Biomaterials strategies to balance inflammation and tenogenesis for tendon repair

Piezoelectric nanofilms fabricated by coaxial electrospun polycaprolactone/Barium titanate promote Achilles tendon regeneration by reducing IL-17A/NF-κB-mediated inflammation

Tendon injuries are often exacerbated by persistent inflammation, which hampers tissue regeneration. In this study, we developed a noninvasive, wirelessly controlled, and self-powered piezoelectric nanofilm fabricated by coaxial electrospinning of polycaprolactone (PCL) and tetragonal barium titanate nanoparticles (BTO), and investigated its roles in modulating inflammation and repairing Achilles tendon defects as well as the mechanism in a rat model. In vitro study and in vivo study upon subcutaneous implantation showed that the piezoelectric PCL/BTO nanofilms could inhibit M1 macrophage polarization and reduce the secretion of inflammatory factors. Moreover, when bridging an Achilles tendon defect, the nanofilms could promote tenogenic gene expression including collagen deposition, and collagen remodeling, facilitate functional tendon recovery and significantly reduce tissue inflammation by suppressing M1 macrophage polarization and promoting M2 polarization. Moreover, the piezoelectric stimulation could also enhance tendon regeneration by inhibiting angiogenesis, reducing lipid deposition, and decreasing ectopic ossification. Mechanistically, the piezoelectric nanofilms reduced tissue inflammation mainly via inhibiting the nuclear factor (NF)-κB signaling pathway that is mediated by interleukin (IL)-17A secreted from CD3+ T cells, and thus to reduce proinflammatory factors, such as IL-1β and IL-6, inducible nitric oxide synthase, monocyte chemoattractant protein-1, and tumor necrosis factor-α. These findings indicate the potential of piezoelectric stimulation in immunomodulation, and in promoting tendon regeneration via IL-17A/NF-κB-mediated pathway.

Advanced Biomimetic Materials in the Prevention of Tendon Adhesions: Design, Preparation, and Application of Hydrogel and Electrospun fiber Membranes

Tendon adhesion formation results from a fibrotic process between the tendon and surrounding tissues, typically occurring after tendon injury or surgery. This condition significantly impacts the quality of life and motor function. Currently, treating adhesions following the repair of injured tendons remains challenging and is a prominent clinical issue that needs to be addressed. This review compiles the existing pathophysiological mechanisms underlying tendon adhesion formation, with a particular focus on the critical roles of inflammation and inflammatory pathways, growth factors and their associated pathways, as well as peritendinous cellular behaviors in promoting adhesion formation. Furthermore, this paper is dedicated to summarizing the evaluation of hydrogels and electrospun fiber membranes as anti-adhesion materials, emphasizing their design, preparation, and application. Additionally, the success of composite patches created by combining these two materials in preventing tendon adhesions is reviewed, which demonstrates the broad applicability of the hydrogel and electrospun film combination. Finally, the review provides insights into future directions for preventing tendon adhesion formation, focusing on material structure and functional design.

Effect of Tyrosine-Containing Self-Assembling β-Sheet Peptides on Macrophage Polarization and Inflammatory Response

Self-assembling peptides (SAPs) are fully defined nanobiomaterials offering unprecedented opportunities to control nanostructure and chemical attributes to investigate and manipulate cellular signals. To investigate the influence of chemical and morphological characteristics on inflammatory signaling in native immunity, we designed five β-sheet SAPs: EFEFKFEFK (EF8), YEFEFKFEFK (YEF8), EFEFKFEFKY (EF8Y), YEFEFKFEFKY (YEF8Y), and EYEFKFEFK (EYF8) (F: phenylalanine; E: glutamic acid; K: lysine, Y: tyrosine). The position of tyrosine in the peptide sequence dictated the self-assembly into nanostructures, with all SAPs self-assembling into thin constituent nanofibers with d ≈ 3.8 ± 0.4 nm, and sequences YEF8 and EF8 showing a propensity for associative bundling. These distinct SAPs induced contrasting inflammatory responses of monocytic model THP-1 cells-derived macrophages (MΦs). Presence of soluble EF8 nanofibers (at 2 mM) induced an anti-inflammatory response and polarization toward an M2 state, whereas YEF8 (at 2 mM) displayed a tendency for inducing a pro-inflammatory response and polarization toward an M1 state. EF8Y, YEF8Y, and EYF8 SAPs did not induce an inflammatory response in our models. These results were validated using peripheral blood mononuclear cells (PBMCs)-derived MΦs from human donors, confirming the critical role of EF8 and YEF8 SAPs as possible orchestrators of the repair of tissues or inducers of pro-inflammatory state, respectively. The same MΦs polarization responses from THP-1-derived MΦs cultured on 20 mM hydrogels were obtained. These findings will facilitate the utilization of this family of SAPs as immunomodulatory nanobiomaterials potentially changing the course of inflammation during the progression of various diseases.

Decellularised amniotic membrane-TDSCs composite promotes Achilles tendon healing

Adhesions and poor healing are major complications after Achilles tendon injury, and there is no effective solution to this problem. The purpose of this study is to determine whether the biomimetic peritoneurosis can solve the above problems in the process of Achilles tendon healing; This study investigated the adhesion and proliferation of tendon-derived stem cells (TDSCs) on dAM in vitro, as well as their tenogenic differentiation. The effects of Achilles tendon rupture on tendon regeneration were assessed in vivo by using an Achilles tendon rupture model in rats; Finally, from in vitro mRNA transcriptome sequencing combined with in vivo Achilles tendon protein to omics analysis to explore the repair mechanism of Achilles tendon rupture. Student's t-tests were used to assess the significance of observed differences between the two experimental groups. Multiple groups were compared using one-way analysis of variances (ANOVAs), followed by post hoc Bonferroni comparisons. The main findings of this study are that cell adhesion, proliferation, and differentiation of TDSCs were enhanced by dAM. Implanted dAM + TDSCs significantly accelerated tendon regeneration in vivo. In addition, extracellular matrix-related differential genes and proteins were screened by mRNA transcriptometry in vitro and proteomic analysis of Achilles tendon in vivo, and ERK signaling pathway was further explored to participate in the repair of Achilles tendon rupture. The dAM-TDSCs composite biomimetic peritendinous membrane material can effectively promote the healing of Achilles tendon. It provides a new direction for the development of biomimetic peritendinous membrane materials.

Modeling and fabrication of MEW-3D tubular scaffolds with tendon mechanical behavior for tenocyte differentiation

Tendon injuries present substantial challenges in clinical settings, where traditional treatments often lead to suboptimal outcomes, including scar tissue formation, reduced strength, limited range of motion, and re-ruptures. These difficulties primarily arise from the complex hierarchical structure of tendons and their limited healing capacity. Tissue engineering offers promising solutions for tendon regeneration and muscle-to-bone reconnection, typically through the use of biodegradable scaffolds that mimic the extracellular matrix of tendons, thereby supporting cell growth and tissue formation. However, several obstacles must be addressed to develop tendon reconstruction strategies suitable for clinical application. In this study, we developed computational models to design and produce Melt Electrowriting (MEW)-3D tubular structures that replicate the mechanical properties of native mouse Achilles tendons, improving the guidance of tenocyte growth and differentiation. These models incorporated a new approach to consider the non-continuum nature of printed scaffolds formed by sets of fibers interacting with one another. Moreover, these structures facilitated cell confinement, expansion, and alignment, resulting in bundle-like formations with enhanced tensile properties. Importantly, tenocyte differentiation was achieved through a two-step cell culture protocol, which involved transitioning cells from high to low serum media. This approach effectively mitigated the phenotypic drift that often occurs with long-term cell cultures. Statement of Significance Tendon injuries are a common issue in healthcare, often leading to scar tissue, weaker tendons, limited mobility, and frequent re-injury. Tendon engineering aims to address these challenges by using biodegradable "scaffolds" that mimic the natural tendon environment, supporting cell growth and tissue repair. In this study, we developed computational models to design 3D tubular structures using a printing method called Melt Electrowriting (MEW). These structures replicate the mechanical properties of mouse Achilles tendons, guiding tendon cells to grow and differentiate effectively. By creating scaffolds from interwoven fibers, we closely replicated how natural tendon fibers interact, allowing cells to form strong, organized bundles. This approach may improve engineered tendon strength and durability, addressing key challenges in tendon reconstruction strategies.

