Stem Cells and Extrusion 3D Printing for Hyaline Cartilage Engineering

Nanomaterials-Based Hybrid Bioink Platforms in Advancing 3D Bioprinting Technologies for Regenerative Medicine

3D bioprinting is recognized as the ultimate additive biomanufacturing technology in tissue engineering and regeneration, augmented with intelligent bioinks and bioprinters to construct tissues or organs, thereby eliminating the stipulation for artificial organs. For 3D bioprinting of soft tissues, such as kidneys, hearts, and other human body parts, formulations of bioink with enhanced bioinspired rheological and mechanical properties were essential. Nanomaterials-based hybrid bioinks have the potential to overcome the above-mentioned problem and require much attention among researchers. Natural and synthetic nanomaterials such as carbon nanotubes, graphene oxides, titanium oxides, nanosilicates, nanoclay, nanocellulose, etc. and their blended have been used in various 3D bioprinters as bioinks and benefitted enhanced bioprintability, biocompatibility, and biodegradability. A limited number of articles were published, and the above-mentioned requirement pushed us to write this review. We reviewed, explored, and discussed the nanomaterials and nanocomposite-based hybrid bioinks for the 3D bioprinting technology, 3D bioprinters properties, natural, synthetic, and nanomaterial-based hybrid bioinks, including applications with challenges, limitations, ethical considerations, potential solution for future perspective, and technological advancement of efficient and cost-effective 3D bioprinting methods in tissue regeneration and healthcare.

Increasing Collagen to Bioink Drives Mesenchymal Stromal Cells-Chondrogenesis from Hyaline to Calcified Layers

The bioextrusion of mesenchymal stromal cells (MSCs) directly seeded in a bioink enables the production of three-dimensional (3D) constructs, promoting their chondrogenic differentiation. Our study aimed to evaluate the effect of different type I collagen concentrations in the bioink on MSCs' chondrogenic differentiation. We printed 3D constructs using an alginate, gelatin, and fibrinogen-based bioink cellularized with MSCs, with four different quantities of type I collagen addition (0.0, 0.5, 1.0, and 5.0 mg per bioink syringe). We assessed the influence of the bioprinting process, the bioink composition, and the growth factor (TGF-ꞵ1) on the MSCs' survival rate. We confirmed the biocompatibility of the process and the bioinks' cytocompatibility. We evaluated the chondrogenic effects of TGF-ꞵ1 and collagen addition on the MSCs' chondrogenic properties through macroscopic observation, shrinking ratio, reverse transcription polymerase chain reaction, glycosaminoglycan synthesis, histology, and type II collagen immunohistochemistry. The bioink containing 0.5 mg of collagen produces the richest hyaline-like extracellular matrix, presenting itself as a promising tool to recreate the superficial layer of hyaline cartilage. The bioink containing 5.0 mg of collagen enhances the synthesis of a calcified matrix, making it a good candidate for mimicking the calcified cartilaginous layer. Type I collagen thus allows the dose-dependent design of specific hyaline cartilage layers.

Advanced 3D imaging and organoid bioprinting for biomedical research and therapeutic applications

Organoid cultures offer a valuable platform for studying organ-level biology, allowing for a closer mimicry of human physiology compared to traditional two-dimensional cell culture systems or non-primate animal models. While many organoid cultures use cell aggregates or decellularized extracellular matrices as scaffolds, they often lack precise biochemical and biophysical microenvironments. In contrast, three-dimensional (3D) bioprinting allows precise placement of organoids or spheroids, providing enhanced spatial control and facilitating the direct fusion for the formation of large-scale functional tissues in vitro. In addition, 3D bioprinting enables fine tuning of biochemical and biophysical cues to support organoid development and maturation. With advances in the organoid technology and its potential applications across diverse research fields such as cell biology, developmental biology, disease pathology, precision medicine, drug toxicology, and tissue engineering, organoid imaging has become a crucial aspect of physiological and pathological studies. This review highlights the recent advancements in imaging technologies that have significantly contributed to organoid research. Additionally, we discuss various bioprinting techniques, emphasizing their applications in organoid bioprinting. Integrating 3D imaging tools into a bioprinting platform allows real-time visualization while facilitating quality control, optimization, and comprehensive bioprinting assessment. Similarly, combining imaging technologies with organoid bioprinting can provide valuable insights into tissue formation, maturation, functions, and therapeutic responses. This approach not only improves the reproducibility of physiologically relevant tissues but also enhances understanding of complex biological processes. Thus, careful selection of bioprinting modalities, coupled with appropriate imaging techniques, holds the potential to create a versatile platform capable of addressing existing challenges and harnessing opportunities in these rapidly evolving fields.

Stem-Cell-Driven Chondrogenesis: Perspectives on Amnion-Derived Cells

Regenerative medicine harnesses stem cells' capacity to restore damaged tissues and organs. In vitro methods employing specific bioactive molecules, such as growth factors, bio-inductive scaffolds, 3D cultures, co-cultures, and mechanical stimuli, steer stem cells toward the desired differentiation pathways, mimicking their natural development. Chondrogenesis presents a challenge for regenerative medicine. This intricate process involves precise modulation of chondro-related transcription factors and pathways, critical for generating cartilage. Cartilage damage disrupts this process, impeding proper tissue healing due to its unique mechanical and anatomical characteristics. Consequently, the resultant tissue often forms fibrocartilage, which lacks adequate mechanical properties, posing a significant hurdle for effective regeneration. This review comprehensively explores studies showcasing the potential of amniotic mesenchymal stem cells (AMSCs) and amniotic epithelial cells (AECs) in chondrogenic differentiation. These cells exhibit innate characteristics that position them as promising candidates for regenerative medicine. Their capacity to differentiate toward chondrocytes offers a pathway for developing effective regenerative protocols. Understanding and leveraging the innate properties of AMSCs and AECs hold promise in addressing the challenges associated with cartilage repair, potentially offering superior outcomes in tissue regeneration.

