Hybrid Bioprinting of Chondrogenically Induced Human Mesenchymal Stem Cell Spheroids

Microneedles Constructed by Swellable Hydrogels Loaded with Celastrol for Efficient Treatment of Skin Infections Induced by Drug-Resistant Bacterial Strains

The urgent need for new antimicrobial drugs arises from the limited efficacy of traditional antibiotics against emerging drug-resistant strains. Celastrol (CSL) demonstrates an exceptional antibacterial property that remains unaffected by bacterial resistance, but its poor water solubility limits its wide applications. This study uses the hydrophobic inner cavity of mono-(6-diethylenetriamine-6-deoxy)-β-cyclodextrin (mβ-CD) (a derivative of cyclodextrin) to encapsulate CSL, constructing an inclusion complex (CSL@mβ-CD) to enhance the water solubility of CSL. The obtained inclusion complex is further incorporated into a swellable hydrogel microneedle (MN) to obtain CSL@mβ-CD/MN. The fabricated CSL@mβ-CD/MN can enable the sustained release of CSL, achieving effective bacterial eradication at infected sites. In vivo experiments demonstrate that CSL@mβ-CD/MN has a remarkable efficacy in the treatment of methicillin-resistant Staphylococcus aureus-induced subcutaneous abscesses and wound infections. Specifically, CSL@mβ-CD/MN can effectively penetrate the stratum corneum of the skin to realize rapid elimination of the bacteria in wounds. Moreover, CSL@mβ-CD/MN can efficiently scavenge reactive oxygen species, promote M2 polarization of macrophages, and relieve local inflammation at the wound sites. These results reveal that CSL@mβ-CD/MN holds great promise in the clinical treatment of acute skin infections induced by drug-resistant bacteria.

Rapidly Degrading Hydrogels to Support Biofabrication and 3D Bioprinting Using Cartilage Microtissues

In recent years, there has been increased interest in the use of cellular spheroids, microtissues, and organoids as biological building blocks to engineer functional tissues and organs. Such microtissues are typically formed by the self-assembly of cellular aggregates and the subsequent deposition of a tissue-specific extracellular matrix (ECM). Biofabrication and 3D bioprinting strategies using microtissues may require the development of supporting hydrogels and bioinks to spatially localize such biological building blocks in 3D space and hence enable the engineering of geometrically defined tissues. Therefore, the aim of this work was to engineer scaled-up, geometrically defined cartilage grafts by combining multiple cartilage microtissues within a rapidly degrading oxidized alginate (OA) supporting hydrogel and maintaining these constructs in dynamic culture conditions. To this end, cartilage microtissues were first independently matured for either 2 or 4 days and then combined in the presence or absence of a supporting OA hydrogel. Over 6 weeks in static culture, constructs engineered using microtissues that were matured independently for 2 days generated higher amounts of glycosaminoglycans (GAGs) compared to those matured for 4 days. Histological analysis revealed intense staining for GAGs and negative staining for calcium deposits in constructs generated by using the supporting OA hydrogel. Less physical contraction was also observed in constructs generated in the presence of the supporting gel; however, the remnants of individual microtissues were more observable, suggesting that even the presence of a rapidly degrading hydrogel may delay the fusion and/or the remodeling of the individual microtissues. Dynamic culture conditions were found to modulate ECM synthesis following the OA hydrogel encapsulation. We also assessed the feasibility of 3D bioprinting of cartilage microtissues within OA based bioinks. It was observed that the microtissues remained viable after extrusion-based bioprinting and were able to fuse after 48 h, particularly when high microtissue densities were used, ultimately generating a cartilage tissue that was rich in GAGs and negative for calcium deposits. Therefore, this work supports the use of OA as a supporting hydrogel/bioink when using microtissues as biological building blocks in diverse biofabrication and 3D bioprinting platforms.

Strategies to decouple cell micro-scale and macro-scale environments for designing multifunctional biomimetic tissues

The regulation of cellular behavior within a three-dimensional (3D) environment to execute a specific function remains a challenge in the field of tissue engineering. In native tissues, cells and matrices are arranged into 3D modular units, comprising biochemical and biophysical signals that orchestrate specific cellular activities. Modular tissue engineering aims to emulate this natural complexity through the utilization of functional building blocks with unique stimulation features. By adopting a modular approach and using well-designed biomaterials, cellular microenvironments can be effectively decoupled from their macro-scale surroundings, enabling the development of engineered tissues with enhanced multifunctionality and heterogeneity. We overview recent advancements in decoupling the cellular micro-scale niches from their macroenvironment and evaluate the implications of this strategy on cellular and tissue functionality.

Bioprinting Using Organ Building Blocks: Spheroids, Organoids, and Assembloids

Three-dimensional (3D) bioprinting, a promising advancement in tissue engineering technology, involves the robotic, layer-by-layer additive biofabrication of functional 3D tissue and organ constructs. This process utilizes biomaterials, typically hydrogels and living cells, following digital models. Traditional tissue engineering uses a classic triad of living cells, scaffolds, and physicochemical signals in bioreactors. A scaffold is a temporary, often biodegradable, support structure. Tissue engineering primarily falls into two categories: (i) scaffold based and (ii) scaffold free. The latter, scaffold-free 3D bioprinting, is gaining increasing popularity. Organ building blocks (OBB), capable of self-assembly and self-organization, such as tissue spheroids, organoids, and assembloids, have begun to be utilized in scaffold-free bioprinting. This article discusses the expanding range of OBB, presents the rapidly evolving collection of bioprinting and bioassembly methods using these OBB, and finally, outlines the advantages, challenges, and future perspectives of using OBB in organ printing.

An overview of nasal cartilage bioprinting: from bench to bedside

Nasal cartilage diseases and injuries are known as significant challenges in reconstructive medicine, affecting a substantial number of individuals worldwide. In recent years, the advent of three-dimensional (3D) bioprinting has emerged as a promising approach for nasal cartilage reconstruction, offering potential breakthroughs in the field of regenerative medicine. This paper provides an overview of the methods and challenges associated with 3D bioprinting technologies in the procedure of reconstructing nasal cartilage tissue. The process of 3D bioprinting entails generating a digital 3D model using biomedical imaging techniques and computer-aided design to integrate both internal and external scaffold features. Then, bioinks which consist of biomaterials, cell types, and bioactive chemicals, are applied to facilitate the precise layer-by-layer bioprinting of tissue-engineered scaffolds. After undergoing in vitro and in vivo experiments, this process results in the development of the physiologically functional integrity of the tissue. The advantages of 3D bioprinting encompass the ability to customize scaffold design, enabling the precise incorporation of pore shape, size, and porosity, as well as the utilization of patient-specific cells to enhance compatibility. However, various challenges should be considered, including the optimization of biomaterials, ensuring adequate cell viability and differentiation, achieving seamless integration with the host tissue, and navigating regulatory attention. Although numerous studies have demonstrated the potential of 3D bioprinting in the rebuilding of such soft tissues, this paper covers various aspects of the bioprinted tissues to provide insights for the future development of repair techniques appropriate for clinical use.

Collagen and Alginate Hydrogels Support Chondrocytes Redifferentiation In Vitro without Supplementation of Exogenous Growth Factors

Focal cartilage defects are a prevalent knee problem affecting people of all ages. Articular cartilage (AC) possesses limited healing potential, and osteochondral defects can lead to pain and long-term complications such as osteoarthritis. Autologous chondrocyte implantation (ACI) has been a successful surgical approach for repairing osteochondral defects over the past two decades. However, a major drawback of ACI is the dedifferentiation of chondrocytes during their in vitro expansion. In this study, we isolated ovine chondrocytes and cultured them in a two-dimensional environment for ACI procedures. We hypothesized that 3D scaffolds would support the cells' redifferentiation without the need for growth factors so we encapsulated them into soft collagen and alginate (col/alg) hydrogels. Chondrocytes embedded into the hydrogels were viable and proliferated. After 7 days, they regained their original rounded morphology (aspect ratio 1.08) and started to aggregate. Gene expression studies showed an upregulation of COL2A1, FOXO3A, FOXO1, ACAN, and COL6A1 (37, 1.13, 22, 1123, and 1.08-fold change expression, respectively) as early as day one. At 21 days, chondrocytes had extensively colonized the hydrogel, forming large cell clusters. They started to replace the degrading scaffold by depositing collagen II and aggrecan, but with limited collagen type I deposition. This approach allows us to overcome the limitations of current approaches such as the dedifferentiation occurring in 2D in vitro expansion and the necrotic formation in spheroids. Further studies are warranted to assess long-term ECM deposition and integration with native cartilage. Though limitations exist, this study suggests a promising avenue for cartilage repair with col/alg hydrogel scaffolds.

