# Organic Electrochemical Transistor Microplate for Real-Time Cell Culture Monitoring
## 3 Results and Discussion  
The first experiment tested the OECT parameters as the transfer characteristics and the derived transconductance \(g\). The fix drain potential was set to \(V_{\mathrm{ds}}=-0.735\) V and the output source current was measured continuously at the triangular symmetric gate voltage \(V_{\mathrm{gs}}\) in a range of 0.2 to \(-0.8\)V. The results can be found in Figure 3. As can be seen from the figure, due to relaxations, the transfer characteristics and transconductance \(g=\mathrm{d}I_{\mathrm{s}}/\mathrm{d}V_{\mathrm{gs}}\) included are strongly dependent on the sweep rate of the testing gate voltage \(V_{\mathrm{gs}}\) and its direction. The sharp transconductance maximum was observed first by [PERSON] et al. [4]. Its position on the \(V_{\mathrm{gs}}\) axis depends on the channel geometry, but no sweep rate influence was reported. The transfer characteristics in our case show significant hysteresis. In the case of a short period of about \(T=1\) s, the EDL capacitance charging delays the output current \(I_{\mathrm{s}}\), so the hysteresis is oriented counter-clockwise. In the case of a long period of \(T=20\) s, slow relaxations of the polymer network in the channel, reducing the output current \(I_{\mathrm{s}}\), dominate so that the hysteresis is oriented clockwise. In the case of a medium period of \(T=5\) s, both effects are compensated. A second experiment, investigating the response of an OECT's channel current on a simulated signal of electronic cells, was carried out (Figure 4). The standard PBS solution was used as an electrolyte. In looking for the optimum working point of the OECT with the highest transconductance, the gate voltage \(V_{\mathrm{gs}}\) from the function generator was modulated by rectangular pulses of 10 mV\({}_{\mathrm{pp}}\) over a 5 s period. This was superposited on the offset voltage, as can be seen in Figure 4. While the response on the offset setting showed negative relaxation with a time constant of approximately 25 s, the amplification of the pulsing signal did not change significantly according to the offset. This means that the transconductance remains constant across a wide range of gate offsets, in our case a transconductance of \(g=0.01\) mA/\(0.01\) V = 1 mS deduced from the plot. However, this value is limited by the frequency band pass \(f=(0.2\)-\(8.0)\) Hz corresponding to the time constants of the gate circuit and slow relaxation processes. The measured time constant \(\tau=0.12\) s achieved by the simple exponential fitting corresponds to charging-discharging the channel EDL capacitance of \(C=0.1\)\(\mu\)F and the entire gate circuit resistance of \(R=1.2\) M\(\Omega\). The potential on the capacitance varies and it also controls the ion doping-dedoping process. The measured circuit current, charging the gate, achieved 1 \(\mu\)A in accordance with the circuit resistance. It is theoretically possible to lower the time constant by decreasing the channel area and hence its capacitance or by decreasing the gate-channel distance and hence its resistance. The resistivity of the electrolyte is determined by its physiological function and so it cannot be variated. The speed of the device is then limited by the screen-printing resolution. The lithographic technique enables the development of much smaller dimensions and quicker devices. This is necessary for recording the shape of the electrogenic cell (cardiomycytte) signal. Figure 3: Transfer characteristics measured at the triangular gate periodic voltage at various periods. Using a short time range display, we found one further time constant, namely 3 ms caused by hole extraction and injection into the channel [1]. This might play a more significant role in a smaller channel area with a lower capacitance and time constant. The faster rectangular modulation, applied to the \(V_{\mathrm{gs}}\) simulating a signal of electrogenic cells (cardiomyocytes), lent importance to the measurement.