Ionic transistor using ion exchange membranes.

Ionic transistors can be used to modulate ionic current in a way that is analogous to their electronic counterparts. An ionic transistor can reversibly change its ionic conduction to control ionic current by injecting electrical charges. To facilitate its applications in biomedical devices (e.g., controlled drug delivery, rectification of ionic current, and signal processing), an ionic transistor should maintain high performance of ionic current control within physiological solutions (e.g., 0.9% NaCl) for long durations. Here, we introduce an ionic transistor using cation and anion exchange membranes (CEM and AEM). It could impose a 10× impedance change in a channel filled with 0.9% NaCl solution and we observed a stable modulation of ionic current throughout a test of 1000 cycles of on/off switching of the ionic transistor.

On-chip ionic current sensor.

Neural implants that deliver drugs or electrical stimuli via microfluidic ports are promising in providing therapy for various disorders such as epilepsy, chronic pain, and vestibular diseases. To deliver the stimuli to a neural target, these devices incorporate two or more electrodes that apply an electric field to drive charged particles or ions along an aqueous route provided by microfluidic channels. The amount of drug/current delivered is determined by measuring the ionic current flow. When the ionic current can only travel from one electrode to another via a single route or channel, the amount of therapeutic current is stoichiometrically equal to the electronic current applied by the device and therefore can be measured with an electronic current sensor. However, some recently developed devices contain networks of branched channels. In this case, the presence of multiple parallel ionic current paths makes it so that the current through any one individual channel is no longer measurable by observing electronic current alone. Here, we present an on-chip sensor that uses two Pt/Ir electrodes to transduce the ionic current through a target channel into a measurable voltage signal. The size of the metal wires did not impact the measured voltage, the size of the channel between the two sensing electrodes determines sensitivity of the sensor, change in temperature can cause a change in readings, and input impedance of the voltage measuring equipment must be greater than 1 GΩ to maintain measurement stability. The sensor showed stability of reading in a one-week longevity test.

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.