Increased preferential activation of small cutaneous nerve fibers by optimization of electrode design parameters

Abstract

Objective. Electrical preferential activation of small nociceptive fibers may be achieved with the use of specialized small area electrodes, however, the existing electrodes are limited to low stimulation intensities. As existing electrodes have been developed empirically, the present study aimed to use computational modeling and optimization techniques to investigate if changes in electrode design parameters could improve the preferential activation of small fibers. Approach. Two finite element models; one of a planar concentric and one of an intra-epidermal electrode were combined with two multi-compartmental nerve fiber models of an Aδ-fiber and an Aβ-fiber. These two-step hybrid models were used for the optimization of four electrode parameters; anode area, anode–cathode distance, cathode area, and cathode protrusion. Optimization was performed using a gradient-free bounded Nelder–Mead algorithm, to maximize the current activation threshold ratio between the Aβ-fiber model and the Aδ-fiber model. Main results. All electrode parameters were optimal at their lower bound, except the cathode protrusion, which was optimal a few micrometers above the location of the Aδ-fiber model. A small cathode area is essential for producing a high current density in the epidermal skin layer enabling activation of small fibers, while a small anode area and anode–cathode distance are important for the minimization of current spread to deeper tissues, making it less likely to activate large fibers. Combining each of the optimized electrode parameters improved the preferential activation of small fibers in comparison to existing electrodes, by increasing the activation threshold ratio between the two nerve fiber types. The maximum increase in the activation threshold ratio was 289% and 595% for the intra-epidermal and planar concentric design, respectively. Significance. The present study showed that electrical preferential small fiber activation can be improved by electrode design. Additionally, the results may be used for the production of an electrode that could potentially be used for clinical assessment of small fiber neuropathy.

Evoked sensations with transcutaneous electrical stimulation with different frequencies, waveforms, and electrode configurations

“…Several studies confirmed these findings, as it was shown that Aδ‐type nociceptors could be selectively activated using intraepidermal electrical stimulation with a concentric electrode configuration, provided that low stimulation intensities were applied 21,22 . A recent study has validated the design of a planar concentric electrode which resulted in an increased activation threshold ratio (between the Aβ and Aδ‐fibers) of 92%–595% 23 …”

“…21,22 A recent study has validated the design of a planar concentric electrode which resulted in an increased activation threshold ratio (between the Aβ and Aδ-fibers) of 92%-595%. 23 On the other hand, electrotactile feedback has been suggested for a wide variety of human-machine/computerinteraction applications, predominantly for providing prosthetic hand users with somatosensory feedback but also for other applications such as robotic teleoperation or virtual reality. 24 In the case of hand prostheses, electrotactile feedback was found successful rendering tactile sensations corresponding to touch and texture, which resulted in a more accurate prosthesis control.…”

Measuring pain and nociception: Through the glasses of a computational scientist. Transdisciplinary overview of methods

“…Electrodes with a small surface lead to high current densities and are more likely to activate C-fibers than electrodes with a large surface. Special configurations of anode and cathode, e.g., concentric electrodes with an intraepidermal part, seem to preferentially activate A-delta fibers mediating pain and thermal stimuli compared to A-beta fibers, which mediate non-painful sensations (Poulsen et al, 2020;Poulsen et al, 2021). High frequency stimulation activates preferentially A-fibers, while C-fibers cannot follow higher stimulation frequencies over 100 Hz for longer time periods.…”

Novel surface electrode design for preferential activation of cutaneous nociceptors

“…Previous modelling studies have shown that smaller cathodes increase the current density within the epidermal skin layer, while the anode size and anode-cathode distance decrease the current spread to deeper tissues and thereby decrease the probability of activating large fibers [11,20]. The overall result for minimizing the electrode dimensions would thus be increased preferential activation of small fibers.…”

“…Nonetheless, this recommendation may only be relevant to one specific electrode design since current density is dependent on the electrode shape and type [11]. Moreover, all of the existing electrodes have been developed empirically and may be further optimized to increase nociceptive specificity and the applicable range of stimulation intensities [20]. Poulsen et al [20] showed that minimizing the electrode dimensions would increase preferential activation of small fibers.…”