Assessment of Materials for Use in Chronically Implantable Microelectrodes.

Nerve damage is the major cause of disability worldwide. Unlike other cells in the body, neurons do not divide and reproduce themselves when they are mature and have limited capability to regenerate themselves when they are damaged. Nerve damage due to spinal cord injury (SCI), stroke, and disease in the central nervous system often results in paralysis. It is estimated that the annual incidence of SCI is over 92,000 new cases worldwide. The estimated annual incidence of stroke is around 100,000 in the UK alone. In most cases the nerves below and above the level of damage remain intact and active after nerve damage. At the University of Surrey research has been on going to overcome the nerve damage using chronically implantable microelectrodes (neuroprobes) to record signals coming from the body’s own sensors to provide feedback to functional electrical stimulators. This project is a part of on going research programme and the purpose of this project was to identify an optimum material for electrode recording sites on implantable microelectrodes. The most common materials for electrode sites gold (Au), platinum (Pt), iridium (Ir), and indium tin oxide (ITO) along with a reference material (titanium, Ti) were investigated. In this project, the biocompatibility of materials for electrode sites has been investigated using two measures: adsorption of proteins onto materials using atomic force microscopy, ellipsometry, Fourier transform infrared spectroscopy and Fourier transform infrared ellipsometry, and cytotoxic effects of materials on cells after 72 hours using neutral red assay. The electrical characteristics of these materials have been assessed by measuring impedance under physiological conditions, and by recording electrical activity from neuroblastoma cells cultured on electrodes made of these materials. The results of the studies reported here suggest that Au is the material of choice for electrode recoding sites on implantable microelectrodes. ITO is the material of choice for in vitro planar electrode arrays and suitable for cell-based biosensors. Other components of the neuroprobe such as insulating materials, wires and adhesives must be tested for cytotoxicity and all the materials must be tested for genotoxicity before in vivo tests of the whole device could be performed