These surfaces, made by self-assembly of carbon nanotubes, could exhibit a variety of useful 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,” says MIT's John Hart, the first author of a paper describing the new technique in the journal Nature Communications.
The technique works by inducing carbon nanotubes to bend as they grow. The mechanism is analogous to the bending of a bimetallic strip as it warms: one material expands faster than another bonded to it. But in this new process, the material bends 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,” says Hart. Where nanotubes growing at different rates are adjacent, “they push and pull on each other,” he says, producing more complex forms. “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. This technique 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 says 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 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 explains. “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, and could make it possible to mass-produce surfaces with specialised characteristics, such as the water-repellent and adhesive ability of some insects.
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.