The length scale on which we focus in this work is the human scale, motivated by the challenge of human climbing with a gecko-inspired dry adhesive. The adhesive chosen for this task consists of polydimethylsiloxane (PDMS) slanted microwedges (roughly 100 μm tall), which adhere well to glass and can be repositioned many times without degradation [4,5] (figure 1, inset). While other dry adhesive materials produce larger adhesive stresses, PDMS microwedges are especially suitable for climbing because they exhibit controllable adhesion that can be effectively switched on or off in a fraction of a second by applying a shear stress. Therefore, a climber can attach PDMS microwedges simply by transferring weight to the adhesive, and can detach by removing the weight, requiring nearly-zero added effort. However, this work, on efficient scaling, is not specific to PDMS microwedges and may be applicable to a wide range of other adhesive materials.
Figure 1.
The red squares (n = 5) indicate the maximum adhesive shear stress σmax that can be achieved on a flat, smooth surface by tokay gecko adhesion systems as the adhesive area A increases [6]. From left, these data correspond to a single seta, a setal array, a toe and two feet. The red line shows the least-squares power law fit to the data (log σmax = –0.24 log A + 1.1). Similarly, for the PDMS microwedge synthetic adhesion system, the blue diamonds (n = 6) represent the maximum adhesive stress produced by a 1.5 mm2 patch, a 12 mm2 patch, a 6.5 cm2 tile and a 24-tile system. The least-squares power law is plotted as a blue dashed line (log σmax = –0.02 log A + 1.8).
There have been notable efforts to scale adhesives beyond small laboratory tests, but there has been little work dealing specifically with scaling efficiency. Researchers have created small robots that climb well with dry adhesives [7–11], yet these systems cannot support as large a load as predicted by their total area of adhesive. For instance, Stickybot had an area of adhesive that should have supported 5 kg based on small-scale tests, but could only support 500 g owing to inefficient scaling [11]. In other work, adhesive anchors capable of holding impressive loads have been created [12,13], but the scaling efficiency of these adhesion systems has not been reported. Additionally, a demonstration of human climbing with gecko-inspired adhesives has been recently revealed by the Defense Advanced Research Projects Agency (DARPA) Z-Man programme [14], but details of the adhesive material, climbing device and total area of adhesive remain undisclosed.
Without scaling efficiency numbers in the literature, it is useful to set a scaling efficiency benchmark by turning towards the adhesion systems of geckos, as designers of dry adhesives have done previously to judge the relative merits of their synthetic materials. In the tokay gecko (Gekko gecko), the adhesion decreases as the length scale increases from the seta-scale to the toe- and foot-scale [6]. This adhesion system has been found to approximately follow a scaling power law, σmax ∝ A−1/4, where σmax is the maximum shear stress supported by the adhesive and A is the adhesive area (figure 1, red squares).
If the benchmark power law of the tokay gecko were applied to the PDMS microwedge adhesive, an impractically large area of more than 1200 cm2 of adhesive per hand would be required to support a 70 kg human climber with no safety factor (a modern tennis racket is approx. 675 cm2). This is partly because the area of adhesive required for climbing increases disproportionately as the scale increases (even with perfectly efficient scaling) owing to the climber's surface area and mass following a square-cube law.
Therefore, to allow a human to climb with a practical area of controllable dry adhesive, it is necessary to attain significantly more efficient scaling than that of the gecko's adhesion system. To achieve this goal, we developed a synthetic adhesion system which ensures that the load distribution across a large adhesive area is nearly uniform, using a concept referred to as degressive load-sharing. In degressive load-sharing, elastic elements which have decreasing stiffness with increasing displacement support independent patches of adhesive and help equalize the load on all patches. The synthetic adhesion system was found to sustain adhesive stress with little decrease across four orders of magnitude of area, approximately following the power law σmax ∝ A−1/50 (figure 1, blue diamonds). Furthermore, the system was found to resist catastrophic failure by preventing stress concentrations when a simulated failure was induced on a portion of the adhesive (see §2.3), and the system also requires little effort to attach or detach (see §2.5). Thus, with efficient scaling, robustness to failure and controllable adhesion, the synthetic adhesion system enables a human to ascend a vertical glass surface with a hand-sized1 area of adhesive (figure 2).
Figure 2.
Three frames from a video (electronic supplementary material, movie S1) showing a 70 kg climber ascending a 3.7 m vertical glass surface using a synthetic adhesion system with degressive load-sharing and gecko-inspired adhesives. The time between (a) and (c) is about 90 s and includes six steps.
No comments:
Post a Comment