Petroleum Derived Carbons

19 Carbon In Prosthetic Devices

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 2, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch019

J. C. BOKROS, R. J. AKINS, H. S. SHIM, A. D. HAUBOLD, and Ν. K. AGARWAL General Atomic Co., San Diego, Calif. 92138

During the past decade, carbon has been evaluated and ac cepted as a material for use in the construction of implantable prosthetic devices. The chain of events that led to this devel opment is intriguing because, as with many developments, the pathway of progress contained curious combinations of coinci dences and misinterpretations. In the early 1960s, Gott and his colleagues at the University of Wisconsin set out to study the effect of electrical charges on the blood-clotting properties of plastic materials (1). Recalling Abramson's observations that the important components of blood are negatively charged (2) and Sawyer's findings that the inside lin ing of blood vessels is negative relative to the outside (3-4), Gott reasoned that the clotting of implanted foreign materials should be influenced by their surface charge. He prepared con ductive test specimens by coating them with a graphite paint* and, using a dry-cell battery, applied electrical charges to specimens implanted in blood vessels. He found that when a positive charge was applied to a specimen, it thrombosed, whereas when a negative charge was imposed, the specimen remained clot-free. More impor tantly, however, was the chance finding that specimens with no electrical connection did not clot (5). Apparently, the graphite paint was antithrombogenic. At about this same time, there was a parallel effort in the nuclear field to develop radiation-stable, impermeable carbon coatings for the nuclear fuel of high-temperature, gas-cooled, atomic reactors (6). Soon after Gott's results were reported, a mutual interest in carbon prompted, in 1965, a collaborative effort between Gott's group at the University of Wisconsin and the carbon group at General Atomic Company in San Diego to study the compatibility of pyrolytic carbons and blood and to relate the compatibility of carbon to its crystal structure and surface properties. *Dag 154 colloidal graphite paint manufactured by Acheson Colloids Company, Port Huron, Michigan 237

Deviney and O'Grady; Petroleum Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

PETROLEUM


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DERIVED

CARBONS

I r o n i c a l l y , the i n f e r e n c e by the e a r l y data that p a i n t p i g mented with c o l l o i d a l g r a p h i t e was antithrombogenic l a t e r proved to be f a l s e , and the antithrombogenic property of the graphite coatings was found to be due to a c o i n c i d e n t a l pretreatment procedure that was used b e f o r e the specimens were implanted. In the e a r l y t e s t s , the coated specimens were s t e r i l i z e d by soaking i n benzalkonium c h l o r i d e and, j u s t before Implantation, were r i n s e d i n a d i l u t e s o l u t i o n of heparin!zed s a l i n e i n order to minimize c l o t t i n g during the placement (1). I t was not u n t i l l a t e r that i t was found that g r a p h i t e p a i n t alone or g r a p h i t e paint treated with only a heparin r i n s e was thrombogenic and that only specimens that were coated w i t h g r a p h i t e p a i n t , soaked In benzalkonium c h l o r i d e , and then r i n s e d with heparin were a n t i thrombogenic (7). The antithrombogenicity r e s u l t e d from formation of a g e l a t i n o u s complex of the benzalkonium i o n with heparin that i s not very s o l u a b l e and was somehow bound t o the graphite s u r f a c e (8). I n i t i a l l y , the bonding o f heparin to graphite p a i n t was thought t o be due t o a weak chemical l i n k a g e between the hydrophob i c p o r t i o n o f the benzalkonium Ion and the a d s o r p t i v e , o l e o p h i l i c graphite s u r f a c e which, i n t u r n , was l i n k e d to the h i g h l y negative heparin molecule v i a the c a t l o n i c quaternary ammonium group. A c t u a l l y , however, subsequent s t u d i e s showed t h a t the bonding between the g e l a t i n o u s quaternary ammonium s a l t of heparin and the graphite p a i n t was mechanical. The g e l a t i n o u s benzalkoniumheparin s a l t was h e l d i n small s u r f a c e pores i n the graphite p a i n t , and the amount of the complex t h a t could be r e t a i n e d was d i r e c t l y dependent on the s u r f a c e p o r o s i t y ( j M O ) * In f a c t , seve r a l porous s u b s t r a t e s were found to be temporarily rendered a n t i thrombogenic by s u c c e s s i v e pretreatments w i t h benzalkonium c h l o r i d e and heparin. Only by chance was the conductive graphite p a i n t s e l e c t e d f o r use i n the e a r l y t e s t s porous and rendered antithrombogenic by pretreatment w i t h benzalkonium c h l o r i d e and heparin; the h i g h l e v e l o f thromboreslstance was not an Inherent property of the g r a p h i t e p a i n t . In any event, Gott's e a r l y experiments Q ) sparked i n t e r e s t i n using a v a r i e t y o f carbons i n p r o s t h e t i c s

(5, 11).

The p e c u l i a r chain o f events l e a d i n g from the e a r l y s t u d i e s of e l e c t r i c a l e f f e c t s using conductive graphite p a i n t to the misl e a d i n g r e s u l t that suggested t h a t graphite p a i n t was i n h e r e n t l y antithrombogenic provided the impetus t o study the c o m p a t i b i l i t y of the newly developed nuclear carbons. T h i s , i n t u r n , l e d to the s u r p r i s i n g f i n d i n g that c l e a n , p o l i s h e d i s o t r o p i c p y r o l y t i c c a r bons deposited a t r e l a t i v e l y low temperature (LTI carbons)* i n f l u i d i z e d beds a r e i n h e r e n t l y thromboresistant and t h a t the

* A v a i l a b l e commercially of General Atomic Company

under PYROLITE, r e g i s t e r e d trademark



19.

