# Design and Development of OECT Logic Circuits for Electrical Stimulation Applications
## 4 Discussion
This was evident for the 40 V maximal output voltage but became gradually even more pronounced for the lowered voltage levels of 30 V and 20 V. This illustrates the need for a high output voltage in applications relying on transcutaneous electrodes with pads of relatively small contact areas between the electrode and the skin, requiring current levels exceeding 3-4 mA. Another factor that will affect the current leakage is the output current. This effect is noticeable in the current leakage vs. total current graphs shown in Figure 8. With a 30 V output voltage, the current leakage was considerable (>10% for all OECT architectures) for low currents (1 and 2 mA), but the current leakage was reduced and stabilized when the current was further increased (around 5% for the 1 mm, 2 mm and 3 mm OECT architectures and around 13% for 6 \(\times\) 1 mm OECT architecture). A similar behavior was observed when the maximal voltage was set to 40 V, but in this case, the transistor performance was significantly degraded (consistent 50% current leakage for the 6 \(\times\) 1 mm architecture and 20-30% leakage for the other architectures). Furthermore, there was a notable inter-device variability reflected through the wide ranges of the recorded leakage currents. This was potentially a result of variations originating from the manufacturing process (e.g.,ink homogeneity, non-uniform curing process, thickness variations and moisture level fluctuations in the channel). Humidity is clearly another factor that influences the performance of printed OECT-based circuits, and our results demonstrated this effect for the current-controlled electrical stimulation pulses. The testing was performed in humidity conditions just outside the range recommended by most professional bodies but still within the range required by the majority of the regulatory bodies [32]. From Figure 10, by directly comparing the dashed lines with the solid lines, it can be observed that for the same set of OECTs, the leakage current in the closed branch was minimized in high-humidity conditions. For a maximal output voltage of 40 V, there were two current levels at which the OECTs were unable to function (i.e., current dissipation close to 50%) at 25 %RH, but the increase in the relative humidity to 65 %RH resulted in a reduction of the current leakage to the level of 5%. It should be noted that the electrodes for transcutaneous electrical stimulation were located on the skin, which usually implies a moist microenvironment [33; 34]. Hence, improved OECT operation in high-RH conditions is desirable with respect to the application scenario, and one solution to mitigate the RH dependence is to combine the OECT screen-printing process with an appropriate encapsulation process. The advantage of properly encapsulated OECT-based circuits is that an identical drive protocol can be used throughout a wider RH range, which in turn results in a prolonged operational lifetime. The final demonstration of the performance of an OECT-based screen-printed demultiplexer circuit (shown in Figure 8) further evidences that this technology can serve as a basis for smart biomedical electrodes that integrate logic circuits to allow for direct pad addressing. Considering that the 2-4 demultiplexer circuit integrates 16 printed OECTs, and by also taking into account the potential influence of inter-device variability within the circuit, the stimulation requirements were reduced compared with the previous tests done with the standalone OECTs (i.e., the maximal voltage was set to 20 V and the input current level to 2 mA). From the currents in the four output channels of the demultiplexer, it can be concluded that the current in the active channel was very close to the desired output current level and that the leakage currents in the remaining three output channels were minimal.