Wearable technologies for personalized monitoring require sensors that track biomarkers often present at low levels. Cortisol-a key stress biomarker-is present in sweat at low nanomolar concentrations. Previous wearable sensing systems are limited to analytes in the micromolar-millimolar ranges. To overcome this and other limitations, we developed a flexible field-effect transistor (FET) biosensor array that exploits a previously unreported cortisol aptamer coupled to nanometer-thin-film In 2 O 3 FETs. Cortisol levels were determined via molecular recognition by aptamers where binding was transduced to electrical signals on FETs. The physiological relevance of cortisol as a stress biomarker was demonstrated by tracking salivary cortisol levels in participants in a Trier Social Stress Test and establishing correlations between cortisol in diurnal saliva and sweat samples. These correlations motivated the development and on-body validation of an aptamer-FET array-based smartwatch equipped with a custom, multichannel, self-referencing, and autonomous source measurement unit enabling seamless, real-time cortisol sweat sensing.

Recent advances in functional low‐melting liquid metals (LMs) provide routes to quickly fabricate wearable devices, which can offer excellent mechanical compliance on human skin compared to conventional rigid wafer‐based electronics. Particularly, the Gallium‐based LM mixtures are promising materials for use as stretchable and flexible circuits in wearable healthcare electronics due to their fantastic electrical conductivity, favorable fluidity, super compliance, and benign biocompatibility. Here, the authors develop a kind of directly‐printed Ni‐GaIn functional amalgams, which can efficiently construct wearable healthcare monitors. Unlike EGaIn, Ni‐GaIn amalgam owns a tunable and enhanced adhesion, which makes it easy to directly paint on various substrate for customized circuits. A wireless power transfer coil printed with these amalgams is presented to demonstrate the electrical stability under different tensile states. Finally, the authors design a wearable healthcare monitor for pulse wave measurement via collecting pulse wave signals from the wrist steadily. With the advantages of stretchability and rapid manufacture, the functional Ni‐GaIn amalgams as currently presented shows a promising approach for individualized wearable electronics.

Active biofluid management is central to the realization of wearable bioanalytical platforms that are poised to autonomously provide frequent, real-time, and accurate measures of biomarkers in epidermally-retrievable biofluids (e.g., sweat). Accordingly, here, a programmable epidermal microfluidic valving system is devised, which is capable of biofluid sampling, routing, and compartmentalization for biomarker analysis. At its core, the system is a network of individually-addressable microheater-controlled thermo-responsive hydrogel valves, augmented with a pressure regulation mechanism to accommodate pressure built-up, when interfacing sweat glands. The active biofluid control achieved by this system is harnessed to create unprecedented wearable bioanalytical capabilities at both the sensor level (decoupling the confounding influence of flow rate variability on sensor response) and the system level (facilitating context-based sensor selection/protection). Through integration with a wireless flexible printed circuit board and seamless bilateral communication with consumer electronics (e.g., smartwatch), contextually-relevant (scheduled/on-demand) on-body biomarker data acquisition/display was achieved.