A Compact Hydraulic Head Auto-Regulating Module (CHARM) for long-term constant gravity-driven flow microfluidics

Fluid flow is a ubiquitous aspect of microfluidic systems. Gravity-driven flow is one microfluidic flow initiation and maintenance mechanism that is appealing because it is simple, requires no external power source, and is easy to use. However, the driving forces created by hydraulic head differences gradually decrease during operation, resulting in decreasing flow rates that are undesirable in many microfluidic applications such as perfusion culture, droplet microfluidics, etc. Existing methods to maintain a constant gravity-driven flow either require additional control equipment, involve complex fabrication or operation, are incompatible with miniaturization, or introduce interfaces that lack robustness. Here we tackled those problems by introducing a 3D-printed compact hydraulic head auto-regulating module that automatically maintains a constant fluid level at the microfluidic inlet port without human intervention. Our module successfully maintained a constant hydraulic head for more than 24 h, with the operation time solely limited by the reservoir capacity. A comparison with the conventional gravity-driven flow demonstrated our device's capability to produce a more stable flow over the perfusion period. Overall, our module creates a simple, robust solution to produce a stable flow rate in gravity-driven flow systems. The compactness of the design allows easy parallelization and compatibility with high-throughput applications, and the biocompatibility of the materials enables the device's use with life science applications.