Diamond: The Biomaterial of the 21st Century?

Two main groups of ventricular assist devices exist: active deformable devices and blood pumps fitted out with undeformable chamber. The left ventricular assist device (LVAD) which was initially proposed by J.R. Monties et P. Havlik (1) is valveless and based on the Maillard-Wenkel rotary principle so it is a non centrifugal device which produces a partially pulsatile flow. As this system is intended to be fully implantable construction materials must meet stringent requirements. Physically they must be light but rigid, hard and have a low friction coefficient while being wear resistant and chemically inert. Physiologically they must be biocompatible (2, 3). In particular they must not a) be toxic or allergenic, b) alter the osmotic and acidlbase balance of the blood, c) damage blood cells, d) modify the quantity or quality of plasmatic porteins or e) activate the complement and hemostasis systems. The materials which fit the best these requirements belong to many categories: steels, cemented carbides, composites (C/C, C/SiC), coated materials and ceramics. If the materials presenting a high density are discarded, composites, coated materials and ceramics appeared to feature the best chemical resistance and the best tribological properties. Carbon-carbon composites have been selected along a first stage (4-6) but morphological studies revealed a too disturbed surface state; that is the main reason why such materials were given up. Coated materials were then selected. Choice of the substrate depends on bulk characteristics while that of the coating is clearly determined by its surface properties and biocompatibility. Four ceramic coatings were then considered: titanium nitride (TiN), boron carbide (B4C), diamond-like carbon (DLC) and diamond. Substrates could be either graphite or aluminium coated by vapor deposition process. At the present time, TiN has been elaborated (7) and its total qualification is on hand. Boron carbide and diamond-like carbon studies are still in progress. Although studies of pure crystalline diamonds have not yet been undertaken in our laboratory, some data are available from literature. Thus comparisons can be established between the four ceramic coatings: hardness, density, friction coefficient, wear resistance, thermal conductivity and chemical resistance (Tab. I). Diamond exhibits exceptional properties and is therefore intended for projects associated with the durability concept. What is diamond? What are its potential applications? How to elaborate diamond films and control them? Diamond (8) is an allotropic form of carbon that crystallizes in a cubic or hexagonal structure in which each carbon atom is linked by strong, rigid chemical Sp3 bonds (9) to four other carbon atoms arranged tetrahedrally around it. Each crystal is a rigid structure and diamonds is the hardest material (10). Apart from its excellent thermal conductivity (11), diamond is the most chemically inert of all known materials: diamonds are resistant to all acids Diamond is perfect for wear-resistant applications (12). Its low friction coefficient varies from 0.05 to 0.16 depending on the surface (13). Imperfections or small amounts of impurities in the diamond can effect its properties (8). Let us recall that diamonds films must not be mistaken for diamond-like carbon films (14); DLC contains mixed Sp2 and Sp3 bonds and is amorphous, hydrogen can be present at an atomic abundancy reaching 30 to 50%. Many methods allow diamond films synthesis (15). Among them we chose low pressure chemical vapor deposition processes (CVD). Such methods make diamond formation possible within a well defined domain of the phase diagram (16). Diamond can be vapor deposited by (17) Hot Filament CVD (HFCVD) (18), by Electron-Assisted CVD (EACVD) (19), by Laser Assisted CVD (LCVD), laser excited CVD and by the various plasma-assisted CVD (PACVD): dc jet chemical vapor deposition (20), discharge plasma CVD (21, 22), radiofrequency plasma assisted CVD and microwave plasma assisted CVD (MPACVD) (19, 23-29). The net chemical reaction that occurs: