Kirigami coating could make shoes safer
MIT researchers have drawn on kirigami, a variation of origami that involves cutting paper as well as folding it, to create a friction-boosting material that could be used to coat the soles of shoes, giving them a stronger grip on ice and other slippery surfaces.
The Japanese art of kirigami, which, like origami, involves folding paper to create interesting designs and shapes but with the added skill of cutting the material at strategic points, has inspired new soling technology for shoes. The development can enhance footwear’s resistance to slippery conditions and could, therefore, function well in work and safety footwear in environments such as construction, food processing and manufacturing.A team of engineers working at the Massachusetts Institute of Technology (MIT) has developed this idea, with the MIT Department of Mechanical Engineering, the US National Science Foundation and the Swiss National Science Foundation funding the research.
Cutting edge
Artists use kirigami to create designs. Using a single sheet of paper, they fold, cut and bend the paper to create pop-up, three-dimensional images and models. The MIT engineers decided to apply a similar technique to sheets of plastic or metal, which they then applied as flexible coatings on the soles of shoes. Possible future developments will include a rubber-like polymer with a reinforced steel tip. Testers wore the kirigami-coated shoes in laboratory experiments, walking on an ice-covered plate that was wired up to measure the force the wearers were exerting on it. The tests showed that people wearing these shoes generated more friction than those wearing shoes without the specially coated soles. Generating extra friction makes the shoes more resistant to slips and fall.
MIT research scientist Sahab Babaee, Simo Pajovic, an MIT graduate student, and Ahmad Rafsanjani, a former Harvard student, are among the lead authors of a paper detailing this development that a journal, Nature Biomedical Engineering, published in June. They describe the sole of a prototype shoe as looking flat when held still, with the lines of the kirigami incisions appearing as no more than a visual effect. But when flexed, an intricate pattern of spikes forms, a feature inspired by nature, by the scales on snakeskin, for example. “We have a shape transition from a 2D flat surface to a 3D geometry with needles that come out,” Sahab Babaee says. “You can use those elements to control friction, because the sharp needles can pop in and out, based on the stretch that you apply.”
Slips and falls can be a health threat to people in all walks of life, the paper argues. The MIT team says that by taking inspiration from claws and scales found in nature it has created “buckling kirigami structures” that can generate higher friction forces in the forefoot and transversally to the direction of movement, helping footwear manufacturers and employers keep people safe. “We anticipate that lightweight kirigami metasurfaces applied to footwear outsoles could help mitigate the risk of slips and falls in a range of environments,” the authors say.
Pattern of success
The researchers experimented with different patterns and by cutting various shapes (squares, triangles and curves) of various sizes into the soling material. To assess the different combinations, they measured the stiffness of the material and the angle at which the spikes jutted out when the sole was stretched. As part of the same work, they measured the levels of friction generated by each design on wood, vinyl flooring and artificial turf, as well as on the ice-covered plate.
All of the designs they tried increased friction but one stood out: a pattern of concave curves gave the best results. It was this design that went into the soles of the prototype shoes the testers wore in the trials. In these tests, kirigami coatings increased the levels of friction by between 20% and 35% compared to the levels generated by the same shoes without the special soles.
MIT has said that its research team is continuing its work on this idea and will now attempt to determine if it will function better as an integrated part of the sole design or as a separate element that workers could attach to their footwear when conditions demand extra protection against slips. Assistant professor of mechanical engineering Giovanni Traverso, another of the co-authors of the paper, confirms that MIT will now attempt to commercialise the idea.
Unhappy accidents
Slips, trips and falls are the largest cause of accidents in all sectors, from heavy manufacturing through to office work, according to the European Agency for Safety and Health at Work. The agency has identified slips and falls as the main causes of accidents that result in more than three days’ absence from work. In the US, the National Safety Council has falls, slips and trips as the number-two cause of occupational injuries that lead to people taking time off work (number one is over-exertion). It says slips or a falls were the cause of more than 240,000 workplace accidents in the US in 2018, with an average of 12 lost work-days as a result of a strains, sprains and ligament tears; of course, more serious injuries and even fatalities can happen too. The National Safety Council has identified transportation, warehousing and agriculture as some of the sectors in which these accidents seem most frequent.
Employers know the importance of preventing accidents like these from taking place and the crucial role that specialist footwear plays in making work environments safe. Taking nature as an inspiration, the MIT team may have found a way of helping footwear become even safer.
The flexed sole of a prototype shoe showing the curves cut into the material by kirigami- and nature-inspired researchers from the Massachusetts Institute of Technology.
Credit: Diemut Strebe
