3.1.1 The relationship between the radius of working electrode (Ri) and cell impedance

During impedance measurements, ions move through the cell monolayer between the working and counter electrodes which follow many paths. The counter electrodes must have adequate sensing area in order to provide adequate circuit connection. The larger Ri working electrode provides more current paths, which decreases the corresponding impedance. Higher impedance values can improve the data quality of the measured impedance by increasing signal-to-noise ratio, which is useful particularly for sensing small changes in cell behavior. However, the working electrode should not be too small in order to measure adequate number of cells and to guarantee sufficient cell-to-cell contact area.

In this study, the ECIS sensors with Ri from 100 to 400 μm were fabricated to analyze the relationship between Ri and measured cell impedance. Figure 2 illustrates the cell morphology on those sensors. The simulated cell impedance by using Ri within the same range was also obtained from the mathematical model. The experimental and simulated impedance of cell were shown in Figure 3. The

Figure 2. BAEC monolayer on ECIS sensors with different Ri (dio = 3.5 mm).

dio only slightly influences the simulated impedance because the natural logarithm

Relationships between dio and experimental impedance, and between dio and simulated impedance, measured at 8000 Hz (n = 6 � 7, Ri = 100 μm). The three images show the cell morphology of BAECs on the ECIS sensors

The Modeling, Design, Fabrication, and Application of Biosensor Based on Electric Cell…

impedance more slightly in Eq. (7). The simulated impedance is consistent with the experimental data with maximum difference 0.63%, which validates the model. The experimental and simulated impedance indicates that dio in the range of 1000–3500 μm has only a little influence on the impedance because dio influenced the impedance of medium, which is only a small portion of measured impedance. Thus, dio cannot dramatically influence the measured impedance. However, dio should be large enough to avoid the current bypassing the cell monolayer between

3.2 The influence of electrode dimensions on the detection sensitivity of ECIS

between cell density and impedance. Figure 5 shows the impedance shifts versus

corresponding cell morphology on different ECIS sensors. When the cell density

increased 597 and 350 Ω for the sensors with Ri of 100 and 150 μm, respectively.

Ri of 100 and 150 μm, respectively. The experimental results indicate that the sensors with larger Ri illustrate less impedance changes with the same amount of cell density changes. Therefore, the sensors with smaller Ri are able to detect more

), the impedance increased 1336 and 880 Ω for the sensors with

the cell density changes with Ri of 100 and 150 μm. Figure 6 shows the

changes are 10,000 cells/cm<sup>2</sup> (from 90,000 to 100,000 cells/cm<sup>2</sup>

When the cell density changes are 20,000 cells/cm<sup>2</sup> (from 90,000 to

Detection sensitivity reflects the fineness of impedance response to the changes of cell behavior in cell-based assay environmental monitoring. The detection sensitivity of ECIS sensors is influenced by Ri. According to the previous experimental results, ECIS sensors with Ri larger than 200 μm do not respond sensitively and quickly on cell morphology changes. So, ECIS sensors with Ri of 100 and 150 μm were fabricated to study the influence of Ri on the detection sensitivity. Cell densi-

, were used to study the relationship

), the impedance

of the quotient of ð Þ Ri þ dio and Ri makes the influence of dio on simulated

sensing electrodes.

Figure 4.

110,000 cells/cm<sup>2</sup>

25

ties, 90,000, 100,000, and 110,000 cells/cm<sup>2</sup>

with dio of 1 mm, 2 mm, and 3.5 mm, respectively.

DOI: http://dx.doi.org/10.5772/intechopen.81178

#### Figure 3.

Relationships between Ri and experimental impedance, and between Ri and simulated impedance at 8000 Hz (n = 4 � 6, dio = 3.5 mm).

simulated impedance curve matches the experimental data closely with maximum difference 13.29%, which is acceptable when considering the fluctuation of measured impedance. The consistency of the simulated impedance with the experimental impedance validates this model's ability to optimize the Ri according to the range of measured cell number and expected output impedance level during sensor designing.
