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. 2020 Mar 20;11(3):325.
doi: 10.3390/mi11030325.

A Flow-Through Microfluidic Relative Permittivity Sensor

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Free PMC article

A Flow-Through Microfluidic Relative Permittivity Sensor

Yaxiang Zeng et al. Micromachines (Basel). .
Free PMC article

Abstract

In this paper, we present the design, simulation, fabrication and characterization of a microfluidic relative permittivity sensor in which the fluid flows through an interdigitated electrode structure. Sensor fabrication is based on an silicon on insulator (SOI) wafer where the fluidic inlet and outlet are etched through the handle layer and the interdigitated electrodes are made in the device layer. An impedance analyzer was used to measure the impedance between the interdigitated electrodes for various non-conducting fluids with a relative permittivity ranging from 1 to 41. The sensor shows good linearity over this range of relative permittivity and can be integrated with other microfluidic sensors in a multiparameter chip.

Keywords: capacitance sensor; interdigitated electrodes; relative permittivity sensor.

Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Figures

Figure 1
Figure 1
(a) Schematic cross-sectional drawing of the sensor. Fluidic inlets and outlets are realized through the handle layer of an SOI wafer. Interdigitated electrode pairs are suspended in the device layer above the inlet and outlet. A fluidic channel between inlet and outlet is realized by means of a MEMpax glass cap. (b) Schematic top view of the sensor (not to scale). The chips contains a total of eight electrode pairs that can be connected independently from each other.
Figure 2
Figure 2
Design of the interdigitated electrode pattern. The dashed line indicates the opening in the handle layer. The electrodes are attached to the handle layer at both sides of the opening by means of 50 μm × 50 μm anchors. As a result the heart-to-heart spacing between the electrodes is 35 μm and fingers are attached to the sides of the electrodes to further increase the capacitance. The total length of the electrode is 210 μm. The width of the electrode is 10 μm without trapezoid shape fingers. The height of the electrode is similar to the thickness of the device layer, which is 25 μm.
Figure 3
Figure 3
Equivalent circuit of the sensor including parasitic capacitance and series resistance.
Figure 4
Figure 4
(a) Electrode geometry used for finite element simulations. The geometry is repeated 13 times above each fluid inlet and outlet as shown in Figure 2. (b) Simulated electric potential 12.5 μm below the chip surface. The voltage between the two electrodes is set to 0.5 V. The liquid between the electrodes is set to isopropanol.
Figure 5
Figure 5
(af)Fabrication process of the relative permittivity sensor. (g) SEM image of a fluid inlet/outlet and the interdigitated electrode pairs above the two fluid openings. The silicon connectors going to the bond pad are isolated by etched trenches. These trenches are filled with PDMS when the cap is glued on.
Figure 6
Figure 6
Schematic drawing of the measurement setup. The blue lines represent the fluid path. The light blue areas represent the fluid channel on chip. The photograph shows the chip glued and wire bonded on a printed circuit board.
Figure 7
Figure 7
Nyquist plot for each selected fluids between 100 kHz and 1 MHz. On each curve, Z decreases as the frequency increases. Note the difference in scale between x-axis and y-axis.
Figure 8
Figure 8
Measured absolute value of impedance as a function of frequency. The results clearly show a capacitive behavior with impedance inversely proportional to frequency.
Figure 9
Figure 9
(a) Measurement and simulation capacitance versus reference relative permittivity. Black stars represent the capacitance measured with different fluids. The red line represents the simulated result without the correction for PDMS inside the electrical insulation channels. The blue line represents the simulated result when the electrical insulating channels are partially filled with PDMS. (b) Linear fit residual error of (a). The length of the error bars equal four times the standard deviation.

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