Engineering orthopedic tissue interfaces

Paracrine signals influence patterns of fibrocartilage differentiation in a lyophilized gelatin hydrogel for applications in rotator cuff repair

Rotator cuff injuries present a clinical challenge for repair due to current limitations in functional regeneration of the native tendon-to-bone enthesis. A biomaterial that can regionally instruct unique tissue-specific phenotypes offers potential to promote enthesis repair. We have recently demonstrated the mechanical benefits of a stratified triphasic biomaterial made up of tendon- and bone-mimetic collagen scaffold compartments connected via a continuous hydrogel, and we now explore the potential of a biologically favorable enthesis hydrogel for this application. Here we report in vitro behavior of human mesenchymal stem cells (hMSCs) within thiolated gelatin (Gel-SH) hydrogels in response to chondrogenic stimuli as well as paracrine signals derived from MSC-seeded bone and tendon scaffold compartments. Chondrogenic differentiation media promoted upregulation of cartilage and entheseal fibrocartilage matrix markers COL2, COLX, and ACAN as well as the enthesis-associated transcription factors SCX, SOX9, and RUNX2 in hMSCs within Gel-SH. Similar effects were observed in response to TGF-β3 and BMP-4, enthesis-associated growth factors known to play a role in entheseal development and maintenance. Conditioned media generated by hMSCs seeded in tendon- and bone-mimetic collagen scaffolds influenced patterns of gene expression regarding enthesis-relevant growth factors, matrix markers, and tendon-to-bone transcription factors for hMSCs within the material. Together, these findings demonstrate that a Gel-SH hydrogel provides a permissive environment for enthesis tissue engineering and highlights the significance of cellular crosstalk between adjacent compartments within a spatially graded biomaterial.

Cell-free bilayer functionalized scaffold for osteochondral tissue engineering

Osteochondral tissue engineering using layered scaffolds is a promising approach for treating osteochondral defects as an alternative to microfracture procedure, autologous chondrocyte implantation, and cartilage-bone grafting. The team previously investigated the chondrogenesis of mesenchymal stem cells (MSCs) on a polycaprolactone (PCL)/acetylated hyaluronic acid scaffold. The present study first focused on fabricating a novel osteoconductive scaffold utilizing bismuth-nanohydroxyapatite/reduced graphene oxide (Bi-nHAp/rGO) nanocomposite and electrospun PCL. The osteoconductive ability of the scaffold was investigated by evaluating the alkaline phosphatase (ALP) activity and the osteogenic genes expression in the adipose-derived MSCs. The expression of Runx2, collagen I, ALP, and osteocalcin as well as the result of ALP activity indicated the osteoconductive potential of the Bi-nHA-rGO/PCL scaffold. In the next step, a bilayer scaffold containing Bi-nHAp/rGO/PCL as an osteogenic layer and acetylated hyaluronic acid/PCL as a chondrogenic layer was prepared by the electrospinning technique and transplanted into osteochondral defects of rats. The chondrogenic and osteogenic markers corresponding to the surrounding tissues of the transplanted scaffold were surveyed 60 days later by real-time polymerase chain reaction (PCR) and immunohistochemistry methods. The results showed increased chondrogenic (Sox9 and collagen II) and osteogenic (osteocalcin and ALP) gene expression and augmented secretion of collagens II and X after transplantation. The results strongly support the efficacy of this constructed cell-free bilayer scaffold to induce osteochondral defect regeneration.

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.

Optimized design of an enthesis-mimicking suture anchor-tendon hybrid graft for mechanically robust bone-tendon repair

