Covalently conjugated transforming growth factor-β1 in modular chitosan hydrogels for the effective treatment of articular cartilage defects

Vanillin-based functionalization strategy to construct multifunctional microspheres for treating inflammation and regenerating intervertebral disc

Intervertebral disc degeneration (IVDD) is one of the main causes of low back pain. Although local delivery strategies using biomaterial carriers have shown potential for IVDD treatment, it remains challenging for intervention against multiple adverse contributors by a single delivery platform. In the present work, we propose a new functionalization strategy using vanillin, a natural molecule with anti-inflammatory and antioxidant properties, to develop multifunctional gelatin methacrylate (GelMA) microspheres for local delivery of transforming growth factor β3 (TGFβ3) toward IVDD treatment. In vitro, functionalized microspheres not only improved the release kinetics of TGFβ3 but also effectively inhibited inflammatory responses and promoted the secretion of extracellular matrix (ECM) in lipopolysaccharide-induced nucleus pulposus (NP) cells. In vivo, functionalized platform plays roles in alleviating inflammation and oxidative stress, preserving the water content of NP and disc height, and maintaining intact structure and biomechanical functions, thereby promoting the regeneration of IVD. High-throughput sequencing suggests that inhibition of the phosphatidylinositol 3-kinase (PI3K)-Akt signaling might be associated with their therapeutic effects. In summary, the vanillin-based functionalization strategy provides a novel and simple way for packaging multiple functions into a single delivery platform and holds promise for tissue regeneration beyond the IVD.

Application of chitosan with different molecular weights in cartilage tissue engineering

Cartilage tissue engineering involves the invention of novel implantable cartilage replacement materials to help heal cartilage injuries that do not heal themselves, aiming to overcome the shortcomings of current clinical cartilage treatments. Chitosan has been widely used in cartilage tissue engineering because of its similar structure to glycine aminoglycan, which is widely distributed in connective tissues. The molecular weight, as an important structural parameter of chitosan, affects not only the method of chitosan composite scaffold preparation but also the effect on cartilage tissue healing. Thus, this review identifies methods for the preparation of chitosan composite scaffolds with low, medium and high molecular weights, as well as a range of chitosan molecular weights appropriate for cartilage tissue repair, by summarizing the application of different molecular weights of chitosan in cartilage repair in recent years.

Progressive use of nanocomposite hydrogels materials for regeneration of damaged cartilage and their tribological mechanical properties

Osteoarthritis (OA) is a non-inflammatory deteriorating debilitating state that bring about remarkable health and economic issues globally. Break down/deterioration of the articular cartilage (AC) is one of the pathologic characteristics of osteoarthritis (OA). Nanocomposite hydrogels (NCH) materials are evolving as a potential class of scaffolds for organ regeneration and tissue engineering. In recent years, innovative hydrogels specifically loaded with nanoparticles have been developed and synthesized with the goal of changing conventional cartilage treatments. The detailed development of a tailored nanocomposite hydrogels (NCH) material utilized for tissue engineering is presented in this review study. Also, the mechanical characteristics, particularly the tribological behavior, of these produced NCH have been highlighted. Large amounts of research and data on the hydrogel substance utilized in cartilage healing are summarized in the current review study. When determining future research gaps in the area of hydrogels for cartilage regeneration, such information will provide researchers an advantage to further develop NCH.

Advances in the Treatment of Partial-Thickness Cartilage Defect

Partial-thickness cartilage defects (PTCDs) of the articular surface is the most common problem in cartilage degeneration, and also one of the main pathogenesis of osteoarthritis (OA). Due to the lack of a clear diagnosis, the symptoms are often more severe when full-thickness cartilage defect (FTCDs) is present. In contrast to FTCDs and osteochondral defects (OCDs), PTCDs does not injure the subchondral bone, there is no blood supply and bone marrow exudation, and the nearby microenvironment is unsuitable for stem cells adhesion, which completely loses the ability of self-repair. Some clinical studies have shown that partial-thickness cartilage defects is as harmful as full-thickness cartilage defects. Due to the poor effect of conservative treatment, the destructive surgical treatment is not suitable for the treatment of partial-thickness cartilage defects, and the current tissue engineering strategies are not effective, so it is urgent to develop novel strategies or treatment methods to repair PTCDs. In recent years, with the interdisciplinary development of bioscience, mechanics, material science and engineering, many discoveries have been made in the repair of PTCDs. This article reviews the current status and research progress in the treatment of PTCDs from the aspects of diagnosis and modeling of PTCDs, drug therapy, tissue transplantation repair technology and tissue engineering ("bottom-up").

Within or Without You? A Perspective Comparing In Situ and Ex Situ Tissue Engineering Strategies for Articular Cartilage Repair

Human articular cartilage has a poor ability to self-repair, meaning small injuries often lead to osteoarthritis, a painful and debilitating condition which is a major contributor to the global burden of disease. Existing clinical strategies generally do not regenerate hyaline type cartilage, motivating research toward tissue engineering solutions. Prospective cartilage tissue engineering therapies can be placed into two broad categories: i) Ex situ strategies, where cartilage tissue constructs are engineered in the lab prior to implantation and ii) in situ strategies, where cells and/or a bioscaffold are delivered to the defect site to stimulate chondral repair directly. While commonalities exist between these two approaches, the core point of distinction-whether chondrogenesis primarily occurs "within" or "without" (outside) the body-can dictate many aspects of the treatment. This difference influences decisions around cell selection, the biomaterials formulation and the surgical implantation procedure, the processes of tissue integration and maturation, as well as, the prospects for regulatory clearance and clinical translation. Here, ex situ and in situ cartilage engineering strategies are compared: Highlighting their respective challenges, opportunities, and prospects on their translational pathways toward long term human cartilage repair.

Manipulating Cell Fates with Protein Conjugates

The homeostasis of cellular activities is essential for the normal functioning of living organisms. Hence, the ability to regulate the fates of cells is of great significance for both fundamental chemical biology studies and therapeutic development. Despite the notable success of small-molecule drugs that normally act on cellular protein functions, current clinical challenges have highlighted the use of macromolecules to tune cell function for improved therapeutic outcomes. As a class of hybrid biomacromolecules gaining rapidly increasing attention, protein conjugates have exhibited great potential as versatile tools to manipulate cell function for therapeutic applications, including cancer treatment, tissue engineering, and regenerative medicine. Therefore, recent progress in the design and assembly of protein conjugates used to regulate cell function is discussed in this review. The protein conjugates covered here are classified into three different categories based on their mechanisms of action and relevant applications: (1) regulation of intercellular interactions; (2) intervention in intracellular biological pathways; (3) termination of cell proliferation. Within each genre, a variety of protein conjugate scaffolds are discussed, which contain a diverse array of grafted molecules, such as lipids, oligonucleotides, synthetic polymers, and small molecules, with an emphasis on their conjugation methodologies and potential biomedical applications. While the current generation of protein conjugates is focused largely on delivery, the next generation is expected to address issues of site-specific conjugation, in vivo stability, controllability, target selectivity, and biocompatibility.

