Methacrylated Cartilage ECM-Based Hydrogels as Injectables and Bioinks for Cartilage Tissue Engineering

Injectable Biomimetic Gels for Biomedical Applications

Biomimetic gels are synthetic materials designed to mimic the properties and functions of natural biological systems, such as tissues and cellular environments. This manuscript explores the advancements and future directions of injectable biomimetic gels in biomedical applications and highlights the significant potential of hydrogels in wound healing, tissue regeneration, and controlled drug delivery due to their enhanced biocompatibility, multifunctionality, and mechanical properties. Despite these advancements, challenges such as mechanical resilience, controlled degradation rates, and scalable manufacturing remain. This manuscript discusses ongoing research to optimize these properties, develop cost-effective production techniques, and integrate emerging technologies like 3D bioprinting and nanotechnology. Addressing these challenges through collaborative efforts is essential for unlocking the full potential of injectable biomimetic gels in tissue engineering and regenerative medicine.

3D Bioprinting Strategies for Articular Cartilage Tissue Engineering

Articular cartilage is the avascular and aneural tissue which is the primary connective tissue covering the surface of articulating bone. Traumatic damage or degenerative diseases can cause articular cartilage injuries that are common in the population. As a result, the demand for new therapeutic options is continually increasing for older people and traumatic young patients. Many attempts have been made to address these clinical needs to treat articular cartilage injuries, including osteoarthritis (OA); however, regenerating highly qualified cartilage tissue remains a significant obstacle. 3D bioprinting technology combined with tissue engineering principles has been developed to create biological tissue constructs that recapitulate the anatomical, structural, and functional properties of native tissues. In addition, this cutting-edge technology can precisely place multiple cell types in a 3D tissue architecture. Thus, 3D bioprinting has rapidly become the most innovative tool for manufacturing clinically applicable bioengineered tissue constructs. This has led to increased interest in 3D bioprinting in articular cartilage tissue engineering applications. Here, we reviewed current advances in bioprinting for articular cartilage tissue engineering.

Chemistry matters: A side-by-side comparison of two chemically distinct methacryloylated dECM bioresins for vat photopolymerization

Decellularized extracellular matrix (dECM) is an excellent natural source for 3D bioprinting materials due to its inherent cell compatibility. In vat photopolymerization, the use of dECM-based bioresins is just emerging, and extensive research is needed to fully exploit their potential. In this study, two distinct methacryloyl-functionalized, photocrosslinkable dECM-based bioresins were prepared from digested porcine liver dECM through functionalization with glycidyl methacrylate (GMA) or conventional methacrylic anhydride (MA) under mild conditions for systematic comparison. Although the chemical modifications did not significantly affect the structural integrity of the dECM proteins, mammalian cells encapsulated in the respective hydrogels performed differently in long-term culture. In either case, photocrosslinking during 3D (bio)printing resulted in transparent, highly swollen, and soft hydrogels with good shape fidelity, excellent biomimetic properties and tunable mechanical properties (~ 0.2-2.5 kPa). Interestingly, at a similar degree of functionalization (DOF ~ 81.5-83.5 %), the dECM-GMA resin showed faster photocrosslinking kinetics in photorheology resulting in lower final stiffness and faster enzymatic biodegradation compared to the dECM-MA gels, yet comparable network homogeneity as assessed via Brillouin imaging. While human hepatic HepaRG cells exhibited comparable cell viability directly after 3D bioprinting within both materials, cell proliferation and spreading were clearly enhanced in the softer dECM-GMA hydrogels at a comparable degree of crosslinking. These differences were attributed to the additional hydrophilicity introduced to dECM via methacryloylation through GMA compared to MA. Due to its excellent printability and cytocompatibility, the functional porcine liver dECM-GMA biomaterial enables the advanced biofabrication of soft 3D tissue analogs using vat photopolymerization-based bioprinting.

