# Design and Development of OECT Logic Circuits for Electrical Stimulation Applications
## 4 Discussion
The pulse in the second phase--the compensation phase--extracted the charge introduced in the activation phase, thus preventing local charge build-up that can lead to reduced stimulation effectiveness or even tissue damage. The same principle is beneficial for the use of printed OECT switches. PEDOT:PSS-based OECTs are operated in the third quadrant (i.e., a negative drain voltage is required to reach a state in which the OECT channel is completely switched off). On the contrary, a positive drain voltage turned the OECT device toward a diode-switching behavior. However, switching the polarity of the biphasic pulse during stimulation increased the efficacy, as well as the expected operationallifetime, to a large extent due to the fact that the pulse length of the respective pulse was in the sub-ms range. It should be noted though that the shape of the recorded current during the stimulation was not ideally symmetrical in the two phases. This peculiarity may be caused by the behavior inherent to the electrical stimulation current control circuitry. The external load, which is an analog to a simplified model of the human skin, had both resistive and capacitive elements that acted as an RC filter to the current source. Here, the current amplitudes of the pulses were controlled by changing (increasing) the output voltage to maintain the desired level. Once the maximal output voltage was reached, the current dropped while the capacitor was charging. This can be seen in the shape of the first pulse. However, in the second pulse phase, the current drop was delayed, as the capacitor was already fully charged by the opposite polarity, acting as a voltage buffer. Since a symmetrical pulse is desired, this effect can be compensated for by tuning the current control algorithm after the OECT circuit has been characterized. This poses an interesting challenge for future research and development activities in this domain. Additionally, a presence of ripples can be noted in the signals recorded with the first testbed, which are shown in Figures 7, 9 and 10. These were caused by the implemented current-controlled system, which was not optimized for the additional dynamics introduced by the OECT circuit and the specific HMEC, resulting in minor overshoots of the setpoint. Conversely, in the second testbed, the output module was specifically designed to be used with the OECT-based smart electrodes with a 2-4 decoder. As can be noted in Figure 11, the ripples in the signals were significantly lower and were on a level comparable to the measurement noise. These could be further suppressed by additional system tuning, but they were already inconsequential to the targeted application at the presented level. Due to the current-controlled method that increases the voltage until the desired current level is reached, the maximal voltage level is a critical parameter that will dictate the performance of the OECTs and hence also the current leakage in the closed branches. The graphs shown in Figure 9 demonstrate that for voltage levels of 50 V and above, the OECT devices were not able to control the current propagation, as the current in the closed branch was comparable to the current in the open branch (approximately 2.5 mA). For voltages of 40 V and below, the current was propagating primarily through the branch with the OECT in the open state, with leakage currents of approximately 0.5 mA in the branch with the OECT in the closed state. It should be noted that these results were analyzed for OECT devices with a 1 mm channel width, since they showed comparable performance with the 2 mm channel width devices and better performance compared with other architectures for the testing scenario at 40 V. From the current signals shown in the right panel, it can be noted that due to the high capacitance of the HMEC, the required 5-mA current level was not generated, nor did it reach the steady state throughout the pulse for some of the voltage output levels.