Nerve cuff electrode pressure estimation via electrical impedance measurement.

OBJECTIVE: Implantable neuromodulation devices that have cuff electrodes are known to exert mechanical pressure on the target nerves. The amount of pressure exerted by cuff enclosures is one of the key determinants of physiological safety of these devices since excess pressures can cause neural damage. Because direct measurements of pressure on a nerve are challenging, the current cuff design approaches rely heavily on theoretical models or numerical computations for pressure predictions. An experimental approach to test these devices for pressure can complement existing theoretical models and can also serve as a quality control step to screen cuff electrode designs before implantation. APPROACH: We hypothesize that the pressure exerted on a nerve by a cuff can be estimated by measuring the resulting changes to the nerve's electrical impedance. MAIN RESULTS: We investigated ten 1 cm-long explanted rat sciatic nerves: five that were used within an hour after surgery, and five after 50 h of storage in physiological saline. For each experiment we applied variable pressure on the nerve ex vivo and measured the resulting changes in its impedance. We found a strong correlation between the external pressure on the nerve and its impedance and generated a pressure-impedance calibration curve. At the upper limit of physiologically safe pressure, the nerve impedance increased by ~2 kΩ, whereas, a rise of ~3 kΩ corresponded to pressure value that onsets irreversible nerve damage. SIGNIFICANCE: As a proof-of-concept, we used this protocol to generate a pressure-impedance calibration curve for a monkey tibial nerve and estimated pressure exerted by a commercial silicone cuff electrode on the explanted nerve. This single-point measurement was in an agreement with an independent estimate of the pressure measured using a mechanical pull test within 3 mmHg.

Normally closed plunger-membrane microvalve self-actuated electrically using a shape memory alloy wire.

Various microfluidic architectures designed for in vivo and point-of-care diagnostic applications require larger channels, autonomous actuation, and portability. In this paper, we present a normally closed microvalve design capable of fully autonomous actuation for wide diameter microchannels (tens to hundreds of µm). We fabricated the multilayer plunger-membrane valve architecture using the silicone elastomer, poly-dimethylsiloxane (PDMS) and optimized it to reduce the force required to open the valve. A 50-µm Nitinol (NiTi) shape memory alloy wire is incorporated into the device and can operate the valve when actuated with 100-mA current delivered from a 3-V supply. We characterized the valve for its actuation kinetics using an electrochemical assay and tested its reliability at 1.5-s cycle duration for 1 million cycles during which we observed no operational degradation.

Miniature Elastomeric Valve Design for Safe Direct Current Stimulator.

For safety reasons, commercial neural implants use charge-balanced biphasic pulses to interact with target neurons using metal electrodes. Short biphasic pulses are used to avoid irreversible 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 delivered to metal electrodes, it causes toxic electrochemical reactions. We recently introduced Safe Direct Current Stimulator (SDCS) technology, which can excite or inhibit neurons without violating the safety criteria. Instead of direct current, SDCS generates an ionic direct current (iDC) from a biphasic input signal using a network of fluidic channels and mechanical valves. A key enabler towards transforming SDCS concept from a benchtop design to an implantable neural prosthesis is the design of a miniature valve. In this work, we present poly-dimethylsiloxane (PDMS) based elastomeric valves, squeeze valve (SV) and plunger valve (PV) capable of being actuated using a shape memory alloy wire.