Nanostructured Biopolymer-Based Constructs for Cartilage Regeneration: Fabrication Techniques and Perspectives

The essential functions of cartilage, such as shock absorption and resilience, are hindered by its limited regenerative capacity. Although current therapies alleviate symptoms, novel strategies for cartilage regeneration are desperately needed. Recent developments in three-dimensional (3D) constructs aim to address this challenge by mimicking the intrinsic characteristics of native cartilage using biocompatible materials, with a significant emphasis on both functionality and stability. Through fabrication methods such as 3D printing and electrospinning, researchers are making progress in cartilage regeneration; nevertheless, it is still very difficult to translate these advances into clinical practice. The review emphasizes the importance of integrating various fabrication techniques to create stable 3D constructs. Meticulous design and material selection are required to achieve seamless cartilage integration and durability. The review outlines the need to address these challenges and focuses on the latest developments in the production of hybrid 3D constructs based on biodegradable and biocompatible polymers. Furthermore, the review acknowledges the limitations of current research and provides perspectives on potential avenues for effectively regenerating cartilage defects in the future.

Injectable hydrogels as promising in situ therapeutic platform for cartilage tissue engineering

Injectable hydrogels are gaining prominence as a biocompatible, minimally invasive, and adaptable platform for cartilage tissue engineering. Commencing with their synthesis, this review accentuates the tailored matrix formulations and cross-linking techniques essential for fostering three-dimensional cell culture and melding with complex tissue structures. Subsequently, it spotlights the hydrogels' enhanced properties, highlighting their augmented functionalities and broadened scope in cartilage tissue repair applications. Furthermore, future perspectives are advocated, urging continuous innovation and exploration to surmount existing challenges and harness the full clinical potential of hydrogels in regenerative medicine. Such advancements are crucial for validating the long-term efficacy and safety of hydrogels, positioning them as a promising direction in regenerative medicine to address cartilage-related ailments.

Rise of tissue- and species-specific 3D bioprinting based on decellularized extracellular matrix-derived bioinks and bioresins

Thanks to its natural complexity and functionality, decellularized extracellular matrix (dECM) serves as an excellent foundation for creating highly cell-compatible bioinks and bioresins. This enables the bioprinted cells to thrive in an environment that closely mimics their native ECM composition and offers customizable biomechanical properties. To formulate dECM bioinks and bioresins, one must first pulverize and/or solubilize the dECM into non-crosslinked fragments, which can then be chemically modified as needed. In bioprinting, the solubilized dECM-derived material is typically deposited and/or crosslinked in a layer-by-layer fashion to build 3D hydrogel structures. Since the introduction of the first liver-derived dECM-based bioinks, a wide variety of decellularized tissue have been employed in bioprinting, including kidney, heart, cartilage, and adipose tissue among others. This review aims to summarize the critical steps involved in tissue-derived dECM bioprinting, starting from the decellularization of the ECM to the standardized formulation of bioinks and bioresins, ultimately leading to the reproducible bioprinting of tissue constructs. Notably, this discussion also covers photocrosslinkable dECM bioresins, which are particularly attractive due to their ability to provide precise spatiotemporal control over the gelation in bioprinting. Both in extrusion printing and vat photopolymerization, there is a need for more standardized protocols to fully harness the unique properties of dECM-derived materials. In addition to mammalian tissues, the most recent bioprinting approaches involve the use of microbial extracellular polymeric substances in bioprinting of bacteria. This presents similar challenges as those encountered in mammalian cell printing and represents a fascinating frontier in bioprinting technology.

An injectable and 3D printable pro-chondrogenic hyaluronic acid and collagen type II composite hydrogel for the repair of articular cartilage defects

