# Organic Electrochemical Transistor Microplate for Real-Time Cell Culture Monitoring
## 2 Materials and Methods
The field strength at the channel vicinity determines the effective ion drift in and out of the channel and consequently the channel conductance. The small channel area facing the electrolyte compared with the gate electrode area enhances the field and lowers the electrical double layer (EDL) capacitance per unit area at the PEDOT:PSS, which is responsible for the device speed. The capacitance of the electrode in a physiological solution of phosphate-buffered saline (PBS) based on a 0.15 M solution of NaCl was estimated at \(C=0.2\)\(\mu\)F\(\cdot\)mm\({}^{-2}\). Together with the electrolyte conductance \(G=1\)\(\mu\)S, the time constant \(\tau=C/G=0.2\) s was anticipated. The capacitance of the much larger gate charge EDL can be neglected due to its serial combination with the channel capacitance. The speed of the OECT device is limited by the capacitance of the gate circuit. It is controlled by the effective capacitance of the channel. The label 'effective' here takes into account that the potential at the channel is not uniform but distributed non-linearly along its length from the source to the drain electrode (from 0 to \(-0.725\) V in our case). It can be, to some degree, considered a consequence of the channel aspect ratio. Moreover, the geometry of the entire system including the gate electrode and electrolyte can influence the channel potential distribution. Contrary to the majority of authors, we use the planar arrangement of the electrodes for an easy optical and camera investigation. The computer modelling of the system has still not been satisfactorily solved. The testing and gate offset voltage, as well the modulation (simulating the cardiomyocyte pulsing), were delivered from a function generator to the Analog IN input. An OECT was selected by a couple of switches. The drain is supplied by a negative voltage down to \(V_{\mathrm{ds}}=-0.735\) V. This value is a factory setting and it represents a compromise between the requirement of a safety voltage against electrode redox reactions in the event of a positive \(V_{\mathrm{gs}}>0\) V gate voltage and the requirement of high amplification (transconductance). Its fixed value enables comparing the amplification of our various OECT devices at any point in time. The source is connected to a virtual zero input of the \(I/V\) converter. The amplified signal was recorded by a digital memory scope. The offset of the stable channel current was compensated for by a potentiometer \(R_{\mathrm{offset}}\) so the Analog OUT DC signal component could be eliminated. Mouse 3T3 fibroblasts (cat. No. CRL-1658, ATCC, Manassas, VA, USA) were routinely grown in Dulbecco modified Eagle medium, high glucose supplemented with 10% fetal calf serum, 100 U/mL penicillin, and 0.1 mg/mL streptomycin (all from Gibco, Gaithersburg, MD, USA) in standard 100 mm cell culture dishes (cat. No. 93100, TPP, Trasadingen Switzerland), as described previously [22]. The inocula for experiments ranged from 8300 to 28,000 cells per 1 cm\({}^{2}\). Viability was routinely checked by means of a cytometer (CASY, Roche Diagnostics Ltd., Rotkreuz, Switzerland). Cells were grown in sensors for 48 h.  
To provide proof of sensor function, cells grown within the experiment were washed with physiological buffered saline two times and 200 \(\mu\)L of 0.25% trypsin (cat. No. R001100, Gibco, Gaithersburg, MD, USA) was added. Together with the addition of trypsin, video recording was started.