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thromboresistance i s not enhanced by pretreatment w i t h benzalkonium c h l o r i d e or heparin (10). Independently, i n the autumn of 1967, Benson, w h i l e involved with the NASA Biomedical A p p l i c a t i o n s Team, became i n t e r e s t e d i n p o s s i b l e biomedical a p p l i c a t i o n s of carbons developed f o r the space program and began an extensive c o l l a b o r a t i v e e f f o r t to e v a l uate the b i o c o m p a t i b i l i t y of v i t r e o u s and other carbons (12)« This work was instrumental i n advancing v i t r e o u s carbon as a mat e r i a l u s e f u l i n a v a r i e t y of transcutaneous a p p l i c a t i o n s i n c l u d ing a r t i f i c i a l tooth r o o t s . Although the usefulness of carbon as a b i o m a t e r i a l i s a d i r e c t r e s u l t of i t s b i o c o m p a t i b i l i t y , the breadth of i t s p o s s i b l e a p p l i c a t i o n s i s a consequence of i t s other p r o p e r t i e s , which are t a i l o r a b l e and i n combination make the m a t e r i a l q u a l i f i e d f o r use i n a v a r i e t y of d i f f e r e n t a p p l i c a t i o n s . For example, the benign p r o p e r t i e s of carbon have been demons t r a t e d by e a r l y uses of lampblack as a pigment f o r tatoos (12). This ancient a p p l i c a t i o n not only demonstrates the b i o c o m p a t i b i l i t y of carbon but a l s o provides evidence that carbon has the a b i l i t y to r e s i s t the c o r r o s i v e environment of the l i v i n g body and not be biodegraded by i t . Furthermore, the d e s i r a b l e engineering p r o p e r t i e s of carbon and i t s f a b r i c a b i l i t y i n t o a wide v a r i e t y of u s e f u l shapes c o n t r i b u t e to i t s v e r s a t i l i t y (13); e.g., carbon may be e i t h e r porous or impermeable (14), and c e r t a i n forms (the LTI carbons) have h i g h strength (15-16) as w e l l as wear and f a t i g u e r e s i s t a n c e (17-18). Benson's e a r l y survey (12) l i s t e d a wide v a r i e t y of carbonaceous forms and p r o j e c t e d many p o s s i b l e biomedi c a l a p p l i c a t i o n s , some of which have been r e a l i z e d . This paper i s devoted p r i m a r i l y to the uses of p y r o l y t i c c a r bons (the LTI carbons) s i n c e they have been c l i n i c a l l y accepted and are used e x t e n s i v e l y i n the c o n s t r u c t i o n of c a r d i o v a s c u l a r p r o s t h e t i c devices. A p p l i c a t i o n s of v i t r e o u s carbon as d e n t a l imp l a n t s that are i n a new-technique phase and some new forms of carbon t h a t are under development are a l s o discussed. Properties B i o c o m p a t i b i l i t y . The b i o c o m p a t i b i l i t y of LTI carbons and i t s dependence on s u r f a c e chemistry and surface topography have been studied e x t e n s i v e l y . Most of the e a r l y data was generated by NIH's A r t i f i c i a l Heart Program Contractors, the bulk of which has been p u b l i s h e d . The most important r e s u l t s have been summarized by Bruck and are l i s t e d i n Table I (19-25). The pure and s i l i c o n - a l l o y e d v a r i e t i e s of LTI carbons e x h i b i t a good to e x c e l l e n t degree of thromboresistance i n both the vena cava t e s t of Gott (20) and the r e n a l embolus t e s t of Kusserow (26). Although thromboresistance i s of prime importance f o r b i o m a t e r i a l s to be used i n c a r d i o v a s c u l a r devices handling blood, other c h a r a c t e r i s t i c s are a l s o important. A m a t e r i a l may be thromboresistant but s t i l l cause adverse r e a c t i o n s w i t h c e l l s or


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TABLE I BIOLOGICAL PROPERTIES OP LTI CARBONS Q 9 ) Test


In-Vivo

Result

(10, 20-21)

Vena cava 2 hr 2 weeks Renal embolus

Excellent Excellent Very good

In-Vitro E f f e c t on plasma p r o t e i n s (22) E f f e c t on plasma enzymes (22) Calcium replacement c l o t t i n g time (22) Adherence of : Erythrocytes (WB) Leukocytes (WB) P l a t e l e t s (PRP) (23) P l a t e l e t aggregation and a c t i v a t i o n (24) C e l l growth (amnion) (23) Zeta p o t e n t i a l (Krebs) C r i t i c a l s u r f a c e t e n s i o n (25)

