Highly Sensitive Flexible Sensors using Autonomously Self-Healable and Temperature-Tolerant Eutectogel

With the increased demand for advanced wearable electronics, hydrogel-based devices have attracted enormous attention. Hydrogels have many advantages such as excellent flexibility, biocompatibility, ionic conductivity, and self-healing property, adequate for wearable devices. On the other hand, hydrogel-based devices exhibit limit in the performance due to the freezing and evaporation of water in the polymer network at low and high temperatures, respectively. To address such a thermal instability issue, there has been extensive effort on replacing the water within the hydrogel network with ionic liquids of low vapor pressure and high ionic conductivity. However, most ionic liquids are expensive and toxic to hamper the practical application to wearable devices. Therefore, research on deep eutectic solvents, mixtures of two or more components characterized by significant depressions in melting points compared to their neat constituent components, has been recently activated. Deep eutectic solvents show properties similar to those of ionic liquids as well as additional advantages of low-cost, non-toxicity, biocompatibility, and biodegradability. Furthermore, eutectogels having polymer networks composed of deep eutectic solvents have demonstrated the high potential application to wearable devices.<br/>In this paper, we report on the fabrication of resistive-type strain and pressure sensors based on newly synthesized autonomous self-healing and temperature-tolerant eutectogel. Our synthesized eutectogel is prepared by UV polymerization of acrylamide with trehalose and phytic acid as crosslinkers, and choline chloride, glycerol and a small amount of water as solvents. In addition to the inherent low melting point of deep eutectic solvent, inclusion of bio-derived trehalose and phytic acid makes abundant hydrogen bonding, resulting in improved anti-freezing and anti-drying properties. Also, the dynamic hydrogen bond interactions between the polymer network and trehalose, phytic acid, and glycerol endowed the eutectogel with self-healing properties. The synthesized eutectogel exhibits high stretchability (&gt;300%), temperature-tolerance, and good self-healing efficiency (&gt;90%) after 24 hours at room temperature. By incorporating conducting polymer onto the eutectogel, highly sensitive resistive-type strain and pressure sensors are fabricated for detecting bio-signals after attachment onto a finger or wrist. The eutectogel-based wearable sensors demonstrate stable signal detection over a wide temperature range even after repetitive self-healing, thereby expanding their potential application to wearable electronics with longevity under extreme conditions.