(K-415) Improved long-term stimulation stability of thin flexible microelectrode arrays for intracortical microstimulation via carbon nanotube doped polyethylenedioxythiophene conducting polymer coatings

Introduction:: The use of microelectrode arrays (MEAs) to both record from and electrically stimulate neurons has rapidly growing impact in the medical field. Applications range from control of robotic prostheses to the treatment of neurological disorders such as epilepsy [1], with recent advances in intracortical microstimulation (ICMS) showing the potential to provide somatosensory feedback to patients controlling a robotic prosthesis [2]. A primary challenge for ICMS is long-term stability, with devices shown to break down after extensive use [3]. Thin flexible MEAs are emerging as a potential alternative to traditional rigid electrodes, having better tissue integration which could improve electrode performance and longevity [4]. Another method for improving the performance and longevity of ICMS is coating electrodes in organic conducting polymers such as polyethylenedioxythiophene (PEDOT) doped with carbon nanotubes (CNTs) to improve their electrochemical properties [5]. Here, we perform an in-depth evaluation of electrode failure under long-term stimulation using thin flexible MEAs with small (30-40um) sites, suitable for ICMS, comparing bare Pt to PEDOT-CNT coated arrays. In addition, we present a rigorous and multi-modal approach to the failure analysis of MEAs, with a particular emphasis on conservative and accurate characterization which we hope will help to standardize the evaluation of long-term ICMS devices.

Materials and Methods:: Flexible MEAs were fabricated via photolithography on silicon wafers. They were comprised of polyimide insulation with Pt/Au/Pt metal interconnects and traces leading to 16 exposed circular Pt sites 30 micrometers in diameter. Electrochemical coating and characterization were carried out on a potentiostat using an Ag/AgCl reference and a Pt wire counter. For coating, poly(3,4)-ethylenedioxythiophene doped with acid functionalized CNTs was electrochemically deposited onto Pt microelectrode sites , at a constant voltage of 0.9 V until charge density reaches 200uC/cm2. Stimulation was performed using a Ripple Grapevine Scout in an electrolyte of 1X phosphate-buffered saline (PBS) using a Pt counter. Cathodic charge storage capacity (CSCc) was determined using cyclic voltammetry at a scan rate of 100mV/s, and impedance measurements were performed over a frequency range of 0 Hz to 100 kHz. Charge injection limit (CIL) was determined by establishing a profile of the Emc (maximum negative polarization voltage) across a range of currents within the window of the electrodes’ reduction and oxidation potentials (Figure 1). The two points closest to the window’s edge were used to extrapolate the current at which the Emc reaches reduction potential to ensure the most accurate analysis (Figure 2). The Emc was determined as the potential at which the current became zero after the end of the cathodic pulse [6]. Injected currents were single biphasic pulses with a leading cathodic phase of 200us, an anodic phase of 400us, and an interphase of 100us. The same waveform was used for all stimulations.

Results, Conclusions, and Discussions::

Comparing the PEDOT-CNT coated MEA to the bare Pt MEA, prior to stimulation it was evident that the PEDOT-CNT coating improved the electrochemical characteristics of the MEA, as expected based on previous studies. Here, we show that even for thin flexible microelectrodes the enhanced electrochemical properties of the PEDOT-CNT coating were maintained under long-term electrical stimulation. Although there were initial changes in electrochemical properties for PEDOT-CNT coated electrodes (Figure 3), they maintained a higher CIL and CSCc and a lower impedance than the bare Pt array throughout prolonged stimulation and the trend across days of stimulation was for changes to become smaller as stimulation progressed, suggesting that CIL, CSCc and impedance may eventually stabilize under constant stimulation load. The bare Pt MEA’s decrease followed by an increase in CIL and reverse trend for CSC and 1 kHz impedance beginning at 48 hours (Figure 4) is suggestive of possible deterioration, perhaps related to insulation loss, as mentioned in prior studies [1], and may be an early sign of failure. Scanning electron microscopy of the MEA will be necessary to confirm this.

In addition to empirical tests of the stability of PEDOT-CNT coatings on thin flexible MEAs under prolonged stimulation, we have developed rigorous and conservative methods for monitoring the electrochemical changes during stimulation which are sensitive and also minimize the potential to damage the material by aggressive measurement of the CIL. Studies characterizing electrode stability often use impedance and CSC as measures of integrity, although there is some evidence here that CIL may be a more sensitive measure, as at the 48-hour timepoint we saw changes in all sites for CIL, whereas only a subset of sites showed large changes in impedance and CSCc. This highlights the importance of carefully observing and evaluating individual electrode sites and on multiple electrochemical measurements, including CIL. Standardized methods of evaluating an electrode’s performance online without risk of damage must be established to further the use of ICMS devices. Here we present a thorough multi-modal approach to the failure analysis of flexible MEAs that will provide a detailed assessment of their suitability as ICMS devices.

Acknowledgements (Optional): : This work was funded by BRAIN initiative R01NS110564 awarded to Dr. Cui.

References (Optional): :

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