Parylene-Based Electrochemical-MEMS Force Sensor for Studies of Intracortical Probe Insertion Mechanics

To investigate the mechanical interactions between the implanted cortical multielectrode probes and brain tissue, a Parylene C-based electrochemical-microelectromechanical systems force sensor array was developed. The array consists of seven linearly distributed sensor units arranged along the length of a flexible Parylene C microchannel. The seven sensor units are formed by eight fluidically coupled and adjacent platinum electrode pairs enclosed within the microchannel. Deformation of the top surface of the mechanically compliant microchannel changes the volumetric conduction path between the pairs of sensing electrodes, and therefore, the measured electrochemical impedance, which is proportional to the magnitude of the contacting force. Each sensor unit demonstrated a linear response from 0 to 60 mN with a sensitivity of 0.13 ± 0.01 percentage change in impedance/μN (%/μN; mean ± SE, n = 6). The sensor arrays were mounted onto a ceramic intracortical probe and inserted into the tissue phantoms to verify in situ functionality and assess interfacial probe mechanics. Probe surface force distribution was measured under various insertion speeds and the results indicated that interfacial forces are distributed nonuniformly along the probe shaft length, concentrating within the first 1 mm of the advancing probe tip. Faster insertion speeds were also found to decrease the magnitude of the interfacial forces, suggesting that the tissue strain during cortical implantation may be minimized through appropriate selection of the insertion speed.