Tendon tissue engineering: Cells, growth factors, scaffolds and production techniques

SDF-1α/BMP-12 loaded biphasic sustained-release SIS hydrogel/SA microspheres composite for tendon regeneration

Due to the inherent limited regenerative capacity of tendons, rendering countermeasures for tendon injury remains challenging. The pathophysiology of tendon healing is complex and contains three sequential phases including inflammation, proliferation and remodeling. Aiming at the treatment of different stages of tendon injury, in our work, an injectable small intestinal submucosa hydrogel/sodium alginate microspheres (SIS/SA) composite co-encapsulating stromal cell derived factor-1α (SDF-1α) and bone morphogenetic protein-12 (BMP-12) was developed for effective tendon regeneration. BMP-12 was initially embedded into SA microspheres by microfluid method, and then, microspheres were subsequently encapsulated into the SDF-1α loaded SIS hydrogel. The two bioactive molecules were released in a biphasic and controlled manner to facilitate cell recruitment in the early stage and tendon differentiation in the long-time stage, respectively. Meanwhile, with the degradation of hydrogel/microspheres composite, the regeneration process was accelerated through multiple pathways encompassing immune regulation, angiogenesis, and extracellular matrix (ECM) synthesis. The findings of this study present a compelling translational strategy with significant clinical potential for advancing tendon regeneration therapies.

Magnetic scaffolds for the mechanotransduction stimulation in tendon tissue regeneration

Nowadays, tendon injuries represent a global health issue that annually affects millions of individuals. An innovative approach for their treatment is represented by the development of tissue engineered scaffolds able to support the host cells adhesion, differentiation, and proliferation. However, the scaffold alone could be insufficient to guarantee an improvement of healing control. Magnetite nanoparticles (Fe3O4 NPs) are gaining interest due to their unique properties. In particular, when combined with bio-mimetic scaffolds, they should lead to the cells mechano-stimulation, improving the tenogenic differentiation and allowing a deeper tissue reparation. The aim of this work is the study and the development of scaffolds based on polyhydroxybutyrate and gelatin and doped with Fe3O4 NPs. The scaffolds are characterized by an aligned fibrous shape able to mimic the tendon fascicles. Moreover, they possess a superparamagnetic behavior and a slow degradation rate that should guarantee structural support during the tissue regeneration. The magnetic scaffolds promote cell proliferation and alignment onto the matrix, in particular when combined with the application of an external magnetic field. Also, the cells are able to differentiate and produce collagen I extracellular matrix. Finally, the magnetic scaffold in vivo promotes complete tissue healing after 1 week of treatment when combined with the external magnetic stimulation.

Strong and Tough Tendon-Mimetic Silk Fibroin for Tissue Regeneration

Tendons exhibit outstanding mechanical performance due to their hierarchical and anisotropic structures. Despite notable progress in mimicking anisotropy, developing biomaterials that simultaneously exhibit high strength, and excellent biocompatibility remains a significant challenge. In this study, a novel strategy is proposed to fabricate multiscale, tendon-mimetic regenerated silk fibroin (RSF) scaffold with a preferentially aligned microstructure and superior mechanical properties. This strategy integrates ethanol-annealing to induce β-sheet crystallization (nanoscale), freeze-casting to create aligned microstructures (microscale), and densification to enhance interfacial bonding (macroscale). The synergistic effect of these multiscale structures effectively resists crack propagation, yielding impressive mechanical properties: a tensile strength of 7.8 MPa, elongation at break of 206%, and toughness of 14.52 MJ m-3. Notably, the scaffold is fabricated through a purely physical process, free of chemical modifications or additives, preserving its biocompatibility and suggesting promising clinical potential. This biomimetic RSF scaffold promotes tendon cell adhesion, directional migration, and upregulated expression of tendon-specific proteins. After 8 weeks of implantation in a rabbit full-thickness Achilles tendon defect model, the scaffold effectively promoted realignment of the newly formed collagen fibers, resulting in a regenerated tendon structure that closely resembles native tendon architecture. This study provides insights into the design and manufacturing of high-performance biomaterials.

Biomaterial Properties and Differentiation Strategies for Tenogenic Differentiation of Mesenchymal Stem Cells

Tendinopathy is a prevalent musculoskeletal condition that affects both aging populations and individuals involved in repetitive, high-intensity activities, such as athletes. Current treatment options primarily address symptom management or involve surgery, which carries a significant risk of complications and re-injury. This highlights the need for regenerative medicine approaches that combine stem cells, biomaterials, and growth factors. However, achieving effective tenogenic differentiation remains challenging due to the absence of standardized differentiation protocols. Consequently, a review of existing research has been conducted to identify optimal biomaterial properties and growth factor protocols. Findings suggest that the ideal biomaterial for tenogenic differentiation should feature a 3D structure to preserve tenogenic expression, incorporate a combination of aligned micro- and nanofibers to promote differentiation, and require further investigation into optimal stiffness. Additionally, growth factor protocols should include an induction phase to initiate tenogenic lineage commitment, followed by a maintenance phase to support matrix production and maturation.

Design and Characterization of pH-Responsive DGEA-Derived Peptide Scaffolds: A Comprehensive Molecular Dynamics Simulation Study

Peptide-based, functionally active, stimuli-responsive biomaterials hold immense potential for diverse biomedical applications. Functionally active motifs of extracellular matrix (ECM) proteins, when conjugated with self-assembling peptides (SAP) or polymers, demonstrate significant promise in the development of such bioactive scaffolds. However, synthesis complexity, high associated costs, limited functionality, and potential immune responses present significant challenges. This study explores collagen-I-derived DGEA motif-based SAPs, incorporating modifications such as salt bridge pairing, charged and polar residues, hydrophobic residues, amyloidogenic sequences, and non-ECM motifs, to develop stimuli-responsive, functionally active scaffolds. Extensive molecular dynamics (MD) simulations, totaling 16.7 μs, were conducted on 20 systematically designed peptide systems. These simulations also characterized the stimuli-responsive properties of the peptides, focusing on pH and temperature responsiveness. Among the 20 designs, three peptide systems─DGEA-SBD, DGEA-SBE (salt-bridge modifications), and DGEA-F4 (with hydrophobic residue addition at the C-terminus)─successfully formed large, stable, and bioactive scaffolds. These systems exhibited enhanced aggregation (greater than 90%) and improved interpeptide hydrogen bonding (more than 30 bonds) while maintaining the accessibility of functional motifs (60-70% availability) compared to the unmodified DGEA motif. Notably, the DGEA-SBD and DGEA-SBE peptides showed a transition from small, unstable, uneven gel-like structures to large, stable, uniform, and functionally active scaffolds as the pH shifted from 3.0 to physiological pH. Comprehensive MD simulation studies demonstrated that these designed peptides exhibit increased aggregation and enhanced interpeptide hydrogen bonding while retaining their functional activity under various physiological conditions, highlighting their promising potential for biomedical applications.

Erroneous Differentiation of Tendon Stem/Progenitor Cells in the Pathogenesis of Tendinopathy: Current Evidence and Future Perspectives

Tendinopathy is a condition characterized by persistent tendon pain, structural damage, and compromised functionality. Presently, the treatment for tendinopathy remains a formidable challenge, partly because of its unclear pathogenesis. Tendon stem/progenitor cells (TSPCs) are essential for tendon homeostasis, regeneration, remodeling, and repair. An innovative theory has been previously proposed, with insufficient evidence, that the erroneous differentiation of TSPCs may constitute one of the fundamental mechanisms underpinning tendinopathy. Over the past few years, there has been accumulating evidence for plausibility of this theory. In this review, we delve into alterations in the differentiation potential of TSPCs and the underlying mechanisms in the context of injury-induced tendinopathy, diabetic tendinopathy, and age-related tendinopathy to provide updated evidence on the erroneous differentiation theory. Despite certain limitations inherent in the existing body of evidence, the erroneous differentiation theory emerges as a promising and highly pertinent avenue for understanding tendinopathy. In the future, advanced methodologies will be harnessed to further deepen comprehension of this theory, paving the way for prospective developments in clinical therapies targeting TSPCs for the management of tendinopathy.

