God's Gecko Glue
||Bert Thompson, Ph.D.
Branyon May, Ph.D.
For over two millennia, the gecko has been the subject of curiosity, fear, and amazement. In some cultures’ folklore, geckos are believed to hold a medicinal value when dried, boiled, and eaten. Yet others fear them, suspecting their bite to be highly venomous and even deadly. Fortunately for the countless children who have spent their free time and energy chasing them, they are neither poisonous nor harmful, but rather induce a wild squeal of delight from youngsters as the geckos wriggle in their hands. Records of these amazing lizard-like creatures can be found before Christ ever walked the Earth. In his manuscript, Historia Animalium, written in 350 B.C., Aristotle mentioned these curious creatures at least five times. At one point, he described another creature by saying, “It can run up and down a tree in any way, even with the head downwards, like the gecko-lizard” (Aristotle, n.d.). The gecko’s superior ability to climb and cling has become its trademark—its defining characteristic. It has been described as “one of the reptile world’s greatest climbers” (see “Scientists Defy…,” 2003), and the “envy of every serious rock climber” (Pennisi, 2000, 258:1717). However, as a rock climber is dependent upon his equipment, so also the gecko is able to accomplish its extraordinary feats by using some intriguing “equipment.”
The gecko is a quadruped, meaning that it walks on all fours. This is an important fact because everything required to scale the obstacles before it must reside on the feet of this tiny reptile. And, how elegant the foot of the gecko is! It truly is a work of minute precision. Its omnidirectional toes allow the gecko the support needed to ascend a vertical wall without fear of backsliding. The feet and toes then are subdivided into divisions called lamellae, which are lobe-like, overlapping layers. On each lamella, there is a multitude of tiny, hair-like structures called setae, which impart the fine texture that can be readily observed by our human senses. Like most animal hair, setae are keratinous in their composition. Kellar Autumn and colleagues noted: “Microscopy has shown that a gecko’s foot has nearly five hundred thousand keratinous hairs or setae. Each 30–130 µm long seta is only one-tenth the diameter of a human hair” (2000, 405:681). On average, the setae are arranged with a surface density around 5,000 per square millimeter. When you compare this figure with the pelt of the sea otter, which has one of the highest fur densities in the animal kingdom (at approximately 1,000 per square millimeter), the complexity is apparent simply by sheer magnitude. However, this is only the beginning of the complexity of the gecko’s remarkable equipment. By searching beyond the optical confines of light, researchers have discovered the hidden secret of the gecko’s foot. Using electron microscopy, scientists have learned that, attached to the end of every tiny seta, are hundreds of minuscule cilia-like projections. These projections are smaller than a wavelength of light, making it impossible to detect them with optics set in the visible range (i.e., around 400 to 700 nanometers). Furthermore, the projections terminate in spatula-shaped structures, aptly named spatulae (Autumn et al., 405:681). Without these exceedingly intricate structures, the gecko most assuredly would be relegated to a lackluster life on the ground.
Knowing, finally, the anatomy that the gecko has been using to maintain its heights of splendor, scientists then searched for the physiological mechanism underlying these feats—feats that allowed the animal to seemingly defy gravity. Past analysis of the gecko’s climbing ability yielded a multiplicity of hypotheses for adhesion. Scientists have taken these previous hypotheses and re-tested their conclusions in light of recent discoveries. An early proposition involved the gecko using some form of suction between the pads of its feet and the surface being traversed. An example of this mechanism presently at work in nature is the salamander. Testing for adhesion by suction, the scientists changed the environmental pressure of the subjects, both salamander and gecko. If suction were used, then the effectiveness of adherence would be altered. Both an increase and decrease in pressure confirmed that the mechanism the gecko uses does not rely on any form of suction. A second proposed hypothesis was that of micro-interlocking structures working together with friction to achieve adherence and motion. A functional example of this method would be the ubiquitous cockroach, which uses tiny barbs on its legs to clutch irregularities on the surface. However, when placed on a smooth surface (one with less than or equal to 2.5 nanometers in variation and having a low coefficient of friction), the gecko continued to perform, undaunted by the change. A third proposed hypothesis was adhesion by secretion. In nature, there are several creatures, such as the slug or snail, which use secretion of glue-like substances, as is clearly apparent from the slimy residue left in their trail. But as Autumn and his colleagues noted, this would be extremely hard for the gecko, seeing that “skin glands are not present on the feet of lizards” (405:683). A fourth hypothesis proposed that an electrostatic attraction is produced between the surface and gecko. This can be likened, with quite a bit of oversimplification, to the “static cling” produced by a constant rubbing together in a hot, dry environment (and found on laundry that was mechanically dried—as every college student can attest!). However, through a much more technical process, that of x-ray bombardment, this postulation also was proven not to be at work in the gecko’s locomotion.
