Biochemical monitoring of sweat could enable new forms of health care diagnostics and personalized hydration strategies. This design is a waterproof, skin-like, microfluidic and electronic system that adheres to the skin and enables real-time quantitative measurements of sweat loss, sweat biomarkers, and skin temperature. Sweat is driven through microfluidic channels and chambers via sweat gland pressure and provides sweat collection, handling, and analysis capabilities directly on the surface of the skin. Colorimetric reagents provide visual measures of sweat loss and biomarker concentrations via changes in the sweat color. Thin, soft mechanics, waterproof materials, and a skin-safe adhesive enable strong adhesion and sweat collection even while fully underwater. Microscale valves and pumps provide capabilities for purging of sweat as a reset mechanism to coincide with hydration events, and automatically deliver a mild cooling effect to alert users to levels of excessive sweat loss.