Researchers at MIT have developed a kirigami-inspired adhesive film designed to stick to highly deformable areas of the body, such as the elbow and knee. The potential applications of this innovative kirigami film range from bandages to wearables.
The utility of single precise cuts
Low adhesion and grip failures are found in available current medical bandages, or wearable soft electronic devices, when used in body regions subjected to high deformability. Inspired by the kirigami clean-cut patterns, researchers at Massachusetts Institute of Technology (MIT) applied this technique to create a more stretchable material with significant improvement on the adhesion. They developed a lightweight, rubber-textured film that uses a kirigami pattern cut to create an effective adhesive bandage solution. This new development is especially useful, when applied on joints, such as the elbow or knee. Since these regions are highly deformable, as they are subjected to frequent bending movements, this film was created to maintain its integrity and adhesiveness up to 100 bending cycles.
The Japanese paper art of kirigami has been inspirational in the development of innovative material designs for engineering applications. Kirigami is made with precise cutting and folding techniques to create fine patterns. Beyond the 3D compositions and complex patterns, flexibility properties, such as stretchability and deformability, of kirigamis can be tuned.
Three cutting-edge applications
The engineers presented the concept in three different applications: a standard thin rubber adhesive bandage, a heating pad with embedded heating wires capable to maintain a constant temperature of 100°F (38°C), and a wearable electronic adhesive film equipped with light-emitting diodes.
This means a whole improvement in diverse applications such as in heating retaining sleeves more adjustable and efficient in on-demand heating for pain and stiffness relief in joints in cases of osteoarthritis, wearable sensors such as headgear impact sensors, skin U.V. sensors or glucose monitors and innovative adhesive bandages.
The adhesive thin films are prepared in 3D printed molds with a liquid elastomer or a rubber solution, and the materials can range from soft polymers to hard metal sheets.
The innovation behind the cut and the mechanism of adhesion
The innovative working feature of this adhesive film lies in how the material responds to the deformation compared to the response of a plain film without cuts. With the repetitive bending of a subject’s knee wearing the film, each time the knee is in the bent position, the film kirigami segments at the center stretch open, following the curve shape of the bent knee. This allows tension release of the material, keeping it adhered to the skin. The film is capable to stick to the skin even after 100 knee bents.
By performing stretch tests of the kirigami film bind to a plane surface, the researchers determined the “critical energy-release rate” to quantify the extent of stretch the film can be subjected to before it peels off. The findings were interesting, as the degree to which the film responded to the applied force varied. Applying force gradually and stretching by pulling the film from one side, the slits located in the middle opened up, releasing tension energy, while the remaining film surface kept with the segments closed and attached to the plane substrate.
In brief, the most important factors responsible for the high adhesive performance of the developed films are:
- the shear lag, defined as the release of the stretching tension by the segments shear distortion;
- the partial debonding conferred by the segments attached to the substrate combined with the film open cuts;
- the stable film response to a heterogeneous substrate deformation, due to the partial stretching segments.
Wrapping up the cuts
A number of developments using kirigami patterns have been attractive topics of research. Improved flexibility and resistance also applies to power sources designed for wearable electronics in emerging sensing platforms such as artificial electronic skin, physiological monitoring systems applications, flexible electronics in soft robotics , as for example in commercially available grippers to handle delicate materials, or soft materials with increased adhesiveness. Kirigami design in material processing is a simple solution to improve the elastic and adhesive properties of films without changing the chemical composition of the material, including thickness or rigidity, which is especially relevant when applicable to regions with high deformability.
The team of researchers have patented the developed work and are collaborating with a medical supply company (Tibet Cheezheng Tibetan Medicine Co. Ltd.) to produce pain relief medicine patches featuring the kirigami design.
The next step under investigation is to apply the kirigami patterns to hydrogel materials combining this strategy to drug delivery systems.
Featured image caption: 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. Credits: Image courtesy of researchers.
