Microfluidics and Neural Interfaces Development for the Safe Direct Current Stimulator

Safety of commercial neural implants fundamentally limits its working to the use of charge-balanced, biphasic pulses to interact with target neurons using metal electrodes. Short biphasic pulses are used to avoid toxic electrochemical reactions at the electrode-tissue interfaces. Biphasic pulses are effective at exciting neurons, but quite limited in inhibiting their activity. In contrast, direct current can both excite and inhibit neurons, however it leads to the formation of harmful, Faradaic reactions at the metal electrode/tissue interface. To address this challenge of safety over chronic use, we are developing the Safe Direct Current Stimulator (SDCS) technology, that generates an ionic direct current (iDC) from a biphasic input signal using a network of microfluidic channels and mechanical valves. This rectified iDC is applied to the target neural tissue through an ionically conductive neural interface. A key enabler towards transforming the SDCS concept from a benchtop design to an implantable neural prosthesis is the design of a miniature valve. Several valve architectures and actuation mechanism were studied for the development of the microfluidics in SDCS technology, before settling on the plunger-membrane microvalve design. This thesis characterizes a miniature polydimethylsiloxane (PDMS) based elastomeric normally closed (NC) mechanical valve actuated using a shape-memory alloy (SMA) wire through distinct tests and examines its current capability for iDC delivery. The analysis of the test outputs confirmed the feasibility of using this design for rectifying the charge-balanced alternating current (AC) into iDC. As metal electrodes are unsuitable for delivering iDC to the neural tissue safely, an ionic conductive neural lead is built. These gel-based, PDMS electrodes should be designed within the acceptable pressure limits that a nerve can handle safely. Preliminary experiments were conducted to verify the design and conductivity of the lead. While the results suggest that the lead design maintains the pressure below the maximum limit, its high impedance raises concerns. Although this thesis forms a basis for development of the SDCS device, further experimentation and progress is required for a reliable, safe, chronic, and fully functional device