Gecko-inspired electrospun flexible fiber arrays for adhesion

The ability of geckos to adhere to vertical solid surfaces comes from their remarkable feet with millions of projections terminating in nanometer spatulae. We present a simple yet robust method for fabricating directionally sensitive dry adhesives. By using electrospun nylon 6 nanofiber arrays, we create gecko-inspired dry adhesives, that are electrically insulating, and that show shear adhesion strength of 27 N/cm2 on a glass slide. This measured value is 270% that reported of gecko feet and 97-fold above normal adhesion strength of the same arrays. The data indicate a strong shear binding-on and easy normal lifting-off. This anisotropic strength distribution is attributed to an enhanced shear adhesion strength with decreasing fiber diameter (d) and an optimum performance of nanofiber arrays in the shear direction over a specific range of thicknesses.;With use of electrospinning, we report the fabrication of nylon 6 nanofiber arrays that show a friction coefficient (mu) of ~11.5. These arrays possess significant shear adhesion strength and low normal adhesion strength. Increasing the applied normal load considerably enhances the shear adhesion strength and mu, irrespective of d and fiber arrays thickness ( T). Fiber bending stiffness and fiber surface roughness are considerably decreased with diminishing d while fiber packing density is noticeably increased. These enhancements are proposed to considerably upsurge the shear adhesion strength between nanofiber arrays and a glass slide. The latter upsurge is mainly attributed to a sizeable proliferation in van der Waals (vdW) forces. These nanofiber arrays can be alternatively bound-on and lifted-off over a glass slide with a trivial decrease in the initial mu and adhesion strength. By using selective coating technique, we have also created hierarchical structures having closely packed nanofibers with d of 50 nm.;We determine the effects of applied normal load, fiber surface roughness, loading angle, d, T, and repeated adhesion measurements on their corresponding adhesion strength and mu. These effects are determined with the aid of atomic force microscopy (AFM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), macroscopic adhesion testing, and wide angle x-ray diffraction (WAXD) techniques.