Advancements in biomaterials and scaffold design for tendon repair and regeneration

Tendon injuries present a significant clinical challenge due to their limited natural healing capacity and the mechanical demands placed on these tissues. This review provides a comprehensive evaluation of the current strategies and advancements in tendon repair and regeneration, focusing on biomaterial innovations and scaffold design. Through a systematic literature search of databases such as PubMed, Scopus, and Web of Science, key studies were analyzed to assess the efficacy of biocompatible materials like hydrogels, synthetic polymers, and fiber-reinforced scaffolds in promoting tendon healing. Emphasis is placed on the role of collagen fiber architecture, including fiber diameter, alignment, and crimping, in restoring the mechanical strength and functional properties of tendons. Additionally, the review highlights emerging techniques such as electrospinning, melt electrowriting, and hybrid textile methods that allow for precise scaffold designs mimicking native tendon structures. Cutting-edge approaches in regenerative medicine, including stem cell therapies, bioelectronic devices, and bioactive molecules, are also explored for their potential to enhance tendon repair. The findings underscore the transformative impact of these technologies on improving tendon biomechanics and functional recovery. Future research directions are outlined, aiming to overcome the current limitations in scaffold mechanical properties and integration at tendon-bone and tendon-muscle junctions. This review contributes to the development of more effective strategies for tendon regeneration, advancing both clinical outcomes and the field of orthopedic tissue engineering.

Fiber-based biomaterial scaffolds for cell support towards the production of cultivated meat

The in vitro production of animal-derived foods via cellular agriculture is emerging as a key solution to global food security challenges. Here, the potential for fiber-based scaffolds, including silk and cotton, in the cultivation of muscle cells for tissue formation was pursued. Mechanical properties and cytocompatibility with the mouse myoblast cell line C2C12 and immortalized bovine muscle satellite cells (iBSCs) were assessed, as well as pre-digestion options for the materials due to their resilience within the human digestive track. The fibers supported cell adhesion, proliferation, and guided muscle cell orientation, facilitating myotube formation per differentiation. A progressive increase in biomass was also documented. Interestingly, iBSC proliferation was enhanced with coatings of recombinant proteins while C2C12 cells showed minimal response. Thus, both cotton and silk yarns were suitable as fiber-based scaffolds towards cell supportive goals, suggesting an alternative path toward structured protein-rich foods via this initial stage of textile engineering for food. Biomass prediction models were generated, enabling forecasts of cell growth and maturation across various scaffold conditions and cell types. This capability enhances the precision of the cultivation process towards an engineering approach, building on the inherent benefits of hierarchical muscle tissue structure, but here via textile engineering with these initial muscle-coated edible fibers. Further, the approach offers to reduce costs by optimizing cultivation time and media needs. These approaches are part of a foundation for future scalable and sustainable cultivated meat production. STATEMENT OF SIGNIFICANCE: This research investigates the use of one-dimensional fiber-based scaffolds for cultivated meat production, contributing to advancements in cellular agriculture. It introduces a method to measure changes in biomass and scaffold degradation throughout the cultivation process. Additionally, our development of biomass prediction models improves the precision and predictability of cultivated meat production. This research not only aids in scaling up cultivated meats but also enhances the use of textile engineering techniques in tissue engineering, paving the way for producing complex, three-dimensional meat structures more sustainably.

The Application of Photobiomodulation on Mesenchymal Stem Cells and its Potential Use for Tenocyte Differentiation

Tendinopathy is a prevalent and debilitating musculoskeletal disorder. Uncertainty remains regarding its pathophysiology, but it is believed to be a combination of inflammation, damage, degenerative changes, and unsuccessful repair mechanisms. Cell-based therapy is an emerging regenerative medicine modality that uses mesenchymal stem cells (MSCs), their progeny or exosomes to promote tendon healing and regeneration. It is based on the fact that MSCs can be differentiated into tenocytes, the major cell type within tendons, and facilitate tendon repair. Photobiomodulation (PBM) is a non-invasive and potentially promising therapeutic technique that utilizes low-level light to alter intracellular processes and promote tissue healing and regeneration. Recent studies have examined the potential for PBM to improve MSC therapy use in tendinopathy by promoting viability, proliferation, and differentiation. As well as enhance tendon regeneration. This review focuses on Photobiomodulation and MSC therapy applications in regenerative medicine and their potential for tendon tissue engineering.

Integrating electrospun aligned fiber scaffolds with bovine serum albumin-basic fibroblast growth factor nanoparticles to promote tendon regeneration

BACKGROUND

Electrospun nanofiber scaffolds have been widely used in tissue engineering because they can mimic extracellular matrix-like structures and offer advantages including high porosity, large specific surface area, and customizable structure. In this study, we prepared scaffolds composed of aligned and random electrospun polycaprolactone (PCL) nanofibers capable of delivering basic fibroblast growth factor (bFGF) in a sustained manner for repairing damaged tendons.

RESULTS

Aligned and random PCL fiber scaffolds containing bFGF-loaded bovine serum albumin (BSA) nanoparticles (BSA-bFGF NPs, diameter 146 ± 32 nm) were fabricated, respectively. To validate the viability of bFGF-loaded aligned PCL nanofiber scaffold (aPCL + bFGF group) in tendon tissue engineering, we assessed the in vitro differentiation of human amniotic mesenchymal stem cells (hAMSCs) towards a tenogenic lineage and the in vivo regeneration of tendons using a rat Achilles tendon defect model. The encapsulated bFGF could be delivered in a sustained manner in vitro. The aPCL + bFGF scaffold promoted the in vitro differentiation of human amniotic mesenchymal stem cells (hAMSCs) towards a tenogenic lineage. In the repair of a rat Achilles tendon defect model, the aPCL + bFGF group showed a better repair effect. The scaffold offers a promising substrate for the regeneration of tendon tissue.

CONCLUSIONS

The aligned and random PCL fiber scaffolds containing bFGF nanoparticles were successfully prepared, and their physical and chemical properties were characterized. The aPCL + bFGF scaffold could promote the expression of the related genes and proteins of tendon-forming, facilitating tendon differentiation. In the rat Achilles tendon defect experiments, the aPCL + bFGF exhibited excellent tendon regeneration effects.

Mueller matrix polarimetry for quantitative evaluation of the Achilles tendon injury recovery

Achilles tendon injuries, as a widely existing disease, have attracted a lot of research interest. Mueller matrix polarimetry, as a novel label-free quantitative imaging method, has been widely used in various applications of lesion identification and pathological diagnosis. However, focusing on the recovery process of Achilles tendon injuries, current optical imaging methods have not yet achieved the label-free precise identification and quantitative evaluation. In this study, using Mueller matrix polarimetry, various Achilles tendon injury samples were characterized specifically, and the efficacy of different recovery schemes was evaluated accordingly. Experiments indicate that injured Achilles tendons show less phase retardance, larger diattenuation, and relatively disordered orientation. The combination of experiments with Monte Carlo simulation results illustrate the microscopic mechanism of the Achilles tendon recovery process from three aspects, that is, the increased fiber diameter, a more consistent fiber orientation, and greater birefringence induced by more collagen protein. Finally, based on the statistical distribution of polarization measurements, a polarization specific characterization parameter was extracted to construct a label-free image, which cannot only intuitively show the injury and recovery of Achilles tendon samples, but also give a quantitative evaluation of the treatment.

Smart hydrogel-based trends in future tendon injury repair: A review

Despite advances in tissue engineering for tendon repair, rapid functional repair is still challenging due to its specificity and is prone to complications such as postoperative infections and tendon adhesions. Smart responsive hydrogels provide new ideas for tendon therapy with their flexibly designed three-dimensional cross-linked polymer networks that respond to specific stimuli. In recent years, a variety of smart-responsive hydrogels have been developed for the treatment of tendon disorders, showing great research promise and ability to address complex challenges. This article provides a comprehensive review of recent advances in the field of smart-responsive hydrogels for the treatment of tendon disorders, with a special focus on their response properties to different physical, chemical and biological stimuli. The multiple functional properties of these innovative materials are discussed in depth, including excellent biocompatibility and biodegradability, excellent mechanical properties, biomimetic structural design, convenient injectability, and unique self-healing capabilities. These properties enable the smart-responsive hydrogels to demonstrate significant advantages in solving difficult problems in the treatment of tendon disorders, such as precise drug delivery, tendon adhesion prevention and postoperative infection control. In addition, the article looks at the future prospects of smart-responsive hydrogels and analyses the challenges they may face in achieving widespread application.

