Those rugged little bodies whose parts rise
And fall in various inequalities,
Hills in the risings of their surface show,
As valleys in their hollow pits below.
RICHARD LEIGH,’Greatness in Little’
It is a lazy afternoon in the Reptile House at London Zoo: lizards lie motionless on the floor of their cell; a chameleon occasionally shifts from one twig to another. And there is a gecko – what is his idea of relaxation? He hangs on the wall on one side of his glass-box compartment, face downwards for 15 minutes or more, then moves across to do the same on the other side. It seems to cost the gecko no effort to do this; something – not his muscles – is holding him to the wall.
Geckos have always astonished everyone who has ever seen them – and that includes Aristotle, back in the 4th century BC – with their ability to run vertically up and down at will. They can scale a perfectly smooth vertical wall, even glass, and walk across a ceiling. Whether the surface is rough or smooth, wet or dry, it is all the same to the gecko.
So, what are geckos? They are a group of nocturnal lizards, about 850 species in all, found across all the southern continents and as far north as southern California, southern Europe and central Asia. The gecko on which most of the research has been done is the Tokay gecko (Gecko gecko), a large Asian species.
The gecko really began to yield up its secrets in the mid-1990s in Professor Bob Full’s Polypedal Lab in the Department of Integrative Biology at the University of California, Berkeley. Full is a large, ebullient stirrer and shaker at the heart of bio-inspiration today. His lab’s name is apt, given the multitude of projects under his wing and the furious pedalling and promoting of the bio-inspired programme that goes on (fig. 4.1). Full is an expert on animal locomotion, which means he belongs to the biomechanic wing of bio-inspiration. Dynamics is his subject and dynamic he is, being at the centre of collaborative efforts between different disciplines and various universities. Early on he saw the potential for robotics in the way that animals move. Much of his work has focused on insect motion – the six-legged gait of creatures such as cockroaches, for instance – but the adhesion of the gecko on vertical surfaces would obviously be an attractive quality in an autonomous robot. Darpa, the Defense Advanced Research Projects Agency, which has funded so much of the work in bio-inspiration, and the firm iRobot agree. One possible use of gecko adhesion is in a climbing robot: the Mecko Gecko.
Full’s approach is to find the precise principle at work in nature and then work with engineers to fabricate technical systems that do the same job. The gecko man is Kellar Autumn, who began in Full’s department and is now a professor at Lewis and Clark College, Oregon.
Autumn has studied geckos for most of his professional life and his initial interest was not in their adhesion at all. Geckos are pretty remarkable all round. They are nocturnal and that means that they have to be active at low temperatures. Autumn did his Ph.D at Berkeley on the energetics of geckos – ‘cold geckos running up a treadmill’, as he puts it. He also spent time in Tibet, collecting geckos in their natural habitat.
The Tokay gecko is the prime gecko in every respect. It is three times more energy efficient than most creatures and its senses are very finely honed. For nocturnal hunting it has enormous eyes and it can hear the movement of an insect on a wall from across the lab. Unlike other lizards, it also vocalizes. During the Vietnam War it became notorious as the ‘FU lizard’. Jumpy soldiers at night often heard a noise which sounded like the enemy taunting them in a foreign accent: ‘FU, FU, FU’, it screeched. The old hands never told the rookies about the creature until they had discovered it for themselves.
Autumn only started to think about the gecko’s stickability when he was trying to take a break from the creature. One night, on vacation in Hawaii, he was lying on his bed when he saw a large spider on the ceiling. He was starting to think it might be dangerous when help arrived. A gecko walked across the ceiling and devoured the spider. On his return, he decided that perhaps the gecko’s feet were the most interesting thing after all.
Although the gecko’s ability is apparent to anyone who ever saw one, understanding it depends on being able to see the micro-structures on the pads of its feet. An unaided visual inspection offers few clues to their adhesive ability. To our eyes, the pad of the foot is crossed by transverse bands that look like a variation on standard reptile scales. Looking at the gecko’s foot through an ordinary light microscope does not help much either: the pads seem to have some sort of bristly structure. It wasn’t until the application of the electron microscope to the problem that the fine structure of the gecko’s foot was teased out.
