Actuators work by converting external stimuli into mechanical motion that can then be harnessed to do work or move objects. Such a process is at the heart of nanorobotics, artificial muscles and other smart systems. Materials that respond to stimuli in this way include shape-memory alloys, dielectric and electroactive polymers, and polymer hydrogels.

One promising polymer hydrogel is poly(N-isopropylacrylamide), or pNIPAM. An important feature of this gel is that it has a "lower critical solution temperature" (LCST) that occurs between 32 and 33 °C – which is comparable to the temperature range found in many biological environments. When heated to above its LCST, the gel goes from being hydrophilic to hydrophobic and greatly reduces in volume thanks to the fact that it expels water. If loaded with functional materials, such as CNTs, the gel can be made to react to other stimuli as well, such as light or different chemicals.

Ali Javey and colleagues have now fabricated actuators from composites of pNIPAM loaded with single-walled carbon nanotubes at 0.75 mg/mL. The devices, which can be made in cube or flower shapes, respond to heat up to five times better than devices made from pNIPAM alone. What is more, the embedded SWNTs allow the devices to absorb infrared radiation since SWNTs are sensitive to light in this part of the electromagnetic spectrum. This latter property can be exploited to make ultrafast near-IR optically responsive hydrogels.

The researchers used a technique called UV polymerization to deposit a thin film of CNT/pNIPAM on a low-density polyethylene (LDPE) substrate. Before depositing the CNT/pNIPAM layer, they laser-cut hinge lines onto the LDPE substrate to define the actuation sites. They then bored an array of holes into the LDPE substrate around the hinge lines to help anchor the CNT/pNIPAM layer more strongly on the LDPE.

"This work could lead to the development of a new class of programmable devices that can change shape and functionality on command using user-defined external stimuli," Javey told "Some applications include developing smart, self-powered solar tracking devices and engineering tissue, such as artificial muscle."

The team now hopes to further enhance the performance of the actuators and explore more complex functional systems.

The work was published in Nano Letters.