The complexity of the gecko’s anatomy is demonstrated by the extensive list of invalid hypotheses that have been suggested to reveal its secret. More recently, scientists have postulated, based upon elimination of many other possibilities, and in accordance with fairly recent structural discoveries, that the secret to the gecko’s amazing dry adhesion lies in microscopic intermolecular forces known as van der Waals forces.
VAN DER WAALS FORCES
Johannes Diderik van der Waals, a Dutch scientist who lived in the late nineteenth and early twentieth centuries, worked on establishing the relationship between the pressure, volume, and temperature of liquids and gases. During his study, he saw the necessity for including the effects of forces present between adjacent molecules. As gases cool (slowing their kinetic motion), they begin to clump together and, given the opportunity, will form liquids, and eventually solids. So, van der Waals postulated that every molecule experiences some form of molecular attraction with contiguous matter (see “Johannes Diderik…,” n.d.). These intermolecular forces are negligible at relatively large distances, being overpowered by other forces (e.g. gravity and electromagnetism). However, at very short molecular distances, van der Waals forces become quite important.
Within every atom (and thus every molecule) are electrically charged particles, collectively known as subatomic particles. The negatively charged particles are electrons; the neutral particles are called neutrons; and the positively charged particles are protons. The protons and neutrons are packed together to form the nucleus of the atom. The electrons “orbit” around the nucleus, creating an area known as the electron cloud. On the subatomic level, a vast distance separates the nucleus and the electron cloud. The relative distance can be depicted by an analogy. Consider a football stadium. If the uppermost seats were the electron cloud, then the nucleus would be the size of a ping-pong ball placed at the fifty-yard line. Van der Waals forces result from a build-up of similar charge within an atom or molecule. Any atom can become subject to these forces when a partial charge imbalance arises. This can occur anytime the charge equilibrium becomes asymmetrical. This is analogous to the nucleus being at one end of the stadium, while the electrons are clumped together at the other. The new arrangement of the electrons creates a partially negative region. This, in turn, leaves the vacated region with a partially positive charge, produced by the protons in the nucleus of the atom. Sometimes, these charge separations can be temporary, fluctuating in both strength and geometrical arrangement. At other times, they can be permanent, like the separated poles of a magnet. Oftentimes, the temporary partitions in charge (or dipoles) are induced by correlating dipolar characteristics of the surrounding molecules. These temporary dipoles form a division in van der Waals forces referred to as “dispersion forces.” (They also are known by the name of “London forces,” named after Fritz London, the man who first suggested how they might arise.) The strength of the dispersion forces is influenced by two factors—the molecular size and the molecular geometry. The larger the radial volume (usually accompanied by an increase in electrons), the greater the possible proportion of charge separation. Geometrically, the longer a molecule is, the greater the possible distance for the charge to separate (see “Intermolecular Bonding…,” n.d.).
In the gecko’s case, it exploits the fact that van der Waals forces are universal in their affect on atoms. Whether the gecko is walking on a horizontal or a vertical surface, smooth or rough, wet or dry, low-pressure or high, it maintains its ability to adhere by using the tiny spatulae structures to induce infinitesimal charge separations in the peripheral atoms—regardless of the surface. Although each spatula is able to produce only a diminutive force of attraction, the geometrical structure, enormous quantity, and design of the spatulae and setae on a gecko’s foot allow it to produce enough force to enable itself to hang upside down from any given surface, using merely a single toe (Graham-Rowe, 2003, 178:15). In fact, researchers have calculated, using their laboratory measurements on a single gecko seta, that the gecko uses approximately .03 percent of its setae when supporting its body weight. They further noted: “If each seta were attached simultaneously, it could have 120 kilograms (265 pounds) of adhesive force” (Brown, 2002). Is this amazing design—or random chance?
EVOLUTION VS. DESIGN
Due to the amazing abundance of creatures, such as the gecko, which simply are “the best” at what they do, science has developed a specialized field devoted to searching out the design of natural mechanisms. The purpose of this search is to examine nature’s models, with the goal being imitation and replication. This branch of science has become known as “biomimicry.”