Bioprinting of human dermal microtissues precursors as building blocks for endogenousin vitroconnective tissue manufacturing

The advent of 3D bioprinting technologies in tissue engineering has unlocked the potential to fabricatein vitrotissue models, overcoming the constraints associated with the shape limitations of preformed scaffolds. However, achieving an accurate mimicry of complex tissue microenvironments, encompassing cellular and biochemical components, and orchestrating their supramolecular assembly to form hierarchical structures while maintaining control over tissue formation, is crucial for gaining deeper insights into tissue repair and regeneration. Building upon our expertise in developing competent three-dimensional tissue equivalents (e.g. skin, gut, cervix), we established a two-step bottom-up approach involving the dynamic assembly of microtissue precursors (μTPs) to generate macroscopic functional tissue composed of cell-secreted extracellular matrix (ECM). To enhance precision and scalability, we integrated extrusion-based bioprinting technology into our established paradigm to automate, control and guide the coherent assembly ofμTPs into predefined shapes. Compared to cell-aggregated bioink, ourμTPs represent a functional unit where cells are embedded in their specific ECM.μTPs were derived from human dermal fibroblasts dynamically seeded onto gelatin-based microbeads. After 9 days,μTPs were suspended (50% v/v) in Pluronic-F127 (30% w/v) (µTP:P30), and the obtained formulation was loaded as bioink into the syringe of the Dr.INVIVO-4D6 extrusion based bioprinter.µTP:P30 bioink showed shear-thinning behavior and temperature-dependent viscosity (gel atT> 30 °C), ensuringµTPs homogenous dispersion within the gel and optimal printability. The bioprinting involved extruding several geometries (line, circle, and square) into Pluronic-F127 (40% w/v) (P40) support bath, leveraging its shear-recovery property. P40 effectively held the bioink throughout and after the bioprinting procedure, untilµTPs fused into a continuous connective tissue.µTPs fusion dynamics was studied over 8 days of culture, while the resulting endogenous construct underwent 28 days culture. Histological, immunofluorescence analysis, and second harmonic generation reconstruction revealed an increase in endogenous collagen and fibronectin production within the bioprinted construct, closely resembling the composition of the native connective tissues.

In vitro and in vivo evaluation of periosteum-derived cells and iPSC-derived chondrocytes encapsulated in GelMA for osteochondral tissue engineering

Osteochondral defects are deep joint surface lesions that affect the articular cartilage and the underlying subchondral bone. In the current study, a tissue engineering approach encompassing individual cells encapsulated in a biocompatible hydrogel is explored in vitro and in vivo. Cell-laden hydrogels containing either human periosteum-derived progenitor cells (PDCs) or human induced pluripotent stem cell (iPSC)-derived chondrocytes encapsulated in gelatin methacryloyl (GelMA) were evaluated for their potential to regenerate the subchondral mineralized bone and the articular cartilage on the joint surface, respectively. PDCs are easily isolated and expanded progenitor cells that are capable of generating mineralized cartilage and bone tissue in vivo via endochondral ossification. iPSC-derived chondrocytes are an unlimited source of stable and highly metabolically active chondrocytes. Cell-laden hydrogel constructs were cultured for up to 28 days in a serum-free chemically defined chondrogenic medium. On day 1 and day 21 of the differentiation period, the cell-laden constructs were implanted subcutaneously in nude mice to evaluate ectopic tissue formation 4 weeks post-implantation. Taken together, the data suggest that iPSC-derived chondrocytes encapsulated in GelMA can generate hyaline cartilage-like tissue constructs with different levels of maturity, while using periosteum-derived cells in the same construct type generates mineralized tissue and cortical bone in vivo. Therefore, the aforementioned cell-laden hydrogels can be an important part of a multi-component strategy for the manufacturing of an osteochondral implant.

Generating human skeletal myoblast spheroids for vascular myogenic tissue engineering

Engineered myogenic microtissues derived from human skeletal myoblasts offer unique opportunities for varying skeletal muscle tissue engineering applications, such asin vitrodrug-testing and disease modelling. However, more complex models require the incorporation of vascular structures, which remains to be challenging. In this study, myogenic spheroids were generated using a high-throughput, non-adhesive micropatterned surface. Since monoculture spheroids containing human skeletal myoblasts were unable to remain their integrity, co-culture spheroids combining human skeletal myoblasts and human adipose-derived stem cells were created. When using the optimal ratio, uniform and viable spheroids with enhanced myogenic properties were achieved. Applying a pre-vascularization strategy, through addition of endothelial cells, resulted in the formation of spheroids containing capillary-like networks, lumina and collagen in the extracellular matrix, whilst retaining myogenicity. Moreover, sprouting of endothelial cells from the spheroids when encapsulated in fibrin was allowed. The possibility of spheroids, from different maturation stages, to assemble into a more large construct was proven by doublet fusion experiments. The relevance of using three-dimensional microtissues with tissue-specific microarchitecture and increased complexity, together with the high-throughput generation approach, makes the generated spheroids a suitable tool forin vitrodrug-testing and human disease modeling.

Advancements of 3D bioprinting in regenerative medicine: Exploring cell sources for organ fabrication

3D bioprinting has unlocked new possibilities for generating complex and functional tissues and organs. However, one of the greatest challenges lies in selecting the appropriate seed cells for constructing fully functional 3D artificial organs. Currently, there are no cell sources available that can fulfill all requirements of 3D bioprinting technologies, and each cell source possesses unique characteristics suitable for specific applications. In this review, we explore the impact of different 3D bioprinting technologies and bioink materials on seed cells, providing a comprehensive overview of the current landscape of cell sources that have been used or hold potential in 3D bioprinting. We also summarized key points to guide the selection of seed cells for 3D bioprinting. Moreover, we offer insights into the prospects of seed cell sources in 3D bioprinted organs, highlighting their potential to revolutionize the fields of tissue engineering and regenerative medicine.

Gelatin-Based Hybrid Hydrogels as Matrices for Organoid Culture

The application of liver organoids is very promising in the field of liver tissue engineering; however, it is still facing some limitations. One of the current major limitations is the matrix in which they are cultured. The mainly undefined and murine-originated tumor matrices derived from Engelbreth-Holm-Swarm (EHS) sarcoma, such as Matrigel, are still the standard culturing matrices for expansion and differentiation of organoids toward hepatocyte-like cells, which will obstruct its future clinical application potential. In this study, we exploited the use of newly developed highly defined hydrogels as potential matrices for the culture of liver organoids and compared them to Matrigel and two hydrogels that were already researched in the field of organoid research [i.e., polyisocyanopeptides, enriched with laminin-entactin complex (PIC-LEC) and gelatin methacryloyl (GelMA)]. The newly developed hydrogels are materials that have a physicochemical resemblance with native liver tissue. Norbornene-modified dextran cross-linked with thiolated gelatin (DexNB-GelSH) has a swelling ratio and macro- and microscale properties that highly mimic liver tissue. Norbornene-modified chondroitin sulfate cross-linked with thiolated gelatin (CSNB-GelSH) contains chondroitin sulfate, which is a glycosaminoglycan (GAG) that is present in the liver ECM. Furthermore, CSNB-GelSH hydrogels with different mechanical properties were evaluated. Bipotent intrahepatic cholangiocyte organoids (ICOs) were applied in this work and encapsulated in these materials. This research revealed that the newly developed materials outperformed Matrigel, PIC-LEC, and GelMA in the differentiation of ICOs toward hepatocyte-like cells. Furthermore, some trends indicate that an interplay of both the chemical composition and the mechanical properties has an influence on the relative expression of certain hepatocyte markers. Both DexNB-GelSH and CSNB-GelSH showed promising results for the expansion and differentiation of intrahepatic cholangiocyte organoids. The stiffest CSNB-GelSH hydrogel even significantly outperformed Matrigel based on ALB, BSEP, and CYP3A4 gene expression, being three important hepatocyte markers.

