High Flex

Pedro Reis, an instructor in MIT’s Department of Mathematics, did not have to go far to find a subject for his latest study. Strolling past the ­Edgerton Center in Building 4, Reis noticed tiny blisters forming in an MIT logo made from packing tape as it slowly peeled away from a glass door.

“It’s something that’s around you all the time–but if you look at it a different way you can see something new,” he says.

Reis and colleagues at the French National Centre for Scientific Research knew that different rates of heat-induced expansion between a thin film and the surface to which it is attached can cause them to separate, a process known as delamination. For example, sunlight can produce blisters in stickers attached to windows. And they knew that delamination can also occur when a surface is compressed; the film bends with the surface until the compressive forces become too large, causing it to pop away from the surface to form blisters.

To find out more about the mechanics of delamination caused by surface compression, the researchers compressed and stretched surfaces with thin films attached to them and measured the dimensions of the resulting blisters. Their analysis revealed that the formation, size, and evolution of the blisters depend on several factors: the elasticity of the sticker, the elasticity of the surface it’s stuck to, and the strength of adhesion between them.

Though delamination is usually something to be avoided, the researchers realized that they could use their findings to improve the stretchable electronics used in electronic paper and flexible displays. These flexible devices have proved difficult to engineer, because twisting tends to damage the electrical wiring. By partially separating the wires from the material through a carefully controlled delamination process, engineers could make stretchable electronics that won’t break as the substrate is stretched and twisted.

Others have tried to make stretchable electronics by using complex microfabrication techniques to create blisters between wires and surface materials. But this approach can sometimes force the blisters to become too large, leading to structural defects. The model that Reis and his colleagues developed can predict the correct blister size for materials with different characteristics, making it possible to build sturdier flexible materials.

With the new model, which the researchers described in the Proceedings of the National Academy of Sciences, engineers can vary the adhesive strength and elastic properties of surfaces and wires in a controlled way–making it easier to design bendable cell phones, for example, or clothing that incorporates electronic sensors.