Arrays of gecko‐inspired angled polymer microfibers with angled mushroom tips adhere with similar strength to gecko subdigital toe‐pads in their gripping direction (∼100 kPa) on smooth surfaces (see image), but are easily released in the opposite direction. Control of adhesion in the normal direction is also possible by controlling drag distance during loading.

This paper proposes an approximate adhesion model for fibrillar adhesives for developing a fibrillar adhesive design methodology and compares numerical simulation adhesion results with macroscale adhesion data from polymer microfiber array experiments. A technique for fabricating microfibers with a controlled angle is described for the first time. Polyurethane microfibers with different hardnesses, angles, and aspect ratios are fabricated using optical lithography and polymer micromolding techniques and tested with a custom tensile adhesion measurement setup. Macroscale adhesion and overall work of adhesion of the microfiber arrays are measured and compared with the models to observe the effect of fiber geometry and preload. The adhesion strength and work of adhesion behavior of short and long vertical and long angled fiber arrays have similar trends with the numerical simulations. A scheme is also proposed to aid in optimized fiber adhesive design.

Angled polyurethane fiber arrays are modified by adding soft spherical and spatula shaped tips via dipping. These fibers are characterized for adhesion and friction and compared with unmodified fibers and flat material samples. Sphere and spatula tip fiber samples demonstrate increased adhesion, with 10 and 23 times the maximum adhesion of the unmodified fiber sample, respectively. The sphere and spatula tip fiber samples also show increased friction, with 1.6 and 4.7 times the maximum friction of the unmodified fiber sample, respectively. Friction and adhesion are simultaneously observed in a synthetic dry angled fibrillar adhesive sample (spatula tip fiber sample). The direction dependent friction of angled fibers is investigated. The adhesion and friction results reported in this paper suggest that fibers with negligible adhesion can be modified to exhibit both significant adhesion and friction enhancements by the proposed fiber tip modifications.

Previous studies have demonstrated that gecko foot-hair inspired elastomer microfibers with spatulate tips have significant adhesion enhancement compared to the flat elastomer surface. In this study, we report the friction enhancement of these highly adhesive fibers and analyze the relation between adhesion and friction of elastomer microfiber arrays with spatulate tips. Fabricated polyurethane fiber arrays with spatulate tips demonstrate macroscale static friction pressures up to 41 N / cm 2 for a preload pressure of 1.5 N / cm 2 on a 6 mm diameter smooth glass hemisphere.

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SummaryOver the last decade, significant effort has been put into mimicking the ability of the gecko lizard to strongly and reversibly cling to surfaces, by using synthetic structures. Among these structures, mushroom-like elastomer fiber arrays have demonstrated promising performance on smooth surfaces matching the adhesive strengths obtained with the natural gecko foot-pads. It is possible to improve the already impressive adhesive performance of mushroom-like fibers provided that the underlying adhesion mechanism is understood. Here, the adhesion mechanism of bio-inspired mushroom-like fibers is investigated by implementing the Dugdale–Barenblatt cohesive zone model into finite elements simulations. It is found that the magnitude of pull-off stress depends on the edge angle θ and the ratio of the tip radius to the stalk radius β of the mushroom-like fiber. Pull-off stress is also found to depend on a dimensionless parameter χ, the ratio of the fiber radius to a length-scale related to the dominance of adhesive stress. As an estimate, the optimal parameters are found to be β = 1.1 and θ = 45°. Further, the location of crack initiation is found to depend on χ for given β and θ. An analytical model for pull-off stress, which depends on the location of crack initiation as well as on θ and β, is proposed and found to agree with the simulation results. Results obtained in this work provide a geometrical guideline for designing robust bio-inspired dry fibrillar adhesives.

Vertically aligned carbon nanofibers partially embedded inside polyurethane (eVACNFs) are proposed as a robust high friction fibrillar material with a compliant backing. Carbon nanofibers with 50–150nm in diameter and 20–30μm in length are vertically grown on silicon and transferred completely inside an elastomer by vacuum molding. By using time controlled and selective oxygen plasma etching, fibers are partially released up to 5μm length. Macroscale friction experiments show that eVACNFs exhibit reproducible effective friction coefficients up to 1. Besides high friction, the proposed fabrication method improves fiber-substrate bond strength, and enables uniform height nanofibers with a compliant backing.

Abstract-One of the primary impediments to building ensembles of modular robots is the complexity and number of mechanical mechanisms used to construct the individual modules. As part of the Claytronics project-which aims to build very large ensembles of modular robots-we investigate how to simplify each module by eliminating moving parts and reducing the number of mechanical mechanisms on each robot by using force-at-a-distance actuators. Additionally, we are also investigating the feasibility of using these unary actuators to improve docking performance, implement intermodule adhesion, power transfer, communication, and sensing.In this paper we describe our most recent results in the magnetic domain, including our first design sufficiently robust to operate reliably in groups greater than two modules. Our work should be seen as an extension of systems such as Fracta [9], and a contrasting line of inquiry to several other researchers' prior efforts that have used magnetic latching to attach modules to one another but relied upon a powered hinge [10] or telescoping mechanism [12] within each module to facilitate self-reconfiguration.