Engineering large cartilage tissues using dynamic bioreactor culture at defined oxygen conditions

Physiologic Doses of Transforming Growth Factor-β Improve the Composition of Engineered Articular Cartilage

Conventionally, for cartilage tissue engineering applications, transforming growth factor beta (TGF-β) is administered at doses that are several orders of magnitude higher than those present during native cartilage development. While these doses accelerate extracellular matrix (ECM) biosynthesis, they may also contribute to features detrimental to hyaline cartilage function, including tissue swelling, type I collagen (COL-I) deposition, cellular hypertrophy, and cellular hyperplasia. In contrast, during native cartilage development, chondrocytes are exposed to moderate TGF-β levels, which serve to promote strong biosynthetic enhancements while mitigating risks of pathology associated with TGF-β excesses. Here, we examine the hypothesis that physiologic doses of TGF-β can yield neocartilage with a more hyaline cartilage-like composition and structure relative to conventionally administered supraphysiologic doses. This hypothesis was examined on a model system of reduced-size constructs (∅2 × 2 mm or ∅3 × 2 mm) comprised of bovine chondrocytes encapsulated in agarose, which exhibit mitigated TGF-β spatial gradients allowing for an evaluation of the intrinsic effect of TGF-β doses on tissue development. Reduced-size (∅2 × 2 mm or ∅3 × 2 mm) and conventional-size constructs (∅4-∅6 mm × 2 mm) were subjected to a range of physiologic (0.1, 0.3, 1 ng/mL) and supraphysiologic (3, 10 ng/mL) TGF-β doses. At day 56, the physiologic 0.3 ng/mL dose yielded reduced-size constructs with native cartilage-matched Young's modulus (EY) (630 ± 58 kPa) and sulfated glycosaminoglycan (sGAG) content (5.9 ± 0.6%) while significantly increasing the sGAG-to-collagen ratio, leading to significantly reduced tissue swelling relative to constructs exposed to the supraphysiologic 10 ng/mL TGF-β dose. Furthermore, reduced-size constructs exposed to the 0.3 ng/mL dose exhibited a significant reduction in fibrocartilage-associated COL-I and a 77% reduction in the fraction of chondrocytes present in a clustered morphology, relative to the supraphysiologic 10 ng/mL dose (p < 0.001). EY was significantly lower for conventional-size constructs exposed to physiologic doses due to TGF-β transport limitations in these larger tissues (p < 0.001). Overall, physiologic TGF-β appears to achieve an important balance of promoting requisite ECM biosynthesis, while mitigating features detrimental to hyaline cartilage function. While reduced-size constructs are not suitable for the repair of clinical-size cartilage lesions, insights from this work can inform TGF-β dosing requirements for emerging scaffold release or nutrient channel delivery platforms capable of achieving uniform delivery of physiologic TGF-β doses to larger constructs required for clinical cartilage repair.

Fabrication of polymeric microspheres for biomedical applications

Polymeric microspheres (PMs) have attracted great attention in the field of biomedicine in the last several decades due to their small particle size, special functionalities shown on the surface and high surface-to-volume ratio. However, how to fabricate PMs which can meet the clinical needs and transform laboratory achievements to industrial scale-up still remains a challenge. Therefore, advanced fabrication technologies are pursued. In this review, we summarize the technologies used to fabricate PMs, including emulsion-based methods, microfluidics, spray drying, coacervation, supercritical fluid and superhydrophobic surface-mediated method and their advantages and disadvantages. We also review the different structures, properties and functions of the PMs and their applications in the fields of drug delivery, cell encapsulation and expansion, scaffolds in tissue engineering, transcatheter arterial embolization and artificial cells. Moreover, we discuss existing challenges and future perspectives for advancing fabrication technologies and biomedical applications of PMs.

Perfusable Tissue Bioprinted into a 3D-Printed Tailored Bioreactor System

Bioprinting provides a powerful tool for regenerative medicine, as it allows tissue construction with a patient's specific geometry. However, tissue culture and maturation, commonly supported by dynamic bioreactors, are needed. We designed a workflow that creates an implant-specific bioreactor system, which is easily producible and customizable and supports cell cultivation and tissue maturation. First, a bioreactor was designed and different tissue geometries were simulated regarding shear stress and nutrient distribution to match cell culture requirements. These tissues were then directly bioprinted into the 3D-printed bioreactor. To prove the ability of cell maintenance, C2C12 cells in two bioinks were printed into the system and successfully cultured for two weeks. Next, human mesenchymal stem cells (hMSCs) were successfully differentiated toward an adipocyte lineage. As the last step of the presented strategy, we developed a prototype of an automated mobile docking station for the bioreactor. Overall, we present an open-source bioreactor system that is adaptable to a wound-specific geometry and allows cell culture and differentiation. This interdisciplinary roadmap is intended to close the gap between the lab and clinic and to integrate novel 3D-printing technologies for regenerative medicine.

Bioprinting of scaled-up meniscal grafts by spatially patterning phenotypically distinct meniscus progenitor cells within melt electrowritten scaffolds

Meniscus injuries are a common problem in orthopedic medicine and are associated with a significantly increased risk of developing osteoarthritis. While developments have been made in the field of meniscus regeneration, the engineering of cell-laden constructs that mimic the complex structure, composition and biomechanics of the native tissue remains a significant challenge. This can be linked to the use of cells that are not phenotypically representative of the different zones of the meniscus, and an inability to direct the spatial organization of engineered meniscal tissues. In this study we investigated the potential of zone-specific meniscus progenitor cells (MPCs) to generate functional meniscal tissue following their deposition into melt electrowritten (MEW) scaffolds. We first confirmed that fibronectin selected MPCs from the inner and outer regions of the meniscus maintain their differentiation capacity with prolonged monolayer expansion, opening their use within advanced biofabrication strategies. By depositing MPCs within MEW scaffolds with elongated pore shapes, which functioned as physical boundaries to direct cell growth and extracellular matrix production, we were able to bioprint anisotropic fibrocartilaginous tissues with preferentially aligned collagen networks. Furthermore, by using MPCs isolated from the inner (iMPCs) and outer (oMPCs) zone of the meniscus, we were able to bioprint phenotypically distinct constructs mimicking aspects of the native tissue. An iterative MEW process was then implemented to print scaffolds with a similar wedged-shaped profile to that of the native meniscus, into which we deposited iMPCs and oMPCs in a spatially controlled manner. This process allowed us to engineer sulfated glycosaminoglycan and collagen rich constructs mimicking the geometry of the meniscus, with MPCs generating a more fibrocartilage-like tissue compared to the mesenchymal stromal/stem cells. Taken together, these results demonstrate how the convergence of emerging biofabrication platforms with tissue-specific progenitor cells can enable the engineering of complex tissues such as the meniscus.

