A material that rapidly heats up and changes shape when connected to a battery has been developed by US scientists. 

Patrick Mather at Syracuse University, New York, and co-workers blended an electrically conductive network of carbon nanofibres with a shape memory polymer (SMP) - a material that changes from a deformed shape to its original shape induced by a trigger such as a change in temperature. The network of nanofibres enabled the material to heat up very quickly, triggering a change in motion (actuation). This material could be used in deployable mechanical structures which can change shape when commanded to do so. 

Mather explains that SMPs enable simple actuation in applications ranging from mechanical mechanisms and surgery devices to even toys. 'We have found that many plastic items you see at the hardware or department store, or in the hospital, could benefit from shape change functionality. For example, SMP eyeglass frames could be easily adjusted to a perfect fit,' he says. 

However, SMPs aren't exactly ready to win an arm-wrestling match, due to being too soft when actuated, and are quite slow to recover their shape because of their low thermal conductivities, says Mather. They are also not very responsive to electricity since they are electrical insulators. To increase their electrical conductivity, SMPs have been combined with conductive fillers such as carbon materials or conducting polymers. A voltage is applied to these materials, generating heat and triggering shape recovery.

But even with the conductive fillers added these materials are unsuitable for high speed actuation applications because the conductivity is quite low resulting in slow shape recovery. Mather says the carbon nanofibres used by his group were arranged together in a highly interconnected network which provided a continuous path for electron conduction between the nanofibres and enhanced the transfer of heat. This resulted in a polymer composite with surprisingly high electrical actuation speeds - the actuation takes just 2 seconds.

Richard Vaia, who develops adaptive soft matter at the US Air Force Research Laboratory, Ohio, says that tuning SMPs with carbon nanofibre results in more uniform heating than the use of external heaters, increasing the potential for actuators with remote triggering. 'This provides the possibility of faster motion and greater scope for technologies ranging from medical impacts to aerospace,' he adds.

Mather envisages the technology being immediately used in deployable devices where speed is essential. In the future Mather hopes to be able to better understand the relationship between the material's structure and the properties, allowing them to be tailored further. 

Fay Nolan-Neylan 
 

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