troactive polymers, [7] as well as patterned hydrogels that shape-shift on swelling. [8,9] Further development of soft elements capable to perform complex motions or functions requires adequate materials and processing technologies that enable accurate control of the mechanical response. Moreover, on the route toward practical applications, there is often a necessity to have the possibility to miniaturize these elements, produce them in large dimensions or over large areas, or integrate them with other materials, elements, or devices.Crosslinked liquid crystalline polymers (LCPs) have received much attention as candidates for this purpose since they can exhibit large macroscopic scale mechanical response to different external stimuli such as heat, light, pH, or moisture. [10][11][12] Thinfilms of these materials with controlled molecular orientation, defined by the director (n), have predominantly been investigated as building blocks to implement a variety of responsive elements or devices. [13][14][15][16][17][18][19][20][21][22] However the thinfilm character of these elements markedly limits the energy available for actuation. Even more, the reported systems, including their processing toolbox, are limited to one single material and therefore multifunctional and multiresponsive systems are difficult to create. For the true development and incorporation of these LCP structures in real life applications, the capability to generate elements of different sizes, from very small to very large, is fundamental. This needs to be done with a precise control of the material morphology and properties as well as director orientation in well-defined complex geometries, all together leading to an accurate control of the mechanical response.Additive manufacturing techniques enable digital generation of material patterns in surfaces or fabrication of 3D objects. While 3D printing of conventional materials leads to inanimate 3D objects with static shape, 4D printing of responsive materials adds a 4th dimension as it leads to architectures that, with an appropriate stimulus, change their shape over time. [9,23] Here, we report the 4D printing of liquid crystalline elastomer (LCE) macro-and microstructures. Digital control of the local anisotropy of the applied LC material is advantageously achieved through the printing process. This allows to precisely program the magnitude and directionality of the forces in response to the external stimulus, temperature in our case, and therefore well-defined reversible shape-morphing of the structures in space and time. Although 4D printing has been recently described with hydrogels charged with anisotropic cellulose fibrils that get oriented during printing, [9] the mechanical response of these materials is based on water swelling, which limits the applicability due to the specific and stringent environment required for actuation as well as the slow response 3D Printing Soft matter elements undergoing programed, reversible shape change can contribute to fundamental advance in areas such as ...

Remote light exposure of photoresponsive liquid crystalline polymers has drawn great attention over the last years as an attractive strategy to generate mechanical work with high spatial resolution. To tailor these materials into practical engineering devices, it is of key importance to gain control over their morphology and thus precisely program their mechanical response, which must also be fast and relevant in magnitude. In this communication, we report the four-dimensional (4D) printing of azobenzene-containing liquid crystalline elastomers (LCEs) that respond to light. During extrusion of the LCE precursor, mesogen orientation is defined by the needle's moving direction enabling a precise definition of the director, which is later fixed by photopolymerization. Fast mechanical responses have been observed in these 4D printed LCE elements when excited with ultraviolet (UV) light. These 4D printed elements lift objects many times heavier than their own weight, demonstrating a capacity to produce effective work. Photochemical and photothermal contributions to the deformation and force have been identified. Advantageously, the use of blue and UV light excitation enables adjustment of generated forces that can be maintained even in the dark and can be released by light excitation or temperature. The demonstrated ability to generate light-responsive elements quickly delivering sufficient work paves the way for implementing remotely addressed mechanical functions to future soft robotics and engineering.

Inkjet printing of photoacid catalyzed organic–inorganic hybrid formulations has been used in the preparation of low-loss optical waveguides.