Progress in biomechanical stimuli on the cell-encapsulated hydrogels for cartilage tissue regeneration

BACKGROUND

Worldwide, many people suffer from knee injuries and articular cartilage damage every year, which causes pain and reduces productivity, life quality, and daily routines. Medication is currently primarily used to relieve symptoms and not to ameliorate cartilage degeneration. As the natural healing capacity of cartilage damage is limited due to a lack of vascularization, common surgical methods are used to repair cartilage tissue, but they cannot prevent massive damage followed by injury.

MAIN BODY

Functional tissue engineering has recently attracted attention for the repair of cartilage damage using a combination of cells, scaffolds (constructs), biochemical factors, and biomechanical stimuli. As cyclic biomechanical loading is the key factor in maintaining the chondrocyte phenotype, many studies have evaluated the effect of biomechanical stimulation on chondrogenesis. The characteristics of hydrogels, such as their mechanical properties, water content, and cell encapsulation, make them ideal for tissue-engineered scaffolds. Induced cell signaling (biochemical and biomechanical factors) and encapsulation of cells in hydrogels as a construct are discussed for biomechanical stimulation-based tissue regeneration, and several notable studies on the effect of biomechanical stimulation on encapsulated cells within hydrogels are discussed for cartilage regeneration.

CONCLUSION

Induction of biochemical and biomechanical signaling on the encapsulated cells in hydrogels are important factors for biomechanical stimulation-based cartilage regeneration.

Mini- and macro-scale direct perfusion bioreactors with optimized flow for engineering 3D tissues

Bioreactors enabling direct perfusion of cell suspensions or culture media through the pores of 3D scaffolds have long been used in tissue engineering to improve cell seeding efficiency as well as uniformity of cell distribution and tissue development. A macro-scale U-shaped bioreactor for cell culture under perfusion (U-CUP) has been previously developed. In that system, the geometry of the perfusion chamber results in rather uniform flow through most of the scaffold volume, but not in the peripheral regions. Here, the design of the perfusion chamber has been optimized to provide a more homogenous perfusion flow through the scaffold. Then, the design of this macro-scale flow-optimized perfusion bioreactor (macro-Flopper) has been miniaturized to create a mini-scale device (mini-Flopper) compatible with medium-throughput assays. Computational fluid dynamic (CFD) modeling of the new chamber design, including a porous scaffold structure, revealed that Flopper bioreactors provide highly homogenous flow speed, pressure, and shear stress. Finally, a proof-of-principle of the functionality of the Flopper systems by engineering endothelialized stromal tissues using human adipose tissue-derived stromal vascular fraction (SVF) cells has been offered. Preliminary evidence showing that flow optimization improves cell maintenance in the engineered tissues will have to be confirmed in future studies. In summary, two bioreactor models with optimized perfusion flow and complementary sizes have been proposed that might be exploited to engineer homogenous tissues and, in the case of the mini-Flopper, for drug testing assays with a limited amount of biological material.

Chondrogenic differentiation of human bone marrow MSCs in osteochondral implants under kinematic mechanical load is dependent on the underlying osteo component

Chondrogenic models utilizing human mesenchymal stromal cells (hMSCs) are often simplistic, with a single cell type and the absence of mechanical stimulation. Considering the articulating joint as an organ it would be beneficial to include more complex stimulation. Within this study we applied clinically relevant kinematic load to biphasic constructs. In each case, the upper layer consisted of fibrin embedded hMSCs retained within an elastomeric polyurethane (PU) scaffold. These were randomly assigned to five base scaffolds, a cell-free fibrin PU base, viable bone, decellularized bone, 3D printed calcium phosphate or clinically used cement. This allowed the study of cross talk between viable bone and chondrogenically differentiating MSCs, while controlling for the change in stiffness of the base material. Data obtained showed that the bulk stiffness of the construct was not the defining factor in the response obtained, with viable and decellularized bone producing similar results to the softer PU base. However, the stiff synthetic materials led to reduced chondrogenesis and increased calcification in the upper MSC seeded layer. This demonstrates that the underlying base material must be considered when driving chondrogenesis of human cells using a clinically relevant loading protocol. It also indicates that the material used for bony reconstruction of osteochondral defects may influence subsequent chondrogenic potential.

Tracheal reconstruction with pedicled tandem grafts engineered by a radial stretch bioreactor

The engineering of tracheal substitutes is pivotal in improving tracheal reconstruction. In this study, we aimed to investigate the effects of biomechanical stimulation on tissue engineering tracheal cartilage by mimicking the trachea motion through a novel radial stretching bioreactor, which enables to dynamically change the diameter of the hollow cylindrical implants. Applying our bioreactor, we demonstrated that chondrocytes seeded on the surface of Poly (ε-caprolactone) scaffold respond to mechanical stimulation by improvement of infiltration into implants and upregulation of cartilage-specific genes. Further, the mechanical stimulation enhanced the accumulation of cartilage neo-tissues and cartilage-specific extracellular macromolecules in the muscle flap-remodeled implants and reconstructed trachea. Nevertheless, the invasion of fibrous tissues in the reconstructed trachea was suppressed upon mechanical loading.