Synthesis and characterization of growth factor free nanoengineered bioactive scaffolds for bone tissue engineering

BACKGROUND

To address the obstacles that come with orthopedic surgery for biological graft tissues, including immune rejections, bacterial infections, and weak osseointegration, bioactive nanocomposites have been used as an alternative for bone grafting since they can mimic the biological and mechanical properties of the native bone. Among them, PCL-PEG-PCL (PCEC) copolymer has gained much attention for bone tissue engineering as a result of its biocompatibility and ability for osteogenesis.

METHODS

Here, we designed a growth factor-free nanoengineered scaffold based on the incorporation of Fe₃O₄ and hydroxyapatite (HA) nanoparticles into the PCL-PEG-PCL/Gelatin (PCEC/Gel) nanocomposite. We characterized different formulations of nanocomposite scaffolds in terms of physicochemical properties. Also, the mechanical property and specific surface area of the prepared scaffolds, as well as their feasibility for human dental pulp stem cells (hDPSCs) adhesion were assessed.

RESULTS

The results of in vitro cell culture study revealed that the PCEC/Gel Fe₃O₄&HA scaffold could promote osteogenesis in comparison with the bare scaffold, which confirmed the positive effect of the Fe₃O₄ and HA nanoparticles in the osteogenic differentiation of hDPSCs.

CONCLUSION

The incorporation of Fe₃O₄ and HA with PCEC/gelatin could enhance osteogenic differentiation of hDPSCs for possible substitution of bone grafting tissue.

Instructive cartilage regeneration modalities with advanced therapeutic implantations under abnormal conditions

The development of interdisciplinary biomedical engineering brings significant breakthroughs to the field of cartilage regeneration. However, cartilage defects are considerably more complicated in clinical conditions, especially when injuries occur at specific sites (e.g., osteochondral tissue, growth plate, and weight-bearing area) or under inflammatory microenvironments (e.g., osteoarthritis and rheumatoid arthritis). Therapeutic implantations, including advanced scaffolds, developed growth factors, and various cells alone or in combination currently used to treat cartilage lesions, address cartilage regeneration under abnormal conditions. This review summarizes the strategies for cartilage regeneration at particular sites and pathological microenvironment regulation and discusses the challenges and opportunities for clinical transformation.

Combination of mesenchymal stem cells and bioactive molecules in hydrogels for osteoarthritis treatment

Osteoarthritis (OA) is a chronic and inflammatory disease with no effective regenerative treatments to date. The therapeutic potential of mesenchymal stem cells (MSCs) remains to be fully explored. Intra-articular injection of these cells promotes cartilage protection and regeneration by paracrine signaling and differentiation into chondrocytes. However, joints display a harsh avascular environment for these cells upon injection. This phenomenon prompted researchers to develop suitable injectable materials or systems for MSCs to enhance their function and survival. Among them, hydrogels can absorb a large amount of water and maintain their 3D structure but also allow incorporation of bioactive agents or small molecules in their matrix that maximize the action of MSCs. These materials possess advantageous cartilage-like features such as collagen or hyaluronic acid moieties that interact with MSC receptors, thereby promoting cell adhesion. This review provides an up-to-date overview of the progress and opportunities of MSCs entrapped into hydrogels, combined with bioactive/small molecules to improve the therapeutic effects in OA treatment.

Tethered TGF-β1 in a Hyaluronic Acid-Based Bioink for Bioprinting Cartilaginous Tissues

In 3D bioprinting for cartilage regeneration, bioinks that support chondrogenic development are of key importance. Growth factors covalently bound in non-printable hydrogels have been shown to effectively promote chondrogenesis. However, studies that investigate the functionality of tethered growth factors within 3D printable bioinks are still lacking. Therefore, in this study, we established a dual-stage crosslinked hyaluronic acid-based bioink that enabled covalent tethering of transforming growth factor-beta 1 (TGF-β1). Bone marrow-derived mesenchymal stromal cells (MSCs) were cultured over three weeks in vitro, and chondrogenic differentiation of MSCs within bioink constructs with tethered TGF-β1 was markedly enhanced, as compared to constructs with non-covalently incorporated TGF-β1. This was substantiated with regard to early TGF-β1 signaling, chondrogenic gene expression, qualitative and quantitative ECM deposition and distribution, and resulting construct stiffness. Furthermore, it was successfully demonstrated, in a comparative analysis of cast and printed bioinks, that covalently tethered TGF-β1 maintained its functionality after 3D printing. Taken together, the presented ink composition enabled the generation of high-quality cartilaginous tissues without the need for continuous exogenous growth factor supply and, thus, bears great potential for future investigation towards cartilage regeneration. Furthermore, growth factor tethering within bioinks, potentially leading to superior tissue development, may also be explored for other biofabrication applications.

Hydrogels: 3D Drug Delivery Systems for Nanoparticles and Extracellular Vesicles

Synthetic and naturally occurring nano-sized particles present versatile vehicles for the delivery of therapy in a range of clinical settings. Their small size and modifiable physicochemical properties support refinement of targeting capabilities, immune response, and therapeutic cargo, but rapid clearance from the body and limited efficacy remain a major challenge. This highlights the need for a local sustained delivery system for nanoparticles (NPs) and extracellular vesicles (EVs) at the target site that will ensure prolonged exposure, maximum efficacy and dose, and minimal toxicity. Biocompatible hydrogels loaded with therapeutic NPs/EVs hold immense promise as cell-free sustained and targeted delivery systems in a range of disease settings. These bioscaffolds ensure retention of the nano-sized particles at the target site and can also act as controlled release systems for therapeutics over a prolonged period of time. The encapsulation of stimuli sensitive components into hydrogels supports the release of the content on-demand. In this review, we highlight the prospect of the sustained and prolonged delivery of these nano-sized therapeutic entities from hydrogels for broad applications spanning tissue regeneration and cancer treatment. Further understanding of the parameters controlling the release rate of these particles and efficient transfer of cargo to target cells will be fundamental to success.

