Design and Fabrication of a Hierarchically Structured Scaffold for Tendon-to-Bone Repair

A New Tissue Engineering Strategy to Promote Tendon-bone Healing: Regulation of Osteogenic and Chondrogenic Differentiation of Tendon-derived Stem Cells

In the field of sports medicine, repair surgery for anterior cruciate ligament (ACL) and rotator cuff (RC) injuries are remarkably common. Despite the availability of relatively effective treatment modalities, outcomes often fall short of expectations. This comprehensive review aims to thoroughly examine current strategies employed to promote tendon-bone healing and analyze pertinent preclinical and clinical research. Amidst ongoing investigations, tendon-derived stem cells (TDSCs), which have comparatively limited prior exploration, have garnered increasing attention in the context of tendon-bone healing, emerging as a promising cell type for regenerative therapies. This review article delves into the potential of combining TDSCs with tissue engineering methods, with ACL reconstruction as the main focus. It comprehensively reviews relevant research on ACL and RC healing to address the issues of graft healing and bone tunnel integration. To optimize tendon-bone healing outcomes, our emphasis lies in not only reconstructing the original microstructure of the tendon-bone interface but also achieving proper bone tunnel integration, encompassing both cartilage and bone formation. In this endeavor, we thoroughly analyze the transcriptional and molecular regulatory variables governing TDSCs differentiation, incorporating a retrospective analysis utilizing single-cell sequencing, with the aim of unearthing relevant signaling pathways and processes. By presenting a novel strategy rooted in TDSCs-driven osteogenic and chondrogenic differentiation for tendon-bone healing, this study paves the way for potential future research avenues and promising therapeutic applications. It is anticipated that the findings herein will contribute to advancing the field of tendon-bone healing and foster the exploration of TDSCs as a viable option for regenerative therapies in the future.

Facile and rapid fabrication of a novel 3D-printable, visible light-crosslinkable and bioactive polythiourethane for large-to-massive rotator cuff tendon repair

Facile and rapid 3D fabrication of strong, bioactive materials can address challenges that impede repair of large-to-massive rotator cuff tears including personalized grafts, limited mechanical support, and inadequate tissue regeneration. Herein, we developed a facile and rapid methodology that generates visible light-crosslinkable polythiourethane (PHT) pre-polymer resin (∼30 min at room temperature), yielding 3D-printable scaffolds with tendon-like mechanical attributes capable of delivering tenogenic bioactive factors. Ex vivo characterization confirmed successful fabrication, robust human supraspinatus tendon (SST)-like tensile properties (strength: 23 MPa, modulus: 459 MPa, at least 10,000 physiological loading cycles without failure), excellent suture retention (8.62-fold lower than acellular dermal matrix (ADM)-based clinical graft), slow degradation, and controlled release of fibroblast growth factor-2 (FGF-2) and transforming growth factor-β3 (TGF-β3). In vitro studies showed cytocompatibility and growth factor-mediated tenogenic-like differentiation of mesenchymal stem cells. In vivo studies demonstrated biocompatibility (3-week mouse subcutaneous implantation) and ability of growth factor-containing scaffolds to notably regenerate at least 1-cm of tendon with native-like biomechanical attributes as uninjured shoulder (8-week, large-to-massive 1-cm gap rabbit rotator cuff injury). This study demonstrates use of a 3D-printable, strong, and bioactive material to provide mechanical support and pro-regenerative cues for challenging injuries such as large-to-massive rotator cuff tears.

Advances in non-coding RNA in tendon injuries

Tendons serve as important weight-bearing structures that smoothly transfer forces from muscles to skeletal parts, allowing contracted muscle movements to be translated into corresponding joint movements. For body mechanics, tendon tissue plays an important role. If the tendons are damaged to varying degrees, it can lead to disability or pain in patients. That is to say, tendon injuries havea significant impact on quality of life and deserve our high attention. Compared to other musculoskeletal tissues, tendons are hypovascular and hypo-cellular, and therefore have a greater ability to heal, this will lead to a longer recovery period after injury or even disability, which will significantly affect the quality of life. There are many causes of tendon injury, including trauma, genetic factors, inflammation, aging, and long-term overuse, and the study of related mechanisms is of great significance. Currently, tendon there are different treatment modalities, like injection therapy and surgical interventions. However, they have a high failure rate due to different reasons, among which the formation of adhesions severely weakens the tissue strength, affecting the functional recovery and the patient's quality of life. A large amount of data has shown that non coding RNAs can play a huge role in this field, thus attracting widespread attention from researchers from various countries. This review summarizes the relevant research progress on non-coding RNAs in tendon injuries, providing new ideas for a deeper understanding of tendon injuries and exploring new diagnostic and therapeutic approaches.

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.

Putting Hybrid Nanomaterials to Work for Biomedical Applications

Hybrid nanomaterials have found use in many biomedical applications. This article provides a comprehensive review of the principles, techniques, and recent advancements in the design and fabrication of hybrid nanomaterials for biomedicine. We begin with an introduction to the general concept of material hybridization, followed by a discussion of how this approach leads to materials with additional functionality and enhanced performance. We then highlight hybrid nanomaterials in the forms of nanostructures, nanocomposites, metal-organic frameworks, and biohybrids, including their fabrication methods. We also showcase the use of hybrid nanomaterials to advance biomedical engineering in the context of nanomedicine, regenerative medicine, diagnostics, theranostics, and biomanufacturing. Finally, we offer perspectives on challenges and opportunities.

Melt Electrowriting Combined with Fused Deposition Modeling Printing for the Fabrication of Three-Dimensional Biomimetic Scaffolds for Osteotendinous Junction Regeneration

Purpose

This study aims to explore a novel scaffold for osteotendinous junction regeneration and to preliminarily verify its osteogenic and tenogenic abilities in vitro.

Methods

A polycaprolactone (PCL) scaffold with aligned and orthogonal fibers was created using melt electrowriting (MEW) and fused deposition modeling (FDM). The scaffold was coated with Type I collagen, and hydroxyapatite was carefully added to separate the regions intended for bone and tendon regeneration, before being rolled into a cylindrical shape. Human adipose-derived stem cells (hADSCs) were seeded to evaluate viability and differentiation. Scaffold characterization was performed with Scanning Electron Microscope (SEM). Osteogenesis was assessed by alkaline phosphatase (ALP) and Alizarin red staining, while immunostaining and transcription-quantitative polymerase chain reaction (RT-qPCR) evaluated osteogenic and tendogenic markers.

Results

Scaffolds were developed in four variations: aligned (A), collagen-coated aligned (A+C), orthogonal (O), and mineral-coated orthogonal (O+M). SEM analysis confirmed surface morphology and energy-dispersive X-ray spectroscopy (EDS) verified mineral coating on O+M types. Hydrophilicity and mechanical properties were optimized in modified scaffolds, with A+C showing increased tensile strength and O+M improved in compression. hADSCs demonstrated good viability and morphology across scaffolds, withO+M scaffolds showing higher cell proliferation and osteogenic potential, and A and A+C scaffolds supporting tenogenic differentiation.

Conclusion

This study confirms the potential of a novel PCL scaffold with distinct regions for osteogenic and tenogenic differentiation, supporting the regeneration of osteotendinous junctions in vitro.

