While the unique optical properties of liquid crystals (LCs) are already well exploited for flat-panel displays, their intrinsic ability to self-organize into ordered mesophases, which are intermediate states between crystal and liquid, gives rise to a broad variety of additional applications. The high degree of molecular order, the possibility for large scale orientation, and the structural motif of the aromatic subunits recommend liquid-crystalline materials as organic semiconductors, which are solvent-processable and can easily be deposited on a substrate. The anisotropy of liquid crystals can further cause a stimuli-responsive macroscopic shape change of cross-linked polymer networks, which act as reversibly contracting artificial muscles. After illustrating the concept of liquid-crystalline order in this Review, emphasis will be placed on synthetic strategies for novel classes of LC materials, and the design and fabrication of active devices.

Responsive polymers are low-cost, light weight and flexible, and thus an attractive class of materials for the integration into micromechanical and lab-on-chip systems. Triggered by external stimuli, liquid crystalline elastomers are able to perform mechanical motion and can be utilized as microactuators. Here we present the fabrication of one-piece micropumps from liquid crystalline core-shell elastomer particles via a microfluidic double-emulsion process, the continuous nature of which enables a low-cost and rapid production. The liquid crystalline elastomer shell contains a liquid core, which is reversibly pumped into and out of the particle by actuation of the liquid crystalline shell in a jellyfish-like motion. The liquid crystalline elastomer shells have the potential to be integrated into a microfluidic system as micropumps that do not require additional components, except passive channel connectors and a trigger for actuation. This renders elaborate and high-cost micromachining techniques, which are otherwise required for obtaining microstructures with pump function, unnecessary.

In this article new results on the preparation of monodisperse particles from a liquid crystalline elastomer in a microfluidic setup are presnted. For this, droplets from a liquid crystalline monomer are prepared in a microfluidic device and polymerized while they are flowing inside a microtube. The parti­cles obtained by this method possess an internal orientation, which gives them actuating properties. When they are heated into the isotropic phase of the liquid crystalline material they show a reversible change in shape whereby they change their length in one direction by almost 100%. It is shown how the variation of experimental parameters during their synthesis impacts the properties of these micro‐actuators. Influence over their primal shape, the strength of their shape changing properties, their size, and their mechanical properties is demontrated. From the systematic variation of experimental parameters a deep understanding of the complex processes taking place in a flowing droplet of a liquid crystalline material is obtainted. Additionally NMR analysis and swelling experiments on these actuating materials are provided.

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The preparation of thermoresponsive fibers made from a crosslinked liquid‐crystalline (LC) side‐chain polymer is presented. For this, an LC polyacrylate with side‐on attached mesogens and crosslinkable units is synthesized and processed in solution in a microfluidic co‐flowing device. Due to the high viscosity of the polymer and the reduced interfacial tension between the dispersed polymer solution and the immiscible ambient fluid, it becomes possible to produce and stabilize a liquid jet against its breaking into droplets, even at low shear rates. The polymer jet is finally stabilized in the capillary by UV‐initiated photopolymerization leading to well‐oriented, crosslinked LC fibers. If the crosslinking density in these fibers is moderate (LCEs), contractions of several 100% can be observed at the nematic–isotropic phase transition.

Controlling the molecular alignment of liquid crystal monomers (LCMs) within nano- and microstructures is essential in manipulating the actuation behavior of nematic liquid crystal elastomers (NLCEs). Here, we study how to induce uniformly vertical alignment of nematic LCMs within a micropillar array to maximize the macroscopic shape change using surface chemistry. Landau-de Gennes numerical modeling suggests that it is difficult to perfectly align LCMs vertically in every pore within a poly(dimethylsiloxane) (PDMS) mold with porous channels during soft lithography. In an untreated PDMS mold that provides homeotropic anchoring of LCMs, a radially escaped configuration of LCMs is observed. Vertically aligned LCMs, a preferred configuration for actuation, are only observed when using a PDMS mold with planar anchoring. Guided by the numerical modeling, we coat the PDMS mold with a thin layer of poly(2-hydroxyethyl methacrylate) (PHEMA), leading to planar anchoring of LCM. Confirmed by polarized optical microscopy, we observe monodomains of vertically aligned LCMs within the mold, in agreement with modeling. After curing and peeling off the mold, the resulting NLCE micropillars showed a relatively large and reversible radial strain (∼30%) when heated above the nematic to isotropic transition temperature.

We present the synthesis of a polymeric liquid crystalline (LC) crosslinker and its usage in the preparation of soft microactuators. The microactuators, composed of thermoresponsive liquid crystalline elastomers (LCE), are fabricated in a microfluidic device in a continuous “on the fly” process. The LC polymer crosslinker is miscible with the LC monomer and provides for a rapid polymerization and crosslinking. Most importantly, it promotes an enantiotropic mesophase of the mobile monomer phase, which is not provided by conventional nonmesogenic crosslinkers. This allows an isothermal handling inside the microfluidic setup. Temperature‐driven shape changes up to 65% could be achieved by judiciously optimizing the crosslinking density of the LCE particles.

Die einzigartigen optischen Eigenschaften von flüssigkristallinen Phasen (LCs) werden schon seit langem in Flachbildschirmen genutzt. Durch die Besonderheiten dieser Mesophase im Zwischenbereich zwischen dem Kristall (Ordnung) und der isotropen Schmelze (Beweglichkeit) eignen sich flüssigkristalline Materialien aber auch zu einer Vielzahl von weiteren Anwendungen. Der hohe Grad an molekularer Ordnung, die Möglichkeit zur großflächigen Orientierung und das Strukturmotiv aromatischer Einheiten machen Flüssigkristalle zu interessanten Materialien für organische Halbleiter, die in Lösungsmitteln verarbeitet und leicht auf Substraten abgeschieden werden können. Die Anisotropie der Flüssigkristalle kann weiterhin zu einer stimuliresponsiven makroskopischen Formänderung von Polymernetzwerken führen, die als reversibel kontrahierende künstliche Muskeln fungieren. Nach Vorstellung des Konzepts der flüssigkristallinen Ordnung beschäftigt sich dieser Aufsatz schwerpunktmäßig mit Strategien zur Synthese neuer Klassen von LC‐Materialien sowie dem Design und der Herstellung aktiver Bauelemente.