The electron microscope shows that there are very many bristles on the toes of a gecko: almost 500,000 on each foot. And more than that, the gecko has, as Autumn says, ‘a bad case of split ends’: the ends of the bristles fork into between 100 and 1,000 mini-bristles with enlarged and flattened spoon-like endings, spatulas, which, as Bob Full puts it, ‘look like broccoli on the tips of the hairs’. It is these spatulas that make contact with the surface and a single gecko has about one billion of these points of contact (fig. 4.2).
So the gecko has these impressive structures, and lots of them, but how do they work? Before the era of bio-inspiration, studies of something as arcane as the gecko’s foot were the province of zoologists alone. For over 100 years there have been innumerable descriptive papers dealing with gecko feet but they all came up against the limits of microscopy – the Blind Zone. Early speculations on the mechanism involved included suction and capillary force, and there were those who believed that the secret lay in the sheer number of bristle endings – if only we could see them properly.
One question that occurs to most people sooner or later is: can a dead gecko stick? Kellar Autumn tells us: ‘Yes, a dead gecko can stick to vertical glass – even with a single toe. It can stay there for quite some time in fact. A large Tokay gecko died in our facility, and while it might seem morbid, we also wanted to answer this question. So we stuck it to vertical glass by a single digit, and it hung there for the entire day. Indeed, if it wasn’t for our concern about odour, we could have left it there much longer.’
So, the bristles on the foot are not themselves ‘alive’, nor do they depend on any kind of muscular activation to achieve a bond: the foot bristles are adhesive even when not attached to the gecko. Tape can be made from them, which can be harvested harmlessly from a live animal (the bristles then grow back).
In June 2000, Autumn’s team published a paper in Nature that really brought the gecko into the age of bio-inspired solutions. They measured the adhesive force of a single gecko bristle. The experiment was very delicate and precise, given the tiny size of the bristle. This is where contemporary engineering and physics meet biology, because the micro-electronics industry has a range of techniques for working with very small structures. Bob Full’s teams always include engineers, drawn from within Berkeley, Stanford and universities further afield.
Autumn’s team showed that a single bristle did exhibit the force required to account for the adhesion of the foot: more than that, a single gecko bristle had 10 times more force than you would have expected given the force exerted by the whole animal and the number of bristles. This suggested that at any one time only a fraction of the bristles was in contact. It also meant that if a gecko did have all its bristles in contact at the same time, it would be able to support a 120-kg man. In the real world it is unusual for two surfaces to touch at many points – surfaces may look to us as if they are in intimate contact but at the nanolevel hardly anything touches at all. To work effectively, the gecko has a huge margin of error. That is why it moves around so confidently and can suspend itself from a single toe: it knows it has plenty of adhesion in reserve.
But is it simply contact that explains the gecko’s adhesion? Autumn likes to describe the process like this: ‘The split ends merge with the surface at the molecular level.’ It has to be at the molecular level, and there have to be some very small structures like those split ends to make the contact.
What is this force that acts when the gecko’s bristles touch a surface, that, multiplied millions of times by the gecko’s bristles, gives it its remarkable adhesion? It is a universal force of attraction that acts on all things at the bottom end of the nanorange (up to 2 nm). Further than 2 nm the force cannot be felt at all. It is not gravity, electricity or magnetism, and it is not chemical attraction. It is called the van der Waals force, after Johannes van der Waals, the Dutch physicist who first proposed these forces in 1873 to account for the temperature, pressure and volume behaviour of gases.’Van der Waals force’ sounds wonderfully obscure and highfalutin but it is just a sort of background attraction, always there but very faint. It is the Lucretian Leap again: forces on the nanoscale are different from those in the world above.
