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Self-cleaning nanotechnology stickies

(Nanowerk Spotlight) Adhesive tapes are ubiquitous in our lives, whether it's on the back of a yellow sticky note, the tape that closes baby diapers, masking connectors on printed circuit boards, or surgical tape in hospitals. Most adhesive tape will stick to a wide variety of surfaces - provided that they are clean and dry. Adhesive tapes are made up of two components: a carrier which is usually paper or plastic, and an adhesive which is either water or solvent based. Many modern adhesive tapes use pressure sensitive adhesives. When you apply pressure to the tape with your finger, a strong adhesive bond is formed. Most tapes have poor ageing properties and will deteriorate quickly: with time, after several uses, or as the sticky side becomes dirty, they lose their adhesive ability.
As we have reported previously (For super-strong nanotechnology dry adhesives look no further than the gecko), scientists are very interested in exploring the secret of the gecko’s adhesive properties and to use this knowledge to create superior man-made adhesives. But it's not just the stickiness that intrigues researchers: because geckos are able to walk across a dusty or dirty surface and then scale a vertical wall without problems, their feet must also possess some kind of self-cleaning ability. Scientists have now managed to mimic the remarkable self-cleaning abilities of the gecko, as well as lotus leaves, and incorporate this ability into the design of a self-cleaning, carbon nanotube based adhesive material.
"Significant effort in developing synthetic materials inspired by gecko feet show comparable, and in some cases better, shear resistance than natural gecko feet" Dr. Ali Dhinojwala tells Nanowerk. "Still, these measurements were done in controlled environments and limited self-cleaning data of these synthetic materials were reported. For the past five years, my research group has been working on mimicking the remarkable climbing ability of geckos using patterned aligned carbon nanotubes. We have achieved shear resistance that are four times higher than the geckos. Now, for the first time, we have also mimicked the self-cleaning aspect of the gecko foot. By using the patterns of specific dimensions and lengths, we have shown that our adhesive tapes can be cleaned by water and mechanical contact. The cleaning by water is very similar to the mechanism that is found on the leaves of lotus and lady mantle plants."
Dhinojwala, a professor at the University of Akron, together with Pulickel Ajayan's group at Rensselaer, has now demonstrated the self-cleaning ability of carbon nanotube-based flexible gecko tapes. The findings have been reported in the February 12, 2008 online edition of Nano Letters ("Gecko-Inspired Carbon Nanotube-Based Self-Cleaning Adhesives").
Superhydrophobic behavior of micropatterned carbon nanotube pillars
Superhydrophobic behavior of micropatterned carbon nanotube pillars of 250 µm in width and 100 µm in height. (A) A 10 µL water droplet sitting on the surface of vertically aligned carbon nanotubes. (B) Top view of water droplet sitting on the micropatterned carbon nanotube pillars. (C) Schematic diagram showing carbon nanotube pillars held at base by polymer adhesive and a water droplet sitting on top of pillars. (Reprinted wih permission from American Chemical Society)
Previously, the Dhinojwala and Ajayan groups have fabricated hierarchical structures of setae and spatulas found on the gecko foot using aligned multi-walled carbon nanotubes. In a paper in PNAS last year they have shown that a 1 square centimeter area can support nearly 4 kg of weight and that much larger forces can be supported by increasing the area of the tape ("Carbon nanotube-based synthetic gecko tapes"). The researchers found that the length and diameter of carbon nanotubes, size of the pattern, and stiffness of the backing tape are all important parameters that need to be optimized for superior performance.
In his new research, Dhinojwala shows that for optimum length and pattern size, these carbon nanotube-based synthetic tapes exhibit self-cleaning as well as high shear resistance.
"Our tapes can be cleaned by water, as shown by the leaves of lotus plants" he says. "In addition, the synthetic tapes can also be cleaned by a contact mechanism similar to that exhibited by the gecko. After mechanical cleaning, the shear strength recovers back to 90% (and 60% for water-cleaned samples) of the values measured before soiling. In comparison, the gecko recovers back 50% of the shear stress after eight steps. The ability of these synthetic tapes to self-clean and also retain their shear resistance makes it an excellent choice for gecko-inspired adhesives."
For the experiments, MWCNTs with a length of 100 µm and an average diameter of 8 nm (2-5 walls) were used. Flexible polymer tape with tacky coating on one side is pressed against the top surface of the carbon nanotubes. Upon peeling, the carbon nanotubes are transferred from the silicon substrate to the flexible polymer tape.
The CNTs are held by a polymeric glue at the base and this does not allow individual structures to collapse due to capillary forces. "This process eliminated the use of fluorinated coatings (or other nonwetting materials) on the carbon nanotubes to make these structures superhydrophobic" explains Dhinojwala. "To demonstrate the self-cleaning ability of the synthetic tapes, we soiled these tapes with silica particles – to represent dust – ranging from 1 to 100 µm in size. When rinsed with water, the water droplets roll off very easily, carrying with them most of the silica particles."
The researchers also tested the self-cleaning properties of these synthetic tapes by contact mechanics. After a couple of contacts with mica or glass substrate they observed that the majority of these particles are transferred to the mica (or the glass) surface.
"The successful design of a gecko-like adhesive requires both the ability to mimic the stickiness as well as self-cleaning" Dhinojwala sums up his findings. "If you want to use these tapes in robotics, we cannot just test them on clean glass. They need to work in real dusty environments. In addition, the important element of the gecko design is reversibility and this cannot be achieved without some aspect of self-cleaning. Therefore we envisage their is a broad spectrum of applications including robotics, space applications, electronics, and sports."
Several companies already have shown an interest in this new technique and are in discussions with Dhinojwala on how to commercialize his research.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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