A comprehensive review on 3D tissue models: Biofabrication technologies and preclinical applications

The limitations of traditional two-dimensional (2D) cultures and animal testing, when it comes to precisely foreseeing the toxicity and clinical effectiveness of potential drug candidates, have resulted in a notable increase in the rate of failure during the process of drug discovery and development. Three-dimensional (3D) in-vitro models have arisen as substitute platforms with the capacity to accurately depict in-vivo conditions and increasing the predictivity of clinical effects and toxicity of drug candidates. It has been found that 3D models can accurately represent complex tissue structure of human body and can be used for a wide range of disease modeling purposes. Recently, substantial progress in biomedicine, materials and engineering have been made to fabricate various 3D in-vitro models, which have been exhibited better disease progression predictivity and drug effects than convention models, suggesting a promising direction in pharmaceutics. This comprehensive review highlights the recent developments in 3D in-vitro tissue models for preclinical applications including drug screening and disease modeling targeting multiple organs and tissues, like liver, bone, gastrointestinal tract, kidney, heart, brain, and cartilage. We discuss current strategies for fabricating 3D models for specific organs with their strengths and pitfalls. We expand future considerations for establishing a physiologically-relevant microenvironment for growing 3D models and also provide readers with a perspective on intellectual property, industry, and regulatory landscape.

Human Induced Pluripotent Spheroids' Growth Is Driven by Viscoelastic Properties and Macrostructure of 3D Hydrogel Environment

Stem cells, particularly human iPSCs, constitute a powerful tool for tissue engineering, notably through spheroid and organoid models. While the sensitivity of stem cells to the viscoelastic properties of their direct microenvironment is well-described, stem cell differentiation still relies on biochemical factors. Our aim is to investigate the role of the viscoelastic properties of hiPSC spheroids' direct environment on their fate. To ensure that cell growth is driven only by mechanical interaction, bioprintable alginate-gelatin hydrogels with significantly different viscoelastic properties were utilized in differentiation factor-free culture medium. Alginate-gelatin hydrogels of varying concentrations were developed to provide 3D environments of significantly different mechanical properties, ranging from 1 to 100 kPa, while allowing printability. hiPSC spheroids from two different cell lines were prepared by aggregation (⌀ = 100 µm, n > 1 × 104), included and cultured in the different hydrogels for 14 days. While spheroids within dense hydrogels exhibited limited growth, irrespective of formulation, porous hydrogels prepared with a liquid-liquid emulsion method displayed significant variations of spheroid morphology and growth as a function of hydrogel mechanical properties. Transversal culture (adjacent spheroids-laden alginate-gelatin hydrogels) clearly confirmed the separate effect of each hydrogel environment on hiPSC spheroid behavior. This study is the first to demonstrate that a mechanically modulated microenvironment induces diverse hiPSC spheroid behavior without the influence of other factors. It allows one to envision the combination of multiple formulations to create a complex object, where the fate of hiPSCs will be independently controlled by their direct microenvironment.

Photo-cross-linked Hydrogels for Cartilage and Osteochondral Repair

Photo-cross-linked hydrogels, which respond to light and induce structural or morphological transitions, form a microenvironment that mimics the extracellular matrix of native tissue. In the last decades, photo-cross-linked hydrogels have been widely used in cartilage and osteochondral tissue engineering due to their good biocompatibility, ease of fabrication, rapid in situ gel-forming ability, and tunable mechanical and degradable properties. In this review, we systemically summarize the different types and physicochemical properties of photo-cross-linked hydrogels (including the materials and photoinitiators) and explore the biological properties modulated through the incorporation of additives, including cells, biomolecules, genes, and nanomaterials, into photo-cross-linked hydrogels. Subsequently, we compile the applications of photo-cross-linked hydrogels with a specific focus on cartilage and osteochondral repair. Finally, current limitations and future perspectives of photo-cross-linked hydrogels are also discussed.

Gelatin-Based Hybrid Hydrogel Scaffolds: Toward Physicochemical Liver Mimicry

There exists a clear need to develop novel materials that could serve liver tissue engineering purposes. Those materials need to be researched for the development of bioengineered liver tissue as an alternative to donor livers, as well as for materials that could be applied for scaffolds to develop an in vitro model for drug-induced liver injury (DILI) detection . In this paper, the hydrogels oxidized dextran-gelatin (Dexox-Gel) and norbornene-modified dextran-thiolated gelatin (DexNB-GelSH) were developed, and their feasibility toward processing via indirect 3D-printing was investigated with the aim to develop hydrogel scaffolds that physicochemically mimic native liver tissue. Furthermore, their in vitro biocompatibility was assessed using preliminary biological tests using HepG2 cells. Both materials were thoroughly physicochemically characterized and benchmarked to the methacrylated gelatin (GelMA) reference material. Due to inferior properties, Dexox-gel was not further processed into 3D-hydrogel scaffolds. This research revealed that DexNB-GelSH exhibited physicochemical properties that were in excellent agreement with the properties of natural liver tissue in contrast to GelMA. In combination with an equally good biological evaluation of DexNB-GelSH in comparison with GelMA based on an MTS proliferation assay and an albumin quantification assay, DexNB-GelSH can be considered promising in the field of liver tissue engineering.

The Role of Mechanical Properties and Structure of Type I Collagen Hydrogels on Colorectal Cancer Cell Migration

Mechanical interactions between cells and their microenvironment play an important role in determining cell fate, which is particularly relevant in metastasis, a process where cells invade tissue matrices with different mechanical properties. In vitro, type I collagen hydrogels have been commonly used for modeling the microenvironment due to its ubiquity in the human body. In this work, the combined influence of the stiffness of these hydrogels and their ultrastructure on the migration patterns of HCT-116 and HT-29 spheroids are analyzed. For this, six different types of pure type I collagen hydrogels by changing the collagen concentration and the gelation temperature are prepared. The stiffness of each sample is measured and its ultrastructure is characterized. Cell migration studies are then performed by seeding the spheroids in three different spatial conditions. It is shown that changes in the aforementioned parameters lead to differences in the mechanical stiffness of the matrices as well as the ultrastructure. These differences, in turn, lead to distinct cell migration patterns of HCT-116 and HT-29 spheroids in either of the spatial conditions tested. Based on these results, it is concluded that the stiffness and the ultrastructural organization of the matrix can actively modulate cell migration behavior in colorectal cancer spheroids.

Engineering Highly Vascularized Bone Tissues by 3D Bioprinting of Granular Prevascularized Spheroids

The convergence of 3D bioprinting with powerful manufacturing capability and cellular self-organization that can reproduce intricate tissue microarchitecture and function is a promising direction toward building functional tissues and has yet to be demonstrated. Here, we develop a granular aggregate-prevascularized (GAP) bioink for engineering highly vascularized bone tissues by capitalizing on the condensate-mimicking, self-organization, and angiogenic properties of prevascularized mesenchymal spheroids. The GAP bioink utilizes prevascularized aggregates as building blocks, which are embedded densely in extracellular matrices conducive to spontaneous self-organization. We printed various complex structures with high cell density (∼1.5 × 108 cells/cm3), viability (∼80%), and shape fidelity using GAP bioink. After printing, the prevascularized mesenchymal spheroids developed an interconnected vascular network through angiogenic sprouting. We printed highly vascularized bone tissues using GAP bioink and found that prevascularized spheroids were more conducive to osteogenesis and angiogenesis. We envision that the design of the GAP bioink could be further integrated with human-induced pluripotent stem cell-derived organoids, which opens new avenues to create patient-specific vascularized tissues for therapeutic applications..

Optimizing Bioink Composition for Human Chondrocyte Expression of Lubricin

The surface zone of articular cartilage is the first area impacted by cartilage defects, commonly resulting in osteoarthritis. Chondrocytes in the surface zone of articular cartilage synthesize and secrete lubricin, a proteoglycan that functions as a lubricant protecting the deeper layers from shear stress. Notably, 3D bioprinting is a tissue engineering technique that uses cells encapsulated in biomaterials to fabricate 3D constructs. Gelatin methacrylate (GelMA) is a frequently used biomaterial for 3D bioprinting cartilage. Oxidized methacrylated alginate (OMA) is a chemically modified alginate designed for its tunable degradation rate and mechanical properties. To determine an optimal combination of GelMA and OMA for lubricin expression, we used our novel high-throughput human articular chondrocyte reporter system. Primary human chondrocytes were transduced with PRG4 (lubricin) promoter-driven Gaussia luciferase, allowing for temporal assessment of lubricin expression. A lubricin expression-driven Design of Experiment screen and subsequent validation identified 14% GelMA/2% OMA for further study. Therefore, DoE optimized 14% GelMA/2% OMA, 14% GelMA control, and 16% GelMA (total solid content control) were 3D bioprinted. The combination of lubricin protein expression and shape retention over the 22 days in culture, successfully determined the 14% GelMA/2%OMA to be the optimal formulation for lubricin secretion. This strategy allows for rapid analysis of the role(s) of biomaterial composition, stiffness or other cell manipulations on lubricin expression by chondrocytes, which may improve therapeutic strategies for cartilage regeneration.

