Sub-nanoliter, real-time flow monitoring in microfluidic chips using a portable device and smartphone

The ever-increasing need for portable, easy-to-use, cost-effective, and connected point-of-care diagnostics (POCD) has been one of the main drivers of recent research on lab-on-a-chip (LoC) devices. A majority of these devices use microfluidics to manipulate precisely samples and reagents for bioanalysis. However, filling microfluidic devices with liquid can be prone to failure. For this reason, we have implemented a simple, yet efficient method for monitoring liquid displacement in microfluidic chips using capacitive sensing and a compact (75 mm x 30 mm x 10 mm), low-cost ($60), and battery-powered (10-hour autonomy) device communicating with a smartphone. We demonstrated the concept using a capillary-driven microfluidic chip comprising two equivalent flow paths, each with a total volume of 420 nL. Capacitance measurements from a pair of electrodes patterned longitudinally along the flow paths yielded 17 pL resolution in monitoring liquid displacement at a sampling rate of 1 data/s (similar to 1 nL/min resolution in the flow rate). We characterized the system using human serum, biological buffers, and water, and implemented an algorithm to provide real-time information on flow conditions occurring in a microfluidic chip and interactive guidance to the user.