Geckos are nature’s best climbers, but mechanisms underlying geckos’ remarkable climbing ability had remained a mystery since Aristotle first observed it in the fourth century B.C. We finally cracked the secrets of gecko adhesion, opening the door for engineers to fabricate gecko-inspired synthetic adhesives (GSAs).

We demonstrated that weak molecular attractions called van der Waals forces—named for the Dutch physicist Johannes van der Waals—and a unique geometry are primarily responsible for geckos’ amazing climbing ability. Using millions of microscopic hairs (setae) on its toes, a gecko can run up polished glass at a meter per second and easily support its entire body weight with only a single toe.

Because van der Waals interactions depend more on geometry than chemistry, we proposed that the size and shape of the structures—not their chemical composition—determine the stickiness. To test this hypothesis, we used two polymer materials to synthesize the world's first gecko-inspired adhesive nanostructure.

Our research showed that we can enhance adhesion, just as geckos have, simply by dividing a surface into small protrusions to increase surface density. Our work on gecko adhesion has grown into new subfield of research at the interface of biology, physics, and materials science. The study of gecko-like adhesive nanostructure is generating a rapidly growing number of publications.

At least seven possible mechanisms for gecko adhesion have been discussed over the past 175 years: glue, suction, interlocking, friction, static electricity, capillary forces and van der Waals adhesion. All but the latter two mechanisms had been rejected by 1969, and, there was strong evidence that gecko adhesion was in part determined by surface energy.

To test whether capillary adhesion or van der Waals force is a sufficient mechanism of adhesion in geckos, we measured adhesion and friction on two semiconductor surfaces that varied greatly in hydrophobicity. If capillary adhesive forces dominated, we expected a lack of adhesion on the hydrophobic surface. In contrast, we found that adhesion was similar on hydrophobic and hydrophilic semiconductors.

We also found that gecko setae are strongly hydrophobic, with a water droplet contact angle of 161°. Since van der Waals force is the only mechanism that can cause two hydrophobic surfaces to adhere in air, the semiconductor experiments provided the first direct evidence that van der Waals force is a sufficient mechanism of adhesion in gecko setae.

van der Waals force is largely independent of surface chemistry and highly dependent on the distance between surfaces, so it can be said that gecko adhesion depends more on geometry than on chemistry. This means that gecko keratin proteins should not be required for fabrication of gecko-like adhesives.

To test this, my coauthor Ron Fearing, of the Department of Electrical Engineering and Computer Sciences at UC Berkeley, used silicone and polyester to fabricate the first prototype synthetic gecko spatulae that exhibited limited gecko-like adhesion at the nanoscale. Our study paved the way for fabrication of GSAs from a variety of materials.

I became interested in gecko adhesion ten years ago during a vacation in Hawaii. I was in a little hotel near Kealakekua Bay and, while I was in bed, a huge spider crawled out across the ceiling. I’m a bit arachnophobic, so I was trying to work up the courage to deal with the spider when a tiny gecko came out on the ceiling too. The gecko and the spider had an upside-down battle right there above me. The gecko won easily and knocked the spider off (fortunately not on me!). This made me wonder how the gecko could run so easily on the ceiling, and why it was so much better at sticking than the spider.

This moment of curiosity inspired our study of the force generated by a single gecko foot-hair (Autumn et al, "Adhesive force of a single gecko foot-hair," Nature 405: 681-685, 2000). It took an interdisciplinary group of scientists and engineers to solve the mystery. We called ourselves the "Gecko Team," which included myself, UC Berkeley biologist Bob Full, engineers Ron Fearing (also at UC Berkeley), and Tom Kenny at Stanford. Later, for the van der Waals project, Jacob Israelachvili of the UC Santa Barbara (UCSB) also joined our group.

Initially, this project did not go as planned. Once we had isolated one of the microscopic hairs, or setae, on a gecko’s toe, we couldn’t make it stick. The frustration continued for months, leading us to hypothesize that a chemical component secreted by the gecko might be required for setal adhesion, as is the case for many insects. Instead, once we realized that a gecko first places its feet on the surface and then pulls inward toward its body, we discovered that attachment and detachment in gecko setae are controlled mechanically, not chemically.

Adhesion in gecko setae requires a small push perpendicular to the surface, followed by a small parallel drag. When properly oriented, preloaded, and dragged, a single seta can generate over three orders of magnitude more force than required to hold the animal’s body weight. All 6.5 million setae on the toes of one gecko attached simultaneously could lift 133 kg.

