In recent decades, the biomedical applications of mesenchymal stem cells (MSCs) have attracted increasing attention. MSCs are easily extracted from the bone marrow, fat, and synovium, and differentiate into various cell lineages according to the requirements of specific biomedical applications. As MSCs do not express significant histocompatibility complexes and immune stimulating molecules, they are not detected by immune surveillance and do not lead to graft rejection after transplantation. These properties make them competent biomedical candidates, especially in tissue engineering. We present a brief overview of MSC extraction methods and subsequent potential for differentiation, and a comprehensive overview of their preclinical and clinical applications in regenerative medicine, and discuss future challenges.

https://www.mdpi.com/2073-4409/8/8/886/pdf

Mesenchymal Stem Cells for Regenerative Medicine.

Bioprinting is an emerging field in regenerative medicine. Producing cell-laden, three-dimensional structures to mimic bodily tissues has an important role not only in tissue engineering, but also in drug delivery and cancer studies. Bioprinting can provide patient-specific spatial geometry, controlled microstructures and the positioning of different cell types for the fabrication of tissue engineering scaffolds. In this brief review, the different fabrication techniques: laser-based, extrusion-based and inkjet-based bioprinting, are defined, elaborated and compared. Advantages and challenges of each technique are addressed as well as the current research status of each technique towards various tissue types. Nozzle-based techniques, like inkjet and extrusion printing, and laser-based techniques, like stereolithography and laser-assisted bioprinting, are all capable of producing successful bioprinted scaffolds. These four techniques were found to have diverse effects on cell viability, resolution and print fidelity. Additionally, the choice of materials and their concentrations were also found to impact the printing characteristics. Each technique has demonstrated individual advantages and disadvantages with more recent research conduct involving multiple techniques to combine the advantages of each technique.

https://www.mdpi.com/1996-1944/11/11/2199/pdf

An Introduction to 3D Bioprinting: Possibilities, Challenges and Future Aspects

Additive manufacturing (AM) is considered the latest technology that creates breakthrough innovations and addresses complex medical problems. This is clearly demonstrated by the promising results obtained in regenerative medicine, diagnosis, implants, artificial tissues and organs. This paper provides a basic understanding of the fundamentals of 3D/4D printing along with bioprinting processes. We are briefly discussing about the main printing systems including stereolithography, inkjet 3D printing, extrusion, laser-assisted printing, selective laser melting and Poly-Jet printing. The basic requirements for the selection of successful inks based on polymers, polymer blends, and composites are described. Furthermore, the on-going transition from 3D to 4D printing is highlighted with emphasis on the newest applications in the medical area. Also, a glimpse into the future possibilities and benefits provided by machine learning in the additive manufacturing field is emphasized. Machine learning can improve printing efficiency by using generative design and testing in the pre-fabrication stage. Finally, important limitations and prospects are identified. Within the next few years, AM is set to become an important component in patient-specific medical technologies.

https://link.springer.com/content/pdf/10.1007/s10924-020-01722-x.pdf

Trends in 3D Printing Processes for Biomedical Field: Opportunities and Challenges

Engineering three-dimensional (3D) tissues in clinically relevant sizes have demonstrated to be an effective solution to bridge the gap between organ demand and the dearth of compatible organ donors. A major challenge to the clinical translation of tissue-engineered constructs is the lack of vasculature to support an adequate supply of oxygen and nutrients post-implantation. Previous efforts to improve the vascularization of engineered tissues have not been commensurate to meeting the oxygen demands of implanted constructs during the process of homogeneous integration with the host. Maintaining cell viability and metabolic activity during this period is imperative to the survival and functionality of the engineered tissues. As a corollary, there has been a shift in the scientific impetus beyond improving vascularization. Strategies to engineer biomaterials that encapsulate cells and provide the sustained release of oxygen over time are now being explored. This review summarizes different types of oxygen-releasing biomaterials, strategies for their fabrication, and approaches to meet the oxygen requirements in various tissue engineering applications, including cardiac, skin, bone, cartilage, pancreas, and muscle regeneration. Biomaterials that surround artificial tissue and slowly release oxygen can improve survival rates during wound healing and organ regeneration trials. Although researchers have successfully constructed many kinds of tissues using 3D scaffolds, supplying the cells with adequate oxygen and nutrients post-implantation remains challenging. Gulden Camci-Unal from the University of Massachusetts Lowell in the USA and colleagues review how microscale oxygen sources such as calcium peroxide crystals are being combined with flow-regulating polymers to keep engineered tissues healthy until vascular systems are established. Coatings of peroxides and polymers applied to 3D scaffolds, for example, strengthen bone cultures by releasing a steady supply of oxygen to permeate throughout the tissue for extended times. Byproducts from oxygen-releasing materials can also be used to inhibit biofilm growth on implants, lessening chances of infection. The inability to administer oxygen in a controlled and sustained manner into thick artificial tissues has attracted a growing interest towards the design and development of new functional biomaterials. Without a sufficient oxygen supply, tissues suffer from the effects of apoptosis and necrosis. Incorporation of oxygen-releasing materials into scaffolds can help address this challenge. This paper provides an overview of the recent developments and technological advances in engineering oxygen-releasing biomaterials to improve the viability and function of cells and prevent hypoxic tissue death. Recent advances in different types of oxygen-releasing materials, mechanisms of oxygen generation, and their applications are discussed.

