This observation is still effective in a 180-nm-thick Ti film, but the average distance between adjacent secondary cracks is much larger than in the 80-nm-thick Ti film (Figure 3b). The secondary cracks finally disappear when the Ti film attains a 250-nm thickness (Figure 3c). The absence of secondary cracks is further supported by the LSM images (see Figure 3d,e). In actuality, the average crack width in the 250-nm Ti film was measured to be 0.88 μm, which corresponds to a 20% reduction from the 180-nm Ti film. These are because more stress is expended

in propagating cracks through Ti film for full development of the vertical cracks; thus the σ c becomes larger as the film thickness increases. In this respect, the film thickness dependence GSK690693 of cracking is qualitatively consistent with the strain-dependent cracking explained above. Figure 3 Optical microscope and LSM images of Ti films on PDMS Tozasertib purchase substrates at a strain of 50%. Optical microscope images of (a) 80 nm, (b) 180 nm, and (c) 250 nm on PDMS substrates at an identical strain of 50%. In (a, b, c), the straining direction and the directions of primary cracks

and secondary cracks are displayed. LSM images of (d) 180-nm and (e) 250-nm Ti films on PDMS substrates at the same strain (50%). Cracks in the 250-nm sample look narrower compared to the 180-nm sample. Scale bars are 50 μm for (a, b, Cell Cycle inhibitor c) and 10 μm for (d, e). All Ti films on PDMS substrates were transparent Farnesyltransferase in the measured Ti film thickness range of 80 to 250 nm. Figure 4a shows the transparency of flat 180-and 250-nm-thick Ti films on PDMS substrates at both zero strain and 30% strain. Furthermore, the Ti films

on PDMS substrates retained the transparency under the mixed stress state of bending and stretching, as shown in Figure 4b where a 250-nm-thick Ti film/PDMS sample was strained by 30% along the surface of a transparent cylinder with a radius of curvature of 11 mm. From these results, it is confirmed that Ti films on PDMS substrates are transparent irrespective of the strain state. The transparency of the Ti films on PDMS substrates offers a potential that they could be particularly considered for special applications such as flexible electronics, health monitoring, and transparent structure diagnostics. Figure 4 Photographs showing the transparency of Ti films on PDMS substrates. (a) Ti films with thicknesses of 180 nm (upper) and 250 nm (lower) on PDMS substrates at zero strain (left) and 30% strain (right) covering only half of the paper design underneath. (b) A 250-nm-thick Ti film on PDMS substrate wrapped around a transparent cylinder with a radius of curvature of 11 mm. Yellow dotted lines are drawn along the boundaries between the sample-overlaid areas and the bare areas. The resistances of the Ti films on PDMS substrates subjected to varying strains were measured by a simple two-probe method, using an ultrasensitive electrical characterization system.