Self-healing strain-responsive electrochromic display based on a multiple crosslinked network hydrogel

Stretchable electronic devices with self-healing functions that can improve durability are highly recommended as next-generation personal instruments for economic and sustainable society. Here, we report a fabrication of self-healing strain-responsive electrochromic display based on a multiple crosslinked network hydrogel (MCNH) consisting of both hydrophilic and hydrophobic domains. After optimizing the mechanical and self-healing properties of the hydrogel with variation of the chemical crosslinker, N,N'-methylenebisacrylamide, and the ionic crosslinker CaCl2, an extreme mechanical stretchability of up to 2000% strain and shape recovery, and a self-healing efficiency of 83.5% after 8 h at room temperature are obtained. The MCNH-based strain sensor exhibits a fast and linear resistance response with a coefficient of determination of 0.997 over a wide strain range of 100%. The strain sensitivity of the hydrogel remains stable even after 10 repeated self-healing cycles at a single location. As a display application, a novel two-dimensional electrochromic device is fabricated using a hydrogel without depositing an electrochromic material (ECM) on the electrode. ECM-containing gel electrolyte exhibits electrochromic properties through the migration of ions to the electrodes. Coloration/discoloration occurs at a potential bias of 1.7 V with a transmittance change of 76.1% at 547 nm through the chemical oxidation/reduction of ethyl viologen ions in the hydrogel matrix. An integrated system comprising a self-healing strain sensor and an ECD attached to the skin is demonstrated to visually express the applied strain due to finger bending, aided by an external circuit. Such a strain-responsive ECD system preserves a stable performance with the self-healed sensor after a complete bisection. These results suggest the potential application of our newly synthesized hydrogel to various skin-attachable self-healing, and stretchable devices with high durability.