This paper presents three types of planar check valves that uniquely exploit the large compliance of simple PDMS structures. The micro check valves consist of a thin compliant flap and a rigid stopper embedded in a microchannel. The flap is perpendicular to the flow and lies near the stopper, forming a restricted fluid path inside the channel. In particular, a novel "zero flap-stopper gap" valve design was proposed and fabricated using a special technique, in which the flap and stopper are in contact naturally forming a normally-closed device with high diodicity. Testing results show that the check valves can achieve a diodicity up to 10~5. The check valves can be fabricated using the standard replica molding technique from PDMS, and thus are highly amenable to integration with other PDMS-based microfluidic systems.
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