Gecko-inspired dry adhesive, a slow-cooked disruptive innovation

The Biomimicry Column

Gecko-inspired dry adhesive, a slow-cooked disruptive innovation

“Disruptive” conveys such a sense of brutal, even violent, shift, doesn’t it? Shattered planets, chaotic classrooms and collapsing towers come to my mind when I think of the term. But, of course, disruption comes more frequently in the slow-cooked version: climate change, for instance. Similarly, innovation can follow the same typology, the old joke being that he or she was an overnight success 10 years in the making. The model for innovation, as it turns out, is more likely Thomas Edison’s highly planned research and development organization than Archimedes in his bathtub.

The gecko lizard has been a poster child of biomimetics for nearly 15 years since it sparked the interest of a young postdoc researcher in the integrative biology department at the University of California, Berkeley, in the late 1990s. Kellar Autumn, now a professor at Lewis and Clark College in Portland, Ore., was on vacation when he became intrigued with the gecko’s ability to stick to his bungalow’s ceiling. How do they do that? The question led to hundreds of journal papers by laboratories across the globe, more than $30 million in U.S. federal grant funding and over 100 U.S. patents and patent applications.

Autumn demonstrated in 2000 that his vacation bunkmates were clinging to surfaces by a dry action, not some type of wet adhesive or liquid surface tension. He later proved in 2002 what that action was. Geckos stick to surfaces by weak intermolecular attraction called van der Waals forces, and they do it with a hierarchical array of structures on their feet. From the mechanical gripping of their toes to the fringes or lamellae on those toes to the hairs or setae on those fringes to the split ends on those hairs to the pads or spatulae on those split ends, the gecko has made “up close and personal” its way of life.

We cannot stick to that ceiling because we cannot get close enough (with enough surface area). The gecko can. It sports millions of 200-nanometer-diameter spatulae on the slit ends of its toe hairs. As a matter of fact, with 15 to 30 pounds of force per square inch, the typical gecko can support its own weight with just one (highly fringed) toe. Importantly, it can unstick itself at will by changing the angle by which the toes are applied to the surface. The toes will peel back from a surface not unlike a New Year’s noisemaker rolling back.

Slow development

The discovery of this natural phenomenon was a phenomenon itself when first reported in the Proceedings of the National Academies of Science. All sorts of applications have been put forth since, from car parts to surgical sutures.

The development of a universal adhesive device has been a slow slog, however — except when compared to the historically glacial schedule of bringing disruptive inventions to market. What may be popularly called the “two-sided geckotape” some day will remind us that George de Mestral himself labored for a decade to manufacture and sell his first Velcro after inventing it in 1948.

The long journey of a universal gecko-inspired dry adhesive has been marked recently by two important milestones, however. Last month, not-for-profit Draper Laboratories of Cambridge, Mass., working under the aegis of the DARPA Z-Man project demonstrated the utility of their gecko-inspired polymer in a climbing apparatus of two large paddles. It can support an over-200-pound man carrying a 50-pound load as he climbs up a 25-foot high wall of glass. The paddles are meant for military use at this stage. The goal is eventually to develop a field kit for urban climbing.

The other milestone is the development of Geckskin by an academic team from the University of Massachusetts, Amherst, as reported first in a February 2012 issue of Advanced Materials journal, and more recently in April.

The interdisciplinary team (engineering, biology, polymer science) that has developed Geckskin at UMass took a very different approach from previous groups. Unlike other synthetic gecko materials, Geckskin relies on a broader scale hierarchy to stick. The team studied the relationship of the animal’s skin to its tendons and bones, and found it to be as important as the nanoscopic hairs on the feet. They discovered that the animal's tendon is integrated into its skin. They borrowed this feature from the gecko and made it a key feature of their design.

Soft pads and stiffening skin for a new sticky material

Their “draping adhesion” system consists of silicone (or polydimethylsiloxane, PDMS) woven into a synthetic tendon of Kevlar or carbon fiber that maintains stiffness and rotational freedom. PDMS is cheap, non-toxic, stable and acts as a soft pad; the Kevlar or carbon fiber acts as the stiffening skin. Professor Al Crosby, who leads the team, estimates that a 16-inch-square pad able to hold 700 pounds would cost about $0.25 in materials.

Unlike typical visco-elastic adhesives that flow into close contact based on temperature, the material conforms to a surface while still maintaining high, elastic stiffness in directions where forces will be applied. As in the animal, the sticking of this combination is reversible and reusable, for it has to be loaded in one direction to stick. For example, it will adhere while experiencing a downward force, but removing the force and pulling up will free the material from the surface.

The fabric is applied to a surface with a burnisher in order to make tight contact. The inventors promote it as a hanging device for electronic appliances, and are discussing applications for it with both private investors and industry representatives.

The disruptive power of dry adhesive technology to change entire methods of design and manufacturing appears limitless today, as does the applications potential of a cheap, easy to use, reversible, reusable dry attachment device for heavy objects.

Those milestones bring the much anticipated application of a universal dry and tunable adhesive closer to everyday use. They would not have been possible without the slow simmer of years of research, exploration and debate by the dedicated men and women of our scientific community.