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.

High-Performance Artificial Ligament Made from Helical Polyester Fibers Wrapped with Aligned Carbon Nanotube Sheets

Repairing high-load connective tissues, such as ligaments, by surgically implanting artificial grafts after injury is challenging because they lack biointegration with host bones for stable interfaces. Herein, a high-performance helical composite fiber (HCF) ligament by wrapping aligned carbon nanotube (CNT) sheets around polyester fibers is proposed. Anterior cruciate ligament (ACL) reconstruction surgery shows that HCF grafts could induce effective bone regeneration, thus allowing the narrowing of bone tunnel defects. Such repair of the bone tunnel is in strong contrast to the tunnel enlargement of more than 50% for commercial artificial ligaments made from bare polyester fibers. Rats reconstructed with this HCF ligament show normal jumping, walking, and running without limping. This work allows bone regeneration in vivo through a one-step surgery without seeding cells or transforming growth factors, thereby opening an avenue for high-performance artificial tissues.

Biomechanical and histological changes secondary to aging in the human rotator cuff: A preliminary analysis

The high failure rate of rotator cuff repair surgeries is positively correlated with age, yet the biomechanical changes to the tendons of the rotator cuff with age have not been described. As such, we sought to benchmark and characterize the biomechanical and histopathological properties with the accompanying gene expression of human rotator cuff tendons as a function of age and histopathological degeneration. All four rotator cuff tendons from fresh human cadaver shoulders underwent biomechanical, histopathological, and gene expression analyses. Following cadaver availability, samples were grouped into Younger (i.e., less than 36 years of age, n = 2 donors) and Aged (i.e., greater than 55 years of age, n = 3 donors) as a means of characterizing and quantifying the age-related changes exhibited by the tendons. Biomechanical testing and subsequent computational modeling techniques revealed both differences in properties between tendons and greater Young's moduli in the Younger tendons (supraspinatus 3.06x, infraspinatus 1.76x, subscapularis 1.25x, and teres minor 1.32x). Histopathological scoring using the semi-quantitative Bonar scoring scheme revealed a positive correlation with age across all tendons (r = 0.508, p < 0.001). These data contextualize the biomechanical and histopathological changes to tendons that occurs naturally with aging, highlighting the innate differences in biomechanical properties of all four rotator cuff tendons, as well as the difference in their degenerative trajectories. Additionally, the histopathological scoring revealed moderate signs of degeneration within the Younger supraspinatus tendons, suggesting tissue quality may decrease in this specific tendon in patients less than 40 years old, before clinical symptoms or tears.

Integrated Piezoelectric/Conductive Composite Cryogel Creates Electroactive Microenvironment for Enhanced Bone Regeneration

Natural bone tissue possesses inherent electrophysiological characteristics, displaying conductivity and piezoelectricity simultaneously; hence, the reconstruction of local electrical microenvironment at defect site provides an effective strategy to enhance osteogenesis. Herein, a composite cryogel-type scaffold (referred to as Gel-PD-CMBT) is developed for bone regeneration, utilizing gelatin (Gel) in combination with a conductive poly(ethylene dioxythiophene)/polystyrene sulfonate matrix and Ca/Mn co-doped barium titanate (CMBT) nanofibers as the piezoelectric filler. The incorporation of these components results in the formation of an integrated piezoelectric/conductive network within the scaffold, facilitating charge migration and yielding a conductivity of 0.59 S cm-1 . This conductive scaffold creates a promising electroactive microenvironment, which is capable of up-regulating biological responses. Furthermore, the interconnected porous structure of the Gel-PD-CMBT scaffold not only provides mechanical stability but also offered ample space for cellular and tissue ingrowth. This Gel-PD-CMBT scaffold demonstrates a greater capacity to promote cellular osteogenic differentiation in vitro and neo-bone formation in vivo. In summary, the Gel-PD-CMBT scaffold, with its integrated piezoelectricity and conductivity, effectively restores the local electroactive microenvironment, offering an ideal platform for the regeneration of electrophysiological bone tissue.

Layered Double Hydroxides: A Novel Promising 2D Nanomaterial for Bone Diseases Treatment

Bone diseases including bone defects, bone infections, osteoarthritis, and bone tumors seriously affect life quality of the patient and bring serious economic burdens to social health management, for which the current clinical treatments bear dissatisfactory therapeutic effects. Biomaterial-based strategies have been widely applied in the treatment of orthopedic diseases but are still plagued by deficient bioreactivity. With the development of nanotechnology, layered double hydroxides (LDHs) with adjustable metal ion composition and alterable interlayer structure possessing charming physicochemical characteristics, versatile bioactive properties, and excellent drug loading and delivery capabilities arise widespread attention and have achieved considerable achievements for bone disease treatment in the last decade. However, to the authors' best knowledge, no review has comprehensively summarized the advances of LDHs in treating bone disease so far. Herein, the advantages of LDHs for orthopedic disorders treatment are outlined and the corresponding state-of-the-art achievements are summarized for the first time. The potential of LDHs-based nanocomposites for extended therapeutics for bone diseases is highlighted and perspectives for LDHs-based scaffold design are proposed for facilitated clinical translation.

The diversified hydrogels for biomedical applications and their imperative roles in tissue regeneration

Repair and regeneration of tissues after injury are complex pathophysiological processes. Microbial infection, malnutrition, and an ischemic and hypoxic microenvironment in the injured area can impede the typical healing cascade. Distinguished by biomimicry of the extracellular matrix, high aqueous content, and diverse functions, hydrogels have revolutionized clinical practices in tissue regeneration owing to their outstanding hydrophilicity, biocompatibility, and biodegradability. Various hydrogels such as smart hydrogels, nanocomposite hydrogels, and acellular matrix hydrogels are widely used for applications ranging from bench-scale to an industrial scale. In this review, some emerging hydrogels in the biomedical field are briefly discussed. The protective roles of hydrogels in wound dressings and their diverse biological effects on multiple tissues such as bone, cartilage, nerve, muscle, and adipose tissue are also discussed. The vehicle functions of hydrogels for chemicals and cell payloads are detailed. Additionally, this review emphasizes the particular characteristics of hydrogel products that promote tissue repair and reconstruction such as anti-infection, inflammation regulation, and angiogenesis.

Micropattern Silk Fibroin Film Facilitates Tendon Repair In Vivo and Promotes Tenogenic Differentiation of Tendon Stem/Progenitor Cells through the α2β1/FAK/PI3K/AKT Signaling Pathway In Vitro

Background

Tendon injuries are common clinical disorders. Due to the limited regeneration ability of tendons, tissue engineering technology is often used as an adjuvant treatment. This study explored the molecular pathways underlying micropattern SF film-regulated TSPC propensity and their repairing effects to highlight the application value of micropattern SF films.

Methods

First, we characterized the physical properties of the micropattern SF films and explored their repairing effects on the injured tendons in vivo. Then, we seeded TSPCs on SF films in vitro and determined the micropattern SF film-induced gene expression and activation of signaling pathways in TSPCs through high-throughput RNA sequencing and proteomics assays.

