Novel nanofiber-based scaffold for rotator cuff repair and augmentation

Diseased Tendon Models Demonstrate Influence of Extracellular Matrix Alterations on Extracellular Vesicle Profile

Tendons enable movement through their highly aligned extracellular matrix (ECM), predominantly composed of collagen I. Tendinopathies disrupt the structural integrity of tendons by causing fragmentation of collagen fibers, disorganization of fiber bundles, and an increase in glycosaminoglycans and microvasculature, thereby driving the apparent biomechanical and regenerative capacity in patients. Moreover, the complex cellular communication within the tendon microenvironment ultimately dictates the fate between healthy and diseased tendon, wherein extracellular vesicles (EVs) may facilitate the tendon's fate by transporting biomolecules within the tissue. In this study, we aimed to elucidate how the EV functionality is altered in the context of tendon microenvironments by using polycaprolactone (PCL) electrospun scaffolds mimicking healthy and pathological tendon matrices. Scaffolds were characterized for fiber alignment, mechanical properties, and cellular activity. EVs were isolated and analyzed for concentration, heterogeneity, and protein content. Our results show that our mimicked healthy tendon led to an increase in EV secretion and baseline metabolic activity over the mimicked diseased tendon, where reduced EV secretion and a significant increase in metabolic activity over 5 days were observed. These findings suggest that scaffold mechanics may influence EV functionality, offering insights into tendon homeostasis. Future research should further investigate how EV cargo affects the tendon's microenvironment.

Mechanobiological Strategies to Enhance Ovine (Ovis aries) Adipose-Derived Stem Cells Tendon Plasticity for Regenerative Medicine and Tissue Engineering Applications

Adipose-derived stem cells (ADSCs) hold promise for tendon repair, even if their tenogenic plasticity and underlying mechanisms remain only partially understood, particularly in cells derived from the ovine animal model. This study aimed to characterize oADSCs during in vitro expansion to validate their phenotypic properties pre-transplantation. Moreover, their tenogenic potential was assessed using two in vitro-validated approaches: (1) teno-inductive conditioned media (CM) derived from a co-culture between ovine amniotic stem cells and fetal tendon explants, and (2) short- (48 h) and long-term (14 days) seeding on highly aligned PLGA (ha-PLGA) electrospun scaffold. Our findings indicate that oADSCs can be expanded without senescence and can maintain the expression of stemness (Sox2, Oct4, Nanog) and mesenchymal (CD29, CD166, CD44, CD90) markers while remaining negative for hematopoietic (CD31, CD45) and MHC-II antigens. Of note, oADSCs' tendon differentiation potential greatly depended on the in vitro strategy. oADSCs exposed to CM significantly upregulated tendon-related genes (COL1, TNMD, THBS4) but failed to accumulate TNMD protein at 14 days of culture. Conversely, oADSCs seeded on ha-PLGA fleeces quickly upregulated the tendon-related genes (48 h) and in 14 days accumulated high levels of the TNMD protein into the cytoplasm of ADSCs, displaying a tenocyte-like morphology. This mechano-sensing cellular response involved a complete SOX9 downregulation accompanied by YAP activation, highlighting the efficacy of biophysical stimuli in promoting tenogenic differentiation. These findings underscore oADSCs' long-term self-renewal and tendon differentiative potential, thus opening their use in a preclinical setting to develop innovative stem cell-based and tissue engineering protocols for tendon regeneration, applied to the veterinary field.

Biomimetic gradient scaffolds for the tissue engineering and regeneration of rotator cuff enthesis

Rotator cuff tear is one of the most common musculoskeletal disorders, which often results in recurrent shoulder pain and limited movement. Enthesis is a structurally complex and functionally critical interface connecting tendon and bone that plays an essential role in maintaining integrity of the shoulder joint. Despite the availability of advanced surgical procedures for rotator cuff repair, there is a high rate of failure following surgery due to suboptimal enthesis healing and regeneration. Novel strategies based on tissue engineering are gaining popularity in improving tendon-bone interface (TBI) regeneration. Through incorporating physical and biochemical cues into scaffold design which mimics the structure and composition of native enthesis is advantageous to guide specific differentiation of seeding cells and facilitate the formation of functional tissues. In this review, we summarize the current state of research in enthesis tissue engineering highlighting the development and application of biomimetic scaffolds that replicate the gradient TBI. We also discuss the latest techniques for fabricating potential translatable scaffolds such as 3D bioprinting and microfluidic device. While preclinical studies have demonstrated encouraging results of biomimetic gradient scaffolds, the translation of these findings into clinical applications necessitates a comprehensive understanding of their safety and long-term efficacy.

Electroconductivity, a regenerative engineering approach to reverse rotator cuff muscle degeneration

Muscle degeneration is one the main factors that lead to the high rate of retear after a successful repair of rotator cuff (RC) tears. The current surgical practices have failed to treat patients with chronic massive rotator cuff tears (RCTs). Therefore, regenerative engineering approaches are being studied to address the challenges. Recent studies showed the promising outcomes of electroactive materials (EAMs) on the regeneration of electrically excitable tissues such as skeletal muscle. Here, we review the most important biological mechanism of RC muscle degeneration. Further, the review covers the recent studies on EAMs for muscle regeneration including RC muscle. Finally, we will discuss the future direction toward the application of EAMs for the augmentation of RCTs.

Biofabrication of Composite Tendon Constructs with the Fibrous Arrangement, High Cell Density, and Enhanced Cell Alignment

Current tissue-engineered tendons are mostly limited to the replication of fibrous organizations of native tendons, which lack the biomimicry of a densely packed cell arrangement. In this study, composite tendon constructs (CTCs) with fibrous arrangement, high cell density, and enhanced cell alignment were developed by integrating the electrohydrodynamic jet 3D printing (e-jetting) technique and the fabrication of tissue strands (TSs). A tubular polycaprolactone (PCL) scaffold was created using e-jetting, followed by coating a thin layer of alginate. Human mesenchymal stem cells were then microinjected into the PCL scaffolds, aggregated into TSs, and formed CTCs with a core-shell structure. Owing to the presence of TSs, CTCs demonstrated the anatomically relevant cell density and morphology, and cells migrated from the TSs onto e-jetted scaffolds. Also, the mechanical strength of CTCs approached that of native tendons due to the existence of e-jetted scaffolds (Young's modulus: ∼21 MPa, ultimate strength: ∼5 MPa). During the entire culture period, CTCs maintained high survival rates and good structural integrity without the observation of necrotic cores and disintegration of two portions. In addition, CTCs that were cultured with uniaxial cyclic stretching revealed not only the increased expression of tendon-related proteins but also the enhanced cellular orientation. The promising results demonstrated the potential of this novel biofabrication strategy for building tissue-engineered tendon constructs with the proper biological, mechanical, and histological relevance..

Efficacy of a Novel Electroconductive Matrix To Treat Muscle Atrophy and Fat Accumulation in Chronic Massive Rotator Cuff Tears of the Shoulder

The high retear rate after a successful repair of the rotator cuff (RC) is a major clinical challenge. Muscle atrophy and fat accumulation of RC muscles over time adversely affect the rate of retear. Since current surgical techniques do not improve muscle degenerative conditions, new treatments are being developed to reduce muscle atrophy and fat accumulation. In the previous study, we have shown the efficacy of aligned electroconductive nanofibrous fabricated by coating poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) nanoparticles onto aligned poly(ε-caprolactone) (PCL) electrospun nanofibers (PEDOT:PSS matrix) to reduce muscle atrophy in acute and subacute models of RC tears (RCTs). In this study, we further evaluated the efficacy of the PEDOT:PSS matrix to reduce muscle atrophy and fat accumulation in a rat model of chronic massive full-thickness RCTs (MRCTs). The matrices were transplanted on the myotendinous junction to the belly of the supraspinatus and infraspinatus muscles at 16 weeks after MRCTs. The biomechanics and histological assessments showed the potential of the PEDOT:PSS matrix to suppress the progression of muscle atrophy, fat accumulation, and fibrosis in both supraspinatus and infraspinatus muscles at 24 and 32 weeks after MRCTs. We also demonstrated that the PEDOT:PSS matrix implantation significantly improved the tendon morphology and tensile properties compared with current surgical techniques.

