With a wide range of healthcare, energy, and military applications, stretchable electronics are valued for their ability to be compressed, twisted and conformed to uneven surfaces without losing functionality. By using the elasticity of polymers such as silicone, these emerging technologies are made to move in ways that mimic skin. This sheds light on why Smooth-On Ecoflex, a substance most commercially used to create molds, movie masks, and prosthetics, is the most prominent silicone elastomer found in research.
While handling a sample of the material, Professor Matt Pharr and graduate student Seunghyun Lee, recently discovered a new type of fracture they call sideways cracking. This phenomenon is when a fracture branches from a crack tip and extends perpendicular to the original tear.
“Initially this material is isotopic, meaning it has the same properties in all directions. But once you start to stretch it, you cause some microstructural changes in the material that makes it anisotropic — different properties in all different directions,” said Pharr. This conceptualization is critical to innovation and advancement in stretchable electronics.
As Pharr explained, upon loading, polymers with incisions tend to be ripped apart from one end to another. However, materials that exhibit sideways cracking stop the fracture from deepening. Instead, the incision simply expands alongside the rest of the elastomer and eventually, once stretched enough, looks like nothing more than a small dent in the surface of the material — negating further threat from the original crack. This allows the unharmed section of an elastomer to retain its load-bearing and functional properties, all while increasing stretchability.
Going forward, by investigating how to reverse engineer microstructures that lead to sideways cracking, researchers can develop application methods to materials that do not normally exhibit such fractures. This would lead to better fracture resistance in the very thin layers of elastomers used in stretchable electronics, as well as greater stretchability — both of which are key to the advancement and future usability of such technologies.