The 4DP materials have been used as smart materials in drug delivery using 4DP containers, stent fabrication, splint fabrication and others. One example is the magnetic nanoparticle-incorporated microgripper, which was printed using hydrogels. 19 The 4DP materials are also found to respond to magnetic fields. 18 An electric current is also used to absorb or desorb water to control the volume of the matrix of the 4DP materials. In particular, when heat is generated after the passage of an electric current, ethanol evaporates and the volume of the 4DP materials increases thereafter due to the increase in the volume of ethanol. 17 The electro-responsive 4DP materials also attracted attention after the introduction of the silicon elastomer and ethanol-based artificial muscle. 16 By the virtue of anisotropy, the 4DP materials bend when exposed to UV rays. 15 When light was incident over certain parts of a polymer gel block, the part swelled due to the infiltration of chromophores (photo-responsive) at the illuminated site. 10–14 The smart 4DP shape-folding structures can change shape under heat and light. The swelling of the hydrogel is controlled by varying the temperature of the aqueous environment. The hydrogel can be immersed to absorb water until it is saturated. The bio-inspired 4DP materials such as hydrogels (which can expand to twice their original volume) are capable of changing their shapes when immersed in water. 5–9 The 4D printed (4DP) smart material recovers its permanent shape once the temperature is raised above its T g. The shape memory polymers (SMPs) are programmed above their glass transition temperature ( T g) and further cooled to set their temporary shape, which is free of any external loading. Nowadays, researchers are freely using such shape memory alloys for the ease in the printing of such smart bio-inspired materials. 4 In the case of thermo-responsive smart materials, the shape memory effect (SME) is the key mechanism to drive deformation. 1 The materials for such bio-inspired structures are selected on the basis of their responses to environmental and temporal stimuli. 2 Several methods have been introduced for the printing of such smart materials such as laser-assisted bioprinting, 3 fused deposition modeling, and other methods common to the 3D printing of CAD models. These intelligent materials were modeled using a bi-exponential mathematical model to predict their fourth dimension. Such stimuli-responsive materials have one extra dimension (fourth dimension) as compared to 3D printed materials. The concept of 4D printed (4DP) smart materials was coined by Tibbits in the year 2013 1 in his speech at the MIT (USA) conference. The future applications would be based on these smart and intelligent materials thus, it is important to modify the existing voxel-based modeling and simulation approach and discuss efficient printing methods to fabricate such bio-inspired materials. This paper also outlines a review of the 4D printing of (a) smart photocurable and biocompatible scaffolds with renewable plant oils, which can be a better alternative to traditional polyethylene glycol diacrylate (PEGDA) to support human bone marrow mesenchymal stem cells (hMSCs), and (b) a biomimetic dual shape-changing tube having applications in biomedical engineering as a bioimplant. Such plant-inspired architectures can change shapes when immersed in water. The shape-changing materials are inspired from biological objects, such as flowers, which are temperature-sensitive or touch-sensitive, and can be 4D printed in such a way that they are encrypted with a decentralized, anisotropic enlargement feature under a restrained alignment of cellulose fibers as in the case of composite hydrogels. The voxel-based modeling and simulation approach is further modified using bi-exponential expressions to encode the time-dependent behavior of the bio-inspired 4D printed materials. The voxel-based modeling and simulation approach has the enhanced features for the rapid testing (prior to moving into design procedures) of the given distribution of such 4D printed smart materials (SMs) while checking for behaviors, particularly when these intelligent materials are exposed to a stimulus. This paper encompasses two recent approaches to explore the conceptual design of 4D printed objects in detail: (a) an application-based modeling and simulation approach for phytomimetic structures and (b) a voxel-based modeling and simulation approach. For this, the designing space has to be explored in the initial stages, which is lagging so far. However, the manufacturing of such objects is still a challenging task. The 4D printed materials are stimulus-responsive and have shape-changing features. 4D printed objects are indexed under additive manufacturing (AM) objects.
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