Autonomous Capillary Microfluidic Devices with Constant Flow Rate and Temperature-Controlled Valving

In this work, we introduce a Poly(N-isopropylacrylamide) (PNIPAm) grafted PDMS (PNIPAm-g-PDMS) capillary flow-driven microfluidic device with integrated valving function. Due to the thermo-sensitive properties of PNIPAm, the device possesses a temperature-switchable surface wettability between 20 and 36 °C. By locally integrating a heating wire, a hydrophobic valving function can thus be obtained. The device provides large operational freedom, enables single-valve control, and operates in a convenient temperature range. In addition, this device is characterized by a capillary filling rate that is constant in time. The constant flow velocities ranging from 1 µm/s to 240 µm/s can be obtained in dry PNIPAm-g-PDMS and freshly treated PNIPAm-g-PDMS devices with different channel geometry. We explained the constant flow rate with diffusive hydration of PNIPAm at the liquid front. This device thus provides both stop valving and accurate flow control functions, being potentially applied for diagnostic assay performance.