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.