Watching geckos dart with equal ease over walls, floors and ceilings must have engaged the attention of countless travellers. How do these little animals do it? It is only in recent years that the answers have been forthcoming. The adhesion comes from van der Waals forces between nano-sized spatulae located on hairs on the gecko's foot and the substrate. The gecko provides a superb demonstration of how nanofibres can be organised to have a macroscopic impact. Research to reproduce the adhesion effects has taken place in many countries and two significant papers report recent progress.
Lee and colleagues have sought to enhance the wet performance of adhesion by incorporating a "synthetic polymer that mimics the wet adhesive proteins found in mussel holdfasts". They report greatly improved adhesion properties. "The hybrid material, which they call a geckel nanoadhesive, proved in initial testing to be adherent under dry and wet conditions. It also adhered much longer under both extremes than previous gecko-based synthetic adhesives, a major issue in this area of research."
The other paper, by Ge and colleagues, reports on a gecko-inspired adhesive tape. "We have demonstrated for the first time a macroscopic flexible patch that can be used repeatedly with peeling and adhesive properties better than the natural gecko foot. The carbon nanotube-based tape offers an excellent synthetic option as a dry conductive reversible adhesive in microelectronics, robotics, and space applications."
One thing we can learn from this particular exercise in biomimetics is that the gecko does not demonstrate just a single trait with enhanced performance. There are issues of adhesion and delamination, self-cleaning, and achieving a sustained adhesive performance. What we have in the gecko is exquisite design and, for that, biomimetics needs a methodology that can relate well to intelligent engineering design concepts. For more on this, go here.
One report of the adhesive tape research says: "Work is now underway to make the tape self-cleaning as well". Interestingly, in 2005, Hansen and Autumn speculated that the nano-sized setae might be self-cleaning, but this ongoing activity suggests that there are more gecko secrets yet to emerge in this area!
A reversible wet/dry adhesive inspired by mussels and geckos
Haeshin Lee, Bruce P. Lee & Phillip B. Messersmith
Nature, 448, 338-341 (19 July 2007) | doi:10.1038/nature05968
The adhesive strategy of the gecko relies on foot pads composed of specialized keratinous foot-hairs called setae, which are subdivided into terminal spatulae of approximately 200 nm (ref. 1). Contact between the gecko foot and an opposing surface generates adhesive forces that are sufficient to allow the gecko to cling onto vertical and even inverted surfaces. Although strong, the adhesion is temporary, permitting rapid detachment and reattachment of the gecko foot during locomotion. Researchers have attempted to capture these properties of gecko adhesive in synthetic mimics with nanoscale surface features reminiscent of setae2, 3, 4, 5, 6, 7; however, maintenance of adhesive performance over many cycles has been elusive2, 8, and gecko adhesion is greatly diminished upon full immersion in water9, 10. Here we report a hybrid biologically inspired adhesive consisting of an array of nanofabricated polymer pillars coated with a thin layer of a synthetic polymer that mimics the wet adhesive proteins found in mussel holdfasts. Wet adhesion of the nanostructured polymer pillar arrays increased nearly 15-fold when coated with mussel-mimetic polymer. The system maintains its adhesive performance for over a thousand contact cycles in both dry and wet environments. This hybrid adhesive, which combines the salient design elements of both gecko and mussel adhesives, should be useful for reversible attachment to a variety of surfaces in any environment.
Carbon nanotube-based synthetic gecko tapes
Liehui Ge, Sunny Sethi, Lijie Ci, Pulickel M. Ajayan, and Ali Dhinojwala
Proceedings of the National Academy of Sciences USA, June 26, 2007, vol. 104, no. 26, 10792-10795 | 10.1073/pnas.0703505104
We have developed a synthetic gecko tape by transferring micropatterned carbon nanotube arrays onto flexible polymer tape based on the hierarchical structure found on the foot of a gecko lizard. The gecko tape can support a shear stress (36 N/cm2) nearly four times higher than the gecko foot and sticks to a variety of surfaces, including Teflon. Both the micrometer-size setae (replicated by nanotube bundles) and nanometer-size spatulas (individual nanotubes) are necessary to achieve macroscopic shear adhesion and to translate the weak van der Waals interactions into high shear forces. We have demonstrated for the first time a macroscopic flexible patch that can be used repeatedly with peeling and adhesive properties better than the natural gecko foot. The carbon nanotube-based tape offers an excellent synthetic option as a dry conductive reversible adhesive in microelectronics, robotics, and space applications.
Gould, P. Nanotube tape mimics gecko's sticky feet, NanoToday, Volume 2, Issue 4, August 2007, Page 12.
Hansen, W.R. and Autumn, K., Evidence for self-cleaning in gecko setae, Proceedings of the National Academy of Sciences USA, January 11, 2005, vol. 102, no. 2, 385-389 | 10.1073/pnas.0408304102
Nature's secrets yield new adhesive material, EurekAlert, 18 July 2007.
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