Optical flow sensor for droplet-based Lab-on-PCB devices

Advancements on Lab-on-a-PCB devices nowadays focus on design goals such as Affordable, Sensitive, Specific, User-friendly, Rapid and robust, Equipment-free, Deliverable to end-users (ASSURED) devices. However, most of these new systems present external equipment dependencies, complex set-up processes, low reproducibility factors, and intricate manufacturing processes. For many industries (medical, pharmaceutical, cosmetics), Lab-on-a-PCB devices are capable of characterizing multiphase systems such as cell-in-droplets identification, flow-phase characterization, and micromixing detection. Thus, this work presents a new optical droplet detector, employing common and cost-effective electronics components. The device consists of a fluid channel between a light-emitting diode (LED) and a photo-resistor (LDR), whose voltage variation is measured and then processed with an ARDUINO microcontroller. This new sensor can determine different multiphase flow properties such as velocity, flow, droplet lengths, and volume with high-speed throughput up to 1000 droplets per second. Furthermore, this sensor presents a modular electronic design that provides a simple calibration, high adaptability, and a standardized fabrication process. Therefore, it creates a cost-effective, portable, easy-to-fabricate, and plug-and-play environment for the alignment with the ASSURED criteria. Droplet detection and characterization showed MRE values ranging from 2.4% up to 17%. The lowest MRE value was obtained using a two-phase flow system with water-in-air droplets at a sampling rate of 2.3 kHz for flow rates starting at 20 up to 425 μL/min. In contrast, the highest MRE value reported was under a three-phase flow system for dyed and pure water-in-air droplets at a 5 kHz sampling rate at a 250 µL/min flow rate.