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.

Translation of nanotechnology-based implants for orthopedic applications: current barriers and future perspective

The objective of bioimplant engineering is to develop biologically compatible materials for restoring, preserving, or altering damaged tissues and/or organ functions. The variety of substances used for orthopedic implant applications has been substantially influenced by modern material technology. Therefore, nanomaterials can mimic the surface properties of normal tissues, including surface chemistry, topography, energy, and wettability. Moreover, the new characteristics of nanomaterials promote their application in sustaining the progression of many tissues. The current review establishes a basis for nanotechnology-driven biomaterials by demonstrating the fundamental design problems that influence the success or failure of an orthopedic graft, cell adhesion, proliferation, antimicrobial/antibacterial activity, and differentiation. In this context, extensive research has been conducted on the nano-functionalization of biomaterial surfaces to enhance cell adhesion, differentiation, propagation, and implant population with potent antimicrobial activity. The possible nanomaterials applications (in terms of a functional nanocoating or a nanostructured surface) may resolve a variety of issues (such as bacterial adhesion and corrosion) associated with conventional metallic or non-metallic grafts, primarily for optimizing implant procedures. Future developments in orthopedic biomaterials, such as smart biomaterials, porous structures, and 3D implants, show promise for achieving the necessary characteristics and shape of a stimuli-responsive implant. Ultimately, the major barriers to the commercialization of nanotechnology-derived biomaterials are addressed to help overcome the limitations of current orthopedic biomaterials in terms of critical fundamental factors including cost of therapy, quality, pain relief, and implant life. Despite the recent success of nanotechnology, there are significant hurdles that must be overcome before nanomedicine may be applied to orthopedics. The objective of this review was to provide a thorough examination of recent advancements, their commercialization prospects, as well as the challenges and potential perspectives associated with them. This review aims to assist healthcare providers and researchers in extracting relevant data to develop translational research within the field. In addition, it will assist the readers in comprehending the scope and gaps of nanomedicine's applicability in the orthopedics field.

Textile Design of an Intervertebral Disc Replacement Device from Silk Yarn

Low back pain is often due to degeneration of the intervertebral discs (IVD). It is one of the most common age- and work-related problems in today's society. Current treatments are not able to efficiently restore the full function of the IVD. Therefore, the aim of the present work was to reconstruct the two parts of the intervertebral disc-the annulus fibrosus (AF) and the nucleus pulposus (NP)-in such a way that the natural structural features were mimicked by a textile design. Silk was selected as the biomaterial for realization of a textile IVD because of its cytocompatibility, biodegradability, high strength, stiffness, and toughness, both in tension and compression. Therefore, an embroidered structure made of silk yarn was developed that reproduces the alternating fiber structure of +30° and -30° fiber orientation found in the AF and mimics its lamellar structure. The developed embroidered ribbons showed a tensile strength that corresponded to that of the natural AF. Fiber additive manufacturing with 1 mm silk staple fibers was used to replicate the fiber network of the NP and generate an open porous textile 3D structure that may serve as a reinforcement structure for the gel-like NP.

Designing of gradient scaffolds and their applications in tissue regeneration

Gradient scaffolds are isotropic/anisotropic three-dimensional structures with gradual transitions in geometry, density, porosity, stiffness, etc., that mimic the biological extracellular matrix. The gradient structures in biological tissues play a major role in various functional and metabolic activities in the body. The designing of gradients in the scaffold can overcome the current challenges in the clinic compared to conventional scaffolds by exhibiting excellent penetration capacity for nutrients & cells, increased cellular adhesion, cell viability & differentiation, improved mechanical stability, and biocompatibility. In this review, the recent advancements in designing gradient scaffolds with desired biomimetic properties, and their implication in tissue regeneration applications have been briefly explained. Furthermore, the gradients in native tissues such as bone, cartilage, neuron, cardiovascular, skin and their specific utility in tissue regeneration have been discussed in detail. The insights from such advances using gradient-based scaffolds can widen the horizon for using gradient biomaterials in tissue regeneration applications.

Triphasic 3D In Vitro Model of Bone-Tendon-Muscle Interfaces to Study Their Regeneration

The transition areas between different tissues, known as tissue interfaces, have limited ability to regenerate after damage, which can lead to incomplete healing. Previous studies focussed on single interfaces, most commonly bone-tendon and bone-cartilage interfaces. Herein, we develop a 3D in vitro model to study the regeneration of the bone-tendon-muscle interface. The 3D model was prepared from collagen and agarose, with different concentrations of hydroxyapatite to graduate the tissues from bones to muscles, resulting in a stiffness gradient. This graduated structure was fabricated using indirect 3D printing to provide biologically relevant surface topographies. MG-63, human dermal fibroblasts, and Sket.4U cells were found suitable cell models for bones, tendons, and muscles, respectively. The biphasic and triphasic hydrogels composing the 3D model were shown to be suitable for cell growth. Cells were co-cultured on the 3D model for over 21 days before assessing cell proliferation, metabolic activity, viability, cytotoxicity, tissue-specific markers, and matrix deposition to determine interface formations. The studies were conducted in a newly developed growth chamber that allowed cell communication while the cell culture media was compartmentalised. The 3D model promoted cell viability, tissue-specific marker expression, and new matrix deposition over 21 days, thereby showing promise for the development of new interfaces.

3D cell-printing of gradient multi-tissue interfaces for rotator cuff regeneration

Owing to the prevalence of rotator cuff (RC) injuries and suboptimal healing outcome, rapid and functional regeneration of the tendon–bone interface (TBI) after RC repair continues to be a major clinical challenge. Given the essential role of the RC in shoulder movement, the engineering of biomimetic multi-tissue constructs presents an opportunity for complex TBI reconstruction after RC repair. Here, we propose a gradient cell-laden multi-tissue construct combined with compositional gradient TBI-specific bioinks via 3D cell-printing technology. In vitro studies demonstrated the capability of a gradient scaffold system in zone-specific inducibility and multi-tissue formation mimicking TBI. The regenerative performance of the gradient scaffold on RC regeneration was determined using a rat RC repair model. In particular, we adopted nondestructive, consecutive, and tissue-targeted near-infrared fluorescence imaging to visualize the direct anatomical change and the intricate RC regeneration progression in real time in vivo. Furthermore, the 3D cell-printed implant promotes effective restoration of shoulder locomotion function and accelerates TBI healing in vivo. In summary, this study identifies the therapeutic contribution of cell-printed constructs towards functional RC regeneration, demonstrating the translational potential of biomimetic gradient constructs for the clinical repair of multi-tissue interfaces.

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A biomimetic cellular TBI scaffold was 3D bioprinted with dECM bioinks.

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A gradient multi-tissue construct was implanted for RC repair in vivo.

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Targeted NIR fluorescence imaging facilitated real-time monitoring of TBI regeneration.

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The scaffolds had therapeutic contribution on gradient TBI regeneration and functional recovery.

