Gecko Adhesion Research Articles

D-119 Characterization of the Structure and Composition of Gecko Adhesive Setae: A Microdiffraction Study

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Influence of surface roughness on gecko adhesion

In this study we show the influence of surface roughness on gecko adhesion on both the nano- and macroscales. We present experimental data for the force necessary to pull off single spatulae from hard rough substrates and also detail observations on living geckos clinging to various surfaces. Both experiments consistently show that the effective adhesion shows a minimum for a root mean square roughness ranging from 100 to 300 nm.

Peel-Zone Model of Tape Peeling Based on the Gecko Adhesive System

A tape-peeling model based on the geometry of the peel zone (PZ) is derived to predict the peeling behavior of adhesive tapes at peel angles less than or equal to 90°. The PZ model adds an angle-dependent multiplier to the Kendall equation that takes into account the geometrical changes within the peel zone. The model is compared with experimental measurements of the peel force at different angles for a model tape and two commercial tapes, each with different bending moduli, stretch moduli, and adhesive strengths. Good agreement is found for a wide range of peel angles. The PZ model is also applied to the gecko adhesive system and predicts a spatula peel angle of 18.4° to achieve the adhesion forces reported for single setae. The PZ model captures the fact that adhesive forces can be significantly enhanced by peeling at an angle, thereby exploiting high friction forces between the detaching material and the substrate.

Mechanical principles of robust and releasable adhesion of gecko

Robust attachment to rough surfaces and controllable detachment are two features that must coexist for gecko to locomote on walls and ceilings. While robust adhesion ensures that the animal can stick reliably in the presence of random surface roughness, the same adhesion must be readily releasable upon its movement. What are the mechanical principles of such robust and releasable adhesion in nature? In this paper, we review some of the most essential results obtained in our previous studies on this question and also add a number of new results aimed to facilitate a more complete understanding. We begin with addressing the shape and size effects in adhesion between single asperities, and then extend the discussion to contact between rough surfaces, focusing on strategies to achieve robust (flaw-tolerant) and releasable adhesion across multiple length scales. Based on the fundamental principles in fracture and adhesion mechanics, we show that flaw tolerant adhesion is controlled by a dimensionless parameter which suggests a combination of strategies including size confinement, hierarchical energy dissipation, graded elasticity and strength degradation over multiple size scales for robust attachment to rough surfaces. At the same time, we show that strong elastic anisotropy allows adhesion to be controllable depending on the direction of pulling. These results and their future extensions may lead to a theoretical basis to understand robust and releasable adhesion in a variety of biological systems.

Adhesion of multi-level hierarchical attachment systems in gecko feet

Several creatures including insects, spiders and lizards have developed a unique clinging ability that utilizes dry adhesion. Geckos, in particular, have developed the most complex adhesive structures capable of smart adhesion — the ability to cling to different smooth and rough surfaces as well as detach at will. These animals make use of about three million microscale hairs (setae) (about 14 000/mm2) that branch off into hundreds of nanoscale spatulae (about a billion spatulae). Van der Waals forces are the primary mechanism utilized to adhere to surfaces, and capillary forces are a secondary effect that can further increase adhesion force. The hierarchical surface construction gives the gecko the adaptability to create a large real area of contact with surfaces. Modeling of the gecko attachment system as a hierarchical spring model has provided insight into adhesion enhancement generated by this system. It is shown that a multi-level hierarchical structure produces adhesion enhancement, and this enhancement increases with an increase in applied load and a decrease in the stiffness of springs. A significant adhesion enhancement occurs when the maximum spring deformation is greater than two to three times the root mean square amplitude of surface roughness. Although a gecko is capable of producing on the order of 20 N of adhesion force, it retains the ability to remove its feet from an attachment surface at will. Adhesion design databases for biomimetic attachment systems are presented. These adhesion design databases are useful for understanding biological systems and for guiding the fabrication of biomimetic attachment systems. A man-made fibrillar structure capable of replicating gecko adhesion has the potential for use in dry, superadhesive tapes that would be of use in a wide range of applications. These adhesives could be created using micro/nanofabrication techniques or self-assembly.

Towards friction and adhesion from high modulus microfiber arrays

Unlike traditional pressure sensitive adhesives, the natural setal arrays of gecko lizards achieve dry adhesion with stiff, keratinous material. This remarkable property has inspired a new class of adhesive and high friction microstructures composed of stiff materials that, like natural setae, have an elastic modulus greater than 1 GPa. In contrast to softer materials, such as rubber and low molecular weight acrylates, stiff materials have the advantage of wear and creep resistance and represent a wide range of polymers, metals, and ceramics that include materials that are also temperature resistant and biocompatible. This work presents progress in the design and fabrication of synthetic gecko adhesives with particular attention to the principles of contact mechanics and elasticity that are essential in formulating accurate design criteria.

Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements

The hairy attachment system on a gecko's toes, consisting of one billion spatulae in the case of Gekko gecko [Ruibal, R. & Ernst, V. (1965) J. Morphol. 117, 271-294], allows it to adhere to nearly all surface topographies. The mechanistic basis for gecko adhesion has been intensely investigated, but the lowest hierarchical level, that of the spatula, has become experimentally accessible only recently. This report details measurements of the adhesion force exerted by a single gecko spatula for various atmospheric conditions and surface chemistries. Through judicious choice and modification of substrates, the short- and long-range adhesive forces are separated. In contrast to previous work [Autumn, K., Sitti, M., Liang, Y. C. A., Peattie, A. M., Hansen, W. R., Sponberg, S., Kenny, T. W., Fearing, R., Israelachvili, J. N. & Full, R. J. (2002) Proc. Natl. Acad. Sci. USA 99, 12252-12256], our measurements clearly show that humidity contributes significantly to gecko adhesion on a nanoscopic level. These findings are crucial for the development of artificial biomimetic attachment systems.