The gecko has now become a subject for those involved in the scientific study of biomimicry. Industry has fueled monetary support for the desired replication of dry adhesives that could imitate the gecko’s natural capabilities. Researchers, together with engineers, have initiated studies, fabricating materials that could be coated with synthetic gecko-like hairs, which would mimic the amazing utilization of van der Waals adhesion. This offers a possible mechanism for the first dry, micro-structural adhesive. Like the gecko’s foot, this adhesive would be for continual use, not just a one-time placement. Several groups have worked on the production of a “gecko-tape” adhesive. In their attempts to produce the tiny structures, the scientists have called upon their colleagues from the area of microelectronics. Although computer chips and gecko feet are very different, the two research groups have decided to collaborate in order to enhance their manufacturing capabilities. However, scientists have noted that the exact duplication of these complex anatomical structures simply may not be possible. Researchers may have to settle for “second best,” as Elizabeth Pennisi suggested in an article she wrote for Science: “Instead, they (researchers—BM/BT) hope that studies of other lizards and also of kissing bugs, which have setae with few and sometimes only one spatula, will help them design simplified setae that can be manufactured” (2000, 288:1718, emp. added). In an article published in the September 17, 2002 Proceedings of the National Academy of Sciences (PNAS), the researchers responsible for the initial gecko discoveries echoed a similar conclusion when they wrote: “We find it remarkable, however, that the geometrically simple Johnson-Kendall-Roberts model and our physical models were sufficient to approximate the function of setal tips.” The point of all of this, of course, is that while it might be possible to “approximate the function” of the gecko’s setae by manufacturing “simplified setae,” being able to perfect and duplicate the exact design found in every living gecko is too much to hope for. Notice what the gecko researchers went on to say in their PNAS article: “We suggest that development of biologically inspired dry adhesive microstructures will not require direct biomimicry of complex gecko setal structures, but rather application by analogy of the essential design principles underlying their evolution” (Autumn, et al., 2002, 99:12255).
Although the gecko has been the inspiration for profound new research inquiries, its magnificent design has been cited as the main reason that duplication of the complex structures and interacting forces cannot be readily accomplished. When the scientists involved in this type of research repeatedly make statements such as, “geckos have evolved one of the most versatile and effective adhesives known” (99:12252) and “geckos have been able to…evolve elaborate microstructures with phenomenal adhesive properties” (99:12255), they readily admit that the little gecko demonstrates incredible “design principles.” Yet they nevertheless contend that the gecko is simply a random product of evolutionary processes.
The gecko’s “elaborate,” “phenomenal,” “most versatile and effective” adhesive is a “design principle” (as the evolutionists have so well put it) that has enabled the gecko to survive, and to do so in a rather impressive fashion. However, design demands a designer—a simple-but-profound concept that the evolutionists have glossed over in order to protect and ensure the sanctity of their theory, which has absolutely no position for The Designer.
As the apostle Paul observed of some of the people of his generation, “they have exchanged the truth of God for a lie, and worshipped and served the creature rather than the Creator” (Romans 1:25). Many have traded their faith in God, and the spiritual blessings endowed by Him, for the vacuous errors of evolution. Would it not be better to imitate the humble nature of the psalmist, and follow his inspired wisdom: “Let them praise the name of Jehovah, for he commanded, and they were created” (Psalm 148:5)?
Aristotle (no date), The History of Animals, trans. D’Arcy Wentworth Thompson. [On-line], URL: http://www.philosophy.ru/library/aristotle/history_anim_en/history_anim.9.ix.html.
Autumn, Kellar, et al. (2000), “Adhesive Force of a Single Gecko Foot-Hair,” Nature, 405:681-685, June 8.
Autumn, Kellar, et al. (2002), “Evidence for van der Waals Adhesion in Gecko Setae,” Proceedings of the National Academy of Sciences, 99:12252-12256, September 17.
Brown, Irene (2002), “Researchers Creating Gecko Glue,” [On-line], URL: http://dsc.discovery.com/news/briefs/20020826/gecko.html.
Graham-Rowe, Duncan (2003), “Fancy a Walk on the Ceiling?” New Scientist, 178:15, May 17.
“Intermolecular Bonding—Van der Waals Forces” (no date), [On-line], URL: http://www.chemguide.co.uk/atoms/bonding/vdw.html.
“Johannes Diderik van der Waals—Biography” (no date), [On-line], URL: http://www.nobel.se/physics/laureates/1910/waals-bio.html.
Pennisi, Elizabeth (2000), “Geckos Climb by the Hairs of Their Toes,” Science, 288:1717-1718, June 9.
“Scientists Defy Gravity with ‘Spider-Man’ Gloves” (2003), [On-line], URL: http://www.cnn.com/2003/TECH/science/06/02/offbeat.spiderman.reut/index.html.