EVIDENCE FOR VAN DER WAALS ADHESION IN GECKO SETAE PDF

Liang Anne M. Peattie Wendy R. Israelachvili Robert J. E-mail: ude. Abstract Geckos have evolved one of the most versatile and effective adhesives known.

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Robert Full Y. The mechanism of dry adhesion in the millions setae 24, 25 are consistent with the use of capillary adhesion of setae on the toes of geckos has been the focus of scientific study 2 , and suggest that the strength of setal adhesion may be for over a century. In the case of two phobic and strongly hydrophilic, polarizable surfaces. However, if the two of the size and shape of the tips, and are not strongly affected by adhering surfaces are of different materials, as for gecko setae surface chemistry.

Estimates using a standard adhesion model and our Waals dispersion force is strong between polarizable surfaces, measured forces come remarkably close to predicting the tip size and is only weakly dependent on the hydrophobicity of the of Tokay gecko seta. We verified the dependence on size and not interacting surfaces Previous studies found that geckos fail surface type by using physical models of setal tips nanofabricated to adhere to hydrophobic, weakly polarizable surfaces [polytet- from two different materials.

I n the 4th century B. Two millennia later, we are uncovering To test directly whether capillary adhesion or van der Waals dispersion force is the primary mechanism of adhesion in geckos, we separated polarizability from polarity hydrophobicity and the secrets of how geckos use millions of tiny foot-hairs to adhere hydrophilicity by measuring adhesion on two polarizable semi- to even molecularly smooth surfaces.

We tested the two currently conductor surfaces that varied greatly in hydrophobicity. We competing hypotheses 2, 3 of adhesion mechanisms in gecko measured the parallel force of single gecko toes on a gallium setae: i thin-film capillary forces or other mechanisms relying arsenide GaAs semiconductor surface that is highly hydro- on hydrophilicity and ii van der Waals forces.

As the capillary and van der Waals hypotheses experimentally. We also compared the perpendicular diction of the size of a setal tip. If wet, capillary adhesive forces domi- two different materials. We then compared the adhesive func- nate, we expect a lack of adhesion on the strongly hydrophobic tion of the physical model to predicted force values from the GaAs and Si MEMS surfaces.

In contrast, if van der Waals forces mathematical model. In either case we measurements of a single gecko seta 3 were consistent with expect strong adhesion to the hydrophilic SiO2 semiconductor adhesion by van der Waals forces, but we could not reject the and MEMS control surfaces Fig.

Capillary forces in gecko setae, then adhesive force should depend more on size contribute to adhesion in many insects 6—13 , frogs 14—16 , and of the setal tips spatulae than on the nature of the setal even some mammals Unlike many insects, geckos lack glands on the surfaces of their feet 18— E-mail: autumn lclark.

The first studies CA Force of gecko setae on highly polarizable surfaces versus for surface hydrophobicity. A Wet adhesion prediction. B van der Waals prediction. C Results from toe on highly polarizable semiconductor wafer surfaces differing in hydrophobicity.

D Results from single seta attaching to highly polarizable MEMS cantilevers differing in hydrophobicity. Adhesion to hydrophilic and hydrophobic polarizable surfaces was similar. Therefore, we reject the hypothesis that wet, capillary interactions are necessary for gecko adhesion in favor of the van der Waals hypothesis.

This theoretical dependence on size and not setal surface were measured in CANVAS 8 Deneba on a Macin- surface type encouraged our nanofabrication of synthetic spatu- tosh computer. If van der Waals forces are responsible for gecko adhesion, then we should be able to fabricate physical models of spatulae Setal Adhesion. Fabrication of MEMS cantilevers. The backside of the cantilevers were then Materials and Methods patterned, again using contact lithography, and released using a Foot Adhesion.

Hydrophobic and hydrophilic semiconductor wafers. DRIE deep reactive ion etching process. The buried oxide We placed a single toe of nine live Tokay geckos Gekko gecko layer was removed in hydrofluoric acid, leaving a released against the surface of a vertically oriented mm diameter cantilever. Silicon forms a thin native oxide layer on its surface semiconductor wafer Wafer World, West Palm Beach, FL , when exposed to humid air. This silicon dioxide layer is hydro- which was embedded in a Plexiglas plate fixed with a rigid rod philic.

To gecko was pulled down an oxidized silicon SiO2, orienta- produce a hydrophobic surface on the MEMS sensor, we used a tion or GaAs type N-Si, orientation wafer until the toe vapor-phase hydrofluoric HF acid etch. HF removes the native slipped off. We measured shear force on the toes of the front and oxide layer and produces a hydrogen-terminated silicon surface. To measure only a single toe, we restrained the The surface of silicon can be passivated this way to prevent geckos by hand, and held the other toes in a flexed position.

We further oxidation for days. In the absence of a native oxide layer, excluded any trial in which the gecko struggled or moved its toe. Because force of adhesion is strongly sensors. We carefully peeled the outer epidermal layer of a single correlated with pad area 27 , we standardized the data by seta-bearing toe pad scansor off the toe of a restrained, live, dividing the maximum force generated at each toe by the pad nonmolting gecko.

With the tip of a finely etched tungsten pin, area of that toe to determine shear stress. We measured pad area we scraped the epidermal surface to break off individual setae for each gecko by scanning the animals on a flatbed scanner at the base of the stalk. The isolated seta was then glued to the Agfa and analyzing the images with a commercial program end of a no. We analyzed all data with a Danvars, MA. To prevent the epoxy from creeping up the stalk of the seta, Water droplet contact angle measurements.

We carefully peeled outer epidermal layer of single specimen. The seta was oriented such that the spatular surface scansors from five restrained, nonmolting geckos. The scansors was approximately perpendicular to the axis of the pin.