Particulate ECM biomaterial ink is 3D printed and naturally crosslinked to form structurally-layered and lubricated cartilage tissue mimics

Articular cartilage is a layered tissue with a complex, heterogenous structure and lubricated surface which is challenging to reproduce using traditional tissue engineering methods. 3D printing techniques have enabled engineering of complex scaffolds for cartilage regeneration, but constructs fail to replicate the unique zonal layers, and limited cytocompatible crosslinkers exist. To address the need for mechanically robust, layered scaffolds, we developed an extracellular matrix particle-based biomaterial ink (pECM biomaterial ink) which can be extruded, polymerizes via disulfide bonding, and restores surface lubrication. Our cartilage pECM biomaterial ink utilizes functionalized hyaluronan, a naturally occurring glycosaminoglycan, crosslinked directly to decellularized tissue particles (ø 40-100 µm). We experimentally determined that hyaluronan functionalized with thiol groups (t-HA) forms disulfide bonds with the ECM particles to form a 3D network. We show that two inks can be co-printed to create a layered cartilage scaffold with bulk compressive and surface (friction coefficient, adhesion, and roughness) mechanics approaching values measured on native cartilage. We demonstrate that our printing process enables the addition of macropores throughout the construct, increasing the viability of introduced cells by 10%. The delivery of these 3D printed scaffolds to a defect is straightforward, customizable to any shape, and adheres to surrounding tissue.

Application of Alginate Hydrogels for Next-Generation Articular Cartilage Regeneration

The articular cartilage has insufficient intrinsic healing abilities, and articular cartilage injuries often progress to osteoarthritis. Alginate-based scaffolds are attractive biomaterials for cartilage repair and regeneration, allowing for the delivery of cells and therapeutic drugs and gene sequences. In light of the heterogeneity of findings reporting the benefits of using alginate for cartilage regeneration, a better understanding of alginate-based systems is needed in order to improve the approaches aiming to enhance cartilage regeneration with this compound. This review provides an in-depth evaluation of the literature, focusing on the manipulation of alginate as a tool to support the processes involved in cartilage healing in order to demonstrate how such a material, used as a direct compound or combined with cell and gene therapy and with scaffold-guided gene transfer procedures, may assist cartilage regeneration in an optimal manner for future applications in patients.

Additive manufacturing of cartilage-mimetic scaffolds as off-the-shelf implants for joint regeneration

Biomimetic scaffolds that provide a tissue-specific environment to cells are particularly promising for tissue engineering and regenerative medicine applications. The goal of this study was to integrate emerging additive manufacturing and biomaterial design strategies to produce articular cartilage (AC) mimetic scaffolds that could be used as 'off-the-shelf' implants for joint regeneration. To this end alginate sulfate, a sulfated glycosaminoglycan (sGAG) mimic, was used to functionalize porous alginate-based scaffolds and to support the sustained release of transforming growth factor-β3 (TGF-β3). Covalent crosslinking dramatically improved the elasticity of the alginate/alginate sulfate scaffolds, while scaffold architecture could be tailored using a directional freezing technique. Introducing such an anisotropic architecture was found to promote mesenchymal stem cell (MSC) infiltration into the scaffold and to direct the orientation of the deposited extracellular matrix, leading to the development of cartilage tissue with a biomimetic zonal architecture. In vitro experiments also demonstrated the capacity of the sulfated scaffolds to both enhance chondrogenesis of MSCs and to control the release of TGF-β3, leading to the development of a tissue rich in sGAG and type II collagen. The scaffolds were further reinforced with a 3D printed PLCL framework, leading to composite implants that were more elastic than those reinforced with PCL, and which better mimicked the bulk mechanical properties of native cartilage tissue. The ability of this composite scaffold to support chondrogenesis was then confirmed within a dynamic culture system. Altogether, these findings demonstrate the potential of such biomimetic scaffolds as putative 'single-stage' or 'off-the-shelf' strategies for articular cartilage regeneration.

Bioprinting of biomimetic self-organised cartilage with a supporting joint fixation device

Despite sustained efforts, engineering truly biomimetic articular cartilage (AC) via traditional top-down approaches remains challenging. Emerging biofabrication strategies, from 3D bioprinting to scaffold-free approaches that leverage principles of cellular self-organisation, are generating significant interest in the field of cartilage tissue engineering as a means of developing biomimetic tissue analoguesin vitro.Although such strategies have advanced the quality of engineered cartilage, recapitulation of many key structural features of native AC, in particular a collagen network mimicking the tissue's 'Benninghoff arcade', remains elusive. Additionally, a complete solution to fixating engineered cartilagesin situwithin damaged synovial joints has yet to be identified. This study sought to address both of these key challenges by engineering biomimetic AC within a device designed to anchor the tissue within a synovial joint defect. We first designed and fabricated a fixation device capable of anchoring engineered cartilage into the subchondral bone. Next, we developed a strategy for inkjet printing porcine mesenchymal stem/stromal cells (MSCs) into this supporting fixation device, which was also designed to provide instructive cues to direct the self-organisation of MSC condensations towards a stratified engineered AC. We found that a higher starting cell-density supported the development of a more zonally defined collagen network within the engineered tissue. Dynamic culture was implemented to further enhance the quality of this engineered tissue, resulting in an approximate 3 fold increase in glycosaminoglycan and collagen accumulation. Ultimately this strategy supported the development of AC that exhibited near-native levels of glycosaminoglycan accumulation (>5% WW), as well as a biomimetic collagen network organisation with a perpendicular to a parallel fibre arrangement (relative to the tissue surface) from the deep to superficial zones via arcading fibres within the middle zone of the engineered tissue. Collectively, this work demonstrates the successful convergence of novel biofabrication methods, bioprinting strategies and culture regimes to engineer a hybrid implant suited to resurfacing AC defects.

