Non-oxide ceramics as biomaterials

This article outlines the use of silicon nitride and silicon carbide ceramics as biomaterials. Silicon nitride is densified using Y2O3 and Al2O3 as sintering additives and is fired in nitrogen at >1700°C followed by hot-isostatic pressing. Commercially, it has been developed for spinal reconstruction devices. Using various chemical or heat treatments its surface chemistry can be varied from a silica-rich to a predominantly N-rich composition, which can influence the response of cells, tissues, and bacteria in vivo. Si3N4 stimulates human cells to produce a “chemically modified” bony apatite, the amount of which is orders of magnitude greater than with other implants. Reactive nitrogen species developed on Si3N4 substrates were more efficient in inducing bacterial lysis than reactive oxygen species developed on TiO2. Thus, while Si3N4 retains all of its engineered mechanical properties, it has unusual surface chemistry such that when it is immersed in an aqueous environment, slow elution of Si and N from its surface enhances healing of soft and osseous tissue and inhibits bacterial proliferation. Both α- and β-SiC powders can be densified at temperatures of 1850–2000°C with the addition of Al2O3 and Y2O3 which involves liquid-phase-sintering. Bioactivity, based on hydroxyapatite formation in simulated body fluid, was only achieved by surface modification. Silicon carbide can also be fabricated by controlled pyrolysis of wood to obtain a carbon scaffold and subsequent reactive infiltration with molten silicon at 1550°C. The microstructure mimics that of the starting organic material with porosity in the range 30%–70% and resembles the cellular microstructure of bone. Cell proliferation occurs with no signs of cytotoxicity. SiC is a wide-band-gap semiconductor biocompatible material that offers higher power densities and lower energy losses, enabling lighter, more compact and higher efficiency products for biocompatible and long-term in vivo applications.