4D Printed Actuators with Soft‐Robotic Functions

Abstract

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 ...

Shape changing hydrogels and their applications as soft actuators

“…The locally different responsive properties of the hydrogels can be achieved by local adjusting the compositions and/or structures of the hydrogels. Among the reported methods, photolithographic 55,56 and three-dimensional (3D) printing methods [57][58][59][60][61] can produce hydrogels with macroscale inhomogeneous structures by introducing a double-network hydrogel or inducing different orientations of additive cellulose fibrils, respectively. Photolithographic, 62-66 electrochemical ionprinting, [67][68][69][70] ion-transfer-printing (ITP), 50 and ion-inkjetprinting (IIP) 71 methods can prepare hydrogels with molecular scale inhomogeneity by introducing crosslinking gradients into as-prepared hydrogels.…”

Photoreversible Soft Azo Dye Materials: Toward Optical Control of Bio‐Interfaces

“…Additive manufacturing techniques, such as two‐photon lithography and 3D printing, are becoming more and more relevant in biotechnology, giving the possibility to produce complex 3D structures on different length‐scales and becoming amenable to many different materials . It has been shown that LC materials can be utilized with these techniques with the characteristic molecular order remaining preserved during the photo‐crosslinking process, or even induced and patterned due to shear forces in the 3D printing process . Van Oosten et al achieved the construction of bioinspired artificial cilia for mixing applications and microfluidic pumping (Figure d) .…”

Advances and Future Perspectives in 4D Bioprinting

“…López‐Valdeolivas et al prepared cross‐linkable liquid crystal polymers as inks and used them to print stimuli‐responsive structures. The printed elastomers were able to undergo shape changes in response to thermal stimulus and served as actuators or soft‐robots . Nishiguchi et al used a direct laser writing method to generate sophisticated and controlled folding of thermosensitive hydrogels.…”

“…The printed elastomers were able to undergo shape changes in response to thermal stimulus and served as actuators or soft-robots. [97] Nishiguchi et al [98] used a direct laser writing method to generate sophisticated and controlled folding of thermosensitive hydrogels. The designed hydrogels were capable of biomimetic actuation.…”