None None to s l i g h t Not prolonged Light None Moderate Slight Near 100% Negative 50 dynes/cm

a l t e r molecular elements of blood. R e s u l t s of H e g y e l l (23) and Schoen (24) I n d i c a t e that c l e a n , p o l i s h e d LTI surfaces are not only thromboresistant but a l s o compatible w i t h c e l l u l a r elements of blood. H a l b e r t (22) and h i s coworkers showed f u r t h e r that LTI carbon surfaces do not i n f l u e n c e plasma p r o t e i n s or a l t e r the a c t i v i t y of plasma enzymes. There has been considerable s p e c u l a t i o n about the s p e c i f i c combinations of s u r f a c e p r o p e r t i e s t h a t are r e s p o n s i b l e f o r the c o m p a t i b i l i t y of LTI carbons. The problem of i n t e r p r e t a t i o n i s a many-faceted and d i f f i c u l t one because blood c o m p a t i b i l i t y i n f e r s not only a c o m p a t i b i l i t y with the v a r i o u s c l o t t i n g f a c t o r s but a l s o a broader c o m p a t i b i l i t y with the formed c e l l u l a r elements of blood as w e l l as a c o m p a t i b i l i t y with the molecular species not involved i n c l o t t i n g . I t i s thought t h a t the c o m p a t i b i l i t y of carbon w i t h blood may be due to an a b i l i t y to maintain an i n t e r mediate adsorbed l a y e r of p r o t e i n a t the carbon-blood I n t e r f a c e without causing u n d e s i r a b l e a l t e r a t i o n s which, f o r example, could t r i g g e r the complex s e r i e s of r e a c t i o n s that lead to thrombus formation ( 2 5 ) . Recent r e p o r t s of the a d s o r p t i v i t y of albumin on LTI carbons and the p a s s i f l e a t i o n of the s u r f a c e with regard to p l a t e l e t i n t e r a c t i o n s are c o n s i s t e n t w i t h t h i s general view ( 2 7 29).



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A l l of the r e s u l t s taken together suggest t h a t the L T I carbons represent a s i g n i f i c a n t advance In the search f o r b l o o d compatible m a t e r i a l s . Because blood I s probably the most f i n i c k y of the body's t i s s u e s , one may I n f e r that the L T I carbons w i l l a l s o be i n e r t i n s o f t or hard t i s s u e . T h i s Inference i s being confirmed i n a p p l i c a t i o n s described i n l a t e r s e c t i o n s of t h i s paper. As a l l u d e d t o above, b i o c o m p a t i b i l i t y and n o n b l o d e g r a d a b i l i t y are not s u f f i c i e n t f o r m a t e r i a l s that a r e t o serve In devices t h a t are subjected t o mechanical abuse. In the f o l l o w i n g s e c t i o n s , the p h y s i c a l and mechanical p r o p e r t i e s t h a t a r e r e l e v a n t t o usages i n prosthetics are described. Strength. The s t r e n g t h p r o p e r t i e s of carbon are s t r u c t u r e s e n s i t i v e and vary with s t r u c t u r e , e.g., d e n s i t y , degree of graphi t i z a t l o n , p r e f e r r e d o r i e n t a t i o n , c r y s t a l l i t e s i z e , and micros t r u c t u r e (6). Of a l l o f the bulk forms o f carbons, I n c l u d i n g the p y r o l y t i c graphites used i n r o c k e t r y (30) and v i t r e o u s carbons (31), the p y r o l y t i c carbons deposited i n f l u i d i z e d beds e x h i b i t the h i g h e s t s t r e n g t h and toughness. Kaae has s t u d i e d the mechanical p r o p e r t i e s of the L T I carbons and has r e l a t e d t h e i r s t r e n g t h to t h e i r d e n s i t y and c r y s t a l l i t e s i z e (15, 32). H i s data show that the f r a c t u r e s t r e s s of the h i g h e s t - d e n s i t y L T I carbons i s above 70,000 p e l , i . e . , three or four times that o f bone. The s t r e n g t h o f the L T I carbons i s en hanced by a l l o y i n g with s i l i c o n (16), and, f o r concentrations l e s s than about 20 wt % S i , the a l l o y i n g a d d i t i o n does n o t d e t r a c t from the b i o c o m p a t i b i l i t y (10). I t i s worth comparing the s t r e n g t h of the L T I carbons with that o f v i t r e o u s carbon s i n c e the l a t t e r has been used c l i n i c a l l y i n d e n t a l a p p l i c a t i o n s f o r s e v e r a l years (33) and i s c u r r e n t l y considered by the American Dental A s s o c i a t i o n ' s C o u n c i l on Dental M a t e r i a l s and Devices and C o u n c i l on Dental Research to be In a new-technique phase. The s t r e n g t h p r o p e r t i e s of v i t r e o u s carbon and a h i g h - d e n s i t y L T I carbon a r e l i s t e d i n Table I I . The combi n a t i o n of h i g h s t r e n g t h and r e l a t i v e l y low e l a s t i c modulus o f the LTI carbon r e s u l t s i n a high value of σ2/2Ε, the s t r a i n energy t o f r a c t u r e , implying t h a t i t i s tougher and l e s s b r i t t l e than v i t r e ous carbon. The f a c t that the s t r e n g t h of L T I carbon i s higher than t h a t of v i t r e o u s carbon has been a t t r i b u t e d t o a b a s i c d i f f e r e n c e i n the m i c r o s t r u c t u r e s that may be traced t o d i f f e r e n c e s i n process i n g (34-35). V i t r e o u s carbon i s derived from a polymeric precur sor by p y r o l y z l n g a t a slow r a t e , d r i v i n g o f f v o l a t i l e c o n s t i t uents, and l e a v i n g a glassy-appearing carbonaceous r e s i d u e (31). These m a t e r i a l s a r e t y p i f i e d by a low d e n s i t y near 1.5 g/cm . T h e i r p u r i t y i s v a r i a b l e , depending on the p u r i t y of the precursor polymer. A v a r i e t y of d e f e c t s i d e n t i f i e d i n these m a t e r i a l s have been a t t r i b u t e d t o r e s i d u a l i m p u r i t i e s t h a t a r e presumably present In the parent polymer (36). The presence of these d e f e c t s may be J