Enhanced cellular migration and prolonged chondrogenic differentiation in decellularized cartilage scaffolds under dynamic culture conditions

Lesions of aural, nasal and tracheal cartilage are frequently reconstructed by complex surgeries which are based on harvesting autologous cartilage from other locations such as the rib. Cartilage tissue engineering (CTE) is regarded as a promising alternative to attain vital cartilage. Nevertheless, CTE with nearly natural properties poses a significant challenge to research due to the complex reciprocal interactions between cells and extracellular matrix which have to be imitated and which are still not fully understood. Thus, we used a custom-made glass bioreactor to enhance cell migration into decellularized porcine cartilage scaffolds (DECM) and mimic physiological conditions. The DECM seeded with human nasal chondrocytes (HPCH) were cultured in the glass reactor for 6 weeks and examined by histological and immunohistochemical staining, biochemical analyses and real time-PCR at 14, 28 and 42 days. The migration depth and the number of migrated cells were quantified by computational analysis. Compared to the static cultivation, the dynamic culture (DC) fostered migration of HPCH into deeper tissue layers. Furthermore, cultivation in the bioreactor enhanced differentiation of the cells during the first 14 days, but differentiation diminished in the course of further cultivation. We consider the DC in the presented bioreactor as a promising tool to facilitate CTE and to help to better understand the complex physiological processes during cartilage regeneration. Maintaining differentiation of chondrocytes and improving cellular migration by further optimizing culture conditions is an important prerequisite for future clinical application.

Mechano-Hypoxia Conditioning of Engineered Human Meniscus

Meniscus fibrochondrocytes (MFCs) experience simultaneous hypoxia and mechanical loading in the knee joint. Experimental conditions based on these aspects of the native MFC environment may have promising applications in human meniscus tissue engineering. We hypothesized that in vitro “mechano-hypoxia conditioning” with mechanical loading such as dynamic compression (DC) and cyclic hydrostatic pressure (CHP) would enhance development of human meniscus fibrocartilage extracellular matrix in vitro. MFCs from inner human meniscus surgical discards were pre-cultured on porous type I collagen scaffolds with TGF-β3 supplementation to form baseline tissues with newly formed matrix that were used in a series of experiments. First, baseline tissues were treated with DC or CHP under hypoxia (HYP, 3% O₂) for 5 days. DC was the more effective load regime in inducing gene expression changes, and combined HYP/DC enhanced gene expression of fibrocartilage precursors. The individual treatments of DC and HYP regulated thousands of genes, such as chondrogenic markers SOX5/6, in an overwhelmingly additive rather than synergistic manner. Similar baseline tissues were then treated with a short course of DC (5 vs 60 min, 10–20% vs 30–40% strain) with different pre-culture duration (3 vs 6 weeks). The longer course of loading (60 min) had diminishing returns in regulating mechano-sensitive and inflammatory genes such as c-FOS and PTGS2, suggesting that as few as 5 min of DC was adequate. There was a dose-effect in gene regulation by higher DC strains, whereas outcomes were inconsistent for different MFC donors in pre-culture durations. A final set of baseline tissues was then cultured for 3 weeks with mechano-hypoxia conditioning to assess mechanical and protein-level outcomes. There were 1.8–5.1-fold gains in the dynamic modulus relative to baseline in HYP/DC, but matrix outcomes were equal or inferior to static controls. Long-term mechano-hypoxia conditioning was effective in suppressing hypertrophic markers (e.g., COL10A1 10-fold suppression vs static/normoxia). Taken together, these results indicate that appropriately applied mechano-hypoxia conditioning can support meniscus fibrocartilage development in vitro and may be useful as a strategy for developing non-hypertrophic articular cartilage using mesenchymal stem cells.

Perichondrium-inspired permeable nanofibrous tube well promoting differentiation of hiPSC-derived pellet toward hyaline-like cartilage pellet

The pellet formation has been regarded as a golden standard forin vitrochondrogenic differentiation. However, a spatially inhomogeneous chondrogenic microenvironment around a pellet resulted from the use of a traditional impermeable narrow tube, such as the conical tube, undermines the differentiation performance and therapeutic potential of differentiated cartilage pellet in defective articular cartilage treatment. To address this drawback, a perichondrium-inspired permeable nanofibrous tube (PINaT) well with a nanofibrous wall permeable to gas and soluble molecules is proposed. The PINaT well was fabricated with a micro deep drawing process where a flat thin nanofibrous membrane was transformed to a 3.5 mm deep tube well with a ∼50µm thick nanofibrous wall. Similar toin vivoperichondrium, the PINaT well was found to allow oxygen and growth factor diffusion required for chondrogenic differentiation across the entire nanofibrous wall. Analyses of gene expressions (COL2A1, COL10A1, ACAN, and SOX9), proteins (type II and X collagen), and glycosaminoglycans contents were conducted to assess the differentiation performance and clinical efficacy of differentiated cartilage pellet. The regulated spatially homogeneous chondrogenic microenvironment around the human induced pluripotent stem cell-derived pellet (3 × 10⁵cells per pellet) in the PINaT well remarkably improved the quality of the differentiated pellet toward a more hyaline-like cartilage pellet. Furthermore, an accelerated chondrogenic differentiation process of the pellet produced by the PINaT well was achieved for 14 days, demonstrating a hyaline cartilage-specific marker similar to the control pellet differentiated for 20 days. Finally, the enhanced clinical efficacy of the hyaline-like cartilage pellet was confirmed using an osteochondral defect rat model, with the repaired tissue resembling hyaline cartilage rather than fibrous cartilage after 8 weeks of regeneration.

Advances in biomechanical and biochemical engineering methods to stimulate meniscus tissue

Meniscal injuries can cause cartilage degeneration, which usually leads to the development of osteoarthritis (OA) and results in progressive destruction of the knee joint. Therefore, it is important to identify methods to stop or slow the development of OA after the onset of meniscal defects. The current surgical techniques for meniscal injuries are insufficient to prevent the progression of knee OA, which has accelerated the development of alternative tissue engineering strategies. Much progress has been made in the use of biomechanical and biochemical stimuli in the past decades to engineer neotissue akin to native meniscus. In this review, we focus on the current progress in biomechanical and biochemical stimuli-based strategies applied to meniscal tissue engineering, and explore how these factors influence meniscal regeneration. By understanding the functional mechanism that can stimulate regeneration in the meniscus, we hope that this review will provide a theoretical basis and strategies for meniscus tissue engineering.

