Joint Design of Self-Tuning UHF RFID Antenna and Microfluidic Channel for Liquid Sensing

TL;DR

This work addresses the challenge of wireless liquid sensing by jointly designing a UHF RFID antenna and a microfluidic channel, leveraging self-tuning RFID ICs to digitally quantify impedance changes as liquid content varies. The authors formulate a fitness-based optimization over both antenna and microfluidic geometries, and demonstrate two optimized designs—sensitivity-optimized and gain-optimized—achieving high sensitivity ($S ext{ around } 20~ ext{mg}^{-1}$) with controllable gain degradation, validated by experiments on paper-based microfluidics. The study shows that co-optimization yields robust, digitally readable sensing in compact RFID-enabled devices, with potential applications in healthcare (e.g., sweating monitoring) and food quality assessment. The results highlight a clear trade-off between sensitivity and reading distance and establish a framework for extending joint designs to more complex microfluidics and multiphysics simulations.

Abstract

Microfluidic has been an enabling technology for over a decade, particularly in the field of medical and wearable devices, allowing for the manipulation of small amounts of fluid in confined spaces. Micro-channels can also be used for wireless sensing thanks to the variations in antenna properties when the fluid flows near it. However, up to now, microfluidic channels and sensing antennas have always been designed separately; instead, since the liquid flow and the antenna geometry both contribute to the overall performance, they should be considered simultaneously when optimizing the antenna-microfluidic system. In this paper, the joint design of the antenna and microfluidic channels is investigated for liquid quantification. Self-tuning RFID microchips are exploited to minimize communication degradation due to the increase of lossy liquid amount over the sensing antenna while digitalizing the impedance mismatch itself. To experimentally corroborate the joint design technique, two different geometries are obtained and prototyped starting from a given antenna-microfluidic layout by setting different goals for an optimization function. The two flexible RFID prototypes returned performance in agreement with the simulated ones, achieving a maximum sensitivity of about 20 units of the digital metric per milligram increase of water.

Figures (11)

• Figure 1: Concept of a UHF RFID sensor-antenna with microfluidic for liquid quantification. The sensitive area of the system is constituted by the superposition of the microfluidic channel with the AA.
• Figure 2: Model of the fluid diffusion in paper-based microfluidic.
• Figure 3: System layout composed by an RFID sensor-antenna and microfluidic for liquid quantification. The sensitive area of the system is highlighted in red. (a) Exploded view with highlighted AA and $c_1$ parameter, and (b) above view with the highlighted sensitive area. Parametrization of the antenna-microfluidic system. (c) Geometrical parameters of the RFID antenna and the pair to be optimized $\left\{a_1,a_2\right\}$. (d) Zoomed-in view of the system's sensitive area and geometrical parameters of the microfluidic channels to be optimized $\left\{c_2,c_3,c_4\right\}$. $\left\{a_3,c_1\right\}$ are fixed.
• Figure 4: Flowchart detailing the two-step hierarchical grid search performed. $\left\{a_{1,O1},a_{2,O1},c_{2,O1}\right\}$ are the optimum values of the $\left\{a_{1},a_{2},c_{2}\right\}$ triplet found in the first search, and $\left\{a_{1,O2},a_{2,O2}\right\}$ are the values of $\left\{a_1,a_2\right\}$ refined in the second grid search.
• Figure 5: Numerical optimization for the two different geometries to be designed: (a) sensitivity-optimized and (b) gain-optimized. For each geometry are shown, from the top to the bottom: the parameters' space with the goal function values evaluated in the points of the hierarchical grid and the $c_2$ value of the optimal solution highlighted; the slice including the best solution; the exploded numerical model of the antenna-microfluidic system, omitting the antenna substrate and the ovoidal input and waste pads of the microfluidic.