Chronic stability of activated iridium oxide film voltage transients from wireless floating microelectrode arrays

Abstract

Successful monitoring of the condition of stimulation electrodes is critical for maintaining chronic device performance for neural stimulation. As part of pre-clinical safety testing in preparation for a visual prostheses clinical trial, we evaluated the stability of the implantable devices and stimulation electrodes using a combination of current pulsing in saline and in canine visual cortex. Specifically, in saline we monitored the stability and performance of 3000 μm2 geometric surface area activated iridium oxide film (AIROF) electrodes within a wireless floating microelectrode array (WFMA) by measuring the voltage transient (VT) response through reverse telemetry. Eight WFMAs were assessed in vitro for 24 days, where n = 4 were pulsed continuously at 80 μA (16 nC/phase) and n = 4 remained in solution with no applied stimulation. Subsequently, twelve different WFMAs were implanted in visual cortex in n = 3 canine subjects (4 WFMAs each). After a 2-week recovery period, half of the electrodes in each of the twelve devices were pulsed continuously for 24 h at either 20, 40, 63, or 80 μA (200 μs pulse width, 100 Hz). VTs were recorded to track changes in the electrodes at set time intervals in both the saline and in vivo study. The VT response of AIROF electrodes remained stable during pulsing in saline over 24 days. Electrode polarization and driving voltage changed by less than 200 mV on average. The AIROF electrodes also maintained consistent performance, overall, during 24 h of pulsing in vivo. Four of the in vivo WFMA devices showed a change in polarization, access voltage, or driving voltage over time. However, no VT recordings indicated electrode failure, and the same trend was typically seen in both pulsed and unpulsed electrodes within the same device. Overall, accelerated stimulation testing in saline and in vivo indicated that AIROF electrodes in the WFMA were able to consistently deliver up to 16 nC per pulse and would be suitable for chronic clinical use.

In‐vivo testing of a novel wireless intraspinal microstimulation interface for restoration of motor function following spinal cord injury

“…The current WFMA is equipped with 16 channels for stimulation and recording. As previously reported, 9 the WFMA is composed of a ceramic substrate, an application‐specific integrated circuit (ASIC), a coil, and a microelectrode array. Stimulation electrodes are insulated iridium microwires with 3000 μm 2 of exposed activated iridium oxide film (AIROF).…”

“…That is, the AIROF electrode potential must remain more positive than the expected potential for the onset of water reduction (−0.6 V vs. Ag|AgCl in pH 7.1 phosphate-buffered saline), which has been shown to lead to delamination of the AIROF on the surface of the electrode. 9 The magnitude of the driving voltage V drive must be less than 3.5 V, which is the maximum voltage supply for the WFMA. If the magnitude of the driving voltage exceeds 3.5 V, there will not be enough voltage compliance available to drive the desired current.…”

“…9 In ongoing preclinical and clinical trials, the WFMA devices provided a stable neural interface. 9,10 Furthermore, a previous study in a surrogate spinal cord suggested that the strain caused by a WFMA-based spinal interface may fall below the threshold for axonal injury. 11 Despite this, the mechanical and electrochemical stability of the WFMA devices in the spinal cord remain unknown.…”

“…The proposed wireless ISMS interface is supported by a 5 mm‐diameter wireless floating microelectrode array (WFMA) which is currently in use in an FDA‐approved clinical trial (NCT04634383) aimed at eliciting visual precepts in persons with profound blindness. A wireless inductive link is used to transfer power and enable bidirectional telemetry 9 . In ongoing preclinical and clinical trials, the WFMA devices provided a stable neural interface 9,10 .…”

“…A wireless inductive link is used to transfer power and enable bidirectional telemetry 9 . In ongoing preclinical and clinical trials, the WFMA devices provided a stable neural interface 9,10 . Furthermore, a previous study in a surrogate spinal cord suggested that the strain caused by a WFMA‐based spinal interface may fall below the threshold for axonal injury 11 …”