# Organic Electrochemical Transistor Microplate for Real-Time Cell Culture Monitoring
## 2 Materials and Methods  
The device (see Figure 1) was constructed on a 96-well polystyrene microplate (chimney-well, no bottom) from Greiner Bio-One (GmbH Frickenhausen, Frickenhausen, Germany), for the automated analysis of a multitude of substances. We provided the bottom with the all-printed 12 \(\times\) 8 array of Organic Electrochemical Transistors (OECT) based on the Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) polymer (Clevios(tm S V3, Heraeus Holding GmbH, Hanau, Germany). The screen-printing method was used. Printed organic electronic technology has generated the possibility of printing biosensor patterns on biocompatible polymer foils of PET (Poly(Ethylene Terephthalate)) or PEN (Poly(Ethylene 2,6-Naphthalate) (both Goodfellow Cambridge Ltd., Huntingdon, UK), the latter of which enables processing at a temperature of up to 170 \({}^{\circ}\)C. The bottom substrate was a 0.25 mm thick PEN foil due to better thermal stability during the subsequent annealing processing of the prints. The first printed pattern (screen mesh count 77 threads/cm) of silver paste CB115v2 (@DuPont) Photopolymer and Electronic Materials, Wilmington, DE, USA) creates the contact and conductive field and the paths that are 0.18 mm in width (see Figure 1d). The foil with the silver pattern was then annealed at 120 \({}^{\circ}\)C for fixation. The second pattern of PEDOT:PSS paste Clevios(tm) S V3 (screen mesh count 140 threads/cm) created a functional OECT gate and channel. The paste was first stirred intensively before printing in order to obtain a lower viscosity and better homogeneity of the resulting film. An ARG2 rheometer (TA Instruments, New Castle, DE, USA) was used for viscosity testing at various shear rates and always at 25 \({}^{\circ}\)C, but only the initial low rate viscosity is presented. The paste of gel consistency exhibits thixotropy. The static viscosity (at shear rates of 0.1/s) falls from the initial value above 1000 Pa-s down to ultimately 14 Pa-s after five days of stirring in a magnetic mixer at a laboratory temperature of 25 \({}^{\circ}\)C. The stirring temperature of 90 \({}^{\circ}\)C shortened the time needed to 24 h. The viscosity did not then decrease further by further stirring. After the stirring was ceased, the viscosity returned to the initial value, with 100 Pa-s measured after about 1 h, illustrating that the printing has to be performed within a few minutes, before the consistency of the paste returns back to the initial gel state. The channel dimensions are 1.5 \(\times\) 1.5 mm width \(\times\) length and the gate is created by printing a ring broken on one side for source and drain contacting. The silver conductive paste and the polymer patterns were annealed using a hot plate at 150 \({}^{\circ}\)C for 15 and 30 min, respectively, in air for the film's stabilization and adhesion improvement. The stirring effect slightly decreased the final thickness by an average of 20% from 250 nm to 200 nm. It also improved the thickness homogeneity in roughness from 25 nm to 18 nm and waviness from 25 nm to 10 nm, measured by a DetakXT profilemeter (Bruker, Billerica, MA, USA) (see Figures S1 and S2). Additionally, the sheet resistance decreased significantly from about 800 \(\Omega\)/\(\square\) to 400 \(\Omega\)/\(\square\). It is considered that intensive stirring acts similarly to alcohol treatment. Furthermore, the core-shell molecular structure changes in a linear fashion as was already reported in [20] and the process is reversible. This can also explain the resistance, roughness, and waviness reduction. The improvement in homogeneity can be observed by an optical microscope according to the round interference fringes on the PEDOT:PSS layer (see Figure S3).