Synthetic setae emulate the setae found on the toes of a gecko and scientific research in this area is driven towards the development of dry adhesives. Geckos have no difficulty mastering vertical walls and are apparently capable of adhering themselves to just about any surface. The five-toed feet of a gecko are covered with elastic hairs called setae and the ends of these hairs are split into nanoscale structures called spatulae (because of their resemblance to actual spatulas). The sheer abundance and proximity to the surface of these spatulae make it sufficient for van der Waals forces alone to provide the required adhesive strength. Following the discovery of the gecko's adhesion mechanism in 2002, which is based on van der Waals forces, biomimetic adhesives have become the topic of a major research effort. These developments are poised to yield families of novel adhesive materials with superior properties which are likely to find uses in industries ranging from defense and nanotechnology to healthcare and sport.
Basic principles
Geckos are renowned for their exceptional ability to stick and run on any vertical and inverted surface (excluding TeflonGecko Tape – 6 Jan 2005 – Sciencentral). However gecko toes are not sticky in the usual way like chemical adhesives. Instead, they can detach from the surface quickly and remain quite clean around everyday contaminants even without grooming.
Extraordinary adhesion The two front feet of a tokay gecko can withstand 20.1 N of force parallel to the surface with 227 mm2 of pad area,Irschick DJ, Austin CC, Petren K, Fisher R, Losos JB, Ellers 0 (1996) A comparative analysis of clinging ability among pad-bearing lizards. Eiol J Linnaean Soc 59:21–35 a force as much as 40 times the gecko's weight. Scientists have been investigating the secret of this extraordinary adhesion ever since the 19th century, and at least seven possible mechanisms for gecko adhesion have been discussed over the past 175 years. There have been hypotheses of glue, friction, suction, electrostatics, micro-interlocking and intermolecular forces. Sticky secretions were ruled out first early in the study of gecko adhesion since geckos lack glandular tissue on their toes. The friction hypothesis was also dismissed quickly because the friction force only acts in shear which cannot explain the adhesive capabilities of geckos on inverted surfaces. The hypothesis that the toe pads act as suction cups was dispelled in 1934 by experiments carried out in a vacuum in which the gecko's toes remained stuck. Similarly, the electrostatic hypothesis was refuted by an experiment showing that geckos could still adhere even when the build-up of electrostatic charge was impossible (such as on a metal surface in air ionized by a stream of x-rays). The mechanism of microinterlocking which suggested that the curved tips of setae could act as microscale hooks was also challenged by the fact that geckos generate large adhesive forces even on molecularly smooth surfaces.
The possibilities finally narrowed down to intermolecular forces, and the development of electron microscopy in the 1950s, which revealed the micro-structure of the setae on the gecko's foot, provided further proof to support this hypothesis. The problem was finally solved in 2000 by a research team led by biologists Kellar Autumn of Lewis & Clark College in Portland, Oregon, and Robert Full at the University of California at Berkeley.Autumn, K., Liang, Y.A., Hsieh, S.T., Zesch, W., Chan, W-P., Kenny, W.T., Fearing, R. and Full, R.J. (2000), "Adhesive force of a single gecko foot-hair", Nature, Vol. 405, pp. 681–5. They showed that the underside of a gecko toe typically bears a series of ridges, which are covered with uniform ranks of setae, and each seta further divides into hundreds of split ends and flat tips called spatulas (see figure on the right). A single seta of the tokay gecko is roughly 110 micrometers long and 4.2 micrometers wide. Each of a seta's branches ends in a thin, triangular spatula connected at its apex. The end is about 0.2 micrometers long and 0.2 micrometers wide.Autumn, K. (2006). How gecko toes stick. American Scientist 94, 124–132. The adhesion between gecko's foot and the surfaces is exactly the result of the Van der Waals force between each seta and the surface molecules. A single seta can generate up to 200 μN of force.Autumn, K., Liang, Y. A., Hsieh, S. T., Zesch, W., Chan, W.-P., Kenny, W. T., Fearing, R. & Full, R. J. (2000) Nature 405, 681–685. There are about 14,400 setae per square millimeter on the foot of a tokay gecko, which leads to a total number of about 3,268,800 setae on a tokay gecko's two front feet. From the equation for intermolecular potential:
where and are the number of contacts of the two surfaces, R is the radius of each contact and D is the distance between the two surfaces.
We find that the intermolecular force, or the van der Waals force in this case between two surfaces is greatly dominated by the number of contacts. This is exactly the reason why the gecko's feet can generate extraordinary adhesion force to different kinds of surfaces. The combined effect of millions of spatulae provides an adhesive force many times greater than the gecko needs to hang from a ceiling by one foot.
Lift-off mechanism The surprisingly large forces generated by the gecko's toesAutumn K, Peattie AM. (2002) Mechanisms of Adhesion in Geckos. Integr Comp Biol 42:1081–1090 raised the question of how geckos manage to lift their feet so quickly – in just 15 milliseconds – with no measurable detachment forces. Kellar Autumn and his research group found out the 'Lift-off mechanism' of the gecko's feet. Their discovery revealed that gecko adhesive actually works in a 'programmable' way that by increasing the angle between the setal shaft and the substrate to 30 degrees, no matter how big the perpendicular adhesive force is, geckos 'turn off' the stickiness since the increased stress at the trailing edge of the seta causes the bonds between seta and the substrate to break. The seta then returns to an unloaded default state. On the other hand, by applying preload and dragging along the surface, the geckos turn on the modulate stickiness. This 'Lift-off' mechanism can be shown in the figure on the right.
