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 variety of surfaces - provided that they are clean and dry. Adhesive tapes are made up of two components: a carrier, usually paper or plastic materials, 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 aging properties and will deteriorate quickly; with time, after several uses, or as the sticky side becomes dirty, they lose their adhesive ability.
As has been reported, scientists are very interested in exploring the secret of the gecko’s adhesive properties in order to use this knowledge to create superior synthetic 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, a professor at the University of Akron’s Department of Polymer Science, said. “Still, these measurements were done in controlled environments, and limited self-cleaning data for 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 is 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, together with Pulickel Ajayan’s group at Rensselaer Polytechnic Institute, has now demonstrated the self-cleaning ability of carbon nanotube-based flexible gecko tapes. The findings were reported in the February 12, 2008, online edition of Nano Letters ( dx.doi.org/doi:10.1021/nl0727765).
Previously, the Dhinojwala and Ajayan groups had 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 demonstrated that a one-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. The researchers found that the length and diameter of carbon nanotubes, the size of the pattern, and the 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 them 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 was pressed against the top surface of the carbon nanotubes. Upon peeling, the carbon nanotubes were transferred from the silicon substrate to the flexible polymer tape.
The CNTs were held by a polymeric glue at the base, which 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,” Dhinojwala explains. “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 the ability to self-clean,” Dhinojwala sums up. “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 envision a broad spectrum of applications, including robotics, space applications, electronics and sports.”
Several companies have already expressed an interest in this new technique and are in discussions with Dhinojwala on how to commercialize his research.
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