When Ars last examined the state-of-the-art in gecko mechanics, researchers were measuring the strength of single fibers from the bottom of their feet, hoping to gain insights into how these translate weak van der Waals attractions into the ability to scamper up walls. Since then, researchers have been making pretty good progress in fashioning carbon nanotubes into fibers with similar adhesive properties, allowing them to make adhesives that approach the ability of geckos to stick to surfaces.
In a paper that should show up at PNAS later this week1, a research team has discovered that the lessons of the gecko go well beyond the properties of the individual fibers. It turns out that the biological version of these fibers are arranged in hierarchical clusters, and the research team involved sought to mimic this organization. They experimented with creating various bundles of carbon nanotubes, and compared their adhesive properties with both unbundled nanotubes and live geckos (join me, if you will, in imagining the gecko harness involved...).
Unbundled, their nanotube tape was nearly as adhesive as a live gecko, but as these same tubes were clustered into bundles, their strength went up. By the time the authors optimized the combination of fiber length and bundle width, their tape was over four times stronger than a gecko: a square centimeter was sufficient to support nearly four kilograms. Although this was weaker than the initial strength of a standard piece of adhesive tape, the "gecko tape" had staying power. Its adhesive properties remained stable over time, while those of the adhesive tape dropped below those of the gecko tape after about five minutes.
Because of its reliance on van der Waals forces, the gecko tape had some unusual properties. These forces can work between any two surfaces, allowing the tape to stick to Teflon with roughly half the efficiency of its adhesion to a charged surface. Because the forces are proportional to surface area, peeling the tape works remarkably well: for most angles, peeling gently reduced the surface area, allowing the tape to come off with little force and no damage.
The only downside seems to be the consequences of overloading the tape. The failures tend to be catastrophic, and many of the fibers break and are left behind on the surface. Long term, this will degrade the performance of what is an otherwise reusable adhesive.
A second paper that should also be released soon2 has a potential solution to that. Instead of having the nanofibers contact the surface directly, it uses them to support a flat surface of similar material. That flat surface maximizes the area capable of undergoing van der Walls attractions, while the fibers create many individual points of failure that have to be overcome before the surface peels away. In tests, having the nanofiber backing improved the adhesive properties of the surface by over nine-fold compared to the surface alone, while failure of the adhesion left the fibers involved intact.
1: When PNAS releases the paper, it should appear here.
2: The draft of this paper uses a DOI that has been assigned to an unrelated paper, so I cannot link to it. Those interested can watch PNAS for Glassmaker et. al., Biologically inspired crack trapping for enhanced adhesion.