/pdf/breathing-life-into-engineered-tissues-using-oxygen-3n3ns38l9a.pdf

Breathing life into engineered tissues using oxygen-releasing biomaterials

It is difficult to fabricate tubular-shaped tissues and organs (e.g., trachea, blood vessel, and esophagus tissue) with traditional biofabrication techniques (e.g., electrospinning, cell-sheet engineering, and mold-casting) because these have complicated multiple processes. In addition, the tubular-shaped tissues and organs have their own design with target-specific mechanical and biological properties. Therefore, the customized geometrical and physiological environment is required as one of the most critical factors for functional tissue regeneration. 3D bioprinting technology has been receiving attention for the fabrication of patient-tailored and complex-shaped free-form architecture with high reproducibility and versatility. Printable biocomposite inks that can facilitate to build tissue constructs with polymeric frameworks and biochemical microenvironmental cues are also being actively developed for the reconstruction of functional tissue. In this review, we delineated the state-of-the-art of 3D bioprinting techniques specifically for tubular tissue and organ regeneration. In addition, this review described biocomposite inks, such as natural and synthetic polymers. Several described engineering approaches using 3D bioprinting techniques and biocomposite inks may offer beneficial characteristics for the physiological mimicry of human tubular tissues and organs.

3D Bioprinting Strategies for the Regeneration of Functional Tubular Tissues and Organs.

Tracheal resection has limited applicability. Although various tracheal replacement strategies were performed using artificial prosthesis, synthetic stents and tissue transplantation, the best method in tracheal reconstruction remains to be identified. Recent advances in tissue engineering enabled 3D bioprinting using various biocompatible materials including living cells, thereby making the product clinically applicable. Moreover, clinical interest in mesenchymal stem cell has dramatically increased. Here, rabbit bone marrow-derived mesenchymal stem cells (bMSC) and rabbit respiratory epithelial cells were cultured. The chondrogenic differentiation level of bMSC cultured in regular media (MSC) and that in chondrogenic media (d-MSC) were compared. Dual cell-containing artificial trachea were manufactured using a 3D bioprinting method with epithelial cells and undifferentiated bMSC (MSC group, n = 6) or with epithelial cells and chondrogenic-differentiated bMSC (d-MSC group, n = 6). d-MSC showed a relatively higher level of glycosaminoglycan (GAG) accumulation and chondrogenic marker gene expression than MSC in vitro. Neo-epithelialization and neo-vascularization were observed in all groups in vivo but neo-cartilage formation was only noted in d-MSC. The epithelial cells in the 3D bioprinted artificial trachea were effective in respiratory epithelium regeneration. Chondrogenic-differentiated bMSC had more neo-cartilage formation potential in a short period. Nevertheless, the cartilage formation was observed only in a localized area.

3D Bioprinted Artificial Trachea with Epithelial Cells and Chondrogenic-Differentiated Bone Marrow-Derived Mesenchymal Stem Cells

Various treatment methods for tracheal defects have been attempted, such as artificial implants, allografts, autogenous grafts, and tissue engineering; however, no perfect method has been established. We attempted to create an effective artificial trachea via a tissue engineering method using 3D bio-printing. A multi-layered scaffold was fabricated using a 3D printer. Polycaprolactone (PCL) and hydrogel were used with nasal epithelial and auricular cartilage cells in the printing process. An artificial trachea was transplanted into 15 rabbits and a PCL scaffold without the addition of cells was transplanted into 6 rabbits (controls). All animals were followed up with radiography, CT, and endoscopy at 3, 6, and 12 months. In the control group, 3 out of 6 rabbits died from respiratory symptoms. Surviving rabbits in control group had narrowed tracheas due to the formation of granulation tissue and absence of epithelium regeneration. In the experimental group, 13 of 15 animals survived, and the histologic examination confirmed the regeneration of epithelial cells. Neonatal cartilage was also confirmed at 6 and 12 months. Our artificial trachea was effective in the regeneration of respiratory epithelium, but not in cartilage regeneration. Additional studies are needed to promote cartilage regeneration and improve implant stability.

/pdf/experimental-tracheal-replacement-using-3-dimensional-52vue2akar.pdf

Kang-Woog Lee

Papers

3D Bioprinted Artificial Trachea with Epithelial Cells and Chondrogenic-Differentiated Bone Marrow-Derived Mesenchymal Stem Cells

Experimental Tracheal Replacement Using 3-dimensional Bioprinted Artificial Trachea with Autologous Epithelial Cells and Chondrocytes.