| This dissertation presents the development of microfabrication processes for the integration of process-incompatible, dissimilar materials in MEMS devices with application to neural interfaces for long-term implantation. Micromachining techniques were developed for a novel, mechanically-dynamic, chemo-responsive polymer nanocomposite based on a poly(vinyl acetate) matrix with embedded cellulose nanofibers designed to address issues related to the mechanical mismatch between the neural probe and cortical tissue. Microtensile testing was performed to assess the mechanical properties of laser-patterned nanocomposite microstructures. Within 3--7 minutes of immersion in DI water, the nanocomposite displayed a reduction in Young's modulus by a factor of 140--200, from 1.2--3.4 GPa to 4.9--22 MPa, depending upon the cellulose nanofiber source and concentration. A comparable reduction in Young's modulus was measured for samples that had been implanted in a rat cortex, suggesting this material can penetrate through the pia without buckling, but then softens and better matches the mechanics of cortical tissue. A process utilizing a parylene moisture barrier was developed to fabricate nanocomposite-based intracortical probes with integrated thin-film metal electrodes and interconnects. The electrode impedance was found to be 95.0 +/- 4.8 kO for an electrode 6000 microm2 in area.;To develop a microfabricated neural array technology with on-board recording, stimulation and chemical sensing capabilities, a process was developed to integrate semi-metallic diamond microelectrodes into a flexible polymer substrate. Diamond-on-polymer microelectrode arrays were fabricated using a "diamond-first" fabrication process whereby the diamond electrodes were first grown on a Si/SiO 2 substrate, followed by the deposition and patterning of a polynorbornene capping layer, thin-film metal interconnects, and a polynorbornene substrate layer. A chemical release process in 49% HF was used to transfer the diamond from the Si substrate to the polymer substrate. Ohmic contact was made to diamond microelectrodes using Cr/Au metallization, without requiring an annealing step. Current pulse testing demonstrated potential for neural stimulation with diamond microelectrodes. The electrode impedance at 1 kHz was found to range from 300 kO to 8 MO, indicating that there are some electrodes suitable for neural recording. Finally, electrochemical detection of 100 microM dopamine in saline was achieved with a diamond-on-polymer electrode. |
