Many insects and some larger animals, such as geckos, skinks, and tree frogs, can easily climb vertical walls and even walk on the ceiling. These abilities require a method to attach the feet strongly but reversibly to a variety of surfaces—smooth or rough, hydrophilic or hydrophobic, clean or containing contaminants. This issue of MRS Bulletin examines how fibrils, absorbed water layers, geometry, and other factors make reversible adhesion possible, and how this understanding might be applied to robots and other artificially created structures that can climb walls, walk on ceilings, and get to other hard-to-reach places.

Geckos attach and detach their adhesive toes in milliseconds while running with reckless abandon on nearly any surface. The adhesive on gecko toes differs dramatically from that of conventional adhesives. Conventional pressure-sensitive adhesives (PSAs) are soft viscoelastic polymers that degrade, foul, self-adhere, and attach accidentally to inappropriate surfaces. In contrast, gecko toes bear angled arrays of branched, hair-like fibers (setae) formed from stiff, hydrophobic keratin that act as a bed of angled springs with an effective stiffness similar to that of PSAs. Setae are selfcleaning and maintain function for months during repeated use in dirty conditions. Setae are an anisotropic “frictional adhesive” in that adhesion requires maintenance of a proximally directed shear load. Thus, gecko setae resist inappropriate bonding and are capable of easy and rapid attachment and detachment. Engineered adhesive nanostructures inspired by geckos may become the glue of the future—and perhaps the screw of the future as well.

Smooth adhesive pads are found among the arthropods, amphibians (particularly tree frogs), and in some mammals. They are used for dynamic adhesion when an animal is climbing steep or overhanging smooth surfaces. There is a need for strong attachment to avoid falling and easy detachment to enable the animal to move. This article describes the morphology and physical properties of smooth adhesive pads, stressing how there is little variation in structure, within tree frogs at least, even among pads that have evolved independently. This is clear evidence of an optimum design; best adhesion occurs when there is a continuous, thin film of fluid between the pad and the surface. Smooth adhesive pads adhere by wet adhesion, the main force component being capillarity, produced by the air/liquid interface (meniscus) around the edge of each pad. Smooth adhesive pads also produce substantial friction forces, probably because of actual contact between the pad surface and substrate (tree frogs) or non-Newtonian properties of the secreted fluid (insects). This is possible because the fluid layer beneath the pad has an average thickness of only a few nanometers. The article also discusses the scaling of adhesive force with size and, finally, implications for biomimetics.

Surface roughness is the main reason why macroscopic solids usually do not adhere to each other with any measurable strength, and a root-mean-square roughness amplitude of only 1 μm is enough to completely remove the adhesion between normal rubber (with an elastic modulus E ≈ 1 MPa) and a hard, nominally flat substrate. Biological adhesion systems used by insects and geckos for locomotion are built from a relatively stiff material (keratin or chitin–protein composite with E ≈ 1 GPa). Nevertheless, strong adhesion is possible even to very rough substrate surfaces by using noncompact solid structures consisting of fibers (setae) and plates (spatulae). Biological systems use a hierarchical building principle, where the thickness of the fibers or plates decreases as one approaches the outer surface of the attachment pad, to optimize the binding to rough surfaces while simultaneously avoiding elastic instabilities, for example, lateral bundling of fibers.

Materials that are inspired by or are mimics of natural fibrillar surfaces in lizards and insects aim to achieve enhanced interfacial adhesion and contact properties by means of a fibrillar architecture. Studies of the mechanics of deformation and adhesion of such materials help to explain how they work and are aiding the design of their architecture. This article discusses some of the issues addressed by these studies, such as how can a fibrillar interface be made stronger and tougher than a flat control, and how does one enhance its ability to make contact to a variety of substrates?

Nature provides inspiration for enhanced control of adhesion through numerous examples ranging from geckos to jumping spiders. The primary strategy in these examples is the incorporation of patterns, specifically high-aspect-ratio topographic features, to ingeniously maximize adhesion forces while maintaining ease of release. Recently, considerable research efforts have been devoted toward the understanding, development, and optimization of synthetic analogues to these examples in nature. In this article, we provide insight into the mechanisms that lead to enhanced control of interfacial properties through patterning, the strategies that can be used for fabricating synthetic patterns, and an overview of experimental results that have been used to gain understanding and guidance in this emerging field.

Novel insect-foot–inspired materials may enable future robots to walk on surfaces regardless of the direction of gravity. Mini-Whegs™, a small robot that uses four wheel-legs for locomotion, was converted to a wall-walking robot with compliant, adhesive feet. First, the robot was tested with conventional adhesive feet. Then a new, reusable insect-inspired adhesive was tested on the robot. This structured polymer adhesive has less adhesive strength than conventional pressure-sensitive adhesives, but it has two important advantages: the foot material maintains its properties for more walking cycles before becoming contaminated, and the feet can then be washed and reused with similar results, which is not feasible with conventional adhesives. After the addition of a tail and widening the feet, the robot is capable of ascending vertical smooth glass surfaces using the structured polymer adhesive.