Compact, Flexible Harmonic Transponder Sensor With Multiplexed Sensing Capabilities for Rapid, Contactless Microfluidic Diagnosis

In this article, we design and experimentally demonstrate a flexible, low-profile harmonic transponder sensor, capable of performing multiplexed fluidic sensing. The harmonic transponder comprises multiple reconfigurable electrically small antennas (ESAs) integrated with microfluidic channels for receiving radio frequency (RF) signals at the fundamental frequencies (f
₀
), as well as a broadband microstrip monopole for retransmitting the second-harmonic signal (2 f
₀
) to a sniffer. Such a frequency orthogonality can help suppressing possible echoes, clutters, and crosstalks in the rich-scattering environment. We show that injection of different liquid samples into a microfluidic channel, which tunes the operating frequency of an ESA, can be precisely recognized by analyzing the shift of peak second-harmonic received signal strength indicator (RSSI) in the frequency-hopping spread spectrum (FHSS). We also demonstrate the possibility of performing rapid and multiplexed noncontact detection with the proposed harmonic transponder sensor. We envision that this battery-free and lightweight multiplexing wireless sensor may benefit various Internet-of-Things (IoTs) and healthcare applications, such as rapid contactless point-of-care (POC) and drive-through tests.