## 3 Results and Discussion
### Characterization of PDMS Spiral-Shaped Microdevices  
Prior to analyze the performances of spiral-shaped microdevices for the capture of microRNAs, a morphological and chemical characterization was performed. Several microdevices were sectioned and measured at the Field Emission Scanning Electron Microscope (FESEM) (Figure 3) in order to check for the real dimensions of microdevice channels. For Chip A, channels were \(182\pm 13\)\(\upmu\)m wide and \(325\pm 88\)\(\upmu\)m deep, with a mean volume of 20 \(\upmu\)L, while for Chip B channels were \(60\pm 4\)\(\upmu\)m wide, \(197\pm 13\)\(\upmu\)m deep and 12 \(\upmu\)L as total volume. The two microdevices differed for their surface to volume ratio (S/V for Chip A is 18.0 mm\({}^{-1}\) and for Chip B is 46.2 mm\({}^{-1}\)).  
The chemical composition of the microdevice inner surfaces was also analyzed and compared to bulk PDMS (i.e., freshly-cut surfaces). During the replication process, in order to easily detach the PDMS, the mold was coated by a trichloromethylsilane anti-adhesion layer, which was deposited by a wet phase deposition. Since this reagent could contaminate the inner surface of channels of the PDMS replica, and possibly interfere with the downstream biological applications, a thorough XPS analysis and a PCR-compatibility assay were performed.  
It is indeed well known that materials composing microdevices and residues of reagents employed in their processing, can inhibit microdevice performances, in particular when specific amplification of nucleic acids is desired, as in this case [36; 37]. The presence of chlorine, deriving from trichloromethylsilane, was investigated via XPS. XPS survey spectra taken inside a microdevice channel and on bulk PDMS material (Figure 4) showed the presence of oxygen, carbon and silicon, as expected for PDMS, while no chlorine was detected. The quantification of elements, reported as relative elemental percentage in Table 1, revealed a similar composition for all the measured samples. Similar amount of elements are present both on inner channel surfaces and on bulk microdevice material, as well as on PDMS planar surface and no chlorine was detected in all the tested conditions. The overall elemental composition highlighted by XPS measurements reflects indeed the molecular composition of PDMS [(C\({}_{2}\)H\({}_{6}\)OSi)n], with C/O and C/Si ratios around 1.7 and 2.5, respectively, in agreement with the literature [17; 20; 38].  
Figure 3: FESEM images of microdevice A (Panel (**a**)) and B (Panel (**b**)). Microdevices were cut to expose microdevice channels. White bars represent 100 \(\upmu\)m.  
Beside XPS measurements, a PCR was performed in order to verify the compatibility of the microdevice material with an amplification reaction. Small pieces of microdevice exposing the channel surface were submerged in the PCR mix and the reaction was run. PCR products were loaded on an agarose gel (Figure S1) and compared with controls (both tubes without PDMS and tubes containing small pieces of a PDMS microdevice known for its compatibility with PCR [18; 19]). The microdevice internal surfaces were confirmed as perfectly compatible with the PCR reaction.