Biologically inspired crack trapping for enhanced adhesion

Abstract

We present a synthetic adaptation of the fibrillar adhesion surfaces found in nature. The structure consists of protruding fibrils topped by a thin plate and shows an experimentally measured enhancement in adhesion energy of up to a factor of 9 over a flat control. Additionally, this structure solves the robustness problems of previous mimic structures and has preferred contact properties (i.e., a large surface area and a highly compliant structure). We show that this geometry enhances adhesion because of its ability to trap interfacial cracks in highly compliant contact regimes between successive fibril detachments. This results in the requirement that the externally supplied energy release rate for interfacial separation be greater than the intrinsic work of adhesion, in a manner analogous to lattice trapping of cracks in crystalline solids.interface ͉ fracture ͉ fibrillar ͉ lattice trapping ͉ biomechanics T he ability to adhere two surfaces strongly together and then reversibly separate them, repeatedly, is a desirable capability that is rarely achievable using conventional fabrication techniques and materials. Nevertheless, fibrillar surfaces with these properties have evolved in nature on the adhesive surfaces of the feet of many lizards and insects. With such natural surfaces as inspiration, we have developed a fibrillar structure that produces a robust, reusable material with strongly enhanced adhesion compared with a flat control of the same material, with the enhancement resulting only from the modification of surface geometry.The essential feature that our surfaces borrow from biology is the seta, a hair-like bristle having a diameter of 0.5-10 m and terminating in one or more flattened, expanded tips (''spatulas'') at its contacting end. A number of biological studies (1-16) have found that arrays of setae are a common feature on the adhesion surfaces of many lizards and insects. In the biological literature, the shape, dimensions, and composition of setae from various species are described (1-5, 9-16). Also, the mechanical properties and adhesion force of a single gecko seta (7) and even a single spatula (17,18) were the subjects of recent investigations. An important conclusion to emerge from these two studies is that setal arrays use noncovalent surface forces to achieve adhesion, and evidence suggests that geckos rely primarily on van der Waals and capillary forces (8, 18). As a result, the surface architecture is the primary design variable that has been adjusted in biological systems by evolution.Because of the extraordinary adhesion ability of animals that possess setal arrays, several researchers have recently made an effort to mimic the biological setal geometry by using synthetic materials (19)(20)(21)(22)(23)(24)(25)(26). It has been established theoretically that a fibrillar interface can increase both strength and interfacial toughness, compared with a flat control (21, 27-30). However, simple arrays of micropillars (19-21) have not exhibited stronger adhesion than flat control surfaces o...

Buckling of sheared and compressed microfibrils

“…It is important to note that the last two boundary conditions are satisfied only for fibrils that are well adhered to the indenter. Most investigators use fibrils that have structures at their tips to improve contact and adhesion; for example, Varenberg & Gorb (2007) and Kim & Sitti (2006) used terminal contact plates, whereas Glassmaker et al (2007) and Yao et al (2007) used a terminal continuous thin film. For these fibrils, it is expected that the aforementioned boundary conditions would be satisfied.…”

Compliance of a microfibril subjected to shear and normal loads

“…Recent interest in bio-inspired adhesives has motivated many researchers to fabricate microfibril arrays (Liu & Bhushan 2003;Peressadko & Gorb 2004;Chung & Chaudhury 2005;Crosby et al 2005;Glassmaker et al 2005Glassmaker et al , 2007Huber et al 2005;Northen & Turner 2005;Yurdumakan et al 2005;Kim & Sitti 2006;Majidi et al 2006;Aksak et al 2007;Gorb et al 2007;Greiner et al 2007; and to study their contact mechanics and adhesion (Jagota & Bennison 2002;Gao et al 2003Gao et al , 2005Persson & Gorb 2003;Hui et al 2004;Persson et al 2005;Spolenak et al 2005a,b;Tang et al 2005;Bhushan et al 2006;Tian et al 2006;Yao & Gao 2006;Chen & Gao 2007). Most of these studies focus on how the interface between the microfibrils and a smooth, hard substrate separates under a normal load.…”

It's Not a Bug, It's a Feature: Functional Materials in Insects

“…For example, many insects perform sliding movements while their feet are in contact with a surface in order to induce shear stresses that not only increase viscous adhesive forces, but help to dislodge contaminants. [63,64] Secreted adhesive fluid also plays a key role in self-cleaning by making it possible to deposit contaminating particles with each step, essentially washing the epidermal layer. [26] The ability of certain insects to strongly fasten themselves to a variety of surfaces is also remarkable.…”