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116 D. Q. Loc, H. B. Anh, “Development of a passive wireless ... electrode (PCE) configuration.”
Development of a passive wireless sensor
for fluidic detection and characterization utilizing the PCB-based
coplanar electrode (PCE) configuration
Do Quang Loc1*
, Hoang Bao Anh2
1Faculty of Physics, VNU University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam;
2Center for Electronics and Telecommunications Research, VNU University of Engineering and
Technology, 144 Xuan Thuy, Cau Giay, Hanoi, Vietnam.
*Corresponding author: locdq@hus.edu.vn
Received 13 Apr. 2024; Revised 29 May 2024; Accepted 12 Jun. 2024; Published 25 Jun. 2024.
DOI: https://doi.org/10.54939/1859-1043.j.mst.96.2024.116-123
ABSTRACT
During the global economic development, there's a growing focus on healthcare, especially in
the advancement of medical diagnostic technologies, with a significant emphasis on glucose level
evaluation. Glucose biosensors, predominantly electrochemical, have evolved over four
generations, with the first three being enzyme-based and known for sensitivity and cost- effectiveness, albeit with limitations due to environmental susceptibility and reliance on enzyme
activity. Recent advancements in non-invasive blood glucose monitoring, utilizing optical,
microwave, and electrochemical techniques, offer diverse benefits without tissue penetration.
Among these, impedance sensing stands out due to its flexibility and integration capability in
handheld devices. This study proposes a wireless passive impedance method leveraging the
inductor-capacitor (LC) sensing technique and PCB (Printed Circuit Board)-based coplanar
electrode (PCE) configuration for fluidic sample detection. The proposed system integrates a two- coplanar-electrode layout with a square spiral inductor to assess fluidic conductivity and
characterize various fluid types within samples. The effectiveness of this configuration was
validated through experiments with NaCl and glucose solutions, confirming the feasibility of
integrating PCB-based coplanar electrodes into conventional LC passive wireless sensing designs
for fluidic detection and characterization.
Keywords: Fluidic detection; LC passive wireless sensing; Printed circuit board; Glucose biosensors.
1. INTRODUCTION
Amidst ongoing economic development, there is a growing societal focus on healthcare,
leading to the advancement of various medical diagnostic technologies. Of particular importance
is the evaluation of glucose levels as a key diagnostic indicator. Currently, the predominant glucose
biosensors belong to the category of electrochemical sensors, which have evolved into four
generations over time. The first three generations are enzyme-based biosensors known for their
sensitivity, reproducibility, and cost-effectiveness [1, 2]. However, these biosensors have
limitations, including susceptibility to environmental conditions and reliance on enzyme activity,
restricting their applicability and reliability. Recently, non-invasive blood glucose monitoring has
transformed glucose level measurement by avoiding invasive procedures and tissue harm. This
technology includes optical, microwave, and electrochemical techniques, each offering distinct
benefits for glucose measurement without tissue penetration. Optical methods, such as near- infrared reflectance spectroscopy, polarized optical rotation, Raman spectroscopy, fluorescence,
and optical coherence tomography, utilize light properties to detect glucose concentrations,
providing diverse monitoring options with reduced discomfort and complications [3-6]. Various
approaches and advancements have been made to enhance glucose detection, particularly in
exploring enzyme-free detection methods, resulting in the emergence of the fourth generation of
glucose biosensors—nonenzymatic glucose (NEG) sensors [7, 8]. Among the physical techniques
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Nghiên cứu khoa học công nghệ
Tạp chí Nghiên cứu KH&CN quân sự, 96 (2024), 116-123 117
for NEG sensor development, the impedance sensing approach stands out due to its flexibility,
integration capability in handheld wearable devices, and elimination of bulky optical equipment
and operational staff training. Additionally, with the ability to wirelessly connect to the sensing
part by utilizing the RF power transmission, this method enables miniaturization for the integration
of implantable devices, which holds promise as a solution for patient monitoring and diagnosis in
today's healthcare system, especially for point-of-care applications and wearable devices [9-12].
In this study, a wireless passive impedance method for detecting and characterizing the fluidic
sample leverages the inductor-capacitor (LC) sensing technique is proposed, designed, fabricated
and experimented in conjunction with the PCB (Printed Circuit Board)-based coplanar electrode
(PCE) configuration (figure 1). This fusion, known as passive wireless PCE configuration, offers
distinct advantages, particularly its compatibility with biochips and chemical analysis structures that
are easy fabrication and low-cost. The concept of PCE has resurfaced as a prominent trend in a
growing number of research studies reported in recent years [13]. The dimensions and configuration
of the electrodes are readily adaptable to enhance efficiency and fulfill the precise demands of the
sensor's intended use. In contrast to conventional electrode fabrication techniques like lithography,
employing printed circuit board (PCB) technology substantially diminishes manufacturing expenses.
