Microfluidic-electrochemical sweat patches for athletes: monitoring lactate, electrolytes, and hydration status

Wearable biochemical sensing is shifting sports physiology from intermittent laboratory sampling toward continuous, in situ readouts during training and competition. This review analyzes skin conformal systems that integrate capillary driven fluid routing, flexible sensor arrays, and wireless readout to capture dynamic changes in eccrine secretions relevant to performance management. We compare microchannel layouts that enable controlled filling, time resolved sampling, and evaporation suppression, and we discuss how material selection governs comfort and analytical fidelity, highlighting tradeoffs among polydimethylsiloxane, polyurethane, and polyethylene terephthalate substrates and their compatibility with scalable fabrication. Manufacturing pathways are assessed from soft lithography to laser cutting, three dimensional printing, and roll to roll processing for high throughput multilayer assembly. For transduction, we summarize enzymatic amperometric schemes based on lactate oxidase with hydrogen peroxide detection, including sensitivity, response time, and oxygen dependence, and we contrast these with emerging non enzymatic catalysts. We then detail potentiometric ion selective electrodes for sodium and potassium, focusing on ion selective membrane chemistry, solid state reference electrodes, Nernstian response, and dominant error sources such as drift and biofouling. System integration challenges, including chemical and electrical cross talk in multiplexed layouts, are linked to microfluidic isolation strategies and multiplexed electronics. Finally, we appraise validation practice, emphasizing the debated sweat to blood relationship, the need for synchronized comparative protocols, and the role of data analytics and machine learning for personalization, drift compensation, and prediction of thermoregulatory strain. Remaining barriers include long term stability, adhesion under motion, manufacturability, and regulatory evidence requirements in real world settings. Across these topics, the review emphasizes technology-specific limitations, engineering translation metrics, and realistic breakthrough directions rather than treating wearable sweat patches as a mature plug-and-play platform.