(66e) Normally Closed Valves: Design Considerations, Scope and Applications

Microvalves are one of the most important components of microfluidic devices.  Though a lot significant progress has been made in improving the performance of these valves with respect to their size, response time, leakage, dead volume, power consumption, chemical compatibility, and sensitivity to particulate contamination, improvements need to be done for a better functionality of microfluidic device.

Incorporation of all the  ideal characteristics in a microvalve is challenging, however, depending on the application, a microvalve can be designed to have most of the desired properties. For instance, pneumatic microvalves developed by Quake and coworkers have been successfully used in many applications requiring multi-step and high throughput operations on a single device [1].  One of the biggest advantage is the compatibility of these valves with standard soft lithography which allows easy integration into complex microfluidic devices.

Pneumatic microvalves can be classified into two types of valves: normally open and normally closed.  Although normally open valves are widely used in many microfluidic applications, they have limited portability in applications that require continuous closed state for long periods of time, as these microvalves need bulky ancillaries for actuation. For example, in investigation of protein-antibody  interaction, the valve needs to be open only for a short period of time, when the solutions are being mixed.  In such applications, since normally open valves require continuous pump actuation, they are limited in their portability between filling section and detection ancillaries [2].  Another limitation of normally open microvalves are their high actuation pressures, up to 28 psi, especially for taller channels. Normally closed valves not only address the above limitations, but also retains the ease of fabrication and integration on to microfluidic devices. 

Our focus is on the design rules of these microvalves because actuation of these microvalves involves complex interplay between mechanical deformation and adhesion which cannot be quantified easily.  In addition, design rules become even more important in devices with dense network because pressure losses across the device cause failure of the valves at the end of the network and hence, minimizing actuation pressures becomes crucial.  Finally, since they are compatible with standard soft lithography, optimization for  their integration into complex microfluidic devices is necessary.

References:

[1]Schudel, Benjamin R, et al. "Microfluidic chip for combinatorial mixing and screening of assays." Lab on a Chip (2009): 1676-1680.

[2]Unger, Marc A, et al. "Monolithic Microfabricated Valves and Pumps by Multilayer Soft Lithography." Science (2000): 113-116.