Stable capillary stop valves in highly hydrophilic materials for passive microfluidic systems
Capillary stop valves are passive valves which stop liquid flowing in a microchannel using a sudden divergence of channel sidewalls in the flow path . At the divergence, the liquid needs to undergo a large contact angle change in order to achieve an advancing angle with the new wall. Stability (over time and shock) of such valves therefore improves significantly for large sidewall divergences (135° to 180°) and valve dimensions of 50-100 µm (easy to fabricate) are sufficient for creating large pressure barriers across the valve .
At Micronit Microfluidics we have micro-fabricated stable passive capillary stop valves entirely in glass (without surface treatment or polymers), using simple UV lithography and isotropic wet chemical etching. Glass is cheap, has high wettability (θ < 30° for water on glass) and bio-compatibility, which makes it an interesting choice for stable (limited change in contact angle over time), capillary driven bio-microfluidic systems. Two types of valves relative to the direction of liquid flow are fabricated; in-plane valves with 135° horizontal and 90° vertical divergence and; out-of-plane valves with 180° divergence along all sidewalls. Both valve types showed stable stopping of capillary force driven liquids in glass microchannels. The valves can be triggered on demand via a second liquid, or manually, which is an essential feature for a sequential flow based immunoassay lab-on-a-chip .
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