LTI pyrolytic carbon

LTI pyrolytic carbon is one of the very few synthetic materials generally accepted as suitable for long-term blood contact applications (1 ). Although a number of hypotheses have been formulated with respect to the blood tolerability of materials, a general theory or mechanism is not yet available. Nyilas, et al., ( ) have shown that in certain situations the local hemodynamics can play a predominant role, while in most cases the solid-blood interfacial properties have been shown to be equally important (2, 3). It is assumed that understanding the plasma protein adsorption processes on solids used for blood-contact applications will lead to a better understanding of solid-blood interactions (, 2, ... 

Estimations of Oxygen-Containing Functional Groups on Polished LTI Pyrolytic Carbon and Graphite Surfaces... 

Low tenperature isotropic (LTI) pyrolytic carbon has been studied by X-ray photoelectron spectroscopy, scanning electron microscopy, and energy dispersive X-ray analysis. Both silicon-alloyed and unalloyed carbons were studied, in both as-deposited and polished (finished) forms. The polished materials contain significant amounts of surface oxygen. Approximately 1 in 10 of the carbon atoms in the surface volume analyzed by XPS are... 

Surface Analysis of Silicon-Alloyed and Unalloyed LTI Pyrolytic Carbon," in this volume. 

Isotropic carbon, on the other hand, has no preferred crystal orientation and hence possesses isotropic material properties. There ate three types of isotropic carbon pyrolytic, vitreous, and vapor-deposited carbon. Pyrolytic carbons are formed by the deposition of carbon from a fluidized bed onto a substrate. The fluidized bed is formed from pyrolysis of hydrocarbon gas at between 1000 and 2500°C (Hench and Ethridge, 1982). Low-temperature isotropic (LTI) carbons are formed at temperatures below 1500°C. LTI pyrolytic carbon possesses good frictional and wear properties, and incorporation of silicon can further increase hardness and wear resistance (Bokros, 1978). 

LTI pyrolytic carbons, since their introduction in the late 1960s, have become the material of choice for use in the fabrication of mechanical prosthetic heart valves. Over 90% of the mechanical valves implanted... 

Thomas, K.A. et al.. The effect of surface treatments on the interface mechanics of LTI pyrolytic carbon implants,/. Biomed. Mater. Res. 19,145,1985. 

FIGURE 35.9 LTI pyrolytic carbon-coated heart valve. 

On the microstructural level, several types of pyrolytic carbons may be deposited each with one of four distinctly different structures, ranging from layered, highly anisotropic forms to structures with very small, randomly-oriented crystallites with no preferred orientation. All of these structural variations are a result of modifications in processing conditions. In this particular study, only the isotropic forms of both pure LTI carbon and co-deposited LTI carbon-silicon alloyed carbon (Pyrolite registered trademark of Carbo-Medics, Inc., San Diego, California) were investigated. 

ESCA. The surface elements determined from wide-scan ESCA analysis for the various lots of unalloyed pyrolytic carbon (LTI-A, B, C) are given in Table I. We have adopted Scofield s theoretical cross-sections ( ) for semi-quantitatively normalizing our ESCA spectra with respect to carbon. 

Pyrolytic Carbon. Polished LTI carbon is composed of a crystalline graphitic-like microstructure, combined with amorphous material ( ). The polished samples have been shown to be oxidized with a C 0 ratio of about 10 1, containing three major types of carbon-oxygen functionalities quinone-like, ether-... 

A pyrolytic, low-temperature isotropic LTt) carbon is also used by depositing it on substrates such as metals. For instance, an artificial heart valve is composed of a metal coated with LTI carbon. It has not been found out so far why this carbon has a good blood compatibility... 

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