A couple of interesting papers from PNAS:
D Vella et al
The wrinkling and delamination of stiff thin films adhered to a polymer substrate have important applications in “flexible electronics.” The resulting periodic structures, when used for circuitry, have remarkable mechanical properties because stretching or twisting of the substrate is mostly accommodated through bending of the film, which minimizes fatigue or fracture. To date, applications in this context have used substrate patterning to create an anisotropic substrate-film adhesion energy, thereby producing a controlled array of delamination “blisters.” However, even in the absence of such patterning, blisters appear spontaneously, with a characteristic size. Here, we perform well-controlled experiments at macroscopic scales to study what sets the dimensions of these blisters in terms of the material properties and explain our results by using a combination of scaling and analytical methods. Besides pointing to a method for determining the interfacial toughness, our analysis suggests a number of design guidelines for the thin films used in flexible electronic applications. Crucially, we show that, to avoid the possibility that delamination may cause fatigue damage, the thin film thickness must be greater than a critical value, which we determine.
J A Rogers and Y Huang
Electronics of the future will be soft and rubbery. Devices based on this new technology will be stretchable, twistable, and deformable into curvilinear shapes, thereby enabling applications that would be impossible to achieve by using the hard, rigid electronics of today. Examples range from ultrathin, conformable health-monitoring tapes that seamlessly mount on the skin, “electronic skin,” to advanced imaging devices that use hemispherical detector layouts, “electronic eyeball cameras.” In these and other systems, mechanical design will be as important as circuit design. Vella et al. [see above] in this issue of PNAS introduce simple schemes to measure the mechanical properties of structures for stretchable electronics and they describe powerful analytical approaches that capture the underlying physics.