Biodegradable Temperature Sensors with Enhanced Sensitivity Using Bioderived Ionic Liquid with Sodium Ions

Gels promise the development of flexible sensors for electronic and ionic skins owing to their high affinity for human skin, making them ideal for biomedical monitoring. Bioderived ionic liquids and their gels possess favorable physical and electrochemical characteristics, including extremely low vapor pressure, high ionic conductivity, and biodegradability, which suit applications of transient electronics requiring the autonomous decomposition of materials. However, their low temperature coefficient of resistance (TCR) may hinder the use of bioderived ionic liquids in wearable devices, implants, and environmental sensing. To address this issue, we doped sodium (Na) ions into the bioderived ionic liquid choline lactate to enhance its TCR for biodegradable temperature sensors. The Na ions improve the coupling of choline and lactate, resulting in a Na-doped ionic liquid with a high TCR, which is attributed to enhanced intermolecular interactions. The ionic gel, composed of the Na-doped ionic liquid and poly(vinyl alcohol), demonstrated a high thermal index of 7563 K, an activation energy of 1303 meV, and a TCR of 8.4%/K. The temperature sensor degraded in phosphate-buffered solution over 48 days, leaving the encapsulation layer. This study provides insight into enhancing the sensitivity of temperature sensors with ionic liquids for healthcare, wearable, and environmental applications.