Inspired by the geckos’ ability of climbing on all kinds of natural surfaces, a novel dry adhesive is being developed that can mimic the hairs on a gecko’s foot using vertically aligned multiwalled carbon nanotube (CNT) array. The use of CNT array as dry adhesive material allows reproducing biological adhesion structures and generates fairly strong adhesion strength (over 100 N/cm2). This is because vertically aligned multiwalled carbon nanotube arrays have a structure similar to biological adhesive surfaces with even smaller feature sizes.
Figure 1. Vertically aligned multiwalled carbon nanotube array.
Synthetic Hair Patches
Synthetic hair patches have been fabricated using polymer materials such as rubber, and polyimide. The diameters of these polymer based synthetic structures are in the range of few micrometers. The adhesive strengths generated by those polymer hairs are generally less than 1 N/cm2. The maximum adhesion that has been reported using polymer based micro pillar structures is 3 N/cm2. However, the time consuming and expensive electro-beam lithography involved in defining the pillar patterns are prohibitive for mass production of such structures.
How the Vertically Aligned Multiwalled Carbon Nanotube Arrays Work
CNT arrays can be directly grown on a solid surface which might be well suited as the artificial material mimicking a gecko foot structure. When the CNT array comes into contact with a target surface with certain roughness, the fine structures of CNTs ensure their capability of filling-in the cavities at interface, and make effective contact at mating surfaces.
Forces of Attraction
The adhesion between the CNT array and a target surface is based on van der Waals interaction which is inversely proportional to the feature size of the structure, then a rough estimation suggests that it is possible to generate adhesive strengths of more than 100 N/cm2, assuming that all the tubes make effective contact with the target surface.
Current measurement indicates there are approximately 10% of CNTs making effective contact at interfaces: still a big number (approximate 1010 CNTs per cm2). By varying the tower height of the CNT array and the compliance of the substrate an optimum contact at the interface can be reached, and that could lead to further improvement of adhesion.
Figure 2. Demonstrations of how the vertically aligned multiwalled carbon nanotube arrays work as a dry adhesive.
Density of CNTs
~ 1011 cm-2
Adhesion force generated at interface (with glass)
~ 10 N/cm2
Adhesion force after 7 detach-reattach cycles (with various surfaces including glass, GaAS, Si)
~ 4 N/cm2
Thermal conductance at interface (mating to glass)
Potential for the Vertically Aligned Multiwalled Carbon Nanotube Arrays
Measurements on the adhesion of the CNT array over various surfaces indicate that an adhesion strength of over 10 N/cm2 (~ 1 kg/cm2 or 14 lb/in2) can be achieved, which is superior to other polymer based dry adhesives, and is similar to that a gecko’s foot can generate.
CNTs are known to have extraordinary mechanical, thermal and electrical properties. It can be bend repeatedly with large deflection without failure, which intrinsically makes them suitable as a reusable or re-attachable dry adhesive.
Advantages of Carbon Nanotubes over Synthetic Hair Structures
• The highly hydrophobic property of CNTs makes them free from the clumping problem, which the polymer based synthetic hair structures are usually facing.
• Unlike the synthesized polymer hair structures that the fabrication technique limits the size and the density of the hair patch. The diameters of CNTs are on the order of few tens of nanometers. And a high density of the array (> 1010 cm-2) can be easily achieved using current technology.
• The growth technique for dense CNT arrays is well developed and controllable.
• One unique property of CNT array dry adhesive is its excellent thermal and electrical conductivity.
• A more important advantage over other polymer hair structures is that it can work under some extreme environments, such as vacuum, very low and very high temperature.
Applications for the Vertically Aligned Multiwalled Carbon Nanotube Arrays
The applications cover any climbing devices that need detachable and re-attachable adhesion. Another potential application is for attachment devices in space, since currently available adhesive materials can not effectively perform in the space environment (vacuum and extreme low temperature). Because of CNTs thermal and electrical properties, potential industry applications include using them as thermally or electrically conductive tapes.
This material could be use to fasten a diamond coating onto specific parts of a metal surface. It could enable the manufacture and assembly of electronics without solder. It can create a joint that is stronger than many traditional assembly methods and it can be assembled at room temperature. Moreover, this novel material can create a new electronic assembly process that no longer needs a stencil solder or solder reflow.