Experimental framework for skin-electrode impedance characterization in somatotopic TENS

Transcutaneous Electrical Nerve Stimulation (TENS) has been widely adopted as a non-invasive method for delivering somatotopic sensory feedback (SSF) in neuroprosthetic applications. However, the implementation of portable TENS-based systems is constrained by the high and variable skin-electrode interface impedance (Z), which in turn defines the minimum voltage compliance (VC) a stimulator must guarantee to effectively deliver current-controlled stimuli. While numerous models of skin impedance exist, an accurate quantification of its variability under somatotopic stimulation conditions remains lacking. This study aims to propose and evaluate an experimental framework for identifying impedance ranges and the corresponding voltage-compliance requirements relevant to somatotopic TENS stimulation in the upper and lower limbs. Impedance values were acquired in a repeated-measures experimental design from ten healthy participants under stimulation conditions selected to systematically reproduce key SSF application constraints. The results show that electrode diameter is the strongest experimental factor influencing impedance, supporting its role as a primary determinant in the estimation of voltage-compliance requirements. The upper limb exhibited higher impedance values than the lower limb across all conditions. A maximum VC of 104 V and 50.8 V was recorded for the lower and upper limbs, respectively. These findings provide quantitative requirements critical for the design of low-power, wearable SSF TENS stimulators.