New method creates microstructured surfaces

These surfaces, made by self-assembly of carbon nanotubes, could exhibit a variety of properties including controllable mechanical stiffness and strength, or the ability to repel water in a certain direction.

‘We have demonstrated that mechanical forces can be used to direct nanostructures to form complex three-dimensional microstructures, and that we can independently control…the mechanical properties of the microstructures,’ said A. John Hart, the Mitsui Career Development Associate Professor of Mechanical Engineering at the Massachusetts Institute of Technology (MIT) and senior author of a paper describing the new technique in Nature Communications.

’We have a surface with exceptional stiffness, strength, and toughness relative to its density’

According to MIT, the technique works by inducing carbon nanotubes to bend as they grow with the material bending as it is produced by a chemical reaction.

The process begins by printing two patterns onto a substrate: one is a catalyst of carbon nanotubes; the second material modifies the growth rate of the nanotubes. By offsetting the two patterns, the researchers showed that the nanotubes bend into predictable shapes as they extend.

‘We can specify these simple two-dimensional instructions, and cause the nanotubes to form complex shapes in three dimensions,’ Hart said in a statement.

Where nanotubes growing at different rates are adjacent, ‘they push and pull on each other,’ producing more complex forms, Hart said. ‘It’s a new principle of using mechanics to control the growth of a nanostructured material.’

Few high-throughput manufacturing processes can achieve such flexibility in creating three-dimensional structures, Hart said. This technique, he added, is attractive because it can be used to create large expanses of the structures simultaneously; the shape of each structure can be specified by designing the starting pattern. Hart said the technique could also enable control of other properties, such as electrical and thermal conductivity and chemical reactivity, by attaching various coatings to the carbon nanotubes after they grow.

‘If you coat the structures after the growth process, you can exquisitely modify their properties,’ says Hart. For example, coating the nanotubes with ceramic, using a method called atomic layer deposition, allows the mechanical properties of the structures to be controlled.

‘When a thick coating is deposited, we have a surface with exceptional stiffness, strength, and toughness relative to [its] density,’ Hart said. ‘When a thin coating is deposited, the structures are very flexible and resilient.’

This approach may also enable high-fidelity replication of the intricate structures found on the skins of certain plants and animals, Hart said, and could make it possible to mass-produce surfaces with specialised characteristics, such as the water-repellent and adhesive ability of some insects.

‘We’re interested in controlling these fundamental properties using scalable manufacturing techniques,’ Hart said.

Hart says the surfaces have the durability of carbon nanotubes, which could allow them to survive in harsh environments, and could be connected to electronics and function as sensors of mechanical or chemical signals.