Dual-Barb Microneedle with JAK/STAT Inhibitor-Loaded Nanovesicles Encapsulation for Tendinopathy

Tendon stem/progenitor cells (TSPCs) are crucial for tendon repair, regeneration, and homeostasis. Dysfunction of TSPCs, due to aberrant activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway, contributes to tendinopathy. Unfortunately, the effectiveness of conventional subcutaneous injection targeting at suppressing JAK/STAT signaling pathway is limited due to the passive diffusion of drugs away from the injury site. Herein, a novel poly-gamma-glutamic acid (γ-PGA) dual-barb microneedle (MN) path loaded with TSPCs-derived nanovesicles (NVs) containing JAK/STAT inhibitor WP1066 (MN-WP1066-NVs) for tendinopathy treatment is designed. The dual-barb design of the MN ensures firm adhesion to the skin, allowing for sustained and prolonged release of WP1066-NVs, facilitating enhanced TSPCs self-renewal, migration, and stemness in tendinopathy. In vitro and in vivo experiments demonstrate that the degradation of γ-PGA patch tips facilitates the gradual release of WP1066-NVs at the lesion site. This release alleviates inflammation, suppresses extracellular matrix degradation, and restores normal tendon histological structure by inhibiting the JAK/STAT pathway. These findings suggest that the multifunctional dual-barb MN patch offers a novel and effective therapeutic strategy for tendinopathy treatment.

Human Tendon-on-Chip: Unveiling the Effect of Core Compartment-T Cell Spatiotemporal Crosstalk at the Onset of Tendon Inflammation

The lack of representative in vitro models recapitulating human tendon (patho)physiology is among the major factors hindering consistent progress in the knowledge-based development of adequate therapies for tendinopathy.Here, an organotypic 3D tendon-on-chip model is designed that allows studying the spatiotemporal dynamics of its cellular and molecular mechanisms.Combining the synergistic effects of a bioactive hydrogel matrix with the biophysical cues of magnetic microfibers directly aligned on the microfluidic chip, it is possible to recreate the anisotropic architecture, cell patterns, and phenotype of tendon intrinsic (core) compartment. When incorporated with vascular-like vessels emulating the interface between its intrinsic-extrinsic compartments, crosstalk with endothelial cells are found to drive stromal tenocytes toward a reparative profile. This platform is further used to study adaptive immune cell responses at the onset of tissue inflammation, focusing on interactions between tendon compartment tenocytes and circulating T cells.The proinflammatory signature resulting from this intra/inter-cellular communication induces the recruitment of T cells into the inflamed core compartment and confirms the involvement of this cellular crosstalk in positive feedback loops leading to the amplification of tendon inflammation.Overall, the developed 3D tendon-on-chip provides a powerful new tool enabling mechanistic studies on the pathogenesis of tendinopathy as well as for assessing new therapies.

Leveraging in vivo animal models of tendon loading to inform tissue engineering approaches

Tendon injuries disrupt successful transmission of force between muscle and bone, resulting in reduced mobility, increased pain, and significantly reduced quality of life for affected patients. There are currently no targeted treatments to improve tendon healing beyond conservative methods such as rest and physical therapy. Tissue engineering approaches hold great promise for designing instructive biomaterials that could improve tendon healing or for generating replacement graft tissue. More recently, engineered microphysiological systems to model tendon injuries have been used to identify therapeutic targets. Despite these advances, current tissue engineering efforts that aim to regenerate, replace, or model injured tendons have largely failed due in large part to a lack of understanding of how the mechanical environment of the tendon influences tissue homeostasis and how altered mechanical loading can promote or prevent disease progression. This review article draws inspiration from what is known about tendon loading from in vivo animal models and identifies key metrics that can be used to benchmark success in tissue engineering applications. Finally, we highlight important challenges and opportunities for the field of tendon tissue engineering that should be taken into consideration in designing engineered platforms to understand or improve tendon healing.

Developing fibrin-based biomaterials/scaffolds in tissue engineering

Tissue engineering technology has advanced rapidly in recent years, offering opportunities to construct biologically active tissues or organ substitutes to repair or even enhance the functions of diseased tissues and organs. Tissue-engineered scaffolds rebuild the extracellular microenvironment by mimicking the extracellular matrix. Fibrin-based scaffolds possess numerous advantages, including hemostasis, high biocompatibility, and good degradability. Fibrin scaffolds provide an initial matrix that facilitates cell migration, differentiation, proliferation, and adhesion, and also play a critical role in cell-matrix interactions. Fibrin scaffolds are now widely recognized as a key component in tissue engineering, where they can facilitate tissue and organ defect repair. This review introduces the properties of fibrin, including its composition, structure, and biology. In addition, the modification and cross-linking modes of fibrin are discussed, along with various forms commonly used in tissue engineering. We also describe the biofunctionalization of fibrin. This review provides a detailed overview of the use and applications of fibrin in skin, bone, and nervous tissues, and provides novel insights into future research directions for clinical treatment.

Optimizing Tissue Engineering for Clinical Relevance in Rotator Cuff Repair

Rotator cuff tear (RCT) is the most common cause of disability in the upper extremity. It results in 4.5 million physician visits in the United States every year and is the most common etiology of shoulder conditions evaluated by orthopedic surgeons. Over 460,000 RCT repair surgeries are performed in the United States annually. Rotator cuff (RC) retear and failure to heal remain significant postoperative complications. Literature suggests that the retear rates can range from 29.5% to as high as 94%. Weakened and irregular enthesis regeneration is a crucial factor in postsurgical failure. Although commercially available RC repair grafts have been introduced to augment RC enthesis repair, they have been associated with mixed clinical outcomes. These grafts lack appropriate biological cues such as stem cells and signaling molecules at the bone-tendon interface. In addition, they do little to prevent fibrovascular scar tissue formation, which causes the RC to be susceptible to retear. Advances in tissue engineering have demonstrated that mesenchymal stem cells (MSCs) and growth factors (GFs) enhance RC enthesis regeneration in animal models. These models show that delivering MSCs and GFs to the site of RCT enhances native enthesis repair and leads to greater mechanical strength. In addition, these models demonstrate that MSCs and GFs may be delivered through a variety of methods including direct injection, saturation of repair materials, and loaded microspheres. Grafts that incorporate MSCs and GFs enhance anti-inflammation, osteogenesis, angiogenesis, and chondrogenesis in the RC repair process. It is crucial that the techniques that have shown success in animal models are incorporated into the clinical setting. A gap currently exists between the promising biological factors that have been investigated in animal models and the RC repair grafts that can be used in the clinical setting. Future RC repair grafts must allow for stable implantation and fixation, be compatible with current arthroscopic techniques, and have the capability to deliver MSCs and/or GFs.

Leveraging Machine Learning for Optimized Mechanical Properties and 3D Printing of PLA/cHAP for Bone Implant

This study explores the fabrication and characterisation of 3D-printed polylactic acid (PLA) scaffolds reinforced with calcium hydroxyapatite (cHAP) for bone tissue engineering applications. By varying the cHAP content, we aimed to enhance PLA scaffolds' mechanical and thermal properties, making them suitable for load-bearing biomedical applications. The results indicate that increasing cHAP content improves the tensile and compressive strength of the scaffolds, although it also increases brittleness. Notably, incorporating cHAP at 7.5% and 10% significantly enhances thermal stability and mechanical performance, with properties comparable to or exceeding those of human cancellous bone. Furthermore, this study integrates machine learning techniques to predict the mechanical properties of these composites, employing algorithms such as XGBoost and AdaBoost. The models demonstrated high predictive accuracy, with R2 scores of 0.9173 and 0.8772 for compressive and tensile strength, respectively. These findings highlight the potential of using data-driven approaches to optimise material properties autonomously, offering significant implications for developing custom-tailored scaffolds in bone tissue engineering and regenerative medicine. The study underscores the promise of PLA/cHAP composites as viable candidates for advanced biomedical applications, particularly in creating patient-specific implants with improved mechanical and thermal characteristics.