Tendon regeneration deserves better: focused review on In vivo models, artificial intelligence and 3D bioprinting approaches

Tendon regeneration has been one of the most challenging issues in orthopedics. Despite various surgical techniques and rehabilitation methods, tendon tears or ruptures cannot wholly regenerate and gain the load-bearing capacity the tendon tissue had before the injury. The enhancement of tendon regeneration mostly requires grafting or an artificial tendon-like tissue to replace the damaged tendon. Tendon tissue engineering offers promising regenerative effects with numerous techniques in the additive manufacturing context. 3D bioprinting is a widely used additive manufacturing method to produce tendon-like artificial tissues based on biocompatible substitutes. There are multiple techniques and bio-inks for fabricating innovative scaffolds for tendon applications. Nevertheless, there are still many drawbacks to overcome for the successful regeneration of injured tendon tissue. The most important target is to catch the highest similarity to the tissue requirements such as anisotropy, porosity, viscoelasticity, mechanical strength, and cell-compatible constructs. To achieve the best-designed artificial tendon-like structure, novel AI-based systems in the field of 3D bioprinting may unveil excellent final products to re-establish tendon integrity and functionality. AI-driven optimization can enhance bio-ink selection, scaffold architecture, and printing parameters, ensuring better alignment with the biomechanical properties of native tendons. Furthermore, AI algorithms facilitate real-time process monitoring and adaptive adjustments, improving reproducibility and precision in scaffold fabrication. Thus, in vitro biocompatibility and in vivo application-based experimental processes will make it possible to accelerate tendon healing and reach the required mechanical strength. Integrating AI-based predictive modeling can further refine these experimental processes to evaluate scaffold performance, cell viability, and mechanical durability, ultimately improving translation into clinical applications. Here in this review, 3D bioprinting approaches and AI-based technology incorporation were given in addition to in vivo models.

Tendon regeneration deserves better: focused review on In vivo models, artificial intelligence and 3D bioprinting approaches

Tendon regeneration has been one of the most challenging issues in orthopedics. Despite various surgical techniques and rehabilitation methods, tendon tears or ruptures cannot wholly regenerate and gain the load-bearing capacity the tendon tissue had before the injury. The enhancement of tendon regeneration mostly requires grafting or an artificial tendon-like tissue to replace the damaged tendon. Tendon tissue engineering offers promising regenerative effects with numerous techniques in the additive manufacturing context. 3D bioprinting is a widely used additive manufacturing method to produce tendon-like artificial tissues based on biocompatible substitutes. There are multiple techniques and bio-inks for fabricating innovative scaffolds for tendon applications. Nevertheless, there are still many drawbacks to overcome for the successful regeneration of injured tendon tissue. The most important target is to catch the highest similarity to the tissue requirements such as anisotropy, porosity, viscoelasticity, mechanical strength, and cell-compatible constructs. To achieve the best-designed artificial tendon-like structure, novel AI-based systems in the field of 3D bioprinting may unveil excellent final products to re-establish tendon integrity and functionality. AI-driven optimization can enhance bio-ink selection, scaffold architecture, and printing parameters, ensuring better alignment with the biomechanical properties of native tendons. Furthermore, AI algorithms facilitate real-time process monitoring and adaptive adjustments, improving reproducibility and precision in scaffold fabrication. Thus, in vitro biocompatibility and in vivo application-based experimental processes will make it possible to accelerate tendon healing and reach the required mechanical strength. Integrating AI-based predictive modeling can further refine these experimental processes to evaluate scaffold performance, cell viability, and mechanical durability, ultimately improving translation into clinical applications. Here in this review, 3D bioprinting approaches and AI-based technology incorporation were given in addition to in vivo models.

One-Step Formed Janus Hydrogel with Time-Space Regulating Properties for Suture-Free and High-Quality Tendon Healing

Janus hydrogels have promising applications in tendon healing and anti-peritendinous adhesions. However, their complicated preparation methods, weak mechanical properties, and unstable adhesion interfaces have severely limited their application in suture-free and high-quality tendon healing. In this work, by controlling the interfacial distribution of free -COOH groups and cationic-π structures on both sides of the hydrogels, a series of PZBA-EGCG-ALC Janus hydrogels with varying degrees of asymmetric properties are successfully prepared using a simple and efficient one-step synthesis method. The tensile strength and elongation at the break of the Janus hydrogel are as high as 0.51 ± 0.04 MPa and 922.89 ± 28.59%. In addition, the Janus hydrogel can achieve a high difference in adhesion strength (nearly 20-fold) while maintaining a strong adhesion strength on their bottom sides (up to 524.8 ± 33.1 J m-2). In the spatial dimension, its excellent mechanical compliance and one-sided adhesion behavior can provide effective mechanical support and physical barriers for the injured Achilles tendons. More importantly, the Janus hydrogel can also minimize early inflammation generation in the time dimension via its ROS-responsive PZBA-EGCG prodrug macromolecules. This study provided a more effective and convenient suture-free strategy for constructing Janus hydrogels to promote high-quality tendon healing.

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.

Biocompatible and stretchable chitosan piezoelectric gel with antibacterial capability and motion monitoring function for Achilles tendon rupture treatment

Achilles tendon rupture is a common and serious condition that remains a challenge in the restoration of tendon structure and function. The design and use of high-performance piezoelectric materials serve as an effective solution to enhance repair outcomes, shorten recovery times, and reduce the risk of recurrence. In this study, we prepared a chitosan piezoelectric gel (CSPG) as an organic polymer with excellent biocompatibility, stretchability, and piezoelectric properties as well as excellent antibacterial properties. In vitro experiments showed that CSPG, which induces a piezoelectric effect, can inhibit bacterial growth, promote cell proliferation and migration, upregulate the expression of tendon-related genes, and inhibit the expression of inflammation-related genes. In vivo experiments showed improved outcomes for Achilles tendon repair following CSPG intervention, as evidenced by enhanced animal mobility and improved mechanical test results. In addition, the CSPG exhibited sensory functions capable of monitoring temperature and motion, providing timely feedback on repair efficacy. In summary, this study not only successfully prepared a multifunctional piezoelectric material that can effectively promote Achilles tendon rupture repair and regeneration and control inflammatory response, it also possesses antibacterial and sensing functions, thus offering a new strategy for Achilles tendon rupture repair.

Optimizing Flexor Digitorum Profundus Tendon Repair: A Narrative Review

Zone II flexor digitorum profundus (FDP) tendon injuries are complex, and present significant challenges in hand surgery, due to the need to balance strength and flexibility during repair. Traditional suture techniques often lead to complications such as adhesions or tendon rupture, prompting the exploration of novel strategies to improve outcomes. This review investigates the use of flexor digitorum superficialis (FDS) tendon autografts to reinforce FDP repairs, alongside the integration of biomaterials to enhance mechanical strength without sacrificing FDS tissue. Key biomaterials, including collagen-polycaprolactone (PCL) composites, are evaluated for their biocompatibility, mechanical integrity, and controlled degradation properties. Collagen-PCL emerges as a leading candidate, offering the potential to reduce adhesions and promote tendon healing. Although nanomaterials such as nanofibers and nanoparticles show promise in preventing adhesions and supporting cellular proliferation, their application remains limited by manufacturing challenges. By combining advanced repair techniques with biomaterials like collagen-PCL, this approach aims to improve surgical outcomes and minimize complications. Future research will focus on validating these findings in biological models, assessing tendon healing through imaging, and comparing the cost-effectiveness of biomaterial-enhanced repairs with traditional methods. This review underscores the potential for biomaterial-based approaches to transform FDP tendon repair.