Engineered biomaterials to guide spheroid formation, function, and fabrication into 3D tissue constructs

Cellular spheroids are aggregates of cells that are being explored to address fundamental biological questions and as building blocks for engineered tissues. Spheroids possess distinct advantages over cellular monolayers or cell encapsulation in 3D natural and synthetic hydrogels, including direct cell-cell interactions and high cell densities, which better mimic aspects of many tissues. Despite these advantages, spheroid cultures often exhibit uncontrollable growth and may be too simplistic to mimic complex tissue structures. To address this, biomaterials are being leveraged to further expand the use of cellular spheroids for biomedical applications. In this review, we provide an overview of recent studies that utilize engineered biomaterials to guide spheroid formation and function, as well as their fabrication into tissues for use as tissue models and for therapeutic applications. First, we describe biomaterial strategies that allow the high-throughput fabrication of homogeneously-sized spheroids. Next, we summarize how engineered biomaterials are introduced into spheroid cultures either internally as microparticles or externally as hydrogel microenvironments to influence spheroid behavior (e.g., differentiation, fusion). Lastly, we discuss a variety of biofabrication strategies (e.g., 3D bioprinting, melt electrowriting) that have been used to develop macroscale tissue models and implantable constructs through the guided assembly of spheroids. Overall, the goal of this review is to provide a summary of how biomaterials are currently being engineered and leveraged to support spheroids in biomedical applications, as well as to provide a future outlook of the field. STATEMENT OF SIGNIFICANCE: Cellular spheroids are becoming increasingly used as in vitro tissue models or as 'building blocks' for tissue engineering and repair strategies. Engineered biomaterials and their processing through biofabrication approaches are being leveraged to structurally support and guide spheroid processes. This review summarizes current approaches where such biomaterials are being used to guide spheroid formation, function, and fabrication into tissue constructs. As the field is rapidly expanding, we also provide an outlook on future directions and how new engineered biomaterials can be implemented to further the development of biofabricated spheroid-based tissue constructs.

Bioprinting of Stem Cell Spheroids Followed by Post-Printing Chondrogenic Differentiation for Cartilage Tissue Engineering

Cartilage tissue presents low self-repair capability and lesions often undergo irreversible progression. Structures obtained by tissue engineering, such as those based in extrusion bioprinting of constructs loaded with stem cell spheroids may offer valuable alternatives for research and therapeutic purposes. Human mesenchymal stromal cell (hMSC) spheroids can be chondrogenically differentiated faster and more efficiently than single cells. This approach allows obtaining larger tissues in a rapid, controlled and reproducible way. However, it is challenging to control tissue architecture, construct stability, and cell viability during maturation. Herein, this work reports a reproducible bioprinting process followed by a successful post-bioprinting chondrogenic differentiation procedure using large quantities of hMSC spheroids encapsulated in a xanthan gum-alginate hydrogel. Multi-layered constructs are bioprinted, ionically crosslinked, and post chondrogenically differentiated for 28 days. The expression of glycosaminoglycan, collagen II and IV are observed. After 56 days in culture, the bioprinted constructs are still stable and show satisfactory cell metabolic activity with profuse extracellular matrix production. These results show a promising procedure to obtain 3D models for cartilage research and ultimately, an in vitro proof-of-concept of their potential use as stable chondral tissue implants.

Engineering High-Quality Cartilage Microtissues Using Hydrocortisone Functionalized Microwells

Engineering clinically relevant musculoskeletal tissues at a human scale is a considerable challenge. Developmentally inspired scaffold-free approaches for engineering cartilage tissues have shown great promise in recent years, enabling the generation of highly biomimetic tissues. Despite the relative success of these approaches, the absence of a supporting scaffold or hydrogel creates challenges in the development of large-scale tissues. Combining numerous scaled-down tissue units (herein termed microtissues) into a larger macrotissue represents a promising strategy to address this challenge. The overall success of such approaches, however, relies on the development of strategies which support the robust and consistent chondrogenic differentiation of clinically relevant cell sources such as mesenchymal stem/stromal cells (MSCs) within microwell arrays to biofabricate numerous microtissues rich in cartilage-specific extracellular matrix components. In this article, we first describe a simple method to manufacture cartilage microtissues at various scales using novel microwell array stamps. This system allows the rapid and reliable generation of cartilage microtissues and can be used as a platform to study microtissue phenotype and development. Based on the unexpected discovery that Endothelial Growth Medium (EGM) enhanced MSC aggregation and chondrogenic capacity within the microwell arrays, this work also sought to identify soluble factors within the media capable of supporting robust differentiation using heterogeneous MSC populations. Hydrocortisone was found to be the key factor within EGM that enhanced the chondrogenic capacity of MSCs within these microwell arrays. This strategy represents a promising means of generating large numbers of high-quality, scaffold-free cartilage microtissues for diverse biofabrication applications. Impact statement This study addresses a key challenge facing emerging modular biofabrication strategies that use microtissues as biological building blocks. Namely, achieving the necessary robust and consistent differentiation of clinically relevant cell sources, for example, mesenchymal stem/stromal cells (MSCs), and the accumulation of sufficient tissue-specific extracellular matrix (ECM) to engineer tissue of scale. We achieved this by establishing hydrocortisone as a simple and potent method for improving MSC chondrogenesis, resulting in the biofabrication of high-quality (ECM rich) cartilage microtissues. These findings could enable the generation of more scalable engineered cartilage by ensuring the formation of high-quality microtissue building blocks generated using heterogeneous MSC populations.

Neo-cartilage formation using human nondegenerate versus osteoarthritic chondrocyte-derived cartilage organoids in a viscoelastic hydrogel

Current regenerative cartilage therapies are associated with several drawbacks such as dedifferentiation of chondrocytes during expansion and the formation of fibrocartilage. Optimized chondrocyte expansion and tissue formation could lead to better clinical results of these therapies. In this study, a novel chondrocyte suspension expansion protocol that includes the addition of porcine notochordal cell-derived matrix was used to self-assemble human chondrocytes from osteoarthritic (OA) and nondegenerate (ND) origin into cartilage organoids containing collagen type II and proteoglycans. Proliferation rate and viability were similar for OA and ND chondrocytes and organoids formed had a similar histologic appearance and gene expression profile. Organoids were then encapsulated in viscoelastic alginate hydrogels to form larger tissues. Chondrocytes on the outer bounds of the organoids produced a proteoglycan-rich matrix to bridge the space between organoids. In hydrogels containing ND organoids some collagen type I was observed between the organoids. Surrounding the bulk of organoids in the center of the gels, in both OA and ND gels a continuous tissue containing cells, proteoglycans and collagen type II had been produced. No difference was observed in sulphated glycosaminoglycan and hydroxyproline content between gels containing organoids from OA or ND origin after 28 days. It was concluded that OA chondrocytes, which can be harvested from leftover surgery tissue, perform similar to ND chondrocytes in terms of human cartilage organoid formation and matrix production in alginate gels. This opens possibilities for their potential to serve as a platform for cartilage regeneration but also as an in vitro model to study pathways, pathology, or drug development.

Plug-and-Play Lymph Node-on-Chip: Secondary Tumor Modeling by the Combination of Cell Spheroid, Collagen Sponge and T-Cells

Towards the improvement of the efficient study of drugs and contrast agents, the 3D microfluidic platforms are currently being actively developed for testing these substances and particles in vitro. Here, we have elaborated a microfluidic lymph node-on-chip (LNOC) as a tissue engineered model of a secondary tumor in lymph node (LN) formed due to the metastasis process. The developed chip has a collagen sponge with a 3D spheroid of 4T1 cells located inside, simulating secondary tumor in the lymphoid tissue. This collagen sponge has a morphology and porosity comparable to that of a native human LN. To demonstrate the suitability of the obtained chip for pharmacological applications, we used it to evaluate the effect of contrast agent/drug carrier size, on the penetration and accumulation of particles in 3D spheroids modeling secondary tumor. For this, the 0.3, 0.5 and 4 μm bovine serum albumin (BSA)/tannic acid (TA) capsules were mixed with lymphocytes and pumped through the developed chip. The capsule penetration was examined by scanning with fluorescence microscopy followed by quantitative image analysis. The results show that capsules with a size of 0.3 μm passed more easily to the tumor spheroid and penetrated inside. We hope that the device will represent a reliable alternative to in vivo early secondary tumor models and decrease the amount of in vivo experiments in the frame of preclinical study.

