Bioadhesive Hydrogel Flexible Laser for Sweat Sensing based on Liquid Crystal Microdroplets

Flexible photonics offers the possibility to realize wearable sensors by bridging the advantages of flexible materials and photonic sensing elements. Recently, optical resonators have emerged as a tool to improve its over sensitivity by integrating with flexible photonic sensors. However, direct monitoring of multiple psychological information on human skin remains challenging, due to the subtle biological signals and complex tissue interface. To tackle the current challenges, here we developed a functional thin film laser formed by encapsulating multiple liquid crystal microdroplet laser resonators in a flexible hydrogel for monitoring important metabolites in human sweat (lactate, glucose, and urea). The three-dimensional cross-linked hydrophilic polymer serves as the adhesive layer to allow small molecules to penetrate from human tissue to generate strong light-matter interactions on the interface of whispering gallery modes resonators. Both hydrogel and CLC microdroplets were modified specifically to achieve high sensitivity and selectivity. As a proof-of-concept, wavelength-multiplexed sensing and a prototype were demonstrated on human skin to detect human metabolites from perspiration. These results present a significant advance in the fabrication and potential guidance for wearable and functional microlasers in healthcare.