The Molecular Genetics of Bioadhesion and Biofilm Formation

Abstract

Gecko toe pads are sticky because they feature an extraordinary hierarchy of structure that functions as a smart adhesive. Gecko toe pads (Russell 1975) operate under perhaps the most severe conditions of any adhesives application. Geckos are capable of attaching and detaching their adhesive toes in milliseconds (Autumn et al. 2005 (in press)) while running with seeming reckless abandon on vertical and inverted surfaces, a challenge no conventional adhesive is capable of meeting. Structurally, the adhesive on gecko toes differs dramatically from that of conventional adhesives. Conventional pressure sensitive adhesives (PSAs) such as those used in adhesive tapes are fabricated from materials that are sufficiently soft and sticky to flow and make intimate and continuous surface contact (Pocius 2002). Because they are soft and sticky, PSAs also tend to degrade, foul, self- adhere, and attach accidentally to inappropriate surfaces. Gecko toes typically bear a series of scansors covered with uniform microarrays of hair-like setae formed from ß- keratin (Russell 1986; Wainwright et al. 1982), a material orders of magnitude stiffer than those used to fabricate PSAs. Each seta branches to form a nanoarray of hundreds of spatular structures that make intimate contact with the surface. Functionally, the properties of gecko setae are as extraordinary as their structure: the gecko adhesive is 1) directional, 2) attaches strongly with minimal preload, 3) detaches quickly and easily (Autumn and Peattie 2002; Autumn et al. 2000), 4) sticks to nearly every material, 5) does not stay dirty (Hansen and Autumn 2005) or 6) self-adhere, and 7) is nonsticky by default. While some of the principles underlying these seven functional properties are now well understood, much more research will be necessary to fully map out the parameters of this complex system.