Translational Applications of Hydrogels

Advances in hydrogel technology have unlocked unique and valuable capabilities that are being applied to a diverse set of translational applications. Hydrogels perform functions relevant to a range of biomedical purposes-they can deliver drugs or cells, regenerate hard and soft tissues, adhere to wet tissues, prevent bleeding, provide contrast during imaging, protect tissues or organs during radiotherapy, and improve the biocompatibility of medical implants. These capabilities make hydrogels useful for many distinct and pressing diseases and medical conditions and even for less conventional areas such as environmental engineering. In this review, we cover the major capabilities of hydrogels, with a focus on the novel benefits of injectable hydrogels, and how they relate to translational applications in medicine and the environment. We pay close attention to how the development of contemporary hydrogels requires extensive interdisciplinary collaboration to accomplish highly specific and complex biological tasks that range from cancer immunotherapy to tissue engineering to vaccination. We complement our discussion of preclinical and clinical development of hydrogels with mechanical design considerations needed for scaling injectable hydrogel technologies for clinical application. We anticipate that readers will gain a more complete picture of the expansive possibilities for hydrogels to make practical and impactful differences across numerous fields and biomedical applications.

Injectable Scaffold for Bone Marrow Stem Cells and Bone Morphogenetic Protein-2 to Repair Cartilage

OBJECTIVE

The limits of the microfracture (MFX) treatment in terms of lesion size and long-term tissue functionality makes it necessary to investigate different alternatives to repair focal cartilage lesions. The present study aims at evaluating the efficacy of a minimally invasive approach against the conventional MFX to repair a chondral defect in rabbits. An injectable scaffold of BMP-2 pre-encapsulated in PLGA microspheres dispersed in a Pluronic F-127 solution is proposed as support of cells and controlled delivery system for the growth factor.

DESIGN

MFX was compared versus the injectable system seeded with mesenchymal stem cells (MSCs), both without BMP-2 and under controlled release of BMP-2 at 2 different doses (3 and 12 µg/scaffold). The different treatments were evaluated on a 4-mm diameter chondral defect model using 9 experimental groups of 4 rabbits (8 knees) each, throughout 24 weeks.

RESULTS

Histologically, all the treated groups, except MFX treated, responded significantly better than the control group (nontreated defect). Although no significant differences were found between the treated groups, only BMP(12), MSC-BMP(12), and MFX-BMP(3) groups showed nonsignificant differences when compared with the normal cartilage.

CONCLUSIONS

The hydrogel system proposed to control the release rate of the BMP-2 was safe, easily injectable, and also provided good support for cells. Treatments with MSCs or BMP-2 repaired efficiently the chondral lesion created in rabbits, being less invasive than MFX treatment.

Strategies to Control or Mimic Growth Factor Activity for Bone, Cartilage, and Osteochondral Tissue Engineering

Growth factors play a critical role in tissue repair and regeneration. However, their clinical success is limited by their low stability, short half-life, and rapid diffusion from the delivery site. Supraphysiological growth factor concentrations are often required to demonstrate efficacy but can lead to adverse reactions, such as inflammatory complications and increased cancer risk. These issues have motivated the development of delivery systems that enable sustained release and controlled presentation of growth factors. This review specifically focuses on bioconjugation strategies to enhance growth factor activity for bone, cartilage, and osteochondral applications. We describe approaches to localize growth factors using noncovalent and covalent methods, bind growth factors via peptides, and mimic growth factor function with mimetic peptide sequences. We also discuss emerging and future directions to control spatiotemporal growth factor delivery to improve functional tissue repair and regeneration.

Silk fibroin hydrogel scaffolds incorporated with chitosan nanoparticles repair articular cartilage defects by regulating TGF-β1 and BMP-2

Cartilage defects frequently occur around the knee joint yet cartilage has limited self-repair abilities. Hydrogel scaffolds have excellent potential for use in tissue engineering. Therefore, the aim of the present study was to assess the ability of silk fibroin (SF) hydrogel scaffolds incorporated with chitosan (CS) nanoparticles (NPs) to repair knee joint cartilage defects. In the present study, composite systems of CS NPs incorporated with transforming growth factor-β1 (TGF-β1; TGF-β1@CS) and SF incorporated with bone morphogenetic protein-2 (BMP-2; TGF-β1@CS/BMP-2@SF) were developed and characterized with respect to their size distribution, zeta potential, morphology, and release of TGF-β1 and BMP-2. Bone marrow stromal cells (BMSCs) were co-cultured with TGF-β1@CS/BMP-2@SF extracts to assess chondrogenesis in vitro using a cell counting kit-8 assay, which was followed by in vivo evaluations in a rabbit model of knee joint cartilage defects. The constructed TGF-β1@CS/BMP-2@SF composite system was successfully characterized and showed favorable biocompatibility. In the presence of TGF-β1@CS/BMP-2@SF extracts, BMSCs exhibited normal cell morphology and enhanced chondrogenic ability both in vitro and in vivo, as evidenced by the promotion of cell viability and the alleviation of cartilage defects. Thus, the TGF-β1@CS/BMP-2@SF hydrogel developed in the present study promoted chondrogenic ability of BMSCs both in vivo and in vitro by releasing TGF-β1 and BMP-2, thereby offering a novel therapeutic strategy for repairing articular cartilage defects in knee joints.

Advanced hydrogels for the repair of cartilage defects and regeneration

Cartilage defects are one of the most common symptoms of osteoarthritis (OA), a degenerative disease that affects millions of people world-wide and places a significant socio-economic burden on society. Hydrogels, which are a class of biomaterials that are elastic, and display smooth surfaces while exhibiting high water content, are promising candidates for cartilage regeneration. In recent years, various kinds of hydrogels have been developed and applied for the repair of cartilage defects in vitro or in vivo, some of which are hopeful to enter clinical trials. In this review, recent research findings and developments of hydrogels for cartilage defects repair are summarized. We discuss the principle of cartilage regeneration, and outline the requirements that have to be fulfilled for the deployment of hydrogels for medical applications. We also highlight the development of advanced hydrogels with tailored properties for different kinds of cartilage defects to meet the requirements of cartilage tissue engineering and precision medicine.

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The biotechnology of developing hydrogels for cartilage defects repair is promising.

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The principle for cartilage regeneration using hydrogels and requirements for clinical transformation are summarized.

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Advanced hydrogels with tailored properties for different kinds of cartilage defects are discussed.