The van der Waals force only acts when two objects come within a range of under 2 nm. Most things in the real world never get close enough for this and it may not be immediately obvious why this is so. If you look at an apparently smooth surface through a scanning electron microscope that can see in detail down to 2 nm, the smoothest surface to our eyes is revealed as a ragged mountain range. Put two such ‘smooth surfaces’ together and in reality their area of contact will be very small. Only if a large enough area comes into contact will any adhesion be developed. The gecko’s billions of spatulas can mould themselves to the contours of any surface, achieving van der Waals adhesion over a wide area. Thus it is the opposite of the Lotus-Effect: Lotus-Effect bumps are much larger than the gecko’s bristles: they make sure that very little contact is made between water or dirt and the lotus surface.
Autumn’s Nature paper concluded that the source of the gecko’s fierce pull was almost certainly the van der Waals force. The only other possible candidate – not entirely ruled out – was some form of capillary attraction associated with an ultra-thin film of water on the bristles. Capillary attraction is the force that exists on the surface of a liquid: it is this that makes water curve where it meets the sides of a glass container and causes water droplets to become spherical when placed on a hard surface.
With Autumn’s work, the gecko now became an interesting engineering problem, not just a biological curiosity. Professor Ron Fearing, the microfabrication engineer at Berkeley, University of California, working on dry adhesive structures modelled on the gecko, says: ‘The image of what something is gets your imagination working. You could say: “We’re going to try to make some structures that maximize the van der Waals forces.” But when you see the gecko you go: “Oh, that would work, wouldn’t it, a bunch of small hairs!”’
Ron Fearing is the key member of the gecko team on the engineering side, operating from Cory Hall, across the Berkeley campus from Full’s lab in the Valley Life Science Building. Fearing is an engineer who became interested in biology when he realized that it is not just the nervous systems of creatures that enable them to pull off astounding physical feats: nine times out of ten it is the mechanics. As he says: ‘You can get more bangs for your bucks by having a better mechanism.’
Fearing is an unusual figure in bio-inspiration, combining engineering skills at the millimetre/centimetre scale – which he uses to construct miniature flying vehicles based on the fly (see Chapter 7) – with the microfabrication techniques of computer-chip manufacture. From 1998, when Autumn’s work was pointing strongly in the direction of a purely physical mechanism for gecko adhesion, Fearing set himself to create arrays of bristles similar to those of the gecko. To ‘anyone skilled in the art’, the almost alchemically mysterious phrase that patents employ, the gecko’s mechanism immediately suggests a variety of ways in which they might be fabricated.
The 2000 Nature paper says: ‘Although manufacturing small, closely packed arrays mimicking setae [bristles] are [sic] beyond the limits of human technology, the natural technology of gecko foot-hairs can provide biological inspiration for future design of a remarkably effective adhesive.’ This condition – ‘beyond the limits of human technology’ – did not last very long. It certainly did not deter Ron Fearing and he has produced some pretty good synthetic gecko arrays with the same density as the natural bristles (fig. 4.3) using ceramic moulds with nanoholes to cast polyurethane bristles.
After the paper in Nature, some researchers suggested that the alternative mechanism for capillary attraction needed to be tested, so Autumn’s team set out to evaluate the two rival theories. This was of more than theoretical importance because if the effect depended entirely on the van der Waals force, synthetic versions could be fabricated using almost any materials and the effect would only be dependent on the size, shape and rigidity of the fibres. If capillary forces came into play, the biological processes involved were likely to be hard to reproduce. So, before investing too much in the synthetic programme it was important to be sure of the mechanism.
If water were involved in the adhesive action, different results would be expected on strongly water-repelling and water-attracting surfaces. In fact, the gecko foot bristles adhere equally well to both. These results, published in August 2002, gave the green light to attempts to fabricate artificial gecko adhesives. Only the size, shape, rigidity and, above all, the number of contact points matter.
This work caught the eye of Andre Geim, a Russian-educated physicist of German extraction, and a professor at Manchester University working in the field of condensed matter physics. He has a new purpose-built Institute of Nanotechnology and Mesophysics at his disposal, with state-of-the-art fabrication equipment in clean-rooms.* The gecko mechanism was a challenge for this kind of fabrication and he set his Russian colleagues to work on the problem. Unlike Full’s team, who are committed to long-term development of their concepts, for Geim this was a project off the main line of his research interests: he simply saw an opportunity to ‘prove the concept’. Geim has heretical views about the gecko’s mechanism and does not believe that the case for a mechanism relying solely on van der Waals forces has been made.
