# Design and Development of OECT Logic Circuits for Electrical Stimulation Applications
For example, two addressing bits in a 2-4 decoder can be provided by the external electronics, and the printed 2-4 decoder circuit ensures that proper addressing of its four output channels is achieved. By using such an attempt in a multi-pad electrode, it is therefore possible to lower the number of leads connected with the electrode pads. A screen-printed shift register is another possible circuit that can be used to lower the I/O count from the addressing electronics to a minimum level. For example, in a 4-bit serial in-parallel out shift register, addressing of the electrodes in the multi-pad electrode is achieved by only using a clock signal and a data input signal. In fact, an arbitrary number of electrode pads can be addressed by using such shift register circuits, thereby lowering the number of leads required between the addressing electronics and the electrode pads. The screen-printed OECT technology shows several advantages for further utilization in multi-pad electrodes. First of all, it is very robust technology that allows for high manufacturing yields despite using a relatively rough fabrication method in the form of conventional screen printing. This was demonstrated in a previous report, in which reproducible switching characteristics (transfer, output and dynamic measurements) and a manufacturing yield of at least 99.7% were achieved upon evaluating almost 1000 OECTs screen-printed on a relatively large substrate with the dimensions of approximately 200 \(\times\) 300 mm [20]. This is mainly due to the electrolytic interface between the gate electrode and the OECT channel. In field effect-driven electronic devices, the voltage drop occurs across the complete thickness of the dielectric layer, and it is therefore critical to deposit very thin layers to obtain low-voltage operation. However, here, in the case of electrolyte-gated devices, the voltage drop occurs at the interfaces between the electrolyte and the gate electrode as well as between the electrolyte and the OECT channel. This ensures low-voltage operation (\(\sim\)1 V) without the requirement of thin layer deposition. The conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate), abbreviated PEDOT:PSS, is used as the active OECT channel material in the devices reported herein. PEDOT:PSS is intrinsically conducting and therefore transports chargesthroughout the complete thickness of the channel which, in comparison with field effect transistors, implies much higher current levels in the ON state of the OECT. The thickness of the electrolyte layer in screen-printed OECTs often exceeds 10-20 um, which dramatically simplifies the manufacturing process and hence also implies an increased manufacturing yield. Screen-printed OECT devices have already shown promising results in applications requiring high loads (e.g., as addressing devices used to drive light emitting diodes) [21] and are therefore a good candidate for switching devices capable of delivering the high current levels needed in multi-pad electrodes for stimulation applications. The main functional disadvantage of the screen-printed OECT technology is the relatively long switching times, typically tens to hundreds of ms for standard OECT architectures. Such long switching times may compromise the utilization of OECTs and OECT-based circuits in many applications. However, for the multi-pad electrodes targeted in stimulation applications, this is not considered to be an issue, since the pad selection for most applications and protocols is performed prior to the stimulation session. Once the desired electrode pads have been selected, the same addressing will be maintained. However, the requirements for the current throughput and output voltages in electrical stimulation applications [22] are an order of magnitude higher than what any of the OECT devices described in the available literature can sustain [19; 21]. To address this, the OECT design was further improved and tested in conditions specific for transcutaneous electrical nerve stimulation (TENS).