Hypoxia-Mimicking Microenvironment Scaffold for Enhanced Tendon Regeneration

Tendon, a connective tissue structure, serves the crucial role of transmitting force between muscles and bones. However, tendon injury repair continues to pose a significant challenge in clinical settings. In this study, we utilized single-cell RNA sequencing to delve into the cell populations and signaling pathways that are integral to tendon healing. Our findings suggest that hypoxia plays a pivotal role in activating macrophages, stimulating endothelial cell migration, and fostering fibroblast proliferation. Based on these insights, we have developed a PCL scaffold coated with DFOA, which effectively mimics a hypoxic environment to enhance tendon tissue regeneration. Furthermore, the PCL-DFOA scaffolds exhibit exceptional ability in promoting macrophage polarization and angiogenesis. This research offers a therapeutic strategy that harnesses the regenerative power of hypoxia to accelerate and optimize tendon healing processes.

Autophagy modulates tenogenic differentiation of cartilage-derived stem cells in response to mechanical tension via FGF signaling

BACKGROUND

In our previous study, we demonstrated that cartilage-derived stem cells (CDSCs) possess multi-differentiation potential, enabling direct bone-to-tendon structure regeneration after transplantation in a rat model. Therefore, the objective of this study is to investigate whether CDSCs are a suitable candidate for achieving biological regeneration of tendon injuries.

METHODS

Tenogenic differentiation was evaluated through cell morphology observation, PCR, and Western blot (WB) analysis. Autophagic flux, transmission electron microscopy, and WB analysis were employed to elucidate the role of autophagy during CDSC tenogenic differentiation. Cell survival and tenogenesis of transplanted CDSCs were assessed using fluorescence detection of gross and frozen section images. Heterotopic ossification and quality of tendon healing were evaluated by immunofluorescence, hematoxylin-eosin (H&E), and Safrinin O/Fast Green stains.

RESULTS

We found autophagy is activated in CDSCs when treated with cyclic tensile stress, which facilitates the preservation of their chondrogenic potential while impeding tenogenic differentiation. Inhibiting autophagy with chloroquine promoted tenogenic differentiation of CDSCs in response to cyclic tensile stress through activation of the Fgf2/Fgfr2 signaling pathway. This mechanism was further validated by 2 mouse transplantation models, revealed that autophagy inhibition could enhance the tendon regeneration efficacy of transplanted CDSCs at the patellar tendon resection site.

CONCLUSION

Our findings provide insights into CDSC transplantation for achieving biological regeneration of tendon injuries, and demonstrate how modulation of autophagy in CDSCs can promote tenogenic differentiation in response to tensile stress both in vivo and in vitro.

Controlled TPCA-1 delivery engineers a pro-tenogenic niche to initiate tendon regeneration by targeting IKKβ/NF-κB signaling

Tendon repair remains challenging due to its poor intrinsic healing capacity, and stem cell therapy has emerged as a promising strategy to promote tendon regeneration. Nevertheless, the inflammatory environment following acute tendon injuries disrupts stem cell differentiation, leading to unsatisfied outcomes. Our study recognized the critical role of NF-κB signaling in activating inflammation and suppressing tenogenic differentiation of stem cells after acute tendon injury via multiomics analysis. TPCA-1, a selective inhibitor of IKKβ/NF-κB signaling, efficiently restored the impaired tenogenesis of stem cells in the inflammatory environment. By developing a microsphere-incorporated hydrogel system for stem cell delivery and controlled release of TPCA-1, we successfully engineered a pro-tenogenic niche to initiate tenogenesis for tendon regeneration. Collectively, we recognize NF-κB signaling as a critical target to tailor a pro-tenogenic niche and propose the combined delivery of stem cells and TPCA-1 as a potential strategy for acute tendon injuries.

Regenerative properties of bone marrow mesenchymal stem cell derived exosomes in rotator cuff tears

ABSTRCT

Rotator cuff injury (RCI), characterized by shoulder pain and restricted mobility, represents a subset of tendon-bone insertion injuries (TBI). In the majority of cases, surgical reconstruction of the affected tendons or ligaments is required to address the damage. However, numerous clinical failures have underscored the suboptimal outcomes associated with such procedures. Further investigations have revealed that these failures are largely attributable to delayed healing at the tendon-bone interface, excessive formation of vascularized scar tissue, and inadequate integration of tendon grafts within bone tunnels. As a result, the healing process of rotator cuff injuries faces significant challenges.Bone marrow-derived mesenchymal stem cell exosomes (BMSC-exos) have emerged as a prominent focus of research within the field of bioengineering, owing to their remarkable potential to regulate cellular proliferation and differentiation, modulate immune responses, and facilitate tissue repair and regeneration following cellular damage. In this review, we explore the anti-inflammatory, angiogenic, anti-scarring, and bone metabolism-modulating effects of BMSC-exos in the context of rotator cuff injury. Additionally, we address the limitations and ongoing challenges within current research, offering insights that could guide the clinical application of BMSC-exos in the treatment of rotator cuff injuries in the future.

Matrix Metalloproteinase-Responsive Controlled Release of Self-Assembly Nanoparticles Accelerates Heart Valve Regeneration In Situ by Orchestrating Immunomodulation

In situ tissue engineering heart valves (TEHVs) are the most promising way to overcome the defects of existing valve prostheses. Despite their promising prospects, the clinical translation of TEHVs remains a formidable challenge, mainly due to unpredictable host interactions post-implantation. An immunomodulatory idea based on hydrogel encapsulation of nanoparticle-coated heart valve scaffolds is introduced. Specifically, galactose-modified human serum albumin nanoparticles (miR-93@HSA NPs) to deliver microRNA-93 mimics are utilized, which target macrophages and induce their differentiation into the anti-inflammatory M2 subtype, fostering a conducive immune microenvironment. Matrix metalloproteinase (MMP)-responsive hydrogel is used to encapsulate the nanoparticles, enabling targeted and sustained release. Results show that the miR-93@HSA NPs exhibit excellent ability to induce macrophage polarization toward the M2 phenotype. A decellularized valve modified with hydrogel reveals MMP-response release of the miR-93@HSA NPs. In vitro, the immunomodulatory heart valve possesses good endocytocompatibility and effectively reprograms macrophages when cocultured with HUVECs or RAW264.7 macrophages. In vivo, this valve scaffold promises to mitigate early inflammatory damage and provide a pro-endothelialization niche for scaffolds' constructive remodeling. With the use of cell coculture systems and transcriptome sequencing, the mechanism of immune-modulating scaffold accelerating endothelialization is being elucidated. The immunomodulatory heart valve scaffold holds promising potential for clinical translation.

Electrospun poly(ε-caprolactone)/poly(glycerol sebacate) aligned fibers fabricated with benign solvents for tendon tissue engineering

The electrospinning technique is a commonly employed approach to fabricate fibers intended for various tissue engineering applications. The aim of this study is to develop a novel strategy for tendon repair through the use of aligned poly(ε-caprolactone) (PCL) and poly(glycerol sebacate) (PGS) fibers fabricated in benign solvents, and further explore the potential application of PGS in tendon tissue engineering (TTE). The fibers were characterized for their morphological and physicochemical properties; amniotic epithelial stem cells (AECs) were used to assess the fibers teno-inductive and immunomodulatory potential due to their ability to teno-differentiate undergoing first a stepwise epithelial to mesenchymal transition, and due to their documented therapeutic role in tendon regeneration. The addition of PGS to PCL improved the spinnability of the polymer solution, as well as the uniformity and directionality of the so-obtained fibers. The mechanical properties were in the range of most TTE applications, specifically in the case of PCL/PGS 4:1 and 2:1 ratios. Compared to PCL alone, the same ratios also allowed a better AECs infiltration and growth over 7 days of culture, and triggered the activation of tendon-related genes (SCX, COL1, TNMD) and the expression of tenomodulin (TNMD) at the protein level. Concerning the immunomodulatory properties, both PCL and PCL/PGS fibers negatively affected the immunomodulatory profile of AECs, up-regulating both anti-inflammatory (IL-10) and pro-inflammatory (IL-12) cytokines over 7 days of culture. Overall, PCL/PGS 2:1 fibers fabricated with benign solvents proved to be the most suitable composition for TTE application based on their topographical cues, mechanical properties, biocompatibility, and teno-inductive properties.