Growth and differentiation factor-7 immobilized, mechanically strong quadrol-hexamethylene diisocyanate-methacrylic anhydride polyurethane polymer for tendon repair and regeneration

Biological and mechanical cues are both vital for biomaterial aided tendon repair and regeneration. Here, we fabricated mechanically tendon-like (0 s UV) QHM polyurethane scaffolds (Q: Quadrol, H: Hexamethylene diisocyanate; M: Methacrylic anhydride) and immobilized them with Growth and differentiation factor-7 (GDF-7) to produce mechanically strong and tenogenic scaffolds. In this study, we assessed QHM polymer cytocompatibility, amenability to fibrin-coating, immobilization and persistence of GDF-7, and capability to support GDF-7-mediated tendon differentiation in vitro as well as in vivo in mouse subcutaneous and acute rat rotator cuff tendon resection models. Cytocompatibility studies showed that QHM facilitated cell attachment, proliferation, and viability. Fibrin-coating and GDF-7 retention studies showed that mechanically tendon-like 0 s UV QHM polymer could be immobilized with GDF-7 and retained the growth factor (GF) for at least 1-week ex vivo. In vitro differentiation studies showed that GDF-7 mediated bone marrow-derived human mesenchymal stem cell (hMSC) tendon-like differentiation on 0 s UV QHM. Subcutaneous implantation of GDF-7-immobilized, fibrin-coated, QHM polymer in mice for 2 weeks demonstrated de novo formation of tendon-like tissue while implantation of GDF-7-immobilized, fibrin-coated, QHM polymer in a rat acute rotator cuff resection injury model indicated tendon-like tissue formation in situ and the absence of heterotopic ossification. Together, our work demonstrates a promising synthetic scaffold with human tendon-like biomechanical attributes as well as immobilized tenogenic GDF-7 for tendon repair and regeneration. STATEMENT OF SIGNIFICANCE: Biological activity and mechanical robustness are key features required for tendon-promoting biomaterials. While synthetic biomaterials can be mechanically robust, they often lack bioactivity. To biologically augment synthetic biomaterials, numerous drug and GF delivery strategies exist but the large tissue space within the shoulder is constantly flushed with saline during arthroscopic surgery, hindering efficacious controlled release of therapeutic molecules. Here, we coated QHM polymer (which exhibits human tendon-to-bone-like biomechanical attributes) with fibrin for GF binding. Unlike conventional drug delivery strategies, our approach utilizes immobilized GFs as opposed to released GFs for sustained, localized tissue regeneration. Our data demonstrated that GF immobilization can be broadly applied to synthetic biomaterials for enhancing bioactivity, and GDF-7-immobilized QHM exhibit high clinical translational potential for tendon repair.

Engineering collagenous analogs of connective tissue extracellular matrix

Connective tissue extracellular matrix (ECM) consists of an interwoven network of contiguous collagen fibers that regulate cell activity, direct biological function, and guide tissue homeostasis throughout life. Recently, ECM analogs have emerged as a unique ex vivo culture platform for studying healthy and diseased tissues and in the latter, enabling the screening for and development of therapeutic regimen. Since these tissue models can mitigate the concern that observations from animal models do not always translate clinically, the design and production of a collagenous ECM analogue with relevant chemistry and nano- to micro-scale architecture remains a frontier challenge in the field. Therefore, the objectives of this study are two-fold— first, to apply green electrospinning approaches to the fabrication of an ECM analog with nanoscale mimicry and second, to systematically optimize collagen crosslinking in order to produce a stable, collagen-like substrate with continuous fibrous architecture that supports human cell culture and phenotypic expression. Specifically, the “green” electrospinning solvent acetic acid was evaluated for biofabrication of gelatin-based meshes, followed by the optimization of glutaraldehyde (GTA) crosslinking under controlled ambient conditions. These efforts led to the production of a collagen-like mesh with nano- and micro-scale cues, fibrous continuity with little batch-to-batch variability, and proven stability in both dry and wet conditions. Moreover, the as-fabricated mesh architecture and native chemistry were preserved with augmented mechanical properties. These meshes supported the in vitro expansion of stem cells and the production of a mineralized matrix by human osteoblast-like cells. Collectively these findings demonstrate the potential of green fabrication in the production of a collagen-like ECM analog with physiological relevance. Future studies will explore the potential of this high-fidelity platform for elucidating cell-matrix interactions and their relevance in connective tissue healing.

Rotator Cuff Repair With Patch Augmentation: What Do We Know?

BACKGROUND

Repair of massive rotator cuff tears remains a challenging process with mixed success. There is a growing interest in the use of patches to augment the repair construct and the potential to enhance the strength, healing, and associated clinical outcomes. Such patches may be synthetic, xenograft, or autograft/allograft, and a variety of techniques have been tried to biologically enhance their integration and performance. The materials used are rapidly advancing, as is our understanding of their effects on rotator cuff tissue. This article aims to evaluate what we currently know about patch augmentation through a comprehensive review of the available literature.

METHODS

We explore the results of existing clinical trials for each graft type, new manufacturing methods, novel techniques for biological enhancement, and the histological and biomechanical impact of patch augmentation.

RESULTS

There are promising results in short-term studies, which suggest that patch augmentation has great potential to improve the success rate. In particular, this appears to be true for human dermal allograft, while porcine dermal grafts and some synthetic grafts have also had promising results.

CONCLUSION

However, there remains a need for high-quality, prospective clinical trials directly comparing each type of graft and the effect that they have on the clinical and radiological outcomes of rotator cuff repair.

Topographic Orientation of Scaffolds for Tissue Regeneration: Recent Advances in Biomaterial Design and Applications

Tissue engineering to develop alternatives for the maintenance, restoration, or enhancement of injured tissues and organs is gaining more and more attention. In tissue engineering, the scaffold used is one of the most critical elements. Its characteristics are expected to mimic the native extracellular matrix and its unique topographical structures. Recently, the topographies of scaffolds have received increasing attention, not least because different topographies, such as aligned and random, have different repair effects on various tissues. In this review, we have focused on various technologies (electrospinning, directional freeze-drying, magnetic freeze-casting, etching, and 3-D printing) to fabricate scaffolds with different topographic orientations, as well as discussed the physicochemical (mechanical properties, porosity, hydrophilicity, and degradation) and biological properties (morphology, distribution, adhesion, proliferation, and migration) of different topographies. Subsequently, we have compiled the effect of scaffold orientation on the regeneration of vessels, skin, neural tissue, bone, articular cartilage, ligaments, tendons, cardiac tissue, corneas, skeletal muscle, and smooth muscle. The compiled information in this review will facilitate the future development of optimal topographical scaffolds for the regeneration of certain tissues. In the majority of tissues, aligned scaffolds are more suitable than random scaffolds for tissue repair and regeneration. The underlying mechanism explaining the various effects of aligned and random orientation might be the differences in “contact guidance”, which stimulate certain biological responses in cells.

Solid fraction determines stiffness and viscosity in decellularized pancreatic tissues

The role of extracellular matrix (ECM) composition and turnover in mechano-signaling and the metamorphic fate of cells seeded into decellularized tissue can be elucidated by recent developments in non-invasive imaging and biotechnological analysis methods. Because these methods allow accurate quantification of the composition and structural integrity of the ECM, they can be critical in establishing standardized decellularization protocols. This study proposes quantification of the solid fraction, the single-component fraction and the viscoelasticity of decellularized pancreatic tissues using compact multifrequency magnetic resonance elastography (MRE) to assess the efficiency and quality of decellularization protocols. MRE of native and decellularized pancreatic tissues showed that viscoelasticity parameters depend according to a power law on the solid fraction of the decellularized matrix. The parameters can thus be used as highly sensitive markers of the mechanical integrity of soft tissues. Compact MRE allows consistent and noninvasive quantification of the viscoelastic properties of decellularized tissue. Such a method is urgently needed for the standardized monitoring of decellularization processes, evaluation of mechanical ECM properties, and quantification of the integrity of solid structural elements remaining in the decellularized tissue matrix.

Three-dimensionally printed recombinant human parathyroid hormone-soaked nanofiber sheet accelerates tendon-to-bone healing in a rabbit model of chronic rotator cuff tear

BACKGROUND

Recombinant human parathyroid hormone (rhPTH) promotes tendon-to-bone healing in humans and animals with rotator cuff tear (RCT). However, problems regarding repeated systemic rhPTH injections in humans exist. This study was conducted to evaluate the effect of topical rhPTH administration using 3-dimensionally (3D) printed nanofiber sheets on tendon-to-bone healing in a rabbit RCT model compared to that of direct topical rhPTH administration.

