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Citation information: DOI 10.1109/JSEN.2019.2908179, IEEE Sensors Journal Development of a Passive Capacitively Coupled Contactless Conductivity Detection (PC4D) Sensor System for Fluidic Channel Analysis Toward Point-of-Care Applications Loc Do Quang 1 , Tung Thanh Bui 2 *, Anh Bao Hoang 2 , Thanh Pham Van 1 , Chun-Ping Jen 3 , Trinh Chu Duc 2 * 1 Faculty of Physics, VNU University of Science, Hanoi, Vietnam 2 Faculty of Electronics and Telecommunications, VNU University of Engineering and Technology, Hanoi, Vietnam 3 Department of Mechanical Engineering, National Chung Cheng University, Chia-Yi, Taiwan, R.O.C Abstract—A sensor system based on a modified capacitively coupled contactless conductivity detection sensor is proposed and developed. The proposed system provides a passive and wireless readout technique, through which the conductivity of fluidic flow can be analyzed and foreign objects occurring in the fluidic flow can be detected. The proposed sensor system takes advantage of the series resonance principle and detects shifts in the resonance frequency and reflection coefficient to estimate the conductivity of the fluidic flow. In this study, the working principle of the device is proposed and analyzed using a multiphysics simulation, and its performance is validated experimentally. The sensing performance is confirmed by measuring the conductivity of the fluidic media and the detection of foreign objects, such as air bubbles or water droplets, occurring in the flow. The influence of the distance between the inductors on the resonance frequency for different solution conductivities is also investigated and reported. The proposed sensor system shows its potential for use in various applications in biomedicine and chemistry, particularly in point-of-care applications, where the sensing chip can be easily set up for measurement and disposed of after use. Index Terms—Capacitively coupled contactless conductivity detection (C4D), LC passive sensor, series resonance I. INTRODUCTION LECTRICAL conductivity is one of the main parameters of an electrolyte solution. Fluidic flow detection and measurement for both conductive and nonconductive solution channels are very important and can be found in many academic research and industrial applications. Several fundamental methods have been studied and developed for fluidic flow detection such as optical, ultrasonic, and electrical sensing based on both contact and contactless mechanisms [1], [2]. In the conventional conductivity detection technique, the detection electrodes are directly in contact with the fluidic medium or electrolyte solution. Therefore, this conventional technique is limited by several drawbacks such as the polarization effect, electrochemical erosion, and electrode contamination effect [3]. In order to avoid the issues of the direct contact technique, capacitive contactless sensor structures have been proposed and developed. This technique is a particularly attractive option for its simple fabrication and measurement setup, as well as the miniaturization capability [1], [4]–[6]. The capacitively coupled contactless conductivity detection (C4D) sensor structure uses a conductivity detection technique, which was first reported in the early 1980s by Gas et al. [7]. A significant development of C4D was made independently by Zemann et al. [8] and Fracassi da Silva and do Lago [9] by proposing the axial arrangement of C4D. Several studies concerning the applications of the C4D technique in millimeter- and micrometer-scale channels have been applied and improved for many research fields, including oil and gas phase detection [10], [11], bioanalytical samples [12], [13], microfluidic systems [14], [15], and food analysis applications [16]. Numerous measurement methods have been developed to overcome the difficulties and limitations of the conventional C4D technique, such as utilizing a grounded shield between the excitation electrode and the pick-up electrode, adopting a differential technique in flow detection and taking advantage of parallel and series resonance methods [3], [17]–[24]. The inductor–capacitor (LC) passive wireless sensing technique is a common method utilized for sensing, detection, and measurement. This technique is based on the principle of resonance frequency shift detection, which is known as a high-sensitivity method [25]. The mechanism of LC passive wireless sensing platform has been studied and developed for numerous sensing applications, such as pressure, humidity, temperature, and strain [26]–[32]. Typically, an LC passive sensor is constructed from a spiral inductor connected with a sensitive electrode pair, forming an electrical LC resonant circuit whose resonant frequency changes with the parameter of interest. The advantages of an LC passive sensor include its light weight, low cost, robustness, and portable nature. This type of E