Microfluidics-based fabrication of flexible ionic hydrogel batteries inspired by electric eels

Electric eels in nature can generate high voltage with hundreds of volts based on the mechanism of gradient-induced ion flux, which provides an excellent prototype to inspire the exploration of more efficient and green energy generation strategies in artificial systems. Here we developed a novel microfluidics-based strategy to efficiently fabricate flexible ionic hydrogel batteries by mimicking ion-concentration gradients in biological organs. Microfluidic networks were designed to simultaneously transport four types of ionic hydrogel solutions into pre-defined well arrays and enable the perfusion of 801 hydrogel precursor wells within 1.53 min. A specifically-designed photomask was used to selectively solidify these precursors into separate hydrogel particles, which were further assembled into a flexible ionic hydrogel battery pack by using negative pressure. The voltage output of the resultant ionic hydrogel battery pack was found to increase linearly with the increase of battery unit number, and a maximum voltage of 73.33 V can be achieved by connecting eight ionic hydrogel battery packs in series. The resultant ionic hydrogel battery exhibited unique capability in stably working in wide temperature ranges (0–90°C) and large deformation conditions, and can be further preserved for a long period after dehydration and instantly recover its original discharge capacity once rehydrated. PEDOT:PSS was introduced to the ionic hydrogel battery for reduced internal resistance, resulting in a 1.5-fold improvement in electrical current output. The presented microfluidics-based and high-efficient fabrication strategy exhibits great promise to translate the biomimetic flexible power-generating systems into a viable soft power source for various electrically-driven applications.