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.

Insects foraging on plant surfaces must attach to the layer of lipophilic materials known as epicuticular waxes (EW) that cover these surfaces. In this paper, we briefly review the evidence that variation in EW morphology can influence the ecology of herbivorous insects directly, by affecting their attachment to plant surfaces, and indirectly by affecting attachment by actively foraging predatory insects to plant surfaces. We then present new data examining how EW micromorphology and chemical composition of Brassica oleracea influence attachment by the predatory beetle, Hippodamia convergens (Coccinellidae). Bioassays with genotypes of B. oleracea differing in wax characteristics, and with EW extracts from these plants applied to glass, show that wax crystals disrupt attachment. In addition, bioassays show that attachment by H. convergens differs among EW extracts prepared to have smooth surfaces without crystals. The differences in attachment under these conditions are evidently due to the chemical composition of the waxes. Bioassays with two pure wax constituents show that wax composition can significantly affect attachment by H. convergens. The study opens the way for using a similar approach to understand attachment by insects to waxy plant surfaces.

Many animals that locomote by legs possess adhesive pads. Such organs are rapidly releasable and adhesive forces can be controlled during walking and running. This capacity results from the interaction of adhesive with complex mechanical systems. Here we present an integrative study of the mechanics and adhesion of smooth attachment pads (arolia) in Asian Weaver ants (Oecophylla smaragdina). Arolia can be unfolded and folded back with each step. They are extended either actively by contraction of the claw flexor muscle or passively when legs are pulled toward the body. Regulation of arolium use and surface attachment includes purely mechanical control inherent in the arrangement of the claw flexor system.

Predictions derived from a ‘wet’ adhesion mechanism were tested by measuring attachment forces on a smooth surface using a centrifuge technique. Consistent with the behavior of a viscid secretion, frictional forces per unit contact area linearly increased with sliding velocity and the increment strongly decreased with temperature.

We studied the nature and dimensions of the adhesive liquid film using Interference Reflection Microscopy (IRM). Analysis of ‘footprint’ droplets showed that they are hydrophobic and form low contact angles. In vivo IRM of insect pads in contact with glass, however, revealed that the adhesive liquid film not only consists of a hydrophobic fluid, but also of a volatile, hydrophilic phase. IRM allows estimation of the height of the liquid film and its viscosity. Preliminary data indicate that the adhesive secretion alone is insufficient to explain the observed friction and that rubbery deformation of the pad cuticle is involved.

Several species of sea cucumbers, all belonging to a single family, possess a peculiar and specialized defense system, the Cuvierian tubules. It is mobilized when the animal is mechanically stimulated, resulting in the discharge of a few white filaments, the tubules. In seawater, the expelled tubules lengthen considerably and become sticky upon contact with any object. The adhesiveness of their outer epithelium combined with the tensile strength of their collagenous core make Cuvierian tubules very efficient at entangling and immobilizing most potential predators. We have designed a method to measure the adhesion of holothuroid Cuvierian tubules. Tubule adhesive strength was measured in seven species of sea cucumbers belonging to the genera Bohadschia, Holothuria and Pearsonothuria. The tenacities (force per unit area) varied from 30 to 135 kPa, falling within the range reported for marine organisms using non-permanent adhesion. Two species, H. forskali and H. leucospilota, were selected as model species to study the influence of various factors on Cuvierian tubule adhesive strength. Tubule tenacity varied with substratum, temperature and salinity of the seawater, and time following expulsion. These differences give insight into the molecular mechanisms underlying Cuvierian tubule adhesion. Tenacity differences between substrata of varying surface free energy indicate the importance of polar interactions in adhesion. Variation due to temperature and time after expulsion suggests that an increase of tubule rigidity, presumably under enzymatic control, takes place after tubule elongation and reinforces adhesion by minimizing peeling effects.

In this paper we report on the effect of surface wettability on surface selection and adhesion properties of settled (adhered) spores of the biofouling marine alga Enteromorpha and cells of the diatom Amphora, through the use of patterned self-assembled monolayers (SAMs). The SAMs were formed from alkanethiols terminated with methyl (CH₃) or hydroxyl (OH) groups, or mixtures of the two, creating a discontinuous gradient of wettability as measured by advancing water contact angle. In the case of Enteromorpha, primary adhesion, as measured by the transition from a motile spore to a settled, sessile organism, is strongly promoted by the hydrophobic surfaces. On the other hand, adhesion strength of the settled spores, as measured by resistance to detachment in a turbulent flow cell, is greatest on a hydrophilic surface. In the case of Amphora, there is little influence of surface wettability on the primary adhesion of this organism, but motility is inhibited at contact angles ≥60° and the cells are more strongly adhered to hydrophobic surfaces. Adhesion strength of Enteromorpha spores is also influenced by group size, spores in groups being more resistant to detachment than single spores.

We review some adhesion mechanisms that have been understood in the field of synthetic adhesives, and more precisely for adhesives that adhere instantaneously (a property named tackiness) and whose adhesive strength usually depends on the applied pressure (pressure-sensitive adhesives). The discussion includes effects of surface roughness, elasticity, cavitation, viscous and elastic fingering, substrate flexibility.

Design of attachment devices in insects varies enormously in relation to different functional loads. Many systems, located on different parts of the body, involve surfaces with particular frictional properties. Such systems evolved to attach parts of the body to each other, or to attach an insect to the substratum by providing fast and reversible attachment/detachment. Among these systems, there are some that deal with predefined surfaces, and others, in which one surface remains unpredictable. The first type of system occurs, for example, in wing-locking devices and head-arresting systems and is called probabilistic fasteners. The second type is mainly represented by insect attachment pads of two alternative designs: hairy and smooth. The relationship between surface patterns and/or mechanical properties of materials of contact pairs results in two main working principles of the frictional devices: mechanical interlocking, or maximization of the contact area. We give an overview of the functional design of two main groups of friction-based attachment devices in insects: probabilistic fasteners and attachment pads.

