"The porous nature of the CNT-PVA composite allows the ions of the liquid electrolyte to enter and swell the film when the device is electrically stimulated," Christèle Bartholome of Centre de Recherche Paul Pascal told nanotechweb.org. "The system expands at both positive and negative voltages, but the expansion is more pronounced when we apply a negative voltage."

To prepare the bimorph, the researchers first oxidize a sample of MWCNTs, which encourages the structures to bind strongly with the host polymer. Next, the nanomaterial is dispersed in water and then mixed with various amounts of high molecular weight PVA.

Membrane filtration is used to remove the water and shape the remaining materials into a paper-like sheet. A gold layer is then deposited over the CNT-polymer surface, which helps to distribute charge evenly throughout the film and promotes uniform actuation. Finally, an insulating and inert polymer layer is added on the reverse side to complete the bimorph device.

"Materials comprised solely of nanotubes exhibit a high electrical conductivity, but are too weak to effectively convert electrical energy into mechanical energy," commented Bartholome. "By adding an optimal fraction of polymer binder, we can strengthen the CNT assembly and still obtain a structure that is a sufficiently porous and electrically conductive network to swell when stimulated in an electrolyte."

Tests reveal that the best compromise of mechanical and electrical properties is obtained for a PVA weight fraction of about 30%.

The research forms part of a European project known as NOESIS, which is focused on the development of smart nanocomposites for aerospace applications. "The next step is to develop a dry version of our nanotube actuator that uses solid electrolytes instead of liquid ones," said Bartholome. "We are also interested in using the electromechanical structures to develop sensing systems that could monitor the health of composite materials used in aircraft."

Other areas that could benefit from the group's actuation scheme include robotics, automotive systems and microtools for non-invasive surgery.

The researchers presented their work in Nanotechnology.