Flexible Carbon‐Nanofiber Connectors with Anisotropic Adhesion Properties

The ability of gecko lizards and many insects to climb vertical surfaces relies on the hierarchical micro- and nanofibrillar arrayed features on their feet. [1‐3] The fibrillar structures provide conformal contact with the opposing surfaces to maximize the van der Waals (vdW) interactions. [4] These adhesive systems found in nature have inspired researchers to design synthetic adhesives by using fibrillar arrays of polymers andcarbonnanotubes(CNTs),whichcanuniversallyattachtoa variety of surfaces. [5‐10] In addition to these universal adhesives, the fibrillar arrays have also been utilized to design self-selective connectors to bind morphologically self-similar components together. Specifically, we recently reported selfselective connectors based on inorganic/organic nanowire (NW) arrays, in which the vdW interactions are significantly amplified by the interpenetration of the high-aspect-ratio NW components. [11‐13] In contrast to gecko adhesives, the unisex NW connectors feature self-selective binding with weak adhesiontonon-self-similarsurfacesarisingfromtherelatively stiff structure ofthe hybrid NWs. While thepotency ofthe NW connectors has been shown, previous studies have relied on hard and fragile backing layers, such as silicon substrates used for the growth of the inorganic NWs, which is not practical for applications requiring lightweight, robust, and bendable backing layers. Herein, we introduce bendable carbon nanofiber (CNF) connectors with mechanically flexible backing and excellent self-selective adhesion properties. CNFs are similar to multiwalled (MW) CNTs but are distinguished by their stacked graphitic, conelike structures, and are often tapered. [14] This structure allows individual CNFs to be free-standing and more effective as interpenetrating connectors than similarly grown CNT forests, which suffer from significant entanglement. The flexible CNF connectors are enabled by the direct transfer of vertical CNF arrays (i.e., CNF forests) grown on silicon substrates to plastic substrates. The vertical geometry of the CNF arrays provides strong shear adhesion strength due to the efficient interpenetration of the CNFs with minimal engagement and disengagement forces. Furthermore, by controlling the tilt angle of the CNF arrays, directional shear adhesion properties are enabled.