A Comparative Assessment of Evoked Compound Action Potentials Measured by Optrode and Conventional Bioamplifier Systems

Reliable signal detection of biopotentials in excitable tissues has been one of the longest-standing challenges in neural and cardiac electrophysiology. While using standard electrodes and bioamplifiers provides insight into the cell activity with a high temporal resolution, electrical recording still suffers from several limitations that can hinder long-term operation and clinical translation. The recently developed liquid crystal optical-electrode or ‘optrode’ has exhibited powerful diagnostic potential for multiple applications by using light to passively sense biopotentials in a fluorophore-free manner. While it has been demonstrated that this optrode device can detect electrically evoked compound action potentials from rabbit sciatic nerves, the microvolt-level features within the response that represent different nerve fibers inside the sciatic nerve bundle have not been investigated. Herein, we report a detailed investigation of the optrode's recording performance by analyzing the conduction velocity of different detected nerve fibers. We show that the optrode can record various nerve fiber responses to electrical stimulation, including $\mathbf{A}\delta, \mathbf{B}$, and C fibers as verified by comparison to measurements via a conventional bioamplifier. We found that there are some challenges encountered when recording nerve responses of low amplitudes $\boldsymbol{(< 300 \ \mu} \mathbf{V})$ or high conduction velocities $(> 30 \ \mathrm{m}/\mathrm{s})$ using the optrode due to its limited signal-to-noise ratio and bandwidth, highlighting the necessity for design improvement.