Mapping knowledge structure and themes trends of non-surgical treatment in intervertebral disc degeneration

Background

Intervertebral disc degeneration (IDD) is a chronic disabling disease caused by degeneration of nucleus pulposus cells, decreased activity and the number of nucleus pulposus cells, decreased extracellular matrix, and infiltration of inflammatory factors, resulting in low back and leg pain. Recent studies have shown that non-surgical treatment is of great significance in reversing the progression of degenerative disc disease, and there are more relevant literature reports. However, there is no bibliometric analysis in this area. This study aimed to describe the knowledge structure and thematic trends of non-surgical treatment methods for IDD through bibliometrics.

Methods

Articles and reviews on non-surgical treatment of disc degeneration from 1998 to 2022 were collected on the Web of Science. VOSviewer 1.6.18, CiteSpace 6.1.R3, R package "bibliometrix" and two online analysis platforms were used for bibliometric and visual literature analysis.

Results

961 articles were screened for inclusion, including 821 articles and 140 reviews. The analysis of our study shows that publications in the non-surgical treatment of disc degeneration are increasing annually, with publications coming mainly from North America and Asia, with China and the United States dominating. Huazhong Univ Sci & Technol and Wang K are the most prolific institutions and authors, respectively, and Le Maitre CL is the most co-cited author. However, there is less collaboration between institutions in different countries. Spine is both the most published and the most cited journal. According to the co-citation and co-occurrence analysis results, "mesenchymal stem cells," "exosomes," "medication," and "tissue engineering" are the current research hotspots in this field.

Conclusions

This study employs bibliometric analysis to explore the knowledge structure and trends of non-surgical treatments for IDD from 2013 to 2022. Key research hotspots include mesenchymal stem cells, exosomes, medication, and tissue engineering. The number of publications, especially from China and the USA, has increased significantly, though international collaboration needs improvement. Influential contributors include Wang K and the journal Spine. These findings provide a comprehensive overview and highlight important future directions for the field.

Application of 3D printing on the design and development of pharmaceutical oral dosage forms

3D printing technologies confer an unparalleled degree of control over the material distribution on the structures they produce, which has led them to become an extremely attractive research topic in pharmaceutical dosage form development, especially for the design of personalized treatments. With fine tuning in material selection and careful design, these technologies allow to tailor not only the amount of drug administered but the biopharmaceutical behaviour of the dosage forms as well. While fused deposition modelling (FDM) is still the most studied 3D printing technology in this area, others are gaining more relevance, which has led to many new and exciting dosage forms developed during 2022 and 2023. Considering that these technologies, in time, will join the current manufacturing methods and with the ever-increasing knowledge on this topic, our review aims to explore the advantages and limitations of 3D printing technologies employed in the design and development of pharmaceutical oral dosage forms, giving special focus to the most important aspects governing the resulting drug release profiles.

Drug-Loaded Bioscaffolds for Osteochondral Regeneration

Osteochondral defect is a complex tissue loss disease caused by arthritis, high-energy trauma, and many other reasons. Due to the unique structural characteristics of osteochondral tissue, the repair process is sophisticated and involves the regeneration of both hyaline cartilage and subchondral bone. However, the current clinical treatments often fall short of achieving the desired outcomes. Tissue engineering bioscaffolds, especially those created via three-dimensional (3D) printing, offer promising solutions for osteochondral defects due to their precisely controllable 3D structures. The microstructure of 3D-printed bioscaffolds provides an excellent physical environment for cell adhesion and proliferation, as well as nutrient transport. Traditional 3D-printed bioscaffolds offer mere physical stimulation, while drug-loaded 3D bioscaffolds accelerate the tissue repair process by synergistically combining drug therapy with physical stimulation. In this review, the physiological characteristics of osteochondral tissue and current treatments of osteochondral defect were reviewed. Subsequently, the latest progress in drug-loaded bioscaffolds was discussed and highlighted in terms of classification, characteristics, and applications. The perspectives of scaffold design, drug control release, and biosafety were also discussed. We hope this article will serve as a valuable reference for the design and development of osteochondral regenerative bioscaffolds and pave the way for the use of drug-loaded bioscaffolds in clinical therapy.

Role of tendon-derived stem cells in tendon and ligament repair: focus on tissue engineer

This review offered a comprehensive analysis of tendon and ligament injuries, emphasizing the crucial role of tendon-derived stem cells (TDSCs) in tissue engineering as a potential solution for these challenging medical conditions. Tendon and ligament injuries, prevalent among athletes, the elderly, and laborers, often result in long-term disability and reduced quality of life due to the poor intrinsic healing capacity of these avascular structures. The formation of biomechanically inferior scar tissue and a high rate of reinjury underscore the need for innovative approaches to enhance and guide the regenerative process. This review delved into the complexities of tendon and ligament structure and function, types of injuries and their impacts, and the limitations of the natural repair process. It particularly focused on the role of TDSCs within the context of tissue engineering. TDSCs, with their ability to differentiate into tenocytes, are explored in various applications, including biocompatible scaffolds for cell tracking, co-culture systems to optimize tendon-bone healing, and graft healing techniques. The review also addressed the challenges of immunoreactivity post-transplantation, the importance of pre-treating TDSCs, and the potential of hydrogels and decellularized matrices in supporting tendon regeneration. It concluded by highlighting the essential roles of mechanical and molecular stimuli in TDSC differentiation and the current challenges in the field, paving the way for future research directions.

Plunge-Freezing Cryopreservation of Tendons

Allograft transplantation is an important method for tendon reconstruction after injury, and its clinical success highly relies on the storage and transportation of the grafts. Cryopreservation is a promising strategy for tendon storage. In this study, we report a novel cryopreservation agent (CPA) formulation with a high biocompatibility for tendon cryopreservation. Mainly composed of natural zwitterionic betaine and the biocompatible polymer poly(vinylpyrrolidone) (PVP), it exhibited ideal abilities to depress the freezing point and inhibit ice growth and recrystallization. Notably, after cryopreservation via plunge-freezing for 1 month, Young's modulus (144 MPa, 98% of fresh tendons) and ultimate stress (46.7 MPa, 99% of fresh tendons) remained stable, and the cross-linking of collagen microfibers, protein structures, and glycosaminoglycan (GAG) contents changed slightly. These results indicate that the formulation (5 wt % betaine and 5 wt % PVP in phosphate-buffered saline, PBS solution) effectively maintains the biomechanical properties and tissue structure. This work offers a novel cryopreservation method for tendons and may also provide insights into the long-term preservation of various other tissues.

Reprogramming tendon healing: a guide to novel molecular tools

Tendons are a frequent site of injury, which greatly impairs the movement and locomotion of patients. Regrettably, injuries at the tendon frequently require surgical intervention, which leads to a long path to recovery. Moreover, the healing of tendons often involves the formation of scar tissue at the site of injury with poor mechanical properties and prone to re-injury. Tissue engineering carries the promise of better and more effective solutions to the improper healing of tendons. Lately, the field of regenerative medicine has seen a significant increase in the focus on the potential use of non-coding RNAs (e.g., siRNAs, miRNAs, and lncRNAs) as molecular tools for tendon tissue engineering. This class of molecules is being investigated due to their ability to act as epigenetic regulators of gene expression and protein production. Thus, providing a molecular instrument to fine-tune, reprogram, and modulate the processes of tendon differentiation, healing, and regeneration. This review focuses particularly on the latest advances involving the use of siRNAs, miRNAs, and lncRNAs in tendon tissue engineering applications.

Macromolecular crowding in human tenocyte and skin fibroblast cultures: A comparative analysis

Although human tenocytes and dermal fibroblasts have shown promise in tendon engineering, no tissue engineered medicine has been developed due to the prolonged ex vivo time required to develop an implantable device. Considering that macromolecular crowding has the potential to substantially accelerate the development of functional tissue facsimiles, herein we compared human tenocyte and dermal fibroblast behaviour under standard and macromolecular crowding conditions to inform future studies in tendon engineering. Basic cell function analysis made apparent the innocuousness of macromolecular crowding for both cell types. Gene expression analysis of the without macromolecular crowding groups revealed expression of tendon related molecules in human dermal fibroblasts and tenocytes. Protein electrophoresis and immunocytochemistry analyses showed significantly increased and similar deposition of collagen fibres by macromolecular crowding in the two cell types. Proteomics analysis demonstrated great similarities between human tenocyte and dermal fibroblast cultures, as well as the induction of haemostatic, anti-microbial and tissue-protective proteins by macromolecular crowding in both cell populations. Collectively, these data rationalise the use of either human dermal fibroblasts or tenocytes in combination with macromolecular crowding in tendon engineering.

