# A Review on the Deformation Behavior of Silver Nanowire Networks under Many Bending Cycles
## 2 Concept of a Cyclic Bending Tester
The superior mechanical reliability of the silver nanowire network is derived from two factors. In the case of the metal thin films, dislocations accumulate on the interface between the plane and the thin film, creating the defect where the strain is concentrated and ultimately causing fatigue damage as shown in Figure 3[32; 33]. However, according to the size-dependent plasticity, the creation of dislocation is difficult in a nanowire network due to the small diameter of nanowires. Furthermore, even if dislocation is created, it easily moves to the nanowire surface, causing dislocation starvation states in the nanowire [34; 35]. Due to these unique characteristics of nanowires, nanowire networks exhibit a high mechanical strength that is close to the theoretical value, referring to where the silver nanowire itself is more resistant to stress than bulk silver [36; 37]. In addition, the dislocations do not accumulate at the interface between the substrate and the silver nanowire network; thus, unlike the silver thin film, cracks that result from the accumulation of dislocations at the interface are less likely to happen for a silver nanowire network, resulting in less increase in resistance [32]. The silver thin film with a thickness below the diameter of a single nanowire, approximately \(<\)35 nm, will show less increase in the resistance under cyclic bending than those with 100 nm thickness. Because the fatigue failure in the thin film is initiated by the accumulation of dislocations at the film-substrate interfaces, which are avoidable in the silver nanowire network, the silver nanowire is expected to still have better mechanical reliability than the silver thin film even with the thickness equal to the diameter of a single nanowire. However, the confirmation of the mechanical reliability of the silver thin film with a few tens of nanometers will be an interesting topic for future research. Furthermore, the network structure itself contributes to the high mechanical reliability. As Figure 4 shows, when tensile strain is applied, the network structure deforms. As suggested in Figure 4[38], which is a COMSOL (COMSOL Inc., Stockholm, Sweden) numerical simulation of the deformation of a silver nanowire network, tensile deformation is accommodated by the network structure, which results in large deformability. Despite the number of junctions existing between nanowires diminishing and the wires elongating, the network structure is retained and there are sufficient junctions to form an electrical percolation pathway. Furthermore, the stress applied to individual nanowires dissipates and fewer cracks are created by the deformation of the network itself. As a result, it minimizes the increase in the resistance of the silver nanowire network. Nevertheless, a silver nanowire network shows increased resistance when it is subjected to repeated deformations. The reasons behind this increase in resistance can be explained through microstructural analysis. Silver nanowires are characterized by a polyvinylpyrrolidone (PVP) coating as a result of their fabrication process [39; 40]. This thin polymer layer increases the interfacial resistance between the individual nanowires. Therefore, to reduce the interfacial resistance, a thermal annealing process for high electrical conductivity is required. Figure 5a shows that silver nanowires adhere with other silver nanowires and form thermal junctions during the thermal annealing process at 150-200 \({}^{\circ}\)C. Interestingly, through repetitive bending, cracks were created at junctions as a result of high stress concentration, and the crack propagation resulted in the failure of the silver nanowires, as shown in Figure 5b. As shown in Figure 5c,d, failure in the silver nanowire junctions caused the failure of nearby junctions, resulting in a serious increase in the resistance as the number of bending cycles increased.