Bilayer Actuator Film for Urea Biosensing with Dual Responsiveness: Bending Actuation and Photonic Color Change

A noninvasive sweat-based biosensor was developed for urea detection using a photonic bilayer actuator film (BAF) consisting of an interpenetrating polymer network (IPN) as the active layer and a flexible poly(ethylene terephthalate) (PET) substrate as the passive layer (IPN/PET). The active IPN layer comprises intertwined solid-state cholesteric liquid crystal and poly(acrylic acid) (PAA) networks. Urease was immobilized in the PAA network in the IPN layer of the photonic BAF. The interaction with aqueous urea altered the curvature and photonic color of the photonic urease-immobilized IPN/PET (IPN_urease/PET) BAF. The curvature (and wavelength of the photonic color) of the IPN_urease/PET BAF increased linearly with urea concentration (C_urea) in the range of C_urea = 20-65 (and 30-65) mM with a limit of detection value of 1.42 (and 1.34) mM. The developed photonic IPN_urease/PET BAF exhibited high selectivity toward urea and excellent spike test results with real human sweat. This novel IPN_urease/PET BAF is promising because it enables battery-free, cost-effective, and visual detection-based analysis without the use of sophisticated instruments. Furthermore, the application of this photonic IPN/PET BAF can be easily extended to other biosensors by immobilizing other receptors on the IPN.