# Design and Development of OECT Logic Circuits for Electrical Stimulation Applications
## 4 Discussion  
The results confirmed that printed OECT devices can be used for the routing of current-controlled pulses in the voltage and current ranges used in a number of transcutaneous electrical stimulation applications. Despite the clear limitations in the voltage levels and the current throughput of the tested OECT architectures, the results indicate that the proposed concept of electrode arrays with integrated logic circuits indeed provides great potential for the next generation of multi-pad electrodes. The working range of the tested architectures can already be recognized as sufficient in the domain of several applications of interest, like iontophoresis, proliferation, regenerative applications and electroactile stimulation. In electroactile applications, current amplitudes of up to 10 mA at maximal output voltages below 40 V could be considered to be at the limit of the more general requirements, but for specific electrode sizes and placements at these levels, they could be considered sufficient to produce clear tactile feedback [15]. For TENS and Functional Electrical Stimulation (FES) applications [3; 7; 8; 17; 30], which require higher voltages and currents, the design of the OECT architectures can be adopted to reduce the demand from a single output transistor. This can be achieved by stimulation configurations and protocols in which multiple branches are conductive at the same time (i.e., by smart electrodes with integrated shift register circuitry designed to activate several pads on the multi-pad electrode at the same time), thus reducing the demand on the single-output transistor. This limits the flexibility and imposes slight limitations in terms of possible electrode designs (i.e., pad size and arrangement) but can cover many application scenarios of interest in which the targeted activation points are not densely distributed. The experiments conducted in this study have shown that none of the devices tested were able to completely switch off the current passing through the non-conductive branch of the testbed. As can be seen in Figures 7 and 8, in each architecture, there was a small leakage current through the non-conductive branch. Both the 1 mm and the 2 mm OECT channel width the architectures showed similar, satisfactory performance in the tested conditions. The 3 mm OECT channel width architecture showed significantly higher leakage currents, indicating that there might be a (non-linear) relation between the channel size and the leakage current. Furthermore, this property seemed to be cumulative, as can be observed when comparing the 1 mm and the interdigitated 6 \(\times\) 1 mm OECT channel width architectures. The latter architecture was designed to increase the maximal current throughput capacity of the device by leveraging the principle of current distribution through parallel structures. However, the same principle seems to apply to the leakage currents, which in the tested conditions reached approximately 50% of the 5-mA output current, meaning that in the application of interest the device acted more or less as a short-circuit in both states. In all other architectures tested, the current that passed through the non-conductive branch could be considered negligible for the application of interest, as current levels below 1 mA, generated by voltages below 40 V, were unable to produce action potentials in the subcutaneous structures with the pulses used, since these values were below the thresholds defined by the strength-duration curves for peripheral nerves [31]. The 40 V value seemed to be near the maximal voltage of the operating range, as some of the 2 mm and 3 mm OECT channel widths maintained the switching capacity by exhibiting a leakage current below 1 mA, while other devices failed to cut off the current when acting as a parallel open branch, as evidenced by the 50% current throughout being allowed. The use of current-controlled biphasic pulses is common practice in electrical stimulation.