3D printed bio-inspired adhesives, friction force generation by modulating stiffness

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For over a decade the use of bio inspired adhesives have been explored to achieve high reversable attachment on a wide range of surfaces. Based on the fibrillar adhesive toepads of geckos, many patterned adhesives have been produced that are able to form and preserve contact with rigid and smooth substrates. But on other types of surfaces, such as soft and deformable substrates, the performance of these micro patterned adhesives diminishes. Alternative methods have to be investigated to create adhesives that optimize the ability to form and preserve contact on these types of substrates. In this study, 3D printing is used to produce a series of adhesives with internal cylindrical porous structures of various geometry. The internal structure modulates the adhesives stiffness in the normal and shearing direction, designed to affect the friction force. Friction forces are measured on glass, soft elastomeric substrates and cloth. On glass, the highest friction forces are generated by the adhesive with the lowest normal stiffness, unaffected by their shear stiffness. On soft substrates, the highest friction forces are achieved with adhesives that combine a low normal stiffness, promoting contact formation, with a high sear stiffness, enhancing contact preservation. The effect of this anisotropic stiffness becomes more pronounced when the stiffness of the substrate decreases, contributing to contact formation. On cloth, generation of friction forces remains challenging, more research is required to create adhesives that excel on these kinds of substrates.