Flexible wood-derived hydrogel patch for smartphone-assisted interval sweat cortisol profiling

Wearable biochemical sensors offer a promising route to noninvasive cortisol analysis. However, existing sweat-cortisol sensing platforms often rely on complex collection devices, electronic readout modules, or dedicated instrumentation. Here, we report a multilayer wood-derived hydrogel patch for portable optical analysis of sweat cortisol. The patch integrates the anisotropic porous framework of delignified balsa wood with a polyacrylamide/poly(vinyl alcohol)/glycerol (PAAm/PVA/glycerol) hydrogel network to form a flexible, hydrophilic sensing layer for pump-free sweat uptake. A cortisol-responsive Förster resonance energy transfer (FRET) probe embedded in the sensing layer enables molecular recognition, while adhesive and encapsulation layers improve wet-skin attachment and water retention. A companion smartphone application provides portable cortisol quantification. Under the tested conditions, the complete patch produced a distinguishable response at the lowest experimentally evaluated concentration of 1 pM in human sweat matrices. Pilot human studies supported the feasibility of interval sweat cortisol analysis during exercise-associated, time-of-day-related, and repeated-measurement scenarios. This work extends wood-derived materials into wearable biosensing and provides a bio-derived integration strategy for portable biochemical analysis.