A new twist on actuator architecture

Scientists have built a novel type of actuator - a device that converts energy into motion - with an artificial opal embedded within it.
To make the actuator, Kwang-Un Jeong at Chonbuk National Unversity, Jeonju, South Korea, and colleagues put colloidal silica in water, where it formed a strip of single crystalline opal one tenth of a millimetre thick. They embedded the strip in a layer of poly(dimethylsiloxane) polymer then added a layer of polyurethane (PU)/2-hydroxyethyl methacrylate polymer on top. Finally, they sealed the bi-layer strip inside a layer of glass.

The team placed the strip in different solvents and found that it responded differently depending on the solvent's hydrophobicity. The strip curled up to form a right handed spiral in hydrophilic solvents, such as acetic acid, and a left handed spiral in hydrophobic solvents, such as hexane. The curling effect is due to the difference in swelling ratios between the two polymer layers, explains Jeong. He showed that the right-handed spirals display different colours depending on the angle they are viewed from but the left-handed ones do not.
'Using a bilayer polymer photonic system to create colour-tunable spiral photonic actuators is a simple and elegant idea,' says Christopher Li, an expert in soft matter and hybrid materials at Drexel University, Philadelphia, US. 'Being able to control the macroscopic handedness of the spiral is fascinating. Slightly changing the sample architecture could lead to profound shapes, such as a helix.'
According to Jeong, the actuators even respond to gaseous solvents. He says that the device may have varied applications: 'These colour-tunable, reversible spiral photonic switches can be useful as mechanical actuators and electrical devices, as well as optical components. Our ultimate goal is to construct complicated 3D objects from programmed 2D structures.'