Repair of functionally graded biological interfaces requires joining dissimilar materials such as hard bone to soft tendon/ligament, with re-injuries/re-tears expected to be minimized by incorporating biomimicking, stress-reducing features within grafts. At bone-tendon interfaces (entheses), stress can be reduced via angled insertion, geometric flaring, mechanical gradation, and interdigitation of tissues. Here, we incorporated enthesis attributes into 3D in silico and physical models of a unique suture anchor-tendon hybrid graft (SATHG) and investigated their effects on stress reduction via finite element analyses (FEA) studies. Over 20 different simulations altering SATHG angulation, flaring, mechanical gradation, and interdigitation identified an optimal design, which included 90° angulation, 25° flaring, and a compliant (ascending then descending) mechanical gradient in SATHG's bone-to-tendon-like transitional region. This design reduced peak stress concentration factor (SCF) by 43.6 % relative to an ascending-only mechanical gradient typically used in hard-to-soft tissue engineering. To verify FEA results, SATHG models were fabricated using a photocrosslinkable bone-tendon-like polyurethane (QHM polymer) for ex vivo tensile assessment. Tensile testing showed that ultimate load (132.9 N), displacement-at-failure (1.78 mm), stiffness (135.4 N/mm), and total work-to-failure (422.1 × 10-3 J) were highest in the optimized design. Furthermore, to assess envisioned usage, SATHG pull-out testing and 6-week in vivo implantation into large, 0.5-cm segmental supraspinatus tendon defects was performed. SATHG pull-out testing showed secure bone attachment while histological assessment such as hematoxylin and eosin (H&E) together with Safranin-O staining showed biocompatibility including enthesis regeneration. This work demonstrates that engineering biomaterials with FEA-optimized, enthesis-like attributes shows potential for enhancing hard-to-soft tissue repair. STATEMENT OF SIGNIFICANCE: Successful repair of hard-to-soft tissue injuries is challenging due to high stress concentrations within bone-tendon/ligament grafts that mechanically compromise repair strength. While stress-reducing gradient biomaterials have been reported, little-to-no attention has focused on other bone-tendon/ligament interface (enthesis) features. To this end, a unique bone-tendon graft (SATHG) was developed by combining two common orthopaedic devices along with biomimetic incorporation of four enthesis-like features to reduce stress and encourage widespread clinician adoption. Notably, utilizing designs based on natural stress dissipation principles such as anchor insertion angle, geometric flaring, and mechanical gradation reduced stress by 43.6 % in silico, which was confirmed ex vivo, while in vivo studies showed SATHG's ability to support native enthesis regeneration. Thus, SATHG shows promise for hard-to-soft tissue repairs.

Anatomical design and production of a novel three-dimensional co-culture system replicating the human flexor digitorum profundus enthesis

The enthesis, the specialized junction between tendon and bone, is a common site of injury. Although notoriously difficult to repair, advances in interfacial tissue engineering techniques are being developed for restorative function. Most notably are 3D in vitro co-culture models, built to recreate the complex heterogeneity of the native enthesis. While cell and matrix properties are often considered, there has been little attention given to native enthesis anatomical morphometrics and replicating these to enhance clinical relevance. This study focuses on the flexor digitorum profundus (FDP) tendon enthesis and, by combining anatomical morphometrics with computer-aided design, demonstrates the design and construction of an accurate and scalable model of the FDP enthesis. Bespoke 3D-printed mould inserts were fabricated based on the size, shape and insertion angle of the FDP enthesis. Then, silicone culture moulds were created, enabling the production of bespoke anatomical culture zones for an in vitro FDP enthesis model. The validity of the model has been confirmed using brushite cement scaffolds seeded with osteoblasts (bone) and fibrin hydrogel scaffolds seeded with fibroblasts (tendon) in individual studies with cells from either human or rat origin. This novel approach allows a bespoke anatomical design for enthesis repair and should be applied to future studies in this area.

The morphology of the interfacial tissue between bighorn sheep horn and bony horncore increases contact surface to enhance strength and facilitate load transfer from the horn to the horncore

The horns of bighorn sheep rams are permanent cranial appendages used for high energy head-to-head impacts during interspecific combat. The horns attach to the underlying bony horncore by a layer of interfacial tissue that facilitates load transfer between the impacted horn and underlying horncore, which has been shown to absorb substantial energy during head impact. However, the morphology and mechanical properties of the interfacial tissue were previously unknown. Histomorphometry was used to quantify the interfacial tissue composition and morphology and lap-shear testing was used to quantify its mechanical properties. Histological analyses revealed the interfacial tissue is a complex network of collagen and keratin fibers, with collagen being the most abundant protein. Sharpey's fibers provide strong attachment between the interfacial tissue and horncore bone. The inner horn surface displayed microscopic porosity and branching digitations which increased the contact surface with the interfacial tissue by approximately 3-fold. Horn-horncore samples tested by lap-shear loading failed primarily at the horn surface, and the interfacial tissue displayed non-linear strain hardening behavior similar to other soft tissues. The elastic properties of the interfacial tissue (i.e., low- and high-strain shear moduli) were comparable to previously measured values for the equine laminar junction. The interfacial tissue contact surface was positively correlated with the interfacial tissue shear strength (1.23 ± 0.21 MPa), high-strain shear modulus (4.5 ± 0.7 MPa), and strain energy density (0.38 ± 0.07 MJ/m3). STATEMENT OF SIGNIFICANCE: The bony horncore in bighorn sheep rams absorbs energy to reduce brain cavity accelerations and mitigate brain injury during head butting. The interfacial zone between the horn and horncore transfers energy from the impacted horn to the energy absorbing horncore but has been largely neglected in previous models of bighorn sheep ramming since interfacial tissue properties were previously unknown. This study quantified the morphology and mechanical properties of the horn-horncore interfacial tissue to better understand structure-property relationships that contribute to energy transfer during ramming. Results from this study will improve models of bighorn sheep ramming used to study mechanisms of brain injury mitigation and may inspire novel materials and structures for brain injury prevention in humans.

