# A Review on the Deformation Behavior of Silver Nanowire Networks under Many Bending Cycles
## 2 Concept of a Cyclic Bending Tester  
Figure 1 shows a schematic of a cyclic bending tester. For cyclic bending tests, a silver nanowire network coated on a flexible substrate is fixed between the upper and bottom plates with metal screws. Strain is applied homogeneously across the sample by controlling the distance between the two plates. The strain applied to the sample is determined by the equation \(\varepsilon=y/R\), where \(\varepsilon,y\), and \(R\) represent the strain, the distance from the neutral plane, and the curvature, respectively. The neutral plane and the degree of deformation are determined by the thickness of the sample and the distance between the upper and lower planes, respectively. The bending strain in the sample is induced by moving the lower plate periodically from left to right. In addition, by controlling the displacement of the lower plane, the area to which the strain is applied is determined. Due to these features of a cyclic bending tester, samples can be sectioned into three different zones: A, B, and C. Zone A refers to the strain-free region, zone B refers to the region in which repetitive strain is applied, and finally, zone C refers to the region in which constant strain exists. Since zone A is strain-free and zone C experiences constant strain, the test results for these two zones are negligible. Therefore, in the cyclic bending test, the actual result is primarily drawn in zone B where repetitive and periodic strain is applied to samples. As mentioned above, the areas of zone B and zone C are determined by the displacement of the lower plane. The edges of the upper and lower plates where the sample is fixed are covered with an electrically conductive material, such as copper or silver. Thus, fluctuations in the electric signal during cyclic bending can be collected in real time [28,29]. In summary, a cyclic bending tester can vary the speed and degree of the deformation and the area of the sample to which strain is applied. In addition, a fatigue tester can apply strain to a sample over 1,000,000 times while precisely measuring the electric signal depending on the bending cycles. Thus, various mechanical reliability tests with different deformation conditions are possible using a cyclic bending tester. Deformation Behavior and Mechanical Reliability of Silver Nanowire Transparent Electrodes under Many Bending Cycles  
Figure 2a shows the change in the resistance of the silver nanowire network under repetitive deformation. The x- and y-axes indicate the number of bending cycles and the increase in resistance, compared to the initial resistance, respectively. Figure 2b shows the same test results for silver thin films for comparison. The initial resistance of the silver nanowire network was 3.2-3.6 ohm/sq and the strain applied to the sample varied at 1-4%. As more strain was applied to the samples, the change in resistance of the silver nanowire network increased accordingly. However, the resistance change in the silver nanowire network was \(<\)20% after 500,000 cycles, even  
Figure 1: Schematic illustration of a cyclic bending tester. Reproduced with permission from ref. [28]. Copyright 2014, Wiley. Figure 2: Resistance change over 500,000 bending cycles for (**a**) silver nanowire networks and (**b**) silver thin films with 1% strain, and (**c**) rescaled image (**a**) within 2000 cycles. Reproduced with permission from ref. [28]. Copyright 2014, Wiley. with 4% strain, which was much less than that of the silver thin film or ITO film [30; 31]. In the case of silver thin films with 100 nm of thickness, the resistance increased up to \(\sim\)80% under only 1% strain, as shown in Figure 2b. In addition, for ITO, the increase in resistance was known to be hundreds and thousands of percent, under even 1% strain [31]. Given the fact that failure of electronics happens when the resistance change exceeds 20% of the initial resistance, the silver nanowire network showed superior mechanical reliability and potential for use in real industry.