Furthermore, the sensor design proposed herein, leveraging PCB circuits, enables label-free detection
in biomedical contexts. This obviates the necessity for incorporating specific markers for sensing, as
the test substance can be directly deposited onto the sensor's detection region. This streamlines and
enhances the detection process [13]. By utilizing this configuration, the antenna for
transmitting/receiving RF power, capacitive sensors, and electronics can be swiftly and affordably
integrated onto the same PCB platform, thanks to the widespread availability of PCB technologies
[14-17]. The proposed passive wireless PCE sensor system integrates a two-coplanar-electrode
layout with a square spiral inductor to evaluate fluidic conductivity and characterize various fluid
types within samples. By detecting subtle changes in fluid conductivity and other electrical
properties, variations in impedance between sensing electrodes impact the resonance frequency of
the LC resonator. These frequency shifts are identified using a network analyzer and analyzed using
customized software, enabling a comprehensive understanding of fluidic behavior and composition
for use in biotechnology, environmental monitoring, and industrial processes. In this study, the
fabricated sensing structure underwent examination, and the effectiveness of the proposed
configuration was validated through implementation with NaCl and glucose solutions ranging in
concentration from 10 mM to 100 mM. This research indirectly confirms the efficacy and feasibility
of integrating PCB-based coplanar electrodes into conventional LC passive wireless sensing designs
for fluidic detection and characterization.
Figure 1. Conceptualization of LC passive wireless sensor with PCE configuration.
2. THEORY AND DESIGN
2.1. Theoretical foundations
The suggested configuration integrates capacitively coupled contactless detection with the LC
passive sensing approach. LC passive wireless sensing relies on the mutual inductance coupling
between two inductors, as depicted in figure 2 (a). The reading inductor LR, serving as the
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118 D. Q. Loc, H. B. Anh, “Development of a passive wireless ... electrode (PCE) configuration.”
transmitting antenna, is magnetically linked to the sensing inductor LS. LR performs dual roles,
compromising to act as the energy transmitter, provide power to LS, and as the signal receiver,
reflecting the resonance frequency of the LC sensing circuit comprising LS and sensing capacitor CF.
The parasitic resistances of the sensing inductor are identified RS. The resonance frequency (fres) can
theoretically be defined as Eq. 1 while CF serves as the fluidic sensing capacitance [18]. Variations
in the dielectric material between electrodes can cause fluctuations in the capacitance of CF, leading
to changes in the resonance frequency of the LC circuit. These frequency shifts can be easily detected
by analyzing the reflection coefficient S11 on LR. The sensing capacitor is formed by capacitively
coupling contactless electrode structures, allowing for the utilization of their inherent advantages.
fres =
1
2π√LSCF
(1)
2.2. Experiment preparation
2.2.1. Sample preparation
An LC sensor is built by combining a square spiral inductor with a sensing capacitor, creating a
resonance LC tank. Figure 2(b) illustrates the proposed experimental setup of LC passive wireless
sensing designed for detecting the fluidic presenting at the capacitive sensing area. The sensing
system comprises a capacitor formed by two copper coplanar electrodes encircling the capacitive
sensing region, along with two inductors serving as transmitters and receivers of radiation. These
inductors are positioned apart from each other in the air, with a defined distance between them. In
this study, the primary and secondary inductors were fabricated using the Printed Circuit Board
(PCB) etching technique on a single-sided copper FR4 PCB. The parameters of the implemented
antennas, which are constructed from coplanar spiral-shaped inductors, are detailed in table 1. In
this study, solutions of NaCl and glucose with varying concentrations spanning from 10 mM to 100
mM were employed to evaluate the efficacy of the proposed system and structure. NaCl and glucose
were diluted in deionized (DI) water to achieve the desired concentrations for the experiments.
Table 1. PCE integrated passive wireless sensor parameters.
Parameter Value Unit
Inductor’s outermost length 12.3 mm
Number of turn 10
Line width 0.3 mm
FR4 substrate’s thickness 1 mm
Electrode gap 0.3 mm
Electrodes length 5 mm
2.2.2. Experimental setup
In the experimental setup, instruments and software are utilized for precise data acquisition and
analysis. The Agilent E5061A ENA Series network analyzer measures the reflection coefficient
S11 within a specific frequency range to determine the passive sensor's resonance frequency. A
custom Visual Basic program monitors resonance frequency shifts corresponding to different
solution dropped, streamlining data analysis. Additionally, the sensing electrodes are serially
connected to the sensor inductor to enhance sensitivity in a series resonance configuration.
To verify the effectiveness of the proposed measurement system, a series of experiments were
undertaken to assess the variation in resonance frequency upon the manual deposition of a 10 μL
droplet of the target fluidic sample onto the sensing region of the capacitive sensing electrode
structure. The experiments further investigated the relationship between the resonance frequency
change and the type of fluidic sample deposited, as well as the alterations in the reflection
coefficient across different excitation sweeping frequency ranges as the distance between the two
inductors was adjusted. The data collected from these experiments were meticulously recorded
and subjected to thorough analysis.