Materials that morph

C&I Issue 1, 2014

Materials controlled by patterns written into their layers could prove useful for optics, three-dimensional biological scaffolds and the controlled encapsulation and release of drugs, among other applications.

Flat slabs of material designed by Rice University, Texas, researchers can be made to morph into different shapes when heated. The team report that by controlling certain conditions they could predict how the material would react, allowing them to design certain configurations (Soft Matter, 2014; doi:10.1039/C3SM51654G).

The material consists of two layers: a liquid crystal elastomer (LCE) of crosslinked polymers that line up along a single ‘nematic’ axis; and a thin layer of polystyrene, placed above or below the LCE.

On its own, the LCE would expand or contract along its nematic axis as the temperature changes, explains team leader Rafael Verduzco. But the stiffer polystyrene layer prevents this, with the result that the entire material wrinkles, bends or folds.

The primary direction of folding or wrinkling of the material – which occurs at the micrometre-scale – depends on the temperature at which the polystyrene is deposited. The team produced spirals, curls and X-shapes in the material.

Placing polystyrene on top of one half of a strip of LCE and on the bottom of the other half produced an S shape. There’s no limit to the complexity of the shapes that could be teased from the material with proper patterning, Verduzco suggests.

‘For any application, you would want to be able to change shape and then go back,’ says Verduzco. ‘LCEs are reversible, unlike shape-memory polymers that change shape only once and cannot go back to their initial shape.’ This is important for biomedical applications, such as dynamic substrates for cell cultures or implantable materials that contract and expand in response to stimulus, he adds. ‘We already know the materials are biocompatible, stable and inert, so they have great potential for biological applications.’

This is not the first example of bending due to bilayer structure, points out Eugene Terentjev of the Cavendish Laboratory, Cambridge University, UK. ‘But the difficulty is in aligning the LCE in these shapes, and this paper doesn’t suggest any new ways to do it: they simply glued a uniformly aligned strip to polystyrene (it will delaminate after a few dozen cycles).’

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