Exquisite design of injectable Hydrogels in Cartilage Repair

Cartilage damage is still a threat to human beings, yet there is currently no treatment available to fully restore the function of cartilage. Recently, due to their unique structures and properties, injectable hydrogels have been widely studied and have exhibited high potential for applications in therapeutic areas, especially in cartilage repair. In this review, we briefly introduce the properties of cartilage, some articular cartilage injuries, and now available treatment strategies. Afterwards, we propose the functional and fundamental requirements of injectable hydrogels in cartilage tissue engineering, as well as the main advantages of injectable hydrogels as a therapy for cartilage damage, including strong plasticity and excellent biocompatibility. Moreover, we comprehensively summarize the polymers, cells, and bioactive molecules regularly used in the fabrication of injectable hydrogels, with two kinds of gelation, i.e., physical and chemical crosslinking, which ensure the excellent design of injectable hydrogels for cartilage repair. We also include novel hybrid injectable hydrogels combined with nanoparticles. Finally, we conclude with the advances of this clinical application and the challenges of injectable hydrogels used in cartilage repair.

Smoothened agonist sterosome immobilized hybrid scaffold for bone regeneration

Biomaterial delivery of bioactive agents and manipulation of stem cell fate are an attractive approach to promote tissue regeneration. Here, smoothened agonist sterosome is developed using small-molecule activators [20S-hydroxycholesterol (OHC) and purmorphamine (PUR)] of the smoothened protein in the hedgehog pathway as carrier and cargo. Sterosome presents inherent osteoinductive property even without drug loading. Sterosome is covalently immobilized onto three-dimensional scaffolds via a bioinspired polydopamine intermediate to fabricate a hybrid scaffold for bone regeneration. Sterosome-immobilized hybrid scaffold not only provides a favorable substrate for cell adhesion and proliferation but also delivers bioactive agents in a sustained and spatially targeted manner. Furthermore, this scaffold significantly improves osteogenic differentiation of bone marrow stem cells through OHC/PUR-mediated synergistic activation of the hedgehog pathway and also enhances bone repair in a mouse calvarial defect model. This system serves as a versatile biomaterial platform for many applications, including therapeutic delivery and endogenous regenerative medicine.

Microporous methacrylated glycol chitosan-montmorillonite nanocomposite hydrogel for bone tissue engineering

Injectable hydrogels can fill irregular defects and promote in situ tissue regrowth and regeneration. The ability of directing stem cell differentiation in a three-dimensional microenvironment for bone regeneration remains a challenge. In this study, we successfully nanoengineer an interconnected microporous networked photocrosslinkable chitosan in situ-forming hydrogel by introducing two-dimensional nanoclay particles with intercalation chemistry. The presence of the nanosilicates increases the Young’s modulus and stalls the degradation rate of the resulting hydrogels. We demonstrate that the reinforced hydrogels promote the proliferation as well as the attachment and induced the differentiation of encapsulated mesenchymal stem cells in vitro. Furthermore, we explore the effects of nanoengineered hydrogels in vivo with the critical-sized mouse calvarial defect model. Our results confirm that chitosan-montmorillonite hydrogels are able to recruit native cells and promote calvarial healing without delivery of additional therapeutic agents or stem cells, indicating their tissue engineering potential.

Injectable hydrogels could be used to repair bone defects. Here the authors incorporate nanoclay particles into chitosan creating an interconnected microporous hydrogel and show that this hydrogel can support MSC proliferation and differentiation in vitro, and support the recruitment of native cells and bone regeneration in a mouse calvarial defect model.

An in vitro and in vivo comparison of cartilage growth in chondrocyte-laden matrix metalloproteinase-sensitive poly(ethylene glycol) hydrogels with localized transforming growth factor β3

While matrix-assisted autologous chondrocyte implantation has emerged as a promising therapy to treat focal chondral defects, matrices that support regeneration of hyaline cartilage remain challenging. The goal of this work was to investigate the potential of a matrix metalloproteinase (MMP)-sensitive poly(ethylene glycol) (PEG) hydrogel containing the tethered growth factor, transforming growth factor β3 (TGF-β3), and compare cartilage regeneration in vitro and in vivo. The in vitro environment comprised chemically-defined medium while the in vivo environment utilized the subcutaneous implant model in athymic mice. Porcine chondrocytes were isolated and expanded in 2D culture for 10 days prior to encapsulation. The presence of tethered TGF-β3 reduced cell spreading. Chondrocyte-laden hydrogels were analyzed for total sulfated glycosaminoglycan and collagen contents, MMP activity, and spatial deposition of aggrecan, decorin, biglycan, and collagens type II and I. The total amount of extracellular matrix (ECM) deposited in the hydrogel constructs was similar in vitro and in vivo. However, the in vitro environment was not able to support long-term culture up to 64 days of the engineered cartilage leading to the eventual breakdown of aggrecan. The in vivo environment, on the other hand, led to more elaborate ECM, which correlated with higher MMP activity, and an overall higher quality of engineered tissue that was rich in aggrecan, decorin, biglycan and collagen type II with minimal collagen type I. Overall, the MMP-sensitive PEG hydrogel containing tethered TGF-β3 is a promising matrix for hyaline cartilage regeneration in vivo. STATEMENT OF SIGNIFICANCE: Regenerating hyaline cartilage remains a significant clinical challenge. The resultant repair tissue is often fibrocartilage, which long-term cannot be sustained. The goal of this study was to investigate the potential of a synthetic hydrogel matrix containing peptide crosslinks that can be degraded by enzymes secreted by encapsulated cartilage cells (i.e., chondrocytes) and tethered growth factors, specifically TGF-β3, to provide localized chondrogenic cues to the cells. This hydrogel led to hyaline cartilage-like tissue growth in vitro and in vivo, with minimal formation of fibrocartilage. However, the tissue formed in vitro, could not be maintained long-term. In vivo this hydrogel shows great promise as a potential matrix for use in regenerating hyaline cartilage.

Adipose-Derived Mesenchymal Stem Cells: A Promising Tool in the Treatment of Musculoskeletal Diseases

Chronic musculoskeletal (MSK) pain is one of the most common medical complaints worldwide and musculoskeletal injuries have an enormous social and economical impact. Current pharmacological and surgical treatments aim to relief pain and restore function; however, unsatiscactory outcomes are commonly reported. In order to find an accurate treatment to such pathologies, over the last years, there has been a significantly increasing interest in cellular therapies, such as adipose-derived mesenchymal stem cells (AMSCs). These cells represent a relatively new strategy in regenerative medicine, with many potential applications, especially regarding MSK disorders, and preclinical and clinical studies have demonstrated their efficacy in muscle, tendon, bone and cartilage regeneration. Nevertheless, several worries about their safety and side effects at long-term remain unsolved. This article aims to review the current state of AMSCs therapy in the treatment of several MSK diseases and their clinical applications in veterinary and human medicine.

