Electromechanical responses in water-rich self-assembled amphiphile gels and hydrated ionogels: reassessing piezoelectric attributions

Abstract

Water-rich, self-assembled amphiphile gels have recently been reported to exhibit electrical responses under mechanical deformation and hysteretic cyclic voltammetry behavior, leading to their interpretation as piezoelectric or ferroelectric soft materials. Such claims are particularly compelling given the relevance of hydrated, compliant electromechanical materials to biological, biomedical, and soft-sensing applications. However, in ion-containing soft matter, the electrical signatures commonly used to infer piezoelectricity are not uniquely diagnostic and may arise from multiple coupled mechanisms. Here, we re-examine the electromechanical behavior of fatty acid-amine, fatty acid-amino acid, and surfactant-fatty alcohol hydrogels previously described as piezoelectric. We show that capacitive cyclic voltammetry responses and mechanically induced voltage or current transients can be rationalized within a broader framework of electromechanical coupling in hydrated ionic gels, encompassing electrokinetic charge redistribution, electrode double-layer and interfacial effects, and strain-dependent dielectric responses, in addition to intrinsic polarization. As an instructive non-amphiphile comparator, ionotropically crosslinked carboxymethyl cellulose gels exhibit closely analogous electrical signatures only in the presence of multivalent ionic crosslinkers, reinforcing the non-uniqueness of piezoelectric interpretations in hydrated ionic systems. To clarify these distinctions, we introduce a minimal coupled electromechanical model that explicitly separates instantaneous elastic polarization from rate-dependent ionic contributions and identifies their distinct experimental signatures. We further propose discriminating experiments required to unambiguously distinguish intrinsic piezoelectricity from electrokinetic and interfacial effects. This reassessment does not diminish the functional relevance of these materials; rather, it provides a rigorous mechanistic foundation for their continued development as soft electromechanical transducers for sensing, biointerfacing, and biomedical applications.