METHODS

Eighty rabbits were randomly assigned to 5 groups (n = 16 each). To create the chronic RCT model, we induced complete supraspinatus tendon tears in both shoulders and left them untreated for 6 weeks. All transected tendons were repaired in a transosseous manner with saline injection in group A, hyaluronic acid (HA) injection in group B, 3D-printed nanofiber sheet fixation in group C, rhPTH and HA injection in group D, and 3D-printed rhPTH- and HA-soaked nanofiber sheet fixation in group E. Genetic (messenger RNA expression evaluation) and histologic evaluations (hematoxylin and eosin and Masson trichrome staining) were performed in half of the rabbits at 4 weeks postrepair. Genetic, histologic, and biomechanical evaluations (mode of tear and load to failure) were performed in the remaining rabbits at 12 weeks.

RESULTS

For genetic evaluation, group E showed a higher collagen type I alpha 1 expression level than did the other groups (P = .008) at 4 weeks. However, its expression level was downregulated, and there was no difference at 12 weeks. For histologic evaluation, group E showed greater collagen fiber continuity, denser collagen fibers, and more mature tendon-to-bone junction than did the other groups (P = .001, P = .001, and P = .003, respectively) at 12 weeks. For biomechanical evaluation, group E showed a higher load-to-failure rate than did the other groups (P < .001) at 12 weeks.

CONCLUSION

Three-dimensionally printed rhPTH-soaked nanofiber sheet fixation can promote tendon-to-bone healing of chronic RCT.

New frontiers of tendon augmentation technology in tissue engineering and regenerative medicine: a concise literature review

Tissue banking programs fail to meet the demand for human organs and tissues for transplantation into patients with congenital defects, injuries, chronic diseases, and end-stage organ failure. Tendons and ligaments are among the most frequently ruptured and/or worn-out body tissues owing to their frequent use, especially in athletes and the elderly population. Surgical repair has remained the mainstay management approach, regardless of scarring and adhesion formation during healing, which then compromises the gliding motion of the joint and reduces the quality of life for patients. Tissue engineering and regenerative medicine approaches, such as tendon augmentation, are promising as they may provide superior outcomes by inducing host-tissue ingrowth and tendon regeneration during degradation, thereby decreasing failure rates and morbidity. However, to date, tendon tissue engineering and regeneration research has been limited and lacks the much-needed human clinical evidence to translate most laboratory augmentation approaches to therapeutics. This narrative review summarizes the current treatment options for various tendon pathologies, future of tendon augmentation, cell therapy, gene therapy, 3D/4D bioprinting, scaffolding, and cell signals.

Advanced Nanofiber-Based Scaffolds for Achilles Tendon Regenerative Engineering

The Achilles tendon (AT) is responsible for running, jumping, and standing. The AT injuries are very common in the population. In the adult population (21–60 years), the incidence of AT injuries is approximately 2.35 per 1,000 people. It negatively impacts people’s quality of life and increases the medical burden. Due to its low cellularity and vascular deficiency, AT has a poor healing ability. Therefore, AT injury healing has attracted a lot of attention from researchers. Current AT injury treatment options cannot effectively restore the mechanical structure and function of AT, which promotes the development of AT regenerative tissue engineering. Various nanofiber-based scaffolds are currently being explored due to their structural similarity to natural tendon and their ability to promote tissue regeneration. This review discusses current methods of AT regeneration, recent advances in the fabrication and enhancement of nanofiber-based scaffolds, and the development and use of multiscale nanofiber-based scaffolds for AT regeneration.

Hydrogel Development for Rotator Cuff Repair

Rotator cuff tears (RCTs) are common in shoulder disease and disability. Despite significant advances in surgical repair techniques, 20–70% of patients still have postoperative rotator cuff dysfunction. These functional defects may be related to retear or rotator cuff quality deterioration due to tendon retraction and scar tissue at the repair site. As an effective delivery system, hydrogel scaffolds may improve the healing of RCTs and be a useful treatment for irreparable rotator cuff injuries. Although many studies have tested this hypothesis, most are limited to laboratory animal experiments. This review summarizes differences in hydrogel scaffold construction, active ingredients, and application methods in recent research. Efforts to determine the indications of hydrogel scaffolds (with different constructions and cargos) for various types of RCTs, as well as the effectiveness and reliability of application methods and devices, are also discussed.

The tendon microenvironment: Engineered in vitro models to study cellular crosstalk

Tendinopathy is a multi-faceted pathology characterized by alterations in tendon microstructure, cellularity and collagen composition. Challenged by the possibility of regenerating pathological or ruptured tendons, the healing mechanisms of this tissue have been widely researched over the past decades. However, so far, most of the cellular players and processes influencing tendon repair remain unknown, which emphasizes the need for developing relevant in vitro models enabling to study the complex multicellular crosstalk occurring in tendon microenvironments. In this review, we critically discuss the insights on the interaction between tenocytes and the other tendon resident cells that have been devised through different types of existing in vitro models. Building on the generated knowledge, we stress the need for advanced models able to mimic the hierarchical architecture, cellularity and physiological signaling of tendon niche under dynamic culture conditions, along with the recreation of the integrated gradients of its tissue interfaces. In a forward-looking vision of the field, we discuss how the convergence of multiple bioengineering technologies can be leveraged as potential platforms to develop the next generation of relevant in vitro models that can contribute for a deeper fundamental knowledge to develop more effective treatments.

Texturing Hierarchical Tissues by Gradient Assembling of Microengineered Platelet-Lysates Activated Fibers

The heterogeneity of hierarchical tissues requires designing multipart engineered constructs as suitable tissue replacements. Herein, the incorporation of platelet lysate (PL) within an electrospun fiber core is proposed aiming for the fabrication of functionally graded 3D scaffolds for heterotypic tissues regeneration, such as tendon-to-bone interfaces. First, anisotropic yarns (A-Yarns) and isotropic threads with nanohydroxyapatite (I-Threads/PL@nHAp) are fabricated to recreate the tendon- and bone-microstructures and both incorporated with PL using emulsion electrospinning for a sustained and local delivery of growth factors, cytokines, and chemokines. Biological performance using human adipose-derived stem cells demonstrates that A-Yarns/PL induce a higher expression of scleraxis, a tenogenic-marker, while in I-Threads/PL@nHAp, higher alkaline phosphatase activity and matrix mineralization suggest an osteogenic commitment without the need for biochemical supplementation compared to controls. As a proof-of-concept, functional 3D gradient scaffolds are fabricated using a weaving technique, resulting in 3D textured hierarchical constructs with gradients in composition and topography. Additionally, the precise delivery of bioactive cues together with in situ biophysical features guide the commitment into a phenotypic gradient exhibiting chondrogenic and osteochondrogenic profiles in the interface of scaffolds. Overall, a promising patch solution for the regeneration of tendon-to-bone tissue interface through the fabrication of PL-functional 3D gradient constructs is demonstrated.

Scaffold-Mediated Immunoengineering as Innovative Strategy for Tendon Regeneration

Tendon injuries are at the frontier of innovative approaches to public health concerns and sectoral policy objectives. Indeed, these injuries remain difficult to manage due to tendon’s poor healing ability ascribable to a hypo-cellularity and low vascularity, leading to the formation of a fibrotic tissue affecting its functionality. Tissue engineering represents a promising solution for the regeneration of damaged tendons with the aim to stimulate tissue regeneration or to produce functional implantable biomaterials. However, any technological advancement must take into consideration the role of the immune system in tissue regeneration and the potential of biomaterial scaffolds to control the immune signaling, creating a pro-regenerative environment. In this context, immunoengineering has emerged as a new discipline, developing innovative strategies for tendon injuries. It aims at designing scaffolds, in combination with engineered bioactive molecules and/or stem cells, able to modulate the interaction between the transplanted biomaterial-scaffold and the host tissue allowing a pro-regenerative immune response, therefore hindering fibrosis occurrence at the injury site and guiding tendon regeneration. Thus, this review is aimed at giving an overview on the role exerted from different tissue engineering actors in leading immunoregeneration by crosstalking with stem and immune cells to generate new paradigms in designing regenerative medicine approaches for tendon injuries.