Current treatments for repairing articular cartilage defects are limited. However, pro-chondrogenic hydrogels formulated using articular cartilage matrix components (such as hyaluronic acid (HA) and collagen type II (Col II)), offer a potential solution if they could be injected into the defect via minimally invasive arthroscopic procedures, or used as bioinks to 3D print patient-specific customised regenerative scaffolds-potentially combined with cells. However, HA and Col II are difficult to incorporate into injectable/3D printable hydrogels due to poor physicochemical properties. This study aimed to overcome this by developing an articular cartilage matrix-inspired pro-chondrogenic hydrogel with improved physicochemical properties for both injectable and 3D printing (3DP) applications. To achieve this, HA was methacrylated to improve mechanical properties and mixed in a 1:1 ratio with Col I, a Col I/Col II blend or Col II. Col I possesses superior mechanical properties to Col II and so was hypothesised to enhance hydrogel mechanical properties. Rheological analysis showed that the pre-gels had viscoelastic and shear thinning properties. Subsequent physicochemical analysis of the crosslinked hydrogels showed that Col II inclusion resulted in a more swollen and softer polymer network, without affecting degradation time. While all hydrogels exhibited exemplary injectability, only the Col I-containing hydrogels had sufficient mechanical stability for 3DP applications. To facilitate 3DP of multi-layered scaffolds using methacrylated HA (MeHA)-Col I and MeHA-Col I/Col II, additional mechanical support in the form of a gelatin slurry support bath freeform reversible embedding of suspended hydrogels was utilised. Biological analysis revealed that Col II inclusion enhanced hydrogel-embedded MSC chondrogenesis, thus MeHA-Col II was selected as the optimal injectable hydrogel, and MeHA-Col I/Col II as the preferred bioink. In summary, this study demonstrates how tailoring biomaterial composition and physicochemical properties enables development of pro-chondrogenic hydrogels with potential for minimally invasive delivery to injured articular joints or 3DP of customised regenerative implants for cartilage repair.

Technologies for the fabrication of crosslinked polysaccharide-based hydrogels and its role in microbial three-dimensional bioprinting - A review

Three-Dimensional bioprinting has recently gained more attraction among researchers for its wide variety of applicability. This technology involving in developing structures that mimic the natural anatomy, and also aims in developing novel biomaterials, bioinks which have a better printable ability. Different hydrogels (cross-linked polysaccharides) can be used and optimized for good adhesion and cell proliferation. Manufacturing hydrogels with adjustable characteristics allows for fine-tuning of the cellular microenvironment. Different printing technologies can be used to create hydrogels on a micro-scale which will allow regular, patterned integration of cells into hydrogels. Controlling tissue constructions' structural architecture is the important key to ensuring its function as it is designed. The designed tiny hydrogels will be useful in investigating the cellular behaviour within the environments. Three-Dimensional designs can be constructed by modifying their shape and behaviour analogous concerning pressure, heat, electricity, ultraviolet radiation or other environmental elements. Yet, its application in in vitro infection models needs more research and practical study. Microbial bioprinting has become an advancing field with promising potential to develop various biomedical as well as environmental applications. This review elucidates the properties and usage of different hydrogels for Three-Dimensional bioprinting.

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

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

Collagen and Beyond: A Comprehensive Comparison of Human ECM Properties Derived from Various Tissue Sources for Regenerative Medicine Applications

Collagen, along with proteoglycans, glycosaminoglycans, glycoproteins, and various growth factors, forms the extracellular matrix (ECM) and contributes to the complexity and diversity of different tissues. Herein, we compared the physicochemical and biological properties of ECM hydrogels derived from four different human tissues: skin, bone, fat, and birth. Pure human collagen type I hydrogels were used as control. Physical characterization of ECM hydrogels and assessment of cell response of cord-tissue mesenchymal stem cells (CMSCs) were performed. Decellularization efficiency was found to be >90% for all ECM. Hydroxyproline quantification assay showed that collagen content in birth ECM was comparable to collagen control and significantly greater than other sources of ECM. Sodium dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed the presence of γ, β, α₁ and α₂ collagen chains in all ECMs. Gelation kinetics of ECM hydrogels was significantly slower than collagen control. Compressive modulus of skin ECM was the highest and birth ECM was the lowest. Skin and birth ECM hydrogels were more stable than bone and fat ECM hydrogels. CMSCs encapsulated in birth ECM hydrogels exhibited the highest metabolic activity. Rheological characterization revealed that all ECM-derived inks exhibited shear thinning properties, and skin-derived ECM inks were most suitable for extrusion-based bioprinting for the concentration and printing conditions used in this study. Overall, results demonstrate that the physicochemical and biological properties of ECM hydrogels vary significantly depending on the tissue source. Therefore, careful selection of tissue source is important for development of ECM-based biomimetic tissue constructs for regenerative medicine applications.

