A perspective on wearable strain sensors featuring ionogel or eutectogel electrolytes

The most familiar gel materials are hydrogels, where water is the main component. While the aqueous electrolytes used in hydrogels tend to dry out quickly owing to the volatility of water, two unconventional non-aqueous electrolyte classes, ionic liquids (ILs) and deep eutectic mixtures (DEMs), can allow one to realize gels that do not readily evaporate at all; these are referred to as ionogels and eutectogels, respectively. By contacting an ionogel or eutectogel with two electrodes on either end, a resistive strain sensor can be formed whose relative ionic resistance change, upon axial deformation, is dictated by the resulting change in the transverse gel cross-section. While it is possible to demonstrate an essentially unlimited number of (co)polymer-supported ionogel or eutectogel formulations, additional future efforts should be made to improve sensor stability, utilize non-toxic materials, demonstrate recyclability of both the polymer and liquid electrolyte components and consider potential costs of implementation. This article is part of the discussion meeting issue 'Ionic liquids and the future of soft materials'.