Darkening of Laser‐Induced Graphene in Wet for Readable In Situ Real Time Sweating Rate Analysis

Abstract Developing a simple technique for in situ monitoring of sweating rate can provide real time feedback of individual's dehydration status and further assist in human health management. This work demonstrates the visible darkening of laser‐induced graphene (LIG) after wetting and its wearable practicability in monitoring sweating rate. LIGs with various porous morphologies are generated and their color differences are quantitatively assessed with hue‐saturation‐value model, correlated with the sheet resistance and the porous architecture. Embedding LIG pattern within microchannel enables the direct readout of water‐filled region and arranging length scale marks along microchannel allows direct readout of fluid volume with human eye for more simple measurements. Flexible design of length scale resolution can balance the detection frequency (i.e., real time ability) and its accuracy. The in situ and real time flow rate monitoring performance is successfully adopted on‐body for sweating rate analysis with reasonable accuracy. Apart from the simplicity and low cost, compared to other flow rate sensing methods, the proposed platform is reusable with no special cleaning needed for the microchannel but just drying, and no visible decline of the performance occurs. LIG's darkening strategy endows an avenue for the development of powerful in situ and real time sweating analytical platform.