Monday, June 20, 2011

Gecko nanotechnology

Animals that cling to walls & walk on ceilings owe this ability to micro- & nanoscale attachment elements. The highest adhesion forces are encountered in geckos. A gecko is the heaviest animal that can 'stand' on a ceiling, with its feet over its head. This is why scientists are intensely researching the adhesive technique of the small hairs on its feet. On the sole of a gecko's toes there's some billion small adhesive hairs, about 200 nanometers in both width & length. These hairs put the gecko in direct physical contact with its surroundings. The shape of the fibers is also significant; for example, spatula-shaped ends on the hairs provide strong adhesion. Researching how insect & gecko feet have evolved to optimize adhesion strength is leading to bio-inspired development of artificial dry adhesive systems. Potential applications range from protective foil for delicate glasses to reusable adhesive fixtures - say goodbye to fridge magnets, here comes the hairy stuff, which will also stick to your mirror, your cupboard & your windows.
Researchers at the Max Planck Institute for Metals Research in Stuttgart/Germany have explored the bizarre adhesion force of gecko feet for some time now. Back in 2004 they found that there exists an optimal shape of the contact surface of the tip of such hairs which gives rise to optimal adhesion to a substrate by molecular interaction forces ("Shape insensitive optimal adhesion of nanoscale fibrillar structures").

The nanoscale fibrillar structures in the hairy attachment pads of beetle, fly, spider & gecko. The density of surface hairs increases with the body weight of animal, & the gecko has the highest density among all animal species. (Picture: Max Planck Institute for Metals Research/Gorb) 

 For macroscopic objects, such optimal shape design tends to be unreliable because the adhesion strength is sensitive to small geometrical variations. It is shown that this limitation can be remedied by size reduction.
The key finding of this research is that there exists a critical contact size around 100 nanometers below which optimal adhesion can be reliably achieved independent of small variations in the shape of the contact surface. In general, optimal adhesion can be achieved by a mix of size reduction & shape optimization. The smaller the size, the less significant the shape.
This result provides a believable explanation why the characteristic size of hairy attachment systems in biology fall in a narrow range between a few hundred nanometer & a few micrometers & suggests a few useful guidelines for designing adhesive structures in engineering.
Continuing this research, in 2005 the Max-Planck researchers discovered that the adhesiveness of geckos increases with the amount of humidity ("Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements" & "Resolving the nanoscale adhesion of individual gecko spatulae by atomic force microscopy").
Its foot's adhesive method, whose branches become increasingly smaller over levels, allows the gecko to stick to any ceiling & walk with its feet over its head. Until then, scientists were uncertain as to what mechanism was responsible for the extreme adhesive ability of the gecko. What was clear is that the adhesive method was in other words, that it functioned without secreting anything of its own. In lieu, it makes use of water, which is present as a narrow film on every terrestrial surface.
The researchers found that as humidity increases, the capillary forces strengthen & that ultra-thin water layers, like those between a gecko spatula & a substrate, influence the strength of adhesive forces.
Copying the biological adhesive mechanism, the Max-Planck scientists used the insights gained from their years of research to create a material with a biomimetic structure that exhibits excellent adhesive qualities. The special surface structure of the material allows it to stick to smooth walls without any adhesives. Potential applications range from reusable adhesive tape to shoe soles for climbing robots & are therefore of considerable relevance to expertise.

Microscope picture of the biomimetic surface structure of the new adhesive material. The material (green), which was inspired by the soles of insects' feet, sticks to the glass (blue). (Picture: Max Planck Institute for Metals Research)

In rigorous tests carried out by the Max Planck researchers with measuring instruments developed for the purpose, the artificial adhesive technique gave an impressive performance & demonstrated lots of benefits. It lasts for hundreds of applications, does not leave any visible marks & can be thoroughly cleaned with soap & water. The researchers found that square centimeters of the material can hold objects weighing up to hundred grams on walls. However, this limit is much lower for ceilings. Smooth structures, such as glass or polished wood, are nice bases but woodchip wallpaper is not suitable.
"Insects also struggle to travel over slightly roughened surfaces - it is a essential issue for adhesion mechanisms," explained Project Leader Stanislav Gorb from the Evolutionary Biomaterials Group at the Max-Planck-Institute for Metals Research.
To manufacture the material, a mold, similar to a cake tin in baking, is used in which the necessary surface is embossed as a negative picture. The mould is filled with a polymerizing mixture which is allowed to cure & then released from the mould. This sounds simple, but is the result of a "great deal of trial & error." The researchers found the construction of the microstructural "cake tin" challenging & exactly the way it works remains a trade secret. Optimizing the polymer mixture also taxed the researchers: if it is liquid it runs out of the mold; if it is viscose, it won't even go in.
Potential applications range from protective foil for delicate glasses to reusable adhesive fixtures. For example, the new material will soon be present in industrial production processes in the manufacture of glass parts. It's already been shown to perform in higher weight categories: the artificial adhesive fibers on the soles of a 120 gram robot helped it to climb a vertical glass wall ("Climbing & Jogging Robots : Proceedings of the 8th International Conference on Climbing & Jogging Robots & the Support Technologies for Mobile Machines").
In their current research, the scientists are trying to improve the adhesion by refining the structures even further.
"However, there is still lots of work to be completed. Something that functions smoothly in the laboratory is a long way away from large-scale production," explained Stanislav Gorb.

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