New gecko insights inspire even stronger adhesives

But researchers at the University of Massachusetts Amherst have made the connection.

First they showed  the previously unappreciated role of geckos' tendons and bones in the little lizards' ability to climb up walls without slipping. Then they used that knowledge -- plus a large helping of human ingenuity -- to create an adhesive device that can hold the television securely on a wall.

"Our 'Geckskin' device is about the size of an index card and can hold a maximum force of about 700 pounds while adhering to a smooth surface such as glass," said Alfred Crosby, associate professor of polymer science and engineering at the University of Massachusetts Amherst.

To produce it, Crosby added, "We focused on the properties and attributes of the gecko: high capacity, easy release, reliability, and the ability to stick to a variety of surfaces."

"This is definitely an important contribution," said Metin Sitti, professor of mechanical engineering at Carnegie Mellon University and an expert on small-scale locomotion and manipulation, who did not participate in the project.

Crosby carried out the research with his doctoral candidate Michael Bartlett and biology professor Duncan Irschick, with support from the Pentagon's Defense Advanced Projects Research Agency.

Scientists have long recognized that so-called van der Waals forces, which produce weak electrical attraction among molecules, cause adhesion between tiny hairs in geckos' toes, known as setae, and vertical surfaces on which the geckos climb.

However, efforts to apply that process on a large scale have had limited success. Scotch tape gains its stickiness through the van der Waals forces.

"But you can't make the forces stronger," Crosby said. "People have tried to produce artificial setae," Irschick added. "But they don't scale up effectively."

To develop a different approach, the UMass team studied the large-scale structure of geckos' feet.

Expanding on research by University of Calgary biologist Anthony Russell, the team discovered how tendons, bones, and skin work together to produce the easily reversible adhesion that causes a gecko's feet to stick to a wall briefly and then release from it as the tiny lizard moves up, down, or sideways on the wall. The process works in large part because of the role of the tendons. In most creatures, tendons connect bones to muscles.

"But in geckos' feet, uniquely, the tendons stretch from bone into skin," Irschick explained.

The group used that knowledge as the basis of an adhesive system stronger than any relying on van der Waals forces.

"We wanted something that would cover a large area and would become increasingly stiff," Crosby recalled. "But those demands are contradictory."

Scotch tape, for example, covers a large area, but is soft and thus unable to hold significant weight. The geckos' anatomy suggested that the team could overcome the contradiction by using a specially treated fabric. A fabric can be both soft and stiff. A tablecloth, for instance, can drape over a table and conform to the shape of anything underneath it while remaining stiff if you try to pull it. For their Geckskin, the researchers mimicked the anatomy of geckos' feet.

"We took a fabric, put a bit of rubber around it, and sewed another piece of fabric -- the 'tendon' -- into that 'skin'," Crosby explained.

Since the fabric is stiff and the rubber soft, the combination yields a stiff but flexible system that drapes over a large surface area, permitting maximum contact and adhesion.

Geckskin's strength does not apply in all directions. While it is almost impossible to move it along any surface on which it is mounted, Crosby said, "a gentle peel from one edge allows it to be effortlessly removed from the surface on command."

It can be removed and stuck onto another surface as often as needed without leaving any residue or losing adhesive strength.

The team has used a variety of ingredients for the rubber component. In particular polydimethylsiloxane, a component of silly putty, holds the promise, in combination with fabric, of developing an inexpensive, strong, and durable dry adhesive.

The researchers also tried a variety of fabrics.

"Those with the greatest load capacity use the fibers such as Kevlar and fiber-based fabrics that are most stiff," Crosby said.

According to Crosby, Geckskin stacks up well against current commercial adhesives.

"The force per area is definitely higher than all the pressure-sensitive adhesives," Crosby said. "The combination of high force and user release is not there in available adhesive systems. And unlike Velcro, Geckskin doesn't need a matching surface."

The team, which reported its advance in the journal Advanced Materials, is now discussing possible commercialization of the technology. 

Source: Inside Science News Service (news : web)

The gecko walks on sticky pads

Dancing on the ceiling. Lionel Ritchie's smash hit in the eighties. The song's video clip shows him walking upside down on the ceiling. Just an illusion, of course. Flies, however, can do that, like countless other insects. Among the bigger animals, the gecko stands out as an example. It can run upside down effortlessly.

