## 3 Results
### Design of the Conditioning Sections for Electrochemical Gas Sensors  
This section describes the low-power modules integrated into the smart garment for conditioning the current signal generated by the electrochemical gas sensors described above. In particular, a multisensor board for detecting the environmental concentrations of CO, CH\({}_{2}\)O, and O\({}_{2}\) was developed using two-electrode electrochemical sensors (ME2-CO, ME2-CH2O, and ME2-O2). The electrochemical sensors of carbon monoxide (CO) and formaldehyde (CH\({}_{2}\)O), when in contact with the aforementioned gaseous species, produce oxidation reactions; therefore, electron transfer occurs from the gas to the working electrode (WE), causing a complementary reaction on the counter electrode (CE). Therefore, the sensor generates a current that flows from CE to WE in proportion to the concentration of the gaseous species. For the electrochemical oxygen sensor (O\({}_{2}\)), a reduction-type reaction on the WE occurs, with consequent electron accumulation on the counter electrode, thus generating an output current from the WE. Furthermore, the used electrochemical sensors require an aging time of 24/48 h when they are calibrated or after long storage periods but ensure a response time of less than 30 s during the measurement phase. For the CO and CH\({}_{2}\)O sensors, potentiostatic stages for two-electrode sensors are employed. In this circuit configuration, a voltage buffer fixes the d.d.p. between the CE and WE (Figure 9). In addition, a load resistor R\({}_{\text{L}}\) between WE and CE is needed, which constitutes an RC smoothing filter with an intrinsic sensor capacitance for reducing the output noise. In addition, the value of load resistance affects the response time of the sensing module in the presence of gas. The manufacturer suggests load resistances between 120 \(\Omega\) and 300 \(\Omega\) for both sensors [35; 36]. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Scenario 1** & **Scenario 2** & **Scenario 3** & **Scenario 4** & **Scenario 5** \\ \hline Illuminance (lux) & 27918 a  & 29322 a  & 530 a  & 530 b  & 530 b  \\ \hline \(\Delta\)T (\({}^{\circ}\)C) c  & 10.4 & 11.8 & 12 & 12.7 & 11.4 \\ \hline Action & steady & walking d  & walking & walking & pushups e  \\ \hline \end{tabular}
\end{table} 
Table 2: Table reporting the five operational conditions used to test the multisource energy harvesting system integrated into the smart garment. Additionally, a transimpedance stage converts the current supplied by the sensor into an output voltage proportional to the concentration of the gaseous species. The reference voltage is generated by a voltage divider that allows obtaining a voltage of 300 mV from a supply voltage of +3.3 V.  
The gains of the transimpedance stages were sized according to the STEL limits for the corresponding gas species (Table 1). In particular, a CO concentration limit of 100 ppm was considered for sizing the gain of the transimpedance stage, taking into account that the ME2-CO sensor is characterized by a sensitivity value equal to 0.023 \(\pm\) 0.008 \(\upmu\)A/ppm. To obtain a full-scale voltage equal to +3.3 V, the resistance on the feedback loop can be calculated as:  
\[\mathrm{R_{feedback}}=\frac{\mathrm{V_{o,Max}-V_{o,Min}}}{\mathrm{I_{sense}}}= \frac{+3.3\ \mathrm{V}\ -\ 0.3\ \mathrm{V}}{1.5\ \upmu\mathrm{A}}=2\ \mathrm{M}\Omega \tag{1}\]  
Furthermore, the gain of the transimpedance stage was dimensioned, taking into account a CH\({}_{2}\)O concentration limit of 0.4 ppm (STEL limit, Table 1) and sensor sensitivity of 0.45 \(\pm\) 0.15 \(\upmu\)A/ppm, thus resulting in a sensor current of 0.18 \(\upmu\)A (I\({}_{\mathrm{sense}}\)).