# Design and Development of OECT Logic Circuits for Electrical Stimulation Applications
## 2 Materials and Methods  
### OECT Architectures and Production Process  
The OECT architecture used in the WEARPLEX project is presented in Figure 1, depicting the device used in the addressing circuits. As in this case the switching time was more important than current throughput, an OECT channel width (W) and length (L) of 150 um and 100 um, respectively, were used. In a WEARPLEX multi-pad electrode, each addressing circuit also has a number of driving OECT devices equal to the number of electrode pads. The requirement on these OECT devices is a high current throughput rather than a short switching time. Due to this, various channel geometries have been evaluated to optimize the devices toward multi-pad electrodes for stimulation applications. The current throughput scales with the W/L ratio, but the channel length is more or less limited by the resolution of the screen-printing equipment. However, the channel width is easily adjusted, and a larger width implies higher current levels in the ON state of the OECT. The correlation between the switching characteristics and the channel width of the OECT has briefly been reported previously by combining both experiments and simulations [21]. In the work reported herein, driving OECT devices with channel widths of 1, 2, 3 and 6 mm were evaluated (see Figure 2). The 6 mm wide channel obviously allowed the highest ON current level, but increasing the channel width also brought a trade-off with respect to the footprint. It should also be mentioned that an interdigitated design was used for the 6 mm wide channel (i.e., it was split into 6 parallel channels, each 1 mm wide). The use of an interdigitated design might be beneficial in other applications, but here it was considered disadvantageous since the leakage current was increased and the footprint of each device was enlarged. In order to not exceed the maximum dimensions of the multi-pad electrode, W = 1 mm was chosen for the drive OECTs in the circuits reported herein. This trade-off allowed for ON current levels up to a few mA, which at least exceeded the sensory threshold of the human body. The transfer and output characteristics of the five different OECT architectures evaluated herein (L = 100 \(\upmu\)m and W = (0.15, 1, 2, 3, 6) mm) are shown in Figure 3.  
Figure 1: The specific architecture of OECT devices developed in the WEARPLEX project shown (**A**) in the cross-section and (**B**) from a top view with printing layers. The gate layer has been omitted to reveal the areas of interest. Figure 2: Driving OECT devices with different channel widths were evaluated in the presented work. The gate layer has been omitted to reveal the areas of interest. Figure 3: **(A)**: The transfer characteristics (forward sweep direction, V\({}_{\rm D}\) = \(-\)1 V) of the five OECT channel widths being evaluated. The narrowest channel clearly resulted in the lowest current throughput and switching voltage, which are both advantageous from the points of view of power consumption and operational lifetime. The wider channels resulted in higher current throughputs suitable for stimulation application. The interdigitated 6 mm wide channel showed a relatively low switching voltage; however, its increased leakage current level was considered disadvantageous. **(B)**: The output characteristics for the same OECTs are shown in this panel upon subsequently applying gate voltages of 0, 0.5, 1.0 and 1.5 V to each OECT. **(C)**: The graphs are based on the same data already shown in the middle panel, but here the minimum current levels are shown upon applying V\({}_{\rm G}\) = 1.0 or 1.5 V.  
The inks used in the OECTs and OECT-based circuits were screen-printed on plastic substrates based on polyethylene terephthalate (PET, Polifoil Bias, Policrom Screen, Carvico, Italy). PEDOT:PSS (Clevios S V4, Heraeus Epurio, Leverkusen, Germany) was used for the electrochemically active transistor channels and gate electrodes. The electrolyte (E003, developed by RISE, Norrkoping, Sweden) was deposited by screen-printing, followed by UV-curing.