Scaling Effects on The Electrochemical Performance of Ti₃C₂T_x MXene Thin-Film Bioelectronics for Neural Recording and Stimulation

In the last few decades there has been a growing interest in the development of technologies for neural recordings and therapeutic electrical brain stimulation. In these applications, microelectrode arrays offer the advantage of high-selectivity and sensitivity approaching the spatiotemporal scales of individual neurons. Recently, 2D Ti₃C₂T<i>_x</i>MXene has emerged as a promising candidate for microscale recording and stimulation, due to its high electrical conductivity and volumetric capacitance. Here, we investigate the scaling effects on the electrochemical performance of thin-film 2D Ti₃C₂T<i>_x</i>MXene microelectrodes with diameters ranging from 500 µm to 25 µm and film thicknesses varying from 100 nm to 700 nm. Prior to all electrochemical characterization measurements, we measured contact diameters, surface roughness, and thicknesses with confocal microscopy, atomic force microscopy (AFM), and 2D profilometry, respectively, in order to accurately assess geometric surface areas and available volumes. We then tested the electrodes with electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), voltage transients (VT), as well as 10,000 continuous cycles of biphasic stimulation pulses. Through these experiments, we calculated impedance modulus at 10 Hz and 1 kHz, charge storage capacity (CSC), and charge injection capacity (CIC) across the various electrode contact sizes and thicknesses. We also investigated the effects of pulse-width and waveform asymmetry on the calculated CIC values to determine the optimal stimulation parameters for the safe and effective delivery of electrical stimulation in thin-film Ti₃C₂T<i>_x</i> MXene microelectrodes. We found there was an expected increase in the magnitude of impedances at 1 kHz as the contact diameters decreased from 500 µm to 25 µm with an average thickness of 100 nm, increasing from 9.1 ± 1.9 kΩ in the 500 µm contacts to 506 ± 220 kΩ in the 25 µm contacts. Similarly, CIC values also increased as the contact size decreased, increasing from 40 ± 10 µC cm^-2 to 400 ± 50 µC cm^-2in the 500 µm and 25 µm contacts, respectively. Ultimately, our findings indicate that Ti₃C₂T<i>_x</i>MXene microelectrodes are able to safely deliver electrical microstimulation, opening the door to further diagnostic and therapeutic research as well as improve our biological understanding of neural activity through electrophysiological recording and electrical stimulation.