Syndicate content

How Geckos Adhere to Surfaces Under Water—Clues to Future Synthetics That Will Retain Adhesion in Water

Geckos are known for their sticky adhesive toes that allow them to stick to, climb on, and run along surfaces in any orientation--even upside down! But until recently, it was not well understood how geckos kept their sticking ability even on wet surfaces, as are common in the tropical regions in which most geckos live. A 2012 study in which geckos slipped on wet glass perplexed scientists trying to unlock the key to gecko adhesion in climates with plentiful rain and moisture.A study supported by the National Science Foundation and published online on April 1, 2013 in PNAS solves the mystery, showing that wet, water-repellant surfaces, like those of leaves and tree trunks, actually secure a gecko's grip in a manner similar to dry surfaces. Researchers from the University of Akron, led by integrated bioscience doctoral candidate Alyssa Stark, tested geckos on four different surfaces. The surfaces ranged from hydrophilic--those that liquids spread across when wet, like glass--to hydrophobic--water-repellent surfaces on which liquids bead, like the natural leaves geckos walk on--and intermediate ones, like acrylic sheets. Geckos were tested on these surfaces both when the surfaces were dry and when they were submerged underwater, and water completely covered the gecko's feet. Fitting a small harness around the pelvis, geckos were gently pulled along the substrate until their feet began to slip. At this point the maximum force with which a gecko could stick was measured. On wet glass, geckos slipped and could not maintain adhesion. However when tested on more hydrophobic surfaces, geckos stuck just as well to the wet surface as they did to the dry ones. When tested, geckos stuck even better to wet Teflon than dry. To understand these findings, researchers developed a model that explains the results from the gecko study and may also help inform future bio-inspired gecko-like adhesives that can maintain adhesion underwater. To help explain their results, the authors developed models based on thermodynamic theory of adhesion for contacting surfaces in different media and found that they could predict the ratio of shear adhesion in water to that in air. They said theirr findings provide insight into how geckos may function in wet environments and also have significant implications for the development of a synthetic gecko mimic that retains adhesion in water. The image shows a tokay gecko (Gekko gecko) clinging to a leaf stem wet with water droplets. (Credit: Alyssa Stark, The University of Akron). [Press release] [PNAS abstract]