Chondrogenesis of Adipose-Derived Stem Cells Using an Arrayed Spheroid Format

Objective

The chondrogenic response of adipose-derived stem cells (ASCs) is often assessed using 3D micromass protocols that use upwards of hundreds of thousands of cells. Scaling these systems up for high-throughput testing is technically challenging and wasteful given the necessary cell numbers and reagent volumes. However, adopting microscale spheroid cultures for this purpose shows promise. Spheroid systems work with only thousands of cells and microliters of medium.

Methods

Molded agarose microwells were fabricated using 2% w/v molten agarose and then equilibrated in medium prior to introducing cells. ASCs were seeded at 50, 500, 5k cells/microwell; 5k, 50k, cells/well plate; and 50k and 250k cells/15 mL centrifuge tube to compare chondrogenic responses across spheroid and micromass sizes. Cells were cultured in control or chondrogenic induction media. ASCs coalesced into spheroids/pellets and were cultured at 37 °C and 5% CO2 for 21 days with media changes every other day.

Results

All culture conditions supported growth of ASCs and formation of viable cell spheroids/micromasses. More robust growth was observed in chondrogenic conditions. Sulfated glycosaminoglycans and collagen II, molecules characteristics of chondrogenesis, were prevalent in both 5000-cell spheroids and 250,000-cell micromasses. Deposition of collagen I, characteristic of fibrocartilage, was more prevalent in the large micromasses than small spheroids.

Conclusions

Chondrogenic differentiation was consistently induced using high-throughput spheroid formats, particularly when seeding at cell densities of 5000 cells/spheroid. This opens possibilities for highly arrayed experiments investigating tissue repair and remodeling during or after exposure to drugs, toxins, or other chemicals.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-022-00746-8.

Design, Fabrication, and Application of Mini-Scaffolds for Cell Components in Tissue Engineering

The concept of "lockyballs" or interlockable mini-scaffolds fabricated by two-photon polymerization from biodegradable polymers for the encagement of tissue spheroids and their delivery into the desired location in the human body has been recently introduced. In order to improve control of delivery, positioning, and assembly of mini-scaffolds with tissue spheroids inside, they must be functionalized. This review describes the design, fabrication, and functionalization of mini-scaffolds as well as perspectives on their application in tissue engineering for precisely controlled cell and mini-tissue delivery and patterning. The development of functionalized mini-scaffolds advances the original concept of "lockyballs" and opens exciting new prospectives for mini-scaffolds' applications in tissue engineering and regenerative medicine and their eventual clinical translation.

Tools for manipulation and positioning of microtissues

Complex three-dimensional (3D) in vitro models are emerging as a key technology to support research areas in personalised medicine, such as drug development and regenerative medicine. Tools for manipulation and positioning of microtissues play a crucial role in the microtissue life cycle from production to end-point analysis. The ability to precisely locate microtissues can improve the efficiency and reliability of processes and investigations by reducing experimental time and by providing more controlled parameters. To achieve this goal, standardisation of the techniques is of primary importance. Compared to microtissue production, the field of microtissue manipulation and positioning is still in its infancy but is gaining increasing attention in the last few years. Techniques to position microtissues have been classified into four main categories: hydrodynamic techniques, bioprinting, substrate modification, and non-contact active forces. In this paper, we provide a comprehensive review of the different tools for the manipulation and positioning of microtissues that have been reported to date. The working mechanism of each technique is described, and its merits and limitations are discussed. We conclude by evaluating the potential of the different approaches to support progress in personalised medicine.

Articular Cartilage Regeneration through Bioassembling Spherical Micro-Cartilage Building Blocks

Articular cartilage lesions are prevalent and affect one out of seven American adults and many young patients. Cartilage is not capable of regeneration on its own. Existing therapeutic approaches for articular cartilage lesions have limitations. Cartilage tissue engineering is a promising approach for regenerating articular neocartilage. Bioassembly is an emerging technology that uses microtissues or micro-precursor tissues as building blocks to construct a macro-tissue. We summarize and highlight the application of bioassembly technology in regenerating articular cartilage. We discuss the advantages of bioassembly and present two types of building blocks: multiple cellular scaffold-free spheroids and cell-laden polymer or hydrogel microspheres. We present techniques for generating building blocks and bioassembly methods, including bioprinting and non-bioprinting techniques. Using a data set of 5069 articles from the last 28 years of literature, we analyzed seven categories of related research, and the year trends are presented. The limitations and future directions of this technology are also discussed.

3D in-vitro cultures of human bone marrow and Wharton’s jelly derived mesenchymal stromal cells show high chondrogenic potential

In this study, chondrogenic potentials of 3D high-density cultures of Bone Marrow (BM) and Wharton’s Jelly (WJ)-derived mesenchymal stromal cells (MSCs) was investigated by chondrogenesis- and cytokine-related gene expression over a 16-day culture period supplemented with human transforming growth factor (hTGF)-β1 at 10 ng/ml. In BM-MSC 3D models, a marked upregulation of chondrogenesis-related genes, such as SOX9, COL2A1, and ACAN (all p < 0.05) and formation of spherical pellets with structured type II collagen fibers were observed. Similarly, WJ-based high-density culture appeared higher in size and more regular in shape, with a significant overexpression of COL2A1 and ACAN (all p < 0.05) at day 16. Moreover, a similar upregulation trend was documented for IL-6 and IL-10 expression in both BM and WJ 3D systems. In conclusion, MSC-based high-density cultures can be considered a promising in vitro model of cartilage regeneration and tissue engineering. Moreover, our data support the use of WJ-MSCs as a valid alternative for chondrogenic commitment of stem cells in regenerative medicine.

Systematic review on the application of 3D-bioprinting technology in orthoregeneration: current achievements and open challenges

BACKGROUND

Joint degeneration and large or complex bone defects are a significant source of morbidity and diminished quality of life worldwide. There is an unmet need for a functional implant with near-native biomechanical properties. The potential for their generation using 3D bioprinting (3DBP)-based tissue engineering methods was assessed. We systematically reviewed the current state of 3DBP in orthoregeneration.

METHODS

This review was performed using PubMed and Web of Science. Primary research articles reporting 3DBP of cartilage, bone, vasculature, and their osteochondral and vascular bone composites were considered. Full text English articles were analyzed.

RESULTS

Over 1300 studies were retrieved, after removing duplicates, 1046 studies remained. After inclusion and exclusion criteria were applied, 114 articles were analyzed fully. Bioink material types and combinations were tallied. Cell types and testing methods were also analyzed. Nearly all papers determined the effect of 3DBP on cell survival. Bioink material physical characterization using gelation and rheology, and construct biomechanics were performed. In vitro testing methods assessed biochemistry, markers of extracellular matrix production and/or cell differentiation into respective lineages. In vivo proof-of-concept studies included full-thickness bone and joint defects as well as subcutaneous implantation in rodents followed by histological and µCT analyses to demonstrate implant growth and integration into surrounding native tissues.

CONCLUSIONS

Despite its relative infancy, 3DBP is making an impact in joint and bone engineering. Several groups have demonstrated preclinical efficacy of mechanically robust constructs which integrate into articular joint defects in small animals. However, notable obstacles remain. Notably, researchers encountered pitfalls in scaling up constructs and establishing implant function and viability in long term animal models. Further, to translate from the laboratory to the clinic, standardized quality control metrics such as construct stiffness and graft integration metrics should be established with investigator consensus. While there is much work to be done, 3DBP implants have great potential to treat degenerative joint diseases and provide benefit to patients globally.

Hydrogel Nanosheets Confined 2D Rhombic Ice: A New Platform Enhancing Chondrogenesis

Nanoconfinement within flexible interfaces is a key step towards exploiting confinement effects in several biological and technological systems wherein flexible 2D materials are frequently utilized but are arduous to prepare. Hitherto unreported, the synthesis of 2D Hydrogel nanosheets (HNS) using a template- and catalyst-free process is developed representing a fertile ground for fundamental structure-property investigations. In due course of time, nucleating folds propagating along the edges trigger co-operative deformations of HNS generating regions of nanoconfinement within trapped water islands. These severely constricting surfaces force water molecules to pack within the nanoscale regime of HNS almost parallel to the surface bringing about phase transition into puckered rhombic ice with AA and AB Bernal stacking pattern, which was mostly restricted to Molecular dynamics (MD) studies so far. Interestingly, under high lateral pressure and spatial inhomogeneity within nanoscale confinement, bilayer rhombic ice structures were formed with an in-plane lattice spacing of 0.31 nm. In this work, a systematic exploration of rhombic ice formation within HNS has been delineated using High-resolution transmission electron microscopy (HRTEM), and its ultrathin morphology was examined using Atomic Force Microscopy (AFM). Scanning Electron Microscopy (SEM) images revealed high porosity while mechanical testing presented young's modulus of 155 kPa with ~84% deformation, whereas contact angle suggested high hydrophilicity. The combinations of nanosheets, porosity, nanoconfinement, hydrophilicity, and mechanical strength, motivated us to explore their application as a scaffold for cartilage regeneration, by inducing chondrogenesis of human Wharton Jelly derived mesenchymal stem cells (hWJ MSCs). HNS promoted the formation of cell aggregates giving higher number of spheroid formation and a marked expression of chondrogenic markers (ColI, ColII, ColX, ACAN and S-100), thereby providing some cues for guiding chondrogenic differentiation.

