Laser-Microfabricated Polymer Multielectrodes for Intraspinal Microstimulation

Objective: The overall goal of this study was to design, fabricate, and characterize a new polymer-based multielectrode for the spinal cord for the application of intraspinal microstimulation (ISMS). Methods: Three-channel multielectrodes were fabricated from modified poly(dimethylsiloxane) (PDMS) and platinum-iridium (Pt-Ir) foil using nanosecond laser microfabrication techniques. These devices were compared against traditional 50 μm diameter Pt-Ir microwire electrodes mechanically and electrochemically in bench environments, and were assessed electrochemically and functionally in vivo in a domestic pig model. Results: Polymer-based multielectrodes were significantly more flexible than microwire electrodes (p < 0.05) and had greater charge storage capacities in phosphate buffered saline (p < 0.05). In a domestic pig model, multielectrodes had significantly greater charge injection limits than microwire electrodes (p < 0.05). When stimulating within the quadriceps motor pool in the spinal cord, multielectrodes generated strong knee extensor joint torques of up to 4.4 ± 0.3 Nm and were able to extend the knee by up to 26 ± 1°. However, histological analyses showed that polymer-based multielectrodes, implanted with half-needle insertion aids, produced greater acute tissue damage compared to microwire electrodes (p < 0.05). Alternative insertion methods for these flexible electrodes should be explored to reduce acute tissue damage. Conclusion: The PDMS-based three-channel multielectrodes demonstrated improved flexibility and charge injection capabilities over traditional microwire electrodes, and were able to produce functional responses in vivo. Significance: Polymer-based multielectrodes demonstrate improved functionality over microwire electrodes while remaining more flexible than silicon multielectrode designs. These features may in the future permit polymer-based multielectrodes to implement ISMS with greater efficacy and biocompatibility compared to traditional technologies.