Self-cleaning ability Unlike conventional adhesives, gecko adhesive becomes cleaner with repeated use, and thus stays quite clean around everyday contaminants such as sand, dust, leaf litter and pollen. In addition, unlike some plants and insects that have the ability of self-cleaning by droplets, geckos are not known to groom their feet in order to retain their adhesive properties – all they need is only a few steps to recover their ability to cling to vertical surfaces.
Kellar Autumn and his research group have conducted experiments to test and demonstrate this ability of the gecko.Hansen, W. and Autumn, K. (2005). Evidence for self-cleaning in gecko setae. Proc. Natl. Acad. Sci. U.S.A. 102, 385–389. They also use the contact mechanical model to suggest that self-cleaning occurs by an energetic disequilibrium between the adhesive forces attracting a dirt particle to the substrate and those attracting the same particle to one or more spatulae. In other words, the Van der Waals interaction energy for the particle-wall system requires a sufficiently great number of particle-spatula systems to counterbalance; however, relatively few spatulae can actually attach to a single particle, therefore the contaminant particles tend to attach to the substrate surface rather than the gecko's toe due to this disequilibrium. Figure on the right shows the model of interaction between N spatulas, a dirt particle and a planar wall.
It is important to know that this property of self-cleaning appears intrinsic to the setal nano-structure and therefore should be replicable in synthetic adhesive materials. In fact, Kellar Autumn's group observed how self-cleaning still occurred in arrays of setae when isolated from the geckos used.
Development and approaches
The discoveries about gecko's feet led to the idea that these structures and mechanisms might be exploited in a new family of adhesives, and research groups from around the world are now investigating this concept. And thanks to the development of nano science and technology, people are now able to create biomimetic adhesive inspired by gecko's setae using nanostructures. Indeed, interest and new discoveries in gecko-type adhesives are booming, as illustrated by the growing number of papers published on this topic. however, synthetic setae are still at a very early stage.
Effective design Effective design of geckolike adhesives will require deep understanding of the principles underlying the properties observed in the natural system. These properties, principles, and related parameters of the gecko adhesive system are shown in the following table.Autumn, K. (2006). Properties, principles, and parameters of the gecko adhesive system. In Biological Adhesives, eds. A. Smith and J. Callow), pp. 225–255. Berlin Heidelberg: Springer Verlag. This table also gives us an insight into how scientists translate those good properties of gecko's setae (as shown in the first column) into the parameters they can actually control and design (as shown in the third column).
Properties Principles parameters 1. Anisotropic attachment<br/ >2. High μ' (pulloff/preload) Cantilever beam Shaft length, radius, density, shaft angle 3. Low detachment force Low effective stiffness Shaft modulus, spatular shape 4. Material independence stickiness Van der Waals mechanism<br/ >JKR-like* contact mechanics<br/ >Nanoarray (divided contact) Spatular size, spatular shape, spatular density 5. Self-cleaning ability Nanoarray (divided contact) Spatular bulk modulus 6. Anti-self-stickiness Small contact area Particle size, shape, surface energy 7. Nonsticky default state Nonsticky spatulae, hydrophobic, Van der Waals force Spatular size, shape, surface energy *JKR refers to the Johnson, Kendall, Roberts model of adhesionJohnson KL, Kendall K, Roberts AD (1973) Surface energy and the contact of elastic solids. Proc R Soc Lond Ser A 324310-313
In summary, the key parameters in the design of synthetic gecko adhesive include:
Pattern and periodicity of the synthetic setae Hierarchical structure Length, diameter, angle and stiffness of the shafts Size, shape and stiffness of the spatulas (end of the satae) Flexibility of the substrate
There is a growing list of benchmark properties that can be used to evaluate the effectiveness of synthetic setae, and the adhesion coefficient, which is defined as:
where is the applied preload force, and is the generated adhesion force. The adhesion coefficient of real gecko setae is typically 8~16.
Materials In the first developments of synthetic setae, polymers like polyimide, polypropylene and polydimethylsiloxane (PDMS) are frequently used since they are flexible and easily fabricated. Later, as nanotechnology rapidly developed, Carbon Nanotubes (CNTs) are preferred by most research groups and used in most recent projects. CNTs have much larger possible length-to-diameter ratio than polymers, and they exhibit both extraordinary strength and flexibility, as well as good electrical properties. It is these novel properties that make synthetic setae more effective.
Fabrication techniques A number of MEMS/NEMS fabrication techniques are applied to the fabrication of synthetic setae, which include photolithography/electron beam lithography, plasma etching, deep reactive ion etching (DRIE), chemical vapor deposition (CVD), and micro-molding, etc.
Examples In this section, several typical examples will be given to show the design and fabrication process of synthetic setae. We can also gain an insight into the development of this biomimetic technology over the past few years from these examples.
Gecko tape