Geckos can do that because of complex structures on their feet, explains Kamperman. These are bundles of tiny hairs each ending in a sort of little flap. Hundreds of thousands of these little flaps stick to every surface by adhesion and without any other material aid. The work is done by so-called Van der Waals forces. Theoretically speaking, anyway. Kamperman says that it is not exactly clear what the mechanism is.

Not that it matters. Kamperman isn't planning to make gecko feet in exact detail. 'That won't be wise. A gecko foot is a very complex organic system. One should focus on the major issues, get to the essence of the design and copy that. This is what I'm doing now.' And this essence lies in the tiny hairs with the flaps.

Kamperman tries to capture this essence in plastic. She describes her first attempts in the latest issue of Acta Biomaterialia. Kamperman's gecko feet is a little plastic sheet (polydimetylsiloxane) covered with countless tiny rods which resemble studs measuring about ten micrometres in diameter. A gecko would not recognize this, but it works.

To a certain extent, that is. Kampermans' gecko skin sticks well to a base of pure silicon. But hardly any surface is as glassy smooth as that. The wheels fall off when the surface is a little rougher. 'So more is needed than just making tiny rods,' she concludes. Kamperman looks for that something extra in a higher resolution in the material: even more tiny rods. Kamperman now tries to attain that higher resolution by, for example, making rods from so-called block copolymers. These are polymers which take on a certain shape through self-assembly. In this case, it is a helix. The result is a surface densely covered with countless spirals, like a cut-open mattress. Being flexible, the spirals can make firm contact and also let go easily again. Eat your heart out, gecko!

Provided by Wageningen University

Scientists trace gecko footprint, find clue to glue

Geckos' ability to scamper up walls with ease has long inspired scientists who study the fine keratin hairs on these creatures' footpads, believed responsible for the adhesion. Researchers at The University of Akron have discovered that geckos' ability to adhere to surfaces is not all about keratin. Clues lie in the lipids left behind in gecko footprints.

This discovery by researchers Ping Yuan Hsu and Liehui Ge, both UA polymer science graduate students; Alyssa Stark, UA integrated bioscience graduate student; Xiaopeng Li, chemistry research scientist; Chrys Wesdemiotis, distinguished professor ofchemistry; Peter Niewiarowski, interim director, UA Integrated Bioscience, Ph.D. Program; and Ali Dhinojwala, chair of the UA Department of Polymer Science, is published in Interface, the Journal of the Royal Society under the title: Direct evidence of phospholipids in gecko footprints and spatula–substrate contact interface detected using surface-sensitive spectroscopy.

The researchers' analysis of the near-invisible gecko footprints reveals the presence of phospholipids, according to Dhinojwala. This material, he says, has not been considered in current models of gecko adhesion and now provides the missing link in understanding superhydrophobicity, self-cleaning and fluid-like adhesion and release of gecko feet.

Dhinojwala, a pioneer in gecko-inspired adhesive research, says the lipids in gecko footprints have significant implications for scientists working to design synthetic adhesives that could be reused thousands of times over, such as for wall-climbing robots, microelectronics, adhesive tapes and bioadhesives.

More information: Direct evidence of phospholipids in gecko footprints and spatula–substrate contact interface detected using surface-sensitive spectroscopy, J. R. Soc. Interface, Published online before print August 24, 2011, doi: 10.1098/?rsif.2011.0370

Abstract
Observers ranging from Aristotle to young children have long marvelled at the ability of geckos to cling to walls and ceilings. Detailed studies have revealed that geckos are ‘sticky’ without the use of glue or suction devices. Instead, a gecko's stickiness derives from van der Waals interactions between proteinaceous hairs called setae and substrate. Here, we present surprising evidence that although geckos do not use glue, a residue is transferred on surfaces as they walk—geckos leave footprints. Using matrix-free nano-assisted laser desorption-ionization mass spectrometry, we identified the residue as phospholipids with phosphocholine head groups. Moreover, interface-sensitive sum-frequency generation spectroscopy revealed predominantly hydrophobic methyl and methylene groups and the complete absence of water at the contact interface between a gecko toe pad and the substrate. The presence of lipids has never been considered in current models of gecko adhesion. Our analysis of gecko footprints and the toe pad–substrate interface has significant consequences for models of gecko adhesion and by extension, the design of synthetic mimics.

Provided by University of Akron