3D printed arrowroot starch-gellan scaffolds for wound healing applications

Skin, the largest organ in the body, blocks the entry of environmental pollutants into the system. Any injury to this organ allows infections and other harmful substances into the body. 3D bioprinting, a state-of-the-art technique, is suitable for fabricating cell culture scaffolds to heal chronic wounds rapidly. This study uses starch extracted from Maranta arundinacea (Arrowroot plant) (AS) and gellan gum (GG) to develop a bioink for 3D printing a scaffold capable of hosting animal cells. Field emission scanning electron microscopy (FE-SEM) and X-ray diffraction analysis (XRD) prove that the isolated AS is analogous to commercial starch. The cell culture scaffolds developed are superior to the existing monolayer culture. Infrared microscopy shows the AS-GG interaction and elucidates the mechanism of hydrogel formation. The physicochemical properties of the 3D-printed scaffold are analyzed to check the cell adhesion and growth; SEM images have confirmed that the AS-GG printed scaffold can support cell growth and proliferation, and the MTT assay shows good cell viability. Cell behavioral and migration studies reveal that cells are healthy. Since the scaffold is biocompatible, it can be 3D printed to any shape and structure and will biodegrade in the requisite time.

Natural based hydrogels promote chondrogenic differentiation of human mesenchymal stem cells

Background: The cartilage tissue lacks blood vessels, which is composed of chondrocytes and ECM. Due to this vessel-less structure, it is difficult to repair cartilage tissue damages. One of the new methods to repair cartilage damage is to use tissue engineering. In the present study, it was attempted to simulate a three-dimensional environment similar to the natural ECM of cartilage tissue by using hydrogels made of natural materials, including Chitosan and different ratios of Alginate. Material and methods: Chitosan, alginate and Chitosan/Alginate hydrogels were fabricated. Fourier Transform Infrared, XRD, swelling ratio, porosity measurement and degradation tests were applied to scaffolds characterization. After that, human adipose derived-mesenchymal stem cells (hADMSCs) were cultured on the hydrogels and then their viability and chondrogenic differentiation capacity were studied. Safranin O and Alcian blue staining, immunofluorescence staining and real time RT-PCR were used as analytical methods for chondrogenic differentiation potential evaluation of hADMSCs when cultured on the hydrogels. Results: The highest degradation rate was detected in Chitosan/Alginate (1:0.5) group The scaffold biocompatibility results revealed that the viability of the cells cultured on the hydrogels groups was not significantly different with the cells cultured in the control group. Safranin O staining, Alcian blue staining, immunofluorescence staining and real time PCR results revealed that the chondrogenic differentiation potential of the hADMSCs when grown on the Chitosan/Alginate hydrogel (1:0.5) was significantly higher than those cell grown on the other groups. Conclusion: Taken together, these results suggest that Chitosan/Alginate hydrogel (1:0.5) could be a promising candidate for cartilage tissue engineering applications.

Human Chondrocytes, Metabolism of Articular Cartilage, and Strategies for Application to Tissue Engineering

Hyaline cartilage, which is characterized by the absence of vascularization and innervation, has minimal self-repair potential in case of damage and defect formation in the chondral layer. Chondrocytes are specialized cells that ensure the synthesis of extracellular matrix components, namely type II collagen and aggregen. On their surface, they express integrins CD44, α1β1, α3β1, α5β1, α10β1, αVβ1, αVβ3, and αVβ5, which are also collagen-binding components of the extracellular matrix. This article aims to contribute to solving the problem of the possible repair of chondral defects through unique methods of tissue engineering, as well as the process of pathological events in articular cartilage. In vitro cell culture models used for hyaline cartilage repair could bring about advanced possibilities. Currently, there are several variants of the combination of natural and synthetic polymers and chondrocytes. In a three-dimensional environment, chondrocytes retain their production capacity. In the case of mesenchymal stromal cells, their favorable ability is to differentiate into a chondrogenic lineage in a three-dimensional culture.

Recent advances in 3D bioprinted cartilage-mimicking constructs for applications in tissue engineering

Human cartilage tissue can be categorized into three types: hyaline cartilage, elastic cartilage and fibrocartilage. Each type of cartilage tissue possesses unique properties and functions, which presents a significant challenge for the regeneration and repair of damaged tissue. Bionics is a discipline in which humans study and imitate nature. A bionic strategy based on comprehensive knowledge of the anatomy and histology of human cartilage is expected to contribute to fundamental study of core elements of tissue repair. Moreover, as a novel tissue-engineered technology, 3D bioprinting has the distinctive advantage of the rapid and precise construction of targeted models. Thus, by selecting suitable materials, cells and cytokines, and by leveraging advanced printing technology and bionic concepts, it becomes possible to simultaneously realize multiple beneficial properties and achieve improved tissue repair. This article provides an overview of key elements involved in the combination of 3D bioprinting and bionic strategies, with a particular focus on recent advances in mimicking different types of cartilage tissue.