Current advancements in bio-ink technology for cartilage and bone tissue engineering

In tissue engineering, the fate of a particular organ/tissue regeneration and repair mainly depends on three pillars - 3D architecture, cells used, and stimulus provided. 3D cell supportive structure development is one of the crucial pillars necessary for defining organ/tissue geometry and shape. In recent years, the advancements in 3D bio-printing (additive manufacturing) made it possible to develop very precise 3D architectures with the help of industrial software like Computer-Aided Design (CAD). The main requirement for the 3D printing process is the bio-ink, which can act as a source for cell support, proliferation, drug (growth factors, stimulators) delivery, and organ/tissue shape. The selection of the bio-ink depends upon the type of 3D tissue of interest. Printing tissues like bone and cartilage is always challenging because it is difficult to find printable biomaterial that can act as bio-ink and mimic the strength of the natural bone and cartilage tissues. This review describes different biomaterials used to develop bio-inks with different processing variables and cell-seeding densities for bone and cartilage 3D printing applications. The review also discusses the advantages, limitations, and cell bio-ink compatibility in each biomaterial section. The emphasis is given to bio-inks reported for 3D printing cartilage and bone and their applications in orthopedics and orthodontists. The critical/important performance and the architectural morphology requirements of desired bone and cartilage bio-inks were compiled in summary.

3D Bioprintable Hypoxia-Mimicking PEG-Based Nano Bioink for Cartilage Tissue Engineering

As hypoxia plays a significant role in the formation and maintenance of cartilage tissue, aiming to develop native hypoxia-mimicking tissue engineering scaffolds is an efficient method to treat articular cartilage (AC) defects. Cobalt (Co) is documented for its hypoxic-inducing effects in vitro by stabilizing the hypoxia-inducible factor-1α (HIF-1α), a chief regulator of stem cell fate. Considering this, we developed a novel three-dimensional (3D) bioprintable hypoxia-mimicking nano bioink wherein cobalt nanowires (Co NWs) were incorporated into the poly(ethylene glycol) diacrylate (PEGDA) hydrogel system as a hypoxia-inducing agent and encapsulated with umbilical cord-derived mesenchymal stem cells (UMSCs). In the current study, we investigated the impact of Co NWs on the chondrogenic differentiation of UMSCs in the PEGDA hydrogel system. Herein, the hypoxia-mimicking nano bioink (PEGDA+Co NW) was rheologically optimized to bioprint geometrically stable cartilaginous constructs. The bioprinted 3D constructs were evaluated for their physicochemical characterization, swelling-degradation behavior, mechanical properties, cell proliferation, and the expression of chondrogenic markers by histological, immunofluorescence, and reverse transcription-quantitative polymerase chain reaction (RT-qPCR) methods. The results disclosed that, compared to the control (PEGDA) group, the hypoxia-mimicking nano bioink (PEGDA+Co NW) group outperformed in print fidelity and mechanical properties. Furthermore, live/dead staining, double-stranded DNA (dsDNA) content, and glycosaminoglycans (GAGs) content demonstrated that adding low amounts of Co NWs (<20 ppm) into PEGDA hydrogel system supported UMSC adhesion, proliferation, and differentiation. Histological and immunofluorescence staining of the PEGDA+Co NW bioprinted structures revealed the production of type 2 collagen (COL2) and sulfated GAGs, rendering it a feasible option for cartilage repair. It was further corroborated by a significant upregulation of the hypoxia-mediated chondrogenic and downregulation of the hypertrophic/osteogenic marker expression. In conclusion, the hypoxia-mimicking hydrogel system, including PEGDA and Co²⁺ ions, synergistically directs the UMSCs toward the chondrocyte lineage without using expensive growth factors and provides an alternative strategy for translational applications in the cartilage tissue engineering field.

