On the Mechanics of Bioimpedance Signals Under Deformation

Abstract

Bioimpedance provides a noninvasive means of characterizing biological tissues and monitoring physiological function. In wearable and skin-integrated systems, however, electrical measurements are acquired while the underlying tissues undergo extension, bending, twisting, compression, and physiological motion. These deformations alter the conducting domain, electrode placement, and redistribute the three-dimensional current field. The measured impedance, therefore, reflects both the tissue's electrical properties and its mechanical state, yet the mechanics governing this dependence remain poorly understood. Here, we combine tetrapolar measurements on conductive hydrogel phantoms, coupled mechanics and electrical finite element analysis, and reduced-order theory to determine how distinct deformation modes enter the impedance response. Deformation alone produces systematic impedance changes reaching approximately 30% under extension, 5% under bending, and 3% under torsion. These relations reproduce the experimental means with root-mean-square errors below 1.2 percentage points and demonstrate that response magnitude is governed by the order at which deformation perturbs electrically weighted current paths, rather than by the apparent complexity of the deformation field. Finite element calculations further show that, for uniform and variable conductivity, conductivity sets the absolute impedance scale while deformation produces a distinct normalized geometric offset. Small conductivity uncertainties can nevertheless equal or exceed the complete bending and torsional signals, and this separation breaks down when conductivity varies spatially or evolves during deformation. These findings establish mechanics as a governing state variable in bioimpedance and provide a basis for distinguishing deformation-induced signals from physiological changes in wearable measurements of highly deformable tissues.