Advances of injectable hydrogel-based scaffolds for cartilage regeneration

Articular cartilage is an important load-bearing tissue distributed on the surface of diarthrodial joints. Due to its avascular, aneural and non-lymphatic features, cartilage has limited self-regenerative properties. To date, the utilization of biomaterials to aid in cartilage regeneration, especially through the use of injectable scaffolds, has attracted considerable attention. Various materials, therapeutics and fabrication approaches have emerged with a focus on manipulating the cartilage microenvironment to induce the formation of cartilaginous structures that have similar properties to the native tissues. In particular, the design and fabrication of injectable hydrogel-based scaffolds have advanced in recent years with the aim of enhancing its therapeutic efficacy and improving its ease of administration. This review summarizes recent progress in these efforts, including the structural improvement of scaffolds, network cross-linking techniques and strategies for controlled release, which present new opportunities for the development of injectable scaffolds for cartilage regeneration.

Universal Biofactor-Releasing Scaffold Enabling in Vivo Reloading

The supply of growth factors to engineered tissues is essential for many physiological processes. These processes include the proper organization of the cells into functioning tissues, maintenance of their viability, vasculogenesis, proliferation, and differentiation. Systems to efficiently control the release of growth factors were previously incorporated into tissue engineering scaffolds to affect cells. However, because the initial concentration of the factors in these systems is finite, their ability to provide a long-term physiological effect is limited. Here, we report on a new reloadable system in which 3D fibrous scaffolds conjugated with an anti His-tag antibody enable the retention and controlled release of any His-tag-modified proteinaceous growth factor. The scaffolds can be reloaded in vitro or in vivo with any His-tagged biomolecule at any time according to the physiological need. We show the ability of the scaffolds to release angiogenic factors in a static cell culture or under flow in a microfluidics device and effect on endothelial cells. We also demonstrate the potential of the system to be sequentially reloaded in vivo with various factors, and as a proof of concept, we provide evidence for the efficient in vivo vascularization of scaffolds after reloading with tagged VEGF.

Chitin/Chitosan: Versatile Ecological, Industrial, and Biomedical Applications

Chitin is a linear polysaccharide of N-acetylglucosamine, which is highly abundant in nature and mainly produced by marine crustaceans. Chitosan is obtained by hydrolytic deacetylation. Both polysaccharides are renewable resources, simply and cost-effectively extracted from waste material of fish industry, mainly crab and shrimp shells. Research over the past five decades has revealed that chitosan, in particular, possesses unique and useful characteristics such as chemical versatility, polyelectrolyte properties, gel- and film-forming ability, high adsorption capacity, antimicrobial and antioxidative properties, low toxicity, and biocompatibility and biodegradability features. A plethora of chemical chitosan derivatives have been synthesized yielding improved materials with suggested or effective applications in water treatment, biosensor engineering, agriculture, food processing and storage, textile additives, cosmetics fabrication, and in veterinary and human medicine. The number of studies in this research field has exploded particularly during the last two decades. Here, we review recent advances in utilizing chitosan and chitosan derivatives in different technical, agricultural, and biomedical fields.

Materials Science and Design Principles of Growth Factor Delivery Systems in Tissue Engineering and Regenerative Medicine

Growth factors (GFs) are signaling molecules that direct cell development by providing biochemical cues for stem cell proliferation, migration, and differentiation. GFs play a key role in tissue regeneration, but one major limitation of GF-based therapies is dosage-related adverse effects. Additionally, the clinical applications and efficacy of GFs are significantly affected by the efficiency of delivery systems and other pharmacokinetic factors. Hence, it is crucial to design delivery systems that provide optimal activity, stability, and tunable delivery for GFs. Understanding the physicochemical properties of the GFs and the biomaterials utilized for the development of biomimetic GF delivery systems is critical for GF-based regeneration. Many different delivery systems have been developed to achieve tunable delivery kinetics for single or multiple GFs. The identification of ideal biomaterials with tunable properties for spatiotemporal delivery of GFs is still challenging. This review characterizes the types, properties, and functions of GFs, the materials science of widely used biomaterials, and various GF loading strategies to comprehensively summarize the current delivery systems for tunable spatiotemporal delivery of GFs aimed for tissue regeneration applications. This review concludes by discussing fundamental design principles for GF delivery vehicles based on the interactive physicochemical properties of the proteins and biomaterials.

Chitosan-Lysozyme Conjugates for Enzyme-Triggered Hydrogel Degradation in Tissue Engineering Applications

Tuning hydrogel degradation enables effective and successful tissue regeneration by modulating cellular behaviors and matrix formation. In this work, we develop a novel degradable hydrogel scaffold on the basis of a unique enzyme-substrate complex by photocrosslinking. Chitosan and lysozyme are chemically modified with methacrylate moieties to be tethered in hydrogels, and in the presence of riboflavin initiator, these hydrogels are cured by blue light irradiation. The incorporation of lysozyme to chitosan hydrogels accelerates the degradation rate of the crosslinked hydrogels in a dose-dependent manner, as evidenced by an increase in pore size and interconnectivity through cryogenic scanning electron microscopy over time. Those noncytotoxic materials significantly enhance cellular proliferation and migration, which contribute to osteogenic differentiation of encapsulated mesenchymal stem cells in vitro and bone formation in mouse calvarial defects. These findings suggest a promising strategy to modulate the degradation behavior of hydrogels for use in tissue engineering.

Adipose-Derived Mesenchymal Stem Cells: Are They a Good Therapeutic Strategy for Osteoarthritis?

Osteoarthritis (OA) is a major cause of disability in elderly population around the world. More than one-third of people over 65 years old shows either clinical or radiological evidence of OA. There is no effective treatment for this degenerative disease, due to the limited capacity for spontaneous cartilage regeneration. Regarding the use of regenerative therapies, it has been reported that one option to restore degenerated cartilage are adipose-derived mesenchymal stem cells (ASCs). The purpose of this review is to describe and compare the efficacy of ASCs versus other therapies in OA. Methods: Recent studies have shown that ASCs exert paracrine effects protecting against degenerative changes in chondrocytes. According to the above, we have carried out a review of the literature using a combination of osteoarthritis, stem cells, and regenerative therapies as keywords. Results: Conventional pharmacological therapies for OA treatment are considered before the surgical option, however, they do not stop the progression of the disease. Moreover, total joint replacement is not recommended for patients under 55 years, and high tibia osteotomy (HTO) is a viable solution to address lower limb malalignment with concomitant OA, but some complications have been described. In recent years, the use of mesenchymal stem cells (MSCs) as a treatment strategy for OA is increasing considerably, thanks to their capacity to improve symptoms together with joint functionality and, therefore, the patients’ quality of life. Conclusions: ASC therapy has a positive effect on patients with OA, although there is limited evidence and little long-term follow-up.