The Role of Nanomaterials and Biological Agents on Rotator Cuff Regeneration

The rotator cuff is a musculotendon unit responsible for movement in the shoulder. Rotator cuff tears represent a significant number of musculoskeletal injuries in the adult population. In addition, there is a high incidence of retear rates due to various complications within the complex anatomical structure and the lack of proper healing. Current clinical strategies for rotator cuff augmentation include surgical intervention with autograft tissue grafts and beneficial impacts have been shown, but challenges still exist because of limited supply. For decades, nanomaterials have been engineered for the repair of various tissue and organ systems. This review article provides a thorough summary of the role nanomaterials, stem cells and biological agents have played in rotator cuff repair to date and offers input on next generation approaches for regenerating this tissue.

Nonwoven-based gelatin/polycaprolactone membrane loaded with ERK inhibitor U0126 for treatment of tendon defects

BACKGROUND

Tendon is a major component of musculoskeletal system connecting the muscles to the bone. Tendon injuries are very common orthopedics problems leading to impeded motion. Up to now, there still lacks effective treatments for tendon diseases.

METHODS

Tendon stem/progenitor cells (TSPCs) were isolated from the patellar tendons of SD rats. The expression levels of genes were evaluated by quantitative RT-PCR. Immunohistochemistry staining was performed to confirm the presence of tendon markers in tendon tissues. Bioinformatics analysis of data acquired by RNA-seq was used to find out the differentially expressed genes. Rat patellar tendon injury model was used to evaluate the effect of U0126 on tendon injury healing. Biomechanical testing was applied to evaluate the mechanical properties of newly formed tendon tissues.

RESULTS

In this study, we have shown that ERK inhibitor U0126 rather PD98059 could effectively increase the expression of tendon-related genes and promote the tenogenesis of TSPCs in vitro. To explore the underlying mechanisms, RNA sequencing was performed to identify the molecular difference between U0126-treated and control TSPCs. The result showed that GDF6 was significantly increased by U0126, which is an important factor of the TGFβ superfamily regulating tendon development and tenogenesis. In addition, NBM (nonwoven-based gelatin/polycaprolactone membrane) which mimics the native microenvironment of the tendon tissue was used as an acellular scaffold to carry U0126. The results demonstrated that when NBM was used in combination with U0126, tendon healing was significantly promoted with better histological staining outcomes and mechanical properties.

CONCLUSION

Taken together, we have found U0126 promoted tenogenesis in TSPCs through activating GDF6, and NBM loaded with U0126 significantly promoted tendon defect healing, which provides a new treatment for tendon injury.

Biomimetic Approaches for the Design and Fabrication of Bone-to-Soft Tissue Interfaces

Bone-to-soft tissue interfaces are responsible for transferring loads between tissues with significantly dissimilar material properties. The examples of connective soft tissues are ligaments, tendons, and cartilages. Such natural tissue interfaces have unique microstructural properties and characteristics which avoid the abrupt transitions between two tissues and prevent formation of stress concentration at their connections. Here, we review some of the important characteristics of these natural interfaces. The native bone-to-soft tissue interfaces consist of several hierarchical levels which are formed in a highly specialized anisotropic fashion and are composed of different types of heterogeneously distributed cells. The characteristics of a natural interface can rely on two main design principles, namely by changing the local microarchitectural features (e.g., complex cell arrangements, and introducing interlocking mechanisms at the interfaces through various geometrical designs) and changing the local chemical compositions (e.g., a smooth and gradual transition in the level of mineralization). Implementing such design principles appears to be a promising approach that can be used in the design, reconstruction, and regeneration of engineered biomimetic tissue interfaces. Furthermore, prominent fabrication techniques such as additive manufacturing (AM) including 3D printing and electrospinning can be used to ease these implementation processes. Biomimetic interfaces have several biological applications, for example, to create synthetic scaffolds for osteochondral tissue repair.

Development of a conduit of PLGA-gelatin aligned nanofibers produced by electrospinning for peripheral nerve regeneration

A promising alternative to conventional nerve grafting is the use of artificial grafts made from biodegradable and biocompatible materials and support cells. The aim of this study has been to produce a biodegradable nerve conduit and investigate the cytocompatibility with stem cells and its regeneration promoting properties in a rat animal model. A poly (lactic-co-glycolic acid) (PLGA) conduit of aligned nanofibers was produced by the electrospinning method, functionalized with gelatin and seeded either with mouse embryonic stem cells (mESCs) or with human mesenchymal stem cells (SHED). The cell proliferation and viability were analyzed in vitro. The conduits were implanted in a rat model of sciatic nerve lesion by transection. The functional recovery was monitored for 8 weeks using the Sciatic Functional Index (SFI) and histological analyses were used to assess the nerve regeneration. Scaffolds of aligned PLGA fibers with an average diameter of 0.90 ± 0.36 μm and an alignment coefficient of 0.817 ± 0.07 were produced. The treatment with gelatin increased the fiber diameter to 1.05 ± 0.32 μm, reduced the alignment coefficient to 0.655 ± 0.045 and made the scaffold very hydrophilic. The cell viability and Live/dead assay showed that the stem cells remained viable and proliferated after 7 days in culture. Confocal images of phalloidin/DAPI staining showed that the cells adhered and proliferated widely, in fully adaptation with the biomaterial. The SFI values of the group that received the conduit were similar to the values of the control lesioned group. In conclusion, conduits composed of PLGA-gelatin nanofibers were produced and promoted a very good interaction with the stem cells. Although in vitro studies have shown this biomaterial to be a promising biomaterial for the regeneration of nerve tissue, in vivo studies of this graft have not shown significant improvements in nerve regeneration.

Electrohydrodynamic jet 3D printing in biomedical applications

Electrohydrodynamic Jet 3D Printing (e-jetting) is a promising technique developed from electrospinning, which enables precise fiber deposition in a layer-by-layer fashion with customized designs. Several studies have verified that e-jetted scaffolds were able to support cell attachment, proliferation, and extracellular matrix formation, as well as cell infiltration into the scaffold due to the well-defined pores. Besides, e-jetting has also been combined with other techniques to incorporate biomaterials (e.g., hydrogels and cell spheroids) that could not be e-jetted, to promote the biological performance of the scaffold. In the recent decade, applying e-jetting in the fabrication of tissue-engineered scaffolds has drawn a lot of interest. Moreover, efforts have been put to develop varied scaffolds for some specific biomedical applications such as cartilage, tendon, and blood vessel, which exhibited superior mechanical properties and promoted cell behaviors including cellular alignment and differentiation. This review article also provides the reader with some crucial considerations and major limitations of e-jetting, such as scaffold design, printability of large-scale constructs, applicable biomaterials, and cell behaviors. Overall, this review article expounds on perspectives in the context of development and biomedical applications of this technique. STATEMENT OF SIGNIFICANCE: E-jetting technique is able to produce fibers with diameter in micrometer scale, which has been considered as a promising 3D printing technique. This technique has shown promise for regeneration of tissue engineered scaffolds with well-defined structures, which has been reported to apply in regeneration of different tissue types. The superior controllability of the process endows the feasibility of constructing multi-scale scaffolds with great biological mimicry and cellular infiltration. The incorporation of other biomaterials into the e-jetted networks further reinforces the scope of applications as compared to e-jetted scaffolds only. There is no doubt that e-jetting will be a great tool for tissue engineered scaffolding, and this review article will give overall perspectives in this topic.

