Gecko Nanoadhesion Research – A classical paradigm of biomimetics

Biomimetics is perhaps the oldest form of scientific plagiarism – science plagiarizing nature. It is "the science of systems which have some function copied from nature, or which represent characteristics of natural systems or their analogues." It is also one of the most fascinating, most fertile areas of engineering and applied science (from airplane wings to velcro or echolocation). Living organisms have evolved particularly well-adapted structures and materials over geological time by means of natural selection. Especially in the area of micro- and nanotechnology, it is difficult to imagine operating without the remarkable opportunities of biomimetic methodology. Harvard established the Wyss Institute for Biologically Inspired Engineering precisely with that objective in mind. Biomimetic synthesis is an entire field of organic chemistry.

Geckos, an ectothermic infraorder of lizards, are one of nature’s most inspiring evolutionary mysteries: they reach in length from 1.6 cm to 60 cm. Some species are parthenogenic, perhaps one reason why they occur throughout the world in warm climates, even on remote islands.

Geckos are capable of running on almost any surface smooth or rough, wet or dry, clean or dirty. They do this at any orientation: up a wall as well as inverted along a ceiling. But not very well on Teflon, and not very well under water. Miraculously, their toes are covered by millions of micron-scale bristles-like structures (setae) that constitute a self-cleaning dry adhesive. On their foot pads, the micrometer-scale setae branch out into nanometer-scale projections (spatulae). It is generally assumed that the exceptional adhesive power – that does not rely on any “sticky” substance – exploits molecular attraction (Van der Waals forces) between the gecko’s toe pads and the surface it is walking on. But very recent research suggests that electrostatic forces may be primarily responsible.

At many research institutions including Oxford, UC Berkeley, Stanford, Northwestern, Carnegie Mellon, UMass, UAlberta, UManchester and numerous other places, Gecko research has become a very exciting topic for biomimeticists, especially in nanotechnology and adhesives research. No wonder DARPA became interested early as well in its potential military applications for scaling vertical surfaces, but so is NASA, the NIH and BAe. Millions of micron scale setae on each toe form combine to a dry adhesive that is self-cleaning and does not involve a glue-like substance. The microfibers or setae are activated by dragging or sliding the toe parallel to the surface. The tip of a seta ends in 100 to 1000 spatulae measuring just 100 nanometers in diameter and 0.2 μm in length. In the gecko, evolution has formed intricate nanostructures that work together in a hierarchy of spatula, spatula stalks, setal stalks, setal arrays, and toe mechanics. Each square mm contains about 14,000 setae with a diameter of 5 μm. An average gecko’s setae can support a weight of 133 kg, while its body weight is 70 g. The adhesion force of a spatula varies with the surface energy of the substrate to which it adheres. The surface energy that originates from long-range forces, e.g., van der Waals forces, follows from the material's structure below the outermost atomic layers at up to 100 nm beneath the surface. The setae are lubricated by phospholipids generated by the gecko’s body that also enable it to detach its feet prior to each subsequent step without slowing down.

Principal commercial applications of Gecko research have focused to date on biomimetic adhesives, a kind of superglue that attaches equally well to wet surfaces and to dry ones. “Geckel,” as a first product is called, has combined a coating of fibrous silicone with a polymer mimicking the “glue” employed by mussels that allows them to stick to rocks while they and the surfaces they adhere to are being pounded by giant ocean waves in perfect storms.

“Gecko tape” was developed already in 2003 at the University of Manchester but only produced in small quantities while scaling up production has proved commercially difficult. In the life sciences, sheets of elastic, sticky polymers could soon replace sutures and staples, including use in laparoscopic surgeries, and provide long-term drug delivery patches to expanding and contracting areas such as cardiac tissue and provide stem cell attracting factors for tissue regeneration.

The gecko’s ability to” run up and down a tree in any way”, as Aristoteles observed in his History of Animals (Περὶ τὰ Ζῷα Ἱστορίαι), continues to inspire even in the robotic age: Stanford’s “Stickybot” has likely applications primarily in outer space and security. One common characteristic of visionary biomimetic technology is, however, that its potential becomes plausible and sometimes obvious far sooner than its commercial viability. It is – and will likely remain for considerable time – one of the central challenges of technology assessment.

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