Together with the upcoming market for wearable consumer technologies, noninvasive and continuous health monitoring has become a new trend in the healthcare landscape. In recent years, significant research has been targeted toward the development of wearable sensing devices for monitoring biomarker levels in nonobtrusively accessible biofluids such as tears, urine, saliva, and sweat. Sweat could be an ideal candidate for prolonged, semicontinuous, and nonobtrusive health monitoring because sweat is a continuously accessible biofluid containing physiologically and metabolically rich information. However, challenges still remain toward commercialized wearable sweat-sensing devices, and the correlation between biomarker concentrations in sweat with health conditions still seems to be not fully understood. This review article aims to display the full scope of sweat sensing for health monitoring, starting from the fundamentals of human sweat, via modeling of the sweat gland physiology toward wearable sweat-sensing devices in research and commercialization efforts. Finally, the challenges of sweat sensing that still have to be overcome toward the utilization of sweat sensing in the conventional healthcare settings are discussed.

Advanced optical imaging techniques address important biological questions in neuroscience, where structures such as synapses are below the resolution limit of a conventional microscope. At the same time, microelectrode arrays (MEAs) are indispensable in understanding the language of neurons. Here, the authors show transparent MEAs capable of recording action potentials from neurons and compatible with advanced microscopy. The electrodes are made of the conducting polymer poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS) and are patterned by optical lithography, ensuring scalable fabrication with good control over device parameters. A thickness of 380 nm ensures low enough impedance and >75% transparency throughout the visible part of the spectrum making them suitable for artefact-free recording in the presence of laser illumination. Using primary neuronal cells, the arrays record single units from multiple nearby sources with a signal-to-noise ratio of 7.7 (17.7 dB). Additionally, it is possible to perform calcium (Ca 2+ ) imaging, a measure of neuronal activity, using the novel transparent electrodes. Different biomarkers are imaged through the electrodes using conventional and super-resolution microscopy (SRM), showing no qualitative differences compared to glass substrates. These transparent MEAs pave the way for harnessing the synergy between the superior temporal resolution of electrophysiology and the selectivity and high spatial resolution of optical imaging.

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Research targeting the development of on-body sensors has been significantly growing in recent years - an example is on-skin sweat sensing. However, the wide inter and intra person variability of...