Freeform Stimulator (FS) Implant Design for Non-Pulsatile Arbitrary Waveform Neuromodulation

In contrast to conventional pulsatile stimulation that evokes action potentials in phase with pulse presentations, ionic direct current (iDC) or low frequency current is capable of excitation, inhibition, sensitization to synaptic input, and synaptic connectivity control. Despite its versatility, iDC cannot be delivered via metal electrodes for safety concerns such as pH changes, electrolysis, and corrosion. Our lab has previously proposed a microfluidic device design that rectifies pulses delivered safely to embedded metal electrodes into iDC. However, this design lacked reliability and consistency that were linked to using fluid electrolyte as the ionic charge carrier. Here, we overcame these limitations by designing a microfluidic chip not filled with liquid but with agar gel as the means of ionic conduction. The device has two plungers coated with agar gel that act as ionic switches to control the direction of ionic current flow. When the plungers are closed, the agar on the plunger and the agar on the chip connect to create a gel bridge for the ionic current to pass through. Movement of the plungers is actuated with a $50\mu m$ diameter nitinol shape memory alloy wire (Flexinol HT) which shrinks when activated with electrical current. To open the plungers, the device has a silicone ring on the plunger that functions as a spring to pull the plungers back to the open position after the nitinol wire is deactivated. With this design, the device can deliver up to 100 $\mu \mathrm{A}$ of iDC and has a reduced form factor of 75% from the previous generation, to facilitate behavioral animal experiments with FS. We fabricated three devices to compare their performance. The switches had conducting and non-conducting impedances of $3.62 k\Omega\pm\theta.43k\Omega$ and $980 k\Omega\pm 12k\Omega$ respectively. We also simulated the output characteristics of the FS by using actual impedance data collected from the devices.