Design of hydrogels to stabilize and enhance bone morphogenetic protein activity by heparin mimetics

Although bone morphogenetic protein-2 (BMP-2) is known to be the most potent stimulator available for bone formation, a major barrier to widespread clinical use is its inherent instability and absence of an adequate delivery system. Heparin is being widely used in controlled release systems due to its strong binding ability and protective effect for many growth factor proteins. In this work, we developed a hydrogel surface that can mimic heparin to stabilize BMP-2 and to enhance osteogenesis by introducing heparin-mimicking sulfonated molecules such as poly-vinylsulfonic acid (PVSA) or poly-4-styrenesulfonic acid (PSS), into photo-crosslinkable hydrogel. Bioactivity of BMP-2 was well preserved in the presence of polysulfonates during exposure to various therapeutically relevant stressors. The heparin-mimicking sulfonated hydrogels were effective to bind BMP-2 compared to unmodified MeGC hydrogel and significantly enhanced osteogenic differentiation of encapsulated bone marrow stromal cells (BMSCs) without the addition of exogenous BMP-2. The sulfonated hydrogels were effective in delivering exogenous BMP-2 with reduced initial burst and increased BMSCs osteogenesis induced by BMP-2. These findings suggest a novel hydrogel platform for sequestering and stabilizing BMP-2 to enhance osteoinductive activity in bone tissue engineering.

STATEMENT OF SIGNIFICANCE

Although bone morphogenetic protein-2 (BMP-2) is believed to be the most potent cytokine for bone regeneration, its clinical applications require supraphysiological BMP dosage due to its intrinsic instability and fast enzymatic degradation, leading to worrisome side effects. This study demonstrates a novel hydrogel platform that mimics a natural protector of BMPs, heparin, to sequester and stabilize BMP-2 for increased osteoinductive signaling. This study will achieve the stabilization of BMPs with prolonged bioactivity by a synthetic heparin mimic that has not been examined previously. Moreover, the heparin mimetic hydrogel surface can augment endogenous BMP activity by sequestering and localizing the cell-produced BMPs. The additional knowledge gained from this study may suggest basis for future development of material-based therapeutics for tissue engineering.

Targeted delivery of FGF2 to subchondral bone enhanced the repair of articular cartilage defect

It is reported that growth factor (GF) is able to enhance the repair of articular cartilage (AC) defect, however underlying mechanisms of which are not fully elucidated yet. Moreover, the strategy for delivering GF needs to be optimized. The crosstalk between AC and subchondral bone (SB) play important role in the homeostasis and integrity of AC, therefore SB targeted delivery of GF represents one promising way to facilitate the repair of AC defect. In this study, we firstly investigated the effects and mechanism of FGF2 on surrounding SB and cartilage of detect defects in rabbits by using a homogenous collagen-based membranes. It was found that FGF2 had a modulating effect on the defect-surrounding SB via upregulation of bone morphogenetic protein (BMP)-2, BMP4 and SOX9 at the early stage. Low dose FGF2 improved the repair upon directly injected to SB. Inhibition of BMP signaling pathway compromised the beneficial effects of FGF2, which indicated the pivotal roles of BMP in the process. To facilitate SB targeted FGF2 delivery, a double-layered inhomogeneous collagen membrane was prepared and it induced increase of BMP2 and BMP4 in the synovial fluid, and subsequent successful repair of AC defect. Taken together, this targeted delivery of FGF2 to SB provides a promising strategy for AC repair owing to the relatively clear mechanism, less amount of it, and short duration of delivery.

STATEMENT OF SIGNIFICANCE

Articular cartilage (AC) and subchondral bone (SB) form an integral functional unit. The homeostasis and integrity of AC depend on its crosstalk with the SB. However, the function of the SB in AC defect repair is not completely understood. The application of growth factors to promote the repair articular cartilage defect is a promising strategy, but still under the optimization. Our study demonstrate that SB plays important roles in the repair of AC defect. Particularly, SB is the effective target of fibroblast growth factor 2 (FGF2), and targeted delivery of FGF2 can modulate SB and thus significantly enhances the repair of AC defect. Therefore, targeted delivery of growth factor to SB is a novel promising strategy to improve the repair of AC defect.

Microenvironmental Regulation of Stem Cell Behavior Through Biochemical and Biophysical Stimulation

Stem cells proliferate by undergoing self-renewal and differentiate into multiple cell lineages in response to biochemical and biophysical stimuli. Various biochemical cues such as growth factors, nucleic acids, chemical reagents, and small molecules have been used to induce stem cell differentiation or reprogramming or to maintain their pluripotency. Moreover, biophysical cues such as matrix stiffness, substrate topography, and external stress and strain play a major role in modulating stem cell behavior. In this chapter, we have summarized microenvironmental regulation of stem cell behavior through biochemical and biophysical stimulation.

Hydrogels for Cartilage Regeneration, from Polysaccharides to Hybrids

The aims of this paper are: (1) to review the current state of the art in the field of cartilage substitution and regeneration; (2) to examine the patented biomaterials being used in preclinical and clinical stages; (3) to explore the potential of polymeric hydrogels for these applications and the reasons that hinder their clinical success. The studies about hydrogels used as potential biomaterials selected for this review are divided into the two major trends in tissue engineering: (1) the use of cell-free biomaterials; and (2) the use of cell seeded biomaterials. Preparation techniques and resulting hydrogel properties are also reviewed. More recent proposals, based on the combination of different polymers and the hybridization process to improve the properties of these materials, are also reviewed. The combination of elements such as scaffolds (cellular solids), matrices (hydrogel-based), growth factors and mechanical stimuli is needed to optimize properties of the required materials in order to facilitate tissue formation, cartilage regeneration and final clinical application. Polymer combinations and hybrids are the most promising materials for this application. Hybrid scaffolds may maximize cell growth and local tissue integration by forming cartilage-like tissue with biomimetic features.

Transforming growth factor-beta1 promotes articular cartilage repair through canonical Smad and Hippo pathways in bone mesenchymal stem cells

AIMS

Transforming growth factor-β1 (TGF-β1) is a chondrogenic factor and has been reported to be able to enhance chondrocyte differentiation from bone marrow mesenchymal stem cells (BMSCs). Here we investigate the molecular mechanism through which TGF-β1 chronically promotes the repair of cartilage defect and inhibit chondrocyte hypertrophy.