Collagen scaffold-seeded iTenocytes accelerate the healing and functional recovery of Achilles tendon defects in a rat model

Introduction

Tendon injuries represent an ongoing challenge in clinical practice due to poor regenerative capacity, structure, and biomechanical function recovery of ruptured tendons. This study is focused on the assessment of a novel strategy to repair ruptured Achilles tendons in a Nude rat model using stem cell-seeded biomaterial.

Methods

Specifically, we have used induced pluripotent stem cell (iPSC)-derived mesenchymal stem cells (iMSCs) overexpressing the early tendon marker Scleraxis (SCX, iMSCSCX+, iTenocytes) in combination with an elastic collagen scaffold. Achilles tendon defects in Nude rat models were created by isolating the tendon and excising 3 mm of the midsection. The Achilles tendon defects were then repaired with iTenocyte-seeded scaffolds, unseeded scaffolds, or suture only and compared to native Nude rat tendon tissue using gait analyses, biomechanical testing, histology, and immunohistochemistry.

Results

The results show faster functional recovery of gait in iTenocyte-seeded scaffold group comparing to scaffold only and suture only groups. Both iTenocyte-seeded scaffold and scaffold only treatment groups had improved biomechanical properties when compared to suture only treatment group, however no statistically significant difference was found in comparing the cell seeding scaffold an scaffold only group in terms of biomechanical properties. Immunohistochemistry staining further demonstrated that iTenocytes successfully populated the collagen scaffolds and survived 9 weeks after implantation in vivo. Additionally, the repaired tissue of iTenocyte-treated injuries exhibited a more organized structure when compared to tendon defects that were repaired only with suturing or unseeded scaffolds.

Conclusion

We suggest that iTenocyte-seeded DuRepair™ collagen scaffold can be used as potential treatment to regenerate the tendon tissue biomechanically and functionally.

The current status of various preclinical therapeutic approaches for tendon repair

Tendons are fibroblastic structures that link muscle and bone. There are two kinds of tendon injuries, including acute and chronic. Each form of injury or deterioration can result in significant pain and loss of tendon function. The recovery of tendon damage is a complex and time-consuming recovery process. Depending on the anatomical location of the tendon tissue, the clinical outcomes are not the same. The healing of the wound process is divided into three stages that overlap: inflammation, proliferation, and tissue remodeling. Furthermore, the curing tendon has a high re-tear rate. Faced with the challenges, tendon injury management is still a clinical issue that must be resolved as soon as possible. Several newer directions and breakthroughs in tendon recovery have emerged in recent years. This article describes tendon injury and summarizes recent advances in tendon recovery, along with stem cell therapy, gene therapy, Platelet-rich plasma remedy, growth factors, drug treatment, and tissue engineering. Despite the recent fast-growing research in tendon recovery treatment, still, none of them translated to the clinical setting. This review provides a detailed overview of tendon injuries and potential preclinical approaches for treating tendon injuries.

Effect of hydroalcoholic extract of Sambucus nigra on superficial digital flexor tendon repair in rabbit by ultrasonography and histopathology

BACKGROUND

Adult tendon tissue has limited and slow regenerative capacity. Sambucus nigra plant possesses antioxidant and anti-inflammatory attributes.

OBJECTIVES

This study aimed to evaluate the effect of hydroalcoholic extract of this plant's fruit on superficial digital flexor tendon repair in rabbits (SDFT).

METHODS

Twenty-five male New Zealand white rabbits weighing 1.5-2 kg were selected, quarantined and randomly divided into four groups of six. By performing a partial left posterior limb tenotomy, differentiating the SDFT and creating multiple scrapes were performed. During 3 consecutive days post-surgery, the positive control group was injected with 0.5 mg/kg dexamethasone, whereas the treatment groups received extract doses of 200 and 400 mg/kg, respectively. The negative control group did not receive any medication. Evaluation of sonographic and histopathological parameters was conducted on days 0, 7 and 28 post-surgeries. Findings were analysed and compared using SPSS22.

RESULTS

Both treatment groups showed significant differences in echogenicity, and collagen fibre alignment compared to the control and positive control groups, in sonographic evaluation. Histopathological examination revealed fewer inflammatory cells and increased collagen fibre formation in the treatment groups compared to the other two groups. No significant difference in angiogenesis was observed among the groups on days 7 and 28 (p value <0.05).

CONCLUSIONS

The results indicate that the S. nigra fruit extract, by stimulating collagen synthesis and reducing inflammation, effectively accelerates the healing process of injured tendons.

Engineered Tendon-Fibrocartilage-Bone Composite With Mechanical Stimulation for Augmentation of Rotator Cuff Repair: A Study Using an In Vivo Canine Model With a 6-Month Follow-up

BACKGROUND

Rotator cuff repair augmentation using biological materials has become popular in clinical practice to reduce the high retear rates associated with traditional repair techniques. Tissue engineering approaches, such as engineered tendon-fibrocartilage-bone composite (TFBC), have shown promise in enhancing the biological healing of rotator cuff tears in animals. However, previous studies have provided limited long-term data on TFBC repair outcomes. The effect of mechanical stimulation on TFBC has not been explored extensively.

PURPOSE

To evaluate functional outcomes after rotator cuff repair with engineered TFBC subjected to mechanical stimulation in a 6-month follow-up using a canine in vivo model.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 40 canines with an acute infraspinatus (ISP) tendon transection model were randomly allocated to 4 groups (n =10): (1) unilateral ISP tendon undergoing suture repair only (control surgery); (2) augmentation with engineered TFBC alone (TFBC); (3) augmentation with engineered TFBC and bone marrow-derived stem cells (BMSCs) (TFBC+C); and (4) augmentation with engineered TFBC and BMSCs, as well as mechanical stimulation (TFBC+C+M). Outcome measures-including biomechanical evaluations such as failure strength, stiffness, failure mode, gross appearance, ISP tendon and muscle morphological assessment, and histological analysis-were performed 6 months after surgery.

RESULTS

As shown in the mechanical test, the TFBC+C+M group exhibited higher failure strength compared with other repair techniques. The most common failure mode was avulsion fracture in the TFBC+C+M group, but tendon-bone junction rupture was observed predominantly in different groups. Engineered TFBC with mechanical stimulation showed over 70% relative failure strength compared with normal ISP, and the other groups showed about 50% relative failure strength. Histological analysis revealed less fat infiltration and closer-to-normal muscle fiber structure in the mechanical stimulation group.

CONCLUSION

This study provides evidence that mechanical stimulation of engineered TFBC promotes rotator cuff regeneration, thus supporting its potential for rotator cuff repair augmentation.

CLINICAL RELEVANCE

This study provides valuable evidence supporting the use of a novel tissue-engineered material (TFBC) in rotator cuff repair and paves the way for advancements in the field of rotator cuff regeneration.

Matrix stiffness regulates macrophage polarisation via the Piezo1-YAP signalling axis

Macrophages play a pivotal role in the immunological cascade activated in response to biomedical implants, which predetermine acceptance or rejection of implants by the host via pro- and anti-inflammatory polarisation states. The role of chemical signals in macrophage polarisation is well-established, but how physical cues regulate macrophage function that may play a fundamental role in implant-bone interface, remains poorly understood. Here we find that bone marrow-derived macrophages (BMDM) cultured on polyacrylamide gels of varying stiffness exhibit different polarisation states. BMDM are 'primed' to a pro-inflammatory M1 phenotype on stiff substrates, while to an anti-inflammatory M2 phenotype on soft and medium stiffness substrates. It is further observed that matrix stiffening increases Piezo1 expression, as well as leads to subsequent activation of the mechanotransduction signalling effector YAP, thus favouring M1 polarisation whilst suppressing M2 polarisation. Moreover, upon treatment with YAP inhibitor, we successfully induce macrophage re-polarisation to the M2 state within the implant site microenvironment, which in turn promotes implant osseointegration. Collectively, our present study thus characterises the critical role of the Piezo1-YAP signalling axis in macrophage mechanosensing and stiffness-mediated macrophage polarisation and provides cues for the design of immuno-modulatory biomaterials that can regulate the macrophage phenotype.