In vitro and in vivo degradation behavior of Mg-0.45Zn-0.45Ca (ZX00) screws for orthopedic applications

Magnesium (Mg) alloys have become a potential material for orthopedic implants due to their unnecessary implant removal, biocompatibility, and mechanical integrity until fracture healing. This study examined the in vitro and in vivo degradation of an Mg fixation screw composed of Mg-0.45Zn-0.45Ca (ZX00, in wt.%). With ZX00 human-sized implants, in vitro immersion tests up to 28 days under physiological conditions, along with electrochemical measurements were performed for the first time. In addition, ZX00 screws were implanted in the diaphysis of sheep for 6, 12, and 24 weeks to assess the degradation and biocompatibility of the screws in vivo. Using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), micro-computed tomography (μCT), X-ray photoelectron spectroscopy (XPS), and histology, the surface and cross-sectional morphologies of the corrosion layers formed, as well as the bone-corrosion-layer-implant interfaces, were analyzed. Our findings from in vivo testing demonstrated that ZX00 alloy promotes bone healing and the formation of new bone in direct contact with the corrosion products. In addition, the same elemental composition of corrosion products was observed for in vitro and in vivo experiments; however, their elemental distribution and thicknesses differ depending on the implant location. Our findings suggest that the corrosion resistance was microstructure-dependent. The head zone was the least corrosion-resistant, indicating that the production procedure could impact the corrosion performance of the implant. In spite of this, the formation of new bone and no adverse effects on the surrounding tissues demonstrated that the ZX00 is a suitable Mg-based alloy for temporary bone implants.

Biological approaches to the repair and regeneration of the rotator cuff tendon-bone enthesis: a literature review

Entheses are highly specialised organs connecting ligaments and tendons to bones, facilitating force transmission, and providing mechanical strengths to absorb forces encountered. Two types of entheses, fibrocartilaginous and fibrous, exist in interfaces. The gradual fibrocartilaginous type is in rotator cuff tendons and is more frequently injured due to the poor healing capacity that leads to loss of the original structural and biomechanical properties and is attributed to the high prevalence of retears. Fluctuating methodologies and outcomes of biological approaches are challenges to overcome for them to be routinely used in clinics. Therefore, stratifying the existing literature according to different categories (chronicity, extent of tear, and studied population) would effectively guide repair approaches. This literature review supports tissue engineering approaches to promote rotator cuff enthesis healing employing cells, growth factors, and scaffolds period. Outcomes suggest its promising role in animal studies as well as some clinical trials and that combination therapies are more beneficial than individualized ones. It then highlights the importance of tailoring interventions according to the tear extent, chronicity, and the population being treated. Contributing factors such as loading, deficiencies, and lifestyle habits should also be taken into consideration. Optimum results can be achieved if biological, mechanical, and environmental factors are approached. It is challenging to determine whether variations are due to the interventions themselves, the animal models, loading regimen, materials, or tear mechanisms. Future research should focus on tailoring interventions for different categories to formulate protocols, which would best guide regenerative medicine decision making.

Bioactive Nanostructured Scaffold-Based Approach for Tendon and Ligament Tissue Engineering

An effective therapeutic strategy to treat tendon or ligament injury continues to be a clinical challenge due to the limited natural healing capacity of these tissues. Furthermore, the repaired tendons or ligaments usually possess inferior mechanical properties and impaired functions. Tissue engineering can restore the physiological functions of tissues using biomaterials, cells, and suitable biochemical signals. It has produced encouraging clinical outcomes, forming tendon or ligament-like tissues with similar compositional, structural, and functional attributes to the native tissues. This paper starts by reviewing tendon/ligament structure and healing mechanisms, followed by describing the bioactive nanostructured scaffolds used in tendon and ligament tissue engineering, with emphasis on electrospun fibrous scaffolds. The natural and synthetic polymers for scaffold preparation, as well as the biological and physical cues offered by incorporating growth factors in the scaffolds or by dynamic cyclic stretching of the scaffolds, are also covered. It is expected to present a comprehensive clinical, biological, and biomaterial insight into advanced tissue engineering-based therapeutics for tendon and ligament repair.