Green electrospinning for biomaterials and biofabrication

Green manufacturing has emerged across industries, propelled by a growing awareness of the negative environmental and health impacts associated with traditional practices. In the biomaterials industry, electrospinning is a ubiquitous fabrication method for producing nano- to micro-scale fibrous meshes that resemble native tissues, but this process traditionally utilizes solvents that are environmentally hazardous and pose a significant barrier to industrial scale-up and clinical translation. Applying sustainability principles to biomaterial production, we have developed a 'green electrospinning' process by systematically testing biologically benign solvents (U.S. Food and Drug Administration Q3C Class 3), and have identified acetic acid as a green solvent that exhibits low ecological impact (global warming potential (GWP) = 1.40 CO₂eq. kg/L) and supports a stable electrospinning jet under routine fabrication conditions. By tuning electrospinning parameters, such as needle-plate distance and flow rate, we updated the fabrication of widely utilized biomedical polymers (e.g. poly-α-hydroxyesters, collagen), polymer blends, polymer-ceramic composites, and growth factor delivery systems. Resulting 'green' fibers and composites are comparable to traditional meshes in terms of composition, chemistry, architecture, mechanical properties, and biocompatibility. Interestingly, material properties of green synthetic fibers are more biomimetic than those of traditionally electrospun fibers, doubling in ductility (91.86 ± 35.65 vs. 45 ± 15.07%,n= 10,p< 0.05) without compromising yield strength (1.32 ± 0.26 vs. 1.38 ± 0.32 MPa) or ultimate tensile strength (2.49 ± 0.55 vs. 2.36 ± 0.45 MPa). Most importantly, green electrospinning proves advantageous for biofabrication, rendering a greater protection of growth factors during fiber formation (72.30 ± 1.94 vs. 62.87 ± 2.49% alpha helical content,n= 3,p< 0.05) and recapitulating native ECM mechanics in the fabrication of biopolymer-based meshes (16.57 ± 3.92% ductility, 33.38 ± 30.26 MPa elastic modulus, 1.30 ± 0.19 MPa yield strength, and 2.13 ± 0.36 MPa ultimate tensile strength,n= 10). The eco-conscious approach demonstrated here represents a paradigm shift in biofabrication, and will accelerate the translation of scalable biomaterials and biomimetic scaffolds for tissue engineering and regenerative medicine.

Hard Dental Tissues Regeneration-Approaches and Challenges

With the development of the modern concept of tissue engineering approach and the discovery of the potential of stem cells in dentistry, the regeneration of hard dental tissues has become a reality and a priority of modern dentistry. The present review reports the recent advances on stem-cell based regeneration strategies for hard dental tissues and analyze the feasibility of stem cells and of growth factors in scaffolds-based or scaffold-free approaches in inducing the regeneration of either the whole tooth or only of its component structures.

Micropatterned growth surface topography affects extracellular vesicle production

Extracellular vesicles (EVs) are micro and nanoscale packages that circulate in all bodily fluids and play an important role in intercellular communication by shuttling biomolecules to nearby and distant cells. However, producing sufficient amounts of EVs for many types of in vitro studies using standard culture methods can be challenging, and despite the success of some bioreactors in increasing EV-production, it is still largely unknown how individual culture conditions can alter the production and content of EVs. In this study, we demonstrate a simple and inexpensive micropatterning technique that can be used to produce polystyrene microtracks over a 100 mm diameter growth surface area. We then demonstrate that these microtracks can play a significant role in increasing EV production using a triple-negative breast cancer cell line (MDA-MB-231) and that these changes in EV production correlate with increases in cellular aspect ratio, alignment of the cells' long axes to the microtracks, and single-cell migration rates. These findings have implications in both biomanufacturing of EVs and potentially in enhancing the biomimicry of EVs produced in vitro.

Fibrous Systems as Potential Solutions for Tendon and Ligament Repair, Healing, and Regeneration

Tendon and ligament injuries caused by trauma and degenerative diseases are frequent and affect diverse groups of the population. Such injuries reduce musculoskeletal performance, limit joint mobility, and lower people's comfort. Currently, various treatment strategies and surgical procedures are used to heal, repair, and restore the native tissue function. However, these strategies are inadequate and, in some cases, fail to re-establish the lost functionality. Tissue engineering and regenerative medicine approaches aim to overcome these disadvantages by stimulating the regeneration and formation of neotissues. Design and fabrication of artificial scaffolds with tailored mechanical properties are crucial for restoring the mechanical function of tendons. In this review, the tendon and ligament structure, their physiology, and performance are presented. On the other hand, the requirements are focused for the development of an effective reconstruction device. The most common fiber-based scaffolding systems are also described for tendon and ligament tissue regeneration like strand fibers, woven, knitted, braided, and braid-twisted fibrous structures, as well as electrospun and wet-spun constructs, discussing critically the advantages and limitations of their utilization. Finally, the potential of multilayered systems as the most effective candidates for tendon and ligaments tissue engineering is pointed out.

Engineering multi-tissue units for regenerative Medicine: Bone-tendon-muscle units of the rotator cuff

Our body systems are comprised of numerous multi-tissue units. For the musculoskeletal system, one of the predominant functional units is comprised of bone, tendon/ligament, and muscle tissues working in tandem to facilitate locomotion. To successfully treat musculoskeletal injuries and diseases, critical consideration and thoughtful integration of clinical, biological, and engineering aspects are necessary to achieve translational bench-to-bedside research. In particular, identifying ideal biomaterial design specifications, understanding prior and recent tissue engineering advances, and judicious application of biomaterial and fabrication technologies will be crucial for addressing current clinical challenges in engineering multi-tissue units. Using rotator cuff tears as an example, insights relevant for engineering a bone-tendon-muscle multi-tissue unit are presented. This review highlights the tissue engineering strategies for musculoskeletal repair and regeneration with implications for other bone-tendon-muscle units, their derivatives, and analogous non-musculoskeletal tissue structures.

Change in Collagen Fibril Diameter Distribution of Bovine Anterior Cruciate Ligament upon Injury Can Be Mimicked in a Nanostructured Scaffold

More than 200,000 people are suffering from Anterior Cruciate Ligament (ACL) related injuries each year in the US. There is an unmet clinical demand for improving biological attachment between grafts and the host tissue in addition to providing mechanical support. For biological graft integration, it is important to provide a physiologically feasible environment for the host cells to enable them to perform their duties. However, behavior of cells during ACL healing and the mechanism of ACL healing is not fully understood partly due to the absence of appropriate environment to test cell behavior both in vitro and in vivo. This study aims at (i) investigating the change in fibril diameter of bovine ACL tissue upon injury and (ii) fabricating nanofiber-based scaffolds to represent the morphology and structure of healthy and injured ACL tissues. We hypothesized that distribution and mean diameter of ACL fibrils will be altered upon injury. Findings revealed that the collagen fibril diameter distribution of bovine ACL changed from bimodal to unimodal upon injury with subsequent decrease in mean diameter. Polycaprolactone (PCL) scaffold fiber diameter distribution exhibited similar bimodal and unimodal distribution behavior to qualitatively represent the cases of healthy and injured ACL, respectively. The native ACL tissue demonstrated comparable modulus values only with the aligned bimodal PCL scaffolds. There was significant difference between mechanical properties of aligned bimodal and unaligned unimodal PCL scaffolds. We believe that the results obtained from measurements of diameter of collagen fibrils of native bovine ACL tissue can serve as a benchmark for scaffold design.

Surfactant location and internal phase volume fraction dictate emulsion electrospun fiber morphology and modulate drug release and cell response

Emulsion electrospinning is a versatile technique used to create fibrous meshes for applications in drug delivery and tissue engineering. In this study, the effects of surfactant and increasing internal phase volume fraction on emulsion electrospun fiber morphology were investigated. The fiber diameter, surface topography, internal architecture, mesh hydrophobicity, and fiber volume fraction were all characterized and the resulting effects on model drug release and cell response were determined. Surfactant relocation to the fiber surface resulted in alterations to fiber surface topography and internal morphology, increased rate of water adsorption into the mesh, and reduced burst effects of drug release. Increasing the internal phase volume fraction within the emulsion resulted in minimal change to fiber diameter, surface morphology, fiber volume fraction, and rate of water adsorption illustrating the ability to increase drug loading without affecting fiber properties. Lastly, all meshes promoted cell adhesion and good viability with a trend of increased MTT absorbance from cells on the surfactant and emulsion fibers possibly suggesting that an increase in surface area via smaller fiber diameter and fiber volume fraction increases metabolic activity. Overall, these studies indicate that fiber morphology and mesh hydrophobicity can be tuned by controlling surfactant location within fibers and internal phase volume fraction. Modulating fiber properties within the emulsion electrospun mesh is important to achieve controlled drug release and cell response for tissue engineering applications.

