A miniaturized SIW circular cavity resonator for microfluidic sensing applications☆
Introduction
In recent years, microwave sensor systems have attracted tremendous attention [[1], [2], [3]], due to their stable sensing performance and applicability for various environments. The interaction between materials under test (MUT) and electromagnetic waves is used to manufacture various types of microwave sensors [[4], [5], [6], [7], [8]]. Some microwave sensors work with biological and chemical systems, dedicated to characterizing the electromagnetic properties of various materials [[9], [10], [11]]. Different from inspecting materials based on invasive technologies such as cell staining in traditional sensing systems (optical, electrochemical, etc.), microwave sensors allow non- destructive, which makes for the possibility of sample reusing. To this series of requirements, microwave sensors for characterizing microliter or nanoliter of liquid samples have been proposed, which results the concept of microwave microfluidic sensors.
Various types of resonators can be used as classic building blocks for microwave microfluidic sensors [[12], [13], [14], [15], [16]]. Among these topologies, cavity resonators are a most attractive type due to their high quality factor. To reduce the size of conventional waveguide cavity, substrate integrated waveguide (SIW) structures are widely used recently due to its low profile, ease of integration, and simple manufacturing, which providing a new solution for microwave microfluidic sensors [[17], [18], [19]]. However, the cavity resonator is larger in geometry compared with other planar microstrip resonators, which will increase the sample volume required for the sensor characterizing of liquid. The miniaturized structures of the microwave microfluidic sensors can reduce the amount of sample to be used for verification effectively, and facilitates the setting of portable monitoring points. An example of miniaturized SIW for liquid sensing is given in [20], which proposed a rectangular SIW cavity loaded with composite right/left-handed transmission line (CRLH TL).
This letter presents a novel miniaturized SIW circular resonator which can be used for microfluidic sensing. The transmission characteristic of the proposed sensor is influenced by samples introduced in the microfluidic channel. Different with the rectangular or other shaped cavities, circular cavities can help to concentrate the electric field in the center position, which will result a high quality factor (Q factor), and contribute a high sensitivity. Besides, a slot loading technology is employed to miniaturize the cavity, which will reduce the consumed volume of the samples in each test, and also improve the sensitivity. The operating mechanism is analyzed by studying the equivalent circuit model. The analyses of the field distribution shown that the slot loading structure will not change the original mode of the cavity. The manufacture and measurement of the sensor has been completed. The results show that the sensor can possess higher sensitivity than the same type of sensors while miniaturizing. The commercial electromagnetic simulation solver HFSS is used in the design.
Section snippets
Miniaturized SIW sensor
The circular SIW cavity has a more concentrated field distribution and its quality factor is relatively higher compared to SIW cavities with other geometric shapes, thus it is more suitable for sensor designs requiring higher sensitivity. Fig. 1 (a) shows a typical SIW circular cavity model. The surrounding vias are arranged at an appropriate distance to reduce energy leakage, which can be considered as the side wall of a circular waveguide. The top and bottom surfaces are completely metalized
Measurement and results
The proposed miniaturized SIW sensor has been manufactured and tested. Fig. 5 shows the sensor and test bench used to perform the measurements, and bottom of the hole is sealed to be able to inject fluid. Fig. 6 depicts the relevant measurement without any samples, which can be seen that the measured S parameters are basically consistent with the simulated results. Compared to the SIW cavity without slot loading, the resonant frequency of the cavity with slot is decreased from 4.93 GHz to
Conclusion
A miniaturized SIW circular cavity resonator is proposed for microfluidic sensing in this letter. The cavity works in the fundamental mode TM010 with the maximum electric field at the center. Slot-line loading technology is employed to reduce the resonant frequency of the cavity, which can reduce the volume of the liquid samples, and improve the sensitivity. The field distributions and the circuit model are analyzed to discuss the miniaturization mechanism of the SIW cavity. The measured
CRediT authorship contribution statement
Xinyue Song: Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Sen Yan: Conceptualization, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that there are no conflicts of interest.
Xinyue Song received her B.E. degree in Information and Communication Engineering from Beijing University of Chemical Technology, Beijing, China, in 2018. She is currently working toward the Ph.D. degree in electronic science and technology at Xi’an Jiaotong University (XJTU), Xi’an, China. Her current research interest is microwave-microfluidic devices design and fabrication.
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Xinyue Song received her B.E. degree in Information and Communication Engineering from Beijing University of Chemical Technology, Beijing, China, in 2018. She is currently working toward the Ph.D. degree in electronic science and technology at Xi’an Jiaotong University (XJTU), Xi’an, China. Her current research interest is microwave-microfluidic devices design and fabrication.
Sen Yan received his Bachelor and Master degrees in Information and Communication Engineering from Xi’an Jiaotong University (XJTU), China, in 2007 and 2010, respectively, and the Ph.D. degree in Electrical Engineering from KU Leuven, Belgium in 2015. After that, he continued his research work in the same institute as a post-doctor. From 2016, he worked as a research fellow supported by Research Foundation - Flanders (FWO), Belgium. In February 2016, he worked as a visiting scholar in EPFL, Switzerland. In August and September 2016, he worked as a visiting researcher in the University of Texas at Austin, U.S. From 2017, he joined in Xi’an Jiaotong University, and currently a full professor there. From 2019, he served as the deputy director of the Engineering Training Center, XJTU. Prof. Yan’s current research is focused on metamaterials and metasurfaces, wearable devices and textile antennas, reconfigurable antennas, antenna diversity, and also biosensors. He has been invited to given 8 invited talks in the academic conferences in the recent 3 years, and authored 48 international journal papers and 50 conference contributions. He was successful in achieving the post-doctoral fellowship from KU Leuven and FWO in 2015 and 2016, respectively. In 2017, He obtained “Young Talent Support Plan” of Xi’an Jiaotong University. In 2019, he received the Young Scientist Award of International Union of Radio Science (URSI). He also received the best paper of 2019 Cross Strait Quad-Regional Radio Wireless Conference.