Gas penetration through pneumatically driven PDMS micro valves

Integration of valve like structures into microfluidic chips is a key factor for the usability of many microfluidic systems. One approach is utilization of thin PDMS layers as flexible membranes which can be closely packed on the microfluidic chips. These flexible membranes can be deformed by pneumatic actuation, closing or opening the fluidic channel of the corresponding valve. However, PDMS is permeable to gases and this permeability gets non-negligible for common valve layer thicknesses and pressure gradients. This factor restricts the usage of PDMS valves in microfluidic systems, as gas bubbles are strictly prohibited in many applications. Here two methods avoiding the intrusion of gas into the microfluidic channels are introduced and investigated. The first is based on an oil droplet placed in the dead end pneumatic channel of the valve, the second utilizes a parylene coating to make PDMS impermeable to gases. The parylene layer could not completely suppress the gas intrusion into the fluidics of the valve and additionally influenced the sealing capabilities of the valve. The oil droplet, in contrast, suppressed the permeability of the valve completely and provides a straight forward solution. In the experimental setup a novel valve design is introduced and characterized. Sealing capabilities, hydraulic resistance and lifetime are measured, dead and internal volume are calculated to describe the capabilities of thin membrane PDMS valves. These values are of interest if the valves are to be used industrially. The design introduced here is easy to fabricate and requires low alignment accuracy: it uses a Polycarbonate/PDMS sandwich, bonded by a APTES coating. Combining the simplicity of the valve design and the low cost fabrication methods and materials needed, the valve is perfectly suited as a disposable consumable.