On-chip resistive microfluidic flow sensor with reduced analysis time using transient analysis

In the realm of microfluidics, a critical issue prevails: The commercially available flow sensors are situated external to the microfluidic chip, resulting in an inherent deficiency of precise operational insights. Applications such as biochemistry, drug development, single cell analyses, gradient generators, biomedical devices and proactive healthcare demand dependable data for broader utilization. In the present work, we introduce a microfluidic flow sensor designed to gauge electrolyte flow rates on-chip inside a microchannel by measuring electrochemical resistance of electrolytic bubbles within a circuit comprising electrolytic fluid and inter-digitated electrodes. This sensor design features a Si wafer substrate with micropatterned Ti/Pt electrodes bonded to a polydimethylsiloxane (PDMS) microchannel using oxygen plasma. The sensor has been characterized for electrolytic fluid (0.1 M NaCl), 600 \(\upmu\) $\mathrm{\mu }$ m wide microchannel, 0–500 \(\upmu\) $\mathrm{\mu }$ l/min range of flow, 3V dc power input, and 25 \(^\circ\) ${}^{\circ }$ C room temperature conditions. Sensor response has been compared for electrolyte concentrations (0.1 M to 0.4 M NaCl), channel width (400–1200 \(\upmu\) $\mathrm{\mu }$ m), and input power (1–5 V) corresponding to a given flow rate. The sensor 3 \(\sigma\) $\sigma$ resolution calculated from calibration curve is observed to be 5 \(\upmu\) $\mathrm{\mu }$ l/min for lower flow rates (0–100 \(\upmu\) $\mathrm{\mu }$ l/min) and 100 \(\upmu\) $\mathrm{\mu }$ l/min for higher flow rates (100–500 \(\upmu\) $\mathrm{\mu }$ l/min). The limit of detection (LoD) of the sensor is 1 \(\upmu\) $\mathrm{\mu }$ l/min. An algorithm based on the trapezoidal rule enabling flow rate prediction within 5 s has been implemented. The proposed flow sensor works over a broad range of flow rates, although with different sensitivity, presenting a novel method employing the electrolysis bubble size and volume for flow rate detection.