Strategies to promote tendon-bone healing after anterior cruciate ligament reconstruction: Present and future

At present, anterior cruciate ligament (ACL) reconstruction still has a high failure rate. Tendon graft and bone tunnel surface angiogenesis and bony ingrowth are the main physiological processes of tendon-bone healing, and also the main reasons for the postoperative efficacy of ACL reconstruction. Poor tendon-bone healing has been also identified as one of the main causes of unsatisfactory treatment outcomes. The physiological process of tendon-bone healing is complicated because the tendon-bone junction requires the organic fusion of the tendon graft with the bone tissue. The failure of the operation is often caused by tendon dislocation or scar healing. Therefore, it is important to study the possible risk factors for tendon-bone healing and strategies to promote it. This review comprehensively analyzed the risk factors contributing to tendon-bone healing failure after ACL reconstruction. Additionally, we discuss the current strategies used to promote tendon-bone healing following ACL reconstruction.

Design and bioprinting for tissue interfaces

Tissue interfaces include complex gradient structures formed by transitioning of biochemical and mechanical properties in micro-scale. This characteristic allows the communication and synchronistic functioning of two adjacent but distinct tissues. It is particularly challenging to restore the function of these complex structures by transplantation of scaffolds exclusively produced by conventional tissue engineering methods. Three-dimensional (3D) bioprinting technology has opened an unprecedented approach for precise and graded patterning of chemical, biological and mechanical cues in a single construct mimicking natural tissue interfaces. This paper reviews and highlights biochemical and biomechanical design for 3D bioprinting of various tissue interfaces, including cartilage-bone, muscle-tendon, tendon/ligament-bone, skin, and neuro-vascular/muscular interfaces. Future directions and translational challenges are also provided at the end of the paper.

Biomaterial-based 3D bioprinting strategy for orthopedic tissue engineering

The advent of three-dimensional (3D) bioprinting has enabled impressive progress in the development of 3D cellular constructs to mimic the structural and functional characteristics of natural tissues. Bioprinting has considerable translational potential in tissue engineering and regenerative medicine. This review highlights the rational design and biofabrication strategies of diverse 3D bioprinted tissue constructs for orthopedic tissue engineering applications. First, we elucidate the fundamentals of 3D bioprinting techniques and biomaterial inks and discuss the basic design principles of bioprinted tissue constructs. Next, we describe the rationale and key considerations in 3D bioprinting of tissues in many different aspects. Thereafter, we outline the recent advances in 3D bioprinting technology for orthopedic tissue engineering applications, along with detailed strategies of the engineering methods and materials used, and discuss the possibilities and limitations of different 3D bioprinted tissue products. Finally, we summarize the current challenges and future directions of 3D bioprinting technology in orthopedic tissue engineering and regenerative medicine. This review not only delineates the representative 3D bioprinting strategies and their tissue engineering applications, but also provides new insights for the clinical translation of 3D bioprinted tissues to aid in prompting the future development of orthopedic implants. STATEMENT OF SIGNIFICANCE: 3D bioprinting has driven major innovations in the field of tissue engineering and regenerative medicine; aiming to develop a functional viable tissue construct that provides an alternative regenerative therapy for musculoskeletal tissue regeneration. 3D bioprinting-based biofabrication strategies could open new clinical possibilities for creating equivalent tissue substitutes with the ability to customize them to meet patient demands. In this review, we summarize the significance and recent advances in 3D bioprinting technology and advanced bioinks. We highlight the rationale for biofabrication strategies using 3D bioprinting for orthopedic tissue engineering applications. Furthermore, we offer ample perspective and new insights into the current challenges and future direction of orthopedic bioprinting translation research.

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.

Magnetic fields enable precise spatial control over electrospun fiber alignment for fabricating complex gradient materials

Musculoskeletal interfacial tissues consist of complex gradients in structure, cell phenotype, and biochemical signaling that are important for function. Designing tissue engineering strategies to mimic these types of gradients is an ongoing challenge. In particular, new fabrication techniques that enable precise spatial control over fiber alignment are needed to better mimic the structural gradients present in interfacial tissues, such as the tendon-bone interface. Here, we report a modular approach to spatially controlling fiber alignment using magnetically-assisted electrospinning. Electrospun fibers were highly aligned in the presence of a magnetic field and smoothly transitioned to randomly aligned fibers away from the magnetic field. Importantly, magnetically-assisted electrospinning allows for spatial control over fiber alignment at sub-millimeter resolution along the length of the fibrous scaffold similar to the native structural gradient present in many interfacial tissues. The versatility of this approach was further demonstrated using multiple electrospinning polymers and different magnet configurations to fabricate complex fiber alignment gradients. As expected, cells seeded onto gradient fibrous scaffolds were elongated and aligned on the aligned fibers and did not show a preferential alignment on the randomly aligned fibers. Overall, this fabrication approach represents an important step forward in creating gradient fibrous materials, where such materials are promising as tissue-engineered scaffolds for regenerating functional musculoskeletal interfacial tissues.

