# Design and Development of OECT Logic Circuits for Electrical Stimulation Applications
## 2 Materials and Methods
### Testbed for Characterization of Individual OECT Devices  
The aim of the tests was to identify the functional operation range of the novel OECT circuit designs when implemented to control current propagation. The two primary parameters of interest were the output voltage and the output current, since the biphasic pulses were generated in the current-controlled high-voltage regime in electrical stimulation applications. This led to the design of a special testbed circuit built on the TY51822R3 mbed development platform (Switch Science, Shinjuku City, Japan) shown in the left part of Figure 4. It included a custom-designed pulse generator circuit with voltage step up and a current feedback loop and allowed for manipulation of all stimulation parameters of interest through the firmware application: current amplitude (range: 0-10 mA, 100 uA step), pulse width (0-1000 uS, 1 uS step), stimulation frequency (1-100 Hz, 1 Hz step) and maximum output voltage (20-100 V, 1 V step). Note that the current and voltage levels targeted here were very high. Most other OECT applications described in the literature rely on lower current levels generated by a voltage typically ranging from several mV to a few volts. Hence, the requirement of the electrical stimulation application to deliver current levels in the mA range at a voltage supply of up to 100 V adds new challenges to the OECT technology. The means for adjusting the high and low voltage levels of the signals, which drive the gate electrodes of the OECTs in two parallel branches, were included to account for the variability present in the prototypes produced in this stage of development. Based on earlier observations with similar OECT architectures, there was a strive to achieve a narrow OECT input voltage window, since this prolongs the operational lifetimes of the devices [29]. Hence, the low and high input voltage levels were set to 0.3 V and 1.3 V, respectively, thereby resulting in an OECT input voltage window of only 1 V.  
The testbed included multiple points for connection of oscilloscope probes, which were selected to enable measurement and recording of the voltages and currents in the parallel branches needed for characterization of the printed OECTs. To facilitate the testing of different OECT architectures and samples, the testbed included two flexible flat cable (FFC) docks, as shown on the right side of Figure 4.  
The switching set-up, with the scheme presented in Figure 5, leveraged a parallel configuration of two printed OECT devices, in which at least one device was open whenever a pulse was generated. This arrangement lowered the stress on each individual OECT device in switching and was in line with a realistic scenario (e.g., when the printed OECT circuit is part of a multi-pad electrode device). Activation of a specific OECT branch was performed through the voltage level adjustment circuit controlled directly by the testbed microcontroller (MCU). Each branch included a simplified human model equivalent circuit (HMEC) to allow measurement of the leakage current in the branch that should be closed. The model from the active branch (HMEC1 or HMEC2) together with the HMEC reference represented the output load equivalent to a human connected through two biomedical electrode pads. All the HMECs were equal, with a resistor in series with a capacitor and a resistor connected in parallel, as shown in the right panel of Figure 5.  
In current-controlled sources, such as the one used in the testbed, the output voltage is variable to account for the variability of the electrode skin contact and individual properties that result in different loads. A common approach in transcutaneous stimulation devices is to connect the external load between the high voltage and a point that floats to generate the targeted current [30]. Thus, the stimuli generated between the electrode pair has a floating potential dependent on the attached load, which is different from the potential of the OECT controller and would cause significant drift over time.