Acute Patellar Tendon Ruptures: An Update on Management

Patellar tendon ruptures can be debilitating injuries. When incomplete, partial tears can be managed nonsurgically with immobilization and progressive rehabilitation. Although complete ruptures remain a relatively uncommon injury, they portend a high level of morbidity. Ruptures typically result from an acute mechanical overload to the extensor mechanism, such as with forced quadriceps contraction and knee flexion. However, chronically degenerated tendons are also predisposed to failure from low-energy injuries. Diagnosis can often be made clinically with recognition of a palpable defect to the tendon, localized patellar tendon tenderness, and inability to actively extend the knee. Diagnosis and surgical planning can be established with radiograph, ultrasonography, or magnetic resonance imaging. Surgical repair is the mainstay of treatment, and there have been many recent advances in repair technique, optimal reconstruction strategies, and supplemental fixation. Time to surgery for complete tears remains the most important prognosticator for success. Direct primary repair can be completed with transosseous tunnels, suture anchor repair, or end-to-end repair. Tendon reconstruction can be achieved with or without mechanical or biologic augments. Rehabilitation programs vary in specifics, but return to sport can be expected by 6 months postoperatively.

Developing Chinese herbal-based functional biomaterials for tissue engineering

The role of traditional Chinese medicine (TCM) in treating diseases is receiving increasing attention. Chinese herbal medicine is an important part of TCM with various applications and the active ingredients extracted from Chinese herbal medicines have physiological and pathological effects. Tissue engineering combines cell biology and materials science to construct tissues or organs in vitro or in vivo. TCM has been proposed by the World Health Organization as an effective treatment modality. In recent years, the potential use of TCM in tissue engineering has been demonstrated. In this review, the classification and efficacy of TCM active ingredients and delivery systems are discussed based on the TCM theory. We also summarized the current application status and broad prospects of Chinese herbal active ingredients in different specialized biomaterials in the field of tissue engineering. This review provides novel insights into the integration of TCM and modern Western medicine through the application of Chinese medicine in tissue engineering and regenerative medicine.

Advancements in 3D-printed artificial tendon

Millions of people have been reported with tendon injuries each year. Unfortunately, Tendon injuries are increasing rapidly due to heavy exercise and a highly aging population. In addition, the introduction of 3D-printing technology in the area of tendon repair and replacement has resolved numerous issues and significantly improved the quality of artificial tendons. This advancement has also enabled us to explore and identify the most effective combinations of biomaterials that can be utilized in this field. This review discusses the recent development of the 3D-printed artificial tendon; where recently, some research investigated the most suitable pore sizes, diameter, and strength for scaffolds to have high tendon cells ingrowth and proliferation, giving a better understanding of the effects of densities and structure patterns on tendon's mechanical properties. In addition, it presents the divergence between 3D-printed tendons and other tissue and how the different 3D-printing techniques and models participated in this development.

Exploring polysaccharide and protein-enriched decellularized matrix scaffolds for tendon and ligament repair: A review

Tissue engineering (TE) has become a primary research topic for the treatment of diseased or damaged tendon/ligament (T/L) tissue. T/L injuries pose a severe clinical burden worldwide, necessitating the development of effective strategies for T/L repair and tissue regeneration. TE has emerged as a promising strategy for restoring T/L function using decellularized extracellular matrix (dECM)-based scaffolds. dECM scaffolds have gained significant prominence because of their native structure, relatively high bioactivity, low immunogenicity, and ability to function as scaffolds for cell attachment, proliferation, and differentiation, which are difficult to imitate using synthetic materials. Here, we review the recent advances and possible future prospects for the advancement of dECM scaffolds for T/L tissue regeneration. We focus on crucial scaffold properties and functions, as well as various engineering strategies employed for biomaterial design in T/L regeneration. dECM provides both the physical and mechanical microenvironments required by cells to survive and proliferate. Various decellularization methods and sources of allogeneic and xenogeneic dECM in T/L repair and regeneration are critically discussed. Additionally, dECM hydrogels, bio-inks in 3D bioprinting, and nanofibers are briefly explored. Understanding the opportunities and challenges associated with dECM-based scaffold development is crucial for advancing T/L repairs in tissue engineering and regenerative medicine.

Multilayer amnion-PCL nanofibrous membrane loaded with celecoxib exerts a therapeutic effect against tendon adhesion by improving the inflammatory microenvironment

Tendon adhesion is a common complication after tendon surgery. The inflammatory phase of tendon healing is characterized by the release of a large number of inflammatory factors, whose mediated excessive inflammatory response is an important cause of tendon adhesion formation. Nonsteroidal anti-inflammatory drugs(NSAIDs) were used to prevent tendon adhesions by reducing the inflammatory response. However, recent studies have shown that the NSAIDs partially impairs tendon healing. Therefore, optimizing the anti-adhesive membrane loaded with NSAIDs to mitigate the effects on tendon healing requires further in-depth study. Amniotic membranes(AM) are natural polymeric semi-permeable membranes from living organisms that are rich in matrix, growth factors, and other active ingredients. In this study, we used electrostatic spinning technology to construct multifunctional nanofiber membranes of the PCL membrane loaded with celecoxib and AM. In vitro cellular assays revealed that celecoxib-loaded PCL membranes significantly inhibited the adhesion and proliferation of fibroblasts with increasing concentrations of celecoxib. In a rabbit tendon repair model, biomechanical tests further confirmed that the PCL membrane loaded with celecoxib had better anti-adhesion effects. Further experimental studies revealed that the PCL/AM membrane improved the inflammatory microenvironment by downregulating the expression of pro-inflammatory factors such as COX-2, IL-1β, and TNF-α proteins; and inhibiting the synthesis of COL I and COL Ⅲ. The PCL/AM membrane can continuously release celecoxib to reduce the inflammatory response and deliver growth factors to the damaged area to build a suitable microenvironment for tendon repair, which provides a new direction to improve the repair efficiency of tendon.

Some Mechanical Constraints to the Biomimicry with Peripheral Nerves

Novel high technology devices built to restore impaired peripheral nerves should be biomimetic in both their structure and in the biomolecular environment created around regenerating axons. Nevertheless, the structural biomimicry with peripheral nerves should follow some basic constraints due to their complex mechanical behaviour. However, it is not currently clear how these constraints could be defined. As a consequence, in this work, an explicit, deterministic, and physical-based framework was proposed to describe some mechanical constraints needed to mimic the peripheral nerve behaviour in extension. More specifically, a novel framework was proposed to investigate whether the similarity of the stress/strain curve was enough to replicate the natural nerve behaviour. An original series of computational optimizing procedures was then introduced to further investigate the role of the tangent modulus and of the rate of change of the tangent modulus with strain in better defining the structural biomimicry with peripheral nerves.

Effect of Aging on Tendon Biology, Biomechanics and Implications for Treatment Approaches

Tendon aging is associated with an increasing prevalence of tendon injuries and/or chronic tendon diseases, such as tendinopathy, which affects approximately 25% of the adult population. Aged tendons are often characterized by a reduction in the number and functionality of tendon stem/progenitor cells (TSPCs), fragmented or disorganized collagen bundles, and an increased deposition of glycosaminoglycans (GAGs), leading to pain, inflammation, and impaired mobility. Although the exact pathology is unknown, overuse and microtrauma from aging are thought to be major causative factors. Due to the hypovascular and hypocellular nature of the tendon microenvironment, healing of aged tendons and related injuries is difficult using current pain/inflammation and surgical management techniques. Therefore, there is a need for novel therapies, specifically cellular therapy such as cell rejuvenation, due to the decreased regenerative capacity during aging. To augment the therapeutic strategies for treating tendon-aging-associated diseases and injuries, a comprehensive understanding of tendon aging pathology is needed. This review summarizes age-related tendon changes, including cell behaviors, extracellular matrix (ECM) composition, biomechanical properties and healing capacity. Additionally, the impact of conventional treatments (diet, exercise, and surgery) is discussed, and recent advanced strategies (cell rejuvenation) are highlighted to address aged tendon healing. This review underscores the molecular and cellular linkages between aged tendon biomechanical properties and the healing response, and provides an overview of current and novel strategies for treating aged tendons. Understanding the underlying rationale for future basic and translational studies of tendon aging is crucial to the development of advanced therapeutics for tendon regeneration.

