Facile Preparation of Starch Biopolymer-Based Hydrogels for Flexible Electronics—Strain-Sensitive Batteries and Self-Powered Sensors

As a renewable biopolymer, high-amylose starch due to its higher content of linearly structured chains is more interesting to realise enhanced material properties and new functionality. However, the full dissolution of the compact granule structure of high-amylose starch is challenging under moderate conditions, which limits its applications. In this work, we have revealed that high-amylose maize starch (HAMS) granules can be easily destructed by certain concentrations of ZnCl₂, MgCl₂ and CaCl₂ solutions (43 wt%, 34 wt% and 31 wt%, respectively) at a moderate temperature (under 50 °C) without chemical derivatization. In particular, the ZnCl₂ and CaCl₂ solutions resulted in complete dissolution of HAS granules and the regenerated starch from the CaCl₂ solution was completely amorphous.<br/>We found by simple mixing of the HAMS with a CaCl₂ solution followed by heating the mixture at 80 °C for 5 min, a flexible and ionically conductive starch-based hydrogel can be obtained. By varying the starch/CaCl₂ dry mass ratio, the materials exhibited tuneable mechanical strength (500–1300 kPa), elongation at break (15–32%), Young’s modulus (4–9 MPa) and toughness (0.05–0.26 MJ/m³), suitable electrical resistivity (3.7–9.2 Ω m), and strain-responsiveness. With plasticisation, a more flexible starch-based hydrogel was obtained, which can be easily reprocessed and has self-healing ability. The hydrogel was then developed into a galvanic cell-type battery, with an output voltage of 0.81 V, and a self-powered (SP) wearable sensor, which had high sensitivity (1.5371 kPa⁻¹) even under weak compression stress. This SP sensor can be used to detect human activities involving small strain such as wrist pulse and throat vibration. Considering the easy processability, cost-effectiveness, high strain-sensitivity, robustness, and greenness of the starch-based hydrogel and electronics, their brilliant application prospect is foreseen.