Niederhoffer, Thomas; Vanhoestenberghe, Anne; Lancashire, Henry Thomas; (2020) Influence of Aspect Ratio on the Current Density Profile of Recessed Stimulation Electrodes. Presented at: International Society of Electrochemistry: 71st Annual Meeting, Belgrade, Serbia.

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Abstract

Introduction: Electrical stimulation is a therapeutic technique aiming at restoring impaired biological functions by injecting a controlled amount of charge in the body. Commonly used planar stimulation electrodes exhibit a non-uniform current density distribution (j) that can lead to locally exceeding the safe stimulation threshold [1]. An electrode can be recessed into an insulating part, which results in a different j [2], thus a different injection of charge, which can induce damage or insufficient stimulation. However, few studies [3] have focused on the influence of the recess parameters on j and how it could be used in electrode design. / Material & Methods: A finite element model was used to calculate j at recessed and non-recessed planar disc platinum electrodes in saline in response to an applied potential (COMSOL 54 Electrochemistry module). Recess shape (tubular and conical) and aspect ratio (AR), defined as recess depth divided by electrode diameter, were varied. Fig. 1 (c) shows a transverse view of a conical (left) and tubular (right) recess with the electrode covering the entire base of the recess. / Results: The tubular recess presents a uniform j at the electrode surface and at most depths throughout the recess, except at the open end of the recess, where the apparent j is more similar to the one of a non-recessed electrode, fig. 1(a, c). The conical recess presents a distribution more similar to a non-recessed electrode at the electrode surface, which becomes more uniform towards the recess end, fig. 1 (b, c). Increasing the AR makes j more uniform, both at the electrode surface and at the end of the recess, fig. 1 (a, b, d). At a given AR, varying the dimensions of the recess does not affect j, with relative standard deviations typically smaller than 0.5% (7 ARs tested, 3 pairs of dimensions per AR). / Discussion: The non-uniformity of j depended only on the recess shape and AR. Understanding this relationship will allow finetuning the current density profile at the electrode surface and at the recess end. For instance, a tubular recess with an AR larger than 2 will yield a uniform distribution at the electrode surface. Similarly, a conical recess of AR = 1/3 will reproduce the current density profile of a non-recessed electrode with less than 6% error, while being straightforward to manufacture. This study used a purely resistive model under steady-state conditions, which is a simple model for charge injection at an electrode-electrolyte interface. Further studies will consider a more realistic charge-transfer model and conditions closer to real dynamic electrical stimulation experiments.

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