4D Printing of Hydrogels: A Review

Abstract

Abstract3D printing permits the construction of objects by layer‐by‐layer deposition of material, resulting in precise control of the dimensions and properties of complex printed structures. Although 3D printing fabricates inanimate objects, the emerging technology of 4D printing allows for animated structures that change their shape, function, or properties over time when exposed to specific external stimuli after fabrication. Among the materials used in 4D printing, hydrogels have attracted growing interest due to the availability of various smart hydrogels. The reversible shape‐morphing in 4D printed hydrogel structures is driven by a stress mismatch arising from the different swelling degrees in the parts of the structure upon application of a stimulus. This review provides the state‐of‐the‐art of 4D printing of hydrogels from the materials perspective. First, the main 3D printing technologies employed are briefly depicted, and, for each one, the required physico‐chemical properties of the precursor material. Then, the hydrogels that have been printed are described, including stimuli‐responsive hydrogels, non‐responsive hydrogels that are sensitive to solvent absorption/desorption, and multimaterial structures that are totally hydrogel‐based. Finally, the current and future applications of this technology are presented, and the requisites and avenues of improvement in terms of material properties are discussed.

Polylactic Acid (PLA) Biocomposite: Processing, Additive Manufacturing and Advanced Applications

“…For instance, woodbased filaments such as poplar, and non-wood filaments such as bamboo, jute, cork, wood dust kenaf, and so forth are combined with PLA to provide a more organic texture to the hybrid filament which in line with Sustainable Development Goals (SDGs) that are to (I) encourage inclusive and sustainable industrialisation and foster innovation, and (II) safeguard sustainable consumption and production patterns. Four dimensional printing is a relatively recent trend to develop 3D printed structures that can change their shape or properties over time [163][164][165]. The difference is that 4D printed objects can transform themselves over time, while 3D printed objects maintain fixed shape like any plastic or metal parts.…”

“…The self-transformation of the structure is also called self-assembly as the structure can be designed to assemble itself. The concept of 4D printing is a smart structure that consists of rigid materials connected with expandable elements, or it can also be a whole structure made from expandable materials depending on what mate- Four dimensional printing is a relatively recent trend to develop 3D printed structures that can change their shape or properties over time [163][164][165]. The difference is that 4D printed objects can transform themselves over time, while 3D printed objects maintain fixed shape like any plastic or metal parts.…”

“…A hydrogel is a polymeric material that is capable of absorbing a large amount of water [172,173]. It can be programmed to expand or shrink when there are changes in the external environmental conditions [163,174]. Hydrogels are biocompatible and easy to be modified.…”

3D Printable Electrically Conductive Hydrogel Scaffolds for Biomedical Applications: A Review

“…In particular, 4D printing of ECHs systems remains unexplored. Such developments are possible by formulating systems with near-infrared light sensitive rGO [ 72 ], pH-responsive poly(2-vinylpyridine) [ 73 ], temperature-responsive poly(N-isopropylacrylamide) [ 74 ], hydration/solvent-responsive PEG/PEGDA hybrids [ 75 ], electric field-responsive ionized PAA [ 76 ], etc., which could be potentially applied for flow regulating biodevices and soft robotic bioactuators [ 77 ]. In addition, new 3D printing-based hybrid methods, such as EDH cojetting and electrochemical 3D printing, need to be further explored for different materials and formulations.…”

A 3D Printed Morphing Nozzle to Control Fiber Orientation during Composite Additive Manufacturing

“…[11][12][13] One critical challenge in printing fiberladen composite materials, however, is that conventional extrusion nozzles yield identical fiber alignment during the deposition process. [14][15][16] Consequently, for applications that are founded on more sophisticated directional control of fiber orientations, such as in the area of "4D printing," researchers must meticulously design and/or model the material deposition paths to ensure that the resulting fiber alignments generate the performance desired. [17][18][19][20] Thus, new strategies that support dynamic customization of fiber orientation throughout the extrusion process hold promise for extending the efficacy of composite material-based technologies.…”