  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.
