Falls and subsequent complications are major contributors to morbidity and mortality, especially in older adults. Here, by taking inspiration from claws and scales found in nature, we show that buckling kirigami structures applied to footwear outsoles generate higher friction forces in the forefoot and transversally to the direction of movement. We identified optimal kirigami designs capable of modulating friction for a range of surfaces, including ice, by evaluating the performance of the dynamic kirigami outsoles through numerical simulations and in vitro friction testing, as well as via human-gait force-plate measurements. We anticipate that lightweight kirigami metasurfaces applied to footwear outsoles could help mitigate the risk of slips and falls in a range of environments.
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The main data supporting the results of this study are available within the paper and its Supplementary Information. All of the data generated in this study, including the source data and the data used to make the figures, are available from figshare with the identifier https://doi.org/10.6084/m9.figshare.12207992.
The ABAQUS scripts used for the numerical analyses are available as Supplementary Information.
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We thank T. Hua, M. Cruz, N. Inverardi, X. Lu and V. Soares for their help with the experimental studies; S. Cotreau, C. Haynes and A. Wentworth for help with manufacturing the experimental specimens; and R. Langer for fruitful discussions. This work was funded in part by a start-up grant from the Deparment of Mechanical Engineering, MIT to G.T. K.B. acknowledges support from the National Science Foundation under grant nos. DMR-1420570 (Materials Research Science and Engineering Center) and EFRI C3 SoRo 1830896. A.R. acknowledges support from Swiss National Science Foundation grant no. P300P2-164648.
A.R. and K.B. are inventors on a patent application (patent no. US2019/0232598A1) describing buckling-induced kirigami. S.B., S.P., A.R., K.B. and G.T. are co-inventors on a provisional patent application (no. 62913419) for the technology described. Complete details of all relationships for profit and not for profit for G.T. can be found at the following link: https://www.dropbox.com/sh/szi7vnr4a2ajb56/AABs5N5i0q9AfT1IqIJAE-T5a?dl=0. The remaining authors disclose no competing interests.
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Supplementary notes, figures and video captions
Steel kirigami shoe grips attached to a shoe sole.
Finite-element simulation of a kirigami unit cell.
Linear post-buckling analysis for the kirigami simulator.
ABAQUS script for simulating buckling-induced kirigami.
Nonlinear post-buckling analysis and compression for the kirigami simulator.
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Babaee, S., Pajovic, S., Rafsanjani, A. et al. Bioinspired kirigami metasurfaces as assistive shoe grips. Nat Biomed Eng 4, 778–786 (2020). https://doi.org/10.1038/s41551-020-0564-3
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