Addressing Patient Heterogeneity and Cost in tVNS Wearables: Electrode Spacing Optimization for Noninvasive Brain–Computer Interfaces

Transcutaneous neuromodulation and noninvasive brain–computer interfaces hold promise for accessible neurorehabilitation and wearable technology, yet their real-world implementation remains constrained by patient tissue heterogeneity and the cost associated with individualized computational modeling. To establish an accessible, low-cost hardware optimization framework, this study deployed a physical hydrogel phantom platform to evaluate the effect of interelectrode spacing on electrical transmission under controlled conductive conditions. An Arduino Nano-based instrumentation architecture delivered controlled 5 V electrical impulses into agarose-based phantoms across three NaCl concentrations (0.5%, 0.9%, and 2.0%). Four interelectrode distances (1 mm, 2 mm, 3 mm, and 4 mm) were evaluated through 60 randomized trials. Peak-to-peak voltage potential (VP) and coefficient of variation were evaluated across the experimental conditions. The measured VP decreased with increasing interelectrode spacing, while higher NaCl concentrations generally produced greater voltage retention. Two-way analysis of variance demonstrated significant main effects of electrode spacing, F(3, 48) = 2394.99, p < 0.001, and NaCl concentration, F(2, 48) = 847.62, p < 0.001, as well as a significant spacing × concentration interaction, F(6, 48) = 36.43, p < 0.001. Post hoc Tukey comparisons demonstrated statistically significant differences among all tested spacing levels under the pooled experimental conditions. These findings demonstrate that electrode spacing materially influences voltage transmission within the tested hydrogel conditions and that its effect varies with phantom concentration. The results provide an experimentally accessible basis for preliminary electrode-geometry evaluation prior to more complex computational, ex vivo, or in vivo validation. Because the experimental platform consists of passive saline-doped agarose, the findings do not directly establish neural recruitment, vagal selectivity, therapeutic efficacy, or clinical safety. Received: 22 June 2026 | Revised: 7 September 2026 | Accepted: 13 October 2026 Conflicts of Interest The author declares that he has no conflicts of interest to this work. Data Availability Statement The raw data supporting the conclusions of this study, including individual trial oscilloscope logs and the underlying embedded C++ microcontroller script, are available from the corresponding author upon reasonable request. Author Contribution Statement Anubhav Mohapatra: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing – original draft, Writing – review & editing, Visualization, Project administration.