MAIN METHODS

Animal models of full thickness cartilage defects were divided into three groups: model group, BMSCs group (treated with BMSCs/calcium alginate gel) and BMSCs+TGF-β1 group (treated with Lentivirus-TGF-β1-EGFP transduced BMSCs/calcium alginate gel). 4 and 8weeks after treatment, macroscopic observation, histopathological study and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) were done to analyze phenotypes of the animals. BMSCs were transduced with Lentivirus-TGF-β1-EGFP in vitro and Western blot analysis was performed.

KEY FINDINGS

We found that TGF-β1-expressiing BMSCs improved the repair of the cartilage defect. The impaired cartilage contained higher amount of GAG and type II collagen and was integrated to the surrounding normal cartilage and higher content of GAG and type II collagen. The major events include increased expression of type II collagen following Smad2/3 phosphorylation, and inhibition of cartilage hypertrophy by increasing Yes-associated protein-1 (YAP-1) and inhibiting Runx2 and Col10 after the completion of chondrogenic differentiation.

SIGNIFICANCE

We conclude that TGF-β1 is beneficial to chondrogenic differentiation of BMSCs via canonical Smad pathway to promote early-repairing of cartilage defect. Furthermore, TGF-β1 inhibits chondrocyte hypertrophy by decreasing hypertrophy marker gene expression via Hippo signaling. Long-term rational use of TGF-β1 may be an alternative approach in clinic for cartilage repair and regeneration.

Covalent Incorporation of Heparin Improves Chondrogenesis in Photocurable Gelatin-Methacryloyl Hydrogels

Multicomponent gelatin-methacryloyl (GelMA) hydrogels are regularly adopted for cartilage tissue engineering (TE) applications, where optimizing chemical modifications for preserving biofunctionality is often overlooked. This study investigates the biological effect of two different modification methods, methacrylation and thiolation, to copolymerize GelMA and heparin. The native bioactivity of methacrylated heparin (HepMA) and thiolated heparin (HepSH) is evaluated via thromboplastin time and heparan sulfate-deficient myeloid cell-line proliferation assay, demonstrating that thiolation is superior for preserving anticoagulation and growth factor signaling capacity. Furthermore, incorporating either HepMA or HepSH in chondrocyte-laden GelMA hydrogels, cultured for 5 weeks under chondrogenic conditions, promotes cell viability and chondrocyte phenotype. However, only GelMA-HepSH hydrogels yield significantly greater differentiation and matrix deposition in vitro compared to GelMA. This study demonstrates that thiol-ene chemistry offers a favorable strategy for incorporating bioactives into gelatin hydrogels as compared to methacrylation while furthermore highlighting GelMA-HepSH hydrogels as candidates for cartilage TE applications.

Simultaneous delivery of hydrophobic small molecules and siRNA using Sterosomes to direct mesenchymal stem cell differentiation for bone repair

The use of small molecular drugs with gene manipulation offers synergistic therapeutic efficacy by targeting multiple signaling pathways for combined treatment. Stimulation of mesenchymal stem cells (MSCs) with osteoinductive small molecule phenamil combined with suppression of noggin is a promising therapeutic strategy that increases bone morphogenetic protein (BMP) signaling and bone repair. Our cationic Sterosome formulated with stearylamine (SA) and cholesterol (Chol) is an attractive co-delivery system that not only forms stable complexes with small interfering RNA (siRNA) molecules but also solubilizes hydrophobic small molecules in a single vehicle, for directing stem cell differentiation. Herein, we demonstrate the ability of SA/Chol Sterosomes to simultaneously deliver hydrophobic small molecule phenamil and noggin-directed siRNA to enhance osteogenic differentiation of MSCs both in in vitro two- and three-dimensional settings as well as in a mouse calvarial defect model. These results suggest a novel liposomal platform to simultaneously deliver therapeutic genes and small molecules for combined therapy.

STATEMENT OF SIGNIFICANCE

Application of phenamil, a small molecular bone morphogenetic protein (BMP) stimulator, combined with suppression of natural BMP antagonists such as noggin is a promising therapeutic strategy to enhance bone regeneration. Here, we present a novel strategy to co-deliver hydrophobic small molecule phenamil and noggin-targeted siRNA via cationic Sterosomes formed with stearylamine (SA) and high content of cholesterol (Chol) to enhance osteogenesis and bone repair. SA/Chol Sterosomes demonstrated high phenamil encapsulation efficiency, supported sustained release of encapsulated drugs, and significantly reduced drug dose requirements to induce osteogenic differentiation of mesenchymal stem cells (MSCs). Simultaneous deliver of phenamil and noggin siRNA in a single vehicle synergistically enhanced MSC osteogenesis and calvarial bone repair. This study suggests a new non-phospholipid liposomal formulation to simultaneously deliver small molecules and therapeutic genes for combined treatment.

Fabrication and characterization of hydrothermal cross-linked chitosan porous scaffolds for cartilage tissue engineering applications

The use of various chemical cross-linking agents for the improvement of scaffolds physical and mechanical properties is a common practical method, which is limited by cytotoxicity effects. Due to exerting contract type forces, chondrocytes are known to implement shrinkage on the tissue engineered constructs, which can be avoided by the scaffold cross-linking. In the this research, chitosan scaffolds are cross-linked with hydrothermal treatment with autoclave sterilization time of 0, 10, 20 and 30min, to avoid the application of the traditional chemical toxic materials. The optimization studies with gel content and crosslink density measurements indicate that for 20min sterilization time, the gel content approaches to ~80%. The scaffolds are fully characterized by the conventional techniques such as SEM, porosity and permeability, XRD, compression, thermal analysis and dynamic mechanical thermal analysis (DMTA). FT-IR studies shows that autoclave inter-chain cross-linking reduces the amine group absorption at 1560cm^-1 and increase the absorption of N-acetylated groups at 1629cm^-1. It is anticipated, that this observation evidenced by chitosan scaffold browning upon autoclave cross-linking is an indication of the familiar maillard reaction between amine moieties and carbonyl groups. The biodegradation rate analysis shows that chitosan scaffolds with lower concentrations, possess suitable degradation rate for cartilage tissue engineering applications. In addition, cytotoxicity analysis shows that fabricated scaffolds are biocompatible. The human articular chondrocytes seeding into 3D cross-linked scaffolds shows a higher viability and proliferation in comparison with the uncross-linked samples and 2D controls. Investigation of cell morphology on the scaffolds by SEM, shows a more spherical morphology of chondrocytes on the cross-linked scaffolds for 21days of in vitro culture.

