"We have demonstrated that gecko-type dry adhesives can work underwater if one borrows ideas from classic, wet adhesives like mussel protein glues," team leader Phillip Messersmith of Northwestern University in Evanston told nanotechweb.org. "The adhesive therefore has the potential to greatly expand the field of temporary gecko-type adhesives to include wet environments. These glues essentially work like a sticky note on both wet and dry surfaces."
Geckos can walk upside down across surfaces thanks to the arrays of hairs on their feet, which typically have diameters of 200 to 500 nm. At this size of hair the geckos can exploit both van der Waals and capillary forces to climb surfaces, depending on the nature of the surface. Each hair produces a force of about 10-7 N, but because there are so many hairs, they can produce an adhesion of roughly 10 N/cm2.
Although researchers have made synthetic gecko-inspired adhesives before, these glues – like geckos – do not work very well in water. To overcome this problem, Messersmith and colleagues decided to combine a gecko-hair structure with a synthetic adhesive, based on the one mussels produce to hold fast onto wet surfaces.
The researchers began by making an array of tiny silicone polymer pillars just 400 nm wide supported on a polymer film – to mimic the arrays of gecko foot hairs. Next, they coated these with another polymer that mimics mussel adhesive proteins.
The new hybrid geckel glue can be applied to a surface and pulled off more than a thousand times without losing its sticking power. More importantly, it works on both wet and dry surfaces. Indeed, the team found that the adhesive power of each silicone pillar increased nearly 15-fold when coated with the synthetic mussel glue.
The researchers would now like to scale up the adhesive so that it can be used for large areas. "This is a significant challenge and critical for commercial development of the technology," said Messersmith. "Right now, we have demonstrated proof of principle on a small area just a few millimetres across, but we need to find a way to make the glue for large areas."
The work was published in Nature.