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TABLE I I COMPARISON OF STRUCTURE AND PROPERTIES OF VITREOUS (GLASSY) AND PYROLYTIC CARBONS Property

V i t r e o u s Carbon

P y r o l y t i c Carbon

3

Density,

g/cm

Crystallite size ( L ) , A c


1.0

1.5

Modulus of rupture, p s i Young's modulus, p s i

10 to 30 20,000 to 30,000 3.5

χ 10

6

to

2.2

20 to

200

50,000 to 100,000 2.0

to 5 χ

10

6

S t r a i n energy to f r a c t u r e , 2

a /

2 E

(x10

V

2

1

7

3

In. / r e s p o n s i b l e f o r the h i g h l y v a r i a b l e , low strength of the v i t r e o u s m a t e r i a l s (37). The LTI p y r o l y t i c carbons are deposited a t elevated tempera tures from a pure hydrocarbon gas onto a preformed r e f r a c t o r y subt r a t e such as g r a p h i t e . Under c e r t a i n s p e c i f i c d e p o s i t i o n con d i t i o n s , s u p e r s a t u r a t i o n occurs and gas-borne d r o p l e t s are formed (38). Deposition of LTI carbon onto the substrate i s p r i m a r i l y due to the d e p o s i t i o n of d r o p l e t s , the remnants of which have a b e n e f i c i a l i n f l u e n c e on the mechanical p r o p e r t i e s (1_5, 34). Be cause of the m i c r o s t r u c t u r a l t e x t u r e conferred by the d r o p l e t s , crack propagation i n LTI carbon i s d i f f i c u l t and t o r t u r o u s . The fractographs i n Figures 1a and 1b compare the smooth conc o i d a l f r a c t u r e s u r f a c e of v i t r e o u s carbon w i t h the rough f r a c t u r e s u r f a c e of LTI carbon. The toughness and strength of the LTI c a r bon have been a t t r i b u t e d to the l a r g e s u r f a c e area and the asso c i a t e d l a r g e amount of energy r e q u i r e d to create the new s u r f a c e during f r a c t u r e . Fatigue and Wear. Although high f r a c t u r e strengths and toughness are important f o r m a t e r i a l s c a l l e d upon to s u s t a i n high s t r e s s e s i n load-bearing devices, f a t i g u e and wear are a d d i t i o n a l c o n s i d e r a t i o n s when m a t e r i a l s are s e l e c t e d f o r use i n devices sub j e c t e d to c y c l i c l o a d i n g o r abrasion, i . e . , dental Implants, orthopedic devices, and a r t i f i c i a l heart v a l v e s . The f a t i g u e p r o p e r t i e s of a v a r i e t y of carbons w i t h medical a p p l i c a t i o n s have been studied as a f u n c t i o n of t h e i r s t r u c t u r e (18, 39). Although the endurance l i m i t v a r i e d over a wide range from 25,000 p s i f o r v i t r e o u s carbon to 80,000 p s i f o r s i l i c o n a l l o y e d LTI carbon, none of the m a t e r i a l s e x h i b i t e d a f a t i g u e - t y p e failure. In every case, e i t h e r the specimen f r a c t u r e d at i t s normal s i n g l e - c y c l e f r a c t u r e s t r e s s or no f a i l u r e occurred. The data i n F i g u r e 2 f o r an LTI carbon with 8 wt % S i are t y p i c a l .




