Mimicking Nature: Gecko-Inspired Hierarchical Nanostructured Adhesives
Ron Fearing (UCB) and Roya Maboudian (UCB)
Synthetic gecko adhesives have been shown to have amazing directional adhesion on planar surfaces. However, on rough surfaces, the synthetic gecko adhesiveâ€™s remarkable directional adhesion is severely diminished. COINS researchers have developed a method to attain the same directional adhesion synthetic gecko adhesives have on smooth surfaces to non-planar surfaces, by creating a hierarchical structure of lamellae (thin plate-like structures) and nanofibers. On non-planar surfaces, the combined lamellae/nanofiber array samples showed adhesion roughly 60 times greater than their non-structured counterparts. This is the first demonstration of a hard polymer synthetic gecko adhesive exhibiting adhesion to a non-planar surface.
In order to provide the controllable adhesion necessary for all-terrain mobility, nanofibrillar adhesive arrays were fabricated in the Maboudian Lab. Single-level nanoscale fiber arrays were constructed by casting a single layer of 25.4 Âµm thick polypropylene into polycarbonate filters with a pore radius ranging from 300 nm to 2.5 Âµm. After etching in methylene chloride, approximately vertically aligned fibers, 20 Âµm long, are released. These fibers are non-sticky by default, but when subjected to a shear load, they increase contact area, and hence adhesion. Static friction measurements were conducted under various preloading conditions for 47 mm diameter sample arrays. Under a normal preload of 0.79 N/cm2, polypropylene fibers of radius 300 nm and length 20 Âµm exhibited a static coefficient of friction of 5.3, well above the unstructured control sample which showed typical friction coefficient of 0.3.
Using a polypropylene fiber array of hairs with radius 100 nm, shear adhesion was demonstrated with rigid polymer fibers. A 47 mm diameter sample area was capable of supporting 100 grams in pure shear, corresponding to a shear adhesion force of 0.2 N/cm2. This was the first demonstration of such adhesion with rigid polymer fibers over macroscopic areas. By carefully controlling the backing curvature and sample uniformity it was possible to achieve nearly 2 N/cm2 of pure shear adhesion without the need for a preload force.