Electrode Materials for Chronic Electrical Microstimulation

Abstract

Electrical microstimulation has enabled partial restoration of vision, hearing, movement, somatosensation, as well as improving organ functions by electrically modulating neural activities. However, chronic microstimulation is faced with numerous challenges. The implantation of an electrode array into the neural tissue triggers an inflammatory response, which can be exacerbated by the delivery of electrical currents. Meanwhile, prolonged stimulation may lead to electrode material degradation., which can be accelerated by the hostile inflammatory environment. Both material degradation and adverse tissue reactions can compromise stimulation performance over time. For stable chronic electrical stimulation, an ideal microelectrode must present 1) high charge injection limit, to efficiently deliver charge without exceeding safety limits for both tissue and electrodes, 2) small size, to gain high spatial selectivity, 3) excellent biocompatibility that ensures tissue health immediately next to the device, and 4) stable in vivo electrochemical properties over the application period. In this review, the challenges in chronic microstimulation are described in detail. To aid material scientists interested in neural stimulation research, the in vitro and in vivo testing methods are introduced for assessing stimulation functionality and longevity and a detailed overview of recent advances in electrode material research and device fabrication for improving chronic microstimulation performance is provided.

Injectable wireless microdevices: challenges and opportunities

“…This challenge is more pronounced in stimulating implants that require a certain voltage across their electrode pair. With the limited area, stimulating electrodes will have to utilize novel rough materials with large charge-injection capacity and small impedance (Pranti et al 2018 ; Khalifa et al 2015 ; Zheng et al 2021 ).…”

Engineering Tissues of the Central Nervous System: Interfacing Conductive Biomaterials with Neural Stem/Progenitor Cells

“…[ 163 ] Several electrode characteristics, including size, probe shape, cross‐sectional area, and surface roughness, have been adapted to create better control over the charge injection limit to reduce irreversible reactions. [ 165 ]…”

“…These irreversible reactions are generally undesirable, as they may damage interfaced cells and tissues, induce inflammation, and impair electrode function. [ 165 ] Thus, applied voltages are typically maintained within the so‐called “water window” (≈‐0.6 V to 0.8 V), which has been established as a safe range for use in tissues to avoid hydrolysis and irreversible electrochemical reactions. [ 164,166 ]…”

“…Perhaps the most difficult challenge arises from the tradeoff between electrical performance and mechanical properties, where conventional electronic materials (e.g., metals) are substantially stiffer than neural tissue, [159] and thus elicit an increased inflammatory response and poor tissue integration when implanted. [160] In Section 9, we detail recent efforts to overcome this challenge by developing new, composite biomaterials that combine those known to be compatible with neural tissues (e.g., soft hydrogels) with those more conventionally used as electrodes (e.g., metals and carbon-based materials) (Figure 5).…”

The Efficient Regeneration of Corneal Nerves via Tunable Transmembrane Signaling Channels Using a Transparent Graphene‐Based Corneal Stimulation Electrode

“…[17] Because of the ability to tune electrical activity, a high requirement on the safe charge injection limit (Q inj , i.e., the electrochemical capacitance) of ES is also required. [18] In the authors' previous work, [19] we showed that the electrically stimulated repair of the un-regenerable optic nerve could be realized through improvements to the electroactivity of the nanoelectrode used. This was achieved by using a nanofiber electrode made from polypyrrole-modified graphene.…”