All were affixed to glass slides, setal side up, using cyanoacrylate gel preparations were completed under a compound microscope. The fixed scansors were oriented such that the setae formed The pin was then mounted on a computer-controlled piezoelec- a flat surface on which water contact angles could be measured tric actuator with closed-loop feedback using capacitive position under a microscope Nikon SMZ The experiments Autumn et al.

Deflections of the cantilever were measured by using captured images at the instant of detachment. Physical Model Adhesion. Fabrication of synthetic setal tips. Synthetic spatulae were fabricated in dimensions similar to natural Tokay gecko spatulae 0. A flat wax surface J. Freeman, Inc. Tokay gecko Gekko gecko adhering to molecularly smooth hydro- height. The punched surface was then filled with polymer. After phobic GaAs semiconductor.

Adhesion measurements for synthetic setal tips. Perpendicular adhesion force of the fabricated spatulae was measured by a rectangular tipless AFM probe. Perpendicular forces were failure of the keratin they are made of. The Results and Discussion unusually large contact angle is likely to be caused by the Experimental Support of van der Waals Adhesion Hypothesis. The micro-roughness of the seta and skin 36, 37 , as has been capillary adhesion hypothesis predicts high attachment forces on discovered for the lotus plant The hydrophobic nature of hydrophilic semiconductors SiO2 and low attachment forces on the seta supports the van der Waals hypothesis, and is inconsis- hydrophobic semiconductors GaAs and Si.

The van der Waals tent with the hypothesis that capillary adhesion determines hypothesis predicts high attachment forces on all semiconduc- adhesive force 24, 25, In fact, high setal hydrophobicity may tors, regardless of hydrophobicity. Our present measurements of aid in decreasing the setal—substrate gap distance by excluding live gecko toes and single setae on hydrophilic and hydrophobic layers of water at points of contact, further reducing the role of semiconductor surfaces Fig.

Let us Parallel stress of live gecko toes on GaAs 0. Adhesion of a single gecko seta on the our empirical measurements of adhesive force, we can apply a hydrophobic MEMS cantilever Therefore, The use of van der Waals dispersion force by geckos suggests that evolution can result in an effective adhesive by simply building an array of small structures rather than by synthesizing a structure with a specialized surface chemistry. Thus, geckos have been able to exploit the peculiarities of their epidermal structure 5, 35, 36 and evolve elaborate microstructures with phenomenal adhesive properties.

Maximizing surface density as Fig. Synthetic gecko spatulae and perpendicular pull-off force measure- predicted by the Johnson—Kendall—Roberts model may repre- ments using a tipless AFM probe.

This is a further indication that geometry, not surface chemistry, is the predicted radii R of individual spatulae in the bundle using our central design principle in the evolution of adhesive setae.

This Our preliminary physical models provide proof of concept that value is close to empirical measurements 3, 5. Even though the humans can fabricate the first dry, adhesive microstructures calculation is only a gross approximation, it shows that our when inspired by biology.

There is a striking contrast between measurements are quantitatively consistent with a van der Waals the simple Johnson—Kendall—Roberts-based models we used and dispersion interaction between setae and substrates.

It predicts the geometrically complex structures evolved by geckos. Each that for a given thermodynamic adhesion energy, smaller spatu- Tokay seta bears hundreds of tips on a curved shaft, and the tips lae will result in a greater adhesive force per unit area.

This themselves consist of a stalk with a thin, roughly triangular end, finding may not only assist us in explaining the function of setal where the apex of the triangle connects the tip to its stalk. It is structures in animals, but also suggests that synthetic adhesives likely that the added complexity of gecko setae provides ease of could be enhanced simply by subdividing their surface into small attachment and detachment 3 , and the ability to adhere to protrusions to increase surface density.

We suggest that devel- with tip radius of — nm Fig. Using We thank S. Attinasi, S. Autumn, W. Federle, G. Hermann, S. Russell, and three anonymous referees. This work was ref. Alan Rudolph to K. Aristotle, Historia Animalium, trans.

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Evidence for van der Waals adhesion in gecko setae

Work started in and 2 years later results were published in Nature Materials. The tape, which had a contact area of around 0. However, the adhesion coefficient was only 0. Synthetic gecko foot hair[ edit ] Micro view of the "Nanotube Synthetic Gecko Foot Hair" As nanoscience and nanotechnology develop, more projects involve the application of nanotechnology, notably the use of carbon nanotubes CNTs. As is shown in the figure on the right.

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Synthetic setae

Robert Full Y. The mechanism of dry adhesion in the millions setae 24, 25 are consistent with the use of capillary adhesion of setae on the toes of geckos has been the focus of scientific study 2 , and suggest that the strength of setal adhesion may be for over a century. In the case of two phobic and strongly hydrophilic, polarizable surfaces. However, if the two of the size and shape of the tips, and are not strongly affected by adhering surfaces are of different materials, as for gecko setae surface chemistry. Estimates using a standard adhesion model and our Waals dispersion force is strong between polarizable surfaces, measured forces come remarkably close to predicting the tip size and is only weakly dependent on the hydrophobicity of the of Tokay gecko seta.

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Gashakar Reduction of water surface tension significantly impacts gecko adhesion underwater. The toes of live Tokay geckos were waalss hydrophobic, and adhered equally well to strongly hydrophobic and strongly hydrophilic, polarizable surfaces. Mechanical Design in Organisms. Geckos have evolved one of the most versatile and effective adhesives known. Biological Mechanisms of Attachment Professor Dr.

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EVIDENCE FOR VAN DER WAALS ADHESION IN GECKO SETAE PDF

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