Microorganisms move in challenging environments by periodic changes in body shape. By contrast, current artificial microrobots cannot actively deform, exhibiting at best passive bending under external fields. Here, by taking advantage of the wireless, scalable and spatiotemporally selective capabilities that light allows, we show that soft microrobots consisting of photoactive liquid-crystal elastomers can be driven by structured monochromatic light to perform sophisticated biomimetic motions. We realized continuum yet selectively addressable artificial microswimmers that generate travelling-wave motions to self-propel without external forces or torques, as well as microrobots capable of versatile locomotion behaviours on demand. Both theoretical predictions and experimental results confirm that multiple gaits, mimicking either symplectic or antiplectic metachrony of ciliate protozoa, can be achieved with single microrobots. The principle of using structured light can be extended to other applications that require microscale actuation with sophisticated spatiotemporal coordination for advanced microrobotic technologies. 3Mobile micro-scale robots are envisioned to navigate within the human body to perform minimally invasive diagnostic or therapeutic tasks 1,2 . Biological microorganisms represent the natural inspiration for this vision. For instance, microorganisms successfully swim and move through a variety of fluids and tissues.Locomotion in this regime, where viscous forces dominate over inertia (low Reynolds number), is only possible through non-reciprocal motions demanding spatiotemporal coordination of multiple actuators 3 . A variety of biological propulsion mechanisms at different scales, from the peristalsis of annelids (Fig. 1a) to the metachrony of ciliates (Fig. 1b), are based on the common principle of travelling waves (Fig. 1c). These emerge from the distributed and self-coordinated action of many independent molecular motors 4,5 .Implementing travelling wave propulsion in an artificial device would require many discrete actuators, each individually addressed and powered in a coordinated fashion (Fig. 1d). The integration of actuators into microrobots that are mobile poses additional hurdles, since power and control need to be distributed without affecting the microrobots' mobility. Existing microscale actuators generally rely on applying external magnetic 6-10 , electric 11 , or optical 12,13 fields globally over the entire workspace. However, these approaches do not permit the spatial selectivity required to independently address individual actuators within a micro-device. Nevertheless, complex non-reciprocal motion patterns have been achieved by carefully engineering the response of different regions in a device to a spatially uniform external field 13,14 .The drawback is that this complicates the fabrication process, inhibits down-scaling and constrains the device to a single predefined behaviour. These challenges mean that most artificial microrobots actually have no actuators. Rather...

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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.

According to a new design strategy for "de Vries-like" liquid crystal materials, which are characterized by a maximum layer contraction of < or = 1% upon transition from the SmA phase to the SmC phase, we report the synthesis and characterization of two homologous series of organosiloxane mesogens. The design of these new materials is based on a frustration between one structural element that promotes the formation of a SmC phase (a trisiloxane-terminated side-chain) and one that promotes the formation of a SmA phase (either a chloro-terminated side-chain or a 5-phenylpyrimidine core). Measurements of smectic layer spacing d as a function of temperature by small-angle X-ray scattering (SAXS) combined with optical tilt angle measurements revealed that the mesogens 5-(4-(11-(1,1,1,3,3,5,5-heptamethyltrisiloxanyl)-undecyloxy)phenyl)-2-(1-alkyloxy)pyrimidine (3(n)) undergo SmA-SmC phase transitions with maximum layer contractions ranging from 0.5% to 1.4%. A comparison of reduction factors R and f suggests that this behavior is due in part to a pronounced negative thermal expansion in the SmC phase that counterbalances the layer contraction caused by increasing tilt. SAXS measurements also revealed that compounds 3(n) are characterized by low orientational and high translational order, which is consistent with theoretical predictions that such materials should exhibit de Vries-like properties. The R values for series 3(n) are comparable to, and even lower than, those reported for established de Vries-like materials such as the perfluorinated 2-phenylpyrimidine material 3M 8422.

In this article, we present the synthesis of highly shape-anisotropic, micrometer-sized particles from liquid crystalline elastomers, which have the ability to reversibly change their shape in response to a certain external stimulus. For their preparation, we utilized a microfluidic setup. We succeeded in preparing sets of particles with differing degrees of shape anisotropy in their ground state including highly anisotropic fiber-like objects. All samples produced movement during the phase transition from the nematic to the isotropic phase of the liquid crystal. Depending on the direction of this shape change, we classified the samples in two groups. One type showed a contraction, while the other showed an expansion during the actuation, generating displacements of 60% and 80%, respectively. Using X-ray diffraction experiments, we could show that the different actuation properties arise from different director patterns of the liquid crystalline moieties in the microparticles. While the weakly shape-anisotropic microparticles possess a concentric director field (director perpendicular to the symmetry axis), the highly anisotropic fiber-like particles show an alignment of the director along the fiber axis. We present an explanation, claiming that this is the result of two different orientation mechanisms involving elongational flow on the one side and "log-rolling" on the other.

A procedure to obtain the smectic translational order parameter Sigma from the temperature-dependent intensity I of the fundamental (001) smectic layer peak, observed in small-angle x-ray scattering experiments, is presented. We report results obtained with this procedure on the smectic-A or chiral smectic-A phases of six different liquid crystals, among them a "de Vries"-type 2-{4'-[1'',1''-dihydro-2''-(2''-perfluorobutoxyperfluoroethoxy)- perfluoroethoxy]}phenyl-5'-octylpyrimidine(3M8422) and the antiferroelectric R-4-(1-methylheptyloxycarbonyl)phenyl-4'-octloxybiphenyl-4-carboxylate (MHPOBC) materials. The smectic order parameters obtained are in the range between 0.5 and 0.9. For the "de Vries"-type 3M 8422 we found remarkably high values of Sigma in the order of approximately 0.9.

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In ionic liquid crystals, the orthogonal smectic A phase is the most common phase whereas the tilted smectic C phase is rather rare. We present a new study with five novel ionic liquid crystals exhibiting both a smectic A as well as the rare smectic C phase. Two of them have a phenylpyrimidine core whereas the other three are imidazolium azobenzenes. Their phase sequences and tilt angles were studied by polarizing microscopy and their temperature-dependent layer spacing as well as their translational and orientational order parameters were studied by X-ray diffraction. The X-ray tilt angles derived from X-ray studies of the layer contraction and the optically measured tilt angles of the five ionic liquid crystals were compared to obtain their de Vries character. Four of our five mesogens turned out to show de Vries-like behavior with a layer shrinkage that is far less than that expected for conventional materials. These materials can thus be considered as the first de Vries-type materials among ionic liquid crystals.