Researchers from the universities of Sheffield in the UK and Bayreuth in Germany, have found a way of mimicking the natural phenomenon of switchable adhesion that allows geckos to climb smooth vertical surfaces and flies to sit upside down on ceilings.
The scientists, led by Mark Geoghegan used one of the surface consisting of a polyacid gel, which is a three-dimensionally cross-linked polymer containing many acid groups. This polymer network is so heavily soaked in liquid that it forms a solid, gelatinous mass.
The second surface is a silicon chip onto which is applied a layer of polymer chains containing basic groups that stand up from the surface like a brush. In water or slightly acidic solution, the acidic groups carry a positive charge while the basic groups are negatively charged, causing them to be attracted each other. In addition to this electrostatic attraction, hydrogen bonds are also formed, which causes the two surfaces to be tightly stuck together.
When the surrounding solution is made more strongly acidic (a pH value of about 1), the hydrogen bonds break up, the basic groups lose their charge, and the electrostatic attraction lets up. The two surfaces can then be slowly and carefully separated from each other without any damage.
This detachment is reversible. If the pH value is raised, making the solution less acidic, the gel and “brush” stick to each other once again. This cycle can be repeated many times by simply changing the pH value.
Possible applications for such “smart” surface pairs include microelectromagnetic components, components for microfluidic systems, or carriers for drugs that could release their cargo under specific physiological conditions.
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