Brief History of Synthetic Gecko Adhesion ResearchHere is a brief background on some of the first work towards synthetic gecko adhesives up through 2003.
2000Autumn et al. (Nature 2000) perform first measurement of shear and normal adhesive forces of single gecko seta. These results showed strong directionality, with detachment of seta at angle greater than 30 degrees, and seta able to withstand perpendicular pull-off force of 20 microNewton. Seta is non-adhesive unless first dragged parallel to surface.
2002Autumn et al. (PNAS 2002) demonstrated that weak molecular attractions called van der Waals forces and a unique geometry are primarily responsible for geckos' amazing climbing ability. Because van der Waals interactions depend more on geometry than chemistry, Autumn et al proposed that the size and shape of the structures determine the stickiness. To test this hypothesis, they used two polymer materials to synthesize the world's first gecko-inspired adhesive nanostructure.
Jagota and Bennison (Integ. and Comp. Bio. 2002) introduce a model of fibrillar adhesion using vertical fibers, which rely on buckling for compliance. The vertical fibers using hard materials would require quite high aspect ratios to contact the surface, and the authors suggest that fibers would need low energy surfaces at the sides to prevent clumping. This vertical fiber model qualitatively explains subsequent carbon nanotube fibrillar adhesive behavior.
Sitti and Fearing (IEEE Nano 2002) demonstrate silicone rubber microfiber arrays with 50 thousand fibers per square centimeter and 2.8 mN per square centimeter normal adhesion after being pressed into the surface with a 25 mN load (adhesion coefficient ~0.1). Polyimide fiber arrays with 1 billion fibers per square centimeter (60 micron length and 200 nm diameter) collapse during release etch.
2003Geim et al. (Nature Materials 2003) demonstrates a centimeter square patch of hard plastic fibers which could support a 3 newton normal load after being pressed into a surface with a 50 newton load (adhesion coefficient ~0.06). Fibers were too short to show the buckled attachment predicted by Jagota and Bennison (2002). Adhesion deteriorated after several cycles.
Persson and Gorb (J. Chem. Phys. 2003) measure natural gecko spatula geometry using high resolution SEM, and show that the estimated spatula plate thickness of 5-10 nanometers plays a key role in adhesion to rough substrates.
Campolo et al. (IEEE Nano 2003) simulates effect of surface roughness on fiber detachment during pulloff, and predicts reduction of adhesion strength with rough surfaces. Polyurethane fiber array fabricated with 1 billion fibers per square centimeter shows vertically aligned fibers.
Sitti and Fearing (2003) introduce inclined cantilever model for fiber array, which provides compliance in contact, specifies fiber spacing to avoid tip-to-tip clumping, and predicts anisotropic adhesion, depending on sliding direction.
Tong et al. introduce first carbon nanotube adhesive. The fibers are also electrically conductive.