(Invited) Scaling Effects on the Electrophysiological Stimulation Capabilities of PEDOT:PSS, Pt, and Au

Abstract

Recent clinical successes of neuromodulator devices such as cochlear implants, deep brain stimulation (DBS) and spinal cord stimulation have generated a wide interest in exploiting emerging state-of-the-art biomaterials for therapeutic electrical stimulation of neurological diseases and disorders. Some of these applications require localized stimulation using microelectrodes with a charge injection density beyond 0.5 mC/cm2 that exceeds Pt and Pt alloys charge injection limits (0.3 mC/cm2). This is where IrOx, TiN and conducting polymers (CPs) emerge as potential candidates as materials for next generation microelectrodes. Here, we present a systematic in-vitro charge injection study to benchmark stimulation performance of PEDOT:PSS on Au, PEDOT:PSS on Pt, and reference Au and Pt electrodes across 20μm to 2000μm electrode diameters. In our study, higher charge injection capacity was measured for PEDOT:PSS/Pt microelectrodes (20 µm to 150 µm diameter size) which was ~3.4 time larger than Pt and ~9 time larger than Au microelectrodes. However, macroscale PEDOT:PSS/Au electrodes (200 µm to 2000 µm diameter size) displayed higher charge injection capacity compare to other electrode materials. Optimal OCP bias voltage was presented at 0.4 V for both PEDOT:PSS/Au and Pt micro/macro electrodes. Particularly, CICs of microelectrodes experienced ~55% and ~135% enhancement compare to zero bias voltage, resulted in maximum 1.88 mC/cm2 and 0.8 mC/cm2 CIC for PEDOT:PSS/Au and Pt microelectrodes respectively. CICs of PEDOT:PSS/Au micro/macro electrodes boosted ~ 13-14% further at 0.5 V OCP bias voltage by pulse engineering (asymmetric pulse injection). Charge injection capacity introduced as frequency-dependent parameter whereas PEDOT:PSS/Au microdots and macrodots demonstrated 40% and 21% larger CIC by pulse width expansion from 200 µs to 2 ms duration; which highlights the importance of accurate regulation of stimulation methodology parameters such as pulse width to make proper balance between safe (higher CIC) and initiative neural stimulation (higher current and frequency). Finally, Charge storage capacity (CSC) evaluation revealed the largest CSC for Pt micro/macro electrodes compare to other electrode materials, whereas PEDOT:PSS/Au micro/macro electrodes offered higher capability to convert those available stored charges into injected electrical pulse (higher CIC/ CSC ratio). We believe that these results could pave the way toward establishing safe and efficient clinically-relevant stimulation protocol for PEDOT:PSS microelectrodes.