To stereoscopically investigate the patterns and sizes of the cracks at the smaller scale, the samples were three-dimensional (3D)-scanned using a 3D laser scanning microscope (Olympus CLS 4000). In addition, scanning electron microscopy (SEM, Hitachi S4800, Hitachi High-Tech, Tokyo, Japan) was utilized to closely observe individual cracks. The resistances of the cracked Ti films on PDMS substrates were measured by a simple two-probe method, using a probe station connected to a high-resolution, multi-purpose electrical characterization system (Keithley 4200-SCS, Keithley Instruments Inc., Cleveland, OH, USA). The Bucladesine extremely high-resolution system enabled to detect a femto-ampere-level

current and to measure a resistance of more than 1 TΩ. The resistance was monitored not only under normal tension, but it also measured under non-planar straining along a curved surface. Results and discussion

Figure 2a,b,c,d,e,f shows optical microscope images of a 180-nm-thick Pd selleck products film on the PDMS substrate, which were obtained under a tensile strain of 0% (Figure 2a), 10% (Figure 2b), 30% (Figure 2c), 50% (Figure 2d), 80% (Figure 2e), and after strain relaxation (Figure 2f). Here, the strain is a length change normalized to the original length, which is simply expressed as ϵ = (L- L 0)/L 0 × 100%, with L 0 and L being the original length and the length under a strain, respectively. It is found from Figure 2a that fine ripples exist on the surface of the Ti film, presumably coming from the small residual strain of the PDMS substrate underneath. Upon applying a 10% strain, cracks begin to form in the direction

perpendicular to the straining direction while buckling occurs at the same time due to the compressive stress acting perpendicularly to the direction of the tensile stress, as shown in Figure 2b. Based on the previous research, the cracks are initiated from the surface of PDMS substrate because the originally soft PDMS surface is modified to a silica-like hard surface during metal sputtering [15]. Once the cracks are initiated at the Ti/PDMS Urease interface, they are supposed to propagate through the Ti film, but the most applied stress is likely to be consumed for PDMS surface cracking at low-strain levels. This is why the crack patterns are not very clear at 10% strain. The cracks become clearer as the strain level increases. This is confirmed by the images shown in Figure 2c,d,e. Wee1 inhibitor Interestingly, the secondary crack patterns that are tilted by certain angles from the vertically formed first cracks begin to appear from a 30% strain. The tilting angle becomes larger with increasing strain (21° to 41° in the strain range of 30% to 80%), reaching an angle of 49° between the crack lines and the straining direction at an 80% strain (Figure 2e).