Engineering Cell-ECM-Material Interactions for Musculoskeletal Regeneration

The extracellular microenvironment regulates many of the mechanical and biochemical cues that direct musculoskeletal development and are involved in musculoskeletal disease. The extracellular matrix (ECM) is a main component of this microenvironment. Tissue engineered approaches towards regenerating muscle, cartilage, tendon, and bone target the ECM because it supplies critical signals for regenerating musculoskeletal tissues. Engineered ECM-material scaffolds that mimic key mechanical and biochemical components of the ECM are of particular interest in musculoskeletal tissue engineering. Such materials are biocompatible, can be fabricated to have desirable mechanical and biochemical properties, and can be further chemically or genetically modified to support cell differentiation or halt degenerative disease progression. In this review, we survey how engineered approaches using natural and ECM-derived materials and scaffold systems can harness the unique characteristics of the ECM to support musculoskeletal tissue regeneration, with a focus on skeletal muscle, cartilage, tendon, and bone. We summarize the strengths of current approaches and look towards a future of materials and culture systems with engineered and highly tailored cell-ECM-material interactions to drive musculoskeletal tissue restoration. The works highlighted in this review strongly support the continued exploration of ECM and other engineered materials as tools to control cell fate and make large-scale musculoskeletal regeneration a reality.

Advances in Cartilage Tissue Engineering Using Bioinks with Decellularized Cartilage and Three-Dimensional Printing

Osteoarthritis, a chronic, debilitating, and painful disease, is one of the leading causes of disability and socioeconomic burden, with an estimated 250 million people affected worldwide. Currently, there is no cure for osteoarthritis and treatments for joint disease require improvements. To address the challenge of improving cartilage repair and regeneration, three-dimensional (3D) printing for tissue engineering purposes has been developed. In this review, emerging technologies are presented with an overview of bioprinting, cartilage structure, current treatment options, decellularization, bioinks, and recent progress in the field of decellularized extracellular matrix (dECM)–bioink composites is discussed. The optimization of tissue engineering approaches using 3D-bioprinted biological scaffolds with dECM incorporated to create novel bioinks is an innovative strategy to promote cartilage repair and regeneration. Challenges and future directions that may lead to innovative improvements to currently available treatments for cartilage regeneration are presented.

Development of highly-reproducible hydrogel based bioink for regeneration of skin-tissues via 3-D bioprinting technology

3-D Bioprinting is employed as a novel approach in biofabrication to promote skin regeneration following chronic-wounds and injury. A novel bioink composed of carbohydrazide crosslinked {polyethylene oxide-co- Chitosan-co- poly(methylmethacrylic-acid)} (PEO-CS-PMMA) laden with Nicotinamide and human dermal fibroblast was successfully synthesized via Free radical-copolymerization at 73 °C. The developed bioink was characterized in term of swelling, structural-confirmation by solid state 13C-Nuclear Magnetic Resonance (NMR), morphology, thermal, 3-D Bioprinting via extrusion, rheological and interaction with DNA respectively. The predominant rate of gelation was attributed to the electrostatic interactions between cationic CS and anionic PMMA pendant groups. The morphology of developed bioink presented a porous architecture satisfying the cell and growth-factor viability across the barrier. The thermal analysis revealed two-step degradation with 85 % weight loss in term of decomposition and molecular changes in the bioink moieties By applying low pressure in the range of 25-50 kPa, the optimum reproducibility and printability were determined at 37 °C in the viscosity range of 500-550 Pa. s. A higher survival rate of 92 % was observed for (PEO-CS-PMMA) in comparison to 67 % for pure chitosan built bioink. A binding constant of K ≈ 1.8 × 10⁶ M^-1 recognized a thermodynamically stable interaction of (PEO-CS-PMMA) with the Salmon-DNA. Further, the addition of PEO (5.0 %) was addressed with better self-healing and printability to produce skin-tissue constructs to replace the infected skin in human.