Results

The results of in vivo studies suggested that micropattern SF films can promote remodeling of the injured tendon. In addition, immunohistochemistry (IHC) results showed that tendon marker genes were significantly increased in the micropattern SF film repair group. Transcriptomic and proteomic analyses demonstrated that micropattern SF film-induced genes and proteins in TSPCs were mainly enriched in the focal adhesion kinase (FAK)/actin and phosphoinositide 3-kinase (PI3K)/AKT pathways. Western blot analysis showed that the expression of integrins α2β1, tenascin-C (TNC), and tenomodulin (TNMD) and the phosphorylation of AKT were significantly increased in the micropattern SF film group, which could be abrogated by applying PI3K/AKT inhibitors.

Conclusion

Micropattern SF films modified by water annealing can promote remodeling of the injured tendon in vivo and regulate the tendon differentiation of TSPCs through the α2β1/FAK/PI3K/AKT signaling pathway in vitro. Therefore, they have great medical value in tendon repair.

Comparison of cortical versus cancellous bone fixation in tendon-to-bone healing with a rat trans-calcaneal suture model for Achilles tendon sleeve avulsion

BACKGROUND

Trans-calcaneal suture technique is an economical and effective method for repairing Achilles tendon sleeve avulsion. Whether cancellous bone fixation upon this technique could accelerate tendon-to-bone healing is unknown. The purpose of this study is to compare the effect of cortical versus cancellous bone fixation on tendon-bone healing with a novel rat trans-calcaneal suture model.

METHODS

Trans-calcaneal suture treatment was carried out on the right hindlimb in male Sprague-Dawley rats (N = 80). They were randomly divided into the cortical group (Achilles fixed to the calcaneal cortical bone, n = 40) and the cancellous group (Achilles fixed to the calcaneal cancellous bone, n = 40). Gait analysis and immunohistochemistry were performed 1, 4, 7, and 14 days after the operation. Gross observation, biomechanical analysis, micro-CT, and histological analysis were performed 4 and 8 weeks after surgery. Independent-samples t tests were used for comparison between groups.

RESULTS

At 1, 4, and 7 days, the swing time of the affected limb in the cancellous group decreased, while the duty cycle, the maximum contact area, the print area, and the mean intensity increased significantly. The cross-sectional area of the tendon-bone junction in the cancellous group was smaller, and the failure load and stiffness were higher 4 weeks after the operation. The cancellous group showed more proportion of new bone and a relatively well-organized and dense connective tissue interface with better fibrocartilage-like tissue at 4 weeks after the operation. The ratio of ED2 + macrophages in the cancellous group was significantly higher than in the cortical group on 1, 4, 7, and 14 days. There were no significant differences in gait at 2 weeks, in appearance, biomechanics, new bone formation, and histology at 8 weeks after surgery between the two groups.

CONCLUSION

In the new rat trans-calcaneal suture model, cancellous fixation can accelerate tendon-to-bone healing in the early stage, which perhaps is related to the abundant bone marrow tissue in the cancellous bone that modulates the inflammatory processes.

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.

•

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

•

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

•

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

•

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

Advanced strategies for constructing interfacial tissues of bone and tendon/ligament

Enthesis, the interfacial tissue between a tendon/ligament and bone, exhibits a complex histological transition from soft to hard tissue, which significantly complicates its repair and regeneration after injury. Because traditional surgical treatments for enthesis injury are not satisfactory, tissue engineering has emerged as a strategy for improving treatment success. Rapid advances in enthesis tissue engineering have led to the development of several strategies for promoting enthesis tissue regeneration, including biological scaffolds, cells, growth factors, and biophysical modulation. In this review, we discuss recent advances in enthesis tissue engineering, particularly the use of biological scaffolds, as well as perspectives on the future directions in enthesis tissue engineering.

Scaffold-based tissue engineering strategies for soft-hard interface regeneration

Repairing injured tendon or ligament attachments to bones (enthesis) remains costly and challenging. Despite superb surgical management, the disorganized enthesis newly formed after surgery accounts for high recurrence rates after operations. Tissue engineering offers efficient alternatives to promote healing and regeneration of the specialized enthesis tissue. Load-transmitting functions thus can be restored with appropriate biomaterials and engineering strategies. Interestingly, recent studies have focused more on microstructure especially the arrangement of fibers since Rossetti successfully demonstrated the variability of fiber underspecific external force. In this review, we provide an important update on the current strategies for scaffold-based tissue engineering of enthesis when natural structure and properties are equally emphasized. We firstly described compositions, structures and features of natural enthesis with their special mechanical properties highlighted. Stimuli for growth, development and healing of enthesis widely used in popular strategies are systematically summarized. We discuss the fabrication of engineering scaffolds from the aspects of biomaterials, techniques and design strategies and comprehensively evaluate the advantages and disadvantages of each strategy. At last, this review pinpoints the remaining challenges and research directions to make breakthroughs in further studies.

Advances toward transformative therapies for tendon diseases

Approved therapies for tendon diseases have not yet changed the clinical practice of symptomatic pain treatment and physiotherapy. This review article summarizes advances in the development of novel drugs, biologic products, and biomaterial therapies for tendon diseases with perspectives for translation of integrated therapies. Shifting from targeting symptom relief toward disease modification and prevention of disease progression may open new avenues for therapies. Deep evidence-based clinical, cellular, and molecular characterization of the underlying pathology of tendon diseases, as well as therapeutic delivery optimization and establishment of multidiscipline interorganizational collaboration platforms, may accelerate the discovery and translation of transformative therapies for tendon diseases.

Biomechanical Effects of 3D-Printed Bioceramic Scaffolds With Porous Gradient Structures on the Regeneration of Alveolar Bone Defect: A Comprehensive Study

In the repair of alveolar bone defect, the microstructure of bone graft scaffolds is pivotal for their biological and biomechanical properties. However, it is currently controversial whether gradient structures perform better in biology and biomechanics than homogeneous structures when considering microstructural design. In this research, bioactive ceramic scaffolds with different porous gradient structures were designed and fabricated by 3D printing technology. Compression test, finite element analysis (FEA) revealed statistically significant differences in the biomechanical properties of three types of scaffolds. The mechanical properties of scaffolds approached the natural cancellous bone, and scaffolds with pore size decreased from the center to the perimeter (GII) had superior mechanical properties among the three groups. While in the simulation of Computational Fluid Dynamics (CFD), scaffolds with pore size increased from the center to the perimeter (GI) possessed the best permeability and largest flow velocity. Scaffolds were cultured in vitro with rBMSC or implanted in vivo for 4 or 8 weeks. Porous ceramics showed excellent biocompatibility. Results of in vivo were analysed by using micro-CT, concentric rings and VG staining. The GI was superior to the other groups with respect to osteogenicity. The Un (uniformed pore size) was slightly inferior to the GII. The concentric rings analysis demonstrated that the new bone in the GI was distributed in the periphery of defect area, whereas the GII was distributed in the center region. This study offers basic strategies and concepts for future design and development of scaffolds for the clinical restoration of alveolar bone defect.

