May 30 2019
With a broad range of energy, healthcare, and military applications, stretchable electronics are valued for their ability to be twisted, compressed, and conformed to irregular surfaces without compromising functionality.
Stretchable electronics are able to move in ways that mimic skin. (image credit: Justin Baetge/Texas A&M Engineering)
By employing the elasticity of polymers such as silicone, these upcoming technologies are fashioned to move in ways that imitate skin.
This offers insight into why Smooth-On Ecoflex, a substance most commercially used to develop molds, movie masks, and prosthetics, is the most extensively used silicone elastomer (a rubber-like material) in research.
While working with a sample of the material, Dr. Matt Pharr, assistant professor in the J. Mike Walker ’66 Department of Mechanical Engineering at Texas A&M University, and graduate student Seunghyun Lee, recently learned about a new kind of fracture.
“I have done some work in the area of stretchable electronics, so I have a lot of materials from when I was a postdoc. We had to store samples in our office and, likewise, I had some here because we were going to use them in a project that we ended up not doing. I’m a nervous fidgeter and while I was playing with it, I noticed something weird,” said Pharr.
This peculiarity is what Pharr and Lee talk about in their latest publication “Sideways and Stable Crack Propagation in a Silicone Elastomer” as sideways cracking. This occurrence is when a fracture branches from the edge of a crack and spreads perpendicular to the original tear.
Their findings not only offer a new, original perspective on the development of factures and how to boost stretchability in elastomers, but also form the basis for more tear- and fracture-resistant materials.
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. Usually, when people think about fracture of a given material, they’re not thinking about fracture resistance being different based on direction.
Dr. Matt Pharr, Assistant Professor, J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University
This conceptualization, however, is important to innovation and progress in stretchable electronics.
As Pharr explained, after loading, polymers having incisions are susceptible to be ripped apart from one end to another. However, materials that display sideways cracking prevent the fracture from deepening. Rather, the incision just expands alongside the rest of the elastomer and ultimately, once stretched enough, resembles nothing more than a small dent in the surface of the material — negating additional threat from the primary crack.
This allows the undamaged area of an elastomer to preserve its load-bearing and functional properties, all while boosting stretchability.
By examining how to reverse engineer microstructures that result in sideways cracking, scientists can harness the advantages related to it and develop application techniques to materials that do not generally display such fractures. This would bring about improved fracture resistance in the very thin layers of elastomers used in stretchable electronics, as well as superior stretchability — both of which are crucial to the progress and future usability of such technologies.
To me, this is scientifically intriguing. It’s not expected. And seeing something that I don’t expect always sparks curiosity. (The material) is literally sitting in a drawer in my desk and this was all inspired by playing around.
Dr. Matt Pharr, Assistant Professor, J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University