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The l a c k of a dependence of the f r a c t u r e s t r e s s on the number of s t r e s s excursions i s , of course, an Important advantage f o r the designer of p r o s t h e t i c devices because l a r g e allowances f o r f a t i g u e e f f e c t s t h a t are normally r e q u i r e d f o r metals and polymers are not r e q u i r e d i n designs t h a t use L T I carbons. The dependence of the endurance l i m i t on d e n s i t y and on s i l i con content i n F i g u r e 3 shows t h a t , f o r a given low d e n s i t y , the endurance l i m i t i s increased by going from the glassy s t r u c t u r e to s t r u c t u r e s c o n t a i n i n g d r o p l e t remnants and t h a t there i s a f u r t h e r Increase with i n c r e a s i n g d e n s i t y to about 50,000 p s i . C©depositing s i l i c o n with carbon r a i s e s the endurance l i m i t to 80,000 p e l . Current s p e c i f i c a t i o n s f o r the LTI carbons used In heart v a l v e prostheses would Include t h e two m a t e r i a l s c o n t a i n i n g 8 and 15 wt % S i . A l l other d e p o s i t s , excepting the deposit with 23 wt % S i , would be too s o f t and would wear a t unacceptably h i g h r a t e s ( d i s cussed below). The m a t e r i a l with 23 wt % S i , although i t s f r a c t u r e s t r e n g t h i s h i g h , would l a c k toughness and would be too b r i t t l e f o r load-bearing a p p l i c a t i o n s . The wear r e s i s t a n c e of carbon i s markedly dependent on I t s s t r u c t u r e s . The data In F i g u r e 4 compare the wear r a t e s caused by a h i g h - d e n s i t y LTI carbon d i s k rubbing on v a r i o u s pure and s i l i c o n - a l l o y e d LTI carbons and v i t r e o u s carbon (17). The I n i t i a l drop i n wear r a t e (open c i r c l e s ) w i t h i n c r e a s i n g hardness i s due to an i n c r e a s e i n the d e n s i t y of the LTI carbon. The low-density LTI carbon and the v i t r e o u s carbon both experienced high wear r a t e s . A f u r t h e r but smaller decrease i n wear r a t e i s a t t a i n a b l e by codepoelting s i l i c o n w i t h carbon; s i l i c o n a l s o enhances strength Q é - 1 2 ) . The s p e c i f i c a t i o n f o r PYROLITE* carbons used c l i n i c a l l y In heart v a l v e a p p l i c a t i o n s r e q u i r e s t h a t the hardness be between about 240 and 370 DPH (50-g l o a d s ) . A v a r i e t y of other p r o p e r t i e s of pure and s i l i c o n - a l l o y e d LTI carbons are l i s t e d i n Table I I I . Applications Cardiovascular. The human heart c o n s i s t s of two pumping chamber β, each of which i s equipped with an i n l e t v a l v e and an o u t l e t v a l v e . Disease can damage these v a l v e s , Impairing t h e i r f u n c t i o n , and can o f t e n r e q u i r e replacement w i t h a p r o s t h e t i c v a l v e (see F i g u r e 5 ) . The f i r s t i m p l a n t a t i o n of an a r t i f i c i a l v a l v e was made In 1952 by Hufnagel In the descending a o r t a (40). The advent of open heart procedures made p o s s i b l e Implantation i n the normal p o s i t i o n , and i n 1960 Harken performed the f i r s t s u c c e s s f u l implanta t i o n of a c a g e d - b a l l p r o s t h e s i s i n the a o r t i c p o s i t i o n (41). In the ensuing year s » the c l i n i c a l use of h e a r t v a l v e prostheses has proved the worth of these d e v i c e s , s a l v a g i n g otherwise hopeless p a t i e n t s . Experience has shown, however, that many problems remain, and the I d e a l p r o s t h e s i s has yet to be proved. •Registered trademark of General Atomic Company



BOKROs E T A L .


10'

1(T

l(f

NUMBER OF CYCLES TO FAILURE Biomatarials, Madical Dtviets, and Artificial Organs

Figure 2. Cycles vs. stress for a silicon-alloyed pyrolytic carbon with 8 wt % silicon, a carbon matrix density of 1.97 g/cm , ana a DPH hardness number of 295; single-cycle fracture strength denoted by hexagon ( 18) s

(23)[430] +

GLASSY CARBON

Ο

PURE PYROLYTIC CARBONS

φ

SILICON-ALLOYED PYROLYTIC CARBONS •[8)[295]

( ) SILICON WT %

60

*(15)[352]

[ ] DPH NUMBER

(16)1212]

1.6

1.7

CARBON MATRIX DENSITY

1.8

1.9

(6/CM ) 3

Biomatarials, Madkal Dtvicas, and Artificial Organs

Figure 3.

Endurance limit vs. carbon matrix density for pure and alloyed LTI carbon ( 18 )




15

100

200

300

400

500

HARDNESS OF FLAT (DPH) Biomaterials, Medical Devices, and Artificial Oigans

Figure 4. Wear rates caused by a high density LTI carbon disk rubbing on a variety of pure and alloyed carbon plates (17)

Figure 5.

Schematic of heart illustrating function of prosthetic heart valves



BOKROS E T A L .

19.

247

TABLE I I I PROPERTIES OF PURE AND SILICON-ALLOYED LTI CARBONS

Property

Silicon-Alloyed LTI Carbon

Pure L T I Carbon

3

Density, g/cm C r y s t a l l i t e s i z e (L ) , A Flexural strength, p s i Young's modulus, p s i Fatigue l i m i t / f l e x u r a l strength Poisson's r a t i o Hardness (DPH) Thermal expansion co e f f i c i e n t (χ 1 0 °C) Electrical resistivity a t 20°C, ohm-cm Thermal c o n d u c t i v i t y a t 20°C, cal/cm-sec-°C S i l i c o n content, v t % Impurity l e v e l , ppm


0

6

1.5 t o 2.1 30 , ν 40,000 t o 7 0 , 0 0 0 2.5 t o 4 χ 1 0 ( >

U ;