Enhanced Repaired Enthesis Using Tenogenically Differentiated Adipose-Derived Stem Cells in a Murine Rotator Cuff Injury Model

Rotator cuff tear (RCT) is among the most common shoulder injuries and is prone to rerupture after surgery. Selecting suitable subpopulations of stem cells as a new specific cell type of mesenchymal stem cells has been increasingly used as a potential therapeutic tool in regenerative medicine. In this study, murine adipose-derived SSEA-4+CD90+PDGFRA+ subpopulation cells were successfully sorted, extracted, and identified. These cells showed good proliferation and differentiation potential, especially in the direction of tendon differentiation, as evidenced by qRT-PCR and immunofluorescence. Subsequently, we established a murine rotator cuff injury model and repaired it with subpopulation cells. Our results showed that the subpopulation cells embedded in a fibrin sealant significantly improved the histological score, as well as the biomechanical strength of the repaired tendon enthesis at four weeks after surgery, compared with the other groups. Hence, these findings indicated that the subpopulation of cells could augment the repaired enthesis and lead to better outcomes, thereby reducing the retear rate after rotator cuff repair. Our study provides a potential therapeutic strategy for rotator cuff healing in the future.

Biomimetic Scaffolds with a Mineral Gradient and Funnel-Shaped Channels for Spatially Controllable Osteogenesis

A facile method is described herein for generating a mineral gradient in a biodegradable polymer scaffold. The gradient is achieved by swelling a composite film made of polycaprolactone (PCL) and hydroxyapatite (HAp) nanoparticles with a PCL solution. During the swelling process, the solvent and PCL polymer chains diffuse into the composite film, generating a gradient in HAp density at their interface. The thickness of the mineral gradient can be tuned by varying the extent of swelling to match the length scale of the natural tendon-to-bone attachment (20-60 µm). When patterned with an array of funnel-shaped channels, the mineral gradient presents stem cells with spatial gradations in both biochemical cues (e.g., osteoinductivity and conductivity associated with the HAp nanoparticles) and mechanical cues (e.g., substrate stiffness) to stimulate their differentiation into a graded distribution of cell phenotypes. This new class of biomimetic scaffolds holds great promise for facilitating the regeneration of the injured tendon-to-bone attachment by stimulating the formation of a functionally graded interface.

3D-printed composite scaffold with anti-infection and osteogenesis potential against infected bone defects

In the field of orthopedics, an infected bone defect is a refractory disease accompanied by bone infection and defects as well as aggravated circulation. There are currently no personalized scaffolds that can treat bone infections using local stable and sustained-release antibiotics while providing mechanical support and bone induction to promote bone repair in the process of absorption in vivo. In our previous study, rifampicin/moxifloxacin-poly lactic-co-glycolic acid (PLGA) microspheres were prepared and tested for sustained release and antibacterial activity. The composite scaffold of poly-l-lactic acid (PLLA)/Pearl had a positive effect on mechanics supports and promoted osteogenesis. Therefore, in this study, the personalized scaffolds of PLLA/Pearl were first prepared by 3D printing. Then, rifampicin/moxifloxacin-PLGA (RM-P) microspheres were loaded into the scaffold pores to prepare the PLLA/Pearl/RM-P scaffolds. In this in vitro study, we investigated the structural characteristics and cytocompatibility of 3D-printed composite scaffolds, which indicates the integrity of the components in the scaffolds. The PLLA/Pearl and PLLA/Pearl/RM-P composite scaffolds can promote adhesion, proliferation, and differentiation of human bone marrow mesenchymal stem cells. Moreover, a rabbit model of infected bone defects of the radius was established. PLLA, PLLA/Pearl, and PLLA/Pearl/RM-P scaffolds were implanted into the bone nidus. The therapeutic effect of the three scaffolds on the infected bone defects was evaluated through imaging and microbiological and histological analysis after surgery. Among the three scaffolds, only the PLLA/Pearl/RM-P scaffold had anti-infection and bone defect repair in vivo. 3D printing provides support for personalized scaffold structures, and composite materials ensure that the scaffolds exert anti-infection and bone repair effects. Our study suggests that the PLLA/Pearl/RM-P scaffold is a promising new material in the clinical treatment of infected bone defects.

Indication the mechanism of dual-functional scaffold in the treatment of infected bone defects.

Bipolar Metal Flexible Electrospun Fibrous Membrane Based on Metal-Organic Framework for Gradient Healing of Tendon-to-Bone Interface Regeneration

Metal ions play a significant role in tissue repair, with widely application in clinical treatment. However, the therapeutic effect of metal ions is always limited due to metabolization and narrow repair capability. Here, a bipolar metal flexible electrospun fibrous membrane based on a metal-organic framework (MOF), which is bioinspired by the gradient structure of the tendon-to-bone interface, with a combination of regulating osteoblasts differentiation and angiogenesis properties, is constructed successfully by a continuous electrospinning technique and matching the longitudinal space morphology for synchronous regeneration. Furthermore, the MOF, acting as carriers, can not only achieve the sustainable release of metal ions, but promote the osteogenesis and tenogenesis on the scaffold. The in vitro data show that this novel hierarchical structure can accelerate the tenogenesis, the biomineralization, and angiogenesis. Moreover, in the in vivo experiment, the flexible fibrous membrane can promote tendon and bone tissue repair, and fibrocartilage reconstruction, to realize the multiple tissue synchronous regeneration at the damaged tendon-to-bone interface. Altogether, this newly developed bipolar metal flexible electrospun fibrous membrane based on a MOF, as a new biomimetic flexible scaffold, has great potential in reconstruct the tissue damage, especially gradient tissue damage.

Oxygen-Carrying and Antibacterial Fluorinated Nano-Hydroxyapatite Incorporated Hydrogels for Enhanced Bone Regeneration

Insufficient oxygen availability in tissue engineering is one of the major factors for the failure of clinical transplantation. One potential strategy to conquer this limitation is the fabrication of spontaneous and continuous oxygen supplying scaffolds for in situ tissue regeneration. In this work, a versatile fluorine-incorporating hydrogel is designed which can not only timely and continuously supply oxygen for mesenchymal stem cells (MSCs) to overcome deficient oxygen before vascularization in scaffolds, but can present a higher antibacterial capability to avoid bacterial infections. The HAp@PDA-F nanoparticles are first prepared and then incorporated with the quaternized and methacrylated chitosan forming CS/HAp@PDA-F by photo-crosslinking. In vitro results indicate that CS/HAp@PDA-F hydrogel has outstanding mechanical performance, moreover, it also has the oxygen-carrying ability to prolong survival ability, enhance proliferation activity, and preserve osteogenic differentiation potency and promote osteogenic-related genes expression of rat bone mesenchymal stem cells (rBMSCs) under hypoxic environment. Furthermore, the CS/HAp@PDA-F hydrogel can inhibit the growth of Staphylococcus aureus and Escherichia coli, providing a good antibacterial activity. Additionally, in vivo experiments demonstrate higher bone volume and bone mineral density, and more new bone tissue generation in CS/HAp@PDA-F group than in CS/HAp@PDA group. These results indicate that the rational design of fluorinated hydrogel possesses a good clinical application prospect for bone regeneration.

