# Investigation of Structural Stability for Monolithic Nano Bridges on Micro Apertures
## 3 Results and Discussion  
In order to handle and use the nano bridges as a mask or filters, supporting layers with micro-sized apertures for free-standing nano bridges are required. It is generally known that as the size of the apertures of the supporting layer gets wider or bigger, the extremely thin bridges get unstable, inducing collapse or fracturing of the bridges. Hence, a proper geometrical design for the fabrication of two-level multiscale polymeric membranes is needed. To investigate the mechanical stability of the monolithic nano bridges on the micro apertures, the diameter of the micro-pillars PDMS holds were varied, while the geometry of the nano-line was fixed. In this study, the micro-pillar PDMS holds with a diameter of 5 \(\upmu\)m, 10 \(\upmu\)m, 50 \(\upmu\)m, 100 \(\upmu\)m, 300 \(\upmu\)m and 500 \(\upmu\)m, and the nano-lines mold with a line width of 800 nm, were used to construct the two-level multiscale polymeric membrane. The spacing ratio (SR: spacing between pillars (or lines) divided by diameter (or line width)) of all the PDMS holds was the same. The scanning electron microscopy (SEM) images of six types of micro-pillars and one type of nano-lines PDMS mold are shown in Figure 1; the detailed information about the fabrication process of the patterned silicon master and PDMS holds is provided in the Materials and Methods section. The fabrication process of the two-level multiscale polymeric membrane is illustrated in Figure 2. The multiscale polymeric membranes were fabricated by using the UV-curable polyurethane acrylate (PUA) resin, micro-pillars PDMS mold and nano-lines PDMS mold. First, the PDMS resin was poured onto the Si master molds of the nano-lines and micro-pillars and cured at 70 \(\lx@math@degree\)C for 2 h to obtain the PDMS patterned molds [13]. Then, PUA pre-polymer was dispensed onto the micro-pillar PDMS mold and the nano-line PDMS mold was placed on it. The contact surface area between the nano-lines and  
Figure 1: (**a**) Definition of the structural features of the micro-pillar and nano-line structures. (**b**) SEM images of the nano-line patterns with a width of 800 nm, spacing of 800 nm and height of 400 nm. (**c**-**h**) SEM images of the diverse dimensions of the micro-pillar pattern PDMS mold. Note: diameter (D), spacing (S) and height (H) for (**c**) 5 \(\upmu\)m, 5 \(\upmu\)m and 5 \(\upmu\)m; (**d**) 10 \(\upmu\)m, 10 \(\upmu\)m and 6 \(\upmu\)m; (**e**) 50 \(\upmu\)m, 50 \(\upmu\)m and 20 \(\upmu\)m; (**f**) 100 \(\upmu\)m, 100 \(\upmu\)m and 120 \(\upmu\)m; (**g**) 300 \(\upmu\)m, 300 \(\upmu\)m and 120 \(\upmu\)m; and (**h**) 500 \(\upmu\)m, 500 \(\upmu\)m and 20 \(\upmu\)m.  
micro-pillars formed overlapped oxygen-infiltrated layers (OILs) due to the gas permeable property of the PDMS, which resulted in delayed UV curing under the ultraviolet irradiation at the contact area [7; 14; 15; 16]. The sandwiched layers (nano-lines PDMS mold-PUA resin-micro-pillars PDMS mold) were properly irradiated with UV light with an intensity of 20 W cm\({}^{-2}\) for 2 min. After the cured PUA membrane was detached from the PDMS mold, the membrane was treated with UV curing for 4 h to fully treat the partially cured resin on the surface. After the fabrication process, the free-standing two-level multiscale polymeric membrane, which includes the monolithic nano bridges over the micro apertures, was obtained, as shown in Figure 1d. The uniform iridescent color of the membrane indicates that the nano-lines were successfully constructed over the large area (\(\sim\)3 cm \(\times\) 3 cm).