Nanofibrous scaffolds for the healing of the fibrocartilaginous enthesis: advances and prospects

With the current developmental advancements in nanotechnology, nanofibrous scaffolds are being widely used. The healing of fibrocartilaginous enthesis is a slow and complex process, and while existing treatments have a certain effect on promoting their healing, these are associated with some limitations. The nanofibrous scaffold has the advantages of easy preparation, wide source of raw materials, easy adjustment, easy modification, can mimic the natural structure and morphology of the fibrocartilaginous enthesis, and has good biocompatibility, which can compensate for existing treatments and be combined with them to promote the repair of fibrocartilaginous enthesis. The nanofibrous scaffold can promote the healing of fibrocartilaginous enthesis by controlling the morphology and ensuring controlled drug release. Hence, the use of nanofibrous scaffold with stimulative response features in the musculoskeletal system has led us to imagine its potential application in fibrocartilaginous enthesis. Therefore, the healing of fibrocartilaginous enthesis based on a nanofibrous scaffold may be a novel therapeutic approach.

Growth factors in the treatment of Achilles tendon injury

Achilles tendon (AT) injury is one of the most common tendon injuries, especially in athletes, the elderly, and working-age people. In AT injury, the biomechanical properties of the tendon are severely affected, leading to abnormal function. In recent years, many efforts have been underway to develop effective treatments for AT injuries to enable patients to return to sports faster. For instance, several new techniques for tissue-engineered biological augmentation for tendon healing, growth factors (GFs), gene therapy, and mesenchymal stem cells were introduced. Increasing evidence has suggested that GFs can reduce inflammation, promote extracellular matrix production, and accelerate AT repair. In this review, we highlighted some recent investigations regarding the role of GFs, such as transforming GF-β(TGF-β), bone morphogenetic proteins (BMP), fibroblast GF (FGF), vascular endothelial GF (VEGF), platelet-derived GF (PDGF), and insulin-like GF (IGF), in tendon healing. In addition, we summarized the clinical trials and animal experiments on the efficacy of GFs in AT repair. We also highlighted the advantages and disadvantages of the different isoforms of TGF-β and BMPs, including GFs combined with stem cells, scaffolds, or other GFs. The strategies discussed in this review are currently in the early stages of development. It is noteworthy that although these emerging technologies may potentially develop into substantial clinical treatment options for AT injury, definitive conclusions on the use of these techniques for routine management of tendon ailments could not be drawn due to the lack of data.

Cell Guiding Multicomponent Nanoyarn Tendon Scaffolds with Tunable Morphology and Flexibility

Nanofibrous scaffolds are widely investigated for tendon tissue engineering due to their porous structure, high flexibility, and the ability to guide cells in a preferred direction. Previous research has shown that providing a microenvironment similar to in vivo settings improves tissue regeneration. Therefore, in this work, ingenious multicomponent nanoyarn scaffolds that mimic the fibrillar and tubular structures of tendons are developed for the first time through electrospinning and bundling nanoyarns followed by electrospinning of a nanofibrous shell around the bundle. Multicomponent nanoyarn scaffolds out of poly(ε-caprolactone) with varying porosity, density, and diameter were successfully produced by coelectrospinning with water-soluble poly(2-ethyl-2-oxazoline) as a sacrificial component. The diameter and fiber orientation of the nanoyarns were successfully tuned based on parameter-morphology models obtained by the design of experiments. Cyclic bending tests were performed, indicating that the flexibility of the multicomponent nanoyarn scaffolds depends on the morphology and can be tuned through controlling the number of nanoyarns in the bundle and the porosity. Indirect and direct cell culture tests using mouse and equine tendon cells revealed excellent cytocompatibility of the nanofibrous products and demonstrated the potential of the nanoyarns to guide the growing cells along the nanofiber direction, which is crucial for tendon tissue engineering.

Biomaterials for Tissue-Engineered Treatment of Tendinopathy in Animal Models: A Systematic Review

To conduct a systematic review of studies reporting the treatment of tendon injury using biomaterials in animal models. A systematic search was conducted to retrieve studies involving animal models of tendon repair using biomaterials, in PubMed (database construction to August 2022) and Ovid-Embase (1946 to August 2022). Data related to tendon repair with biomaterials were extracted by two researchers, respectively. Risk of bias was assessed following the Cochrane Handbook for Systematic Reviews of Interventions. A statistical analysis was performed based on the classification of tendon repair biomaterials included in our study. A total of 8413 articles were retrieved, with 78 studies included in our analysis. For tendon repair in animal models using biomaterials, the most commonly seen characteristics were as follows: naturally derived biomaterials, rabbits and rats as animal models, surgery as the injury model, and the Achilles tendon as the injury site. The histology and biomechanical recovery of tendon injury following repair are affected by different biomaterials. Studies of tendon repair in animal models indicate that biomaterials can significantly improve repair outcomes, including tendon structure and biomechanics. Among effective biomaterial strategies are the use of new composites and incorporation of cells or growth factors into the material, both of which provide obvious benefits for tendon healing. More high-quality preclinical studies are required to encourage the translation of biomaterials into clinical practice for tendon repair.

Natural polysaccharide-based hydrogel bioprinting for articular cartilage repair

Tissue engineering represents a promising approach for impaired articular cartilage tissue regeneration. 3D printed hydrogels have become an emerging tissue engineering strategy because they closely mimic the physical and biochemical characteristics of the extracellular matrix. The formulation of hydrogel ink holds significant importance in attaining a precisely defined scaffold, which could exhibit excellent shape fidelity post-printing. Natural polysaccharide-based hydrogels are a highly promising class of scaffold biomaterials for articular cartilage regeneration in the field of material science and tissue engineering. These hydrogels are particularly advantageous due to their exceptional water absorption capacity, biodegradability, adjustable porosity, and biocompatibility, which closely resemble those of the natural extracellular matrix. This review aims to provide a comprehensive overview of the key characteristics, functions, and research progress in 3D printing technology for natural polysaccharide-based hydrogels. Specifically, this review categorizes the commonly used natural polysaccharide-based hydrogel materials in cartilage tissue engineering, and summarizes the classic literature in this area. In the end, we provide a comprehensive analysis of the challenges and potential applications of natural polysaccharide-based hydrogels in cartilage tissue engineering.

An overview of the material science and knowledge of nanomedicine, bioscaffolds, and tissue engineering for tendon restoration

Tendon wounds are a worldwide health issue affecting millions of people annually. Due to the characteristics of tendons, their natural restoration is a complicated and lengthy process. With the advancement of bioengineering, biomaterials, and cell biology, a new science, tissue engineering, has developed. In this field, numerous ways have been offered. As increasingly intricate and natural structures resembling tendons are produced, the results are encouraging. This study highlights the nature of the tendon and the standard cures that have thus far been utilized. Then, a comparison is made between the many tendon tissue engineering methodologies proposed to date, concentrating on the ingredients required to gain the structures that enable appropriate tendon renewal: cells, growth factors, scaffolds, and scaffold formation methods. The analysis of all these factors enables a global understanding of the impact of each component employed in tendon restoration, thereby shedding light on potential future approaches involving the creation of novel combinations of materials, cells, designs, and bioactive molecules for the restoration of a functional tendon.