High-stiffness, fast-crosslinking, cartilage matrix bioinks

Scaffolds derived from cartilage extracellular matrix may contain intrinsic chondroinductivity and have promise for cartilage regeneration. Cartilage is typically ground into devitalized particles (DVC) and several groups have pioneered innovative methods to rebuild the DVC into a new scaffold. However, challenges remain regarding the fluid and solid biomechanics of cartilage-based scaffolds in achieving 1) high mechanical performance akin to native cartilage and 2) easy surgical delivery/retention. Fortunately, photocrosslinking bioinks may benefit clinical translation: paste-like/injectable precursor rheology facilitates surgical placement, and in situ photocrosslinking enables material retention within any size/shape of defect. While solubilized DVC has been modified with methacryloyls (MeSDVC), MeSDVC is limited by slow crosslinking times (e.g., 5-10 min). Therefore, in the current study, we fabricated a pentenoate-modified SDVC (PSDVC), to enable a faster crosslinking reaction via a thiol-ene click chemistry. The crosslinking time of the PSDVC was faster (∼1.7 min) than MeSDVC (∼4 min). We characterized the solid and fluid mechanics/printabilities of PSDVC, pentenoate-modified hyaluronic acid (PHA), and the PHA or PSDVC with added DVC particles. While the addition of DVC particles enhanced the printed shape fidelity of PHA or PSDVC, the increased clogging decreased the ease of printing and cell viability after bioprinting, and future refinement is needed for DVC-containing bioinks. However, the PSDVC alone had a paste-like rheology/good bioprintability prior to crosslinking, the fastest crosslinking time (i.e., 1.7 min), and the highest compressive modulus (i.e., 3.12 ± 0.41 MPa) after crosslinking. Overall, the PSDVC may have future potential as a translational material for cartilage repair.

Mesenchymal Stromal Cells Laden in Hydrogels for Osteoarthritis Cartilage Regeneration: A Systematic Review from In Vitro Studies to Clinical Applications

This systematic review is focused on the main characteristics of the hydrogels used for embedding the mesenchymal stromal cells (MSCs) in in vitro/ex vivo studies, in vivo OA models and clinical trials for favoring cartilage regeneration in osteoarthritis (OA). PubMED and Embase databases were used to select the papers that were submitted to a public reference manager Rayyan Systematic Review Screening Software. A total of 42 studies were considered eligible: 25 articles concerned in vitro studies, 2 in vitro and ex vivo ones, 5 in vitro and in vivo ones, 8 in vivo ones and 2 clinical trials. Some in vitro studies evidenced a rheological characterization of the hydrogels and description of the crosslinking methods. Only 37.5% of the studies considered at the same time chondrogenic, fibrotic and hypertrophic markers. Ex vivo studies focused on hydrogel adhesion properties and the modification of MSC-laden hydrogels subjected to compression tests. In vivo studies evidenced the effect of cell-laden hydrogels in OA animal models or defined the chondrogenic potentiality of the cells in subcutaneous implantation models. Clinical studies confirmed the positive impact of these treatments on patients with OA. To speed the translation to the clinical use of cell-laden hydrogels, further studies on hydrogel characteristics, injection modalities, chemo-attractant properties and adhesion strength are needed.

3D Bioprinting Technology and Hydrogels Used in the Process

3D bioprinting has gained visibility in regenerative medicine and tissue engineering due to its applicability. Over time, this technology has been optimized and adapted to ensure a better printability of bioinks and biomaterial inks, contributing to developing structures that mimic human anatomy. Therefore, cross-linked polymeric materials, such as hydrogels, have been highly targeted for the elaboration of bioinks, as they guarantee cell proliferation and adhesion. Thus, this short review offers a brief evolution of the 3D bioprinting technology and elucidates the main hydrogels used in the process.

