Reversible Frictional Adhesion of Natural and Bio-Inspired Multi-Scale Structures (NIRT – ) Kimberly Turner**, Thomas Kenny †, Jacob Israelachvili ‡, Mark Cutkosky † **Dept. of Mechanical Engineering, UC Santa Barbara, CA 93106; † Dept. of Mechanical Engineering, Stanford University, CA 94305; ‡ Dept. of Chemical Engineering, UC Santa Barbara, CA Introduction Geckos have the remarkable ability to climb and suspend from several types of rough and smooth surfaces. The hierarchical gecko adhesive system is characterized by a high degree of repeatability, reversibility and control, durability and self-cleaning. The fact that van der Waals forces are always present between any two surfaces in close contact has motivated the fabrication of dry adhesives inspired by the gecko. Application of synthetic adhesive systems to climbing robots requires the development of an adhesive with both high adhesion and friction force components, as also with the ability to both stick and peel off rapidly. Hierarchical multi-scale bio-inspired adhesive systems have been developed to satisfy these requirements. Improved Roughness Adaptation and Patch Scale-Up with Bi-Layer Hierarchical Adhesive Structures Adaptation to a wide variety of surface roughnesses is crucial for applications in climbing robots. Figure (A) on the left shows an SEM image of a hierarchical adhesive made of a polymeric microwedge fine layer supported over compliant directional polymer stalks (DPS). The advantages afforded by structural hierarchy are evident in the increased adhesion measured when compared with non-hierarchical structures on rough surfaces such as granite and wood (Figure B). Improved conformality with rough surfaces using hierarchical adhesive structures also enables more efficient patch scaling to larger areas (Figure C), enabling support of higher payloads. Structural Hierarchy in the Gecko Adhesion System (A) Macrostructure: ventral view of a tokay gecko climbing vertical glass. (B) Mesostructure: ventral view of the foot, with the adhesive lamellae visible as overlapping pads. (C) Microstructure: proximal portion of a single lamella, with individual setae in an array visible. Nanostructures: single seta with branched structure at its upper right (D), terminating in hundreds of spatular tips (E). Each hierarchical level works synergistically to produce high adhesion. ~14,400 per mm 2 setae, ~110 µm in length, 5 µm in diameter. ~ spatulae per setae, 200 nm in length. Each seta withstands 200 µN. Autumn, K. at al. Nature, 405, Microfabrication of Synthetic Polymer Setae Future Hierarchical Fabrication A further refinement to the angled polymeric synthetic setae already fabricated is the ongoing fabrication of a hierarchical structure with both micrometer and sub-micron elements, to obtain larger contact areas and adhesion when dealing with surfaces of differing roughness. Further similarity with the gecko adhesive structure will be achieved with future integration of nanometer and micrometer scale elements in the future. (i) Curved micro-pillars (ii) Integrated nano-structure atop curved micro-pillars Microfabrication of angled gecko seta-like polymeric stalks has been performed by the following method: Contact Dynamics Measurements of Gecko Setae Using Surface Forces Apparatus (SFA) Gripping Direction Releasing Direction Gecko setal arrays are structurally anisotropic, exhibiting strong directional adhesion and friction properties. Shearing in the ‘gripping’ direction leads to maximum adhesion and friction; shearing in the opposite ‘releasing’ direction leads to detachment. Geckos do not slide, except under severe conditions. To mimic gecko adhesive pads and functionalities, anisotropic curved structures are necessary. Zhao, B. et al. Langmuir, (in press). (i) Microfabrication of silicon masters for molding (ii) PDMS mold fabricated using silicon masters. Desired final pillar angle built into mold by shearing before fully curing (iii) Angled polymer micro-pillar structures fabricated using PDMS mold Acknowledgements This research is supported by the Nanoscale Interdisciplinary Research Teams (NIRT) Grant No through the National Science Foundation Future Work Design and construction of a test apparatus for precision friction and adhesion force testing on large meso-scale adhesive patch samples Development of synthetic adhesives focusing on mimicry of adhesion control observed in the natural system using shear forces (Frictional Adhesion) Design of synthetic adhesives capable of conforming and adhering to an even wider variety of surface roughnesses Design of robotic systems capable of systematic actuation to engage and disengage the dry adhesives rapidly Broader Impacts International symposium on gecko-inspired adhesive design conducted in July 2008 at UCSB to promote closer collaboration and idea exchange between research groups in the area at UCSB, Stanford University and the Leibnitz Institute for New Materials, Germany Development of an educational documentary on adhesion and friction, focusing on their everyday life effects on animal and human behavior, is in progress