Over the past seven years, we have discovered seven benchmark functional properties of the gecko adhesive system: 1) anisotropic attachment, 2) high pulloff to preload ratio, 3) low detachment force, 4) material independence / van der Waals adhesion, 5) self cleaning, 6) anti-self matting, and 7) non-sticky default state. We are working hard to understand the fundamental principles underlying these properties.

It’s remarkable that each time we figure out something about the gecko system, multiple new questions emerge. One new opportunity is the use of gecko-inspired synthetic adhesives as physical models to test our understanding of the system. We can learn a great deal by observing how these synthetic analogs can—and cannot—measure up to the material on real gecko toes.

The physics of surfaces in contact is also a hot area of research. New theory and methods are revealing how dynamics at the nanoscale level may affect macroscopic contact forces and fracture energies. There is a real interdisciplinary synergy here: progress in surface science has been fundamental in the study of gecko adhesion, which in turn is advancing understanding of the physical mechanisms that cause adhesion and friction.

  Are there any social or political implications for your research?

Gecko adhesion has become a paradigmatic example of bio-inspired engineering (see "The Gecko’s Foot" by Peter Forbes, Fourth Estate, 2005). Micro-electrical interconnects, wafer alignment, micromanipulation, and robotics are just a few of the many possible applications for GSAs. Because gecko adhesives do not rely on soft polymers or chemical bonds, they may someday replace screws, glues, and interlocking tabs in assembly applications such as automobile dashboards or mobile phones.

Adhesive nanostructures relying on van der Waals forces and made from hard, elastic materials should be able to rebond dynamically following fracture, allowing for self-repair. With a clever joint design that takes advantage of the directionality of gecko adhesives, self-disassembly for repair and recycling should also be possible.

Because they can be self-cleaning, adhesive nanostructures have the potential to reduce our reliance on cleaning solvents and surface preparation, reducing cost and environmental impact. Using nontoxic and nonirritating materials to fabricate synthetic setae may enable biomedical applications such as endoscopy and tissue adhesives.

I think it is a useful lesson that the study of a lizard can have such broad and numerous applications ranging from nanosurgery to aerospace. Genrikh Saulovich Altshuller, the inventor of the so-called "Teoriya Resheniya Izobreatatelskikh Zadatch" (Theory of Solving Inventive Problems—or TRIZ) said: "In nature, there are many hidden patents."

I think what he was getting at was that biological systems arrive at solutions that are unpredictable and, in some cases, unimaginable until we see them. This underscores the value of basic, curiosity-based research. Geckos have solved the problem of sticking to things in a completely different way than engineers do—or at least used to. Would we have considered making an adhesive from a nanostructure if geckos had not evolved?

I like to think of the network created by biodiversity as a huge design library. There are millions of species out there waiting for us to discover their unique solutions to life’s myriad problems. But the scary thing is that extinction is taking these books off the shelves faster than we can open and read them. Even if humanity’s primary aim is economic progress, we must slow the increasingly rapid rate of species extinction before many more valuable secrets of nature are lost forever.

A Closer Look...
	Below are images sent in by Kellar Autumn which correspond with the featured paper, or current research. Note: larger image views will take longer to load.

Figure 1:

Figure 1: Feet of six of the more than 1000 gecko species (image by Dr. Paul Stewart).

Figure 2:

Figure 2: Structural hierarchy of the gecko adhesive system. A. Ventral view of a tokay gecko (Gekko gecko). B. Foot of a tokay gecko, showing a mesoscale array of seta-bearing scansors (adhesive lamellae). C. Array of setae on the ventral surface of each scansor. D. Single gecko seta. E. Nanoscale array of hundreds of spatular tips of a single gecko seta. F. Synthetic spatulae fabricated in the lab of Ronald Fearing using nanomolding.

Figure 3A: Figure 3B:

Figure 3: Frictional adhesion: the anisotropic tribological response of isolated gecko setal arrays (units in mN). A. When dragged across a surface against their natural curvature, setae produce friction forces typical of dry hard materials (with a friction coefficient of 0.25 to 0.3). B. When preloaded and dragged in a direction along their natural curvature, setae adhere and maintain friction even under negative loads. Detachment occurs if friction falls below tan a * times adhesion force, where a * is the critical angle of detachment of the setae (30° in the tokay gecko). This system permits fine control over adhesion and friction. See Autumn et al. 2006. Journal of Experimental Biology 209, 3569-3579.