Strain Gradient Programming in 3D Fibrous Hydrogels to Direct Graded Cell Alignment

Biological tissues experience various stretch gradients which act as mechanical signaling from the extracellular environment to cells. These mechanical stimuli are sensed by cells, triggering essential signaling cascades regulating cell migration, differentiation, and tissue remodeling. In most previous studies, a simple, uniform stretch to 2D elastic substrates has been applied to analyze the response of living cells. However, induction of nonuniform strains in controlled gradients, particularly in biomimetic 3D hydrogels, has proven challenging. In this study, 3D fibrin hydrogels of manipulated geometry are stretched by a silicone carrier to impose programmable strain gradients along different chosen axes. The resulting strain gradients are analyzed and compared to finite element simulations. Experimentally, the programmed strain gradients result in similar gradient patterns in fiber alignment within the gels. Additionally, temporal changes in the orientation of fibroblast cells embedded in the stretched fibrin gels correlate to the strain and fiber alignment gradients. The experimental and simulation data demonstrate the ability to custom-design mechanical gradients in 3D biological hydrogels and to control cell alignment patterns. It provides a new technology for mechanobiology and tissue engineering studies.

Progress in Composite Hydrogels and Scaffolds Enriched with Icariin for Osteochondral Defect Healing

Osteochondral structure reconstruction by tissue engineering, a challenge in regenerative medicine, requires a scaffold that ensures both articular cartilage and subchondral bone remodeling. Functional hydrogels and scaffolds present a strategy for the controlled delivery of signaling molecules (growth factors and therapeutic drugs) and are considered a promising therapeutic approach. Icariin is a pharmacologically-active small molecule of prenylated flavonol glycoside and the main bioactive flavonoid isolated from Epimedium spp. The in vitro and in vivo testing of icariin showed chondrogenic and ostseoinductive effects, comparable to bone morphogenetic proteins, and suggested its use as an alternative to growth factors, representing a low-cost, promising approach for osteochondral regeneration. This paper reviews the complex structure of the osteochondral tissue, underlining the main aspects of osteochondral defects and those specifically occurring in osteoarthritis. The significance of icariin's structure and the extraction methods were emphasized. Studies revealing the valuable chondrogenic and osteogenic effects of icariin for osteochondral restoration were also reviewed. The review highlighted th recent state-of-the-art related to hydrogels and scaffolds enriched with icariin developed as biocompatible materials for osteochondral regeneration strategies.

Recent Developments and Current Applications of Organic Nanomaterials in Cartilage Repair

Regeneration of cartilage is difficult due to the unique microstructure, unique multizone organization, and avascular nature of cartilage tissue. The development of nanomaterials and nanofabrication technologies holds great promise for the repair and regeneration of injured or degenerated cartilage tissue. Nanomaterials have structural components smaller than 100 nm in at least one dimension and exhibit unique properties due to their nanoscale structure and high specific surface area. The unique properties of nanomaterials include, but are not limited to, increased chemical reactivity, mechanical strength, degradability, and biocompatibility. As an emerging nanomaterial, organic nanocomposites can mimic natural cartilage in terms of microstructure, physicochemical, mechanical, and biological properties. The integration of organic nanomaterials is expected to develop scaffolds that better mimic the extracellular matrix (ECM) environment of cartilage to enhance scaffold-cell interactions and improve the functionality of engineered tissue constructs. Next-generation hydrogel technology and bioprinting can be used not only for healing cartilage injury areas but also for extensive osteoarthritic degenerative changes within the joint. Although more challenges need to be solved before they can be translated into full-fledged commercial products, nano-organic composites remain very promising candidates for the future development of cartilage tissue engineering.