The capillary forces that Autumn’s team have rejected might play a role in some situations, he says: ‘No one knows for sure. Of course there is capillary attraction on some surfaces. Keratin is said to be hydrophobic [water-repelling] but many people wouldn’t agree with this. I don’t have experience of geckos but you know from experience that your hair is hydrophilic [water-attracting] otherwise you’d never be able to take a shower. The best guess is that on this level there is no distinction between van der Waals and capillary forces. When there is water on the surface you get what I would call “water-mediated van der Waals force”.’
Geim’s team used an atomic force microscope to create dimples in a wax surface which was then used as a mould to create plastic pillars. In truth, these pillars were not much like the gecko’s bristles. They were very short and squat by comparison. Nevertheless, even though they were far removed from the refinement of the gecko’s system, the plastic pillars worked. Geim was only able to make a little of the ‘gecko tape’ (1 sq cm) but he extracted maximum media impact by attaching it to the hand of a Spiderman toy that could easily be stuck to a horizontal glass plate (fig. 4.4).
The limitations of Geim’s tape were obvious. The plastic used is rather soft and after a few uses the pillars start to stick to each other rather than to the plate. And they are water-attracting and quickly get dirty. Water-repelling, self-cleaning bristles are what is required. A gecko can reuse its bristles thousands of times on any kind of surface – rough, smooth, clean or dirty.
Although this early attempt at making a gecko adhesive was clumsy, the first transistor was also a large, lumpishly crude device, the first jet engines could barely lift a plane into the air, and the first TV pictures were barely discernible. In the history of technical inventions, proof of concept usually leads to dramatic refinements and manifold improvements in performance over a period of up to 30 years or so, when some kind of plateau is reached.
It is not enough to have the fine-scale bristles at sufficient density; enough of them need to make contact and this means that the array must be flexible in order to conform to the shape of the surface – it needs to be compliant, as they call it. This could be achieved by having a flexible backing, or by having flexibility in the stalks. As Ron Fearing says: ‘You don’t have to have soft hairs. You can make them out of hard material and make it soft by making them long and skinny. If you take a chunk of steel, and make it into a hair, it’s compliant.’ But if the stalks are too flexible they are going to fall over, and become entangled.
Fearing admiringly notes the ten different levels of compliance in the gecko: the spatula can bend, then the spatula hair, then the bristle hair, then the foot pad…and so on up the leg to the gecko’s body. Against the ten levels of compliance in the gecko, a standard adhesive tape has only two levels of compliance: the soft sticky surface and the flexible backing.
It is also true to say that beyond the micro-structures, the leg of the gecko has complex mechanisms that enable the sticking and unpeeling to take place. The legs of a gecko on a wall splay widely in almost a Sumo wrestler pose and the precision of its movements is very refined. This is not relevant to adhesive uses because simple peeling mechanisms suffice, but if a gecko-like robot were ever going to climb vertical walls it would need a similarly complex foot mechanism.
The gecko mechanism was patented by the Autumn/Full/Fearing team on 18 May 2004. The patent stage is always a significant and delicate one along the road to the commercialization of an invention. Delicate, because you can neither wait till a technical process is fully realized – by then someone else will almost certainly have pipped you to the post – nor apply too early, because a patent needs to cover every possible ramification of the technique. If, for instance, a patent specifies only one method of achieving a certain end when in fact there are many, or, if very narrow conditions are cited, say, of size, reaction temperature or materials, the patent can be evaded by using a condition falling outside those specified. So a degree of legalistic comprehensiveness is necessary in a patent even though the implications of all the possibilities will not have been worked through at the time. Patents can thus contain a maze of clauses: the gecko adhesive patent application has 34 clauses in its claims section, ranging from taking bristles from living geckos to a range of fabrication techniques.
