## 2 Materials and Methods
### Test Objects
#### 2.1.3 Thermoelectric Module Arrangement and Supply Voltage  
A separate DC converter has been provided for each of the three TEM pairs to minimize the electric supply power discrepancies between them, which is caused by their resistance spread. Within each pair, a series connection might initially seem to be more advantageous because of the higher converter output voltage, which is equal to the sum of the two TEM voltages (Figure 5a):  
\[U_{o}=U_{TEM1}+U_{TEM2}\approx 2U_{TEM}, \tag{1}\]  
To simplify, both TEM voltages have been assumed to be approximately equal at some value \(U_{TEM}\). The converter must deliver to the TEMs a power equal to:  
\[P_{o}=P_{TEM1}+P_{TEM2}=I_{o}\ U_{o}. \tag{2}\]  
Figure 4: Electronic controller block diagram (digital signals: blue; analog signals: green; power paths: red). The currents of the two TEMs are equal, i.e., \(I_{TEM1}=I_{TEM2}=I_{o}\), so this common current is expressed by:  
\[I_{o}=(P_{TEM1}+P_{TEM2})/(2U_{TEM}), \tag{3}\]  
In contrast, in the parallel arrangement (Figure 5b), all the voltages are equal, i.e., \(U_{TEM1}=U_{TEM2}=U_{o}\), while the converter's output current is the sum of the two TEM currents:  
\[I_{o}=I_{TEM1}+I_{TEM2}=P_{TEM1}/V_{TEM}+P_{TEM2}/U_{TEM}=(P_{TEM1}+P_{TEM 2})/U_{TEM}, \tag{4}\]  
which is twice the value yielded by (3). The higher output current in the parallel connection results in two principal drawbacks of this option:  
* higher necessary current ratings of the converter's components; hence, their greater cost and size, as well as, most probably, power losses;
* higher power losses in common wiring, making the efficiency lower and the battery operating time shorter. It must be noted that with worn electronics, the possibility of decreasing losses by increasing the wire diameters is limited by the requirement of flexibility. Nevertheless, when an electronic system is to be integrated with clothing, one must consider that live parts may enter into contact with the skin, either directly (e.g., due to damage to the insulation) or through humidity. When cooling is needed, sweating is especially likely to occur as a natural mechanism of body temperature regulation, and sweat is highly conductive electrically due to the content of NaCl and KCl, which are strong electrolytes [27]. The IEC 60479-1 standard [28] contains data that may be used to determine safe voltage levels for a number of specific cases. As justified above, saltwater-wet conditions should be considered, corresponding to a 3% water solution of NaCl, which, according to the standard, may represent sweat. As far as physiological effects are concerned, only the DC-1 zone may be allowed for worn electronics, where a slight sensation is possible upon rapid changes of current flow, such as by switching the TEMs on or off. The current threshold between the DC-1 zone and the DC-2 zone (where involuntary muscle contractions occur) is given as 2 mA. The standard provides tabulated values of a reference total body resistance \(R_{t(nt)}\) as a function of the touch voltage \(U_{t}\). It is reasonable to use data for a small contact-surface area, which is set at 100 mm\({}^{2}\). To obtain the most conservative voltage limits, data applicable to the 5 th percentile of the population having the lowest body resistance were used. From the point of view of physiology, the critical path is the one between the left hand and the chest, where the resistance \(R_{t}\) is 63% of the reference one. Figure 5: Possible TEM connection topologies within each pair: (**a**) in series; (**b**) parallel. To calculate the touch voltage corresponding to a given current, the \(R_{t}\) versus \(U_{t}\) relationship was approximated using a function that proved suitable for all the cases included in the standard (see Figure 6):  
\[R_{t}=\exp(p_{4}\ (0.5\ -\text{atan}(p_{3}\ (\ln\ U_{t}\ -p_{2}))/\pi)+p_{1}). \tag{5}\]  
The constant parameters \(p_{1}\) through \(p_{4}\) were obtained by data fitting in Octave using the nonlin_curvefit function from the optim package.