Osteochondral Tissue Engineering: The Potential of Electrospinning and Additive Manufacturing

The socioeconomic impact of osteochondral (OC) damage has been increasing steadily over time in the global population, and the promise of tissue engineering in generating biomimetic tissues replicating the physiological OC environment and architecture has been falling short of its projected potential. The most recent advances in OC tissue engineering are summarised in this work, with a focus on electrospun and 3D printed biomaterials combined with stem cells and biochemical stimuli, to identify what is causing this pitfall between the bench and the patients' bedside. Even though significant progress has been achieved in electrospinning, 3D-(bio)printing, and induced pluripotent stem cell (iPSC) technologies, it is still challenging to artificially emulate the OC interface and achieve complete regeneration of bone and cartilage tissues. Their intricate architecture and the need for tight spatiotemporal control of cellular and biochemical cues hinder the attainment of long-term functional integration of tissue-engineered constructs. Moreover, this complexity and the high variability in experimental conditions used in different studies undermine the scalability and reproducibility of prospective regenerative medicine solutions. It is clear that further development of standardised, integrative, and economically viable methods regarding scaffold production, cell selection, and additional biochemical and biomechanical stimulation is likely to be the key to accelerate the clinical translation and fill the gap in OC treatment.

Hydrogels in the treatment of rheumatoid arthritis: drug delivery systems and artificial matrices for dynamic in vitro models

Rheumatoid arthritis (RA) is an autoimmune and chronic inflammatory disorder that mostly affects the synovial joints and can promote both cartilage and bone tissue destruction. Several conservative treatments are available to relieve pain and control the inflammation; however, traditional drugs administration are not fully effective and present severe undesired side effects. Hydrogels are a very attractive platform as a drug delivery system to guarantee these handicaps are reduced, and the therapeutic effect from the drugs is maximized. Furthermore, hydrogels can mimic the physiological microenvironment and have the mechanical behavior needed for use as cartilage in vitro model. The testing of these advanced delivery systems is still bound to animal disease models that have shown low predictability. Alternatively, hydrogel-based human dynamic in vitro systems can be used to model diseases, bypassing some of the animal testing problems. RA dynamic disease models are still in an embryonary stage since advances regarding healthy and inflamed cartilage models are currently giving the first steps regarding complexity increase. Herein, recent studies using hydrogels in the treatment of RA, featuring different hydrogel formulations are discussed. Besides, their use as artificial extracellular matrices in dynamic in vitro articular cartilage is also reviewed.

Applications of nanotechnology in 3D printed tissue engineering scaffolds

Tissue engineering is an interdisciplinary field that aims to combine life sciences and engineering to create therapies that regenerate functional tissue. Early work in tissue engineering mostly used materials as inert scaffolding structures, but research has shown that constructing scaffolds from biologically active materials can help with regeneration by enabling cell-scaffold interactions or release of factors that aid in regeneration. Three-dimensional (3D) printing is a promising technique for the fabrication of structurally intricate and compositionally complex tissue engineering scaffolds. Such scaffolds can be functionalized with techniques developed by nanotechnology research to further enhance their ability to stimulate regeneration and interact with cells. Nanotechnological components, nanoscale textures, and microscale/nanoscale printing can all be incorporated into the manufacture of 3D printed scaffolds. This review discusses recent advancements in the merging of nanotechnology with 3D printed tissue engineering scaffolds, with a focus on applications of nanoscale components, nanoscale texture, and innovative printing techniques and the effects observed in vitro and in vivo.

Turning Nature's own processes into design strategies for living bone implant biomanufacturing: a decade of Developmental Engineering

A decade after the term developmental engineering (DE) was coined to indicate the use of developmental processes as blueprints for the design and development of engineered living implants, a myriad of proof-of-concept studies demonstrate the potential of this approach in small animal models. This review provides an overview of DE work, focusing on applications in bone regeneration. Enabling technologies allow to quantify the distance between in vitro processes and their developmental counterpart, as well as to design strategies to reduce that distance. By embedding Nature's robust mechanisms of action in engineered constructs, predictive large animal data and subsequent positive clinical outcomes can be gradually achieved. To this end, the development of next generation biofabrication technologies should provide the necessary scale and precision for robust living bone implant biomanufacturing.

Oxygen releasing materials: Towards addressing the hypoxia-related issues in tissue engineering

Manufacturing macroscale cell-laden architectures is one of the biggest challenges faced nowadays in the domain of tissue engineering. Such living constructs, in fact, pose strict requirements for nutrients and oxygen supply that can hardly be addressed through simple diffusion in vitro or without a functional vasculature in vivo. In this context, in the last two decades, a substantial amount of work has been carried out to develop smart materials that could actively provide oxygen-release to contrast local hypoxia in large-size constructs. This review provides an overview of the currently available oxygen-releasing materials and their synthesis and mechanism of action, highlighting their capacities under in vitro tissue cultures and in vivo contexts. Additionally, we also showcase an emerging concept, herein termed as "living materials as releasing systems", which relies on the combination of biomaterials with photosynthetic microorganisms, namely algae, in an "unconventional" attempt to supply the damaged or re-growing tissue with the necessary supply of oxygen. We envision that future advances focusing on tissue microenvironment regulated oxygen-supplying materials would unlock an untapped potential for generating a repertoire of anatomic scale, living constructs with improved cell survival, guided differentiation, and tissue-specific biofunctionality.