Bioprinting-Enabled Biomaterials: A Cutting-Edge Strategy for Future Osteoarthritis Therapy

Bioprinting is an advanced technology that allows for the precise placement of cells and biomaterials in a controlled manner, making significant contributions in regenerative medicine. Notably, bioprinting-enabled biomaterials have found extensive application as drug delivery systems (DDS) in the treatment of osteoarthritis (OA). Despite the widespread utilization of these biomaterials, there has been limited comprehensive research summarizing the recent advances in this area. Therefore, this review aims to explore the noteworthy developments and challenges associated with utilizing bioprinting-enabled biomaterials as effective DDS for the treatment of OA. To begin, we provide an overview of the complex pathophysiology of OA, highlighting the shortcomings of current treatment modalities. Following this, we conduct a detailed examination of various bioprinting technologies and discuss the wide range of biomaterials employed in DDS applications for OA therapy. Finally, by placing emphasis on their transformative potential, we discuss the incorporation of crucial cellular components such as chondrocytes and mesenchymal stem cells into bioprinted constructs, which play a pivotal role in promoting tissue regeneration and repair.

Intelligent Vascularized 3D/4D/5D/6D-Printed Tissue Scaffolds

Blood vessels are essential for nutrient and oxygen delivery and waste removal. Scaffold-repairing materials with functional vascular networks are widely used in bone tissue engineering. Additive manufacturing is a manufacturing technology that creates three-dimensional solids by stacking substances layer by layer, mainly including but not limited to 3D printing, but also 4D printing, 5D printing and 6D printing. It can be effectively combined with vascularization to meet the needs of vascularized tissue scaffolds by precisely tuning the mechanical structure and biological properties of smart vascular scaffolds. Herein, the development of neovascularization to vascularization to bone tissue engineering is systematically discussed in terms of the importance of vascularization to the tissue. Additionally, the research progress and future prospects of vascularized 3D printed scaffold materials are highlighted and presented in four categories: functional vascularized 3D printed scaffolds, cell-based vascularized 3D printed scaffolds, vascularized 3D printed scaffolds loaded with specific carriers and bionic vascularized 3D printed scaffolds. Finally, a brief review of vascularized additive manufacturing-tissue scaffolds in related tissues such as the vascular tissue engineering, cardiovascular system, skeletal muscle, soft tissue and a discussion of the challenges and development efforts leading to significant advances in intelligent vascularized tissue regeneration is presented.

Construction of 3D-Bioprinted cartilage-mimicking substitute based on photo-crosslinkable Wharton's jelly bioinks for full-thickness articular cartilage defect repair

Three-dimensional (3D) bioprinted cartilage-mimicking substitutes for full-thickness articular cartilage defect repair have emerged as alternatives to in situ defect repair models. However, there has been very limited breakthrough in cartilage regeneration based on 3D bioprinting owing to the lack of ideal bioinks with printability, biocompatibility, bioactivity, and suitable physicochemical properties. In contrast to animal-derived natural polymers or acellular matrices, human-derived Wharton's jelly is biocompatible and hypoimmunogenic with an abundant source. Although acellular Wharton's jelly can mimic the chondrogenic microenvironment, it remains challenging to prepare both printable and biologically active bioinks from this material. Here, we firstly prepared methacryloyl-modified acellular Wharton's jelly (AWJMA) using a previously established photo-crosslinking strategy. Subsequently, we combined methacryloyl-modified gelatin with AWJMA to obtain a hybrid hydrogel that exhibited both physicochemical properties and biological activities that were suitable for 3D bioprinting. Moreover, bone marrow mesenchymal stem cell-loaded 3D-bioprinted cartilage-mimicking substitutes had superior advantages for the survival, proliferation, spreading, and chondrogenic differentiation of bone marrow mesenchymal stem cells, which enabled satisfactory repair of a model of full-thickness articular cartilage defect in the rabbit knee joint. The current study provides a novel strategy based on 3D bioprinting of cartilage-mimicking substitutes for full-thickness articular cartilage defect repair.

Biofunctionalization of 3D printed collagen with bevacizumab-loaded microparticles targeting pathological angiogenesis

Pathological angiogenesis is a crucial attribute of several chronic diseases such as cancer, age-related macular degeneration, and osteoarthritis (OA). In the case of OA, pathological angiogenesis mediated by the vascular endothelial growth factor (VEGF), among other factors, contributes to cartilage degeneration and to implants rejection. In line with this, the use of the anti-VEGF bevacizumab (BVZ) has been shown to prevent OA progression and support cartilage regeneration. The aim of this work was to functionalize a medical grade collagen with poly (lactic-co-glycolic acid) (PLGA) microparticles containing BVZ via three-dimensional (3D) printing to target pathological angiogenesis. First, the effect of several formulation parameters on the encapsulation and release of BVZ from PLGA microparticles was studied. Then, the anti-angiogenic activity of released BVZ was tested in a 3D cell model. The 3D printability of the microparticle-loaded collagen ink was tested by evaluating the shape fidelity of 3D printed structures. Results showed that the release and the encapsulation efficiency of BVZ could be tuned as a function of several formulation parameters. In addition, the released BVZ was observed to reduce vascularization by human umbilical vein endothelial cells. Finally, the collagen ink with embedded BVZ microparticles was successfully printed, leading to shape-stable meniscus-, nose- and auricle-like structures. Taken altogether, we defined the conditions for the successful combination of BVZ-loaded microparticles with the 3D printing of a medical grade collagen to target pathological angiogenesis.

Hierarchical porous ECM scaffolds incorporating GDF-5 fabricated by cryogenic 3D printing to promote articular cartilage regeneration

BACKGROUND

In recent years, there has been significant research progress on in situ articular cartilage (AC) tissue engineering with endogenous stem cells, which uses biological materials or bioactive factors to improve the regeneration microenvironment and recruit more endogenous stem cells from the joint cavity to the defect area to promote cartilage regeneration.

