## 4 Results and Discussion  
As a proof of concept, we studied the dependency of the impedance on various concentrations of cells in the suspension medium by using this microfluidic device. Figure 3 presents the measured impedance spectra and fitting spectra (on a log scale) of the system for two of microchannels filled with sterilized DI water and PBS at frequency range 1 kHz to 1 MHz. For simulation, 100 data points on the impedance-measured spectrum were used as input to the equivalent circuit (see Figure 1b) and the fitting impedance spectrum was generated using MATLAB. For high-conductivity fluid (PBS), the results show two domains--an electrical double layer (EDL) region and a resistive region [24]. The EDL occurred in the low frequency range from 1 kHz to approximately 300 kHz, whereas the resistive region occurred in high frequency from 300 kHz to 1 MHz. The agreement between the measured and fitting spectra results indicated that our developed circuit model for this system is able to determine the impedance characteristics of solution medium. Figure 4 shows the impedance spectrum measurement of one device when filled with DI water and three different concentrations of PBS solution. Individual curves are the average of five frequency sweeps. Resulting IFC data reflect properties of both the current path around the sample and the material sample in the microchannel (i.e., dielectric properties of the microdevice) [25]. The results show that the sensor can differentiate between solution media due to the change in relative dielectric permittivity of the solution. A rising of the solution conductivity affected the capacitance measurement as the result of an increase in capacitance and thus a decrease in the impedance measured [26]. Yeast cells at different concentrations were utilized to illustrate the cell detection capability of the device. Yeast cell concentrations ranging from 10\({}^{2}\) to 10\({}^{9}\) cfu/mL were infused inside the microchannel with a fixed flow rate 6 \(\upmu\)L/min. As can be seen in Figure 5, the yeast cells were flown passes through the microneedles, which have fixed gap (20 \(\upmu\)m). A sweep frequency (100 kHz to 5 MHz) AC signal (1 Volt) was applied to one side of the microneedle, and the current entering at another side of the microneedle was measured to calculate the impedance of that concentration of yeast cells in DI water Initially, 10\({}^{9}\) cfu/mL were injected, resulting in a drop in impedance by referring to the impedance of DI water as a control. Afterward, the microchannel chip was washed by PBS followed by DI water at maximum flow rate. The maximum flow rate that liquid can flow inside microchannel without leaking is 300 \(\upmu\)L/min. Figure 6a shows the impedance spectra of yeast cell in DI water with different cell concentrations in the range 10\({}^{4}\) to 10\({}^{9}\) cfu/mL, along with DI water as a reference. After washing the microchannel, 10\({}^{8}\) cfu/mL was infused to the microchannel, resulting in an increase in impedance. It can be seen that the impedance spectra of yeast cells in DI water across the sensing area (two microneedles) increase with decreasing cell concentration [14]. According to the observation, it can be said that cell suspensions with high concentration are more conductive than those with lower concentrations. The conductivity of a solution varies proportionally to the cell concentration at a fixed volume of solution [27]. In some studies, the relative dielectric permittivity and charged polyelectrolytes inside the cell may also affect the impedance of solution [14]. The optimum region for the sensing microneedle to differentiate the cell concentration in DI water is between 500 kHz to 5 MHz. The impedance values  
Figure 4: Impedance spectra of DI water and varying concentrations of PBS measured by impedance flow cytometry (IFC). Figure 3: Impedance spectra of sample solution together with their fitting spectra (**a**) deionized (DI) water; (**b**) phosphate-buffered saline (PBS).