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Impedance Properties of Multi-Optrode Biopotential Sensing Arrays

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Recording and monitoring electrically-excitable cells is critical to understanding the complex cellular networking within organs as well as the processes underlying many electrophysiological pathologies. Biopotential recording using an optical-electrode (optrode) is a novel approach which has potential to significantly improve interface-instrumentation impedance mismatching as recording contact-sizes become smaller and smaller. Optrodes incorporate a conductive interface that can sense extracellular potential and an underlying layer of liquid crystals that passively transduces electrical signals into measurable optical signals.

This work demonstrates the advantage of using the optrode system over a conventional electrode and amplifier system in terms of impedance levels and dense-array configurations by varying the diameter of sensing sites and observing the corresponding changes in the impedance values. The results show that the liquid crystals in this optrode platform exhibit input impedance values (1 MΩ – 100 GΩ) that are three orders of magnitude higher than the corresponding interface impedance, which is appropriate for voltage sensing. The automatic scaling of the input impedance enabled within the optrode system maintains a relatively constant ratio between interface input and total system impedance of about one for sensing areas with diameters ranging from 40 µm to 1 mm, at which the calculated signal loss is predicted to be <1%. This feature preserves the interface-transducer impedance ratio, regardless of the size of the sensing site, allowing development of passive optrode arrays capable of very high spatial-resolution recordings. This work is the starting point to optimize the layout and configuration of multi-optrode arrays to target various biomedical applications.

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