METHOD

In this study, we used ECM alone as a bioink in low-temperature deposition manufacturing (LDM) 3D printing and then successfully fabricated a hierarchical porous ECM scaffold incorporating GDF-5.

RESULTS

Comparative in vitro experiments showed that the 7% ECM scaffolds had the best biocompatibility. After the addition of GDF-5 protein, the ECM scaffolds significantly improved bone marrow mesenchymal stem cell (BMSC) migration and chondrogenic differentiation. Most importantly, the in vivo results showed that the ECM/GDF-5 scaffold significantly enhanced in situ cartilage repair.

CONCLUSION

In conclusion, this study reports the construction of a new scaffold based on the concept of in situ regeneration, and we believe that our findings will provide a new treatment strategy for AC defect repair.

Lithium-Containing Biomaterials Stimulate Cartilage Repair through Bone Marrow Stromal Cells-Derived Exosomal miR-455-3p and Histone H3 Acetylation

The repair of damaged cartilage still remains a great challenge in clinic. It is demonstrated that bone marrow stromal cells (BMSCs)-chondrocytes communication is of great significance for cartilage repair. Moreover, BMSCs have been confirmed to enhance biological function of chondrocytes via exosome-mediated paracrine pathway. Lithium-containing scaffolds have been reported to effectively promote cartilage regeneration; however, whether lithium-containing biomaterial could facilitate cartilage regeneration through regulating BMSCs-derived exosomes has not been illustrated. In the study, the model lithium-substituted bioglass ceramic (Li-BGC) is selected and regulatory effects of BMSCs-derived exosomes after Li-BGC treatment (Li-BGC-Exo) are systemically evaluated. The data reveal that Li-BGC-Exo notably promotes chondrogenesis, which attributes to the upregulated exosomal miR-455-3p transfer, consequently leads to suppression of histone deacetylase 2 (HDAC2) and enhanced histone H3 acetylation in chondrocytes. Notably, BMSCs-derived exosomes after LiCl treatment (LiCl-Exo) exhibits the similar regulatory effect with Li-BGC-Exo, indicating that the pro-chondrogenesis capability of them is mainly owing to the lithium ions. Furthermore, the in vivo study proves that LiCl-Exo remarkably facilitates cartilage regeneration. The research may provide novel possibility for the intrinsic mechanism of chondrogenesis trigged by lithium-containing biomaterials, and suggests that application of lithium-containing scaffolds may be a promising strategy for cartilage regeneration.

3D printing of bone and cartilage with polymer materials

Damage and degeneration to bone and articular cartilage are the leading causes of musculoskeletal disability. Commonly used clinical and surgical methods include autologous/allogeneic bone and cartilage transplantation, vascularized bone transplantation, autologous chondrocyte implantation, mosaicplasty, and joint replacement. 3D bio printing technology to construct implants by layer-by-layer printing of biological materials, living cells, and other biologically active substances in vitro, which is expected to replace the repair mentioned above methods. Researchers use cells and biomedical materials as discrete materials. 3D bio printing has largely solved the problem of insufficient organ donors with the ability to prepare different organs and tissue structures. This paper mainly discusses the application of polymer materials, bio printing cell selection, and its application in bone and cartilage repair.

Ex Vivo Maturation of 3D-Printed, Chondrocyte-Laden, Polycaprolactone-Based Scaffolds Prior to Transplantation Improves Engineered Cartilage Substitute Properties and Integration

OBJECTIVE

The surgical management of nasal septal defects due to perforations, malformations, congenital cartilage absence, traumatic defects, or tumors would benefit from availability of optimally matured septal cartilage substitutes. Here, we aimed to improve in vitro maturation of 3-dimensional (3D)-printed, cell-laden polycaprolactone (PCL)-based scaffolds and test their in vivo performance in a rabbit auricular cartilage model.

DESIGN

Rabbit auricular chondrocytes were isolated, cultured, and seeded on 3D-printed PCL scaffolds. The scaffolds were cultured for 21 days in vitro under standard culture media and normoxia or in prochondrogenic and hypoxia conditions, respectively. Cell-laden scaffolds (as well as acellular controls) were implanted into perichondrium pockets of New Zealand white rabbit ears (N = 5 per group) and followed up for 12 weeks. At study end point, the tissue-engineered scaffolds were extracted and tested by histological, immunohistochemical, mechanical, and biochemical assays.

RESULTS

Scaffolds previously matured in vitro under prochondrogenic hypoxic conditions showed superior mechanical properties as well as improved patterns of cartilage matrix deposition, chondrogenic gene expression (COL1A1, COL2A1, ACAN, SOX9, COL10A1), and proteoglycan production in vivo, compared with scaffolds cultured in standard conditions.

CONCLUSIONS

In vitro maturation of engineered cartilage scaffolds under prochondrogenic conditions that better mimic the in vivo environment may be beneficial to improve functional properties of the engineered grafts. The proposed maturation strategy may also be of use for other tissue-engineered constructs and may ultimately impact survival and integration of the grafts in the damaged tissue microenvironment.

Tailoring bioinks of extrusion-based bioprinting for cutaneous wound healing

Extrusion-based bioprinting (EBB) holds potential for regenerative medicine. However, the widely-used bioinks of EBB exhibit some limitations for skin regeneration, such as unsatisfactory bio-physical (i.e., mechanical, structural, biodegradable) properties and compromised cellular compatibilities, and the EBB-based bioinks with therapeutic effects targeting cutaneous wounds still remain largely undiscussed. In this review, the printability considerations for skin bioprinting were discussed. Then, current strategies for improving the physical properties of bioinks and for reinforcing bioinks in EBB approaches were introduced, respectively. Notably, we highlighted the applications and effects of current EBB-based bioinks on wound healing, wound scar formation, vascularization and the regeneration of skin appendages (i.e., sweat glands and hair follicles) and discussed the challenges and future perspectives. This review aims to provide an overall view of the applications, challenges and promising solutions about the EBB-based bioinks for cutaneous wound healing and skin regeneration.

