Experiments by researchers at Rice University, Houston, TX, found that new biocompatible, stable, and inert materials they developed that start as flat slabs can morph into shapes that can be controlled by patterns written into their layers. Materials that can change their shape based on environmental conditions are useful for optics, three-dimensional biological scaffolds, and the controlled encapsulation and release of drugs, they say.
Two layers of the material are needed. The first layer is a liquid crystal elastomer (LCE), a rubber-like material of cross-linked polymers that line up along a single axis, called the nematic director. The other is a thin layer of simple polystyrene, placed either above or below the LCE.
Without the polystyrene layer bonded to it, an LCE would simply expand or contract along its nematic axis when heated. With changing temperature, the LCE tries to contract or expand, but the stiffer polystyrene layer prevents this and instead causes wrinkling, bending or folding of the entire material.
The lab discovered that the layers would react to heat in a predictable and repeatable way, allowing for configurations to be designed into the material depending on a number of parameters: the shape and aspect ratio of the LCE, the thickness and patterning of the polystyrene and even the temperature at which the polystyrene was applied.
The lab made spiraling, curling, and X-shaped materials that alternately closed in or stood up on four legs. Placing polystyrene on top of one half of a strip of LCE and on the bottom of the other half produced an “S” shape. The primary direction of folding or wrinkling of the material was set by the temperature at which the polystyrene layer was deposited.
LCEs are reversible, unlike shape-memory polymers that change shape only once and cannot go back to their initial shape, which is important for biomedical applications, such as dynamic substrates for cell cultures or implantable materials that contract and expand in response to stimulus.