Biophysical and Functional Evaluation of Concentric Electrodes for Localised Non-Invasive FES

Spatial selectivity remains a major limitation of non-invasive functional electrical stimulation (FES), particularly in anatomically dense regions of the upper limb. This study investigates the design, modelling, and validation of concentric surface electrodes to improve spatial selectivity compared to conventional square geometries. A six-stage workflow was implemented, including concentric electrode design, finite element method (FEM) modelling of electric field distributions in a forearm model, axonal activation simulations using the MRG model, additive manufacturing of prototypes, impedance characterisation, and in-vivo evaluation of motor response and subjective discomfort. FEM analysis demonstrated that concentric electrodes enhanced electric field focality and reduced lateral current spread relative to square configurations. Coupled MRG simulations showed improved neural selectivity, with a broader high-selectivity range (Δ = 2.7 mA). Prototypes were fabricated using low-cost, accessible materials (~$1.64 per unit). In-vivo testing in 11 participants revealed significantly higher selectivity indices for concentric electrodes (0.723 ± 0.175) compared to square electrodes (0.546 ± 0.098, p < 0.05), without increased discomfort (p > 0.05). These findings indicate that concentric surface electrodes provide a practical and scalable strategy to enhance spatial selectivity in surface FES, and that the proposed modelling-to-prototyping workflow supports rapid optimisation of personalised neuroprosthetic interfaces.ces.