The role of the tendon ECM in mechanotransduction: disruption and repair following overuse

Purpose: Tendon overuse injuries are prevalent conditions with limited therapeutic options to halt disease progression. The specialized extracellular matrix (ECM) both enables joint function and mediates mechanical signals to tendon cells, driving biological responses to exercise or injury. With overuse, tendon ECM composition and structure changes at multiple scales, disrupting mechanotransduction and resulting in inadequate repair and disease progression. This review highlights the multiscale ECM changes that occur with tendon overuse and corresponding effects on cell-matrix interactions and cellular response to load.Results: Different functional joint requirements and tendon types experience a wide range of loading profiles, creating varied downstream mechanical stimuli. Distinct ECM structure and mechanical properties within the fascicle matrix, interfascicle matrix, and enthesis and their varied disruption with overuse are considered. The pericellular matrix (PCM) comprising the microscale tendon cell environment has a unique composition that changes with overuse injury and exercise, suggesting an important role in mechanotransduction and promoting repair. Cell-matrix interactions are mediated by structures including cilia, integrins, connexins and cytoskeleton that signal downstream homeostasis, adaptation, or repair. ECM disruption with tendon overuse may cause altered mechanical loading and cell-matrix interactions, resulting in mechanobiological understimulation, apoptosis, and ineffective repair. Current interventions to promote repair of tendon overuse injuries including exercise, targeting cell signaling, and modulating inflammation are considered.Conclusion: Future therapeutics should be assessed with regard of their effects on multiscale mechanotransduction in addition to joint function, with consideration of the central role of ECM.

Electrospun Nanofibers for Bone Regeneration: From Biomimetic Composition, Structure to Function

In recent years, a variety of novel materials and processing technologies have been developed to prepare tissue engineering scaffolds for bone defect repair. Among them, nanofibers fabricated via electrospinning technology...

A Nanofiber Mat With Dual Bioactive Components and a Biomimetic Matrix Structure for Improving Osteogenesis Effect

The challenge of effectively regenerating bone tissue through tissue engineering technology is that most tissue engineering scaffolds cannot imitate the three-dimensional structure and function of the natural extracellular matrix. Herein, we have prepared the poly(L-lactic acid)–based dual bioactive component reinforced nanofiber mats which were named as poly(L-lactic acid)/bovine serum albumin/nanohydroxyapatite (PLLA/BSA/nHAp) with dual bioactive components by combining homogeneous blending and electrospinning technology. The results showed that these nanofiber mats had sufficient mechanical properties and a porous structure suitable for cell growth and migration. Furthermore, the results of cell experiments in vitro showed that PLLA/BSA/nHAp composite nanofiber mat could preferably stimulate the proliferation of mouse osteoblastic cells (MC3T3 cells) compared with pure PLLA nanofiber mats. Based on these results, the scaffolds developed in this study are considered to have a great potential to be adhibited as bone repair materials.

Plasma-assisted multiscale topographic scaffolds for soft and hard tissue regeneration

The design of transplantable scaffolds for tissue regeneration requires gaining precise control of topographical properties. Here, we propose a methodology to fabricate hierarchical multiscale scaffolds with controlled hydrophilic and hydrophobic properties by employing capillary force lithography in combination with plasma modification. Using our method, we fabricated biodegradable biomaterial (i.e., polycaprolactone (PCL))-based nitrogen gas (N-FN) and oxygen gas plasma-assisted flexible multiscale nanotopographic (O-FMN) patches with natural extracellular matrix-like hierarchical structures along with flexible and controlled hydrophilic properties. In response to multiscale nanotopographic and chemically modified surface cues, the proliferation and osteogenic mineralization of cells were significantly promoted. Furthermore, the O-FMN patch enhanced regeneration of the mineralized fibrocartilage tissue of the tendon-bone interface and the calvarial bone tissue in vivo in rat models. Overall, the PCL-based O-FMN patches could accelerate soft- and hard-tissue regeneration. Thus, our proposed methodology was confirmed as an efficient approach for the design and manipulation of scaffolds having a multiscale topography with controlled hydrophilic property.

Micro-and-nanometer topological gradient of block copolymer fibrous scaffolds towards region-specific cell regulation

Regulating cell behavior and function by surface topography has drawn significant attention in tissue engineering. Herein, a gradient fibrous scaffold comprising anisotropic aligned fibers and isotropic annealed fibers was developed to provide a controllable direction of cell migration, adhesion, and spreading. The electrospun aligned fibers were engraved to create surface gradients with micro-and-nanometer roughness through block copolymer (BCP) self-assembly induced by selective solvent vapor annealing (SVA). The distinct manipulation of cell behavior by annealed fibrous scaffolds with tailored self-assembled nanostructure and welded fibrous microstructure has been illustrated by in situ/ex situ small angle X-ray scattering (SAXS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and in vitro cell culture. Further insights into the effect of integrated gradient fibrous scaffold were gained at the level of protein expression. From the perspective of gradient topology, this region-specific scaffold based on BCP fibers shows the prospect of guiding cell migration, adhesion and spreading and provides a generic method for designing biomaterials for tissue-engineering.

Gradient bimetallic ion-based hydrogels for tissue microstructure reconstruction of tendon-to-bone insertion

Although gradients play an essential role in guiding the function of tissues, achieving synchronous regeneration of gradient tissue injuries remains a challenge. Here, a gradient bimetallic (Cu and Zn) ion-based hydrogel was first constructed via the one-step coordinative crosslinking of sulfhydryl groups with copper and zinc ions for the microstructure reconstruction of the tendon-to-bone insertion. In this bimetallic hydrogel system, zinc and copper ions could not only act as crosslinkers but also provide strong antibacterial effects and induce regenerative capacity in vitro. The capability of hydrogels in simultaneously promoting tenogenesis and osteogenesis was further verified in a rat rotator cuff tear model. It was found that the Cu/Zn gradient layer could induce considerable collagen and fibrocartilage arrangement and ingrowth at the tendon-to-bone interface. Overall, the gradient bimetallic ion-based hydrogel ensures accessibility and provides opportunities to regenerate inhomogeneous tissue with physiological complexity or interface tissue.

Augmenting Tendon-to-Bone Repair with Functionally Graded Scaffolds

Tendon-to-bone repair often fails because the functionally graded attachment is not regenerated during the healing process. Biomimetic scaffolds that recapitulate the unique features of the native tendon-to-bone attachment hold great promise for enhancing the healing process. Among various types of scaffolds that are developed and evaluated for tendon-to-bone repair, those with gradations (in either a stratified or a continuous fashion) in composition, structure, mechanical properties, and cell phenotype have gained the most attention. In this progress report, the recent efforts in the rational design and fabrication of functionally graded scaffolds based upon electrospun nanofiber mats and inverse opal structures, as well as the evaluation of their applications in augmenting tendon-to-bone repair, are reviewed. This report concludes with perspectives on the necessary future steps for clinical translation of the scaffolds.