Current Advances on the Regeneration of Musculoskeletal Interfaces

The regeneration of the musculoskeletal system has been widely investigated. There is now detailed knowledge about the organs composing this system. Research has also investigated the zones between individual tissues where physical, mechanical, and biochemical properties transition. However, the understanding of the regeneration of musculoskeletal interfaces is still lacking behind. Numerous disorders and injuries can degrade or damage tissue interfaces. Their inability to regenerate can delay the tissue repair and regeneration process, leading to graft instability, high morbidity, and pain. Moreover, the knowledge of the mechanism of tissue interface development is not complete. This review presents an overview of the most recent approaches of the regeneration of musculoskeletal interfaces, including the latest in vitro, preclinical, and clinical studies. Impact statement Interfaces between soft and hard tissues are ubiquitous within the body. These transition zones are crucial for joint motion, stabilisation and load transfer between tissues, but do not seem to regenerate well after injury or deterioration. The knowledge about their biology is vast, but little is known about their development. Various musculoskeletal disorders in combination with risk factors including aging and unhealthy lifestyle, can lead to local imbalances, misalignments, inflammation, pain and restricted mobility. Our manuscript reviews the current approaches taken to promote the regeneration of musculoskeletal interfaces through in vitro, pre-clinical and clinical studies.

Nanofiber Scaffolds by Electrospinning for Rotator Cuff Tissue Engineering

Rotator cuff tears continue to be at risk of retear or failure to heal after surgical repair, despite the use of various surgical techniques, which stimulate development of novel scaffolding strategies. They should be able to address the known causes of failure after the conventional rotator cuff repair: (1) failure to reproduce the normal tendon healing process, (2) resultant failure to reproduce four zones of the enthesis, and (3) failure to attain sufficient mechanical strength after repair. Nanofiber scaffolds are suited for this application because they can be engineered to mimic the ultrastructure and properties of the native rotator cuff tendon. Among various methods for tissue-engineered nanofibers, electrospinning has recently been highlighted in the rotator cuff field. Electrospinning can create fibrous and porous structures that resemble natural tendon's extracellular matrix. Other advantages include the ability to create relatively large surface-to-volume ratios, the ability to control fiber size from the micro to the nano scale, and the flexibility of material choices. In this review, we will discuss the anatomical and mechanical features of the rotator cuff tendon, their potential impacts on improper healing after repair, and the current knowledge of the use of electrospinning for rotator cuff tissue engineering.

Bioinspired Scaffold Designs for Regenerating Musculoskeletal Tissue Interfaces

The musculoskeletal system works at a very advanced level of synchrony, where all the physiological movements of the body are systematically performed through well-organized actions of bone in conjunction with all the other musculoskeletal soft tissues, such as ligaments, tendons, muscles, and cartilage through tissue-tissue interfaces. Interfaces are structurally and compositionally complex, consisting of gradients of extracellular matrix components, cell phenotypes as well as biochemical compositions and are important in mediating load transfer between the distinct orthopedic tissues during body movement. When an injury occurs at interface, it must be re-established to restore its function and stability. Due to the structural and compositional complexity found in interfaces, it is anticipated that they presuppose a concomitant increase in the complexity of the associated regenerative engineering approaches and scaffold designs to achieve successful interface regeneration and seamless integration of the engineered orthopedic tissues. Herein, we discuss the various bioinspired scaffold designs utilized to regenerate orthopedic tissue interfaces. First, we start with discussing the structure-function relationship at the interface. We then discuss the current understanding of the mechanism underlying interface regeneration, followed by discussing the current treatment available in the clinic to treat interface injuries. Lastly, we comprehensively discuss the state-of-the-art scaffold designs utilized to regenerate orthopedic tissue interfaces.

In Vitro Innovation of Tendon Tissue Engineering Strategies

Tendinopathy is the term used to refer to tendon disorders. Spontaneous adult tendon healing results in scar tissue formation and fibrosis with suboptimal biomechanical properties, often resulting in poor and painful mobility. The biomechanical properties of the tissue are negatively affected. Adult tendons have a limited natural healing capacity, and often respond poorly to current treatments that frequently are focused on exercise, drug delivery, and surgical procedures. Therefore, it is of great importance to identify key molecular and cellular processes involved in the progression of tendinopathies to develop effective therapeutic strategies and drive the tissue toward regeneration. To treat tendon diseases and support tendon regeneration, cell-based therapy as well as tissue engineering approaches are considered options, though none can yet be considered conclusive in their reproduction of a safe and successful long-term solution for full microarchitecture and biomechanical tissue recovery. In vitro differentiation techniques are not yet fully validated. This review aims to compare different available tendon in vitro differentiation strategies to clarify the state of art regarding the differentiation process.

Tenogenic adipose-derived stem cell sheets with nanoyarn scaffolds for tendon regeneration

Tissue engineering, especially cell sheets-based engineering, offers a promising approach to tendon regeneration; however, obtaining a sufficient source of cells for tissue engineering applications is challenging. Adipose-derived stem cells (ASCs) are essential sources for tissue regeneration and have been shown to have the potential for tenogenic differentiation in vitro via induction by growth differentiation factor 5 (GDF-5). In this study, we explored the feasibility of ASCs cell sheets stimulated by GDF-5 for engineered tendon repair. As shown by quantitative polymerase chain reaction and western blotting, tenogenesis-related markers (Col I&III, TNMD, biglycan, and tenascin C) were significantly increased in GDF-5-induced ASCs cell sheets compared with the uninduced. Moreover, the levels of SMAD2/3 proteins and phospho-SMAD1/5/9 were significantly enhanced, demonstrating that GDF-5 may exert its functions through phosphorylation of SMAD1/5/9. Furthermore, the cell sheets were combined with P(LLA-CL)/Silk fibroin nanoyarn scaffolds to form constructs for tendon tissue engineering. Terminal deoxynucleotidyl transferase dUTP nick end labeling and immunofluorescence assays demonstrated favorable cell viability and tenogenesis-related marker expression in GDF-5-induced constructs. In addition, the constructs showed the potential for tendon repair in rabbit models, as demonstrated by histological, immunohistochemical, and biomechanical analyses. In our study, we successfully produced a new tissue-engineered tendon by the combination of GDF-5-induced ASCs cell sheets and nanoyarn scaffold which is valuable for tendon regeneration.

3D printing of multilayered scaffolds for rotator cuff tendon regeneration

Repairing massive rotator cuff tendon defects remains a challenge due to the high retear rate after surgical intervention. 3D printing has emerged as a promising technique that enables the fabrication of engineered tissues with heterogeneous structures and mechanical properties, as well as controllable microenvironments for tendon regeneration. In this study, we developed a new strategy for rotator cuff tendon repair by combining a 3D printed scaffold of polylactic-co-glycolic acid (PLGA) with cell-laden collagen-fibrin hydrogels. We designed and fabricated two types of scaffolds: one featuring a separate layer-by-layer structure and another with a tri-layered structure as a whole. Uniaxial tensile tests showed that both types of scaffolds had improved mechanical properties compared to single-layered PLGA scaffolds. The printed scaffold with collagen-fibrin hydrogels effectively supported the growth, proliferation, and tenogenic differentiation of human adipose-derived mesenchymal stem cells. Subcutaneous implantation of the multilayered scaffolds demonstrated their excellent in vivo biocompatibility. This study demonstrates the feasibility of 3D printing multilayered scaffolds for application in rotator cuff tendon regeneration.

