Adhesive designed from gecko toes

In our increasingly environmentally conscious society, consumers often opt for reusable versions of frequently used items such as shopping bags or water bottles. Unfortunately, reusable varieties are not available for all items. For example, tape, one of the most commonly used supplies, does not come in a reusable strain. At least not yet.

A team of researchers from the Karlsruhe Institute of Technology and Carnegie Mellon University has engineered the first adhesive tape that not only sticks to any smooth surface, but also has self-cleaning capabilities. Such tape could be useful for a variety of situations: It can line openings of food packages, keep bandages sticky for extended periods of time and allow snail mail recipients to reseal parcels and letters.

The researchers drew inspiration for their new adhesive tape from nature itself, specifically the gecko, a small, tropics-dwelling lizard. The toes of a gecko are naturally endowed with the gift of self-cleaning. When a gecko walks, its toes drag across the supporting surface. This creates a lateral friction that effectively removes larger dirt particles from the gecko’s toes. Simultaneously, small bristle-like structures called setae, which are located on the sole and skinfolds below the toes, catch smaller dirt particles.

In order to replicate the self-cleaning abilities of gecko toes in a laboratory setting, the researchers devised testing models made with elastic microhairs of various sizes. To model dirt particles, the researchers used tiny glass spheres that were micrometers in diameter. These extremely small spheres were distributed onto a smooth glass plate. Then, to simulate the movement of a gecko taking steps, a piece of artificial adhesive tape covered in microhairs was placed on the plate, moved laterally and peeled off again. The researchers repeated this load-drag-unload cycle several times to measure the adhesive forces present.

Using this model, the researchers demonstrated that the two mechanisms that geckos use for removing large and small dirt particles from their toes accounted for 95 percent of the self-cleaning effect. “This effect is determined by the ratio between particle size and setae diameter,” Hendrik Hölscher, a study leader and a scientist at the Institute of Microstructure Technology, said in a press release from the Karlsruhe Institute of Technology. Experimentally, this means that the size of the glass spheres in relation to the size of the microhairs determines the effectiveness of the engineered adhesive’s self-cleaning capabilities.

If the diameter of the glass spheres was greater than the diameter of the microhairs, the artificial adhesive initially did not behave differently from ordinary adhesive tape. In other words, the adhesive force of the microhairs vanished after the first contact. However, unlike ordinary adhesive tape, after eight to ten test cycles, the artificial adhesive reached 80 to 100 percent of its initial adhesive power again. “In the long term, this effect might be used to develop a low-cost alternative to hook and loop fasteners,” Hölscher said in the press release.

In contrast, when the diameter of the glass spheres was less than the diameter of the microhairs, the artificial adhesive did not fare well. Under these conditions, the researchers were only able to salvage a mere one-third of the tape’s original adhesive force. These results led the researchers to conclude that in order for their artificial adhesive to maintain its sticky properties through multiple uses, the diameter of the microhairs needs to be far smaller than the diameter of dirt particles. “For the perfect gecko-inspired adhesive tape, we. . .need fibers in the nanometer range (10-9 meters), which are smaller than most dirt particles,” Michael Röhrig, a study leader and a scientist at the Institute of Microstructure Technology, said in the press release. An explanation for this result may lie in the anatomy of the gecko foot. Gecko toes have skinfolds that are reproduced by wide grooves between narrow rows of hair, which offer sufficient space for fine dust to deposit. Because of this anatomical restriction, smaller dust particles do not interfere with the stickiness of the gecko’s toes.

The next step for the researchers is to test their artificial tape using real dirt particles of various shapes, sizes and material composition. If the tape is perfected and made available for retail, it may gain a substantial market in numerous industries. “Such tape might be applied in the sports sector, in medicine, automotive industry, or aerospace technology,” Metin Sitti, a study leader, said in the press release.

As long as there are sticky situations in the world, the possibilities for this innovative reusable adhesive are endless.