In vivo study on the repair of rat Achilles tendon injury treated with non-thermal atmospheric-pressure helium microplasma jet

Non-thermal atmospheric-pressure plasma (NTAPP) has been widely studied for clinical applications, e.g., disinfection, wound healing, cancer therapy, hemostasis, and bone regeneration. It is being revealed that the physical and chemical actions of plasma have enabled these clinical applications. Based on our previous report regarding plasma-stimulated bone regeneration, this study focused on Achilles tendon repair by NTAPP. This is the first study to reveal that exposure to NTAPP can accelerate Achilles tendon repair using a well-established Achilles tendon injury rat model. Histological evaluation using the Stoll's and histological scores showed a significant improvement at 2 and 4 weeks, with type I collagen content being substantial at the early time point of 2 weeks post-surgery. Notably, the replacement of type III collagen with type I collagen occurred more frequently in the plasma-treated groups at the early stage of repair. Tensile strength test results showed that the maximum breaking strength in the plasma-treated group at two weeks was significantly higher than that in the untreated group. Overall, our results indicate that a single event of NTAPP treatment during the surgery can contribute to an early recovery of an injured tendon.

Bioactive fiber-reinforced hydrogel to tailor cell microenvironment for structural and functional regeneration of myotendinous junction

Myotendinous junction (MTJ) injuries are prevalent in clinical practice, yet the treatment approaches are limited to surgical suturing and conservative therapy, exhibiting a high recurrence rate. Current research on MTJ tissue engineering is scarce and lacks in vivo evaluation of repair efficacy. Here, we developed a three-dimensional-printed bioactive fiber-reinforced hydrogel containing mesenchymal stem cells (MSCs) and Klotho for structural and functional MTJ regeneration. In a rat MTJ defect model, the bioactive fiber-reinforced hydrogel promoted the structural restoration of muscle, tendon, and muscle-tendon interface and enhanced the functional recovery of injured MTJ. In vivo proteomics and in vitro cell cultures elucidated the regenerative mechanisms of the bioactive fiber-reinforced hydrogel by modulating oxidative stress and inflammation, thus engineering an optimized microenvironment to support the survival and differentiation of transplanted MSCs and maintain the functional phenotype of resident cells within MTJ tissues, including tendon/muscle cells and macrophages. This strategy provides a promising treatment for MTJ injuries.

Assessing Bioprinted Functionalized Grafts for Biological Tendon Augmentation In Vitro

Tendinopathy, characterized by inflammatory and degenerative changes, presents challenges in sports and medicine. In addressing the limitations of conservative management, this study focuses on developing tendon grafts using extrusion bioprinting with platelet-rich plasma (PRP)-infused hydrogels loaded with tendon cells. The objective is to understand paracrine interactions initiated by bioprinted tendon grafts in either inflamed or non-inflamed host tissues. PRP was utilized to functionalize methacrylate gelatin (GelMA), incorporating tendon cells for graft bioprinting. Bioinformatic analyses of overexpressed proteins, predictive of functional enrichment, revealed insights into PRP graft behavior in both non-inflamed and inflamed environments. PRP grafts activated inflammatory pathways, including Interleukin 17 (IL-17), neuroinflammation, Interleukin 33 (IL-33), and chemokine signaling. Interleukin 1 beta (IL-1b) in the graft environment triggered p38 mitogen-activated protein kinase (MAPK) signaling, nuclear factor kappa light chain enhancer of activated B cells (NF-kB) canonical pathway, and Vascular Endothelial Growth Factor (VEGF) signaling. Biological enrichment attributed to PRP grafts included cell chemotaxis, collagen turnover, cell migration, and angiogenesis. Acellular PRP grafts differed from nude grafts in promoting vessel length, vessel area, and junction density. Angiogenesis in cellular grafts was enhanced with newly synthesized Interleukin 8 (IL-8) in cooperation with IL-1b. In conclusion, paracrine signaling from PRP grafts, mediated by chemokine activities, influences cell migration, inflammation, and angiogenic status in host tissues. Under inflammatory conditions, newly synthesized IL-8 regulates vascularization in collaboration with PRP.

The Use of Extracellular Vesicles in Achilles Tendon Repair: A Systematic Review

Achilles tendon (AT) pathologies are common musculoskeletal conditions that can significantly impair function. Despite various traditional treatments, recovery is often slow and may not restore full functionality. The use of extracellular vesicles (EVs) has emerged as a promising therapeutic option due to their role in cell signaling and tissue regeneration. This systematic review aims to consolidate current in vivo animal study findings on the therapeutic effects of EVs on AT injuries. An extensive literature search was conducted using the PubMed, Scopus, and Embase databases for in vivo animal studies examining the effects of EVs on AT pathologies. The extracted variables included but were not limited to the study design, type of EVs used, administration methods, efficacy of treatment, and proposed therapeutic mechanisms. After screening, 18 studies comprising 800 subjects were included. All but one study reported that EVs augmented wound healing processes in the AT. The most proposed mechanisms through which this occurred were gene regulation of the extracellular matrix (ECM), the enhancement of macrophage polarization, and the delivery of therapeutic microRNAs to the injury site. Further research is warranted to not only explore the therapeutic potential of EVs in the context of AT pathologies, but also to establish protocols for their clinical application.

Bioactive 3D Electrohydrodynamic Printed Lattice Architectures Augment Tenogenesis of Tendon Stem/Progenitor Cells

Tendon defect repair remains a tough clinical procedure that hinders functional motion in patients. Electrohydrodynamic (EHD) three-dimensional (3D) printing, as a novel strategy, can controllably fabricate biomimetic micro/nanoscale architecture, but the hydrophobic and bioinert nature of polymers might be adverse to cell-material interplay. In this work, 3D EHD printed polycaprolactone (PCL) was immobilized on basic fibroblast growth factor (bFGF) using polydopamine (PDA), and the proliferation and tenogenic differentiation of tendon stem/progenitor cells (TSPCs) in vitro was researched. A subcutaneous model was established to evaluate the effects of tenogenesis and immunomodulation. We then investigated the in situ implantation and immunomodulation effects in an Achilles tendon defect model. After immobilization of bFGF, the scaffolds profoundly facilitated proliferation and tenogenic differentiation; however, PDA had only a proliferative effect. Intriguingly, the bFGF immobilized on EHD printed PCL indicated a synergistic effect on the highest expression of tenogenic gene and protein markers at 14 days, and the tenogenesis may be induced by activating the transforming growth factor-β (TGF-β) signal pathway in vitro. The subcutaneous engraftment study confirmed a tendon-like structure, similar to that of the native tendon, as well as an M2 macrophage polarization effect. Additionally, the bioactive scaffold exhibited superior efficacy in new collagen formation and repair of Achilles tendon defects. Our study revealed that the topographic cues alone were insufficient to trigger tenogenic differentiation, requiring appropriate chemical signals, and that appropriate immunomodulation was conducive to tenogenesis. The tenogenesis of TSPCs on the bioactive scaffold may be correlated with the TGF-β signal pathway and M2 macrophage polarization.

Janus Membranes Patch Achieves High-Quality Tendon Repair: Inhibiting Exogenous Healing and Promoting Endogenous Healing

The imbalance between endogenous and exogenous healing is the fundamental reason for the poor tendon healing. In this study, a Janus patch was developed to promote endogenous healing and inhibit exogenous healing, leading to improved tendon repair. The upper layer of the patch is a poly(dl-lactide-co-glycolide)/polycaprolactone (PLGA/PCL) nanomembrane (PMCP-NM) modified with poly(2-methylacryloxyethyl phosphocholine) (PMPC), which created a lubricated and antifouling surface, preventing cell invasion and mechanical activation. The lower layer is a PLGA/PCL fiber membrane loaded with fibrin (Fb) (Fb-NM), serving as a temporary chemotactic scaffold to regulate the regenerative microenvironment. In vitro, the Janus patch effectively reduced 92.41% cell adhesion and 79.89% motion friction. In vivo, the patch inhibited tendon adhesion through the TGF-β/Smad signaling pathway and promoted tendon maturation. This Janus patch is expected to provide a practical basis and theoretical guidance for high-quality soft tissue repair.