Nonmulberry silk fibroin-based biomaterials: Impact on cell behavior regulation and tissue regeneration

Silk fibroin (SF) is a promising biomaterial due to its good biocompatibility, easy availability, and high mechanical properties. Compared with mulberry silk fibroin (MSF), nonmulberry silk fibroin (NSF) isolated from typical nonmulberry silkworm silk exhibits unique arginine-glycine-aspartic acid (RGD) sequences with favorable cell adhesion enhancing effect. This inherent property probably makes the NSF more suitable for cell culture and tissue regeneration-related applications. Accordingly, various types of NSF-based biomaterials, such as particles, films, fiber mats, and 3D scaffolds, are constructed and their application potential in different biomedical fields is extensively investigated. Based on these promising NSF biomaterials, this review firstly makes a systematical comparison between the molecular structure and properties of MSF and typical NSF and highlights the unique properties of NSF. In addition, we summarize the effective fabrication strategies from degummed nonmulberry silk fibers to regenerated NSF-based biomaterials with controllable formats and their recent application progresses in cell behavior regulation and tissue regeneration. Finally, current challenges and future perspectives for the fabrication and application of NSF-based biomaterials are discussed. Related research and perspectives may provide valuable references for designing and modifying effective NSF-based and other natural biomaterials. STATEMENT OF SIGNIFICANCE: There exist many reviews about mulberry silk fibroin (MSF) biomaterials and their biomedical applications, while that about nonmulberry silk fibroin (NSF) biomaterials is scarce. Compared with MSF, NSF exhibits unique arginine-glycine-aspartic acid sequences with promising cell adhesion enhancing effect, which makes NSF more suitable for cell culture and tissue regeneration related applications. Focusing on these advanced NSF biomaterials, this review has systematically compared the structure and properties of MSF and NSF, and emphasized the unique properties of NSF. Following that, the effective construction strategies for NSF-based biomaterials are summarized, and their recent applications in cell behavior regulations and tissue regenerations are highlighted. Furthermore, current challenges and future perspectives for the fabrication and application of NSF-based biomaterials were discussed.

Three-Dimensional Bioprinting for Cartilage Tissue Engineering: Insights into Naturally-Derived Bioinks from Land and Marine Sources

In regenerative medicine and tissue engineering, the possibility to: (I) customize the shape and size of scaffolds, (II) develop highly mimicked tissues with a precise digital control, (III) manufacture complex structures and (IV) reduce the wastes related to the production process, are the main advantages of additive manufacturing technologies such as three-dimensional (3D) bioprinting. Specifically, this technique, which uses suitable hydrogel-based bioinks, enriched with cells and/or growth factors, has received significant consideration, especially in cartilage tissue engineering (CTE). In this field of interest, it may allow mimicking the complex native zonal hyaline cartilage organization by further enhancing its biological cues. However, there are still some limitations that need to be overcome before 3D bioprinting may be globally used for scaffolds' development and their clinical translation. One of them is represented by the poor availability of appropriate, biocompatible and eco-friendly biomaterials, which should present a series of specific requirements to be used and transformed into a proper bioink for CTE. In this scenario, considering that, nowadays, the environmental decline is of the highest concerns worldwide, exploring naturally-derived hydrogels has attracted outstanding attention throughout the scientific community. For this reason, a comprehensive review of the naturally-derived hydrogels, commonly employed as bioinks in CTE, was carried out. In particular, the current state of art regarding eco-friendly and natural bioinks' development for CTE was explored. Overall, this paper gives an overview of 3D bioprinting for CTE to guide future research towards the development of more reliable, customized, eco-friendly and innovative strategies for CTE.

Silk fibroin-based biomaterials for cartilage/osteochondral repair

Osteoarthritis (OA) is a common joint disease with a high disability rate. In addition, OA not only causes great physiological and psychological harm to patients, but also puts great pressure on the social healthcare system. Pathologically, the disintegration of cartilage and the lesions of subchondral bone are related to OA. Currently, tissue engineering, which is expected to overcome the defects of existing treatment methods, had a lot of research in the field of cartilage/osteochondral repair. Silk fibroin (SF), as a natural macromolecular material with good biocompatibility, unique mechanical properties, excellent processability and degradability, holds great potential in the field of tissue engineering. Nowadays, SF had been prepared into various materials to adapt to the demands of cartilage/osteochondral repair. SF-based biomaterials can also be functionally modified to enhance repair performance further. In this review, the preparation methods, types, structures, mechanical properties, and functional modifications of SF-based biomaterials used for cartilage/osteochondral repair are summarized and discussed. We hope that this review will provide a reference for the design and development of SF-based biomaterials in cartilage/osteochondral repair field.