6

a

2.0 t o 2.2 30 90,000 5 χ 10 6

1.0 0.2 350

1.0 0.2 150 t o 2 5 0 W a

4 to 6< >

5

0.0005 t o 0.002

0.0003 t o 0.002

0.01 0.00 Less than 100

0.01 12 Less than 100

v

''The higher values a r e f o r carbons with the highest densities. A review of the hemodynamic c h a r a c t e r i s t i c o f p r o s t h e t i c valves has r e c e n t l y been published (42) and may be' consulted f o r f u n c t i o n a l c h a r a c t e r i s t i c s o f v a l v e s that were a v a i l a b l e i n 1973. A s i d e from the hemodynamic problems a s s o c i a t e d with the pump ing of blood, there have been s e r i o u s m a t e r i a l s - r e l a t e d problems of b i o c o m p a t i b i l i t y and d u r a b i l i t y . Problems of thrombosis and mechanical f a i l u r e , by wear o r f a t i g u e , o f rubbing or f l e x i n g com ponents o f v a l v e s have been well-documented (43-62) and summarized (35). The use o f L T I carbon i n the r i g i d components of p r o s t h e t i c valves has v i r t u a l l y e l i m i n a t e d problems of wear, and r e s u l t s from i n - v i t r o (35) and i n - v i v o animal t e s t s suggest t h a t s i g n i f i c a n t wear w i l l not occur even d u r i n g the l i f e t i m e of a young p a t i e n t (35, 63-62). Furthermore, the s u b s t i t u t i o n o f L T I carbon f o r polymeric o c c l u d e r s has e l i m i n a t e d problems of d i s t o r t i o n and deg r a d a t i o n that have been common w i t h polymeric occluders (44-47, 57-58, 62, 70-75). The wear r e s i s t a n c e o f L T I carbon, together w i t h the f a c t that the m a t e r i a l i s not subject t o f a t i g u e o r biodégradation, has made i t the m a t e r i a l of choice f o r long-term Implantation i n valves (63-69) or as the component o f c a r d i a c a s s i s t devices (76-77). A t the time of our l a s t r e p o r t i n g (35) (August 1971),



248


about 2000 components were d e l i v e r e d f o r c l i n i c a l use i n p r o s t h e t i c v a l v e s . Through June 1975, more than 110,000 components were d e l i v e r e d . Very l i k e l y , 100,000 components have been Implanted s i n c e I n i t i a l i n t r o d u c t i o n , i n 1969, of the hollow LTI carboncoated b a l l f o r the DeBakey-Surgitool a o r t i c v a l v e p r o s t h e s i s . About 40,000 components have been Implanted f o r more than 2 y r . Since 1969, there have been no f a i l u r e s of LTI carbon heart v a l v e components i n normal f u n c t i o n . The cause of four deaths due to s t r u t f a i l u r e of the Model 105 B e a l l v a l v e (23-80) was t r a c e d to the a p p l i c a t i o n of e x c e s s i v e l a t e r a l f o r c e s onto the s t r u t s during handling of the p r o s t h e s i s b e f o r e or at Implantation, r e s u l t i n g In cracks In the carbon c o a t i n g w i t h subsequent f a t i g u e f a i l u r e of the u n d e r l y i n g metal wire. As a r e s u l t of t h i s e x p e r i ence, a new package has been designed i n which the v a l v e can be s t e r i l i z e d without being removed u n t i l time of implantation. In a d d i t i o n , the diameter of the s u b s t r a t e w i r e t h a t i s carbon-coated has been increased from 0.030 to 0.045 i n . , i n c r e a s i n g the strength of the s t r u t s i n l a t e r a l bending by a f a c t o r of about 3. During the past 2 y r , there have been s e v e r a l new innovations i n v a l v e s t h a t u t i l i z e LTI carbon. Although the hollow b a l l s f o r the DeBakey a o r t i c v a l v e were rendered opaque by l i n i n g w i t h tungsten screen (35), the o c c l u d e r s In a l l other c l i n i c a l v a l v e s were not r e a d i l y v i s u a l i z a b l e w i t h X r a y s . Recently, methods of f a b r i c a t i n g d i s c - t y p e o c c l u d e r s c o n t a i n i n g opaque markers have been developed, and marked d i s c s are i n t e s t . The most promising method u t i l i z e s a tantalum ribbon embedded i n an annular groove i n the s u b s t r a t e g r a p h i t e (see F i g u r e 6a) which, i n an X ray, i s v i s i b l e as an opaque r i n g (see F i g u r e 6b)· Recently, two new v a l v e s , the Cooley-Cutter a o r t i c and m i t r a l v a l v e s , have been r e l e a s e d f o r general c l i n i c a l use (81). Both v a l v e s use a hollow LTI carbon occluder l i n e d w i t h tungsten screen to a l l o w v i s u a l i z a t i o n of the occluder w i t h X r a y s . The v a l v e s have a double open-ended t i t a n i u m cage w i t h a f u l l o r i f i c e (see F i g u r e 7 ) . Other c l i n i c a l valves t h a t use LTI carbon components are the B e a l l - S u r g i t o o l m i t r a l v a l v e (63), the B j o r k - S h i l e y m i t r a l and a o r t i c v a l v e s (35, 64-67), the Debakey-Surgitool a o r t i c v a l v e (35), and the L i l l e h e l - K a s t e r m i t r a l and a o r t i c valves (35, 68). Transcutaneous Devices. F i f t e e n years ago, Murphy published a s e c t i o n i n Orthopaedic Appliance A t l a s e n t i t l e d "New Developments and Dreams" i n which he p r e d i c t e d : In a l e s s remote f u t u r e , b i o e n g i n e e r i n g research i n bone p l a t e s and hip-replacement prostheses may w e l l lead to a b i l i t y to a t t a c h an i n e r t m a t e r i a l so s e c u r e l y and permanently to bone t h a t loads from a p r o s t h e s i s could be transmitted d i r e c t l y to the s k e l e t o n . Research on eye Implants, s k e l e t a l t r a c t i o n , and implant dentures might even g i v e hope of i n f e c t i o n - r e s i s t a n t passage of the p r o s t h e s i s through the s k i n f o r a l i f e t i m e . A f t e r a l l ,



Figure 6. (a) (top) Photo micrograph of a section through the tantalum radio-opaque marker in a disc-type occluder (25χ); (b) (bottom) radiograph of discs marked with tantalum ribbons embedded in the graphite substrate



