Natural materials such as bone, tooth, and nacre are nanocomposites of proteins and minerals with superior strength. Why is the nanometer scale so important to such materials? Can we learn from this to produce superior nanomaterials in the laboratory? These questions motivate the present study where we show that the nanocomposites in nature exhibit a generic mechanical structure in which the nanometer size of mineral particles is selected to ensure optimum strength and maximum tolerance of flaws (robustness). We further show that the widely used engineering concept of stress concentration at flaws is no longer valid for nanomaterial design.

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 m dimensions of the terminal elements of the setae are sufficient for flies and beetles, geckos must resort to sub-m 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.walking ͉ adhesion ͉ locomotion ͉ legs ͉ insects A ttachment structures have independently developed several times in animal evolution (1, 2). Setose or hairy systems of various animal groups, such as insects, spiders, and lizards contain surfaces covered by fine patterns of protuberances of different origin. These highly specialized structures are not restricted to one particular area of the leg and may be located on different derivatives of the tarsus and pretarsus (3). Even among insects, the protuberances belong to different types: representatives of the Coleoptera and Dermaptera have setae with sockets providing additional mobility of setae, whereas representatives of Diptera have setae without sockets (acanthae). Setae range in their length from several millimeters to a few micrometers (4).Despite Ͼ300 years of studies on hairy attachment systems, there is still a debate concerning the attachment mechanism of animals walking on smooth walls or ceilings. Different hypotheses have been proposed to explain the mechanism of attachment: sticking fluid, microsuckers, and electrostatic forces (5). Based on experimental data, some of these theories have been rejected, and adhesion has been attributed to a combination of molecular interactions and capillary attractive forces mediated by secretions (6) or purely van der Waals interactions (7). Because some animals produce secretory fluids (insects) (8-10) in the contact area, whereas others do not (spiders, geckos) (11, 12), one can expect different basic physical forces contributing to the overall adhesion. Recently, strong evidence has been presented (13) that the adhesion of gecko setae is caused by van der Waals interaction, rejecting mechanisms relying on capillary adhesion. Elements of contact mechanics have also been applied to this problem (13,14); it was predicted that arrays with smaller setae endings should result in greater adhesive strength. In the present study, we combine an extensive microscopical study ¶ of biological surface devices with the theory of contact mechanics based on molecular adhesion. We will show that the scaling of the surface protuberances, for animals differing in weight by 6 orders of magnitude, ca...

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Inspired by biological attachment systems, micropatterned elastomeric surfaces with pillars of different heights (between 2.5 and 80 microm) and radii (between 2.5 and 25 microm) were fabricated. Their adhesion properties were systematically tested and compared with flat controls. Micropatterned surfaces with aspect ratios above 0.5 were found to be more compliant than flat surfaces. The adhesion significantly increases with decreasing pillar radius and increasing aspect ratio of the patterned features. A preload dependence of the adhesion force has been identified and demonstrated to be crucial for obtaining adhesives with tunable adherence.

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Following a recent bioinspired paradigm, patterned surfaces can exhibit better adhesion than flat contacts. Previous studies have verified that finer contact structures give rise to higher adhesion forces. In this study, we report on the effect of the tip shape, which was varied systematically in fibrillar PDMS surfaces, produced by lithographic and soft-molding methods. For fiber radii between 2.5 and 25 microm, it is found that shape exerts a stronger effect on adhesion than size. The highest adhesion is measured for mushroom-like and spatular terminals, which attain adhesion values 30 times in excess of the flat controls and similar to a gecko toe. These results explain the shapes commonly found in biological systems, and help in the exploration of the parameter space for artificial attachment systems.

The amazing adhesion of gecko pads to almost any kind of surfaces has inspired a very active research direction over the last decade: the investigation of how geckos achieve this feat and how this knowledge can be turned into new strategies to reversibly join surfaces. This article reviews the fabrication approaches used so far for the creation of micro- and nanostructured fibrillar surfaces with adhesive properties. In the light of the pertinent contact mechanics, the adhesive properties are presented and discussed. The decisive design parameters are fiber radius and aspect ratio, tilt angle, hierarchical arrangement and the effect of the backing layer. Also first responsive systems that allow thermal switching between nonadhesive and adhesive states are described. These structures show a high potential of application, providing the remaining issues of robustness, reliability, and large-area manufacture can be solved.