Design considerations for elastomeric normally closed microfluidic valves

The ease of fabrication and integration of pneumatic microvalves has enabled extensive miniaturization of microfluidic devices capable of performing massively parallel operations. These valves in their typical open configuration, also known as normally open (NO) valves, require to be actuated to remain closed. As a result, devices employing these valves have limited portability in applications that require valves to be closed continuously. Normally closed (NC) valves based on pneumatic actuation not only address the above issue of portability, but also retain the ease of integrating massively parallel networks of microfluidic elements. In this paper, we report the design and fabrication of elastomeric NC microvalves, along with systematic experimental characterization of NC valve operation. Geometrical parameters of the valve, including shape, fluid channel width, membrane thickness, and valve asymmetry, were examined with the objective of minimizing actuation pressures and ensuring reliable operation. We observed that introduction of asymmetry in the valve geometry created points of weak adhesion between the valve and the substrate, which facilitated opening of the valve at lower actuation pressures. Specifically, valves with a sharp corner feature (v-shaped) actuated at lower pressures (1.5 psi) compared to straight-shaped valves (3 psi). We also observed that membrane thickness does not significantly influence the actuation pressures. An important requirement for microfluidic devices using NC valves is selective irreversible bonding of the fluid layer to the substrate, which we achieved by plasma sealing of the fluid layer to the substrate while simultaneously actuating the valves. Based on our experimental observations, we formulated a set of design considerations with the objective of minimizing actuation pressures, ensuring reliable operation, and facilitating convenient integration into complex microfluidic devices. These NC valves have significant potential in applications where portability is highly desired, such as in on-chip analysis, crystallization screening, and in the study of chemical or biological processes over long durations of time.