If you’ve ever gotten a cut on your elbow or knee, or even on the joint of a finger, you know it’s a nightmare to try to keep those cuts bandaged. No matter how secure the bandage seems at first, it often pops right off the first time you bend the affected joint. It’s incredibly frustrating, and doesn’t bode well for quick healing. But a group of researchers at MIT has developed a bandage that actually stays on, even on joints like the knee. It sounds like a miracle, right? But it’s science!
Many 3D printed breakthroughs have been inspired by the art of origami, but MIT’s bandage, created via 3D printed molds, owes its design to kirigami, an art similar to origami that involves cutting patterns into the paper before folding it. The film of the bandage has a series of slits cut into it so that when it bends, it stretches rather than stiffening and detaching. The MIT researchers attached their “kirigami film” to a volunteer’s knee and found that whenever she bent her knee, the film’s slits opened in the center, releasing tension, while remaining closed at the sides, keeping the film bonded to the skin. The bandage stayed in place for more than 100 bends.
The researchers demonstrated multiple applications by creating not only a kirigami adhesive bandage but a heating pad consisting of kirigami film threaded with heating wires. With the application of a three-volt power supply, the heating pad retains a temperature of 100 ºF. They also created a wearable electronic film with light-emitting diodes. All of the designs demonstrated the same ability to stay in place as the bandage did.
“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. I think kirigami film is one solution to this problem commonly found in adhesives and soft electronics,” said postdoctoral researcher Ruike Zhao.
In 2016, Zhao and colleagues were approached by a Chinese medical supply company that asked them to develop an improved version of its popular pain-relieving bandage.
“Adhesives like these bandages are very commonly used in our daily life, but when you try to attach them to places that encounter large, inhomogenous bending motion, like elbows and knees, they usually detach. It's a huge problem for the company, which they asked us to solve,” said Zhao.
The researchers began looking into kirigami, which some scientists have been considering as a method of developing new functional materials.
“In most cases, people make cuts in a structure to make it stretchable. But we are the first group to find, with a systematic mechanism study, that a kirigami design can improve a material's adhesion,” said Zhao.
The team 3D printed molds with rows of offset grooves of different settings, and filled them with a liquid elastomer. Once the elastomer cured, the sheets were lifted out of the molds to display rows of offset slits. The film, according to the researchers, can be made out of a variety of materials from soft polymers to hard metals.
To find out why kirigami adds to the adhesive qualities of a bandage, the team bonded the film to a polymer surface and then subjected it to stress tests. They measured the amount of stretch a kirigami film can withstand before peeling away; the results varied within a single piece of film. When pulled from either end, the slits in the middle were the first to peel open, while those at the end remained closed and stuck to the underlying surface.
Zhao identified the three main parameters that give kirigami films their adhesive properties: shear-lag, in which shear deformation reduces the strain on other parts of the film; partial debonding, in which the film segments around an open slit maintain a partial bond to the underlying surface; and inhomogenous deformation, in which a film can maintain its overall adhesion even as parts of its underlying surface bend and stretch. Depending on the application, Zhao says that scientists can use this information to design the best pattern of cuts and the optimal balance of parameters.
“These three parameters will help guide the design of soft, advanced materials. You can always design other patterns, just like folk art,” she said. “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.”
Zhao and her colleagues have filed a patent for their technique and are continuing to work with the medical supply company, which is making plans to manufacture medicine patches using kirigami design. The team is now looking to use the technique on different materials.
“The current films are purely elastomers,” said Zhao. “We want to change the film material to gels, which can directly diffuse medicine into the skin. That's our next step.”
The research is published in a paper entitled “Kirigami enhances film adhesion,” which you can access here. Authors include Ruike Zhao, Shaoting Lin, Hyunwoo Yuk and Xuanhe Zhao.
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