Geckos are lizards that can run rapidly up walls and even upside down on polished glass. How can they do that? I wanted to find out. Gecko have millions of microscopic hairs on the bottom of their feet that function as an adhesive. How does the adhesive work? Biologists have been asking this question for over a hundred years. I organized an interdisciplinary team of scientists including Bob Full at Berkeley, Ron Fearing (also at Berkeley), and Tom Kenny at Stanford to answer the question of how gecko foot hairs work. We received funding for the project from DARPA's Controlled Biological Systems Program.
Geckos climb vertical and even inverted surfaces with ease using millions of micron-scale adhesive foot-hairs on each toe. Each foot-hair splits into hundreds of tips only 200 nanometers in diameter, permitting intimate contact with rough and smooth surfaces alike. Geckos' adhesive microstructure requires minimal attachment force, leaves no residue, is directional, detaches without measurable forces, is self-cleaning, and works underwater, in a vacuum, and on nearly every surface material and profile.
We took a single gecko foot hair (seta) and made the first direct measurement of its adhesive function. These tiny setae are only as long as 2 diameters of a human hair. Thats 100 millionths of a meter long. Each seta ends in up to 1000 even tinier tips. The tips are only 200 billionths of a meter wide below the wavelength of visible light. We used a microscopic force sensor designed by Tom Kenny at Stanford to measure the tiny forces of adhesion of the gecko seta.
We discovered that the seta is 10 times more adhesive than predicted from prior measurement on whole animals. The adhesive is so strong that a single seta can lift the weight of an ant 200 µN = 20 mg. A million setae could lift the weight of a child (20kg, 45lbs). A million setae could easily fit onto the area of a Dime. The combined attraction of a billion spatulae is a thousand times more than a gecko needs to hang from the ceiling. Maximum potential force of 2,000,000 setae on 4 feet of a gecko = 2,000,000 x 200 micronewton = 400 newton = 40788 grams force, or about 90 lbs! This is 600 times greater sticking power than friction alone can account for. Weight of a Tokay gecko is approx. 50 to 150 grams.
These exciting results were published in the journal Nature v. 405: 681-685.
Our discovery explains how it is that any gecko can hold up its entire body weight with only a single finger. If the adhesive is so strong, how do they get their feet off? We also discovered how to make the seta detach.We found that if we increased the angle the seta makes to the surface, it just pops off! We think the tip peels off like tape.
What is the molecular mechanism that makes gecko setae so sticky? Your hair (hopefully) isn't sticky, so what makes geckos' foot-hairs adhesive?
Recently, we provided direct support for the van der Waals hypothesis of gecko adhesion, and rejected surface polarity as a predictor of adhesion force, as suggested previously. These results inspired the fabrication of synthetic foot-hair tips that adhere, and was the cover story in the Proceedings of the National Academy of Sciences, USA 99(19): 12252-12256. http://www.pnas.org/content/vol99/issue19/cover.shtml
We are working with Ron Fearing, an engineer at Berkeley, to design and fabricate synthetic gecko setae. Other lizards, and some insects have convergently evolved setae that seem to work in the same way as geckos, but are much simpler in their structure. We chose the "deluxe model" for our study, but we will probably need a simpler model for fabrication of artificial setae. Fortunately we can use Darwins comparative method to choose the best model from biodiveristy. There are countless applications for a synthetic gecko adhesive. Astronauts could use gecko tape and it wouldnt fall off in a vacuum. Single setae could be used in microsurgery and in silicon wafer handling.
We are working with IS Robotics in Boston to design tiny robots that can climb on walls and even upside down. Applications for highly mobile robots are practically limitless. Imagine a firefighter tossing a handful of geckobots into a burning building to search for survivors. A swarm of geckobots could explore Mars. Toy geckobots could be on your child's holiday gift list...
There are still many unanswered questions that we are currently working on. How many setae are attached during locomotion? How is control of attachment and detachment of the single setae integrated with the function of the geckos foot and body? There are about 850 species of gecko, and there are other animals that have evolved gecko-like setae. Each species has a different type of seta. Why? What do the differences mean in terms of function? Some species have much simpler setae without the 1000 tips. Were studying those now.
While we have discovered many of the secrets of gecko adhesion, the millions of dry, adhesive foot-hairs on the toes of geckos continue to produce new questions and valuable answers. Understanding how simultaneous attachment and release of millions of foot-hairs are controlled will require an approach that integrates levels ranging from molecules to lizards. Our work demonstrates how biologists can generate novel designs when relevant principles derived from basic research are transferred to engineers.
Copyright (c) 2003 Kellar Autumn. Do not use without permission of the author.