4. Hemocompatıbılıty

The interaction between blood proteins and the material is regarded as an important source of thrombogenesis. The adsorption of proteins is explained, from the thermodynamic point of view, in terms of the systems free energy or surface energy. However, adsorption itself does not induce thrombosis. Theories regarding correlations between thrombogenicity of a material and its surface charge or its binding properties proved not to be useful.

Thrombus formation on implant materials is one of the first reactions after deployment and may lead to acute failure due to occlusion and serve as a trigger for neointimal formation. Next to the direct activation by the intrinsic or extrinsic coagulation cascade, thrombus formation can also be initiated directly by an electron transfer process while fibrinogen is close to the surface. The electronic nature of a molecule can be defined as semiconductor or insulator. Contact activation is possible in the case of a metal since electrons in the fibrinogen molecule are able to occupy empty electronic states with the same energy. Therefore, the obvious way to avoid this transfer is to use a material with a significantly reduced density of empty electronic states within the range of the valence band of the fibrinogen. This is the case for the used silicon carbide coating.

Hemocompatibility leads to the following physical requirements: (1) to prevent the electron transfer, the solid must have no empty electronic states at the transter level, i.e., deeper than 0.9 eV below Fermi's level. This requirements met by a semiconductor with a sufficiently large band gap (its valence band edge must be deeper than 1.4 eV below Fermi's level) and a low density of states inside the band gap. (2) To prevent electrostatic charging of the interface (which may interfere with requirement 1) the electric conductivity must be higher than 10-3 S/cm. A material that meets these electronic requirements is silicon carbide in an amorphous, heavily n-doped, hydrogen-rich modification (a-SiC:H). The amorphous structure is required in order to avoid any point of increased density of electronic states, especially at grain boundaries.

At present, a-SiC:H is known for its high thromboresistance induced by the optimal barrier that this material presents for protein adhesion. These properties may translate into less protein biofouling and better compatibility for intravascular applications rather than Si. SiC has a relatively low level of fibrinogen and fibrin deposition when contacting blood. These proteins promote local clot formation; thus, the tendency not to adsorb them will resist blood clotting. It is now well established that SiC coatings are resistant to platelet adhesson and clotting both in vitro and in vivo [5]. In the Bolz et al. [8] study, a-SiC:H films were deposited using the glow discharge technique or plasma-enhanced chemical vapour deposition (PECVD). The technique provides the most suitable coating process due to its high inherent hydrogen concentration which satisfies the electronically active defects in the amorphous layers. They used fibrinogen as an example model for thrombogenesss in implants, although most haemoproteins are organic semiconductors. a-SiC:H coatings showed no time-dependent increase in the remaining protein concentration, confirming that no fibrinogen activation and polymerisation had taken place. These results support the electrochemical model for thrombogenesis at artificial surfaces and prove that a proper tailoring of the electronic properties leads to a material with superior hemocompatibility. The in vitro test showed that the morphology of the cells was regular. The a-SiC:H samples showed the same behaviour as the control samples. Blood and membrane proteins have similar band-gaps, because the electronic properties depend mainly on the periodicity of the amino acids, and the proteins %""!.z+\*(5z%\*z0\$!z% z/!-1!\*!\_z\*+0z%\*z0\$!%.z/0.101.(z,!.%+ %%05^z,,.!\*0(5\_z/%)%(.z.!¥ tions inducing a modification of proteins are responsible for the cell culture results.

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A-SiC: H has superior hemocompatibility; its clotting time is 200 percent longer than to that of titanium and pyrolytic carbon. Furthermore, it has been shown that small variations in the preparation conditions cause a significant change in hemocompatibility. Therefore, it is of paramount importance to know the exact physical properties of the material in use. Amorphous silicon carbide can be deposited on any substrate material which is resistant to temperatures of approximately 250 °C. This property makes amorphous silicon carbide a suitable coating material for all hybrid designs of biomedical devices. The substrate material \*z!z"%00! z0+z0\$!z)!\$\*%(z\*!! /\_z %/.!#. %\*#z%0/z\$!)++),0%%(%05\_z3\$!.!/z0\$!z+0¥ %\*#z!\*/1.!/z0\$!z\$!)++),0%%(%05z+"z0\$!z !2%!^z+//%(!z,,(%0%+\*/z.!z0\$!0!./z+.z/!\*¥ sors in blood contact and implants, especially artificial heart valves.

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