Tannic Acid-Modified Decellularized Tendon Scaffold with Antioxidant and Anti-Inflammatory Activities for Tendon Regeneration

Tendon regeneration is greatly influenced by the oxidant and the inflammatory microenvironment. Persistent inflammation during the tendon repair can cause matrix degradation, tendon adhesion, and excessive accumulation of reactive oxygen species (ROS), while excessive ROS affect extracellular matrix remodeling and tendon integration. Herein, we used tannic acid (TA) to modify a decellularized tendon slice (DTS) to fabricate a functional scaffold (DTS-TA) with antioxidant and anti-inflammatory properties for tendon repair. The characterizations and cytocompatibility of the scaffolds were examined in vitro. The antioxidant and anti-inflammatory activities of the scaffold were evaluated in vitro and further studied in vivo using a subcutaneous implantation model. It was found that the modified DTS combined with TA via hydrogen bonds and covalent bonds, and the hydrophilicity, thermal stability, biodegradability, and mechanical characteristics of the scaffold were significantly improved. Afterward, the results demonstrated that DTS-TA could effectively reduce inflammation by increasing the M2/M1 macrophage ratio and interleukin-4 (IL-4) expression, decreasing the secretion of interleukin-6 (IL-6) and interleukin-1β (IL-1β), as well as scavenging excessive ROS in vitro and in vivo. In summary, DTS modified with TA provides a potential versatile scaffold for tendon regeneration.

Immunomodulatory multicellular scaffolds for tendon-to-bone regeneration

Limited motor activity due to the loss of natural structure impedes recovery in patients suffering from tendon-to-bone injury. Conventional biomaterials focus on strengthening the regenerative ability of tendons/bones to restore natural structure. However, owing to ignoring the immune environment and lack of multi-tissue regenerative function, satisfactory outcomes remain elusive. Here, combined manganese silicate (MS) nanoparticles with tendon/bone-related cells, the immunomodulatory multicellular scaffolds were fabricated for integrated regeneration of tendon-to-bone. Notably, by integrating biomimetic cellular distribution and MS nanoparticles, the multicellular scaffolds exhibited diverse bioactivities. Moreover, MS nanoparticles enhanced the specific differentiation of multicellular scaffolds via regulating macrophages, which was mainly attributed to the secretion of PGE2 in macrophages induced by Mn ions. Furthermore, three animal results indicated that the scaffolds achieved immunomodulation, integrated regeneration, and function recovery at tendon-to-bone interfaces. Thus, the multicellular scaffolds based on inorganic biomaterials offer an innovative concept for immunomodulation and integrated regeneration of soft/hard tissue interfaces.

Current clinical opinion on surgical approaches and rehabilitation of hand flexor tendon injury-a questionnaire study

The management of flexor tendon injury has seen many iterations over the years, but more substantial innovations in practice have been sadly lacking. The aim of this study was to investigate the current practice of flexor tendon injury management, and variation in practice from the previous reports, most troublesome complications, and whether there was a clinical interest in potential innovative tendon repair technologies. An online survey was distributed via the British Society for Surgery of the Hand (BSSH) and a total of 132 responses were collected anonymously. Results showed that although most surgeons followed the current medical recommendation based on the literature, a significant number of surgeons still employed more conventional treatments in clinic, such as general anesthesia, ineffective tendon retrieval techniques, and passive rehabilitation. Complications including adhesion formation and re-rupture remained persistent. The interest in new approaches such as use of minimally invasive instruments, biodegradable materials and additive manufactured devices was not strong, however the surgeons were potentially open to more effective and economic solutions.

Local Injection of Rapamycin-Loaded Pcl-Peg Nanoparticles for Enhanced Tendon Healing in Rotator Cuff Tears via Simultaneously Reducing Fatty Infiltration and Drug Toxicity

As a common cause of shoulder pain, rotator cuff tears (RCTs) are difficult to treat clinically because of their unsatisfactory prognosis due to the fatty infiltration caused by muscle-derived stem cells (MDSCs). Previous studies have found that rapamycin (RAPA) can inhibit fatty infiltration. However, systemic administration of RAPA may cause complications such as infection and nausea, while local administration of RAPA may lead to the cytotoxicity of tendon cells, affecting the healing of rotator cuffs. In this study, biocompatible and clinically approved polycaprolactone-polyethylene glycol (PCL-PEG) is formulated into an injectable nanoparticle for the sustained release of RAPA. The results indicate that the RAPA/PCL-PEG nanoparticles (NPs) can efficiently prolong the release of RAPA and significantly reduce the cytotoxicity of tendon cells caused by RAPA. The study of the fatty infiltration model in rats with delayed rotator cuff repair shows that weekly intraarticular injection of RAPA/PCL-PEG NPs can more effectively reduce the fatty infiltration and muscle atrophy of rat rotator cuffs and leads to better mechanical properties and gait improvements than a daily intraarticular injection of RAPA. These findings imply that local injection of RAPA/PCL-PEG NPs in the shoulder joints can be a potential clinical option for RCTs patients with fatty infiltration.

Micro-nano hierarchical scaffold providing temporal-matched biological constraints for tendon reconstruction

Due to the limitations of tendon biology, high-quality tendon repair remains a clinical and scientific challenge. Here, a micro-nano hierarchical scaffold is developed to promote orderly tendon regeneration by providing temporal-matched biological constraints. In short, fibrin (Fb), which provides biological constraints, is loaded into poly (DL-lactide-co-glycolide) nanoyarns with suitable degradation cycles (Fb-loaded nanofiber yarns (Fb-NY)). Then further combined with braiding technology, temporary chemotactic Fb scaffolds with tendon extracellular matrix-like structures are obtained to initiate the regeneration process. At the early stage of healing (2 w), the regeneration microenvironment is regulated (inducing M2 macrophages and restoring the early blood supply necessary for healing) by Fb, and the alignment of cells and collagen is induced by nanoyarn. At the late healing stage (8 w), with the degradation of Fb-NY, non-functional vascular regression occurs, and the newborn tissues gradually undergo load-bearing remodeling, restoring the anvascularous and ordered structure of the tendon. In summary, the proposed repair strategy provides temporal-matched biological constraints, offering a potential pathway to reconstruct the ordered structure and function of tendons.

Macroporous Granular Hydrogels Functionalized with Aligned Architecture and Small Extracellular Vesicles Stimulate Osteoporotic Tendon-To-Bone Healing

Osteoporotic tendon-to-bone healing (TBH) after rotator cuff repair (RCR) is a significant orthopedic challenge. Considering the aligned architecture of the tendon, inflammatory microenvironment at the injury site, and the need for endogenous cell/tissue infiltration, there is an imminent need for an ideal scaffold to promote TBH that has aligned architecture, ability to modulate inflammation, and macroporous structure. Herein, a novel macroporous hydrogel comprising sodium alginate/hyaluronic acid/small extracellular vesicles from adipose-derived stem cells (sEVs) (MHA-sEVs) with aligned architecture and immunomodulatory ability is fabricated. When implanted subcutaneously, MHA-sEVs significantly improve cell infiltration and tissue integration through its macroporous structure. When applied to the osteoporotic RCR model, MHA-sEVs promote TBH by improving tendon repair through macroporous aligned architecture while enhancing bone regeneration by modulating inflammation. Notably, the biomechanical strength of MHA-sEVs is approximately two times higher than the control group, indicating great potential in reducing postoperative retear rates. Further cell-hydrogel interaction studies reveal that the alignment of microfiber gels in MHA-sEVs induces tenogenic differentiation of tendon-derived stem cells, while sEVs improve mitochondrial dysfunction in M1 macrophages (Mφ) and inhibit Mφ polarization toward M1 via nuclear factor-kappaB (NF-κb) signaling pathway. Taken together, MHA-sEVs provide a promising strategy for future clinical application in promoting osteoporotic TBH.

