Thermal Flow Sensor With a Bidirectional Thermal Reference

Conventional thermal flow sensors require samples to be heated; however, the temperature rise of the measuring tools limits their use in biological applications. To address this limitation, in this study, we propose a thermal flow sensor with a bidirectional thermal reference. We fabricated an integrated calorimetric and hot-film thermal flow sensor with a bidirectional thermal reference using a Peltier module to produce a thermal distribution that depends on the flow rate. The integrated thermal flow sensors enable high-resolution and wide-range flow rate measurements in a microfluidic device without the use of heating reagents in the cooling measurement mode. In addition, the sensor can be used as a typical heating thermal flow sensor by inverting the current applied to the Peltier module. Computational fluid dynamics simulations and experiments were performed to evaluate the performance of the integrated sensors in heating and cooling modes. In both modes, the calorimetric sensor measured low flow rates with a resolution of 100 nL/min, whereas the hot-film sensor measured a wide range of flow rates up to 200
$\mu \text{L}$
/min. The proposed sensor expands the use of the thermal flow sensors by switching between the cooling and heating modes according to the sample temperature and maximum temperature limit. [2022-0090]