Noninvasive monitoring of cortisol, a key endocrine biomarker of hypothalamic-pituitary-adrenal (HPA) axis activity, is important for characterizing circadian regulation and physiological responses to exercise and stress. Although sweat provides an accessible alternative to conventional laboratory testing, existing wearable cortisol platforms often rely on complex microfluidic collectors, electronic readout modules, or dedicated instruments. Integrating rapid sweat acquisition, selective molecular recognition, stable attachment to wet skin, and portable signal readout within a single flexible material platform therefore remains a major challenge.
In a study published in Biosensors and Bioelectronics, the research team led by Prof. ZHANG Yun from the Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences reported a flexible multilayer wood-derived hydrogel patch that integrates capillary-driven sweat collection, fluorescence-based cortisol recognition, wet-skin adhesion, and smartphone-assisted analysis.
The researchers removed lignin from balsa wood while preserving its aligned multiscale channels, then infiltrated the scaffold with a polyacrylamide/poly(vinyl alcohol)/glycerol hydrogel and functionalized with a UCNP-apt/cDNA-Au probe. The resulting flexible, skin-conformal patch retained continuous capillary pathways for pump-free sweat transport, achieving a peak water absorption rate of 0.0539 g·s-1 and directional bottom‑to‑top transport within ~0.03 s. An adhesive layer enhanced wet-skin adhesion, while an ultrathin polyethylene terephthalate film improved water retention and fluorescence stability.
In analytical performance, the normalized green fluorescence intensity linearly correlated with the logarithm of cortisol concentration from 1 pM to 1 μM (y = 0.0628x + 0.934, R² = 0.987), with a distinguishable response at the lowest tested concentration of 1 pM in human sweat. Repeatability and spike-and-recovery tests, together with agreement with enzyme‑linked immunosorbent assay (ELISA) and liquid chromatography‑tandem mass spectrometry (LC‑MS/MS), supported its analytical reliability.
For portable quantification, the researchers developed the “VisCor” smartphone application. After a 30-min wearing period, the patch was excited using a 980 nm laser, and its 541 nm emission was collected off-body through a narrow-band filter. Ratiometric self-calibration reduced device-dependent variation, and optical readout could be completed within 2 h after sampling. Pilot human studies supported interval profiling of exercise-associated, diurnal, longitudinal, and inter-individual variations in sweat cortisol, although validation in larger controlled cohorts is still required.
This study presents a wood-derived materials strategy that integrates passive sweat collection, cortisol recognition, wet-skin adhesion, and smartphone quantification without an external pump or on-body electronics. It expands the potential of wearable biosensing and provides a basis for future multiplexed sweat analysis.
Design and application of the multilayer wood-derived hydrogel patch, including (a) patch architecture, (b) fluorescence-based cortisol recognition, (c) pump-free sweat uptake, (d) wet-skin adhesion, (e) smartphone-assisted readout, and (f) pilot interval cortisol profiling (Image by Prof. ZHANG Yun's group)
Contact:
Prof. ZHANG Yun
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
Email:zhangy@fjirsm.ac.cn