In Vitro and In Vivo Evaluation of 3D Constructs Engineered with Human iPSC-Derived Chondrocytes in Gelatin-Methacryloyl Hydrogel

Articular cartilage defects have limited healing potential and, when left untreated, can lead to osteoarthritis. Tissue engineering focuses on regenerating the damaged joint surface, preferably in an early stage. Here we investigate the regenerative potential of 3D constructs consisting of human iPSC-derived chondrocytes in gelatin-methacryloyl (GelMA) hydrogel for stable hyaline cartilage production. iPSC-derived chondrocytes are encapsulated in GelMA hydrogel at low (1x10⁷ mL^-1 ) and high (2x10⁷ mL^-1 ) density. In conventional medium, GelMA hydrogel supports the chondrocyte phenotype, as opposed to cells cultured in 3D in absence of hydrogel. Moreover, encapsulated iPSC-derived chondrocytes preserve their in vivo matrix formation capacity after 21 days in vitro. In differentiation medium, hyaline cartilage-like tissue forms after 21 days, demonstrated by highly sulfated glycosaminoglycans and collagen type II. Matrix deposition is delayed at low encapsulation density, corroborating with lower transcript levels of COL2A1. An ectopic assay in nude mice demonstrates further maturation of the matrix deposited in vitro. Direct ectopic implantation of iPSC-derived chondrocyte-laden GelMA, without in vitro priming, also generates hyaline cartilage-like tissue, albeit less mature. Since it is unclear what maturity upon implantation is desired for joint surface regeneration, this is an attractive technology to generate immature and more mature hyaline cartilage-like tissue. This article is protected by copyright. All rights reserved.

Creating an Optimal In Vivo Environment to Enhance Outcomes Using Cell Therapy to Repair/Regenerate Injured Tissues of the Musculoskeletal System

Following most injuries to a musculoskeletal tissue which function in unique mechanical environments, an inflammatory response occurs to facilitate endogenous repair. This is a process that usually yields functionally inferior scar tissue. In the case of such injuries occurring in adults, the injury environment no longer expresses the anabolic processes that contributed to growth and maturation. An injury can also contribute to the development of a degenerative process, such as osteoarthritis. Over the past several years, researchers have attempted to use cellular therapies to enhance the repair and regeneration of injured tissues, including Platelet-rich Plasma and mesenchymal stem/medicinal signaling cells (MSC) from a variety of tissue sources, either as free MSC or incorporated into tissue engineered constructs, to facilitate regeneration of such damaged tissues. The use of free MSC can sometimes affect pain symptoms associated with conditions such as OA, but regeneration of damaged tissues has been challenging, particularly as some of these tissues have very complex structures. Therefore, implanting MSC or engineered constructs into an inflammatory environment in an adult may compromise the potential of the cells to facilitate regeneration, and neutralizing the inflammatory environment and enhancing the anabolic environment may be required for MSC-based interventions to fulfill their potential. Thus, success may depend on first eliminating negative influences (e.g., inflammation) in an environment, and secondly, implanting optimally cultured MSC or tissue engineered constructs into an anabolic environment to achieve the best outcomes. Furthermore, such interventions should be considered early rather than later on in a disease process, at a time when sufficient endogenous cells remain to serve as a template for repair and regeneration. This review discusses how the interface between inflammation and cell-based regeneration of damaged tissues may be at odds, and outlines approaches to improve outcomes. In addition, other variables that could contribute to the success of cell therapies are discussed. Thus, there may be a need to adopt a Precision Medicine approach to optimize tissue repair and regeneration following injury to these important tissues.

Fundamental and Applied Advances in Stem Cell Therapeutic Research

We are pleased to present this Special Issue of Cells, entitled 'Feature Papers in Stem Cells' [...].

Research progress in seed cells for cartilage tissue engineering

Cartilage defects trouble millions of patients worldwide and their repair via conventional treatment is difficult. Excitingly, tissue engineering technology provides a promising strategy for efficient cartilage regeneration with structural regeneration and functional reconstruction. Seed cells, as biological prerequisites for cartilage regeneration, determine the quality of regenerated cartilage. The proliferation, differentiation and chondrogenesis of seed cells are greatly affected by their type, origin and generation. Thus, a systematic description of the characteristics of seed cells is necessary. This article reviews in detail the cellular characteristics, research progress, clinical translation challenges and future research directions of seed cells while providing guidelines for selecting appropriate seed cells for cartilage regeneration.

A Review on Antibacterial Biomaterials in Biomedical Applications: From Materials Perspective to Bioinks Design

In tissue engineering, three-dimensional (3D) printing is an emerging approach to producing functioning tissue constructs to repair wounds and repair or replace sick tissue/organs. It allows for precise control of materials and other components in the tissue constructs in an automated way, potentially permitting great throughput production. An ink made using one or multiple biomaterials can be 3D printed into tissue constructs by the printing process; though promising in tissue engineering, the printed constructs have also been reported to have the ability to lead to the emergence of unforeseen illnesses and failure due to biomaterial-related infections. Numerous approaches and/or strategies have been developed to combat biomaterial-related infections, and among them, natural biomaterials, surface treatment of biomaterials, and incorporating inorganic agents have been widely employed for the construct fabrication by 3D printing. Despite various attempts to synthesize and/or optimize the inks for 3D printing, the incidence of infection in the implanted tissue constructs remains one of the most significant issues. For the first time, here we present an overview of inks with antibacterial properties for 3D printing, focusing on the principles and strategies to accomplish biomaterials with anti-infective properties, and the synthesis of metallic ion-containing ink, chitosan-containing inks, and other antibacterial inks. Related discussions regarding the mechanics of biofilm formation and antibacterial performance are also presented, along with future perspectives of the importance of developing printable inks.