As the patent puts it: ‘hundreds of thousands of setae [bristles] can be harvested without sacrificing the living being from which the setae are removed.’ Autumn’s team demonstrate a simple gecko sticking plaster in which three strips of gecko bristles along the tape do the job. Gecko bristles leave no gloop behind and can be used again and again, almost ad infinitum. The gecko, meanwhile, grows a new batch of bristles. This is, of course, strictly proof of concept: this is not the way gecko tape is going to be made in the future.
Gecko bristles make especially good sticking plasters because of the way the adhesion takes hold. The gecko pushes its foot pads against the surface and then pulls upwards. Along the line of contact, this produces the authentic gecko force (gecko tape applied to one hand would be powerful enough to stick a man to the ceiling). But if the pad is pulled away at an angle from the surface – which the gecko achieves by means of its foot-peeling mechanism – the bond is broken. In a similar fashion, when a plaster is wrapped around a finger, the pull is strong along the line of the plaster; peeling away in the traditional manner breaks the gecko bond. As Ron Fearing says: ‘It pops straight off. It won’t stick to the hairs, so it doesn’t hurt when it comes off.’
There are already countless potential uses for gecko adhesives and no doubt there are plenty more as yet unforeseen: besides the household uses of a dry version of standard adhesive tape, a group of bristles can grip, carry and release micro-objects such as micro-electronics components; microsurgery is one more obvious application; and, because its grip is so directional, a gecko bristle can act as a clutch for micro-machines. The patent application makes a brave stab at covering the field:
Other applications for the technique of the invention include: insect trapping tape, robot feet or treads, gloves/pads for climbing, gripping, etc., clean room processing tools, micro-optical manipulation that does not scar a surface and leaves no residue or scratches, micro-brooms, micro-vacuums, flake removal from wafers, optical location and removal of individual particles, climbing, throwing, and sticker toys, press-on fingernails, silent fasteners, a substrate to prevent adhesion on specific locations, a broom to clean disk drives, post-it notes, band aids, semiconductor transport, clothes fasteners, and the like.
The gecko story shows strong parallels with that of the Lotus-Effect. For decades everyone thought that there was something magical about the gecko, but the principle of its adhesion does not depend on the creature’s behaviour, on living tissue, or on any particular chemical structure. The gecko’s principle is, like the Lotus-Effect, a universal property of certain nanostructures. In fact, so universal is the gecko effect that after publication of their paper, the Berkeley team heard from many researchers in the nanofabrication field saying that they couldn’t stop their nanostructures sticking to each other – and now they knew why.
In the animal kingdom, a good grip is not confined to geckos. Many creatures – beetles, flies, spiders, and other lizards besides the gecko – have excellent adhesion, due to small hairs on their feet. A team at the Max Planck Institut at Tübingen discovered a blindingly simple but apparently paradoxical rule connecting the size of the creature and the size of the bristles: the larger the creature the finer the division of the bristles. This may seem odd to us because we and other creatures that cannot walk across the ceiling have feet whose surface area grows disproportionately with increasing weight. For instance, an elephant’s feet are disproportionately large compared to those of a human being, if its length alone is taken into account. This is because the weight of any creature grows with the cube of the length, whereas the area of its feet grows only with the square of the length. If a creature doubled in size, including its feet, its weight would increase by a factor of eight whilst its foot area would only have increased by a factor of four. So larger creatures have to have proportionately broader legs and feet to bear the body’s weight.
But creatures that can cling to surfaces need to maximize their adhesive force, and the same rule apples to all: the more bristles, the greater the adhesion. So large creatures like the gecko, with their increased weight ration, need to have a higher density of bristles than smaller ones such as flies. A graph of the number of bristles per area against mass is that simplest of all scientific relations: the straight line. This demonstrates the great generality of nature’s nanoengineering – during evolution, the density of the foot hairs of different creatures has evolved to match the weight of that creature.
