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Review
. 2021 Jun;10(12):e2100119.
doi: 10.1002/adhm.202100119. Epub 2021 May 24.

Electrode Materials for Chronic Electrical Microstimulation

Xin Sally Zheng  1 , Chao Tan  1 , Elisa Castagnola  1 , Xinyan Tracy Cui  1
Affiliations
Review

Electrode Materials for Chronic Electrical Microstimulation

Xin Sally Zheng et al. Adv Healthc Mater. 2021 Jun.

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.

Keywords: carbon materials; chronic neural stimulation; conducting polymers; metals.

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Figures

Figure 1.
Figure 1.
a) Schematic showing electrodes stimulate excitable tissues via applying a voltage, the cellular membrane of the targeted neurons was depolarized, which results in firing of action potentials. Adapted from [38] with permission. b) Faradaic charge transfer occurs via irreversible and reversible faradaic reactions. c) Capacitive transfer is achieved via charging and discharging of the electric double layer (EDL) formed by accumulation of ions on the oppositely charged electrode surface.
Figure 2.
Figure 2.
(a) SEM picture of NanoPt coated PtIr wire. Scale bar is 100 μm. (b) SEM of explanted electrode surface in comparison to pristine NanoPt electrode. The surface was not damaged, but some tissue remained on surface. Scale bar is 500 nm. Adapted from [19] with permission. (c) & d) Optical and SEM picture of Iridium microelectrode after 169 hr electrical aging (6 μA, 100 μs phase−1, 2 billion biphasic). Parylene-C layer was delaminated and cracked while Iridium remain intact. Scale bar is 50 μm in panel (c) and 10 μm in panel (d). Adapted from [144] with permission. (e) SEM picture of explanted AIROF conical electrode pulsed for 7 hours at a high charge density of 3 mC cm−2(100 μA, 600 μs phase−1, 100 pulse/s, +0.4 V bias), the surface was corroded. Scale bar is 20 μm. (f) SEM picture of explanted AIROF conical electrode after 6 months without pulsing. Adapted from [147] with permission.
Figure 3.
Figure 3.
(a) Scanning electron microscopy of PEDOT-PSS on GC substrate after 5 million pulses vs. (b) scanning electron microscopy of PEDOT-PSS on Pt substrate after 1 million pulses of stimulation. Note the crack formation in PEDOT-PSS/Pt due to poor adhesion in (b). Adapted from [166] with permission. (c-h) PEDOT/CNT polymerized on Pt substrate with potentiostatic method and galvanic method respectively. Potentiostatically polymerized coating resulted in a relatively flatter surface d) and exceeded the bounds of the underlying substrate whereas in (e) the galvanostatically polymerized PEDOT/CNT formed a 3D cone shape with increased porosity (f). (g-h) SEM of the same substrate coated with PEDOT-PSS for reference. Adapted from [174] with permission.(i-m) polyimide-based electrode with gold traces was patterned with nanostructured gold polymerized with PEDOT/ox-SWCNH and PEDOT ox-MWCNT. NTs: nanotubes, NH: nanohorns (j-k). (I,m) Nanoporous morphology of NHs and NTs respectively. Adapted from [176] with permission.
Figure 4.
Figure 4.
a) Vertically aligned multiwalled CNT pillars microelectrode arrays (left) and magnification on a single pillars microelectrode (right). From [188]. b) Scanning electron microscopy images of microelectrodes coated with carbon nanotubes grown by chemical vapor deposition. From [27]. c) Scanning electron microscopy imaging of two-channel CNT fiber microelectrodes, fabricated by twisting single filaments of a 43 μm diameter CNT fiber, coated with a 3 μm layer of PS-b-PBD; inset shows a close view of the active site. From [189]. d) GF fibers from [29]: picture of a GF bipolar microelectrode assembly. Inset, SEM image of the GF bipolar microelectrode tip, showing two GFs (bright core) with each one insulated with Parylene-C film (dark shell). Scale bar, 1 cm; inset, 100 μm (left). SEM image of the axial external surface of a GF fiber. Inset, magnified image of the region in the dashed box. Scale bar, 20 μm; inset, 5 μm. (center). SEM image of the exposed cross section acting as the active stimulating site of a GF electrode. Inset, magnified image of the region in the dashed box. Scale bar, 20 μm; inset, 5 μm (right). From [29]. e) liquid crystal graphene oxide (LCGO) fiber with parylene-C insulation: electrode pressed into clay to demonstrate flexibility and elastic deformation (left). Laser treatment leads to an amorphous electrode with extraordinary surface roughness and porosity (right). From [187]
Figure 5.
Figure 5.
(a) GC 12 electrode array and (b) the device is folded to show its flexibility. From [166]. (c) 9-electrode array (curled in the inset) From[207]. (d) Photograph of a fabricated 64 porous graphene electrode array. (e) SEM image of the cross-section view of porous graphene. Scale bar:100 μm. (f) Tilt SEM image of a 64-spot porous graphene array. Scale bar: 1mm. The inset is the SEM image of an individual spot. Scale bar: 100 μm. From [28]. (g)15-channel GC ECoG probe, (h) In inset: Magnified SEM image of a single microelectrode in the ECoG probe. Also, FIB cross-section of the GC microelectrode taken at the edge between GC and Durimide is shown revealing a seamless integration of the layers. (i) Expanded view of FIB cross-section taken at a GC wire trace leading to one of the microelectrodes, (j) High-resolution (50,000×) FIB image of a cross-section taken through a GC wire trace. Box in bottom of (i) shows the location where FIB through GC interconnect was taken. Note that there are no adhesion layers or metal layers in all GC (aGC) probes. From [18]
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