Electrospinning has gained great interest in the field of regenerative medicine, due to its fabrication of a native extracellular matrix-mimicking environment. The micro/nanofibers generated through this process provide cell-friendly surroundings which promote cellular activities. Despite these benefits of electrospinning, a process was introduced to overcome the limitations of electrospinning. Cell-electrospinning is based on the basic process of electrospinning for producing viable cells encapsulated in the micro/nanofibers. In this review, the process of cell-electrospinning and the materials used in this process will be discussed. This review will also discuss the applications of cell-electrospun structures in tissue engineering. Finally, the advantages, limitations, and future perspectives will be discussed.

For muscle regeneration, a uniaxially arranged micropattern is important to mimic the structure of the natural extracellular matrix. Recently, cell electrospinning (CE) has been tested to fabricate cell‐laden fibrous structures by embedding cells directly into micro/nanofibers. Although homogenous cell distribution and a reasonable cell viability of the cell‐laden fibrous structure fabricated using the CE process are achieved, unique topographical cues formed by an aligned fibrous structure have not been applied. In this study, a CE process to achieve not only homogeneous cell distribution with a high cell viability, but also highly aligned cells, which are guided by aligned alginate fibers is employed. To attain the aligned cell‐laden fibrous structure, various processing conditions are examined. The selected condition is applied using C2C12 myoblast cells to ensure the biocompatibility and guidance of cell elongation and alignment. As a control, a cell‐printed scaffold using a 3D bioprinter is used to compare the efficiency of cell alignment and differentiation of myoblasts. Highly arranged, multinucleated cell morphology is confirmed in the CE scaffold, which successively facilitates myogenic differentiation. It is believed that this study will be a new platform for obtaining cell alignment and will significantly benefit the efforts on muscle regeneration.

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. [11] 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. [12] 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 ...

Biomedical scaffolds must be used in tissue engineering to provide physical stability and topological/biochemical properties that directly affect tissue regeneration. In this study, a new cell-laden scaffold was developed that supplies micro/nano-topological cues and promotes efficient release of cells. The hierarchical structure consisted of poly(ε-caprolactone) macrosized struts for sustaining a three-dimensional structural shape, aligned nanofibers obtained with optimized electrospinning, and cell-printed myoblasts. Importantly, the printed myoblasts were fully safe and were efficiently released from the cell-laden struts to neighboring nanofiber networks. The incorporation of micro/nanofibers in the hierarchical scaffold significantly affected myoblast proliferation, alignment, and even facilitated the formation of myotubes. We observed that myosin heavy chain expression and the expression levels of various myogenic genes (MyoD, myogenin, and troponin T) were significantly affected by the fiber alignment achieved in our hierarchical cell-laden structure. We believe that the combination of cell-printing and a hierarchical scaffold that encourages fiber alignment is a highly promising technique for skeletal muscle tissue engineering.

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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.

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