Digital microfluidic meter-on-chip.

The accurate monitoring and control of liquid flow at low flow rates have become increasingly important in contemporary biomedical research and industrial monitoring. Inspired by the drop-counting principle implemented in a clinical gravity drip, we propose a novel microfluidic flowmetry technology for polydimethylsiloxane (PDMS)-based conventional microfluidic devices, known as a microfluidic digital meter-on-chip (DMC), to achieve on-chip and localized microflow measurements with ultrahigh precision and a wide tunable range. The DMC technology primarily relies on capillarity, unlike a gravity drip, to induce a characteristic interfacial droplet pinch-off process, from which digital microflowmetry devices can discretize continuous flow into countable transferred liquid units with consistent quantifiable volumes. Enabled by the passive discretization principle and optical transparency, the DMC device requires no external energy input or bulky control equipment, and a non-contact wireless optical detection scheme using a smartphone can be conveniently used as a readout module. Moreover, the DMC technology achieves an ultrahigh flow-to-frequency sensitivity (6.59 Hz (μL min-1)-1) and resolution (droplet transfer volume down to 2.5 nL, nearly two orders of magnitude smaller than in previously reported work, resulting in ultralow flow rates of 1 μL min-1). In addition, the flow rate measurement range covers up to 80 μL min-1 and down to at least 150 nL min-1 (over 100 times lower than reported similar digital flowmetry on the same time scale) using the current device configuration. Benefiting from its simple device architecture and adaptability, the versatile DMC technology can be seamlessly integrated with various microfluidic and nanofluidic devices for drug delivery and biochemical analysis, serving as a promising technology platform for next-generation highly demanding microflow measurements.