Monolithically Integrated Flexible Multi-Sensor for Flow, Temperature, and Conductivity Measurement in Ionic Solutions

We present a monolithically integrated flexible multi-sensor (FMS) capable of simultaneously measuring flow, temperature, and conductivity in ionic solutions. The sensor design, with a 5 mm spacing between the thermal flow and temperature sensors, is optimized to minimize the thermal crosstalk. Fabricated using a MEMS process, the FMS integrates an impedimetric thermal flow sensor that detects 1X PBS flow up to 1000 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}/\mathrm{s}$</tex>, with a sensitivity of 451.4 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{PPM}/(\mu \mathrm{m}/\mathrm{s})$</tex> and an ultralow limit of detection (LOD) of 4.3 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}/\mathrm{s}$</tex>. The on-chip temperature sensor shows a linear response of 235.3 mV/K over 25–45 °C, while the conductivity sensor, with temperature compensation, achieves a detection range of 2.5-25 mS/cm with an error of <1.5%. The measured crosstalk between the flow and temperature sensors is < 1.4 K in zero-flow and reduced to <0.05 K for flows above <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$&gt; 200\mu \mathrm{m}/\mathrm{s}$</tex>. Despite temperature drift in the MEMS flow sensor, its compensability positions the FMS as a promising tool for multi-modal information sensing of ionic solutions in biomedical applications.