Kirigami inspires better bandages

The key to the film’s clinginess is a pattern of slits that the researchers have cut into the film, similar to the cuts made in a paper-folding art form known as kirigami.

The researchers attached the “kirigami film” to a volunteer’s knee and found that each time she bent her 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 edges remained closed, allowing the film to remain bonded to the skin.

The kirigami cuts give the film not only stretch, but also better grip: The cuts that open release tension that would otherwise cause the entire film to peel away from the skin.

To demonstrate potential applications, the group fabricated a kirigami-patterned adhesive bandage, as well as a heat pad consisting of a kirigami film threaded with heating wires. With the application of a 3V power supply, the pad maintains a steady temperature of 100ºF.

The group has also engineered a wearable electronic film outfitted with light-emitting diodes. All three films can function and stick to the skin, even after 100 knee bends.

Ruike Zhao, a postdoc in MIT’s Department of Mechanical Engineering, says kirigami-patterned adhesives may enable a whole swath 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,” Ruike says. “I think kirigami film is one solution to this problem commonly found in adhesives and soft electronics.”

Ruike is the lead author of a paper published online this month in the journal Soft Matter. Her co-authors are graduate students Shaoting Lin and Hyunwoo Yuk, along with Xuanhe Zhao, the Noyce Career Development Professor in MIT’s Department of Mechanical Engineering.

In August 2016, Ruike and her colleagues were approached by representatives from a medical supply company in China, who asked the group to develop an improved version of a popular pain-relieving bandage that the company currently manufactures.

“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,” Ruike says. “It’s a huge problem for the company, which they asked us to solve.”

The researchers stretched kirigami films and measured their “energy release rate,” or the critical amount of stretch a film can handle before peeling away from its surface. (Credit: MIT)

The team considered kirigami as a potential solution. Originally an Asian folk art, kirigami is the practice of cutting intricate patterns into paper and folding this paper, much like origami, to create beautiful, elaborate three-dimensional structures. More recently, some scientists have been exploring kirigami as a way to develop new, functional materials.

“In most cases, people make cuts in a structure to make it stretchable,” Ruike says. “But we are the first group to find, with a systematic mechanism study, that a kirigami design can improve a material’s adhesion.”

The researchers fabricated thin kirigami films by pouring a liquid elastomer, or rubber solution, into 3D-printed molds. Each mold was printed with rows of offset grooves of various spacings, which the researchers then filled with the rubber solution.

Once cured and lifted out of the molds, the thin elastomer layers were studded with rows of offset slits. The researchers say the film can be made from a wide range of materials, from soft polymers to hard metal sheets.

Ruike applied a thin­­­ adhesive coating, similar to what is applied to bandages, to each film before attaching it to a volunteer’s knee. She took note of each film’s ability to stick to the knee after repeated bending, compared with an elastomer film that had no kirigami patterns. After just one cycle, the plain, continuous film quickly detached, whereas the kirigami film maintained its hold, even after 100 knee bends.

To find out why kirigami cuts enhance a material’s adhesive properties, the researchers first bonded a kirigami film to a polymer surface, then subjected the material to stretch tests.

They measured the amount of stretch a kirigami film can undergo before peeling away from the polymer surface — a measurement they used to calculate the material’s critical “energy-release rate,” a quantity to evaluate detaching.

They found that this energy-release rate varied throughout a single film: When they pulled the film from either end like an accordion, the slits toward the middle exhibited a higher energy-release rate and were first to peel open under less stretch. In contrast, the slits at either end of the film continued to stick to the underlying surface and remained closed.

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Image credit: MIT.