WSense4Cort: A lab-on-skin capillary microfluidic immunosensor with sweat storage for on-body cortisol quantification and parallel validation

WSense4Cort (Wearable Sensor for Cortisol Detection) is a self-powered, wearable electrochemical immunosensor designed to monitor physiological cortisol levels in sweat. Translating traditional immunoassays into wearable electrochemical biosensors is challenging due to the lack of a robust and universal electrochemical signal transduction method. WSense4Cort overcomes this limitation by integrating a passive, diffusion-driven redox-delivery design, eliminating the need for enzymes, labels, or surface-immobilized mediators such as Prussian Blue. The system employs a pair of side-mounted redox reservoirs flanking the sensing region. Upon sweat uptake, these reservoirs hydrate and establish a self-contained redox couple that passively diffuses into the sensing zone, generating a stable electrochemical signal without enzymatic or label-based amplification. The device integrates multilayered microfluidic networks and dual sweat-storage fibers within flexible polymer layers. Mixed hydrophilic–hydrophobic layers guide sweat flow and regulate capillary progression, while the fibers act as absorbent sinks that stabilize sample volume and support controlled, sequential liquid handling. Importantly, the dual-fiber design allows simultaneous on-body detection and off-body enzyme-linked immunosorbent assay (ELISA) validation from the same sample, allowing for direct performance comparisons not possible with current wearable sensors. WSense4Cort's performance was validated through fluid dynamics simulations, image-based analysis, and electrochemical biosensing. It demonstrates a detection range of 1–20 ng/mL, a sensitivity of 1300 Ω·mL/ng, and a detection limit of 1 ng/mL, with a total assay time of 10 min. Strong correlation with ELISA (R 2 = 0.90), detection of diurnal and anatomical variations, and mechanical durability highlight its potential as a scalable “lab-on-skin” platform for immunosensing of sweat biomarkers. WSense4Cort is a wearable immunosensor for sweat cortisol monitoring that employs capillary-guided sweat transport to activate side-mounted redox reservoirs and enable passive, diffusion-driven redox for stable, self-contained, label-free electrochemical detection. Integrated hydrophilic–hydrophobic layers and dual storage fibers precisely regulate fluid progression and sample volume, creating a reliable, self-validating sample-to-result platform for cortisol monitoring within physiological ranges. • Self-powered, lab-on-skin capillary microfluidic electrochemical immunosensor for monitoring physiological cortisol levels in sweat. • Passive, diffusion-driven redox delivery enables robust electrochemical signal transduction for immunosensing, eliminating the need for enzymes, labels, or surface-immobilized mediators. • Multilayer flexible microfluidic network with engineered hydrophilic pathways provides stable fluid handling and consistent signal response. • High-resolution impedance sensing within the physiological cortisol range, enabling detection of subtle fluctuations in on-body measurements. • Integrated sweat-storage fibers preserve the sample for off-device ELISA validation, enabling one-to-one performance comparison from the same sweat sample.