PerspectiveActuating Materials

Shape-shifting liquid crystals

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Science  27 Feb 2015:
Vol. 347, Issue 6225, pp. 949-950
DOI: 10.1126/science.aaa6579

Liquid crystal displays (LCDs) contain tens of thousands of pixels filled with a birefringent fluid known as a liquid crystal, in which molecular orientations fluctuate (like a liquid) but still have an average alignment (like a crystal). The moving images we see on a display are created by controlling the net orientation of the molecules, which changes the optical polarization of the liquid so that it either blocks or transmits light. But what if instead of producing an image on a flat screen, your LCD television could transform into different three-dimensional (3D) objects, and then back to a flat screen? Is it possible for soft materials to reproduce shapes instead of images? On page 982 of this issue, Ware et al. (1) demonstrate this possibility with liquid crystal elastomers (LCEs).

LCEs are soft, elastic solids that can change shape in response to a variety of stimuli: heat, light, and electric and magnetic fields (2). They are made up of liquid crystal molecules chemically bonded to a rubbery (flexible) polymer network. The shape of LCEs is coupled with the liquid crystalline ordering, and vice-versa. Changing the orientation or degree of ordering of the liquid crystals produces shape changes in LCEs. Depending on the structure of the liquid crystals and how they are connected to the polymer network, shape changes can be large—LCEs can double or triple in length (3).

The shaping of things to come?

Schematics for (A) an LCE with (B) patterned liquid crystal “voxels.” (C and D) Analogous to pixels in an LCD screen that define an image, the liquid crystal ordering within the voxels controls the shape of the LCE, resulting in programmed shape change of a thin film.

In theory, LCEs with ideal network structures exhibit what is known as “soft elasticity” and can easily transform between different shapes. The challenge in practice is to dictate and control the liquid crystal ordering in LCEs. A widely used approach for preparing shape-responsive materials, the “two-step” method, simultaneously stretches and cross-links the LCE (4). The molecules align in a single direction, and relatively simple shape changes, such as elongation, contraction, bending, and twisting, are possible (5, 6). Achieving more sophisticated shapes requires more complex liquid crystal orientations. An alternative approach, light-mediated patterning and alignment, offers exquisite control over the liquid crystal orientation (79). However, this produces networks that are stiff and unable to undergo large shape changes.

Ware et al. demonstrate an approach to produce complex director orientations in soft, elastic LCEs, and, as a result, effect complex shape changes. First, photo-alignment defines a complex liquid crystal pattern on a surface. Next, this pattern is imprinted into an LCE with a liquid crystal fluid that polymerizes slowly to form a polymer network. Ware et al. implement chemistry that proceeds without added solvent to form a soft, elastic polymer network. In an analogy to the tens of thousands of pixels in an LCD screen that produce images on a screen, they produce LCEs with 3D patterned elements known as “voxels.” Their light-patterning and polymerization technique can produce more than 20,000 voxels that dictate how the LCE changes shape. By changing the pattern in the alignment layer and within these voxels, they produce a conical actuator, a polymer hinge, and a self-foldable Miura Ori pattern, all from an initially flat film.

The work of Ware et al. is an important step toward realizing materials that can assume arbitrary and programmable shapes, but a number of challenges remain, especially the incorporation of smart, real-time control over the shape of the LCE. This capability would require integration of the LCE with an electronic system capable of turning the liquid crystal orientation within each voxel. Adding nanomaterials could produce a faster and more sensitive shape-response to a variety of signals (10), and reversible chemistry may enable materials that can be reprogrammed to assume different shapes (11). Eventually, we might have access to implantable biomaterials that can respond to their surrounding environment or self-folding devices that can disassemble and shrink to small sizes for storage and transport.

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