Tik Tiki Thik Thik. Does this sound familiar? During this hartal season, people might have spent lazy times lying on bed staring at lizards’ crawling towards each other to fight over an insect prey. I am sure someone must have thought of walking up the wall or the ceiling, like the lizards, with the head hanging down to drop a bomb at the opposite political party headquarters! Then being tired, the person must have fallen asleep!
Well, some scientists rather woke up from sleep in understanding the mechanism of the lizard’s clinging upside down. They did it because such knowledge might be applied on the shoe of an athlete to run faster, or make reversible glue to use in say, surgery, or to give one the ability of a “spiderman” walking up the wall!
From such pursuit of knowledge we now know that there are millions of tiny hairs, called setae, under each tow of the lizard, also called gecko. The tip of each seta is split into hundreds of tinier flattened tips in the shape of a spatula whose widths are smaller than visible light wavelength! This splitting tremendously increases the surface area, bringing the molecular van der Waal’s forces to play. Scientists found that a tip’s adhesive power to the wall is so strong that it each seta could hold an ant (20 mg). With all of its setae, a 150-gram tokay gecko has the theoretical ability to support 14,000 grams, explaining how it clings with a single finger alone! Surface adherence is achieved by pulling the setae towards the body, an opposite action releases the foot. In January 2012 students at UC Barkeley demonstrated by making robots and computer simulations that movement of the tail helps a gecko stabilize itself during jumping to its prey (Nature 481:181).
With such knowledge at hand, engineers and chemists worked with biologists and made very thin (millionth of a millimeter), gecko hair-like structures. They were able to hang a few kilograms of weight with a synthetic hair! Inspired by such success, Nike is investing to create better bouncing balls and other sports gears. Wall clinging toys are already in the market. Other potential application of the knowledge is a non sticking gecko-glue to be used in space because sticking glue does not function there. Automobile tire with better traction is another possibility.
Glue-like property of the setae made other scientists think too. Biologists noticed that some forest gecko can cling or walk across water- submerged leaves. Students at the University of Akron, Ohio, conducted some simple but clever experiments that was published on April 1, 2013 (PNAS DOI 10.1073/pnas.1219317110). They tied a tape around the lower body of a 14-inch tokai Gecko, and allowed it to walk on four wet and dry surfaces of glass (wetting, hydrophilic), plexiglass (partly wetting) and coated, transparent plastic (hydrophobic, similar to the wet surface of plant leaves) and Teflon (hydrophobic). As the gecko tried to walk on these surfaces, a pull on the tape measured the force of the grip on the surface. A thin water layer between glass and gecko’s toe (hydrophilic interaction) weakened the grip while the plexiglass and transparent plastic having leaf-chemistry created air pockets between toes and the surfaces allowing good grip. The scientists concluded that gecko adhesion is highly dependent on surface wettability and the presence of water or air between the toe pad and the contact surface. They expect their data can be used to create a water-resistant adhesive.
The author, a former Dhaka University teacher, is a biomedical scientist working in the USA.