## 3 Results
### Electromechanical Characterization  
Taking advantage of the conformability of the ultra-thin Parylene substrates, the fabricated OFETs can be employed in tattoo electronics applications. The first fundamental property for a tattoo electronic element is the mechanical stability. Indeed, thanks to their ultralow thickness, Parylene substrates conformably adhere to micrometric \"ridged\" features of epidermal crests, thus possibly being subjected to significant surface strain values. The surface strain has the effect of modifying the morphology of the organic semiconductor layer [40; 41], thus determining a variation in the electrical properties of the device, in particular the charge carrier mobility. Surface strain is directly proportional to device thickness, according to the relationship \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Substrate**} & \multirow{2}{*}{**Threshold Voltage**} & **Charge Carrier Mobility** & **Leakage Current** & **Subthreshold** \\  & **[V]** & **[cm\({}^{2}\)V\({}^{-1}\)s\({}^{-1}\)]** & **[A]** & **Slope [Vdec\({}^{-1}\)]** \\ \hline
175 μm-thick PET & \(-0.1\pm 0.2\) & \(0.25\pm 0.08\) & \((1.2\pm 0.08)\times 10^{-12}\) & \(0.13\pm 0.03\) \\
700 nm-thick Parylene C & \(0.0\pm 0.1\) & \(0.13\pm 0.04\) & \((1.0\pm 0.8)\times 10^{-12}\) & \(0.28\pm 0.03\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Average and 1-\(\sigma\) confidence band for the basic electrical parameters of OFETs fabricated on PET (evaluated on 50 devices) and Parylene C substrates (evaluated on 30 devices). Figure 3: (**a**) 1000 double-sweep transfer characteristic curves acquired in saturation regime at VDS = \(-5\) V; (**b**) threshold voltage (black, left axis) and mobility (red, right axis) absolute variations extrapolated by the square root of curves in (**a**). \[\epsilon\%\ =\ \left[\frac{1\ +\ 2\eta+\chi\eta^{2}}{(1+\eta)(1+\chi\eta)}\right] \tag{2}\]  
where \(\mathrm{t_{sub}}\) and \(\mathrm{t_{d}}\) are substrate and device thickness, respectively, \(\eta\) = \(\mathrm{t_{d}}\)/\(\mathrm{t_{sub}}\) and \(\chi\) = Y\(\mathrm{_{d}}\)/Y\(\mathrm{_{sub}}\), being Y\(\mathrm{_{d}}\) and Y\(\mathrm{_{sub}}\) the [PERSON]'s moduli of device and substrate, respectively, and R is the bending radius. It is noteworthy that, as strain is directly proportional to the overall device thickness (\(\mathrm{t_{sub}}\) + \(\mathrm{t_{d}}\)), the strain produced by a certain deformation on a sub-micrometrical OFET can be very low even for low values of R. In order to demonstrate that device performances are substantially stable with bending, ultra-thin OFETs have been bent at different bending radii (R = 3.5, 3, 2.5, 1.5, 1, 0.8, 0.5, 0.1 cm), both in extension and in compression. In extension, a tensile strain is applied on the organic semiconductor, and this would bring a reduction in charge carrier mobility due to the displacement of crystalline domains in the film. On the contrary, when the device is compressed, the local density of crystalline domains in the organic semiconductor film increases, thus bringing an increase in the charge carrier mobility. Results are reported in Figure 4: in Figure 4a, relative charge carrier mobility variations with the applied bending radii are reported, calculated as (\(\mathrm{\mu_{b}}\) - \(\mathrm{\mu_{f}}\))/\(\mathrm{\mu_{f}}\) (\(\mathrm{\mu_{b}}\) and \(\mathrm{\mu_{f}}\) are charge carrier mobility values in bent and flat conditions, respectively). In both extension and compression, average fluctuations of mobility values in the range of 2% were recorded, without any evident trend with applied deformation in both cases, thus demonstrating that complete mechanical stability has been obtained. This is consistent with the fact that, although the significant deformation applied, the surface strain applied in both cases is very low (as high as 0.05%) as the whole device thickness is much lower than the applied deformation.