Fabrication and Plasma Surface Activation of Aligned Electrospun PLGA Fiber Fleeces with Improved Adhesion and Infiltration of Amniotic Epithelial Stem Cells Maintaining their Teno-inductive Potential

Electrospun PLGA microfibers with adequate intrinsic physical features (fiber alignment and diameter) have been shown to boost teno-differentiation and may represent a promising solution for tendon tissue engineering. However, the hydrophobic properties of PLGA may be adjusted through specific treatments to improve cell biodisponibility. In this study, electrospun PLGA with highly aligned microfibers were cold atmospheric plasma (CAP)-treated by varying the treatment exposure time (30, 60, and 90 s) and the working distance (1.3 and 1.7 cm) and characterized by their physicochemical, mechanical and bioactive properties on ovine amniotic epithelial cells (oAECs). CAP improved the hydrophilic properties of the treated materials due to the incorporation of new oxygen polar functionalities on the microfibers' surface especially when increasing treatment exposure time and lowering working distance. The mechanical properties, though, were affected by the treatment exposure time where the optimum performance was obtained after 60 s. Furthermore, CAP treatment did not alter oAECs' biocompatibility and improved cell adhesion and infiltration onto the microfibers especially those treated from a distance of 1.3 cm. Moreover, teno-inductive potential of highly aligned PLGA electrospun microfibers was maintained. Indeed, cells cultured onto the untreated and CAP treated microfibers differentiated towards the tenogenic lineage expressing tenomodulin, a mature tendon marker, in their cytoplasm. In conclusion, CAP treatment on PLGA microfibers conducted at 1.3 cm working distance represent the optimum conditions to activate PLGA surface by improving their hydrophilicity and cell bio-responsiveness. Since for tendon tissue engineering purposes, both high cell adhesion and mechanical parameters are crucial, PLGA treated for 60 s at 1.3 cm was identified as the optimal construct.

Electrospun PLGA Fiber Diameter and Alignment of Tendon Biomimetic Fleece Potentiate Tenogenic Differentiation and Immunomodulatory Function of Amniotic Epithelial Stem Cells

Injured tendons are challenging in their regeneration; thus, tissue engineering represents a promising solution. This research tests the hypothesis that the response of amniotic epithelial stem cells (AECs) can be modulated by fiber diameter size of tendon biomimetic fleeces. Particularly, the effect of electrospun poly(lactide-co-glycolide) (PLGA) fleeces with highly aligned microfibers possessing two different diameter sizes (1.27 and 2.5 µm: ha1- and ha2-PLGA, respectively) was tested on the ability of AECs to differentiate towards the tenogenic lineage by analyzing tendon related markers (Collagen type I: COL1 protein and mRNA Scleraxis: SCX, Tenomodulin: TNMD and COL1 gene expressions) and to modulate their immunomodulatory properties by investigating the pro- (IL-6 and IL-12) and anti- (IL-4 and IL-10) inflammatory cytokines. It was observed that fiber alignment and not fiber size influenced cell morphology determining the morphological change of AECs from cuboidal to fusiform tenocyte-like shape. Instead, fleece mechanical properties, cell proliferation, tenogenic differentiation, and immunomodulation were regulated by changing the ha-PLGA microfiber diameter size. Specifically, higher DNA quantity and better penetration within the fleece were found on ha2-PLGA, while ha1-PLGA fleeces with small fiber diameter size had better mechanical features and were more effective on AECs trans-differentiation towards the tenogenic lineage by significantly translating more efficiently SCX into the downstream effector TNMD. Moreover, the fiber diameter of 1.27 µm induced higher expression of pro-regenerative, anti-inflammatory interleukins mRNA expression (IL-4 and IL-10) with favorable IL-12/IL-10 ratio with respect to the fiber diameter of 2.5 µm. The obtained results demonstrate that fiber diameter is a key factor to be considered when designing tendon biomimetic fleece for tissue repair and provide new insights into the importance of controlling matrix parameters in enhancing cell differentiation and immunomodulation either for the cells functionalized within or for the transplanted host tissue.

Role of Biomaterials and Controlled Architecture on Tendon/Ligament Repair and Regeneration

Engineering synthetic scaffolds to repair and regenerate ruptured native tendon and ligament (T/L) tissues is a significant engineering challenge due to the need to satisfy both the unique biological and biomechanical properties of these tissues. Long-term clinical outcomes of synthetic scaffolds relying solely on high uniaxial tensile strength are poor with high rates of implant rupture and synovitis. Ideal biomaterials for T/L repair and regeneration need to possess the appropriate biological and biomechanical properties necessary for the successful repair and regeneration of ruptured tendon and ligament tissues.

Harnessing Fiber Diameter-Dependent Effects of Myoblasts Toward Biomimetic Scaffold-Based Skeletal Muscle Regeneration

Regeneration of skeletal muscles is limited in cases of volumetric muscle loss and muscle degenerative diseases. Therefore, there is a critical need for developing strategies that provide cellular and structural support for skeletal muscle regeneration. In the present work, a bioengineered cell niche composed of mechanically competent aligned polyester fiber scaffolds is developed to mimic the oriented muscle fiber microenvironment by electrospinning poly(lactide-co-glycolide) (PLGA) using a custom-designed rotating collector with interspaced parallel blades. Aligned fiber scaffolds with fiber diameters ranging from 335 ± 154 nm to 3013 ± 531 nm are characterized for their bioactivities in supporting growth and differentiation of myoblasts. During in vitro culture, polymeric scaffolds with larger fiber diameter support enhanced alignment, growth, and differentiation of myoblasts associated with phosphorylation of p38 MAPK and upregulated expression of myogenin and myosin heavy chain. In vivo studies using a dystrophin-deficient mdx mouse model show that optimized fiber scaffolds seeded with primary myoblasts result in formation of dystrophin-positive myofibers network in tibialis anterior muscles. Collectively, these experiments provide critical insights on harnessing interactions between muscle cells and engineered fiber matrices to develop effective biomaterials for accelerated muscle regeneration.

Biodegradable polymer nanocomposites for ligament/tendon tissue engineering

Ligaments and tendons are fibrous tissues with poor vascularity and limited regeneration capacity. Currently, a ligament/tendon injury often require a surgical procedure using auto- or allografts that present some limitations. These inadequacies combined with the significant economic and health impact have prompted the development of tissue engineering approaches. Several natural and synthetic biodegradable polymers as well as composites, blends and hybrids based on such materials have been used to produce tendon and ligament scaffolds. Given the complex structure of native tissues, the production of fiber-based scaffolds has been the preferred option for tendon/ligament tissue engineering. Electrospinning and several textile methods such as twisting, braiding and knitting have been used to produce these scaffolds. This review focuses on the developments achieved in the preparation of tendon/ligament scaffolds based on different biodegradable polymers. Several examples are overviewed and their processing methodologies, as well as their biological and mechanical performances, are discussed.

Tendon Biomimetic Electrospun PLGA Fleeces Induce an Early Epithelial-Mesenchymal Transition and Tenogenic Differentiation on Amniotic Epithelial Stem Cells

Background. The design of tendon biomimetic electrospun fleece with Amniotic Epithelial Stem Cells (AECs) that have shown a high tenogenic attitude may represent an alternative strategy to overcome the unsatisfactory results of conventional treatments in tendon regeneration. Methods. In this study, we evaluated AEC-engineered electrospun poly(lactide-co-glycolide) (PLGA) fleeces with highly aligned fibers (ha-PLGA) that mimic tendon extracellular matrix, their biocompatibility, and differentiation towards the tenogenic lineage. PLGA fleeces with randomly distributed fibers (rd-PLGA) were generated as control. Results. Optimal cell infiltration and biocompatibility with both PLGA fleeces were shown. However, only ha-PLGA fleeces committed AECs towards an Epithelial-Mesenchymal Transition (EMT) after 48 h culture, inducing their cellular elongation along the fibers' axis and the upregulation of mesenchymal markers. AECs further differentiated towards tenogenic lineage as confirmed by the up-regulation of tendon-related genes and Collagen Type 1 (COL1) protein expression that, after 28 days culture, appeared extracellularly distributed along the direction of ha-PLGA fibers. Moreover, long-term co-cultures of AEC-ha-PLGA bio-hybrids with fetal tendon explants significantly accelerated of half time AEC tenogenic differentiation compared to ha-PLGA fleeces cultured only with AECs. Conclusions. The fabricated tendon biomimetic ha-PLGA fleeces induce AEC tenogenesis through an early EMT, providing a potential tendon substitute for tendon engineering research.