| The gecko adhesive has been of scientific interest for over two millennia, ever since Aristotle observed a gecko running up and down a tree. Since then, advances in optical and electron microscopy have provided increased information on the structure of the pad of the gecko's foot, for it is the structure that leads to the adhesion and not the chemistry. Each toe contains many ridges, or scansors, displaying arrays of ∼100 mum long, ∼5 mum wide setae, each branching into hundreds of smaller fibers called spatulae, ∼200 nm across and 5 nm thick. The combination of the setal flexibility and the nanoscale compliance of the spatulae creates a large amount of intimate surface contact, enhancing van der Waals interactions, and promoting adhesion.; In this work, an adhesive---inspired by the gecko---was micro/nanofabricated. There were two main thrusts in this work. The first was to show the importance of the hierarchical structure on the performance of the gecko adhesive, and the future need to for multiple levels of compliance in synthetic dry adhesives. The second thrust was to create a surface with controllable adhesion. While the adhesion properties of the gecko are of great scientific interest, it is the ability to switch adhesion on and off that provides the technological driving force for mimicking the gecko system.; The microscale setae of the gecko have been replicated by microfabricating flexible silicon dioxide freestanding structures ∼100 mum long and ∼10 mum wide. These structures were coated with aligned vertical polymeric nanorods ∼4 mum tall and ∼200 nm in diameter, analogous to the gecko spatulae. Testing of the synthetic structures shows that the multi-scale system provides a 5-fold increase in adhesion over nanorods alone, demonstrating the need for a hierarchical structure. To create a switchable adhesive, the silicon dioxide microstructures were replaced with nickel micro-paddles. When the nickel structures were placed in a magnetic field, a conformation change was induced, rotating the paddles away from an adherent surface, and adhesion was reduced by a factor of 40. This is the first demonstration of a surface displaying reversible and controllable adhesion. |