Amino Acid- and Growth Factor-Based Multifunctional Nanocapsules for the Modulation of the Local Microenvironment in Tissue Engineering

Oxidative damage to cells from metabolites at a wound site is one of the trickiest factors inhibiting tissue regeneration, especially with bulk damage. In addition, an excessive inflammatory reaction by the body at the wound site can make it even worse. How to scavenge the reactive oxygen species (ROS) produced from metabolism and inflammatory reactions has become a critical issue in tissue engineering. Here, we utilize the natural bioactive small molecules l-arginine and l-phenylalanine and the growth factor inositol to synthesize a branched poly(ester amide) (BPEA) to fabricate BPEA nanocapsules for vitamin E delivery at wound sites. BPEA nanocapsules loaded with vitamin E (BPEA@VE NCs) could protect cells from both extracellular and intracellular damage by scavenging ROS. Simultaneously, the inflammatory reaction could also be downregulated, benefiting from the introduction of l-arginine. Furthermore, the biodegradation products of BPEA are natural metabolites of the body, such as amino acids and growth factors, guaranteeing the biocompatibility of the BPEA@VE NCs. The protective ability of the BPEA@VE NCs was also investigated in vivo for accelerated wound healing. All the results indicate that the BPEA@VE NCs have promising potential for the modulation of the local microenvironment in tissue engineering for excellent antioxidative and anti-inflammatory properties.

Ex Vivo Systems to Study Chondrogenic Differentiation and Cartilage Integration

Articular cartilage injury and repair is an issue of growing importance. Although common, defects of articular cartilage present a unique clinical challenge due to its poor self-healing capacity, which is largely due to its avascular nature. There is a critical need to better study and understand cellular healing mechanisms to achieve more effective therapies for cartilage regeneration. This article aims to describe the key features of cartilage which is being modelled using tissue engineered cartilage constructs and ex vivo systems. These models have been used to investigate chondrogenic differentiation and to study the mechanisms of cartilage integration into the surrounding tissue. The review highlights the key regeneration principles of articular cartilage repair in healthy and diseased joints. Using co-culture models and novel bioreactor designs, the basis of regeneration is aligned with recent efforts for optimal therapeutic interventions.

Optimized Media Volumes Enable Homogeneous Growth of Mesenchymal Stem Cell-Based Engineered Cartilage Constructs

Despite marked advances in the field of cartilage tissue engineering, it remains a challenge to engineer cartilage constructs with homogeneous properties. Moreover, for engineered cartilage to make it to the clinic, this homogeneous growth must occur in a time-efficient manner. In this study we investigated the potential of increased media volume to expedite the homogeneous maturation of mesenchymal stem cell (MSC) laden engineered constructs over time in vitro. We assessed the MSC-laden constructs after 4 and 8 weeks of chondrogenic culture using bulk mechanical, histological, and biochemical measures. These assays were performed on both the intact total constructs and the construct cores to elucidate region-dependent differences. In addition, local strain transfer was assessed to quantify depth-dependent mechanical properties throughout the constructs. Our findings suggest that increased media volume enhances matrix deposition early in culture and ameliorates unwanted regional heterogeneities at later time points. Taken together, these data support the use of higher media volumes during in vitro culture to hasten tissue maturation and increase the core strength of tissue constructs. These findings will forward the field of cartilage tissue engineering and the translation of tissue engineered constructs.

Cartilage tissue engineering using injectable functionalized Demineralized Bone Matrix scaffold with glucosamine in PVA carrier, cultured in microbioreactor prior to study in rabbit model

Using 3D model of injectable scaffolds for cartilage tissue engineering is one of the challenges that should be addressed to avoid invasive surgery for treatment. For this purpose, chondrocytes on Demineralized Bone Matrix (DBM) scaffolds functionalized with glucosamine in 20% polyvinyl alcohol (PVA) as a carrier was applied to the micro-bioreactor in-vitro, then the study was continued on in-vivo stage. Scaffold biocompatibility tests were performed and the mechanical and physicochemical properties were studied showing the fact that DBM was functionalized by Glucosamine, scaffold degradation rate was 53% after 720 h and swelling ratio was 2.5 times after 16 h, injectable scaffold demonstrated better mechanical characteristics (P < 0.05) than other concentrations of PVA. Consequently, in-vitro tests, including live-dead imaging resulting in 99% viability after 14 days (P < 0.001), DAPI staining and scanning electron microscope imaging were performed to determine the number and viability of the cells on the scaffold, showing a cells proliferation property of this group compared with the control after 14 days (P < 0.0001), then relative gene expression was evaluated and protein expression was assessed. The overall chondrogenic gene expression improved (P < 0.05) compared to the control (2D culture). Subsequently, the scaffold were loaded with chondrocytes and injected into the cartilage lesion part After 24 weeks of surgery, MRI and immunocytochemistry were performed. Then all outputs proved that the scaffold plus cell group had a significantly higher topological score (P < 0.0001) than other groups compared to normal cartilage. Finally, studies have shown that transplantation of chondrocytes in DBM, polyvinyl alcohol and glucosamine scaffold through one surgical stage improves cartilage lesion and it can be considered as a breakthrough in tissue engineering.

3D Bioprinting of osteochondral tissue substitutes - in vitro-chondrogenesis in multi-layered mineralized constructs

For the generation of multi-layered full thickness osteochondral tissue substitutes with an individual geometry based on clinical imaging data, combined extrusion-based 3D printing (3D plotting) of a bioink laden with primary chondrocytes and a mineralized biomaterial phase was introduced. A pasty calcium phosphate cement (CPC) and a bioink based on alginate-methylcellulose (algMC) - both are biocompatible and allow 3D plotting with high shape fidelity - were applied in monophasic and combinatory design to recreate osteochondral tissue layers. The capability of cells reacting to chondrogenic biochemical stimuli inside the algMC-based 3D hydrogel matrix was assessed. Towards combined osteochondral constructs, the chondrogenic fate in the presence of CPC in co-fabricated and biphasic mineralized pattern was evaluated. Majority of expanded and algMC-encapsulated cells survived the plotting process and the cultivation period, and were able to undergo redifferentiation in the provided environment to produce their respective extracellular matrix (ECM) components (i.e. sulphated glycosaminoglycans, collagen type II), examined after 3 weeks. The presence of a mineralized zone as located in the physiological calcified cartilage region suspected to interfere with chondrogenesis, was found to support chondrogenic ECM production by altering the ionic concentrations of calcium and phosphorus in in vitro culture conditions.

