Short-and long-term neural biocompatibility of heparin coated sapphire implants

Sapphire is one of the most promising materials for the development of implantable biomedical devices due to its exceptional chemical, mechanical, electrical, thermal and optical properties. Silicon has also been widely used to manufacture neuroprosthetic devices in the past. However, both of these materials have been found to cause the most severe tissue reactions while implanted in vivo in the rat brain, compared with other biomaterials. In order to enhance the biocompatibility of sapphire and silicon, their surfaces were modified by depositing a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS), followed by the photo-immobilization of heparin. To comprehensively evaluate the short- and long-term neural biocompatibility, sapphire and silicon wafers (2.5 mm dia × 0.25 mm thick) with and without heparin coating were implanted on the surface of adult rat's cortex for 10, 28 and 90 days. Specific evaluations of the cell types that contribute to an inflammatory response were performed. The histological results indicate that the biocompatibility of sapphire is dramatically improved by heparin immobilization, while this dramatic improvement is not observed on heparin coated silicon. The failure to improve the biocompatibility of silicon by heparin immobilization can be attributed to the corrosion of the silicon surface in vivo, which was confirmed by atomic force microscopy (AFM). Meanwhile, no corrosion was observed on heparin coated sapphire surfaces and a very thin layer of proteins or extracellular matrix was deposited on the surfaces.