Portable impedance-sensing device for microorganism characterization in the field

Abstract

A variety of biosensors have been proposed to quickly detect and measure the properties of individual microorganisms among heterogeneous populations, but challenges related to cost, portability, stability, sensitivity, and power consumption limit their applicability. This study proposes a portable microfluidic device based on impedance flow-cytometry and electrical impedance spectroscopy that can detect and quantify the size of microparticles larger than 45 µm, such as algae and microplastics. The system is low cost ($300), portable (5 cm [Formula: see text] 5 cm), low-power (1.2 W), and easily fabricated utilizing a 3D-printer and industrial printed circuit board technology. The main novelty we demonstrate is the use of square wave excitation signal for impedance measurements with quadrature phase-sensitive detectors. A linked algorithm removes the errors associated to higher order harmonics. After validating the performance of the device for complex impedance models, we used it to detect and differentiate between polyethylene microbeads of sizes between 63 and 83 µm, and buccal cells between 45 and 70 µm. A precision of 3% is reported for the measured impedance and a minimum size of 45 µm is reported for the particle characterization.

Figure 1

Two microparticles named a and b submerged in a liquid flow from left to right at two different height in a microchannel. (a) Five-electrode configuration proposed by De Ninno. (b) Observed current responses at the first and second pair of electrode based on the position of the two microparticles a and b, with which the impedance and permittivity can be calculated.

Figure 2

(a) Block-diagram and (b) PCB of the impedance-sensing device.

Figure 3

The electronics circuits of the impedance-sensing device.

Figure 4

The different elements that compose the microfluidics system. (a) The diagram of the microfluidics system. (b) The PDMS microchannel. (c) The entire microfluidics system. (d) The PCB electrodes. (e) The 3D-printed mold used to cast the microchannel. (f) The aligned PDMS microchannel on the PCB electrodes.

Figure 5

Bode plot of the impedance magnitude and phase response of a 10-k$\mathrm{\Omega }$ discrete resistor in series with the parallel combination of a 4.47-k$\mathrm{\Omega }$ resistor and a 100-pF capacitor. 320 samples were taken from the SUT for each frequency, and the average and standard deviations are calculated and displayed on the error bar on the left. Four sets of data are displayed, the measured raw impedance, the raw impedance after calibration, the calibrated impedance after transformation using the square to sine spectroscopy algorithm, and the theoretical impedance of the SUT.

Figure 6

Magnitude difference of both electrode pairs (a) when a 78 µm polyethylene microbead passes in the 180 µm wide microchannel. (c) when a 51 µm buccal cell passes in the 180 µm wide microchannel. Distribution of (b) the 63–83 µm microbeads population and (d) the buccal cells population.