The gecko’s is not the first of nature’s adhesive mechanisms to be useful to man. One of the earliest examples of bio-inspiration is the Velcro® brand hook-and-loop fastener. Velcro is such a good name: it is a compound of velours (the vel- part of the name derives from the fact that the weave is technically a velvet: a pile-woven fabric with the threads cut to produce an upstanding fringe or pile) and crochet: the sound of the word mimics the combination of smooth action and raspy spikiness that is its hallmark. Almost too good a name, because the Velcro Corporation, the principal makers of hook-and-loop fasteners, are at pains to point out that the Velcro® brand is the registered name of their product and that there is no such thing as generic ‘Velcro’. The company is the latest manifestation of the enterprise founded by the product’s inventor, George de Mestral, and is clearly the legitimate guardian of the flame. The Velcro Corporation’s naming policy is understandable but it makes discussion of their product in a book such as this difficult. But, as with the complexities of patents, protecting a trademark name is also one of the necessary complications of bringing an invention to market.
There is a persistent urban myth that the hook-and-loop fastener was a spin-off from NASA’s space programme. It wasn’t: NASA had no hand in its invention but astronauts did find it useful to fasten things in weightless conditions and this helped popularize the product.
The hook-and-loop fastener is a story from the pre-history of bio-inspiration and it probably evaded invention for so long because, on the face of it, it doesn’t seem much of an engineering solution. It lacks the precision of most engineering but as a practical application that is exactly its glory. It is the first example of fuzzy logic. The zip fastener is an excellent gizmo but if the zip slips out of its notched channel it is not easy to get it back in again. But the hook-and-loop fastener doesn’t have to be lined up accurately. It is what the German pioneer of bio-inspiration Werner Nachtigall* calls a ‘probabilistic fastening’ – a fancy way of saying that you just fumble with the thing. Whether an individual hook goes through a specific eye is irrelevant: every time you use it, enough hooks will find an eye to achieve a bond. For this reason it is considered slovenly by people who are punctilious about dress code.
As an object, the hook-and-loop fastener is delightfully anomalous. It acts and feels like a glue but it isn’t sticky and is reusable thousands of times. The principle derives from the seed-propagation strategy of plants such as the cocklebur and burdock. The fruits of these plants, known as burs, are covered by thin spines with sharply hooked ends. So sharp are these that they snag anything that passes by. Animal fur is the prime target of burs but they stick to pretty well anything that comes their way.
One day in the 1940s, the target was a dog belonging to George de Mestral. Exactly what species was involved is shrouded in mystery. There are various accounts in the literature, provided by the family and the company: burdock, cocklebur and the mountain thistle have all been cited. It is not the kind of thing that gets recorded precisely in a diary (‘Eureka, today I discovered Velcro’) because its significance only emerges with time.
De Mestral was an inventor, trained as an electrical engineer, who lived in the family chateau near Lausanne, Switzerland. It was his pastime of hunting on the lower slopes of the Jura mountains that led to the discovery. De Mestral’s grandson has said: ‘It was his passion. These rare moments allowed me to be in touch with him, which was difficult. Everyone remarked that he was often lost in his own world.’
In that ‘own world’ of his de Mestral was on the lookout for fasteners. The story is that he had become frustrated with the difficulty of fastening the large hooks and eyes on his wife’s dress before going out for the evening. De Mestral couldn’t bear to be late for anything and he kept thinking that there must be a better way.
When he got back home from his walk, his dog was covered in burs and instead of just picking them off he marvelled at their tenacity, and this started him thinking. Nature maximizes the number of spines on the bur to make attachment more ‘probable’, in the Nachtigallian sense. The bur is spherical because it needs to maximize the angles by which it might catch a passing animal, but if the bur were rolled flat, as it were, a small square of hooks would stick to a rough fabric at whatever angle it was presented – the sort of precision docking required to fasten a single hook and loop would not be necessary.
The hook-and-loop fastener is an example of bio-inspiration from the time just before Feynman’s call to consider the possibilities of nanostructures. The progression from hook-and-loop fastener to gecko adhesion shows a reduction in size from the micro-world to the nanorealm very much in line with the general thrust of engineering practice in the last 50 years. Although it is not nanotechnology, the spines on a bur are microfabrications and considerable work went into developing a viable process for manufacturing their technical equivalent.