Biomechanical Aspects of Osteochondral Regeneration: Implications and Strategies for Three-Dimensional Bioprinting

Osteoarthritis is among the most prevalent of musculoskeletal disorders in the world that causes joint pain, deformity, and limited range of movement. The resulting osteochondral defect can significantly decrease the patient's quality of life, but current treatment options have not demonstrated the capacity to fully regenerate the entire osteochondral microenvironment. Structurally, the osteochondral unit is a composite system composed of three layers-articular cartilage, calcified cartilage, and subchondral bone. Collectively these distinct layers contribute to the distinct biomechanical properties that maintain the health and aid in load transfer during joint articulation. The purpose of this review was to examine the role of the osteochondral interface in tissue engineering. Topics of discussion include the biomechanics of the osteochondral unit and an overview of various strategies for osteochondral interface tissue engineering, with a specific focus on three-dimensional bioprinting. The goal of this review was to elucidate the importance of the osteochondral interface and overview some strategies of developing an interface layer within tissue engineered scaffolds. Impact Statement This review provides an overview of interface tissue engineering for osteochondral regeneration. It offers a detailed investigation into the biomechanics of the osteochondral unit as it relates to tissue engineering, and highlights the strategies that have been utilized to develop the osteochondral interface within tissue engineering scaffolds.

Recent Advances of Natural Polysaccharide-based Double-network Hydrogels for Tissue Repair

Natural polysaccharide hydrogels have been extensively explored for many years due to their outstanding biocompatibility and biodegradability, which are very promising candidates as artificial soft materials for biomedical applications. However, their inferior mechanical performances greatly limited their applications. Introduction of double-network (DN) structure has been well documented to be an efficient strategy for significant improvement of the mechanical property of hydrogels. Here, recent progress of natural polysaccharide-based DN hydrogels is reviewed from the perspective of fundamental concepts on both design rationale and preparation strategies to biomedical application in tissue repair. Retrospect of the DN-strengthened polysaccharide hydrogels can give a deep insight into the fundamental relationship of such bio-based hydrogels among structural design, mechanical properties and practical demands, thereby prompting their translation to clinical application prospects.

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.

Biomaterials and biotechnology for periodontal tissue regeneration: Recent advances and perspectives

Periodontal tissues are organized in a complex three-dimensional (3D) architecture, including the alveolar bone, cementum, and a highly aligned periodontal ligament (PDL). Regeneration is difficult due to the complex structure of these tissues. Currently, materials are developing rapidly, among which synthetic polymers and hydrogels have extensive applications. Moreover, techniques have made a spurt of progress. By applying guided tissue regeneration (GTR) to hydrogels and cell sheets and using 3D printing, a scaffold with an elaborate biomimetic structure can be constructed to guide the orientation of fibers. The incorporation of cells and biotic factors improves regeneration. Nevertheless, the current studies lack long-term effect tracking, clinical research, and in-depth mechanistic research. In summary, periodontal tissue engineering still has considerable room for development. The development of materials and techniques and an in-depth study of the mechanism will provide an impetus for periodontal regeneration.

Advances in Stem Cell Therapies for Rotator Cuff Injuries

Rotator cuff injury is a common upper extremity musculoskeletal disease that may lead to persistent pain and functional impairment. Despite the clinical outcomes of the surgical procedures being satisfactory, the repair of the rotator cuff remains problematic, such as through failure of healing, adhesion formation, and fatty infiltration. Stem cells have high proliferation, strong paracrine action, and multiple differentiation potential, which promote tendon remodeling and fibrocartilage formation and increase biomechanical strength. Additionally, stem cell-derived extracellular vesicles (EVs) can increase collagen synthesis and inhibit inflammation and adhesion formation by carrying regulatory proteins and microRNAs. Therefore, stem cell-based therapy is a promising therapeutic strategy that has great potential for rotator cuff healing. In this review, we summarize the advances of stem cells and stem cell-derived EVs in rotator cuff repair and highlight the underlying mechanism of stem cells and stem cell-derived EVs and biomaterial delivery systems. Future studies need to explore stem cell therapy in combination with cellular factors, gene therapy, and novel biomaterial delivery systems.

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.

Aligned microribbon scaffolds with hydroxyapatite gradient for engineering bone-tendon interface