Chondrogenic Differentiation of Human Mesenchymal Stem Cells and Macrophage Polarization on 3D-Printed Poly(ε-caprolactone)/Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Blended Scaffolds with Different Secondary Porous Structures

This study was aimed to evaluate the chondrogenic differentiation of human mesenchymal stem cells (hMSCs) and polarization of THP-1-derived macrophages cultured on poly(ε-caprolactone) (PC)/poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PH) blended scaffolds with dual primary (PP) and secondary (SP) pores, which were fabricated via a 3D printing technique, i.e., fused deposition modeling, followed by a salt-leaching process at 50 °C for varied times, i.e., 15, 30, and 60 min. Sodium chloride (SC), a porogen, was initially incorporated in the blend at varied weight percentages, i.e., 0, 25, and 50%, whereas 1 M NaOH solution and deionized water were used as salt-leaching agents. To elucidate the surface properties of the developed scaffolds, directly governed by the amount of the salt originally mixed and the salt-leaching efficiency, several characterization techniques, e.g., scanning electron microscopy, X-ray microcomputed tomography, mercury intrusion porosimetry, atomic force microscopy, and contact angle measurement, were used. Meanwhile, the salt-leaching efficiency was determined by means of weight loss measurement and thermogravimetric analysis. It was found that the alkaline solution could satisfactorily leach out the salt particles in 60 min with a mild etching of the polymer framework. The most immensely and homogeneously pitted filament surface was observed in the NaOH-treated scaffold initially integrated with 50% salt, i.e., 60B_PC/PH/50SC; the SP structure was mostly open and interconnected. The size of most of micropores was about 0.14 μm. With its suitable microsurface roughness and hydrophilicity, 60B_PC/PH/50SC could properly support the initial attachment and lamellipodia formation of hMSCs, which was favorable for chondrogenesis. Consequently, a significantly increased ratio of glycosaminoglycans/deoxyribonucleic acid and a superior expression of the COL2A1 gene were detected when cells were grown on this material. Although 60B_PC/PH/50SC induced the macrophages to secrete a slightly high level of IL-1β during the first few days of culture, the polarized M1 cells could return to a nearly normal stage at Day7, suggesting no unfavorable chronic inflammation caused by the material.

Cutting-Edge Technologies for Inflamed Joints on Chip: How Close Are We?

Osteoarthritis (OA) is a painful and disabling musculoskeletal disorder, with a large impact on the global population, resulting in several limitations on daily activities. In OA, inflammation is frequent and mainly controlled through inflammatory cytokines released by immune cells. These outbalanced inflammatory cytokines cause cartilage extracellular matrix (ECM) degradation and possible growth of neuronal fibers into subchondral bone triggering pain. Even though pain is the major symptom of musculoskeletal diseases, there are still no effective treatments to counteract it and the mechanisms behind these pathologies are not fully understood. Thus, there is an urgent need to establish reliable models for assessing the molecular mechanisms and consequently new therapeutic targets. Models have been established to support this research field by providing reliable tools to replicate the joint tissue in vitro. Studies firstly started with simple 2D culture setups, followed by 3D culture focusing mainly on cell-cell interactions to mimic healthy and inflamed cartilage. Cellular approaches were improved by scaffold-based strategies to enhance cell-matrix interactions as well as contribute to developing mechanically more stable in vitro models. The progression of the cartilage tissue engineering would then profit from the integration of 3D bioprinting technologies as these provide 3D constructs with versatile structural arrangements of the 3D constructs. The upgrade of the available tools with dynamic conditions was then achieved using bioreactors and fluid systems. Finally, the organ-on-a-chip encloses all the state of the art on cartilage tissue engineering by incorporation of different microenvironments, cells and stimuli and pave the way to potentially simulate crucial biological, chemical, and mechanical features of arthritic joint. In this review, we describe the several available tools ranging from simple cartilage pellets to complex organ-on-a-chip platforms, including 3D tissue-engineered constructs and bioprinting tools. Moreover, we provide a fruitful discussion on the possible upgrades to enhance the in vitro systems making them more robust regarding the physiological and pathological modeling of the joint tissue/OA.

Mapping Thematic Trends and Analysing Hotspots Concerning the Use of Stem Cells for Cartilage Regeneration: A Bibliometric Analysis From 2010 to 2020

Background: Defects of articular cartilage represent a common condition that usually progresses to osteoarthritis with pain and dysfunction of the joint. Current treatment strategies have yielded limited success in these patients. Stem cells are emerging as a promising option for cartilage regeneration. We aim to summarize the developmental history of stem cells for cartilage regeneration and to analyse the relevant trends and hotspots.

Methods: We screened all relevant literature on stem cells for cartilage regeneration from Web of Science during 2010–2020 and analysed the research trends in this field by VOSviewer and CiteSpace. We also summarized previous clinical trials.

Results: We screened 1,011 publications. China contributed the largest number of publications (317, 31.36%) and citations (81,376, 48.61%). The United States achieved the highest H-index (39). Shanghai Jiao Tong University had the largest number of publications (34) among all full-time institutions. The Journal of Biomaterials and Stem Cell Research and Therapy published the largest number of studies on stem cells for cartilage regeneration (35). SEKIYA I and YANG F published the majority of articles in this field (14), while TOH WS was cited most frequently (740). Regarding clinical research on stem cells for cartilage regeneration, the keyword “double-blind” emerged in recent years, with an average year of 2018.75. In tissue engineering, the keyword “3D printing” appeared latest, with an average year of 2019.625. In biological studies, the key word “extracellular vesicles” appeared latest, with an average year of 2018.9091. The current research trend indicates that basic research is gradually transforming to tissue engineering. Clinical trials have confirmed the safety and feasibility of stem cells for cartilage regeneration.

Conclusion: Multiple scientific methods were employed to reveal productivity, collaborations, and research hotspots related to the use of stem cells for cartilage regeneration. 3D printing, extracellular vesicles, and double-blind clinical trials are research hotspots and are likely to be promising in the near future. Further studies are needed for to improve our understanding of this field, and clinical trials with larger sample sizes and longer follow-up periods are needed for clinical transformation.