Ideas often have to wait for a material in which to clothe them. In de Mestral’s case, he was waiting for nylon – no substance before it could have been fashioned into an effective hook-and-loop fastener. Nylon was invented in 1937 but it was so important to the war effort that it was not available for other uses until after the war. De Mestral then took several years to find a machine process for creating hooks that could snag on the loops. To make the hooks, loops were first formed by passing nylon thread over a bar; the bar was heated to fix the shape and a knife cut the loops to produce an opening, hence a hook.
The original patent was filed in 1951 (fig. 4.5). With help from a weaver at a textile plant in Lyon, France, and a Swiss loom-maker in Basel, de Mestral perfected his hook-and-loop fastener and the product came to market in 1955.
In the original patent, the two opposing strips were more similar than they are in modern-day versions – in fact they were identical in weave but with one strip having the rows at 90° to the first, thus allowing the hooks of one row to make a secure clasp with the loops of the other. It looks as if once de Mestral tackled the engineering problem of making a reliable synthetic equivalent of the bur, for a while he forgot the lesson of the natural solution: the hooks have to have neat structures but the other half can just be tangled fur.
The American patent expired in 1978 and George de Mestral died in 1990. Hook-and-loop fasteners are now a major product worldwide and the Velcro Corporation is still the major producer. There are many variations on the original format, with some tapes using metals and able to withstand temperatures of 800°C for use in aerospace applications. In fact, almost anything that can be stuck can be stuck with a hook-and-loop fastener. It is the only bio-inspired product to have been on the market long enough to have been humanized as a ‘dear and genuine inmate of the household of man’.
There is a persistent thread of humour that likes to relocate such familiar man-made products in the natural world. The television programme Panorama once ran an April Fools Day hoax on the spaghetti fields of Italy. The hook-and-loop fastener achieved this honorary state in a paper by Ken Umbach which reported difficulties with the Californian Velcro crop.
Three problems were encountered in the San Joaquin Valley growing area. Dry and windy conditions caused hook-and-loop spores to commingle, resulting in tangled Velcro bolls combining both strains, unprocessable by any known means. Various pests assailed the crop: the flaccidity virus weakened the hooks causing them to snap; Millepedus minisculus multiplied amongst the crop until it became ensnared in the developing loops and made harvesting impossible. Finally, drought exacerbated crop-stunting salinity. Happily, by late 1996, conditions had returned almost to normal and Velcro today is blossoming. The only faint cloud on the horizon is the suspicion that the static electricity produced by billions of Velcro unzippings every day might be a factor in climate change.
Back in the real world: there is a third creature with remarkable adhesive powers, although it is better known as a culinary treat. The tenacity of mussels clinging on to rocks, ships and pier supports is legendary and they achieve this by means of an adhesive that sets under water, something human glues cannot do (it always says on the tin: ‘surfaces must be clean and dry’). Unlike the gecko, mussels use a wet, sticky glue that gets into cracks in the rock and forms strong elastic cross-linkages. Not only that, but it has a chemical affinity for metals which means that it probably sticks even better to metal piers than it does to rocks. The mussel attaches itself to objects by means of a thread – the byssus – and this spreads out at the end into a plaque that sticks to the rock by means of the glue.
The mussel usually used in research into its adhesive is the blue mussel (Mytilus edulis), the one served in a dish of moules marinières. The glue has been identified as a protein but it is devilishly difficult to work with. It is so sticky from the moment it is formed that getting it to the place where you want to use it is a problem.
Nature gets round this in an ingenious way. The mussel protein is first made in an unfinished form and chemically transformed into the active glue at the last moment – one of nature’s just-in-time manufacturing techniques. There are many odd things about mussel glue and one of the oddest is this last-minute transformation. The transformation creates a DOPA molecule as part of the protein chain and DOPA (dihydroxyphenylalanine) is better known as the drug used to treat Parkinson’s disease and made famous by Oliver Sacks’s casebook in Awakenings of patients who regained consciousness after decades of sleep. However, this dual use is just an interesting coincidence. DOPA has useful properties in two completely different contexts – the mammalian brain and the mussel’s foot: nature is not fussed about the hierarchical divide between the two.
