Bio‐Inspired Hierarchical Polymer Fiber–Carbon Nanotube Adhesives

摘要

Hierarchical pillar arrays consisting of micrometer-sized polymer setae covered by carbon nanotubes are engineered to deliver the role of spatulae, mimicking the fibrillar adhesive surfaces of geckos. These biomimetic structures conform well and achieve better attachment to rough surfaces, providing a new platform for a variety of applications. Natural adhesive systems, consisting of pads covered by dense assemblies of high aspect ratio branched adhesive setae, (Figure 1a) excel in terms of adhesive strength on nearly any surface. Facile contact release is achieved by spatulae at the seta tips that are inclined with respect to the setae axis, requiring a normal preload and shear to establish adhesive contact and enabling low-resistance contact release by peel-off. While technologically valuable, dense hierarchical fibrillar adhesives are difficult to manufacture and no scalable approaches yet exist to create the required spatulae asymmetry. Here we demonstrate the manufacture of biomimetic hierarchical nanostructures based on polymer micro-pillar arrays topped with densely packed, vertically aligned carbon nanotubes (CNTs), which closely resemble gecko toe-pads. A permanent spatula-like asymmetry is introduced into the CNT assembly during a first adhesion-release cycle consisting of a normal preload and a shear motion. The shear adhesion forces of these deformed hierarchical CNTs/polymer pillar arrays on smooth and rough surfaces were found to be considerably higher than those of non-structured (i.e., plain) CNT forests, caused by the conformal attachment of the multilevel adhesive elements to the coarse surface topography, energy dissipation during the deformation of the polymer pillars and the increased contact area provided by the inclined CNTs. Exploitation of the naturally optimized design principles of controllable attachment found in biological systems is highly desirable for synthetic adhesives. If successful, such sophisticated biomimetic adhesives would enable a new platform for a variety of applications, ranging from the micromanipulation in production processes, to microelectronics, robotics and biomedicine. Strong, rapid and robust adhesion mediated by gecko toe pads relies on the conformal contact of a finely structured adhesive area to any surface profile, while maintaining structural integrity and wear-resistance.1-3 While highly efficient in a large number of biological organisms, the biomimetic replication of the “gecko effect” is difficult because of the complex geometry of the adhesive surface and its required hierarchical structure. The toe-pads of geckos consist of millions of branched adhesive setae (Figure 1a), which are arranged in a grid-like pattern on the ventral surface of each scansor, branching out into hundreds of nanometer-sized spatular tips (ca. 200 nm wide), allowing them to deform and adhere to nearly any surface.4 Gecko toe-pads consist of β-keratin (elastic modulus E = 1–3 GPa).5, 6 The intimate contact with surfaces of any roughness7, 8 gives rise to significant van der Waals (vdW) forces,9 and their asymmetric structure allows controlled attachment and detachment during locomotion.10, 11 Although considerable progress has been made in mimicking fibrillar adhesives by utilizing nanofabrication routes including photo and electron-beam lithography,12-14 micro molding15-20 and CNT growth,21-25 only some of the benchmark properties of natural fibrillar adhesives (e.g., anisotropic attachment, vdW adhesion, low detachment force, self-cleaning, anti-self matting, compliance and wear resistance) have been achieved to date. Only few biomimetic adhesives have been produced with a hierarchical structure similar to gecko setae. However, recent work on polymer-based nanostructures has demonstrated that hierarchical design is essential for achieving good adhesion to rough surfaces.16 This is due to the ability of hierarchical structures to compensate for surface roughness on different length scales.