Injuries of the bone-to-tendon interface, such as rotator cuff and anterior cruciate ligament tears, are prevalent yet effective methods for repair remain elusive. Tissue engineering approaches that use cells and biomaterials offer a promising potential solution for engineering the bone-tendon interface, but previous strategies require seeding multiple cell types and use of multiphasic scaffolds to achieve zonal-specific tissue phenotype. Furthermore, mimicking the aligned tissue morphology present in native bone-tendon interface in 3D remains challenging. To facilitate clinical translation, engineering bone-tendon interface using a single cell source and one continuous scaffold with alignment cues would be more attractive, but has not been achieved before. To address these unmet needs, here we develop an aligned gelatin-microribbon (μRB) hydrogel scaffold with hydroxyapatite nanoparticle (HA-np) gradient for guiding zonal-specific differentiation of human mesenchymal stem cell (hMSC) to mimic the bone-tendon interface. We demonstrate aligned μRBs led to cell alignment in 3D, and HA gradient induced zonal-specific differentiation of MSCs that resembles the transition at the bone-tendon interface. Short chrondrogenic priming prior to exposure to osteogenic factors further enhanced the mimicry of bone-cartilage-tendon transition with significantly improved tensile moduli of the resulting tissues. In summary, aligned gelatin μRBs with HA gradient coupled with optimized soluble factors may offer a promising strategy for engineering bone-tendon interface using a single cell source.

Computational investigation of interface printing patterns within 3D printed multilayered scaffolds for osteochondral tissue engineering

Osteoarthritis is a highly prevalent rheumatic musculoskeletal disorder that commonly affects many joints. Repetitive joint overloading perpetuates the damage to the affected cartilage, which undermines the structural integrity of the osteochondral unit. Various tissue engineering strategies have been employed to design multiphasic osteochondral scaffolds that recapitulate layer-specific biomechanical properties, but the inability to fully satisfy mechanical demands within the joint has limited their success. Through computational modeling and extrusion-based bioprinting, we attempted to fabricate a biphasic osteochondral scaffold with improved shear properties and a mechanically strong interface. A 3D stationary solid mechanics model was developed to simulate the effect of lateral shear force on various thermoplastic polymer/hydrogel scaffolds with a patterned interface. Additionally, interfacial shear tests were performed on bioprinted polycaprolactone (PCL)/hydrogel interface scaffolds. The first simulation showed that the PCL/gelatin methacrylate (GelMA) and PCL/polyethylene glycol diacrylate (PEGDA) scaffolds interlocking hydrogel and PCL at interface in a 1:1 ratio possessed the largest average tensile (PCL/GelMA: 80.52 kPa; PCL/PEGDA: 79.75 kPa) and compressive stress (PCL/GelMA: 74.71 kPa; PCL/PEGDA: 73.83 kPa). Although there were significant differences in shear strength between PCL/GelMA and PCL/PEGDA scaffolds, no significant difference was observed among the treatment groups within both scaffold types. Lastly, the hypothetical simulations of potential biphasic 3D printed scaffolds showed that for every order of magnitude decrease in Young's modulus (E) of the soft bioink, all the scaffolds underwent an exponential increase in average displacement at the cartilage and interface layers. The following work provides valuable insights into the biomechanics of 3D printed osteochondral scaffolds, which will help inform future scaffold designs for enhanced regenerative outcomes.

3D Printing for Bone-Cartilage Interface Regeneration

Due to the vasculature defects and/or the avascular nature of cartilage, as well as the complex gradients for bone-cartilage interface regeneration and the layered zonal architecture, self-repair of cartilage and subchondral bone is challenging. Currently, the primary osteochondral defect treatment strategies, including artificial joint replacement and autologous and allogeneic bone graft, are limited by their ability to simply repair, rather than induce regeneration of tissues. Meanwhile, over the past two decades, three-dimension (3D) printing technology has achieved admirable advancements in bone and cartilage reconstruction, providing a new strategy for restoring joint function. The advantages of 3D printing hybrid materials include rapid and accurate molding, as well as personalized therapy. However, certain challenges also exist. For instance, 3D printing technology for osteochondral reconstruction must simulate the histological structure of cartilage and subchondral bone, thus, it is necessary to determine the optimal bioink concentrations to maintain mechanical strength and cell viability, while also identifying biomaterials with dual bioactivities capable of simultaneously regenerating cartilage. The study showed that the regeneration of bone-cartilage interface is crucial for the repair of osteochondral defect. In this review, we focus on the significant progress and application of 3D printing technology for bone-cartilage interface regeneration, while also expounding the potential prospects for 3D printing technology and highlighting some of the most significant challenges currently facing this field.

