Vertically integrated microfluidic sweat sensors encapsulated under a structure-reinforced FPCB with reusable sensing electrodes

Sweat loss monitoring is essential for evaluating hydration and electrolyte balance in sports and clinical settings. Despite advances in microfluidic technology, current systems still face challenges in achieving low-cost and reusable designs suitable for commercialization. Here, we present a vertically integrated strategy for microfluidic sweat sensors in which the microfluidic layer is vertically encapsulated by a flexible printed circuit board (FPCB). Sensing electrodes are fabricated on the backside of the FPCB using scalable, low-cost production methods, independent of microfluidic fabrication. This configuration enables electrode reusability, avoiding the waste of costly materials (e.g., gold) associated with disposable devices, and significantly reduces the consumable cost for long-term sweat monitoring. A thin polyimide (PI) layer is introduced to structurally reinforce the FPCB backplane, preventing liquid leakage through wiring grooves and enhancing flexibility for wearable use. The vertical configuration minimizes the device footprint and eliminates short-circuit risks from sweat contact. On-body tests validate the robust performance and reusability of the vertically integrated sweat sensor, offering a practical route toward commercial, flexible, and sustainable microfluidic sweat monitoring systems.