## 3 Results and Discussions  
### Morphology and Characteristics of Silver Membrane  
The laser-scripted MS transmission line on the PI film is demonstrated in Figure 2a. The strip conductor was fabricated by using a continuous wave laser to irradiate the silver ink on a flexible PI sheet. The laser beam was tightly focused onto the sample surface to promote silver reduction and nucleation. The obtained conductor shows a metallic gray color, which implies the formation of a silver membrane. Figure 2b shows a microscope image of the silver membrane. The ribbon pattern of the silver membrane can be attributed to the line sweep stratagem for the laser patterning. The inset in Figure 2b shows a cross-sectional view of the silver membrane. The thickness of the produced silver membrane was kept around 4 \(\upmu\)m at a laser power above 37.5 mW, due to the sufficient laser power that was able to transfer the entire layer of silver ink to the silver membrane at the irradiated area. The thickness of the silver membrane was dominated by the thickness of the silver ink film ratherthan the laser power. Note that the silver membranes fabricated with laser power below 37.5 mW did not withstand the rinsing process. Therefore, we focused our investigations on laser power larger than 37.5 mW. Detailed investigations of the laser-scripted silver membrane as a function of laser power are shown in Figure 3. At a laser power where reduction of silver begins, the reduction of the silver ink can produce silver nanoparticles (Ag NPs) but is not sintered [7; 18]. At a laser power of 50 mW, necks between Ag NPs with an average size of 61 nm were observed, revealing that sintering of the Ag NPs took place. This sintering led to the formation of silver nanoclusters, as shown in Figure 1a,b. The increase of laser power to 100 mW (Figure 1c,d) led to a significantly smoother and more continuous silver membrane, indicating that both the sintering and melting of silver clusters took place during the laser irradiation. At a laser power of 150 mW (Figure 1e,f), a few silver nanospheres (typical diameter of approximately 250 nm) scattered on top of the silver membrane, revealing that a significant photo-thermal effect led to the melting of the silver membrane. Further increasing laser power to 200 mW (Figure 1g,h) damaged the PI substrate with flake-like porous carbon [8] distributed in the laser irradiation area. The silver membrane was largely ablated from the PI substrate, while a few silver spheres remained among the carbon flakes. The laser irradiation regimes ultimately turned to black. Laser carbonization of PI has been investigated previously [8; 11]. Here, we focused on the laser process of silver, since the design of the MS lines were all based on the silver membrane. From a mechanical prospective, laser reduction of the silver citrate with the presence of PVP has been analyzed in previous studies [18]. In our study, the mix of AgNO\({}_{3}\) to a mixed sodium citrate and PVP aqueous solution enabled an ion-exchange reaction that produced silver citrate. Thus, the obtained silver ink contained silver citrate, sodium nitrate, and PVP. During laser irradiation, the presence of PVP enabled silver reduction and nucleation within the synergy of the PVP\(-\)silver ion complex and produced small silver particles. Meanwhile, PVP capped on the surface of the Ag NPs through chemical and physical bonding [19]. This result produced two consequences. Firstly, the PVP present at the interfaces between Ag NPs and PI can serve as a binder to provide robust bonding between the silver membrane and the substrate [20]. Secondly, however, the thin PVP that presented at the interfaces between Ag NPs can restrain the sintering of Ag NPs. The direct interconnections of Ag NPs are limited, resulting in minor conduction passes for electrons passing through [20]. Laser irritation of the Ag NP membrane promoted sintering. When laser irradiated the Ag NPs, surface plasmon [21] was excited among Ag NPs and generated strong heat through absorption [22; 23].