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In the field of bionics, the gecko is regarded as a true master of adhesion. This reptile can attach itself upside down to almost any surface and is able to walk around on the ceiling – it can even “stick” to polished glass without any problems. The gecko owes this “magical” ability to the millions of fine hairs on its feet which are branched like a tree. The tip of each hair splits into a thousand even finer adhesive hairs which are just 1000 micrometers long and, at a width of six micrometers, have a diameter one tenth of that of a human hair. These are tipped with even finer spatula-shaped structures featuring a diameter of only 200 nanometers.

Visualizing these fine hairs through a microscope presents quite a challenge – since the hairs do not actually conduct any electrical current, it is almost impossible to view them through an electron microscope. In order to find out why they stick so well, researchers at Lewis & Clark College in Portland, Oregon, collected specimens of geckos’ and spiders’ feet which had similarly remarkable properties and examined them using a Carl Zeiss helium-ion microscope. The subject can be scanned using the microscope’s helium-ion beam. This process enables the production of images whose resolution and contrast set new standards.

The secret of geckos’ feet lies in what are known as “van-der-Waals” forces. These arise between atoms and molecules whose electrical charge is asymmetrically divided – the negative charge dominates at one end of the molecule; the positive at the other. Researchers found that the force of the whole toe and the individual fine hair was practically identical on water-repellent and water-absorbent surfaces. Therefore, the gecko’s secret is primarily based on the minute structure of the tiny adhesive hairs on the soles of its feet. In addition, the fineness and density of the structures increase proportionately to the weight of the animal’s body, meaning that it can stay on the ceiling even when it is fully grown.

November 16, 2011