A review of biomimetic scaffolds for bone regeneration: Toward a cell-free strategy

In clinical terms, bone grafting currently involves the application of autogenous, allogeneic, or xenogeneic bone grafts, as well as natural or artificially synthesized materials, such as polymers, bioceramics, and other composites. Many of these are associated with limitations. The ideal scaffold for bone tissue engineering should provide mechanical support while promoting osteogenesis, osteoconduction, and even osteoinduction. There are various structural complications and engineering difficulties to be considered. Here, we describe the biomimetic possibilities of the modification of natural or synthetic materials through physical and chemical design to facilitate bone tissue repair. This review summarizes recent progresses in the strategies for constructing biomimetic scaffolds, including ion-functionalized scaffolds, decellularized extracellular matrix scaffolds, and micro- and nano-scale biomimetic scaffold structures, as well as reactive scaffolds induced by physical factors, and other acellular scaffolds. The fabrication techniques for these scaffolds, along with current strategies in clinical bone repair, are described. The developments in each category are discussed in terms of the connection between the scaffold materials and tissue repair, as well as the interactions with endogenous cells. As the advances in bone tissue engineering move toward application in the clinical setting, the demonstration of the therapeutic efficacy of these novel scaffold designs is critical.

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.

Improvement of ECM-based bioroot regeneration via N-acetylcysteine-induced antioxidative effects

BACKGROUND

The low survival rate or dysfunction of extracellular matrix (ECM)-based engineered organs caused by the adverse effects of unfavourable local microenvironments on seed cell viability and stemness, especially the effects of excessive reactive oxygen species (ROS), prompted us to examine the importance of controlling oxidative damage for tissue transplantation and regeneration. We sought to improve the tolerance of seed cells to the transplant microenvironment via antioxidant pathways, thus promoting transplant efficiency and achieving better tissue regeneration.

METHODS

We improved the antioxidative properties of ECM-based bioroots with higher glutathione contents in dental follicle stem cells (DFCs) by pretreating cells or loading scaffolds with the antioxidant NAC. Additionally, we developed an in situ rat alveolar fossa implantation model to evaluate the long-term therapeutic effects of NAC in bioroot transplantation.

RESULTS

The results showed that NAC decreased H₂O₂-induced cellular damage and maintained the differentiation potential of DFCs. The transplantation experiments further verified that NAC protected the biological properties of DFCs by repressing replacement resorption or ankylosis, thus facilitating bioroot regeneration.

CONCLUSIONS

The following findings suggest that NAC could significantly protect stem cell viability and stemness during oxidative stress and exert better and prolonged effects in bioroot intragrafts.

Adipose Stem Cell-Derived Exosomes Recover Impaired Matrix Metabolism of Torn Human Rotator Cuff Tendons by Maintaining Tissue Homeostasis

BACKGROUND

Adipose stem cell-derived exosomes (ASC-Exos) are reported to effectively prevent muscle atrophy and degeneration of torn rat rotator cuff, but their influence on human samples and their potential mechanism are still unclear.

PURPOSE

We aimed to investigate the effects of ASC-Exos on the metabolic activities of torn human rotator cuff tendons and explore the potential mechanism behind it.

STUDY DESIGN

Controlled laboratory study.

METHODS

Diseased supraspinatus tendons were harvested from 15 patients with a mean ± SD age of 65.8 ± 3.2 years who underwent reverse shoulder arthroplasty for chronic rotator cuff tears associated with glenohumeral pathological changes. Each tendon was dissected into 3 × 4 × 4-mm explants: the ones derived from the same tendon were placed into 12-well plates and cultured in complete culture media (control) or in complete culture media supplemented with ASC-Exos for 72 hours. Afterward, the concentrations of cytokines secreted into the culture media-including interleukin 1β (IL-1β), IL-6, IL-8, and matrix metalloproteinase 9 (MMP-9)-were measured using enzyme-linked immunosorbent assay (ELISA). Tendons were stained with hematoxylin and eosin and immunohistochemistry (type I and III collagens) for histological analyses. Moreover, the expression of anabolic genes (TIMP-1 and TIMP-3; type I and III collagen encoding) and catabolic genes (MMP-9 and MMP-13) in tendons were measured using real-time quantitative polymerase chain reaction. Phosphorylated AMPKα and Wnt/β-catenin pathways were assayed by western blotting to explore the potential mechanism of action of ASC-Exos.

RESULTS

Secretion of proinflammatory cytokines, including IL-1β, IL-6, and MMP-9, was significantly reduced in the ASC-Exos group as compared with the control group. Supraspinatus tendons in the ASC-Exos group exhibited superior histological properties, as demonstrated by higher tendon maturing scores and more type I collagen content, but there was no significant difference in type III collagen content between groups. Expression of MMP-9 and MMP-13 genes was decreased in the ASC-Exos group versus the control group. Increased expression of type I and III collagens and an elevated type I/III ratio were found in the ASC-Exos group when compared with the control group. There was no significant difference in the secretion of IL-8 and expression of TIMP-1 and TIMP-3 genes between the ASC-Exos and control groups. Western blotting revealed that ASC-Exos enhanced phosphorylated AMPKα and decreased β-catenin levels to prevent tendon degeneration.

CONCLUSION

ASC-Exos maintained metabolic homeostasis of torn human rotator cuff tendons to improve their histological properties, which might be achieved by enhancing AMPK signaling to suppress Wnt/β-catenin activity.

CLINICAL RELEVANCE

ASC-Exos could be used as an effective biological tool to promote healing in torn human rotator cuff tendons.

Biomimetic strategies for tendon/ligament-to-bone interface regeneration

Tendon/ligament-to-bone healing poses a formidable clinical challenge due to the complex structure, composition, cell population and mechanics of the interface. With rapid advances in tissue engineering, a variety of strategies including advanced biomaterials, bioactive growth factors and multiple stem cell lineages have been developed to facilitate the healing of this tissue interface. Given the important role of structure-function relationship, the review begins with a brief description of enthesis structure and composition. Next, the biomimetic biomaterials including decellularized extracellular matrix scaffolds and synthetic-/natural-origin scaffolds are critically examined. Then, the key roles of the combination, concentration and location of various growth factors in biomimetic application are emphasized. After that, the various stem cell sources and culture systems are described. At last, we discuss unmet needs and existing challenges in the ideal strategies for tendon/ligament-to-bone regeneration and highlight emerging strategies in the field.

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.

Tremella-Like ZnO@Col-I-Decorated Titanium Surfaces with Dual-Light-Defined Broad-Spectrum Antibacterial and Triple Osteogenic Properties

The growing population of peri-implant diseases (PIDs) has become a public obsession, mainly due to the lack of antibacterial ability and osteogenic promotion of titanium (Ti) implants. Herein, inspired by tremella, we reported zinc oxide (ZnO)@collagen type I (Col-I)-decorated Ti for PIDs treatments. Compared with pure Ti implants, ZnO@Col-I-decorated Ti could be activated by a safe visible yellow light and showed excellent broad-spectrum antibacterial properties. The proliferation and osteogenic gene expression of bone marrow mesenchymal stem cells (BMSCs) indicated that the triple osseointegration of implants was realized through (I) the remarkedly improved surface hydrophilicity of ZnO@Col-I-decorated Ti, (II) the function of Col-I, and (III) the excellent near-infrared (NIR)-induced photothermal performance of ZnO. Collectively, the proposed dual-light-defined ZnO@Col-I coating was a promising implant surface modification system to provide customized treatments for each PID patient.