Fibrocartilage extracellular matrix augmented demineralized bone matrix graft repairs tendon-to-bone interface in a rabbit tendon reconstruction model

Current tendon/ligament reconstructions integrate via scar tissue rather than proper bone-tendon interface regeneration, which affects graft longevity, changes in bone tunnel size, and functional outcomes. The purpose of this study was to develop a functional demineralized bone matrix (DBM) + fibrocartilage extracellular matrix (FCECM) composite scaffold, characterize its physicochemical properties, and evaluate its efficacy in repairing tendon-bone interface in a rabbit tendon reconstruction model. Solubilized FCECM was loaded and crosslinked on to DBM scaffolds via gamma-irradiation to create DBM + FCECM scaffolds. The resulting scaffold showed interconnected pores coated with FCECM and protein cargo similar to FCECM. The addition of FCECM modified the physicochemical properties of the DBM scaffold, including microstructure, biochemical composition, mechanical strength, thermodynamic properties, and degradation period. The DBM + FCECM scaffold was biocompatible for mesenchymal stem cells (MSCs) and resulted in elevation of fibrochondrogenic gene markers compared to DBM scaffolds in vitro. In vivo implantation of DBM + FCECM scaffold resulted in neofibrocartilage formation, better pullout strength, and less bone tunnel widening compared to DBM only group in a rabbit tendon reconstruction model. In conclusion, the FCECM augmented DBM scaffold repairs the tendon-bone interface with osseous-fibrocartilage tissue, which may be utilized to improve current tendon reconstruction surgeries.

Biomimetic Scaffolds for Tendon Tissue Regeneration

Tendon tissue connects muscle to bone and plays crucial roles in stress transfer. Tendon injury remains a significant clinical challenge due to its complicated biological structure and poor self-healing capacity. The treatments for tendon injury have advanced significantly with the development of technology, including the use of sophisticated biomaterials, bioactive growth factors, and numerous stem cells. Among these, biomaterials that the mimic extracellular matrix (ECM) of tendon tissue would provide a resembling microenvironment to improve efficacy in tendon repair and regeneration. In this review, we will begin with a description of the constituents and structural features of tendon tissue, followed by a focus on the available biomimetic scaffolds of natural or synthetic origin for tendon tissue engineering. Finally, we will discuss novel strategies and present challenges in tendon regeneration and repair.

Therapeutic Potential of Exosomes in Tendon and Tendon-Bone Healing: A Systematic Review of Preclinical Studies

Exosomes have been proven to play a positive role in tendon and tendon-bone healing. Here, we systematically review the literature to evaluate the efficacy of exosomes in tendon and tendon-bone healing. Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, a systematic and comprehensive review of the literature was performed on 21 January 2023. The electronic databases searched included Medline (through PubMed), Web of Science, Embase, Scopus, Cochrane Library and Ovid. In the end, a total of 1794 articles were systematically reviewed. Furthermore, a "snowball" search was also carried out. Finally, forty-six studies were included for analysis, with the total sample size being 1481 rats, 416 mice, 330 rabbits, 48 dogs, and 12 sheep. In these studies, exosomes promoted tendon and tendon-bone healing and displayed improved histological, biomechanical and morphological outcomes. Some studies also suggested the mechanism of exosomes in promoting tendon and tendon-bone healing, mainly through the following aspects: (1) suppressing inflammatory response and regulating macrophage polarization; (2) regulating gene expression, reshaping cell microenvironment and reconstructing extracellular matrix; (3) promoting angiogenesis. The risk of bias in the included studies was low on the whole. This systematic review provides evidence of the positive effect of exosomes on tendon and tendon-bone healing in preclinical studies. The unclear-to-low risk of bias highlights the significance of standardization of outcome reporting. It should be noted that the most suitable source, isolation methods, concentration and administration frequency of exosomes are still unknown. Additionally, few studies have used large animals as subjects. Further studies may be required on comparing the safety and efficacy of different treatment parameters in large animal models, which would be conducive to the design of clinical trials.

Instantaneous self-healing and strongly adhesive self-adaptive hyaluronic acid-based hydrogel for controlled drug release to promote tendon wound healing

The treatment of tendon injuries is an important healthcare challenge. Irregular wounds, hypocellularity, and prolonged inflammation impede the rate of healing for tendon injuries. To address these problems, a high-tenacity shape-adaptive, mussel-like hydrogel (PH/GMs@bFGF&PDA) was designed and constructed with polyvinyl alcohol (PVA) and hyaluronic acid grafted with phenylboronic acid (BA-HA) by encapsulating polydopamine and gelatin microspheres containing basic fibroblast growth factor (GMs@bFGF). The shape-adaptive PH/GMs@bFGF&PDA hydrogel can quickly adapt to irregular tendon wounds, and the strong adhesion (101.46 ± 10.88 kPa) can keep the hydrogel adhered to the wound at all times. In addition, the high tenacity and self-healing properties allow the hydrogel to move with the tendon without fracture. Additionally, even if fractured, it can quickly self-heal and continue to adhere to the tendon wound, while slowly releasing basic fibroblast growth factor during the inflammatory phase of the tendon repair process, promoting cell proliferation, migration and shortening the inflammatory phase. In acute tendon injury and chronic tendon injury models, PH/GMs@bFGF&PDA significantly alleviated inflammation and promoted collagen I secretion, enhancing wound healing through the synergistic effects of its shape-adaptive and high-adhesion properties.

Wet-spinnability and crosslinked Fiber properties of alginate/hydroxyethyl cellulose with varied proportion for potential use in tendon tissue engineering

Researchers have examined different bio-inspired materials in tissue engineering and regenerative medicine to fabricate scaffolds to address tendon regeneration requirements. We developed fibers based on alginate (Alg) and hydroxyethyl cellulose (HEC) by wet-spinning technique to mimic the fibrous sheath of ECM. Various proportions (25:75, 50:50, 75:25) of 1 % Alg and 4 % HEC were blended to this aim. Two steps of crosslinking with different concentrations of CaCl₂ (2.5 and 5 %) and glutaraldehyde (2.5 %) were used to improve physical and mechanical properties. The fibers were characterized by FTIR, SEM, swelling, degradation, and tensile tests. The in vitro proliferation, viability, and migration of tenocytes on the fibers were also evaluated. Moreover, the biocompatibility of implanted fibers was investigated in an animal model. The results showed ionic and covalent molecular interactions between the components. In addition, by properly maintaining surface morphology, fiber alignment, and swelling, lower concentrations of HEC in the blending provided good degradability and mechanical features. The mechanical strength of fibers was in the range of collagenous fibers. Increasing the crosslinking led to significantly different mechanical behaviors in terms of tensile strength and elongation at break. Because of good in vitro and in vivo biocompatibility, tenocyte proliferation, and migration, the biological macromolecular fibers could serve as desirable tendon substitutes. This study provides more practical insight into tendon tissue engineering in translational medicine.

Biomechanical and morphological comparison of two interposition graft rotator cuff repair techniques

BACKGROUND

Interposition graft rotator cuff repair is one option for the treatment of massive, otherwise irreparable rotator cuff tears. It is undetermined how different suturing techniques influence morphology at the patch-tendon interface in interposition rotator cuff repairs, particularly with respect to increased cross-sectional area at the repair site post-exposure to cyclic loading, which may influence healing. We aimed to analyze how the morphology of the grafts used in polytetrafluoroethylene (PTFE) interposition rotator cuff repairs differed according to whether the graft was secured using the multiple mattress technique or the weave technique respectively.

METHODS

Twelve PTFE interposition rotator cuff repairs (two groups, n = 6) were cyclically loaded. The thickness, width, cross-sectional area at the individual patches and at the repair site, and patch elongation were compared between repairs using the 'multiple mattress' technique and repairs that used the 'weave' technique.

RESULTS

At all loads, repair site cross-sectional area and thickness was greater in the weave group than in the multiple mattress group (P<0.05), despite repair site width being greater in the multiple mattress group (P<0.05). No significant differences in elongation were found between the multiple mattress and weave groups.

CONCLUSION

Greater repair site cross-sectional area under cyclic loading was observed in polytetrafluoroethylene interposition rotator repairs that used the weave technique than in those that used the multiple mattress technique. Increased repair site cross-sectional area in the weave group occurred due to increased thickness and decreased width relative to the multiple mattress group. No differences in cross-sectional area were found between groups at the individual patches.

