# Organic Electrochemical Transistor Microplate for Real-Time Cell Culture Monitoring
## 2 Materials and Methods
The system is masked and sealed by the third pattern of the Sylgard(r) 184 (Dow Corning, Midland, MI, USA) silicone elastomer (mesh count 120 threads/cm), which insulates the silver conducting paths from the electrolyte environment and also prevents the tested biomaterial from coming into contact with the non-biocompatible parts and layers of the printed device. The silicone elastomer pattern was  
Figure 1: Encapsulated organic electrochemical transistor (OECT) 96 well microplate for electrogenic cell cultivation and investigation (**a**) with a power source-signal amplifier and current/voltage converter (**b**), foil with functional printed patterns of all OECTs in a 12 \(\times\) 8 array (**c**), and a detail of a couple of OECTs with a poly(3,4-ethylenedioxythio-phene): poly(styrene sulfonic acid)—PEDOT:PSS printed channel and gate electrode (**d**). left to polymerize at 60 \({}^{\circ}\)C for at least 8 h before further manipulation. The following screen print provided a layer with a thickness of 12 \(\upmu\)m (see Figure S4). Other parts and materials were tested for biocompatibility [21]. A coating to increase biocompatibility was produced with murine collagen type IV (BD Biosciences, cat. No. 354233) at 10 \(\upmu\)g\(\cdot\)cm\({}^{-2}\) according to the manufacturer's instructions. Patterning and microplate completing followed. The OECT's channel visible in Figure 1d, created by a screen-printed rectangular PEDOT:PSS layer on PEN foil, is surrounded by a planar circular gate electrode with a 6 mm outer diameter. The rotary symmetry is expected to improve the field distribution. The electrodes are contacted by printed silver conductors of a 0.2 mm width. The printed transparent silicone layer covers the surface of the sensing array with the exception of the functional PEDOT:PSS interfacing to the biomaterial and physiological solution. The exposed channel area is of a length of \(L\) = 1 mm and a width of \(W\) = 1.5 mm, the thickness of the PEDOT:PSS layer was 250 nm on average, and the typical resistance of the channel was 500-700 \(\Omega\). The foil was firmly fixed to the polystyrene microplate and bolted down from the bottom by means of a Plexiglas(r) plate in order to enable easier handling and transparency needed for microscope observations. The electrical circuitry designed for OECT testing in schematic form is depicted in Figure 2. The contact array on the microplate foil was contacted by 18-pin Molex connectors and 18-wire ribbon conductors so that eight OECTs were connected by a single connector and individual OECTs can be selected from the array by a proper couple of microswitches. The gate potential \(V_{\text{gs}}\) is set from the outer source in a range from \(-0.8\) V to 0.8 V with respect to preventing redox reactions on electrodes and a source drain voltage \(V_{\text{ds}}\) down to \(-0.725\) V can be set. The source current \(I_{8}\) was converted to voltage in an \(I/V\) converter and recorded by a scope after further amplification. The offset \(I_{8}\) at a stable working point can be compensated for by setting the corresponding opposite current at the input of the operating amplifier. The zero Analog OUT voltage can be set for an input voltage of \(V_{\text{gs}}\) = 0 V with the aid of a \(R_{\text{offset}}\) resistor/potentiometer. The feedback resistor \(R_{\text{sense}}\) = 1 k\(\Omega\) gives a conversion of 1 mA/1 V. Further amplification is enabled by switching the output jumper. Figure 2: The principle of selecting and testing a single OECT from the 96 OECTs array. OECTs are a promising tool for measuring the electrical pulsation of electrogenic cells. Their electric field can penetrate through the cell membrane into the surrounding phosphate-buffered saline (PBS) electrolyte and the channel electrode. Their pulsing potential can be simulated by the modulation of the gate voltage \(V_{\mathrm{gs}}\). Besides the gate voltage, the size of the channel area and electrode arrangement determine the sensitivity and signal to noise ratio.