A Flexible Thermal Flow Sensor Based on Electrochemical Impedance: Design Methodology and Implementation

This article proposes the system-level design methodology and implementation of an ultrathin flexible thermal flow (FTF) sensor based on the electrochemical impedance (EI) of ionic solutions. By integrating concepts from electrochemistry and heat transfer, a comprehensive system-level model that takes into account the transduction principles of flow, temperature, and EI has been developed. Specifically, the equivalent temperature within the active zone of the electrode at different flow rates was derived, thereby correlating the film temperature and solution impedance. The proposed model is validated by finite element method (FEM) results, in terms of film-temperature differential output, current, and temperature profiles within the ionic solution. Accordingly, an EI-based ultrathin FTF sensor is designed by the proposed model, which features a 700- $\mu $ m distance between the microheater and the two pairs of upstream and downstream sensing electrodes. Fabricated by an in-house developed MEMS process and measured in a 1X phosphate-buffered saline (PBS) solution flow, the 8- $\mu $ m-thick FTF sensor achieves an outstanding sensitivity of 1702 PPM/( $\mu $ m/s), with a heating power of 15.8 mW, which is 5X larger than our prior design after power normalization. Besides, the responses of the FTF sensor closely match theoretical predictions. Therefore, the proposed model will be a useful tool for the efficient system-level design of high-performance EI-based thermal flow sensors with integrated circuits in the future.