Bone Regeneration Using Duck's Feet-Derived Collagen Scaffold as an Alternative Collagen Source

Collagen is an important component that makes 25-35% of our body proteins. Over the past decades, tissue engineers have been designing collagen-based biocompatible materials and studying their applications in different fields. Collagen obtained from cattle and pigs has been mainly used until now, but collagen derived from fish and other livestock has attracted more attention since the outbreak of mad cow disease, and they are also used as a raw material for cosmetics and foods. Due to the zoonotic infection using collagen derived from pigs and cattle, their application in developing biomaterials is limited; hence, the development of new animal-derived collagen is required. In addition, there is a religion (Islam, Hinduism, and Judaism) limited to export raw materials and products derived from cattle and pig. Hence, high-value collagen that is universally accessible in the world market is required. Therefore, in this review, we have dealt with the use of duck's feet-derived collagen (DC) as an emerging alternative to solve this problem and also presenting few original investigated bone regeneration results performed using DC.

A microfluidics-derived growth factor gradient in a scaffold regulates stem cell activities for tendon-to-bone interface healing

Treatment of tendon-to-bone interface injury has long been challenging in sports medicine. The major obstacle lies with the complicated three-layer structure of the tissue that consists of a bone region with osteocytes, a tendon region with tenocytes and a transitional region with chondrocytes. Conventional tissue engineering approaches using simply biomaterial scaffolds, stem cells and combinations of them had limited abilities to reconstruct the gradient structure with normal biomechanical properties. We herein aim to construct a three-layer structure with bone marrow-derived stem cells and tendon stem cells cultured in a decellularized tendon scaffold, through application of a gradient of biological cues in the longitudinal direction of the scaffold that guides the stem cells to differentiate and remodel the extracellular matrix in response to different medium concentrations in different regions. A microfluidic chip, on which a tree-like flow pattern was implemented, was adopted to create the concentration gradient in a dichotomous manner. We screened for an optimized seeding ratio between the two stem cell types before incubation of the scaffold in the medium concentration gradient and surgical implantation. Histology and immunohistochemistry assessments, both qualitatively and semi-quantitatively, showed that the microfluidic system provided desired guidance to the seeded stem cells that the healing at 8-week post-implantation presented a similar structure to that of a normal tendon-to-bone interface, which was outstanding compared to treatments without gradient guidance, stem cells or scaffolds where chaotic and fibrotic structures were obtained. This strategy offers a potentially translational tissue engineering approach for better outcomes in tendon-to-bone healing.

Hydrogel to guide chondrogenesis versus osteogenesis of mesenchymal stem cells for fabrication of cartilaginous tissues

The ideal combination of hydrogel components for regeneration of cartilage and cartilaginous interfaces is a significant challenge because control over differentiation into multiple lineages is necessary. Stabilization of the phenotype of stem cell derived chondrocytes is needed to avoid undesired progression to terminal hypertrophy and tissue mineralization. A novel ternary blend hydrogel composed of methacrylated poly(ethylene glycol) (PEG), gelatin, and heparin (PGH) was designed to guide chondrogenesis by bone marrow derived mesenchymal stem cells (BMSCs) and maintenance of their cartilaginous phenotype. The hydrogel material effects on chondrogenic and osteogenic differentiation by BMSCs were evaluated in comparison to methacrylated gelatin hydrogel (GEL), a conventional bioink used for both chondrogenic and osteogenic applications. PGH and GEL hydrogels were loaded with goat BMSCs and cultured in chondrogenic and osteogenic mediums in vitro over six weeks. The PGH showed no sign of mineral deposition in an osteogenic environment in vitro. To further evaluate material effects, the hydrogels were loaded with adult human BMSCs (hBMSCs) and transforming growth factor β-3 and grown in subcutaneous pockets in mice over eight weeks. Consistent with the in vitro results, the PGH had greater potential to induce chondrogenesis by BMSCs in vivo compared to the GEL as evidenced by elevated gene expression of chondrogenic markers, supporting its potential for stable cartilage engineering. The PGH also showed a greater percentage of GAG positive cells compared to the GEL. Unlike the GEL, the PGH hydrogel exhibited anti-osteogenic effects in vivo as evidenced by negative Von Kossa staining and suppressed gene expression of hypertrophic and osteogenic markers. By nature of their polymer composition alone, the PGH and GEL regulated BMSC differentiation down different osteochondral lineages. Thus, the PGH and GEL are promising hydrogels to regenerate stratified cartilaginous interfacial tissues in situ, such as the mandibular condyle surface, using undifferentiated BMSCs and a stratified scaffold design.

Fabrication of Bioactive Inverted Colloidal Crystal Scaffolds Using Expanded Polystyrene Beads

Inverted colloidal crystal (ICC) hydrogel scaffolds have emerged as a new class of three-dimensional cell culture matrix that represents a unique opportunity to reproduce lymphoid tissue microenvironments. ICC geometry promotes the formation of stromal cell networks and their interaction with hematopoietic cells, a core cellular process in lymphoid tissues. When subdermally implanted, ICC hydrogel scaffolds direct unique foreign body responses to form a vascularized stromal tissue with prolonged attraction of hematopoietic cells, which together resemble lymphoid tissue microenvironments. While conceptually simple, fabrication of ICC hydrogel scaffold requires multiple steps and laborious handling of delicate materials. Here, we introduce a facile route for ICC hydrogel scaffold fabrication using expanded polystyrene (EPS) beads. EPS beads shrink and fuse in a tunable manner under pressurized thermal conditions, which serves as colloidal crystal templates for ICC scaffold fabrication. Inclusion of collagen in the precursor solution greatly simplified preparation of bioactive hydrogel scaffolds. The resultant EPS-templated bioactive ICC hydrogel scaffolds demonstrate characteristic features required for lymphoid tissue modeling in both in vitro and in vivo settings. We envision that the presented method will facilitate widespread implementation of ICC hydrogel scaffolds for lymphoid tissue engineering and other emerging applications. Impact statement Inverted colloidal crystal (ICC) hydrogel scaffolds have emerged as a new class of three-dimensional cell culture matrix that represents a unique opportunity for lymphoid tissue modeling and other emerging novel bioengineering applications. While conceptually simple, fabrication of the ICC hydrogel scaffold requires multiple steps and laborious handling of delicate materials with highly toxic chemicals. The presented method for ICC hydrogel scaffold fabrication using expanded polystyrene (EPS) beads is simple, cost-effective, and involves less toxic chemicals than conventional methods, while retaining comparable biological significance. We envision that EPS bead-based hydrogel scaffold fabrication will greatly facilitate the widespread implementation of ICC hydrogel scaffolds and their practical applications.

Three-dimensional printed multiphasic scaffolds with stratified cell-laden gelatin methacrylate hydrogels for biomimetic tendon-to-bone interface engineering

Background

The anatomical properties of the enthesis of the rotator cuff are hardly regained during the process of healing. The tendon-to-bone interface is normally replaced by fibrovascular tissue instead of interposition fibrocartilage, which impairs biomechanics in the shoulder and causes dysfunction. Tissue engineering offers a promising strategy to regenerate a biomimetic interface. Here, we report heterogeneous tendon-to-bone interface engineering based on a 3D-printed multiphasic scaffold.