The mechanisms and functions of TGF-β1 in tendon healing

Tendon injury accounts for 30% of musculoskeletal diseases and often leads to disability, pain, healthcare cost, and lost productivity. Following injury to tendon, tendon healing proceeds via three overlapping healing processes. However, due to the structural defects of the tendon itself, the tendon healing process is characterized by the formation of excessive fibrotic scar tissue, and injured tendons rarely return to native tendons, which can easily contribute to tendon reinjury. Moreover, the resulting fibrous scar is considered to be a precipitating factor for subsequent degenerative tendinopathy. Despite this, therapies are almost limited because underlying molecular mechanisms during tendon healing are still unknown. Transforming Growth Factor-β1 (TGF-β1) is known as one of most potent profibrogenic factors during tendon healing process. However, blockage TGF-β1 fails to effectively enhance tendon healing. A detailed understanding of real abilities of TGF-β1 involved in tendon healing can bring promising perspectives for therapeutic value that improve the tendon healing process. Thus, in this review, we describe recent efforts to identify and characterize the roles and mechanisms of TGF-β1 involved at each stage of the tendon healing and highlight potential roles of TGF-β1 leading to the fibrotic response to tendon injury.

Cerium oxide nanoparticles-carrying human umbilical cord mesenchymal stem cells counteract oxidative damage and facilitate tendon regeneration

BACKGROUND

Tendon injuries have a high incidence and limited treatment options. Stem cell transplantation is essential for several medical conditions like tendon injuries. However, high local concentrations of reactive oxygen species (ROS) inhibit the activity of transplanted stem cells and hinder tendon repair. Cerium oxide nanoparticles (CeONPs) have emerged as antioxidant agents with reproducible reducibility.

RESULTS

In this study, we synthesized polyethylene glycol-packed CeONPs (PEG-CeONPs), which were loaded into the human umbilical cord mesenchymal stem cells (hUCMSCs) to counteract oxidative damage. H2O2 treatment was performed to evaluate the ROS scavenging ability of PEG-CeONPs in hUCMSCs. A rat model of patellar tendon defect was established to assess the effect of PEG-CeONPs-carrying hUCMSCs in vivo. The results showed that PEG-CeONPs exhibited excellent antioxidant activity both inside and outside the hUCMSCs. PEG-CeONPs protect hUCMSCs from senescence and apoptosis under excessive oxidative stress. Transplantation of hUCMSCs loaded with PEG-CeONPs reduced ROS levels in the tendon injury area and facilitated tendon healing. Mechanistically, NFκB activator tumor necrosis factor α and MAPK activator dehydrocrenatine, reversed the therapeutic effect of PEG-CeONPs in hUCMSCs, indicating that PEG-CeONPs act by inhibiting the NFκB and MAPK signaling pathways.

CONCLUSIONS

The carriage of the metal antioxidant oxidase PEG-CeONPs maintained the ability of hUCMSCs in the injured area, reduced the ROS levels in the microenvironment, and facilitated tendon regeneration. The data presented herein provide a novel therapeutic strategy for tendon healing and new insights into the use of stem cells for disease treatment.

Inflammation-related signaling pathways in tendinopathy

Tendon is a connective tissue that produces movement by transmitting the force produced by muscle contraction to the bones. Most tendinopathy is caused by prolonged overloading of the tendon, leading to degenerative disease of the tendon. When overloaded, the oxygen demand of tenocytes increases, and the tendon structure is special and lacks blood supply, which makes it easier to form an oxygen-deficient environment in tenocytes. The production of reactive oxygen species due to hypoxia causes elevation of inflammatory markers in the tendon, including PGE2, IL-1β, and TNF-α. In the process of tendon healing, inflammation is also a necessary stage. The inflammatory environment formed by cytokines and various immune cells play an important role in the clearance of necrotic material, the proliferation of tenocytes, and the production of collagen fibers. However, excessive inflammation can lead to tendon adhesions and hinder tendon healing. Some important and diverse biological functions of the body originate from intercellular signal transduction, among which cytokine mediation is an important way of signal transduction. In particular, NF-κB, NLRP3, p38/MAPK, and signal transducer and activator of transcription 3, four common signaling pathways in tendinopathy inflammatory response, play a crucial role in the regulation and transcription of inflammatory factors. Therefore, summarizing the specific mechanisms of inflammatory signaling pathways in tendinopathy is of great significance for an in-depth understanding of the inflammatory response process and exploring how to inhibit the harmful part of the inflammatory response and promote the beneficial part to improve the healing effect of the tendon.

A Biomimetic Adhesive and Robust Janus Patch with Anti-Oxidative, Anti-Inflammatory, and Anti-Bacterial Activities for Tendon Repair

Early stage oxidative stress, inflammatory response, and infection after tendon surgery are highly associated with the subsequent peritendinous adhesion formation, which may diminish the quality and function of the repaired tendon. Although various anti-inflammatory and/or antibacterial grafts have been proposed to turn the scale, most of them suffer from the uncertainty of drug-induced adverse effects, low mechanical strength, and tissue adhesiveness. Here, inspired by the tendon anatomy and pathophysiology of adhesion development, an adhesive and robust dual-layer Janus patch is developed, whose inner layer facing the operated tendon is a multifunctional electrospun hydrogel patch (MEHP), encircled further by a poly-l-lactic acid (PLLA) fibrous outer layer facing the surrounding tissue. Specifically, MEHP is prepared by gelatin methacryloyl (GelMA) and zinc oxide (ZnO) nanoparticles, which are co-electrospun first and then treated by tannic acid (TA). The inner MEHP exhibits superior mechanical performance, adhesion strength, and outstanding antioxidation, anti-inflammation, and antibacterial properties, and it can adhere to the injury site offering a favorable microenvironment for tendon regeneration. Meanwhile, the outer PLLA acts as a physical barrier that prevents extrinsic cells and tissues from invading the defect site, reducing peritendinous adhesion formation. This work presents a proof-of-concept of a drug-free graft with anisotropic adhesive and biological functions to concert the healing phases of injured tendon by alleviating incipient inflammation and oxidative damage but supporting tissue regeneration and reducing tendon adhesion in the later phase of repair and remodeling. It is envisioned that this Janus patch could offer a promising strategy for safe and efficient tendon therapy.

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.

Drug Delivery Approaches to Improve Tendon Healing

Tendon injuries disrupt the transmission of forces from muscle to bone, leading to chronic pain, disability, and a large socioeconomic burden. Tendon injuries are prevalent; there are over 300,000 tendon repair procedures a year in the United States to address acute trauma or chronic tendinopathy. Successful restoration of function after tendon injury remains challenging clinically. Despite improvements in surgical and physical therapy techniques, the high complication rate of tendon repair procedures motivates the use of therapeutic interventions to augment healing. While many biological and tissue engineering approaches have attempted to promote scarless tendon healing, there is currently no standard clinical treatment to improve tendon healing. Moreover, the limited efficacy of systemic delivery of several promising therapeutic candidates highlights the need for tendon-specific drug delivery approaches to facilitate translation. This review article will synthesize the current state-of-the-art methods that have been used for tendon-targeted delivery through both systemic and local treatments, highlight emerging technologies used for tissue-specific drug delivery in other tissue systems, and outline future challenges and opportunities to enhance tendon healing through targeted drug delivery.

Aligned Ice Templated Biomaterial Strategies for the Musculoskeletal System

Aligned pore structures present many advantages when conceiving biomaterial strategies for treatment of musculoskeletal disorders. Aligned ice templating (AIT) is one of the many different techniques capable of producing anisotropic porous scaffolds; its high versatility allows for the formation of structures with tunable pore sizes, as well as the use of many different materials. AIT has been found to yield improved compressive properties for bone tissue engineering (BTE), as well as higher tensile strength and optimized cellular alignment and proliferation in tendon and muscle repair applications. This review evaluates the work that has been done in the last decade toward the production of aligned pore structures by AIT with an outlook on the musculoskeletal system. This work describes the fundamentals of the AIT technique and focuses on the research carried out to optimize the biomechanical properties of scaffolds by modifying the pore structure, categorizing by material type and application. Related topics including growth factor incorporation into AIT scaffolds, drug delivery applications, and studies about immune system response will be discussed.