250


the present r a t e of r e s i s t a n c e to i n f e c t i o n s u c c e s s f u l l y combats v a s t numbers of b a c t e r i a during a few weeks of s k e l e t a l t r a c t i o n or even a decade of use of Implant dentures w i t h a very crude s e a l . More s u c c e s s f u l s e a l ing techniques, based on b e t t e r engineering as w e l l as b i o l o g i c a l p r i n c i p l e s , might t i p the s c a l e enough to make "limb f i t t i n g " of a s k e l e t a l l y attached device t r u l y a p a r t of the surgeon's s p e c i a l t y . The p r o s t h e t i s t would then be concerned w i t h the alignment of the components to be attached, by a bayonet l o c k , to the peg o u t s i d e the s k i n . Although small research e f f o r t s have been, and again are being devoted to animal experiments on s k e l e t a l attachment and r e l a t e d problems, and a few human e x p e r i ments are s a i d to have been t r i e d , t h i s idea i s s t i l l considered h i g h l y experimental and i s f l a t l y condemned by many surgeons. The work i n i t i a t e d by Benson i n 1967 showed t h a t carbon might provide the means f o r g a i n i n g permanent access through the s k i n (12, 81), not only f o r d i r e c t attachment to the s k e l e t a l system but a l s o f o r p r o v i d i n g a means of t r a n s m i t t i n g e l e c t r i c a l leads f o r powering Implanted c a r d i a c a s s i s t devices, n e u r a l s t i m u l a t i o n , and s t i m u l a t i o n of bone growth, or f o r p r o v i d i n g blood access f o r r e n a l d i a l y s i s . Although the optimum s u r f a c e topography that i s r e q u i r e d to form the beet b a c t e r i a l s e a l has not yet been i d e n t i f i e d (82), there i s evidence that e i t h e r v i t r e o u s carbon or LTI carbon i s s u f f i c i e n t l y compatible w i t h t i s s u e to perform f o r long p e r i o d s i n the transcutaneous s i t u a t i o n (6, V2, 83-89). The schematic In F i g u r e 8 i l l u s t r a t e s the use of a transcutaneous dev i c e to provide a lead-through f o r e l e c t r i c a l l e a d s . V i t r e o u s and LTI carbons have both been used In a v a r i e t y of transcutaneous devices. V i t r e o u s carbon i s somewhat l i m i t e d when load bearing i s required because of i t s extreme b r i t t l e n e s s (90). In the f o l l o w i n g paragraphs, some of the newer transcutaneous dev i c e s t h a t use LTI carbon are d e s c r i b e d . 1. S k e l e t a l Attachment: The R e h a b i l i t a t i o n Engineering Center a t Rancho Los Amigos H o s p i t a l In Downey, C a l i f o r n i a , has p i o neered the search f o r a s a t i s f a c t o r y method of a t t a c h i n g a prost h e s i s to the s k e l e t o n . In November 1974, the group Inserted such a device i n the l e g of an amputee. The p r o s t h e t i c device (see Figures 9 and 10) was designed i n c o l l a b o r a t i o n w i t h the Kennedy Space Center of NASA. The LTI carbon transcutaneous device i s cemented to a m e t a l l i c l a t c h i n g device f o r a t t a c h i n g the belowknee pylon p r o s t h e s i s (see F i g u r e 11). The s k e l e t a l p r o s t h e s i s attachment u n i t Implanted i n the l e g of the amputee i s depicted In F i g u r e 12 as i t appeared Immediately a f t e r surgery. The d e v i c e i s performing s a t i s f a c t o r i l y a t the time of t h i s w r i t i n g (9 months a f t e r i n s e r t i o n ) (91). 2. Dental Implants: D e n t a l surgeons have sought f o r decades f o r a m a t e r i a l t h a t could be fashioned i n t o an a r t i f i c i a l tooth r o o t , implanted through the g i n g i v a i n t o the jawbone, and provide



BOKROs E T A L .


Figure 7. Cooley-Cutter aortic (right) and mitral (left) valve prostheses

Figure 8. Carbon transcutaneous device




Figure 9.

Below-knee prosthetic attachment



253


19. BOKROs E T A L .

Figure 11. Below-knee pylon prosthesis with transparent socket, prepared for attachment