A platform for automated and label-free monitoring of morphological features and kinetics of spheroid fusion

Spheroids are widely applied as building blocks for biofabrication of living tissues, where they exhibit spontaneous fusion toward an integrated structure upon contact. Tissue fusion is a fundamental biological process, but due to a lack of automated monitoring systems, the in-depth characterization of this process is still limited. Therefore, a quantitative high-throughput platform was developed to semi-automatically select doublet candidates and automatically monitor their fusion kinetics. Spheroids with varying degrees of chondrogenic maturation (days 1, 7, 14, and 21) were produced from two different cell pools, and their fusion kinetics were analyzed via the following steps: (1) by applying a novel spheroid seeding approach, the background noise was decreased due to the removal of cell debris while a sufficient number of doublets were still generated. (2) The doublet candidates were semi-automatically selected, thereby reducing the time and effort spent on manual selection. This was achieved by automatic detection of the microwells and building a random forest classifier, obtaining average accuracies, sensitivities, and precisions ranging from 95.0% to 97.4%, from 51.5% to 92.0%, and from 66.7% to 83.9%, respectively. (3) A software tool was developed to automatically extract morphological features such as the doublet area, roundness, contact length, and intersphere angle. For all data sets, the segmentation procedure obtained average sensitivities and precisions ranging from 96.8% to 98.1% and from 97.7% to 98.8%, respectively. Moreover, the average relative errors for the doublet area and contact length ranged from 1.23% to 2.26% and from 2.30% to 4.66%, respectively, while the average absolute errors for the doublet roundness and intersphere angle ranged from 0.0083 to 0.0135 and from 10.70 to 13.44°, respectively. (4) The data of both cell pools were analyzed, and an exponential model was used to extract kinetic parameters from the time-series data of the doublet roundness. For both cell pools, the technology was able to characterize the fusion rate and quality in an automated manner and allowed us to demonstrate that an increased chondrogenic maturity was linked with a decreased fusion rate. The platform is also applicable to other spheroid types, enabling an increased understanding of tissue fusion. Finally, our approach to study spheroid fusion over time will aid in the design of controlled fabrication of “assembloids” and bottom-up biofabrication of living tissues using spheroids.

Engineering bone-forming callus organoid implants in a xenogeneic-free differentiation medium

The field of tissue engineering aspires to provide clinically relevant solutions for patients through the integration of developmental engineering principles with a bottom-up manufacturing approach. However, the manufacturing of cell-based advanced therapy medicinal products is hampered by protocol complexity, lack of non-invasive critical quality controls, and dependency on animal-derived components for tissue differentiation. We investigate a serum-free, chemically defined, xeno- and lipid-free chondrogenic differentiation medium to generate bone-forming callus organoids. Our results show an increase in microtissue homogeneity during prolonged differentiation and the high quality of in vivo bone-forming organoids. The low protein content of the culture medium potentially allows for the monitoring of relevant secreted biomarkers as (critical) quality attributes. Together, we envisage that this xeno- and lipid-free chondrogenic medium is compatible with industrial scale-up and automation while facilitating the implementation of non-invasive imaging and the use of quality control parameters based on secreted biomarkers.

Mathematical and computational models in spheroid-based biofabrication

Ubiquitous in embryonic development, tissue fusion is of interest to tissue engineers who use tissue spheroids or organoids as building blocks of three-dimensional (3D) multicellular constructs. This review presents mathematical models and computer simulations of the fusion of tissue spheroids. The motivation of this study stems from the need to predict the post-printing evolution of 3D bioprinted constructs. First, we provide a brief overview of differential adhesion, the main morphogenetic mechanism involved in post-printing structure formation. It will be shown that clusters of cohesive cells behave as an incompressible viscous fluid on the time scale of hours. The discussion turns then to mathematical models based on the continuum hydrodynamics of highly viscous liquids and on statistical mechanics. Next, we analyze the validity and practical use of computational models of multicellular self-assembly in live constructs created by tissue spheroid bioprinting. Finally, we discuss the perspectives of the field as machine learning starts to reshape experimental design, and modular robotic workstations tend to alleviate the burden of repetitive tasks in biofabrication. STATEMENT OF SIGNIFICANCE: Bioprinted constructs are living systems, which evolve via morphogenetic mechanisms known from developmental biology. This review presents mathematical and computational tools devised for modeling post-printing structure formation. They help achieving a desirable outcome without expensive optimization experiments. While previous reviews mainly focused on assumptions, technical details, strengths, and limitations of computational models of multicellular self-assembly, this article discusses their validity and practical use in biofabrication. It also presents an overview of mathematical models that proved to be useful in the evaluation of experimental data on tissue spheroid fusion, and in the calibration of computational models. Finally, the perspectives of the field are discussed in the advent of robotic biofabrication platforms and bioprinting process optimization by machine learning.

Lipid nano-vesicles for thyroid hormone encapsulation: A comparison between different fabrication technologies, drug loading, and an in vitro delivery to human tendon stem/progenitor cells in 2D and 3D culture

Phosphatidylcholine (PC) vesicles loaded with Triiodothyronine (T3) were fabricated using different manufacturing methods: thin layer hydration plus sonication (TF-UF), supercritical liposome formation (SC), and microfluidic technology (MF). Vesicles obtained by MF had the lowest mean diameter (88.61 ± 44.48 nm) with a Zeta Potential of -20.1 ± 5.90 mV and loading of 10 mg/g (encapsulation efficiency: 57%). In contrast, SC vesicles showed extremely low encapsulation efficiency (<10%) probably due to T3 solubility in ethanol/carbon dioxide mixture; despite TF-UF vesicles exhibiting good size (167.7 ± 90 nm; Zp -8.50 ± 0.60 mV) and loading (10 mg/g), poor mass recovery was obtained (50% loss). MF vesicles had low cytotoxicity, and they were well enough internalized by both HeLa and human tendon stem/progenitor cells (hTSPCs). Their biological activity was also monitored in both 2D and 3D cultures of hTSPCs supplemented with therapeutical concentrations of PC/T3 nano-liposomes. 2D culture showed almost similar constitutive gene expression compared to control culture supplemented with free-T3. On the contrary, when hTPSCs 3D culture was assembled, it showed a more evident homogeneous distribution of FITC labeled vesicles within the high-density structure and a significant upregulation of cell constitutive genes, such as type I Collagen (4.8-fold; p < 0.0001) at day 7, compared to the control, suggesting that T3/PC formulation has increased T3 cytosolic concentration, thus improving cells metabolic activity. The study supported MF technology for nano-carriers fabrication and opens perspectives on the activity of PC/T3 nano-vesicles as innovative formulations for TPSCs stimulation in ECM secretion.

Photo-Crosslinkable Hydrogels for 3D Bioprinting in the Repair of Osteochondral Defects: A Review of Present Applications and Future Perspectives

An osteochondral defect is a common and frequent disease in orthopedics and treatment effects are not good, which can be harmful to patients. Hydrogels have been applied in the repair of cartilage defects. Many studies have reported that hydrogels can effectively repair osteochondral defects through loaded cells or non-loaded cells. As a new type of hydrogel, photo-crosslinked hydrogel has been widely applied in more and more fields. Meanwhile, 3D bioprinting serves as an attractive platform to fabricate customized tissue-engineered substitutes from biomaterials and cells for the repair or replacement of injured tissues and organs. Although photo-crosslinkable hydrogel-based 3D bioprinting has some advantages for repairing bone cartilage defects, it also has some disadvantages. Our aim of this paper is to review the current status and prospect of photo-crosslinkable hydrogel-based 3D bioprinting for repairing osteochondral defects.