An Early Eocene gecko from Baltic amber and its implications for the evolution of gecko adhesion

A new genus and species of gecko from the Lower Eocene of north-western Russia is described from a superbly preserved specimen in Baltic amber. It is the oldest gekkonid lizard to be represented by more than fragmentary skeletal remains. The digits of the specimen are mostly intact and reveal a unique combination of characters not seen in any living form. Expanded sub-digital scansors on the toes, however, are essentially similar to those of modern climbing geckos and verify the existence of a complex adhesive system 20–30 million years earlier than supported by previously discovered fossil geckos.

Flaw tolerant bulk and surface nanostructures of biological systems.

Bone-like biological materials have achieved superior mechanical properties through hierarchical composite structures of mineral and protein. Gecko and many insects have evolved hierarchical surface structures to achieve extraordinary adhesion capabilities. We show that the nanometer scale plays a key role in allowing these biological systems to achieve their superior properties. We suggest that the principle of flaw tolerance may have had an overarching influence on the evolution of the bulk nanostructure of bone-like materials and the surface nanostructure of gecko-like animal species. We demonstrate that the nanoscale sizes allow the mineral nanoparticles in bone to achieve optimum fracture strength and the spatula nanoprotrusions in Gecko to achieve optimum adhesion strength. In both systems, strength optimization is achieved by restricting the characteristic dimension of the basic structure components to nanometer scale so that crack-like flaws do not propagate to break the desired structural link. Continuum modeling and atomistic simulations have been conducted to verify the concept of flaw tolerance at nanoscale. A simple tension-shear chain model has been developed to model the stiffness and fracture energy of biocomposites. It is found that, while the problem of low toughness of mineral crystals is alleviated by restricting the crystal size to nanoscale, the problem of low modulus of protein has been solved by adopting a large aspect ratio for the mineral platelets. The fracture energy of biocomposites is found to be proportional to the effective shear strain and the effective shear stress in protein along its path of deformation to fracture. The bioengineered mineral-protein composites are ideally suited for fracture energy dissipation as the winding paths of protein domain unfolding and slipping along protein-mineral interfaces lead to very large effective strain before fracture. The usual entropic elasticity of biopolymers may involve relatively small effective stress and may not be able to ensure simultaneous domain unfolding and interface slipping. Cross-linking mechanisms such as Ca++ induced sacrificial bonds in bone can increase the shear stress in protein and along the protein-mineral interface, effectively converting the behavior of entropic elasticity to one that resembles metal plasticity. The sacrificial bond mechanism not only builds up a large effective stress in protein but also allows protein deformation and interface slipping to occur simultaneously under similar stress levels, making it possible to engineer a very long range of deformation under significant stress in order to maximize energy absorption. Optimization of mineral platelets near theoretical strength is found to be crucial for allowing a large effective stress to be built up in protein via cross-linking mechanisms such as Ca++ induced sacrificial bonds. Similarly, for gecko adhesion, the strength optimization of individual spatulas is found to play a critical role in enhancing adhesion energy at the higher hierarchical level.

Mechanisms of Adhesion in Geckos

The extraordinary adhesive capabilities of geckos have challenged explanation for millennia, since Aristotle first recorded his observations. We have discovered many of the secrets of gecko adhesion, yet the millions of dry, adhesive setae on the toes of geckos continue to generate puzzling new questions and valuable answers. Each epidermally-derived, keratinous seta ends in hundreds of 200 nm spatular tips, permitting intimate contact with rough and smooth surfaces alike. Prior studies suggested that adhesive force in gecko setae was directly proportional to the water droplet contact angle (θ) , an indicator of the free surface energy of a substrate. In contrast, new theory suggests that adhesion energy between a gecko seta and a surface (W(GS)) is in fact proportional to (1 + cosθ), and only for θ > 60°. A reanalysis of prior data, in combination with our recent study, support the van der Waals hypothesis of gecko adhesion, and contradict surface hydrophobicity as a predictor of adhesion force. Previously, we and our collaborators measured the force production of a single seta. Initial efforts to attach a seta failed because of improper 3D orientation. However, by simulating the dynamics of gecko limbs during climbing (based on force plate data) we discovered that, in single setae, a small normal preload, combined with a 5 μm displacement yielded a very large adhesive force of 200 microNewton (μN), 10 times that predicted by whole-animal measurements. 6.5 million setae of a single tokay gecko attached maximally could generate 130 kg force. This raises the question of how geckos manage to detach their feet in just 15 ms. We discovered that simply increasing the angle that the setal shaft makes with the substrate to 30° causes detachment. Understanding how simultaneous attachment and release of millions of setae are controlled will require an approach that integrates levels ranging from molecules to lizards.

Towards a micromechanical understanding of biological surface devices

Abstract Interesting analogies exist between the science of materials in small dimensions and the mechanics of biological systems. In both instances, mechanical forces and displacements are measured and the results are interpreted in the light of theoretical expectations. The approaches are, however, fundamentally different: consideration of structural diversity and global performance in biology contrasts with more quantitative evaluation of local properties and emphasis on micromechanical modeling in materials science. We report on two examples of our first efforts to combine these approaches in the investigation of biological attachment systems in insects. Nanoindentation studies were performed on the head–neck joint and the wing-locking mechanism in a beetle, and the foot–substrate adhesion in flies and geckos has been examined from the perspective of theoretical contact mechanics. By aiming at a quantitative understanding of the interrelationship between local materaal properties and complex structure...