A simple design concept for check valves has been adopted for microfluidic devices that consist mostly of (1) deformable fluorocarbon polymer membranes sandwiched between (2) borosilicate float glass wafers into which channels, valve seats, and holes have been etched. The first microfluidic devices in which these check valves are intended to be used are micro-capillary electrophoresis (microCE) devices undergoing development for use on Mars in detecting compounds indicative of life. In this application, it will be necessary to store some liquid samples in reservoirs in the devices for subsequent laboratory analysis, and check valves are needed to prevent cross-contamination of the samples. The simple check-valve design concept is also applicable to other microfluidic devices and to fluidic devices in general.
These check valves are simplified microscopic versions of conventional rubber-flap check valves that are parts of numerous industrial and consumer products. These check valves are fabricated, not as separate components, but as integral parts of microfluidic devices. A check valve according to this concept consists of suitably shaped portions of a deformable membrane and the two glass wafers between which the membrane is sandwiched (see figure). The valve flap is formed by making an approximately semicircular cut in the membrane. The flap is centered over a hole in the lower glass wafer, through which hole the liquid in question is intended to flow upward into a wider hole, channel, or reservoir in the upper glass wafer. The radius of the cut exceeds the radius of the hole by an amount large enough to prevent settling of the flap into the hole. As in a conventional rubber-flap check valve, back pressure in the liquid pushes the flap against the valve seat (in this case, the valve seat is the adjacent surface of the lower glass wafer), thereby forming a seal that prevents backflow.
A typical sequence for fabricating a microfluidic device for the original intended microCE application includes the following steps:
- Channels and valve seats are patterned in the two glass wafers between which the deformable membrane is to be sandwiched. (Altogether, there are three glass wafers, but the third wafer is irrelevant to the innovation described here.)
- Holes are drilled through the wafers in predetermined locations for flow paths.
- The deformable membrane is fabricated.
- Holes are punched in the membrane at locations matching those of holes, valve seats, and flow-channel orifices in the upper and lower glass plates. However, holes are not punched at locations where check valves are required.
- At each check-valve location on the membrane, the check-valve flap is formed by use of an approximately semicircular punch. No membrane material is removed.
The ideal cut for forming a check-valve flap is an arc somewhat greater than a semicircle but less than a full circle. The resistance to flow through the check valve can be reduced by increasing the arc length of the punch. It is worth emphasizing that implementation of this concept entails nothing more than the use of additional punches for forming the flaps in the fabrication process.
This work was done by Peter A. Willis, Harold F. Greer, and J. Anthony Smith of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Mechanics/Machinery category. NPO-45933