Stem Cell-Laden Hydrogel-Based 3D Bioprinting for Bone and Cartilage Tissue Engineering

Tremendous advances in tissue engineering and regenerative medicine have revealed the potential of fabricating biomaterials to solve the dilemma of bone and articular defects by promoting osteochondral and cartilage regeneration. Three-dimensional (3D) bioprinting is an innovative fabrication technology to precisely distribute the cell-laden bioink for the construction of artificial tissues, demonstrating great prospect in bone and joint construction areas. With well controllable printability, biocompatibility, biodegradability, and mechanical properties, hydrogels have been emerging as an attractive 3D bioprinting material, which provides a favorable biomimetic microenvironment for cell adhesion, orientation, migration, proliferation, and differentiation. Stem cell-based therapy has been known as a promising approach in regenerative medicine; however, limitations arise from the uncontrollable proliferation, migration, and differentiation of the stem cells and fortunately could be improved after stem cells were encapsulated in the hydrogel. In this review, our focus was centered on the characterization and application of stem cell-laden hydrogel-based 3D bioprinting for bone and cartilage tissue engineering. We not only highlighted the effect of various kinds of hydrogels, stem cells, inorganic particles, and growth factors on chondrogenesis and osteogenesis but also outlined the relationship between biophysical properties like biocompatibility, biodegradability, osteoinductivity, and the regeneration of bone and cartilage. This study was invented to discuss the challenge we have been encountering, the recent progress we have achieved, and the future perspective we have proposed for in this field.

Design and Synthesis of Stem Cell-Laden Keratin/Glycol Chitosan Methacrylate Bioinks for 3D Bioprinting

With the advancements in tissue engineering and three-dimensional (3D) bioprinting, physiologically relevant three-dimensional structures with suitable mechanical and bioactive properties that mimic the biological tissue can be designed and fabricated. However, the available bioinks are less than demanded. In this research, the readily available biomass sources, keratin and glycol chitosan, were selected to develop a UV-curable hydrogel that is feasible for the 3D bioprinting process. Keratin methacrylate and glycol chitosan methacrylate were synthesized, and a hybrid bioink was created by combining this protein-polysaccharide cross-linked hydrogel. While human hair keratin could provide biological functions, the other composition, glycol chitosan, could further enhance the mechanical strength of the construct. The mechanical properties, degradation profile, swelling behavior, cell viability, and proliferation were investigated with various ratios of keratin methacrylate to glycol chitosan methacrylate. The composition of 2% (w/v) keratin methacrylate and 2% (w/v) chitosan methacrylate showed a significantly higher cell number and swelling percentage than other compositions and was designated as the bioink for 3D printing afterward. The feasibility of stem cell loading in the selected formula was examined with an extrusion-based bioprinter. The cells and spheroids can be successfully printed with the synthesized bioink into a specific shape and cultured. This work provides a potential option for bioinks and delivers insights into personalization research on stem cell-laden biofabricated hydrogels in the future.

Two-Dimensional and Three-Dimensional Cartilage Model Platforms for Drug Evaluation and High-Throughput Screening Assays

Osteoarthritis (OA) is a severely painful and debilitating disease of the joint, which brings about degradation of the articular cartilage and currently has few therapeutic solutions. Two-dimensional (2D) high-throughput screening (HTS) assays have been widely used to identify candidate drugs with therapeutic potential for the treatment of OA. A number of small molecules which improve the chondrogenic differentiation of progenitor cells for tissue engineering applications have also been discovered in this way. However, due to the failure of these models to accurately represent the native joint environment, the efficacy of these drugs has been limited in vivo. Screening systems utilizing three-dimensional (3D) models, which more closely reflect the tissue and its complex cell and molecular interactions, have also been described. However, the vast majority of these systems fail to recapitulate the complex, zonal structure of articular cartilage and its unique cell population. This review summarizes current 2D HTS techniques and addresses the question of how to use existing 3D models of tissue-engineered cartilage to create 3D drug screening platforms with improved outcomes. Impact statement Currently, the use of two-dimensional (2D) screening platforms in drug discovery is common practice. However, these systems often fail to predict efficacy in vivo, as they do not accurately represent the complexity of the native three-dimensional (3D) environment. This article describes existing 2D and 3D high-throughput systems used to identify small molecules for osteoarthritis treatment or in vitro chondrogenic differentiation, and suggests ways to improve the efficacy of these systems based on the most recent research.

Meniscus regeneration by 3D printing technologies: Current advances and future perspectives

Meniscal tears are a frequent orthopedic injury commonly managed by conservative strategies to avoid osteoarthritis development descending from altered biomechanics. Among cutting-edge approaches in tissue engineering, 3D printing technologies are extremely promising guaranteeing for complex biomimetic architectures mimicking native tissues. Considering the anisotropic characteristics of the menisci, and the ability of printing over structural control, it descends the intriguing potential of such vanguard techniques to meet individual joints’ requirements within personalized medicine. This literature review provides a state-of-the-art on 3D printing for meniscus reconstruction. Experiences in printing materials/technologies, scaffold types, augmentation strategies, cellular conditioning have been compared/discussed; outcomes of pre-clinical studies allowed for further considerations. To date, translation to clinic of 3D printed meniscal devices is still a challenge: meniscus reconstruction is once again clear expression of how the integration of different expertise (e.g., anatomy, engineering, biomaterials science, cell biology, and medicine) is required to successfully address native tissues complexities.

A Microfluidic Device to Fabricate One-Step Cell Bead-Laden Hydrogel Struts for Tissue Engineering

Cell-laden structures are widely applied for a variety of tissue engineering applications, including tissue restoration. Cell-to-cell interactions in bioprinted structures are important for successful tissue restoration, because cell-cell signaling pathways can regulate tissue development and stem cell fate. However, the low degree of cell-cell interaction in conventional cell-laden bioprinted structures is challenging for the therapeutic application of this modality. Herein, a microfluidic device with cell-laden methacrylated gelatin (GelMa) bioink and alginate as a matrix hydrogel is used to fabricate a functional hybrid structure laden with cell-aggregated microbeads. This approach effectively increases the degree of cell-to-cell interaction to a level comparable to cell spheroids. The hybrid structure is obtained using a one-step process without the exhausting procedure. It consists of cell bead fabrication and an extrusion process for the cell-bead laden structure. Different flow rates are appropriately selected to develop cell-laden struts with homogeneously distributed cell beads for each hydrogel in the process. The hybrid struts exhibit significantly higher cellular activities than those of conventional alginate/GelMa struts, which are bioprinted using similar cell densities and bioink formulations. Furthermore, hybrid struts with adipose stem cells are implanted into mice, resulting in significantly higher myogenesis in comparison to normally bioprinted struts.

The Developing Field of Scaffold-Free Tissue Engineering for Articular Cartilage Repair

Articular cartilage is critical for proper joint mobility as it provides a smooth and lubricated surface between articulating bones and allows for transmission of load to underlying bones. Extended wear or injury of this tissue can result in osteoarthritis, a degenerative disease affecting millions across the globe. Because of its low regenerative capacity, articular cartilage cannot heal on its own and effective treatments for injured joint restoration remain a challenge. Strategies in tissue engineering have been demonstrated as potential therapeutic approaches to regenerate and repair damaged articular cartilage. Although many of these strategies rely on the use of an exogenous three-dimensional scaffolds to regenerate cartilage, scaffold-free tissue engineering provides numerous advantages over scaffold-based methods. This review highlights the latest advancements in scaffold-free tissue engineering for cartilage and the potential for clinical translation.

Chondrocyte Spheroids Laden in GelMA/HAMA Hybrid Hydrogel for Tissue-Engineered Cartilage with Enhanced Proliferation, Better Phenotype Maintenance, and Natural Morphological Structure

Three-dimensional cell-laden tissue engineering has become an extensive research direction. This study aimed to evaluate whether chondrocyte spheroids (chondro-spheroids) prepared using the hanging-drop method could develop better cell proliferation and morphology maintenance characteristics, and be optimized as a micro unit for cartilage tissue engineering. Chondro-spheroids were loaded into a cross-linkable hybrid hydrogel of gelatin methacrylate (GelMA) and hyaluronic acid methacrylate (HAMA) in vivo and in vitro. Cell proliferation, aggregation, cell morphology maintenance as well as cartilage-related gene expression and matrix secretion in vitro and in vivo were evaluated. The results indicated that compared with chondrocyte-laden hydrogel, chondro-spheroid-laden hydrogel enhanced proliferation, had better phenotype maintenance, and a more natural morphological structure, which made it appropriate for use as a micro unit in cartilage tissue engineering.