An antibacterial ε-poly-L-lysine-derived bioink for 3D bioprinting applications

Limited bioinks have hindered applying 3D bioprinting to tissue engineering, and bacterial infection is a serious threat to these applications. Aiming to solve this problem, a novel ε-poly-L-lysine (EPL) derived...

Bioprinting of Complex Multicellular Organs with Advanced Functionality-Recent Progress and Challenges Ahead

Bioprinting has emerged as one of the most promising strategies for fabrication of functional organs in the lab as an alternative to transplant organs. While progress in the field has mostly been restricted to a few miniaturized tissues with minimal biological functionality until a few years ago, recent progress has advanced the concept of building three-dimensional multicellular organ complexity remarkably. This review discusses a series of milestones that have paved the way for bioprinting of tissue constructs that have advanced levels of biological and architectural functionality. Critical materials, engineering and biological challenges that are key to addressing the desirable function of engineered organs are presented. These are discussed in light of the many difficulties to replicate the heterotypic organization of multicellular solid organs, the nanoscale precision of the extracellular microenvironment in hierarchical tissues, as well as the advantages and limitations of existing bioprinting methods to adequately overcome these barriers. In summary, the advances of the field toward realistic manufacturing of functional organs have never been so extensive, and this manuscript serves as a road map for some of the recent progress and the challenges ahead.

Methods of Modification of Mesenchymal Stem Cells and Conditions of Their Culturing for Hyaline Cartilage Tissue Engineering

The use of mesenchymal stromal cells (MSCs) for tissue engineering of hyaline cartilage is a topical area of regenerative medicine that has already entered clinical practice. The key stage of this procedure is to create conditions for chondrogenic differentiation of MSCs, increase the synthesis of hyaline cartilage extracellular matrix proteins by these cells and activate their proliferation. The first such works consisted in the indirect modification of cells, namely, in changing the conditions in which they are located, including microfracturing of the subchondral bone and the use of 3D biodegradable scaffolds. The most effective methods for modifying the cell culture of MSCs are protein and physical, which have already been partially introduced into clinical practice. Genetic methods for modifying MSCs, despite their effectiveness, have significant limitations. Techniques have not yet been developed that allow studying the effectiveness of their application even in limited groups of patients. The use of MSC modification methods allows precise regulation of cell culture proliferation, and in combination with the use of a 3D biodegradable scaffold, it allows obtaining a hyaline-like regenerate in the damaged area. This review is devoted to the consideration and comparison of various methods used to modify the cell culture of MSCs for their use in regenerative medicine of cartilage tissue.

Synergies of Human Umbilical Vein Endothelial Cell-Laden Calcium Silicate-Activated Gelatin Methacrylate for Accelerating 3D Human Dental Pulp Stem Cell Differentiation for Endodontic Regeneration

According to the Centers for Disease Control and Prevention, tooth caries is a common problem affecting 9 out of every 10 adults worldwide. Dentin regeneration has since become one of the pressing issues in dentistry with tissue engineering emerging as a potential solution for enhancing dentin regeneration. In this study, we fabricated cell blocks with human dental pulp stem cells (hDPSCs)-laden alginate/fish gelatin hydrogels (Alg/FGel) at the center of the cell block and human umbilical vascular endothelial cells (HUVEC)-laden Si ion-infused fish gelatin methacrylate (FGelMa) at the periphery of the cell block. ¹H NMR and FTIR results showed the successful fabrication of Alg/FGel and FGelMa. In addition, Si ions in the FGelMa were noted to be bonded via covalent bonds and the increased number of covalent bonds led to an increase in mechanical properties and improved degradation of FGelMa. The Si-containing FGelMa was able to release Si ions, which subsequently significantly not only enhanced the expressions of angiogenic-related protein, but also secreted some cytokines to regulate odontogenesis. Further immunofluorescence results indicated that the cell blocks allowed interactions between the HUVEC and hDPSCs, and taken together, were able to enhance odontogenic-related markers' expression, such as alkaline phosphatase (ALP), dentin matrix phosphoprotein-1 (DMP-1), and osteocalcin (OC). Subsequent Alizarin Red S stain confirmed the benefits of our cell block and demonstrated that such a novel combination and modification of biomaterials can serve as a platform for future clinical applications and use in dentin regeneration.

Tissue-Engineering of Human Intervertebral Disc: A Concise Review

Intervertebral disc (IVD) represents a structure of crucial structural and functional importance for human spine. Pathology of IVD institutes a frequently encountered condition in current clinical practice. Degenerative Disc Disease (DDD), the principal clinical representative of IVD pathology, constitutes an increasingly diagnosed spinal disorder associated with substantial morbidity and mortality in recent years. Despite the considerable incidence and socioeconomic burden of DDD, existing treatment modalities including conservative and surgical methods have been demonstrated to provide a limited therapeutic effect, being not capable of interrupting or reversing natural progress of underlying disease. These limitations underline the requirement for development of novel, innovative and more effective therapeutic strategies for DDD management. Within this literature framework, compromised IVD replacement with a viable IVD construct manufactured with Tissue-Engineering (TE) methods has been recommended as a promising therapeutic strategy for DDD. Existing preliminary preclinical data demonstrate that proper combination of cells from various sources, different scaffold materials and appropriate signaling molecules renders manufacturing of whole-IVD tissue-engineered constructs a technically feasible process. Aim of this narrative review is to critically summarize current published evidence regarding particular aspects of IVD-TE, primarily emphasizing in providing researchers in this field with practicable knowledge in order to enhance clinical translatability of their research and informing clinical practitioners about the features and capabilities of innovative TE science in the field of IVD-TE.