Electrowetting valves for sweat-based microfluidics

Skin-compatible microfluidic valving systems with on-demand sweat capture are necessary to understand the temporal variation of biomarkers. Here, we demonstrate solution-based electrowetting valves with rapid actuation integrated into a flexible microfluidic sweat collection patch. The valve is produced by inkjet-printing a pair of silver electrodes with spacings of 0.2–2 mm and modifying the downstream electrode with a hydrophobic self-assembled monolayer. To complete the valve, a microfluidic channel is fabricated from laser ablation of adhesive layers and pressed over the silver electrodes. Artificial perspiration is driven by capillary action within the channel until stopped by the electrowetting valve. A low voltage is applied to the electrodes, decreasing the surface energy of the hydrophobic monolayer and allowing the fluid front to continue through the microchannel. Statistical analysis demonstrated that applied voltage, and not electrode spacing, influenced valve actuation time, with 17 ± 8 s for 4 V and 40 ± 16 s for 1 V. Inkjet-printing conditions were also optimized to achieve a valve fluid retention time of 9 h. Using four electrowetting valves, an integrated wearable device is designed for artificial perspiration collection through valve actuation at distinct time points over 40 min. Finally, these inexpensive, user-friendly, and disposable electrowetting valves offer exciting opportunities for non-invasive point-of-care sweat monitoring.