Methods

A multiphasic poly(ε-caprolactone) (PCL)–PCL/tricalcium phosphate (TCP)–PCL/TCP porous scaffold was manufactured using 3D printing technology. The three phases of the scaffold were designed to mimic the graded tissue regions in the tendon-to-bone interface—tendon, fibrocartilage, and bone. Fibroblasts, bone marrow–derived mesenchymal stem cells, and osteoblasts were separately encapsulated in gelatin methacrylate (GelMA) and loaded seriatim on the relevant phases of the scaffold, by which a cells/GelMA-multiphasic scaffold (C/G-MS) construct, replicating the native interface, was fabricated. Cell proliferation, viability, and chondrogenic differentiation were evaluated in vitro. The C/G-MS constructs were further examined to determine the potential of regenerating a continuous interface with gradual transition of teno-, fibrocartilage- and osteo-like tissues in vivo.

Results

In vitro tests confirmed the good cytocompatibility of the constructs. After seven days in culture, cellular microfilament staining indicated that not only could cells well proliferate in GelMA hydrogels ​but also efficiently attach to and grow on scaffold fibres. Moreover, by immunolocalizing collagen type II, chondrogenesis was identified in the intermediate phase of the C/G-MS construct that had been treated with transforming growth factor β3 for 21 days. After subcutaneous implantation in mice, the C/G-MS construct exhibited a heterogeneous and graded structure up to eight weeks, with distinguished matrix distribution observed at one week. Overall, gene expression of tenogenic, chondrogenic, and osteogenic markers showed increasing patterns for eight weeks. Among them, expression of collagen type X gene was found drastically increasing during eight weeks, indicating progressive formation of calcifying cartilage within the constructs.

Conclusion

Our findings demonstrate that the stratified manner of fabrication based on the 3D-printed multiphasic scaffold is an effective strategy for tendon-to-bone interface engineering in terms of efficient cell seeding, chondrogenic potential, and distinct matrix deposition in varying phases.

The translational potential of this article

We fabricated a biomimetic tendon-to-bone interface by using a 3D-printed multiphasic scaffold and adopting a stratified cell-seeding manner with GelMA. The biomimetic interface might have applications in tendon-to-bone repair in the rotator cuff. It can not only be an alternative to a biological fixation device ​but also offer an ex vivo living graft to replace the damaged enthesis.

Fabrication and characterization of dextran/nanocrystalline β-tricalcium phosphate nanocomposite hydrogel scaffolds

Design of bioactive three-dimensional scaffolds to support bone tissue repair and regeneration become a key area of research in tissue engineering. Herein, porous hybrid hydrogels composed of dextran incorporated with nanocrystalline β-tricalcium phosphate (β-TCP) particles were tailor made as scaffolds for bone tissue engineering. β-TCP was successfully introduced within the dextran networks crosslinked through intermolecular ionic interactions and hydrogen bonding confirmed by FTIR spectroscopy. The effect of β-TCP content on equilibrium water uptake and swelling kinetics of composite hydrogels was investigated. It was found that the homogeneous distribution of β-TCP nanoparticles through the hydrogel matrix contributes to higher porosity and swelling capacity. In depth swelling measurements revealed that while in the early stage of swelling, water diffusion follows the Fick's law, for longer time swelling behavior of hydrogels undergo the second order kinetics. XRD measurements represented the formation of apatite layer on the surface of nanocomposite hydrogels after immersion in the SBF solution, which implies their bioactivity. Cell culture assays confirmed biocompatibility of the developed hybrid hydrogels in vitro. The obtained results converge to offer dextran/β-TCP nanocomposite hydrogels as promising scaffolds for bone regeneration applications.

Therapeutic "Tool" in Reconstruction and Regeneration of Tissue Engineering for Osteochondral Repair

Repairing osteochondral defects to restore joint function is a major challenge in regenerative medicine. However, with recent advances in tissue engineering, the development of potential treatments is promising. In recent years, in addition to single-layer scaffolds, double-layer or multilayer scaffolds have been prepared to mimic the structure of articular cartilage and subchondral bone for osteochondral repair. Although there are a range of different cells such as umbilical cord stem cells, bone marrow mesenchyml stem cell, and others that can be used, the availability, ease of preparation, and the osteogenic and chondrogenic capacity of these cells are important factors that will influence its selection for tissue engineering. Furthermore, appropriate cell proliferation and differentiation of these cells is also key for the optimal repair of osteochondral defects. The development of bioreactors has enhanced methods to stimulate the proliferation and differentiation of cells. In this review, we summarize the recent advances in tissue engineering, including the development of layered scaffolds, cells, and bioreactors that have changed the approach towards the development of novel treatments for osteochondral repair.

Smart, Elastic, and Nanofiber-Based 3D Scaffolds with Self-Deploying Capability for Osteoporotic Bone Regeneration

It has been a major challenge to treat osteoporotic bone defects with irregular shapes. Although bioactive glass offers an attractive material for bone regeneration, its inherent brittleness has greatly limited its scope of application. Herein, we report the fabrication of bioactive glass (SiO₂-CaO) nanofibers with excellent flexibility to even allow for 180° bending. The bioactive glass nanofibers could be further assembled into 3D fibrous scaffolds with chitosan serving as the linkers. The scaffolds constructed from an assembly of 85SiO₂-15CaO nanofibers and chitosan (85SiO₂-15CaO NF/CS) possessed significantly better mechanical properties when benchmarked against both 75SiO₂-25CaO nanofiber- and chitosan-based scaffolds. Moreover, the 85SiO₂-15CaO NF/CS scaffolds exhibited an elastic behavior, with full recovery from 80% compression and good fatigue resistance over 1000 cycles of compression under water. Upon implantation, the elastic fibrous scaffolds were able to deform and fit irregularly shaped bone defects, followed by a self-deploying behavior to achieve a perfect match with the cavities. When applied to the repair of an osteoporotic calvarial defect in a rat model, the 85SiO₂-15CaO NF/CS scaffolds showed substantial promotion of bone regrowth and vascularization. This new class of 3D fibrous scaffold provides a promising advancement in engineering smart materials for complex bone repair.

Nanocellulose/bioactive glass cryogels as scaffolds for bone regeneration

A major challenge exists in the preparation of scaffolds for bone regeneration, namely, achieving simultaneously bioactivity, biocompatibility, mechanical performance and simple manufacturing. Here, cellulose nanofibrils (CNF) are introduced for the preparation of scaffolds taking advantage of their biocompatibility and ability to form strong 3D porous networks from aqueous suspensions. CNF are made bioactive for bone formation through a simple and scalable strategy that achieves highly interconnected 3D networks. The resultant materials optimally combine morphological and mechanical features and facilitate hydroxyapatite formation while releasing essential ions for in vivo bone repair. The porosity and roughness of the scaffolds favor several cell functions while the ions act in the expression of genes associated with cell differentiation. Ion release is found critical to enhance the production of the bone morphogenetic protein 2 (BMP-2) from cells within the fractured area, thus accelerating the in vivo bone repair. Systemic biocompatibility indicates no negative effects on vital organs such as the liver and kidneys. The results pave the way towards a facile preparation of advanced, high performance CNF-based scaffolds for bone tissue engineering.