its innate properties, or in response to external stimuli. Specifically, the SME can be seen in hybrid structures composed of a smart material and static material due to their inhomogeneity and different properties. [9,10] On the other hand, various stimuli, such as light, temperature, and humidity, can cause SMEs when a smart material converts the stimulus/energy into dynamic movement. With a detailed understanding of the properties of smart materials and their responses to stimuli, 4D printing can be utilized to precisely fabricate a "programmed" dimension that transforms and/or recovers its shape in response to stimuli. Many such smart materials have been developed, thus confirming the feasibility of 4D printing (Figure 1a).In this review, the concept of 4D bioprinting and smart materials will be defined by 4D mechanisms of SMEs and stimulus-responsive mechanisms. Then, 4D bioprinting will be categorized according to the types and cases of biomedical smart materials and applications. The current limitations and future aspects will also be discussed. Four-dimensional Mechanisms: Shape-Morphing EffectsFour-dimensional printing is essentially a 3D printing technique combined with one more dimension, that is, the SME by pre-set stimuli. The SME in 4D printing occurs after printing of the 3D structure, which is printer-independent but still predictable as the effect is programmed beforehand.  The types of SME-inducing stimuli will be discussed in the following section. In this section, SMEs will be classified according to the shape recovery ability of the 4D-printed structure. The SME of 4D structures with no shape recovery ability can be classified as one-way SME, while SMEs with shape recovery ability is classified as two-or multi-way SME. One-Way Shape MorphingOne-way SME refers to a 4D structure that is designed to change its structure once and is not able to recover its original shape by itself. This irreversibility of one-way SME is shown in Figure 1b.  One-way SME is distinguished from normal deformations, such as degradation, contraction, and swelling, as the deformation can be designed and predicted, that is, artificially programmed. One-way SME is a relatively simple mechanism compared to two-or multi-way SME. BioprintingThe development of the three-dimensional (3D) printer has resulted in significant advances in a number of fields, including rapid prototyping and biomedical devices. For 3D structures, the inclusion of dynamic responses to stimuli is added to develop the concept of four-dimensional (4D) printing. Typically, 4D printing is useful for biofabrication by reproducing a stimulus-responsive dynamic environment corresponding to physiological activities. Such a dynamic environment can be precisely designed with an understanding of shape-morphing effects (SMEs), which enables mimicking the functionality or intricate geometry of tissues. Here, 4D bioprinting is investigated for clinical use, for example, in drug delivery systems, tissue engineering, and surgery in vivo. This review presents ...
Three-dimensional (3D) printing in tissue engineering has been studied for the bio mimicry of the structures of human tissues and organs. Now it is being applied to 3D cell printing, which can position cells and biomaterials, such as growth factors, at desired positions in the 3D space. However, there are some challenges of 3D cell printing, such as cell damage during the printing process and the inability to produce a porous 3D shape owing to the embedding of cells in the hydrogel-based printing ink, which should be biocompatible, biodegradable, and non-toxic, etc. Therefore, researchers have been studying ways to balance or enhance the post-print cell viability and the print-ability of 3D cell printing technologies by accommodating several mechanical, electrical, and chemical based systems. In this mini-review, several common 3D cell printing methods and their modified applications are introduced for overcoming deficiencies of the cell printing process.
Extrusion-based bioprinting is one of the most effective methods for fabricating cellladen mesh structures. However, insufficient cellular activities within the printed cylindrical cell-matrix blocks, inducing low cell-to-cell interactions due to the disturbance of the matrix hydrogel, remain to be addressed. Hence, various sacrificial materials or void-forming methods have been used; however, most of them cannot solve the problem completely or require complicated fabricating procedures. Herein, we suggest a bioprinted cell-laden collagen/hydroxyapatite (HA) construct comprising meringue-like porous cell-laden structures to enhance osteogenic activity. A porous bioink is generated using a culinary process, i.e., the whipping method, and the whipping conditions, such as the material concentration, time, and speed, are selected appropriately. The constructs fabricated using the meringue-like bioink with MG63 cells and human adipose stem cells exhibit outstanding metabolic and osteogenic activities owing to the synergistic effects of the efficient cell-to-cell interactions and HA stimulation released from the porous structure. The in vitro cellular responses indicate that the meringue-like collagen bioink for achieving an extremely porous cell-laden construct can be a highly promising cell-laden material for various tissue regeneration applications.
Background Intervertebral disc degeneration (IVDD) is a common cause of chronic low back pain (LBP) and a socioeconomic burden worldwide. Conservative therapies and surgical treatments provide only symptomatic pain relief without promoting intervertebral disc (IVD) regeneration. Therefore, the clinical demand for disc regenerative therapies for disc repair is high. Methods In this study, we used a rat tail nucleotomy model to develop mechanically stable collagen-cryogel and fibrillated collagen with shape-memory for use in minimally invasive surgery for effective treatment of IVDD. The collagen was loaded with hyaluronic acid (HA) into a rat tail nucleotomy model. Results The shape-memory collagen structures exhibited outstanding chondrogenic activities, having completely similar physical properties to those of a typical shape-memory alginate construct in terms of water absorption, compressive properties, and shape-memorability behavior. The treatment of rat tail nucleotomy model with shape-memory collagen-cryogel/HA alleviated mechanical allodynia, maintained a higher concentration of water content, and preserved the disc structure by restoring the matrix proteins. Conclusion According to these results, the collagen-based structure could effectively repair and maintain the IVD matrix better than the controls, including HA only and shape-memory alginate with HA.
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