## 4 Results and Discussion
### Technical Evaluation  
In order to demonstrate the correct performance of the developed system, this section presents the results of the technical evaluation. Firstly, the debugging of the modules using the oscilloscope upholds the expected behaviour of the printed boards. For instance, Figure 7A shows the outputs of the microcontroller CMUX0 in blue and CMUX1 in green. In accordance with the multiplexor's truth table, CMUX0 and CMUX1 generate CNTL0, shown in yellow. The positive and negative amplitudes of the yellow signal are defined by the DAQ analogue outputs, and the digital potentiometers previously described. The amplitude of these pulses controls the LED currents using a Howland pump, the output of which is shown in Figure 7B. Besides the LEDs' supply, the response of each photodiode was tested at the output of the transimpedance amplifier. An impulse response is shown in Figure 7C, where the photodiode was covered from light and exposed very quickly. Figure 6: (**A**) Probe prototype with its 4 LEDs, 3 photodiodes and 15D connector. (**B**) Open ZenTBI view of the instrumentation system. Figure 7: ZenTBI debug tests on the oscilloscope. (**A**) shows the outputs of the microcontroller CMUX0 in blue and CMUX1 in green. In accordance with the multiplexor’s truth table, CMUX0 and CMUX1 generate CNTL0, shown in yellow. (**B**) shows the output of the Howland pump circuits that control the LEDs’ currents. Two LEDs are controlled by the yellow pulses and other two by the green pulses. (**C**) shows the impulse response of a photodetector. (**D**) shows the demux controls generated by the microcontroller, CMUX0 as the green line, CMUX1 as the purple line, and the INHIBIT as the yellow line. Finally, the demultiplexer was evaluated to ensure the effective decomposition of the mixed signal (from all four wavelengths) into four independent signals. Figure 7D displays the demux controls generated by the microcontroller, CMUX0 as the green line, CMUX1 as the purple line, and the INHIBIIT as the yellow line. The output of the demultiplexer is the pink signal that becomes erratic every time the INHIBIT is ON; a constant voltage can then be seen at the different demux combinations according to its true table. The presented debug sequence demonstrates the correct operation of the main ZenTBI stages. During the power consumption assessment, two measurements took place: one without the probe and the second one with the probe connected to the system. The LEDs in the second measurement were driven by 40 mA of current. The load current calculated using Ohms Law was 360 mA without the probe and 420 mA with the system complete (probe connected). From the second measured load current, the battery life of the entire system can be calculated (Equation (7)), and is given as:  
\[ \begin{array}{c}Battery\ life=\frac{Battery\ capacity}{Load\ current}\times 0.7\\ Battery\ life=\frac{26.800\ \mu mA}{420\ mA}\times 0.7\\ Battery\ life=45\ hours\end{array}  \tag{7}\]  
The system aims to assess cerebral oxygenation parameters by computing the DC component of the photodiodes placed at the distal probe. However, arterial saturation from the scalp could also be calculated from both AC and DC PPG components of the photodiode at the proximal probe. Moreover, the system also aims to utilize the NIRS-PPG signals of the proximal photodiode at the distal probe for the estimation of intracranial pressure. Therefore, the consistent acquisition of the DC and AC components from the respective photodiodes is crucial for the proposed applications of the developed probe. The system's performance was validated by the quality assessment of the PPG signals acquired from the proximal detector (proximal probe) and the distal detector (proximal photodiode of the distal probe). Figure 8 displays the raw and filtered PPGs signals from the detectors of interest at all wavelengths. From this Figure, it can be concluded that the acquisition of PPG signals from both detectors at all wavelengths (all LEDs currents were equal) was successful.