Gecko-Inspired Carbon Nanotube-Based Self-Cleaning Adhesives

Abstract

The design of reversible adhesives requires both stickiness and the ability to remain clean from dust and other contaminants. Inspired by gecko feet, we demonstrate the self-cleaning ability of carbon nanotube-based flexible gecko tapes.A gecko has the unique ability to reversibly stick and unstick to a variety of smooth and rough surfaces. The gecko's wall climbing ability, without the use of viscoelastic glue, has attracted significant attention. [1][2][3][4][5][6][7][8][9][10][11] Although the gecko does not groom its feet, its stickiness remains for months between molts. 12 The gecko's dirty feet can recover its ability to climb vertical walls only after a few steps. 12 Our daily experience with sticky tapes has been the opposite. The stickier the adhesive, the more difficult it is to keep it clean from dust and other contaminants. Synthetic self-cleaning adhesives, inspired by the gecko's feet, could be used for many applications including wall climbing robots and microelectronics.The secret of the gecko's adhesive properties lies in the microstructure of gecko feet. 1,[13][14][15] Microscopy shows that gecko feet are covered with millions of small hairs called setae, which further divide into hundreds of smaller spatulas ( Figure 1A). When such a structure is placed against any surface, hairs adapt and allow a very large area of contact with the surface. The van der Waals (vdW) interaction between the hairs and the substrate after contact is sufficient for the gecko to adhere. It has been suggested that this same hairy carpet on the gecko feet also plays an important role in self-cleaning. 12 Some of the other systems found in nature that exhibit self-cleaning properties are the leaves of lotus and lady's mantle plants. 18 The surface of lotus leaves have two levels of microscopic roughness ( Figure 1B). This hierarchical roughness along with a hydrophobic wax coating makes the lotus leaves superhydrophobic. 16,[19][20][21] A water droplet forms a large contact angle with low contact angle hysteresis. This results in the water droplets rolling off the surface, leaving the surface clean. Leaves of a lady's mantle plant have hairs of 10 µm diameter and length of 1 mm ( Figure 1C). It has been suggested that the individual hairs are hydrophilic. However, when acting together on the surface, they make the surface of the leaves superhydrophobic. 17 Even though the surface properties of the spatulas are not known, the hierarchical structure of gecko feet makes the macroscopic structure superhydrophobic. 12 Significant effort in developing synthetic materials inspired by gecko feet show comparable, and in some cases better, shear resistance than natural gecko feet. 2,[6][7][8][9]11 Still, these measurements were done in controlled environments and limited self-cleaning data of these synthetic materials were reported. 7 Recently, we have designed carbon nanotube-based

Dynamic self-cleaning in gecko setae via digital hyperextension

“…Stiff polymer fibrillar adhesives, for example, show self-cleaning properties with gold microspheres (radius 2.5 mm), as samples recovered 25-33% of the original shear adhesion force after 30 simulated steps [43]. Synthetic micro-patterned carbon nanotube-based gecko tapes regained 60 per cent of the shear stress when the tape was dusted and cleaned by water [44]. These synthetic dry adhesives show either lotus effect or similar contact self-cleaning rate as gecko toe pads without DH.…”

Odako growth of dense arrays of single-walled carbon nanotubes attached to carbon surfaces

“…Among these applications are supercapacitors [4], field emitters [1], transparent and aligned conductive films [5], adhesive tapes [6], membrane filters [7], and loudspeaker devices [8], among others. The use of these so-called super-growth processes to rapidly produce ultra-long, aligned SWNTs is continuing to inspire new, important applications.…”

Sticking to the story: outstanding challenges in gecko-inspired adhesives

“…Likewise, understanding the role of hyperextension in cleaning adhesive toe pads may help in designing synthetic materials that remain clean. Although some initial progress has been made in designing self-cleaning surfaces (Lee and Fearing, 2008;Sethi et al, 2008), the performance of current designs still falls short of what is needed for practical applications. Finally, synthetic materials also face additional challenges in that it is difficult to manufacture large quantities of material at prices that can compete with existing and cheaper technologies.…”