A flexible impedance-based microsystem for multi-parameter sensing in ionic solutions

We present a flexible multi-sensor (FMS) microsystem for simultaneous measurement of flow, temperature, and ionic conductivity in liquid environments. The FMS is implemented on a 16 μm thick polyimide (PI) substrate and integrates electrochemical impedance (EI)-based thermal flow sensing, resistive temperature sensing, and impedance-based conductivity sensing with a designed analog-front-end (AFE) CMOS IC. The AFE enables precise constant-temperature control of the flow-sensor microheater and implements a high-SNR impedance readout based on a current-balanced instrumentation amplifier (CBIA) architecture, allowing sensitive detection of small impedance variations induced by flow and ionic conductivity. As a result, the EI-based thermal flow sensor achieves a measurement range up to 1200 μm/s, with a system sensitivity of 0.69 mV/(μm/s) and a detection limit of 3.36 μm/s. The integrated temperature sensor exhibits a linear response over 10-40 °C with an accuracy of ±0.13 °C, while thermal crosstalk from the flow-sensor microheater is suppressed below 0.1 °C at flow velocities above 150 μm/s. Ionic conductivity is measured over a range of 5-35 mS/cm using the same impedance readout circuitry, and temperature compensation maintains the conductivity measurement error below 3% across varying thermal conditions. By integrating flow, temperature, and conductivity sensing within a single flexible microsystem, this work demonstrates a compact, low-noise, and portable platform for multi-parameter sensing of ionic solutions, well suited for microfluidic and biomedical applications.