254

the support f o r a t o o t h , b r i d g e , or complete denture. L T I carbon because of i t s s t r e n g t h , i t s a b i l i t y to perform transcutaneously, and i t s e l a s t i c match with bone i s e s p e c i a l l y s u i t e d f o r t h i s a p p l i c a t i o n . Tests that were begun i n 1971 i n primates a t the Southwest Foundation f o r Research and Education (87) explored a number of c o n f i g u r a t i o n s and provided the b a s i s f o r an expanded, i n t e n s i v e e v a l u a t i o n o f blade-type endosseous implants a t the Tulane U n i v e r s i t y D e l t a Primate Center In L o u i s i a n a . D e t a i l s of the design, mechanical p r o p e r t i e s , and animal r e s u l t s have been reported (92). The approach, i l l u s t r a t e d by the schematic i n F i g u r e 13, uses an L T I carbon-coated g r a p h i t e blade implanted i n the mandible f o r the purpose o f supporting a b r i d g e . The blade i s about 1.5 mm t h i c k , and the base o f the crown support a t the plane of the g i n g i v a i s from 3.0 t o 4.0 mm t h i c k (see F i g u r e s 14a and 14b). The h i g h s t r e n g t h o f the L T I carbon allows much thinner s e c t i o n s than a r e p o s s i b l e with v i t r e o u s carbon, thus r e q u i r i n g that l e s s bone be s a c r i f i c e d when the d e v i c e i s implanted. A c c o r d i n g l y , implants can be made i n a l v e o l a r r i d g e s that have a l r e a d y undergone some degeneration. Even the t h i n n e s t blade designs can support four times the load i n bending that would cause a V l t a l l i u m or t i t a n i u m blade to bend over (92). Human t r i a l s have been i n i t i a t e d . 3. A r t i f i c i a l Hearing: The U n i v e r s i t y o f Utah, i n conjunct i o n w i t h the O t o l o g i c M e d i c a l Group i n Los Angeles, has been exp l o r i n g the f e a s i b i l i t y o f u s i n g the t o n o t o p i c s p a t i a l o r g a n i z a t i o n of the c o c h l e a f o r a r t i f i c i a l h e a r i n g (93). Subjects have been Implanted w i t h cochlear e l e c t r o d e s t o d e f i n e s t i m u l a t i o n parameters and methods f o r p i t c h modulation necessary f o r d e s i g n ing permanent d e v i c e s . L T I carbon transcutaneous buttons (see F i g u r e 15) a r e c u r r e n t l y being used t o house the e l e c t r i c a l conn e c t o r . The L T I carbon button apparently forms a b a c t e r i a l s e a l so that the connector i s w e l l - t o l e r a t e d without I n f e c t i o n . A connector implanted i n a deaf v o l u n t e e r i s shown i n F i g u r e 16 as i t appeared 10 days p o s t - o p e r a t i v e . The d e v i c e i s performing s a t i s f a c t o r i l y a t the present time (4 months a f t e r i n s e r t i o n ) . New Developments There a r e many c a r d i o v a s c u l a r , orthopedic, and d e n t a l devices that would be improved i f carbon components could be used, b u t , because o f shape, cost, and other f a c t o r s , they cannot be made from L T I carbon, v i t r e o u s carbon, or other b u l k forms of carbon. In order to meet these requirements, we a r e developing new p r o c esses that a l l o w d e p o s i t i o n o f an adherent, t h i n , Impermeable b a r r i e r l a y e r of pure I s o t r o p i c carbon on the s u r f a c e of polymeric and i n t r i c a t e m e t a l l i c s u b s t r a t e s . For example, many I n t r i c a t e m e t a l l i c h e a r t v a l v e housings have s e c t i o n s and shapes t h a t make them u n s u i t a b l e f o r c o a t i n g with L T I carbon. The devices shown i n F i g u r e s 17a, 17b, and 17c are experimental components f o r a r t i f i c i a l v a l v e s and an emboli


BOKROS E T

AL.


255


19.

Figure 13. Schematic showing an LTI carbon blade-type implant in a mandible



256

PETROLEUM

DERIVED

Figure 14 (a). LTI carbon endosseous bladetype dental implants

Figure 14 (b). Cross section of dental implant showing internal graphite substrate and LTI carbon structural coating



Figure 16. LTI carbon transcutaneous button installed in a cochlear stimuhtion experiment




Figure 17 (a) and (b). Experimental metallic prosthetic devices—heart valve housings coated with a thin, impermeable layer of isotropic carbon




BOKROs E T A L .

Figure 18. A bone plate and screws coated with an adherent, thin layer of impermeable isotropic carbon




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filter. The impermeable carbon c o a t i n g i s about 5000 Â t h i c k and covers a l l exposed m e t a l l i c s u b s t r a t e s . Because the c o a t i n g * i s so t h i n , i t w i l l i n time be worn o f f i n l o c a l i z e d areas that a r e continuously.rubbed by the occluder. The major f r a c t i o n of the c o a t i n g , however, w i l l remain. Since the c o a t i n g i s too t h i n t o be s t r u c t u r a l , the mechanical p r o p e r t i e s of the coated device a r e the same as those o f an uncoated d e v i c e . Another example i s c o a t i n g f o r orthopedic appliances (see F i g u r e 18). The m e t a l l i c p a r t s r e t a i n a l l o f the p h y s i c a l and strength p r o p e r t i e s o f the uncoated metal, but, b i o c h e m i c a l l y , the surrounding t i s s u e i s aware of only the i n e r t carbon s u r f a c e . In t h i s new process, the substrate m a t e r i a l need not be subj e c t e d t o h i g h temperatures, so polymers may be coated. This means that low-cost molded a r t i c l e s can be coated, f o r example, f o r d i s p o s a b l e blood-handling devices. The present usage of carbon i n medicine t h a t was sparked by Gott's o r i g i n a l experiments w i t h graphite pigmented p a i n t r e p r e sents a s i g n i f i c a n t advance i n the search f o r m a t e r i a l s t o be used i n f a b r i c a t i n g spare p a r t s and a c c e s s o r i e s f o r the body. The s p e c i a l c o m p a t i b i l i t y of carbon, i t s n o n b i o d e g r a d a b i l i t y , i t s mec h a n i c a l l i k e n e s s to bone, i t s good wear r e s i s t a n c e , and i t s r e s i s t a n c e t o f a i l u r e In f a t i g u e taken together o f f e r a unique comb i n a t i o n o f p r o p e r t i e s that makes carbon e s p e c i a l l y s u i t e d f o r use i n p r o s t h e t i c d e v i c e s . The a b i l i t y t o provide a t h i n carbon " s e a l - c o a t " on polymer and i n t r i c a t e metal shapes should expand the usefulness of the m a t e r i a l f o r even more a p p l i c a t i o n s .

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