Thermo-responsive poly(ionic liquid) valves for microfluidic devices

Poly(ionic liquid)s (PIL) are a class of ionic liquids that feature polymerizable groups in the cation, the anion or both. They retain most of the properties present in ionic liquids, including ionic conductivity, low vapour pressure and tunable physico-chemical properties. Several phosphonium ionic liquids have been shown to possess a lower critical solution temperature, making them suitable materials for the synthesis of temperature-responsive smart materials.1,2 Herein, we present the synthesis of a thermo-responsive tributylhexyl phosphonium 3-sulfopropyl acrylate (PSPA) crosslinked PIL, followed by its inclusion in a microfluidic device to be used as a temperature controlled valve. After polymerization, the hydrogels were swollen in deionized water and had their temperature-induced shrinking measured with a digital microscope from 20 °C to 70 °C, in 10 °C intervals. Measurements indicate a relative surface contraction of the hydrogels, in deionized water, of 34.04% ± 4.62% (n = 3) at a temperature of 50 °C, and 53.37% ± 12.55% (n = 3) at a temperature of 70 °C. Following this, microfluidic devices were constructed using poly(methyl methacrylate) and pressure sensitive adhesive. After assembly, the chips were fitted on a heating element and connected to a syringe pump with a flow rate of 500 nL·min-1. A flow microsensor was used to analyze the shrinking-swelling efficiency of the PILc hydrogel valves. A temperature of 50 °C was applied to shrink the hydrogels, followed by a temperature of 25 °C to re-swell the hydrogels. The time required for the PILc valve to open was ~6s, allowing water flow (~140 nL·min-1), while the time required for it to close was ~10s. This process was repeated for six times indicating the possibility of using these valves for multiple times. In conclusion, the results confirm the applicability of PSPA hydrogels as thermally controlled valves in microfluidic devices. Furthermore, the next steps of this study will focus on the optimization of the microfluidic device to ensure maximum efficiency in closing and opening the channel, while also increasing the repeatability of the operation