Inside the mussel, DOPA forms extensive cross-links, a process similar to the coagulation of egg white, but much more concentrated. The glue is remarkably water-repellent, making sure that water cannot interfere with the bond, and has a strong affinity for metal ions, hence the avidity with which mussels bind to metal piers.
Of all the processes in this book, mussel glue is closest to spider silk in being a protein nanoproduced inside living cells. Such processes, in which a cascade of chemical reactions occurs, governed by the specific structures within the cell, are the hardest natural processes to replicate. As with spider silk, genetic engineering techniques are not straightforward in this case, with cloned mussel proteins coming out shorter than those in the natural environment. And then there is the need to reproduce the mussel’s trick of altering the protein after it has been synthesized to create the DOPA molecule.
There are several groups worldwide working on the processes enabling mussels to make the glue and little by little the mussel’s secrets are being prised out. The key to the mussel’s ability seems to be that it is a filter feeder: that is, it strains vast quantities of water to extract what nutrients it can. This means that it is able to concentrate substances such as iron that are only sparsely available in sea water. Iron is part of the mechanism of the glue, forming chemical links with DOPA. But DOPA’s greed for iron is such that it latches on to it wherever it can – other metals also work to some degree. So imagine the amazing eruption into the mussels’ world that man-made metal structures must have been. Mussels are attracted to iron pier-supports, bridges and ships the way moths are drawn to a flame.
In an elegant experiment, the adhesive powers of DOPA have been put to a kind of reverse use. Scientists at Northwestern University, Illinois, have used it to create a non-stick surface for medical applications. This is something that happens time and again in bio-inspiration. Most processes can be reversed and the reverse process can sometimes be more useful than the standard version. Some medical devices and diagnostic kits need to be protected from cells and biological fluids that would naturally stick to them. A chemical compound called polyethylene glycol (PEG) is good for this but it has to be bonded to the device. The Northwestern scientists showed that PEG can first be linked to DOPA; the DOPA then attaches to metal surfaces with its usual greed and the whole is then resistant to fouling by cells.
Meanwhile, work continues on the primary task of making a mussel glue that is easier to handle. A major use would be medical, gluing tissues together to allow healing rather than using stitches. Mussel glue is a project for the long term.
Although bio-inspired adhesives, when they reach fruition, could be used in many ways, it is more likely that the applications will be specialized and medical rather than replacements for the standard reel of sticky tape – after all, sticky tape works well and is cheap. The dryness and special peeling ability of gecko tape should prove attractive. Ron Fearing says: ‘There is going to be someone out there who needs something to stick, say inside the body. They’ll want something that’s non-toxic and will hold for as long as they want it to and come out cleanly when they want it to.’
Although the applications listed in the gecko patent are practical, lurking at the back of the gecko story is an ancient dream: the Spiderman myth. The vertical dimension inspires a mixture of fear and desire. As with flying, some humans would like to climb buildings safely without tackle. The gecko generates such force across a small surface area that gecko gloves, provided they had a safe peeling mechanism, would bring Spiderman’s feats within the realm of reality. It was Spiderman that Andre Geim thought of when he wanted to dramatize his invention, and talking to the press Kellar Autumn often says: ‘Forget Spiderman, what we want is Gecko Girl.’
It is increasingly likely that gecko tape, like Lotus-Effect coatings, is going to make it. The road from proof of concept to full implementation can be a long one – often up to 20 years, and the Full/ Autumn/Fearing team has been on the case since 1998. The big adhesives manufacturers are now seriously interested. If gecko development keeps pace with the example of the Lotus-Effect, we can expect commercial gecko tape around 2009, but it could be much sooner.
The gecko effect and the Lotus-Effect are both concerned with the surface of materials: the lotus presents its wax-encrusted hillocks to the world and the water rolls off; the gecko presents a rippling array of spatulas. But there are structures in many creatures that go beneath the surface: galleries of sculpted subterranean passages. And because they are beneath the surface they are not likely to be involved in surface effects such as adhesion or repelling water. So what are these intricate cave complexes of the nanorealm? The medium they were designed for is light and the effects nature achieves in this element – nature’s iridescent lightshow – are in the vanguard of optical technology.