Recent advances in hydrogels for cartilage tissue engineering

Articular cartilage is a load-bearing tissue that lines the surface of bones in diarthrodial joints. Unfortunately, this avascular tissue has a limited capacity for intrinsic repair. Treatment options for articular cartilage defects include microfracture and arthroplasty; however, these strategies fail to generate tissue that adequately restores damaged cartilage. Limitations of current treatments for cartilage defects have prompted the field of cartilage tissue engineering, which seeks to integrate engineering and biological principles to promote the growth of new cartilage to replace damaged tissue. To date, a wide range of scaffolds and cell sources have emerged with a focus on recapitulating the microenvironments present during development or in adult tissue, in order to induce the formation of cartilaginous constructs with biochemical and mechanical properties of native tissue. Hydrogels have emerged as a promising scaffold due to the wide range of possible properties and the ability to entrap cells within the material. Towards improving cartilage repair, hydrogel design has advanced in recent years to improve their utility. Some of these advances include the development of improved network crosslinking (e.g. double-networks), new techniques to process hydrogels (e.g. 3D printing) and better incorporation of biological signals (e.g. controlled release). This review summarises these innovative approaches to engineer hydrogels towards cartilage repair, with an eye towards eventual clinical translation.

Design of Injectable Materials to Improve Stem Cell Transplantation

Stem cell-based therapies are steadily gaining traction for regenerative medicine approaches to treating disease and injury throughout the body. While a significant body of work has shown success in preclinical studies, results often fail to translate in clinical settings. One potential cause is the massive transplanted cell death that occurs post injection, preventing functional integration with host tissue. Therefore, current research is focusing on developing injectable hydrogel materials to protect cells during delivery and to stimulate endogenous regeneration through interactions of transplanted cells and host tissue. This review explores the design of targeted injectable hydrogel systems for improving the therapeutic potential of stem cells across a variety of tissue engineering applications with a focus on hydrogel materials that have progressed to the stage of preclinical testing.

Photocrosslinkable chitosan hydrogels functionalized with the RGD peptide and phosphoserine to enhance osteogenesis

Hydrogels derived from naturally occurring polymers are attractive matrix for tissue engineering. Here, we report a biofunctional hydrogel for specific use in bone regeneration by introducing Arg-Gly-Asp (RGD)-containing cell adhesive motifs and phosphorylated serine residues, which are prevalent in native bone extracellular matrix and known to promote osteogenesis by enhancing cell-matrix interactions and hydroxyapatite nucleation, into photopolymerizable methacrylated glycol chitosan (MeGC). Incorporation of phosphoserine into MeGC hydrogels increased the ability of the hydrogels to nucleate mineral on their surfaces. RGD incorporation enhanced cell-matrix interactions by supporting attachment, spreading, and proliferation of bone marrow stromal cells (BMSCs) encapsulated in the hydrogels. Moreover, co-modification of MeGC hydrogels with RGD and phosphoserine synergistically increased osteogenic differentiation of encapsulated BMSCs in vitro. The bone healing capacity of the modified hydrogels was further confirmed in a mouse calvarial defect model. These findings suggest a promising hydrogel platform with a specific microenvironment tailored to promote osteogenesis for clinical bone repair.

Macro- and micro-designed chitosan-alginate scaffold architecture by three-dimensional printing and directional freezing

While many tissue-engineered constructs aim to treat cartilage defects, most involve chondrocyte or stem cell seeding on scaffolds. The clinical application of cell-based techniques is limited due to the cost of maintaining cellular constructs on the shelf, potential immune response to allogeneic cell lines, and autologous chondrocyte sources requiring biopsy from already diseased or injured, scarce tissue. An acellular scaffold that can induce endogenous influx and homogeneous distribution of native stem cells from bone marrow holds great promise for cartilage regeneration. This study aims to develop such an acellular scaffold using designed, channeled architecture that simultaneously models the native zones of articular cartilage and subchondral bone. Highly porous, hydrophilic chitosan-alginate (Ch-Al) scaffolds were fabricated in three-dimensionally printed (3DP) molds designed to create millimeter scale macro-channels. Different polymer preform casting techniques were employed to produce scaffolds from both negative and positive 3DP molds. Macro-channeled scaffolds improved cell suspension distribution and uptake overly randomly porous scaffolds, with a wicking volumetric flow rate of 445.6 ± 30.3 mm(3) s(-1) for aqueous solutions and 177 ± 16 mm(3) s(-1) for blood. Additionally, directional freezing was applied to Ch-Al scaffolds, resulting in lamellar pores measuring 300 μm and 50 μm on the long and short axes, thus creating micrometer scale micro-channels. After directionally freezing Ch-Al solution cast in 3DP molds, the combined macro- and micro-channeled scaffold architecture enhanced cell suspension uptake beyond either macro- or micro-channels alone, reaching a volumetric flow rate of 1782.1 ± 48 mm(3) s(-1) for aqueous solutions and 440.9 ± 0.5 mm(3) s(-1) for blood. By combining 3DP and directional freezing, we can control the micro- and macro-architecture of Ch-Al to drastically improve cell influx into and distribution within the scaffold, while achieving porous zones that mimic articular cartilage zonal architecture. In future applications, precisely controlled micro- and macro-channels have the potential to assist immediate endogenous bone marrow uptake, stimulate chondrogenesis, and encourage vascularization of bone in an osteochondral scaffold.

The benefits and limitations of animal models for translational research in cartilage repair

Much research is currently ongoing into new therapies for cartilage defect repair with new biomaterials frequently appearing which purport to have significant regenerative capacity. These biomaterials may be classified as medical devices, and as such must undergo rigorous testing before they are implanted in humans. A large part of this testing involves in vitro trials and biomechanical testing. However, in order to bridge the gap between the lab and the clinic, in vivo preclinical trials are required, and usually demanded by regulatory approval bodies. This review examines the in vivo models in current use for cartilage defect repair testing and the relevance of each in the context of generated results and applicability to bringing the device to clinical practice. Some of the preclinical models currently used include murine, leporine, ovine, caprine, porcine, canine, and equine models. Each of these has advantages and disadvantages in terms of animal husbandry, cartilage thickness, joint biomechanics and ethical and licencing issues. This review will examine the strengths and weaknesses of the various animal models currently in use in preclinical studies of cartilage repair.

In situ-forming click-crosslinked gelatin based hydrogels for 3D culture of thymic epithelial cells

In situ-forming gelatin based hydrogels, which are crosslinked using an efficient nitrile oxide-norbornene click reaction, provide a suitable 3D culture environment for thymic epithelial cells.