A) Images of one front foot of each species used in this study, displayed in proportion to one another, with species ordered by increasing body size. B) Experimental method used to record maximum adhesive force (Fc) and system compliance (C), showing G. vittatus with front feet placed flat on glass plate, just before plate extends upward (denoted by the arrow) away from gecko, and C) sample recordings from each species showing Fc and gecko stiffness (1/C). doi:10.1371/journal.pone.0134604.g001
A few years ago Geckskin,a flexible adhesive was invented . It mimics a gecko’s ability to strongly yet easily attach and detach their feet to walk on walls and ceilings. Geckos employ dry adhesion, using a combination of microscopic hairs on their toe pads, as well as other aspects of internal anatomy, to climb vertical walls and run across ceilings. In particular, it’s a mystery how some species as much as 100X heavier than others can use adhesion so effectively.
Geckos range in body size from about 2 g to over 250 g, a difference in scale greater than 100X. Now a team of biologists and polymer scientists at the University of Massachusetts Amherst show that increasing the surface area of gecko toe pads alone cannot explain this.
METHOD: Gilman measured the ability of 5 gecko species varying in weight from about 0.7 to 3.5 ounces to adhere to glass on a force-extension instrument that measured geckos’ clinging force. She also measured changes in the stiffness of gecko anatomy. Further, the polymer scientists in the group created synthetic 3-toed gecko feet from fabrics and soft elastomers to model adhesive performance.
RESULTS: As gecko body size increased, their complete adhesive system, ie tendons, skin, connective tissue and setae, became stiffer, resulting in the larger animals’ legs and feet being far stiffer than in smaller geckos >> important role in enabling larger geckos to produce sufficient adhesive forces to climb. By incorporating both area of gecko toe pad and stiffness in their model, it explained 92% of the variation in adhesive ability among a variety of gecko species. “As predicted…the gecko adhesive system becomes less compliant (stiffer) as geckos become larger.”
Experiments with synthetic gecko feet showed the same pattern. It will provide the physical ingredients for engineers to build new, better adhesives. The increased stiffness enhances adhesion because it enables the surfaces forces at the surface, produced by van der Waals bonds, to be stored and distributed efficiently. http://www.umass.edu/newsoffice/article/springy-mechanics-large-and-small-gecko
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