A Multiple Crosslinked Network Hydrogel (MCNH)-Based Self-Healing Strain Responsive Electrochromic Display

Self-healing functionalities are studied actively in fields of stretchable electronics applications to improve the device lifespan, to gain economic feasibility, and ensuring environmental friendliness. However, the synthesis of high-efficient healing materials and specific application of these materials to actual devices are still in progress. We report on Multiple Crosslinked Network Hydrogel (MCNH) Based Self-Healing Electrochromic Strain Sensing Device, which is a block copolymer consisted of poly(acrylic acid) hydrophilic and poly(stearyl methacrylate) hydrophobic domains, forming micellar structure by sodium dodecyl sulfate. Micellar structure with of MCNH was confirmed through TEM and DLS, varying micelle size of 50 – 100 nm. The mechanical properties for toughness of MCNH and self-healing property of hydrogel matrix are investigated by controlling chemical and ionic crosslinker of N,N'-methylenebisacrylamide and CaCl₂. The optimized MCNH showed extreme mechanical property, enduring up to 2000% strain and 90% compression, recovering its shape after deformation. Also, the self-healing efficiency showed its highest rate of 83.5% after 8hr of physical contact in room temperature. Synthesized MCNH could be used as a strain sensor based on its elasticity, showing a fast, linear resistance return with coefficient of determination of 0.997 over strain range of 100%. The sensitivity of MCNH strain sensor maintained its sensitivity after 10 repeated healing cycles. Also, by using MCNH as self-healable electrochromic electrolyte, lateral 2D electrochromic devices (2D-ECDs) was fabricated as a display device. Ethyl viologen (EtV) added into MCNH matrix, diffused to each electrode, and showed simultaneous coloration/discoloration when the electrical potential of 1.7 V was applied. 2D-ECD showed transmittance change of 76.1% at 547 nm and also self-healing characteristics. By integrating strain sensors and 2D-ECD using customized Arduino circuit, the strain responsive electrochromic system was fabricated, showing the state of strain applied to human body in electrochromic display, mounted on human hand. Also, since both devices could be self-healed, stable operation after self-healing process was shown. This study proposes a high potential application of our newly synthesized hydrogels to various skin-attached self-healing, deformable and stretchable devices.