Single-layer microfluidic “spring” diodes via optofluidic lithography for ultra-low reynolds number applications

Microfluidic components that are capable of autonomous “on-chip” operations at ultra-low Reynolds number (e.g., Re <; 0.2) are critical to the advancement of integrated fluidic circuitry for chemical and biological applications, including point-of-care (POC) molecular diagnostics and on-site chemical detection. Previously, researchers have utilized dynamic resistive elements, such as suspended microbeads and rotational microstructures, to rectify Re <; 0.2 flow; however, such systems require hydrodynamic forces to return the resistive elements to their “closed state” positions, allowing undesired reverse flow during this process. Conversely, double-layer “flap-type” check valves immediately return to their closed state in the absence of forward flow; unfortunately, such valves have exhibited limited functionality for Re <; ~0.3 flow. To overcome these issues, here we introduce single-layer microfluidic “spring” diodes, which utilize free-standing polymeric microsprings that: (i) compress to promote forward flow, (ii) return to the closed position in the absence of forward flow, and (iii) remain in the closed position to obstruct reverse flow. The free-standing microspring elements were constructed in situ via optofluidic lithography processes. Experimental results revealed an improvement in Di performance with increasing Re for Re <; 0.1; however, Di's were found to decrease for Re > 0.1. At maximum, we observed an experimental average Di of 4.10±0.01, corresponding to 0.075 <; Re <; 0.1 fluid flow.