Oxygenation Profiles of Human Blood, Cell Culture Medium, and Water for Perfusion of 3D-Bioprinted Tissues using the FABRICA Bioreactor Platform

Persistent and saturated oxygen distribution from perfusion media (i.e., blood, or cell culture media) to cells within cell-dense, metabolically-active biofabricated tissues is required to keep them viable. Improper or poor oxygen supply to cells within the tissue bulk severely limits the tissue culturing potential of many bioreactors. We added an oxygenator module to our modular FABRICA bioreactor in order to provide stable oxygenation to biofabricated tissues during culture. In this proof of concept study of an oxygenated and perfused bioreactor, we characterized the oxygenation of water, cell culture medium, and human blood in the FABRICA as functions of augmenting vacuum (air inlet) pressure, perfusion (volumetric flow) rate, and tubing/oxygenator components. The mean oxygen levels for water and cell culture media were 27.7 ± 2.1% and 27.6 ± 4.1%, respectively. The mean oxygen level for human blood was 197.0 ± 90.0 mmHg, with near-physiologic levels achieved with low-permeability PharMed tubing alone (128.0 ± 14.0 mmHg). Hematologic values pre- and post-oxygenation, respectively were (median ± IQR): Red blood cell: 6.0 ± 0.5 (10⁶/μL) and 6.5 ± 0.4 (10⁶/μL); Hemoglobin: 17.5 ± 1.2 g/dL and 19.2 ± 3.0 g/dL; and Hematocrit: 56.7 ± 2.4% and 61.4 ± 7.5%. The relative stability of the hematologic parameters indicates that blood function and thus blood cell integrity were maintained throughout oxygenation. Already a versatile research tool, the now oxygenated FABRICA provides easy-to-implement, in vivo-like perfusion and stable oxygenation culture conditions in vitro semi-independently of one another, which means the bioreactor has the potential to serve as a platform for investigating the behavior of 3D tissue models (regardless of biofabrication method), performing drug toxicity-testing, and testing pharmaceutical efficacy/safety.

Cartilage tissue engineering combining microspheroid building blocks and microneedle arrays

Purpose: Scaffold-free cartilage tissue engineering circumvents issues with scaffold seeding, potential toxicity response, and impaired host integration. However, precisely controlling and maintaining a scaffold-free construct shape have been challenging. We explored the feasibility of microneedle arrays to print tissue using cellular microspheroids as building blocks.Materials and Methods: Human embryonic-derived mesenchymal stem cells or infrapatellar fat pad mesenchymal stem cells were used to create microspheroids of 500 µm in diameter, which were assembled on microneedle arrays in a predefined arrangement using a robotic system under computer vision. Microspheroids on microneedles were cultured to permit fusion into a tissue construct. Infrapatellar fat pad mesenchymal stem cell constructs were either implanted into chondral defects created in human osteoarthritic cartilage explants or maintained on the microneedle array for 3 weeks. Embryonic-derived mesenchymal stem cell constructs were designed to be press-fit into 3 mm subchondral defects in New Zealand White rabbits and maintained for up to 8 weeks to assess retention, early tissue repair, and more mature cartilage regeneration.Results: Microspheroids of both cell types fused together in culture to form neotissues of predefined shape and size. Infrapatellar fat pad mesenchymal stem cell neotissues expressed high levels of chondrogenic genes and integrated with the surrounding osteoarthritic host cartilage. Embryonic-derived mesenchymal stem cell constructs generated chondrogenic neotissue in vivo as early as 2 weeks and more mature tissue by 8 weeks with increased glycosaminoglycan deposition.Conclusions: We constructed defined scaffold-free shapes by bioprinting and fusing microspheroids. Proof of concept was shown in the repair of ex vivo osteoarthritic human cartilage and in vivo rabbit osteochondral (OC) defects.

Hydrogel microparticles for biomedical applications

Hydrogel microparticles (HMPs) are promising for biomedical applications, ranging from the therapeutic delivery of cells and drugs to the production of scaffolds for tissue repair and bioinks for 3D printing. Biologics (cells and drugs) can be encapsulated into HMPs of predefined shapes and sizes using a variety of fabrication techniques (batch emulsion, microfluidics, lithography, electrohydrodynamic (EHD) spraying and mechanical fragmentation). HMPs can be formulated in suspensions to deliver therapeutics, as aggregates of particles (granular hydrogels) to form microporous scaffolds that promote cell infiltration or embedded within a bulk hydrogel to obtain multiscale behaviours. HMP suspensions and granular hydrogels can be injected for minimally invasive delivery of biologics, and they exhibit modular properties when comprised of mixtures of distinct HMP populations. In this Review, we discuss the fabrication techniques that are available for fabricating HMPs, as well as the multiscale behaviours of HMP systems and their functional properties, highlighting their advantages over traditional bulk hydrogels. Furthermore, we discuss applications of HMPs in the fields of cell delivery, drug delivery, scaffold design and biofabrication.

Therapeutic "Tool" in Reconstruction and Regeneration of Tissue Engineering for Osteochondral Repair

Repairing osteochondral defects to restore joint function is a major challenge in regenerative medicine. However, with recent advances in tissue engineering, the development of potential treatments is promising. In recent years, in addition to single-layer scaffolds, double-layer or multilayer scaffolds have been prepared to mimic the structure of articular cartilage and subchondral bone for osteochondral repair. Although there are a range of different cells such as umbilical cord stem cells, bone marrow mesenchyml stem cell, and others that can be used, the availability, ease of preparation, and the osteogenic and chondrogenic capacity of these cells are important factors that will influence its selection for tissue engineering. Furthermore, appropriate cell proliferation and differentiation of these cells is also key for the optimal repair of osteochondral defects. The development of bioreactors has enhanced methods to stimulate the proliferation and differentiation of cells. In this review, we summarize the recent advances in tissue engineering, including the development of layered scaffolds, cells, and bioreactors that have changed the approach towards the development of novel treatments for osteochondral repair.

