Kirigami inspires better bandages27 March 2018, by Jennifer Chu

Small, “kirigami” slits in polymer film enable the materialto stick to the skin, even after 100 knee bends,compared to the same film without slits, which debondsafter just one bending cycle. Credit: MassachusettsInstitute of Technology

Scraped up knees and elbows are tricky places tosecurely apply a bandage. More often than not, theadhesive will peel away from the skin with just afew bends of the affected joint.

Now MIT engineers have come up with a stickiersolution, in the form of a thin, lightweight, rubber-like film. The adhesive film can stick to highlydeformable regions of the body, such as the kneeand elbow, and maintain its hold even after 100bending cycles. The key to the film's clinginess is apattern of slits that the researchers have cut intothe film, similar to the cuts made in a paper-foldingart form known as kirigami.

The researchers attached the "kirigami film" to avolunteer's knee and found that each time she benther knee, the film's slits opened at the center, inthe region of the knee with the most pronouncedbending, while the slits at the edges remainedclosed, allowing the film to remain bonded to theskin. The kirigami cuts give the film not only

stretch, but also better grip: The cuts that openrelease tension that would otherwise cause theentire film to peel away from the skin.

To demonstrate potential applications, the groupfabricated a kirigami-patterned adhesive bandage,as well as a heat pad consisting of a kirigami filmthreaded with heating wires. With the application ofa 3-volt power supply, the pad maintains a steadytemperature of 100 degrees Fahrenheit. The grouphas also engineered a wearable electronic filmoutfitted with light-emitting diodes. All three filmscan function and stick to the skin, even after 100knee bends.

Ruike Zhao, a postdoc in MIT's Department ofMechanical Engineering, says kirigami-patternedadhesives may enable a whole swath of products,from everyday medical bandages to wearable and soft electronics.

"Currently in the soft electronics field, people mostlyattach devices to regions with small deformations,but not in areas with large deformations such asjoint regions, because they would detach," Ruikesays. "I think kirigami film is one solution to thisproblem commonly found in adhesives and softelectronics."

Ruike is the lead author of a paper published onlinethis month in the journal Soft Matter. Her co-authors are graduate students Shaoting Lin andHyunwoo Yuk, along with Xuanhe Zhao, the NoyceCareer Development Professor in MIT'sDepartment of Mechanical Engineering.

Adhesion from an art form

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The researchers stretched kirigami films and measuredtheir “energy release rate,” or the critical amount ofstretch a film can handle before peeling away from itssurface. Credit: Massachusetts Institute of Technology

In August 2016, Ruike and her colleagues wereapproached by representatives from a medicalsupply company in China, who asked the group todevelop an improved version of a popular pain-relieving bandage that the company currentlymanufactures.

"Adhesives like these bandages are very commonlyused in our daily life, but when you try to attachthem to places that encounter large, inhomogenousbending motion, like elbows and knees, theyusually detach," Ruike says. "It's a huge problemfor the company, which they asked us to solve."

The team considered kirigami as a potentialsolution. Originally an Asian folk art, kirigami is thepractice of cutting intricate patterns into paper andfolding this paper, much like origami, to createbeautiful, elaborate three-dimensional structures.More recently, some scientists have been exploring

kirigami as a way to develop new, functionalmaterials.

"In most cases, people make cuts in a structure tomake it stretchable," Ruike says. "But we are thefirst group to find, with a systematic mechanismstudy, that a kirigami design can improve amaterial's adhesion."

The researchers fabricated thin kirigami films bypouring a liquid elastomer, or rubber solution, into3-D-printed molds. Each mold was printed withrows of offset grooves of various spacings, whichthe researchers then filled with the rubber solution.Once cured and lifted out of the molds, the thinelastomer layers were studded with rows of offsetslits. The researchers say the film can be madefrom a wide range of materials, from soft polymersto hard metal sheets.

Ruike applied a thin adhesive coating, similar towhat is applied to bandages, to each film beforeattaching it to a volunteer's knee. She took note ofeach film's ability to stick to the knee after repeatedbending, compared with an elastomer film that hadno kirigami patterns. After just one cycle, the plain,continuous film quickly detached, whereas thekirigami film maintained its hold, even after 100knee bends.

A balance in design

To find out why kirigami cuts enhance a material'sadhesive properties, the researchers first bonded akirigami film to a polymer surface, then subjectedthe material to stretch tests. They measured theamount of stretch a kirigami film can undergobefore peeling away from the polymer surface—ameasurement they used to calculate the material'scritical "energy-release rate," a quantity to evaluatedetaching.

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Ruike Zhao, a postdoc in MIT’s Department ofMechanical Engineering, says kirigami-patternedadhesives may enable a whole swath of products, fromeveryday medical bandages to wearable and softelectronics. Credit: Massachusetts Institute ofTechnology

They found that this energy-release rate variedthroughout a single film: When they pulled the filmfrom either end like an accordion, the slits towardthe middle exhibited a higher energy-release rateand were first to peel open under less stretch. Incontrast, the slits at either end of the film continuedto stick to the underlying surface and remainedclosed.

Through these experiments, Ruike identified threemain parameters that give kirigami films theiradhesive properties: shear-lag, in which sheardeformation of film can reduce the strain on otherparts of the film; partial debonding, in which the filmsegments around an open slit maintain a partialbond to the underlying surface; and inhomogenousdeformation, in which a film can maintain its overalladhesion, even as parts of its underlying surfacemay bend and stretch more than others.

Depending on the application, Ruike saysresearchers can use the team's findings as adesign blueprint to identify the best pattern of cutsand the optimal balance of the three parameters,for a given application.

"These three parameters will help guide the design

of soft, advanced materials," Ruike says. "You canalways design other patterns, just like folk art.There are so many solutions that we can think of.Just follow the mechanical guidance for anoptimized design, and you can achieve a lot ofthings."

Ruike and her colleagues have filed a patent ontheir technique and are continuing to collaboratewith the medical supply company, which is currentlymaking plans to manufacture medicine patchesmade from kirigami films.

"They make this pain-relieving pad that's prettypopular in China—even my parents use it," Ruikesays. "So it's super exciting."

The team is now branching out to explore othermaterials on which to pattern kirigami cuts.

"The current films are purely elastomers," Ruikesays. "We want to change the film material to gels,which can directly diffuse medicine into the skin.That's our next step."

More information: Ruike Zhao et al. Kirigamienhances film adhesion, Soft Matter (2018). DOI:10.1039/C7SM02338C

This story is republished courtesy of MIT News(web.mit.edu/newsoffice/), a popular site thatcovers news about MIT research, innovation andteaching.

Provided by Massachusetts Institute ofTechnology

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APA citation: Kirigami inspires better bandages (2018, March 27) retrieved 5 July 2018 from https://phys.org/news/2018-03-kirigami-bandages.html

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