Translational Application of 3D Bioprinting for Cartilage Tissue Engineering

Cartilage is an avascular tissue with extremely limited self-regeneration capabilities. At present, there are no existing treatments that effectively stop the deterioration of cartilage or reverse its effects; current treatments merely relieve its symptoms and surgical intervention is required when the condition aggravates. Thus, cartilage damage remains an ongoing challenge in orthopaedics with an urgent need for improved treatment options. In recent years, major advances have been made in the development of three-dimensional (3D) bioprinted constructs for cartilage repair applications. 3D bioprinting is an evolutionary additive manufacturing technique that enables the precisely controlled deposition of a combination of biomaterials, cells, and bioactive molecules, collectively known as bioink, layer-by-layer to produce constructs that simulate the structure and function of native cartilage tissue. This review provides an insight into the current developments in 3D bioprinting for cartilage tissue engineering. The bioink and construct properties required for successful application in cartilage repair applications are highlighted. Furthermore, the potential for translation of 3D bioprinted constructs to the clinic is discussed. Overall, 3D bioprinting demonstrates great potential as a novel technique for the fabrication of tissue engineered constructs for cartilage regeneration, with distinct advantages over conventional techniques.

The Application of Cartilage Tissue Engineering with Cell-Laden Hydrogel in Plastic Surgery: A Systematic Review

BACKGROUND

As a contour-supporting material, the cartilage has a significant application value in plastic surgery. Since the development of hydrogel scaffolds with sufficient biomechanical strength and high biocompatibility, cell-laden hydrogels have been widely studied for application in cartilage bioengineering. This systematic review summarizes the latest research on engineered cartilage constructed using cell-laden hydrogel scaffolds in plastic surgery.

METHODS

A systematic review was performed by searching the PubMed and Web of Science databases using selected keywords and Medical Subject Headings search terms.

RESULTS

Forty-two studies were identified based on the search criteria. After full-text screening for inclusion and exclusion criteria, 18 studies were included. Data collected from each study included culturing form, seed cell types and sources, concentration of cells and gels, scaffold materials and bio-printing structures, and biomechanical properties of cartilage constructs. These cell-laden hydrogel scaffolds were reported to show some feasibility of cartilage engineering, including better cell proliferation, enhanced deposition of glycosaminoglycans and collagen type II in the extracellular matrix, and better biomechanical properties close to the natural state.

CONCLUSION

Cell-laden hydrogels have been widely used in cartilage bioengineering research. Through 3-dimensional (3D) printing, the cell-laden hydrogel can form a bionic contour structure. Extracellular matrix expression was observed in vivo and in vitro, and the elastic modulus was reported to be similar to that of natural cartilage. The future direction of cartilage tissue engineering in plastic surgery involves the use of novel hydrogel materials and more advanced 3D printing technology combined with biochemistry and biomechanical stimulation.

Engineering microvasculature by 3D bioprinting of prevascularized spheroids in photo-crosslinkable gelatin

To engineer tissues with clinically relevant dimensions by three-dimensional bioprinting, an extended vascular network with diameters ranging from the macro- to micro-scale needs to be integrated. Extrusion-based bioprinting is the most commonly applied bioprinting technique but due to the limited resolution of conventional bioprinters, the establishment of a microvascular network for the transfer of oxygen, nutrients and metabolic waste products remains challenging. To answer this need, this study assessed the potential and processability of spheroids, containing a capillary-like network, to be used as micron-sized prevascularized units for incorporation throughout the bioprinted construct. Prevascularized spheroids were generated by combining endothelial cells with fibroblasts and adipose tissue-derived mesenchymal stem cells as supporting cells. To serve as a viscous medium for the bioink-based deposition by extrusion printing, spheroids were combined with a photo-crosslinkable methacrylamide-modified gelatin (gelMA) and Irgacure 2959. The influence of gelMA encapsulation, the printing process and photo-crosslinking conditions on spheroid viability, proliferation and vascularization were analyzed by live/dead staining, immunohistochemistry, gene expression analysis and sprouting analysis. Stable spheroid-laden constructs, allowing spheroid outgrowth, were achieved by applying 10 min UV-A photo-curing (365 nm, 4 mW cm^-2), while the construct was incubated in an additional Irgacure 2959 immersion solution. Following implantationin ovoonto a chick chorioallantoic membrane, the prevascular engineered constructs showed anastomosis with the host vasculature. This study demonstrated (a) the potential of triculture prevascularized spheroids for application as multicellular building blocks, (b) the processability of the spheroid-laden gelMA bioink by extrusion bioprinting and (c) the importance of photo-crosslinking parameters post printing, as prolonged photo-curing intervals showed to be detrimental for the angiogenic potential and complete vascularization of the construct post printing.

Cartilaginous spheroid-assembly design considerations for endochondral ossification: towards robotic-driven biomanufacturing

Spheroids have become essential building blocks for biofabrication of functional tissues. Spheroid formats allow high cell-densities to be efficiently engineered into tissue structures closely resembling the native tissues. In this work, we explore the assembly capacity of cartilaginous spheroids (d∼ 150µm) in the context of endochondral bone formation. The fusion capacity of spheroids at various degrees of differentiation was investigated and showed decreased kinetics as well as remodeling capacity with increased spheroid maturity. Subsequently, design considerations regarding the dimensions of engineered spheroid-based cartilaginous mesotissues were explored for the corresponding time points, defining critical dimensions for these type of tissues as they progressively mature. Next, mesotissue assemblies were implanted subcutaneously in order to investigate the influence of spheroid fusion parameters on endochondral ossification. Moreover, as a step towards industrialization, we demonstrated a novel automated image-guided robotics process, based on targeting and registering single-spheroids, covering the range of spheroid and mesotissue dimensions investigated in this work. This work highlights a robust and automated high-precision biomanufacturing roadmap for producing spheroid-based implants for bone regeneration.

miRNA induced co-differentiation and cross-talk of adipose tissue-derived progenitor cells for 3D heterotypic pre-vascularized bone formation

Engineered bone grafts require a vascular network to supply cells with oxygen, nutrients and remove waste. Using heterotypic mature cells to create these graftsin vivohas resulted in limited cell density, ectopic tissue formation and disorganized tissue. Despite evidence that progenitor cell aggregates, such as progenitor spheroids, are a potential candidate for fabrication of native-like pre-vascularized bone tissue, the factors dictating progenitor co-differentiation to create heterotypic pre-vascularized bone tissue remains poorly understood. In this study, we examined a three-dimensional heterotypic pre-vascularized bone tissue model, using osteogenic and endotheliogenic progenitor spheroids induced by miR-148b and miR-210 mimic transfection, respectively. Spheroids made of transfected cells were assembled into heterotypic structures to determine the impact on co-differentiation as a function of micro-RNA (miRNA) mimic treatment group and induction time. Our results demonstrated that miRNAs supported the differentiation in heterotypic structures, and that developing heterotypic structures is determined in part by progenitor maturity, as confirmed by gene and protein markers of osteogenic and endotheliogenic differentiation and the mineralization assay. As a proof of concept, miRNA-transfected spheroids were also bioprinted using aspiration-assisted bioprinting and organized into hollow structures to mimic the Haversian canal. Overall, the presented approach could be useful in fabrication of vascularized bone tissue using spheroids as building blocks.

Towards the Experimentally-Informed In Silico Nozzle Design Optimization for Extrusion-Based Bioprinting of Shear-Thinning Hydrogels

Research in bioprinting is booming due to its potential in addressing several manufacturing challenges in regenerative medicine. However, there are still many hurdles to overcome to guarantee cell survival and good printability. For the 3D extrusion-based bioprinting, cell viability is amongst one of the lowest of all the bioprinting techniques and is strongly influenced by various factors including the shear stress in the print nozzle. The goal of this study is to quantify, by means of in silico modeling, the mechanical environment experienced by the bioink during the printing process. Two ubiquitous nozzle shapes, conical and blunted, were considered, as well as three common hydrogels with material properties spanning from almost Newtonian to highly shear-thinning materials following the power-law behavior: Alginate-Gelatin, Alginate and PF127. Comprehensive in silico testing of all combinations of nozzle geometry variations and hydrogels was achieved by combining a design of experiments approach (DoE) with a computational fluid dynamics (CFD) of the printing process, analyzed through a machine learning approach named Gaussian Process. Available experimental results were used to validate the CFD model and justify the use of shear stress as a surrogate for cell survival in this study. The lower and middle nozzle radius, lower nozzle length and the material properties, alone and combined, were identified as the major influencing factors affecting shear stress, and therefore cell viability, during printing. These results were successfully compared with those of reported experiments testing viability for different nozzle geometry parameters under constant flow rate or constant pressure. The in silico 3D bioprinting platform developed in this study offers the potential to assist and accelerate further development of 3D bioprinting.