Cellular hypoxia promotes osteogenic differentiation of mesenchymal stem cells and bone defect healing via STAT3 signaling

Background

Hypoxia in the vicinity of bone defects triggers the osteogenic differentiation of precursor cells and promotes healing. The activation of STAT3 signaling in mesenchymal stem cells (MSCs) has similarly been reported to mediate bone regeneration. However, the interaction between hypoxia and STAT3 signaling in the osteogenic differentiation of precursor cells during bone defect healing is still unknown.

Methods

In this study, we assessed the impact of different durations of CoCl₂-induced cellular hypoxia on the osteogenic differentiation of MSCs. Role of STAT3 signaling on hypoxia induced osteogenic differentiation was analyzed both in vitro and in vivo. The interaction between cellular hypoxia and STAT3 signaling in vivo was investigated in a mouse femoral bone defect model.

Results

The peak osteogenic differentiation and expression of vascular endothelial growth factor (VEGF) occurred after 3 days of hypoxia. Inhibiting STAT3 reversed this effect. Hypoxia enhanced the expression of hypoxia-inducible factor 1-alpha (HIF-1α) and STAT3 phosphorylation in MSCs. Histology and μ-CT results showed that CoCl₂ treatment enhanced bone defect healing. Inhibiting STAT3 reduced this effect. Immunohistochemistry results showed that CoCl₂ treatment enhanced Hif-1α, ALP and pSTAT3 expression in cells present in the bone defect area and that inhibiting STAT3 reduced this effect.

Conclusions

The in vitro study revealed that the duration of hypoxia is crucial for osteogenic differentiation of precursor cells. The results from both the in vitro and in vivo studies show the role of STAT3 signaling in hypoxia-induced osteogenic differentiation of precursor cells and bone defect healing.

A standalone bioreactor system to deliver compressive load under perfusion flow to hBMSC-seeded 3D chitosan-graphene templates

The availability of engineered biological tissues holds great potential for both clinical applications and basic research in a life science laboratory. A prototype standalone perfusion/compression bioreactor system was proposed to address the osteogenic commitment of stem cells seeded onboard of 3D chitosan-graphene (CHT/G) templates. Testing involved the coordinated administration of a 1 mL/min medium flow rate together with dynamic compression (1% strain at 1 Hz; applied twice daily for 30 min) for one week. When compared to traditional static culture conditions, the application of perfusion and compression stimuli to human bone marrow stem cells using the 3D CHT/G template scaffold induced a sizable effect. After using the dynamic culture protocol, there was evidence of a larger number of viable cells within the inner core of the scaffold and of enhanced extracellular matrix mineralization. These observations show that our novel device would be suitable for addressing and investigating the osteogenic phenotype commitment of stem cells, for both potential clinical applications and basic research.

Modeling, simulations, and optimization of smooth muscle cell tissue engineering for the production of vascular grafts

The paper presents a transient, continuum, two-phase model of the tissue engineering in fibrous scaffolds, including transport equations for the flowing culture medium, nutrient and cell concentration with transverse and in-plane diffusion and cell migration, a novel feature of local in-plane transport across a phenomenological pore and innovative layer-by-layer cell filling approach. The model is successfully validated for the smooth muscle cell tissue engineering of a vascular graft using crosslinked, electrospun gelatin fiber scaffolds for both static and dynamic cell culture, the latter in a dynamic bioreactor with a rotating shaft on which the tubular scaffold is attached. Parametric studies evaluate the impact of the scaffold microstructure, cell dynamics, oxygen transport, and static or dynamic conditions on the rate and extent of cell proliferation and depth of oxygen accessibility. An optimized scaffold of 75% dry porosity is proposed that can be tissue engineered into a viable and still fully oxygenated graft of the tunica media of the coronary artery within 2 days in the dynamic bioreactor. Such scaffold also matches the mechanical properties of the tunica media of the human coronary artery and the suture retention strength of a saphenous vein, often used as a coronary artery graft.

Biofabrication of spatially organised tissues by directing the growth of cellular spheroids within 3D printed polymeric microchambers

Successful tissue engineering requires the generation of human scale implants that mimic the structure, composition and mechanical properties of native tissues. Here, we report a novel biofabrication strategy that enables the engineering of structurally organised tissues by guiding the growth of cellular spheroids within arrays of 3D printed polymeric microchambers. With the goal of engineering stratified articular cartilage, inkjet bioprinting was used to deposit defined numbers of mesenchymal stromal cells (MSCs) and chondrocytes into pre-printed microchambers. These jetted cell suspensions rapidly underwent condensation within the hydrophobic microchambers, leading to the formation of organised arrays of cellular spheroids. The microchambers were also designed to provide boundary conditions to these spheroids, guiding their growth and eventual fusion, leading to the development of stratified cartilage tissue with a depth-dependant collagen fiber architecture that mimicked the structure of native articular cartilage. Furthermore, the composition and biomechanical properties of the bioprinted cartilage was also comparable to the native tissue. Using multi-tool biofabrication, we were also able to engineer anatomically accurate, human scale, osteochondral templates by printing this microchamber system on top of a hypertrophic cartilage region designed to support endochondral bone formation and then maintaining the entire construct in long-term bioreactor culture to enhance tissue development. This bioprinting strategy provides a versatile and scalable approach to engineer structurally organised cartilage tissues for joint resurfacing applications.

New generation of bioreactors that advance extracellular matrix modelling and tissue engineering

Bioreactors hold a lot of promise for tissue engineering and regenerative medicine applications. They have multiple uses including cell cultivation for therapeutic production and for in vitro organ modelling to provide a more physiologically relevant environment for cultures compared to conventional static conditions. Bioreactors are often used in combination with scaffolds as the nutrient flow can enhance oxygen and diffusion throughout the 3D constructs to prevent the formation of necrotic cores. A variety of scaffolds have been fabricated to achieve a structural architecture that mimic native extracellular matrix. Future developments of in vitro models will incorporate the ability to non-invasively monitor the cellular microenvironment to enhance the understanding of in vitro conditions. This review details current advancements in bioreactor and scaffold systems and provides insight on how in vitro models can be augmented for future biomedical applications.