Stanislav N. Gorb

From micro to nano contacts in biological attachment devices.

Animals with widely varying body weight, such as flies, spiders, and geckos, can adhere to and move along vertical walls and even ceilings. This ability is caused by very efficient attachment mechanisms in which patterned surface structures interact with the profile of the substrate. An extensive microscopic study has shown a strong inverse scaling effect in these attachment devices. Whereas μm dimensions of the terminal elements of the setae are sufficient for flies and beetles, geckos must resort to sub-μm devices to ensure adhesion. This general trend is quantitatively explained by applying the principles of contact mechanics, according to which splitting up the contact into finer subcontacts increases adhesion. This principle is widely spread in design of natural adhesive systems and may also be transferred into practical applications.

Biomimetic mushroom-shaped fibrillar adhesive microstructure.

To improve the adhesive properties of artificial fibrillar contact structures, the attachment systems of beetles from the family Chrysomelidae were chosen to serve as a model. Biomimetic mushroom-shaped fibrillar adhesive microstructure inspired by these systems was characterized using a variety of measurement techniques and compared with a control flat surface made of the same material. Results revealed that pull-off force and peel strength of the structured specimens are more than twice those of the flat specimens. In contrast to the control system, the structured one is found to be very tolerant to contamination and able to recover its adhesive properties after being washed in a soap solution. Based on the combination of several geometrical principles found in biological attachment devices, the presented microstructure exhibits a considerable step towards the development of an industrial dry adhesive.

Effects of contact shape on the scaling of biological attachments

Adhesion of biological systems has recently received much research attention: the survival of organisms ranging from single cells and mussels to insects, spiders and geckos relies crucially on their mechanical interaction with their environments. For spiders, lizards and possible other ‘dry’ adhesive systems, explanations for adhesion are based on van der Waals interaction, and the adhesion of single–contact elements has been described by the classical Johnson–Kendall–Roberts (JKR) model derived for spherical contacts. However, real biological contacts display a variety of shapes and only rarely resemble a hemisphere. Here, we theoretically assess the influence of various contact shapes on the pull–off force for single contacts as well as their scaling potential in contact arrays. It is concluded that other shapes, such as a toroidal contact geometry, should lead to better attachment; such geometries are observed in our microscopic investigations of hair–tip shapes in beetles and flies.

Resolving the nanoscale adhesion of individual gecko spatulae by atomic force microscopy

Animals that cling to walls and walk on ceilings owe this ability to micron and nanoscale attachment elements. The highest adhesion forces are encountered in geckoes, which have developed intricate hierarchical structures consisting of toes (millimetre dimensions), lamella (400–600 μm size), setae (micron dimensions) and spatulae (∼200 nm size). Adhesion forces of setae on different substrates have previously been measured by a micro-electromechanical system technique. Here we report the first successful experiments in which the force–displacement curves were determined for individual spatulae by atomic force microscopy. The adhesion force for these smallest elements of the geckos attachment system is reproducibly found to be about 10 nN. This method sheds new light on the nanomechanisms of attachment and will help in the rational design of artificial attachment systems.

Biological Micro- and Nanotribology Nature's Solutions

Introduction.- Physical Principles of Micro- and Nanotribology.- Biological Frictional and Adhesive Systems.- Frictional Devices of Insects.- Microscale Test Equipment.- Nanoscale Probe Techniques.- Microscopy Techniques.- Samples, Sample Preparation and Tester Setup.- Case Study I: Indentation and Adhesion.- Case Study II: Friction.- Case Study III: Material Properties.- Outlook.- Appendix

WHEN LESS IS MORE: EXPERIMENTAL EVIDENCE FOR TENACITY ENHANCEMENT BY DIVISION OF CONTACT AREA

Most recent data on hairy systems demonstrated their excellent adhesion and high reliability of contact. In contrast to smooth systems, some hairy systems seem to operate with dry adhesion and do not require supplementary fluids in the contact area. Contacting surfaces in such devices are subdivided into patterns of micro- or nanostructures with a high aspect ratio (setae, hairs, pins). The size of single points gets smaller and their density gets higher as the body mass increases. Previous authors explained this general trend by applying the JKR theory, according to which splitting up the contact into finer subcontacts increases adhesion. Fundamental importance of contact splitting for adhesion on smooth and rough substrata has been previously explained by a very small effective elastic modulus of the fibre array. This article provides the first experimental evidence of adhesion enhancement by division of contact area. A patterned surface made out of polyvinylsiloxane (PVS) has significantly higher adhesion on a glass surface than a smooth sample made out of the same material. This effect is even more pronounced on curved substrata. An additional advantage of patterned surfaces is the reliability of contact on various surface profiles and the increased tolerance to defects of individual contacts.

A small wall-walking robot with compliant, adhesive feet

The ability to walk on surfaces regardless of the presence or direction of gravity can significantly increase the mobility of a robot for both terrestrial and space applications. Insects and geckos can provide inspiration for both novel adhesive technology and for the locomotory mechanisms employed during climbing. For this work, Mini-Whegs/spl trade/, a small quadruped robot that uses wheel-legs for locomotion, was altered to explore the feasibility of scaling vertical surfaces using compliant, adhesive feet. Modifications were made to reduce its weight, and its legs were redesigned to enable its feet to better attach and detach from the substrate, mimicking homologous actions observed in animals. The resulting vehicle is self-contained, power-autonomous, and weighs only 87 grams. Using pressure-sensitive tape, it is capable of walking up a vertical surface, walking upside-down along an inverted surface, and transitioning between orthogonal surfaces.

Biological microtribology: anisotropy in frictional forces of orthopteran attachment pads reflects the ultrastructure of a highly deformable material

Evolutionarily optimized frictional devices of insects are usually adapted to attach to a variety of natural surfaces. Orthopteran attachment pads are composed of hexagonal outgrowths with smooth flexible surfaces. The pads are designed to balance the weight of the insect in different positions and on different materials. In a scanning electron microscopy study followed by freezing–substitution experiments, the ultrastructural architecture of the pad material was visualized. In friction experiments, the interaction was measured between the attachment pad and a polished silicon surface. The inner structure of this material contains distally directed rods, branching close to the surface, and spaces filled with fluid. The specific design of the pad material provides a higher frictional force in the distal direction. Frictional anisotropy is more enhanced at higher normal forces and lower sliding velocities. It is concluded that optimal mechanical functionality of biosystems is the result of a combination of surface structuring and material design.

Structural Design and Biomechanics of Friction-Based Releasable Attachment Devices in Insects

Abstract 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.

Shearing of fibrillar adhesive microstructure: friction and shear-related changes in pull-off force

To characterize the effect of shearing on function of fibrillar adhesive microstructure, friction and shear-related changes in pull-off force of a biomimetic polyvinylsiloxane mushroom-shaped fibrillar adhesive microstructure were studied. In contrast to a control flat surface, which exhibited pronounced stick–slip motion accompanied with high friction, the fibrillar microstructure demonstrated a stable and smooth sliding with a friction coefficient approximately four times lower. The structured contact also manifested zero pull-off force in a sheared state, while the flat surface exhibited highly scattered and unreliable pull-off force when affected by contact shearing. It appears that the fibrillar microstructure can be used in applications where a total attachment force should be generated in a binary on/off state and, most surprisingly, is suitable to stabilize and minimize elastomer friction.