Polyelectrolyte hydrogel: A versatile platform for mechanical-electric conversion and self-powered sensing

There is an urgent need to develop self-powered technologies for portable strain/pressure sensors. In this work, inspired by salinity-gradient electricity generation, we report the design and fabrication of a mechanical-electric conversion and self-powered sensing platform based on polyelectrolyte hydrogels. Using polyacrylic acid (PAA, whose -COOH group can ionize H+ in water) hydrogel as a model system, we observe that the compression deformation process of the hydrogel leads to a localized increase in the H+ concentration, that induces a directional H+ diffusion as a result of ion concentration difference. In contrast, the PAA chain (negatively charged), as a hydrogel backbone, is hindered from moving freely due to the locking of the hydrogel network. Such drastic difference between ionized H+ and fixed PAA chain defines an opportunity for converting mechanical energy into electricity by compressing the PAA hydrogel. The good elasticity of the PAA hydrogel allows us to repeatedly regenerate and fully recover the electrical signal and finally completes the recycle sensing (>1000 s). PAA hydrogels have also been shown to convert static or low frequency (∼0.05 Hz) pressure (0.05–5 N) to output direct voltage/current. In particular, the hydrogel can yield a voltage output up to ∼6 mV and a current of ∼2 μA when the pressure is 5 N. Taking full advantage of the special sensing principle, we have developed both self-powered and temperature-insensitive PAA hydrogel-based position recognizers and multi-dimensional sound detectors. Importantly, this mechanical-to-electric conversion concept is also applicable to other polyelectrolyte hydrogels. This simple and low-cost soft material system is promising for applications in sensing and energy harvesting.