Advanced injectable hydrogels for cartilage tissue engineering

The rapid development of tissue engineering makes it an effective strategy for repairing cartilage defects. The significant advantages of injectable hydrogels for cartilage injury include the properties of natural extracellular matrix (ECM), good biocompatibility, and strong plasticity to adapt to irregular cartilage defect surfaces. These inherent properties make injectable hydrogels a promising tool for cartilage tissue engineering. This paper reviews the research progress on advanced injectable hydrogels. The cross-linking method and structure of injectable hydrogels are thoroughly discussed. Furthermore, polymers, cells, and stimulators commonly used in the preparation of injectable hydrogels are thoroughly reviewed. Finally, we summarize the research progress of the latest advanced hydrogels for cartilage repair and the future challenges for injectable hydrogels.

Exosomes of mesenchymal stem cells delivered from methacrylated hyaluronic acid patch improve the regenerative properties of endothelial and dermal cells

Wound care management urgently needs the development of innovative smart wound dressings. The complexity of the wound often requires the use of personalized medication and the advent of three-dimensional (3D) bioprinting fits strongly with this need. In this view, in the present work a methacrylated hyaluronic acid (MeHA) bioink was tested for the fabrication of advanced smart patches as a delivery system of exosomes derived from human mesenchymal stem cells (hMSC-EXOs) suitable for wound healing purposes. MeHA patches were realized by 3D bioprinting technique and they were loaded with hMSC-EXOs. The 3D printed MeHA patches revealed improved mechanical performance, appropriate swelling ratio, extended degradation time, and suitable biocompatibility. Furthermore, MeHA patches loaded with hMSC-EXOs improved the proliferation, migration, angiogenic ability, and expression of specific markers related to wound healing process in human fibroblasts and human endothelial cells.

Three-dimensional bio-printing of decellularized extracellular matrix-based bio-inks for cartilage regeneration: a systematic review

Cartilage injuries are common problems that increase with the population aging. Cartilage is an avascular tissue with a relatively low level of cellular mitotic activity, which makes it impossible to heal spontaneously. To compensate for this problem, three-dimensional bio-printing has attracted a great deal of attention in cartilage tissue engineering. This emerging technology aims to create three-dimensional functional scaffolds by accurately depositing layer-by-layer bio-inks composed of biomaterial and cells. As a novel bio-ink, a decellularized extracellular matrix can serve as an appropriate substrate that contains all the necessary biological cues for cellular interactions. Here, this review is intended to provide an overview of decellularized extracellular matrix-based bio-inks and their properties, sources, and preparation process. Following this, decellularized extracellular matrix-based bio-inks for cartilage tissue engineering are discussed, emphasizing cell behavior and in-vivo applications. Afterward, the current challenges and future outlook will be discussed to determine the conclusing remarks.

The Rheology and Printability of Cartilage Matrix-Only Biomaterials

The potential chondroinductivity from cartilage matrix makes it promising for cartilage repair; however, cartilage matrix-based hydrogels developed thus far have failed to match the mechanical performance of native cartilage or be bioprinted without adding polymers for reinforcement. There is a need for cartilage matrix-based hydrogels with robust mechanical performance and paste-like precursor rheology for bioprinting/enhanced surgical placement. In the current study, our goals were to increase hydrogel stiffness and develop the paste-like precursor/printability of our methacryl-modified solubilized and devitalized cartilage (MeSDVC) hydrogels. We compared two methacryloylating reagents, methacrylic anhydride (MA) and glycidyl methacrylate (GM), and varied the molar excess (ME) of MA from 2 to 20. The MA-modified MeSDVCs had greater methacryloylation than GM-modified MeSDVC (20 ME). While GM and most of the MA hydrogel precursors exhibited paste-like rheology, the 2 ME MA and GM MeSDVCs had the best printability (i.e., shape fidelity, filament collapse). After crosslinking, the 2 ME MA MeSDVC had the highest stiffness (1.55 ± 0.23 MPa), approaching the modulus of native cartilage, and supported the viability/adhesion of seeded cells for 15 days. Overall, the MA (2 ME) improved methacryloylation, hydrogel stiffness, and printability, resulting in a stand-alone MeSDVC printable biomaterial. The MeSDVC has potential as a future bioink and has future clinical relevance for cartilage repair.