If you have ever put a Band-Aid on a child, you quickly realize it won’t last long either because the child is going to jump up and down enough to peel it off or rub it away quickly because it is uncomfortable.
But what if that same bandage could adhere to the skin and was flexible enough to move with limbs as they bend?
That’s what MIT engineers have been developing, in the form of a thin, lightweight, rubber-like film that can stick to highly deformable regions of the body — like the knee or elbow — and maintain its hold even after 100 bending cycles.
The key to the toughness of the film is that the researchers cut a pattern of slits into the film, similar to the cuts made in a paper-folding art form known as kirigami. In testing, the researchers found that in a volunteer’s knee that each time they would bend their knee, the film’s slits opened at the center in the region of the knee with the most pronounced bending, while the slits at the edge remained closed, remaining bonded to the skin.
This allowed the film to not just stretch but get a better grip. The cuts that open release tension that would otherwise cause the entire film to peel away from the skin.
Researchers fabricated an adhesive bandage as well as a head pad consisting of a kirigami film threaded with heating wires. The group also engineered a wearable electronic film outfitted with light emitting diodes that stick to the skin.
MIT believes this type of kirigami-patterned adhesives could lead to a whole new type of products from everyday medical bandages to wearable and soft electronics.
“Currently in the soft electronics field, people mostly attach devices to regions with small deformations, but not in areas with large deformations such as joint regions, because they would detach,” said Ruike Zhao, a postdoctoral student in MIT’s Department of Mechanical Engineering. “I think kirigami film is one solution to this problem commonly found in adhesives and soft electronics.”
How They Did It
The MIT team fabricated thin kirigami films by pouring a liquid elastomer into 3D printed molds and each mold was printed with rows to offset grooves by various spacings, which the researchers then filled with the rubber solution. Once cured and lifted out of the molds, the rubber layers were studded with rows of offset slits. Researchers say the film can be made from a wide range of materials from soft polymers to hard metal sheets.
The team then applied a thin, adhesive coating -- similar to what is applied to bandages -- to each film before attaching it to a volunteer’s knee. Compared to an elastomer film with no kirigami patterns, the new film stayed stuck to the knee after repeated bending. After one cycle of bending, the plain continuous film quickly detached while the kirigami film maintained its hold after 100 knee bends.
Through testing the MIT team found three parameters that give kirigami films their adhesive properties: shear-lag, in which shear deformation of film can reduce the strain on other parts of the film, partial debonding, where the film segments around an open slit maintains a partial bond to the underlying surface and inhomogenous deformation where the film maintains its overall adhesion, even as parts of its underlying surface my bend and stretch more than others.
“These three parameters will help guide the design of soft, advanced materials,” Ruike said. “You can always design other patterns, just like folk art. There are so many solutions that we can think of. Just follow the mechanical guidance for an optimized design, and you can achieve a lot of things.”
Future testing will explore other materials where the kirigami cuts could be useful.
“The current films are purely elastomers,” Ruike said. “We want to change the film material to gels, which can directly diffuse medicine into the skin. That’s our next step.”
The full research can be found in the journal Soft Matter.