Recent advances in tendon tissue engineering strategy

Tendon injuries often result in significant pain and disability and impose severe clinical and financial burdens on our society. Despite considerable achievements in the field of regenerative medicine in the past several decades, effective treatments remain a challenge due to the limited natural healing capacity of tendons caused by poor cell density and vascularization. The development of tissue engineering has provided more promising results in regenerating tendon-like tissues with compositional, structural and functional characteristics comparable to those of native tendon tissues. Tissue engineering is the discipline of regenerative medicine that aims to restore the physiological functions of tissues by using a combination of cells and materials, as well as suitable biochemical and physicochemical factors. In this review, following a discussion of tendon structure, injury and healing, we aim to elucidate the current strategies (biomaterials, scaffold fabrication techniques, cells, biological adjuncts, mechanical loading and bioreactors, and the role of macrophage polarization in tendon regeneration), challenges and future directions in the field of tendon tissue engineering.

Rejuvenation of tendon stem/progenitor cells for functional tendon regeneration through platelet-derived exosomes loaded with recombinant Yap1

The regenerative capabilities including self-renewal, migration and differentiation potentials shift from the embryonic phase to the mature period of endogenous tendon stem/progenitor cells (TSPCs) characterize restricted functions and disabilities following tendon injuries. Recent studies have shown that tendon regeneration and repair rely on multiple specific transcription factors to maintain TSPCs characteristics and functions. Here, we demonstrate Yap, a Hippo pathway downstream effector, is associated with TSPCs phenotype and regenerative potentials through gene expression analysis of tendon development and repair process. Exosomes have been proven an efficient transport platform for drug delivery. In this study, purified exosomes derived from donor platelets are loaded with recombinant Yap1 protein (PLT-Exo-Yap1) via electroporation to promote the stemness and differentiation potentials of TSPCs in vitro. Programmed TSPCs with Yap1 import maintain stemness and functions after long-term passage in vitro. The increased oxidative stress levels of TSPCs are related to the phenotype changes in duplicative senescent processes. The results show that treatment with PLT-Exo-Yap1 significantly protects TSPCs against oxidative stressor-induced stemness loss and senescence-associated secretory phenotype (SASP) through the NF-κB signaling pathway. In addition, we fabricate an Exos-Yap1-functioned GelMA hydrogel with a parallel-aligned substrate structure to enhance TSPCs adhesion, promote cell stemness and force regenerative cells toward the tendon lineage for in vitro and in vivo tendon regeneration. The application of Exos-Yap1 functioned implant assists new tendon-like tissue formation with good mechanical properties and locomotor functions in a full-cut Achilles tendon defect model. Thus, PLT-Exo-Yap1-functionalized GelMA promotes the rejuvenation of TSPCs to facilitate functional tendon regeneration. STATEMENT OF SIGNIFICANCE: This is the first study to explore that the hippo pathway downstream effector Yap is involved in tendon aging and repair processes, and is associated with the regenerative capabilities of TSPCs. In this syudy, Platelet-derived exosomes (PLT-Exos) act as an appropriate carrier platform for the delivery of recombinant Yap1 into TSPCs to regulate Yap activity. Effective Yap1 delivery inhibit oxidative stress-induced senescence associated phenotype of TSPCs by blocking ROS-mediated NF-κb signaling pathway activation. This study emphasizes that combined application of biomimetic scaffolds and Yap1 loaded PLT-Exos can provide structural support and promote rejuvenation of resident cells to assist functional regeneration for Achilles tendon defect, and has the prospect of clinical setting.

Evidence for Utilization of Injectable Biologic Augmentation in Primary Rotator Cuff Repair: A Systematic Review of Data From 2010 to 2022

Background

Biologic healing after rotator cuff repair remains a significant challenge. Injectable biologic augmentation may improve tissue quality at the suture-tendon interface.

Purpose

To investigate the effect of injectable biologic supplementation in rotator cuff repair and to assess the quality and adherence to evolving reporting standards.

Study Design

Systematic review; Level of evidence, 3.

Methods

A systematic review was conducted following PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. Included were 40 studies: 29 preclinical (in vivo animal models) and 11 clinical. Each clinical study was assessed for quality, risk of bias, and adherence to relevant MIBO (Minimum Information for Studies Evaluating Biologics in Orthopaedics) guidelines. The outcomes of interest were reported load to failure, load to gap, gap size, and stiffness in the preclinical studies, and healing rate and any patient-reported outcome measures in the clinical studies.

Results

Injectables reported included growth factors (eg, transforming growth factor-beta 3, erythropoietin), bone marrow-derived mesenchymal stem cells and adipose-derived mesenchymal stem cells (ADSCs), and other agents such as platelet-rich plasma (PRP) and hyaluronic acid. The most common findings for preclinical injectables were increased load to failure (16/29 studies; 55.2%) and improved collagen histological quality (11/29 studies; 37.9%). All 11 clinical studies (10 PRP, 1 ADSC) indicated no adverse events, with similar or improved patient-reported outcomes compared with repairs in the control groups. In 1 study utilizing an innovative delivery technique, a concentrated PRP globule with fibrin matrix was shuttled over a suture to maintain concentrated PRP at the repair site and demonstrated a significant decrease in retears (P = .03) at a 31-month follow-up. A matched-cohort study investigating augmentation with ADSCs demonstrated a significantly lower retear rate in the ADSC-augmented group than the control group at a 28-month follow-up (P < .001). On average, the clinical studies adhered to 66% of relevant MIBO reporting guidelines and had a low risk of bias.

Conclusion

Approximately 83% of preclinical studies found a positive biomechanical or histological effect, with no studies showing an overall negative effect. Clinically, utilization of innovative delivery techniques may reduce the risk of arthroscopic washout of PRP and improve retear rates. ADSCs were shown to reduce retear rates at a 28-month follow-up.

Nano-calcium silicate mineralized fish scale scaffolds for enhancing tendon-bone healing

Tendon-bone healing is essential for an effective rotator cuff tendon repair surgery, however, this remains a significant challenge due to the lack of biomaterials with high strength and bioactivity. Inspired by the high-performance exoskeleton of natural organisms, we set out to apply natural fish scale (FS) modified by calcium silicate nanoparticles (CS NPs) as a new biomaterial (CS-FS) to overcome the challenge. Benefit from its “Bouligand” microstructure, such FS-based scaffold maintained excellent tensile strength (125.05 MPa) and toughness (14.16 MJ/m³), which are 1.93 and 2.72 times that of natural tendon respectively, allowing it to well meet the requirements for rotator cuff tendon repair. Additionally, CS-FS showed diverse bioactivities by stimulating the differentiation and phenotypic maintenance of multiple types of cells participated into the composition of tendon-bone junction, (e.g. bone marrow mesenchymal stem cells (BMSCs), chondrocyte, and tendon stem/progenitor cells (TSPCs)). In both rat and rabbit rotator cuff tear (RCT) models, CS-FS played a key role in the tendon-bone interface regeneration and biomechanical function, which may be achieved by activating BMP-2/Smad/Runx2 pathway in BMSCs. Therefore, natural fish scale -based biomaterials are the promising candidate for clinical tendon repair due to their outstanding strength and bioactivity.

•

Nano-calcium silicate mineralized fish scale scaffold was first developed for tendon defect repair.

•CS-FS exhibited excellent mechanical properties superior to those of natural tendon.

•CS-FS showed diverse bioactivities by stimulating the differentiation of multiple types of cells.

•CS NPs accelerated tendon-bone interface tendon-bone healing enhancement and biomechanical recovery.

Natural, synthetic and commercially-available biopolymers used to regenerate tendons and ligaments

Tendon and ligament (TL) injuries affect millions of people annually. Biopolymers play a significant role in TL tissue repair, whether the treatment relies on tissue engineering strategies or using artificial tendon grafts. The biopolymer governs the mechanical properties, biocompatibility, degradation, and fabrication method of the TL scaffold. Many natural, synthetic and hybrid biopolymers have been studied in TL regeneration, often combined with therapeutic agents and minerals to engineer novel scaffold systems. However, most of the advanced biopolymers have not advanced to clinical use yet. Here, we aim to review recent biopolymers and discuss their features for TL tissue engineering. After introducing the properties of the native tissue, we discuss different types of natural, synthetic and hybrid biopolymers used in TL tissue engineering. Then, we review biopolymers used in commercial absorbable and non-absorbable TL grafts. Finally, we explain the challenges and future directions for the development of novel biopolymers in TL regenerative treatment.