Many organisms have evolved a fibrillated interface for contact and adhesion as shown by several of the papers in this issue. For example, in the Gecko, this structure appears to give them the ability to adhere and separate from a variety of surfaces by relying only on weak van der Waals forces. Despite the low intrinsic energy of separating surfaces held together by van der Waals forces, these organisms are able to achieve remarkably strong adhesion. To help understand adhesion in such a case, we consider a simple model of a fibrillar interface. For it, we examine the mechanics of contact and adhesion to a substrate. It appears that this structure allows the organism, at the same time, to achieve good, ‘universal’ contact and adhesion. Due to buckling, a carpet of fibrils behaves like a plastic solid under compressive loading, allowing intimate contact in the presence of some roughness. As an adhesive, we conjecture that energy in the fibrils is lost upon decohesion and unloading. This mechanism can add considerably to the intrinsic work of fracture, resulting in good adhesion even with only van der Waals forces. Analysis of the mechanics of adhesion through such a fibrillar interface provides rules for the design of the microstructure for desired performance as an adhesive.

Octopus suckers consist of a tightly packed three-dimensional array of muscle with three major muscle fiber orientations: 1) radial muscles that traverse the wall; 2) circular muscles arranged circumferentially around the sucker; and 3) meridional muscles oriented perpendicular to the circular and radial muscles. The sucker also includes inner and outer fibrous connective tissue layers and an array of crossed connective tissue fibers embedded in the musculature. Adhesion results from reducing the pressure inside the sucker cavity. This can be achieved by the three-dimensional array of muscle functioning as a muscular-hydrostat. Contraction of the radial muscles thins the wall, thereby increasing the enclosed volume of the sucker. If the sucker is sealed to a surface the cohesiveness of water resists this expansion. Thus, the pressure of the enclosed water decreases instead. The meridional and circular muscles antagonize the radial muscles. The crossed connective tissue fibers may store elastic energy, providing an economical mechanism for maintaining attachment for extended periods. Measurements using miniature flush-mounted pressure transducers show that suckers can generate hydrostatic pressures below 0 kPa on wettable surfaces but cannot do so on non-wettable surfaces. Thus, cavitation, the failure of water in tension, may limit the attachment force of suckers. As depth increases, however, cavitation will cease to be limiting because ambient pressure increases with depth while the cavitation threshold is unchanged. Structural differences between suckers will then determine the attachment force.

Climbing assisted by adhesive subdigital pads in gekkotan lizards has been the subject of intrigue and study for centuries. Many hypotheses have been advanced to explain the mechanism of adhesion, and recently this phenomenon has been investigated at the level of individual setae. The ability to isolate, manipulate and record adhesive forces from individual setae has provided new insights, not only into the mechanism of attachment, but also into the physical orientation of these structures necessary to establish attachment, maximize adhesive force, and effect subsequent release. This, in turn, has enabled a reassessment of the overall morphology and mode of operation of the adhesive system. Digital hyperextension has often been noted as a behavioral characteristic associated with the deployment of the gekkotan adhesive system—this is now understandable in the context of setal attachment and release kinematics, and in the context of the evolution of this pattern of digital movement from the primitive pattern of saurian digital kinematics. The perpendicular and parallel preloads associated with setal attachment are now reconcilable with other morphological aspects of the gekkotan adhesive system—the lateral digital tendon complex and the vascular sinus network, respectively. Future investigations of the integrated adhesive system will help to further elucidate the interdependence of its structural and functional components.

Many marine invertebrates form strong, temporary attachments using viscoelastic gels. To better understand these adhesives, an analysis of what is known of gel structure and function was performed. There are different ways of making gels, ranging from entangling of giant glycoproteins to crosslinking of smaller proteins. The mechanics of the gel depend largely on the size of the polymer, its concentration, and whether it is crosslinked. Compared to gels such as mammalian mucus, the mechanics of adhesive mucous gels often appear to depend more heavily on relatively small proteins than on megadalton-sized glycoproteins. In addition, changes in concentration and the presence of specific proteins have been associated with the change from a non-adhesive to an adhesive form. The attachment strengths produced by different gels at different concentrations were compared with the changes in attachment strength seen in living animals. These data suggest that changes in concentration are not sufficient to account for adhesion. Thus, it is likely that the changes in protein composition may play a large role.

Mussels owe their sessile way of life in the turbulent intertidal zone to adaptive adjustments in the process and biochemistry of permanent attachment. These have understandably attracted scientific interest given that the attachment is rapid, versatile, tough and not subverted by the presence of water. The adhesive pads of mussel byssus contain at least six different proteins all of which possess the peculiar amino acid 3, 4-dihydroxyphenylalanine (DOPA) at concentrations ranging from 0.1 to 30 mol %. Studies of protein distribution in the plaque indicate that proteins with the highest levels of DOPA, such as mefp-3 (20 mol %) and mefp-5 (30 mol %), appear to predominate at or near the interface between the plaque and substratum. Although the presence of DOPA in proteins has traditionally been associated with cross-linking via chelate-mediated or covalent coupling, recent experiments with natural and synthetic DOPA-containing polypeptides suggest that cross-link formation is not the only fate for DOPA. Intact DOPA, particularly near the interface, may be essential for good chemisorption to polar surfaces. Uniformly high DOPA oxidation to cross-links leads to interfacial failure but high cohesive strength, while low DOPA oxidation results in better adhesion at the expense of cohesion. Defining the adaptations involved in balancing these two extremes is crucial to understanding marine adhesion.