M2 Macrophage Membrane-Mediated Biomimetic-Nanoparticle Carrying COX-siRNA Targeted Delivery for Prevention of Tendon Adhesions by Inhibiting Inflammation

Tendon adhesion is the most common outcome of tendon or tendon-to-bone healing after injury. Our group developed a hydrogel-nanoparticle sustained-release system previously to inhibit cyclooxygenases (COXs) expression and consequently prevent tendon adhesion and achieved satisfactory results. However, effective treatment of multiple tendon adhesions is always a challenge in research on the prevention of tendon adhesion. In the present study, an M2M@PLGA/COX-siRNA delivery system is successfully constructed using the cell membranes of M2 macrophages and poly (lactic-co-glycolic acid) (PLGA) nanoparticles. Targeting properties and therapeutic effects are observed in mice or rat models of flexor digitorum longus (FDL) tendon injury combined with rotator cuff injury. The results showed that the M2M@PLGA/COX-siRNA delivery system has low toxicity and remarkable targeting properties to the injured areas. Treatment with the M2M@PLGA/COX-siRNA delivery system reduced the inflammatory reaction and significantly improved tendon adhesion in both the FDL tendon and rotator cuff tissues. These findings indicate that the M2M@PLGA delivery system can provide an effective biological strategy for preventing multiple tendon adhesions.

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.

Material Stiffness in Cooperation with Macrophage Paracrine Signals Determines the Tenogenic Differentiation of Mesenchymal Stem Cells

Stiffness is an important physical property of biomaterials that determines stem cell fate. Guiding stem cell differentiation via stiffness modulation has been considered in tissue engineering. However, the mechanism by which material stiffness regulates stem cell differentiation into the tendon lineage remains controversial. Increasing evidence demonstrates that immune cells interact with implanted biomaterials and regulate stem cell behaviors via paracrine signaling; however, the role of this mechanism in tendon differentiation is not clear. In this study, polydimethylsiloxane (PDMS) substrates with different stiffnesses are developed, and the tenogenic differentiation of mesenchymal stem cells (MSCs) exposed to different stiffnesses and macrophage paracrine signals is investigated. The results reveal that lower stiffnesses facilitates tenogenic differentiation of MSCs, while macrophage paracrine signals at these stiffnesses suppress the differentiation. When exposed to these two stimuli, MSCs still exhibit enhanced tendon differentiation, which is further elucidated by global proteomic analysis. Following subcutaneous implantation in rats for 2 weeks, soft biomaterial induces only low inflammation and promotes tendon-like tissue formation. In conclusion, the study demonstrates that soft, rather than stiff, material has a greater potential to guide tenogenic differentiation of stem cells, which provides comprehensive evidence for optimized bioactive scaffold design in tendon tissue engineering.

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.

Bioactive glass-elicited stem cell-derived extracellular vesicles regulate M2 macrophage polarization and angiogenesis to improve tendon regeneration and functional recovery

Effective countermeasures for tendon injury remains unsatisfactory. Mesenchymal stem cell-derived extracellular vesicles (MSC-EVs)-based therapy via regulation of Mφ-mediated angiogenesis has emerged as a promising strategy for tissue regeneration. Still, approaches to tailor the functions of EVs to treat tendon injuries have been limited. We reported a novel strategy by applying MSC-EVs boosted with bioactive glasses (BG). BG-elicited EVs (EV_B) showed up-regulation of medicinal miRNAs, including miR-199b-3p and miR-125a-5p, which play a pivotal role in M2 Mφ-mediated angiogenesis. EV_B accelerated angiogenesis via the reprogrammed anti-inflammatory M2 Mφs compared with naïve MSC-EVs (EV_N). In rodent Achilles tendon rupture model, EV_B local administration activated anti-inflammatory responses via M2 polarization and led to a spatial correlation between M2 Mφs and newly formed blood vessels. Our results showed that EV_B outperformed EV_N in promoting tenogenesis and in reducing detrimental morphological changes without causing heterotopic ossification. Biomechanical test revealed that EV_B significantly improved ultimate load, stiffness, and tensile modulus of the repaired tendon, along with a positive correlation between M2/M1 ratio and biomechanical properties. On the basis of the boosted nature to reprogram regenerative microenvironment, EV_B holds considerable potential to be developed as a next-generation therapeutic modality for enhancing functional regeneration to achieve satisfying tendon regeneration.

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.

Wnt3a-Modified Nanofiber Scaffolds Facilitate Tendon Healing by Driving Macrophage Polarization during Repair

Inflammation is part of the natural healing response, but persistent inflammatory events tend to contribute to pathology changes of tendon or ligament. Phenotypic switching of macrophages within the inflammatory niche is crucial for tendon healing. One viable strategy to improve the functional and biomechanical properties of ruptured tendons is to modulate the transition from inflammatory to regenerative signals during tendon regeneration at the site of injury. Here, we developed a tendon repair scaffold made of biodegradable polycaprolactone by electrospinning, which was modified to deliver Wnt3a protein and served as an implant to improve tendon healing in a rat model of Achilles tendon defect. During the in vitro study, Wnt3a protein promoted the polarization of M2 macrophages. In the in vivo experiment, Wnt3a scaffold promoted the early recruitment and counting curve of macrophages and increased the proportion of M2 macrophages. Achilles function index and mechanical properties showed that the implantation effect of the Wnt3a group was better than that of the control group. We believe that this type of scaffold can be used to repair tendon defects. This work highlights the beneficial role of local delivery of biological factors in directing inflammatory responses toward regenerative strategies in tendon healing.

Applications of functionally-adapted hydrogels in tendon repair

Despite all the efforts made in tissue engineering for tendon repair, the management of tendon injuries still poses a challenge, as current treatments are unable to restore the function of tendons following injuries. Hydrogels, due to their exceptional biocompatibility and plasticity, have been extensively applied and regarded as promising candidate biomaterials in tissue regeneration. Varieties of approaches have designed functionally-adapted hydrogels and combined hydrogels with other factors (e.g., bioactive molecules or drugs) or materials for the enhancement of tendon repair. This review first summarized the current state of knowledge on the mechanisms underlying the process of tendon healing. Afterward, we discussed novel strategies in fabricating hydrogels to overcome the issues frequently encountered during the applications in tendon repair, including poor mechanical properties and undesirable degradation. In addition, we comprehensively summarized the rational design of hydrogels for promoting stem-cell-based tendon tissue engineering via altering biophysical and biochemical factors. Finally, the role of macrophages in tendon repair and how they respond to immunomodulatory hydrogels were highlighted.

PDGF-loaded microneedles promote tendon healing through p38/cyclin D1 pathway mediated angiogenesis

Tendon injury is one of the most serious orthopedic diseases often leading to disability of patients. Major shortages of tendon healing are due to its multiple comorbidities, uncertainty of therapeutic efficacy and insufficient of angiogenesis. With a deeper understanding of angiogenic mechanism of tendon healing, we investigated an innovative microneedle patch loaded with platelet derived growth factor (PDGF) to achieve a constant systemic administration of PDGF to enhance topical tendon healing. Rat achilles tendon injury model was performed as in vivo animal models. Histological staining showed an enhancement of tendon healing quality, especially angiogenesis. Biomechanical studies demonstrated an increase of tendon stiffness, maximum load and maximum stress with treatment of PDGF-loaded microneedles. Furthermore, MAPK/p38/Cyclin D1 pathway and angiogenesis were found to play an important role in tendon healing process by using a biological high throughput RNA-sequence method and bioinformatic analysis. The high throughput RNA-seq tendon healing results were confirmed by histochemical staining and western blot. These results suggest the novel therapeutic potential of PDGF-loaded microneedle patch in tendon surgery.