Silicon neural interfaces -Design and biomedical aspects-

This thesis covers the development of a silicon neural interface, with focus on silicon sieve electrode fabrication, design, nerve regeneration, signal recording and biocompatibility. A study of how the via hole size and transparency of the perforated sieve membrane influences the nerve regeneration is presented together with a study on soft tissue responses to planar and porous silicon. A method to characterise the electric properties of different sieve electrode configurations in vitro is presented. The development and fabrication of silicon neural interfaces is also described. In vivo, the sieve electrode is implanted between the proximal and distal stumps of a transected nerve, allowing the nerve fibres to regenerate through the sieve and establish a physical contact with microelectrodes on the sieve membrane. Successful regeneration in the peripheral nerve showing registrations of compound action potentials in the rat sciatic nerve are reported. Sieve electrodes with different via hole sizes and transparencies were implanted in the rat sciatic nerve for 12 weeks. It was found that a better nerve regeneration was achieved in configurations with a high transparency (30 %) and small holes (30 µm). Also neural regeneration through a sieve electrode has been demonstrated at spinal cord level. Since sieve electrodes are intended for chronic use the tissue response evoked by the electrode is of importance. Planar and porous silicon elicited a tissue response similar to that observed for titanium. Porous silicon and porous titanium induced a smaller capsule formation as compared to planar implants. Different methods to microfabricate silicon sieve electrodes are also reported. Standard anisotropical etching in KOH was used to etch the pyramid shaped via holes and phosphorous doping was used to generate the neural recording electrodes. To increase the degree of transparency the pn etch stop technique was used to fabricate thin (7 µm) perforated membranes. To be able to integrate individual phosphorous doped recording electrodes on the perforated pn etched stopped membrane a new electrochemical etch process was introduced, i.e. field restricted pn etch stop. Porous silicon is also investigated as a neural electrode material, showing improved impedance characteristics i.e. a high electrode interface capacitance as compared to non-porous silicon electrodes. The findings of a suitable sieve electrode transparency and hole size, improved electrode characteristics and biomaterial properties gives indications for the next generation sieve electrodes