## Materials and Methods
### OPS and LTM Experimental Setups
An electric protection circuit, equipped with a fuse, has also been connected in series, between the power supply and the DUT, to cautiously prevent the passage of a current exceeding 200 mA, which could compromise the device. Two micropositioners have been used (Figure 3) and the microgripper angular displacements have been measured by means of acquired images and collected by a NB50 TS Eurotek trinocular microscope system. The optical resolution is limited by the diffraction of the visible light and it is about 0.45-0.6 \(\upmu\)m, as in conventional light microscopes [46]; in this study, the worst case of 0.6 \(\upmu\)m has been considered. The pixel resolution has been provided by means of calibration procedure; by means of Matlab software, a length of 1000 \(\upmu\)m on an image of a micrometer slide was considered and 15 tests were performed. By means of this procedure, the pixel density of \(1.359\pm 0.007\) pixel/\(\upmu\)m was calculated. Furthermore, the sampling constraints (Nyquist limit) and the density of the photo sites on the digital sensor also limit the whole systems resolution. To achieve a _higher level_ particle characterization (i.e., differentiation based upon higher order measurements such as circularity), the size of the particle or the object under examination must be greater than 4 \(\upmu\)m [46]. In our case, in fact, it is possible to verify that it is impossible to resolve the comb-drive finger gap, that is 3 \(\upmu\)m. However, the distance between two fingers, that is 10 \(\upmu\)m, is clearly discriminated (see also Ref. [27]); for all these reasons, a overall resolution of about 4 \(\upmu\)m for  
Figure 3: The two micropositioners with probe arms and tungsten needles. Figure 2: The two micropositioners with probe arms and tungsten needles (_a_) and (_b_), used in both the experimental setups; a voltage is applied to the comb-drive (_c_) of the DUT, in order to supply the device. the LTM system is assumed. In Figure 4, the entire LTM setup is shown and some specifications of its main components are reported in Table 2. Once the device has been positioned on the DUT stage, it has been powered by means of two probes with tungsten needles. Through the micropositioners, the tungsten needles are approached to the microgripper electrical connections. Considering the OPS measurement chain, a set of images has been acquired, each one corresponding to a specific voltage setting: to calculate the angular displacement of the comb-drive, the first acquired image referred to 0 V, has been compared with the others (i.e., 2 V, 4 V,..., 28 V); Figure 5a illustrates an example of an image that has been processed by the in-house built software. Considering the LTM measurement chain, the same steps have been carried out. However, in order to calculate the angular displacements, the first acquired image (0 V) has been compared with the other images referred to voltages up to 24 volts (i.e., 2 V, 4 V,..., 24 V) instead of 28 V (as in the previous case); Figure 5b shows an example of image that has been processed by the in-house software. Considering the OPS, a set of 12 images has been acquired at each voltage, for a total of 180 images; considering the LTM image acquisition system, a set of 16 images has been acquired at each voltage, for a total of 208 images. Figure 4: Optical Microscope measurement setup. Microgripper prototype (_a_), Supply voltage with protection circuit (_b_), micropositioners with two embedded probe arms and tungsten needles (_c_), Optical Microscope (_d_), embedded camera for images acquisition (_e_), a monitor for displaying and monitoring the device movement in operating conditions (_f_), instrumented support with micrometric screws (_g_), pneumatic suspension table (_l_). Figure 5: Two examples of images of the Microgripper comb-drive, acquired by optical profilometer (**a**), and optical microscope (**b**); both images (**a**,**b**) are referred to 0 V.  
The OPS experimental investigation was made before the LTM campaign.