The Journal of Experimental Biology http://jeb.biologists.org is reporting this morning that the ability of gecko toes to grip surfaces is impacted by water. Gecko toe pads are covered with rows of fine, hair-like filaments called setae. Michael Prowse examined the material properties of the lizard’s feet, knowing that the setae are composed of keratin and keratin is softened by high humidity. Repeatedly stretching and releasing a strip of setae at three different rates (0.5, 5 and 10 Hz) in environments ranging from 10% to 80% humidity, allowed the researchers to measure the force transmitted through the strip to calculate the strip's elastic modulus – how much elastic energy is stored – to see how it changed. As the humidity rose, the elastic modulus decreased by 75% and the strip of setae became softer. So the strip of setae became more deformable as the humidity rose, but could the increased softness explain the gecko's improved attachment under damp conditions?
Puthoff built a mathematical model to see if softer, more deformable, setae could explain the gecko's improved attachment at high humidity and found that it did. Not only did increased softness strengthen the contact between the setae and the surface but also it made it easier for the reptile to peel its foot off. So instead of improving gecko's attachment through microscopic bridges, higher humidity softens the setae that coat the gecko's feet to help them hold fast and peel free with ease.
In 2002, Kellar Autumn found that these dry hairs are in such intimate contact with surfaces that geckos 'glue' themselves to surfaces using van der Waals forces and do not use fluid adhesives. More recently it was suggested that geckos might benefit from additional adhesion in humid environments through capillary action provided by microscopic.
While this study has implications for material research, it may also suggest a reason why some geckos have the habitat preferences and distributions they do.
Citation: Puthoff, J. B., Prowse, M. S., Wilkinson, M. and Autumn, K. (2010). Changes in materials properties explain the effects of humidity on gecko adhesion. Journal of Experimental Biology. 213: 3699-3704.