Cartilage Tissue Engineering Approaches Need to Assess Fibrocartilage When Hydrogel Constructs Are Mechanically Loaded

Chondrocytes that are impregnated within hydrogel constructs sense applied mechanical force and can respond by expressing collagens, which are deposited into the extracellular matrix (ECM). The intention of most cartilage tissue engineering is to form hyaline cartilage, but if mechanical stimulation pushes the ratio of collagen type I (Col1) to collagen type II (Col2) in the ECM too high, then fibrocartilage can form instead. With a focus on Col1 and Col2 expression, the first part of this article reviews the latest studies on hyaline cartilage regeneration within hydrogel constructs that are subjected to compression forces (one of the major types of the forces within joints) in vitro. Since the mechanical loading conditions involving compression and other forces in joints are difficult to reproduce in vitro, implantation of hydrogel constructs in vivo is also reviewed, again with a focus on Col1 and Col2 production within the newly formed cartilage. Furthermore, mechanotransduction pathways that may be related to the expression of Col1 and Col2 within chondrocytes are reviewed and examined. Also, two recently-emerged, novel approaches of load-shielding and synchrotron radiation (SR)–based imaging techniques are discussed and highlighted for future applications to the regeneration of hyaline cartilage. Going forward, all cartilage tissue engineering experiments should assess thoroughly whether fibrocartilage or hyaline cartilage is formed.

Biomimetic Gradient Scaffolds Containing Hyaluronic Acid and Sr/Zn Folates for Osteochondral Tissue Engineering

Regenerative therapies based on tissue engineering are becoming the most promising alternative for the treatment of osteoarthritis and rheumatoid arthritis. However, regeneration of full-thickness articular osteochondral defects that reproduces the complexity of native cartilage and osteochondral interface still remains challenging. Hence, in this work, we present the fabrication, physic-chemical characterization, and in vitro and in vivo evaluation of biomimetic hierarchical scaffolds that mimic both the spatial organization and composition of cartilage and the osteochondral interface. The scaffold is composed of a composite porous support obtained by cryopolymerization of poly(ethylene glycol) dimethacrylate (PEGDMA) in the presence of biodegradable poly(D,L-lactide-co-glycolide) (PLGA), bioactive tricalcium phosphate β-TCP and the bone promoting strontium folate (SrFO), with a gradient biomimetic photo-polymerized methacrylated hyaluronic acid (HAMA) based hydrogel containing the bioactive zinc folic acid derivative (ZnFO). Microscopical analysis of hierarchical scaffolds showed an open interconnected porous open microstructure and the in vitro behaviour results indicated high swelling capacity with a sustained degradation rate. In vitro release studies during 3 weeks indicated the sustained leaching of bioactive compounds, i.e., Sr2+, Zn2+ and folic acid, within a biologically active range without negative effects on human osteoblast cells (hOBs) and human articular cartilage cells (hACs) cultures. In vitro co-cultures of hOBs and hACs revealed guided cell colonization and proliferation according to the matrix microstructure and composition. In vivo rabbit-condyle experiments in a critical-sized defect model showed the ability of the biomimetic scaffold to promote the regeneration of cartilage-like tissue over the scaffold and neoformation of osteochondral tissue.

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.

An injectable hydrogel having proteoglycan-like hierarchical structure supports chondrocytes delivery and chondrogenesis

The ECM of cartilage is composed of proteoglycans (PG) that contain glycosaminoglycan (GAG), aggrecan, hyaluronic acid (HA) and other molecular components which play an important role in regulating chondrocyte functions via cell-matrix interactions, integrin-mediated signalling etc. Implantation of chondrocytes encapsulated in scaffolds that mimic the micro-architecture of proteoglycan, is expected to enhance cartilage repair. With an aim to create a hydrogel having macromolecular structure that resembles the cartilage-specific ECM, we constructed a hierarchal structure that mimic the PG. The bottle brush structure of the aggrecan was obtained using chondroitin sulphate and carboxymethyl cellulose which served as GAG and core protein mimic respectively. A proteoglycan-like structure was obtained by cross-linking it with modified chitosan that served as a HA substitute. The physico-chemical characteristics of the above cross-linked injectable hydrogel supported long term human articular chondrocyte subsistence and excellent post-injection viability. The chondrocytes encapsulated in the PMH expressed significant levels of articular cartilage specific markers like collagen II, aggrecan, GAGs etc., indicating the ability of the hydrogel to support chondrocyte differentiation. The biocompatibility and biodegradability of the hydrogels was confirmed using suitable in vivo studies. The results revealed that the PG-mimetic hydrogel could serve as a promising scaffold for chondrocyte implantation.

Spatial organization of biochemical cues in 3D-printed scaffolds to guide osteochondral tissue engineering

Functional repair of osteochondral (OC) tissue remains challenging because the transition from bone to cartilage presents gradients in biochemical and physical properties necessary for joint function. Osteochondral regeneration requires strategies that restore the spatial composition and organization found in the native tissue. Several biomaterial approaches have been developed to guide chondrogenic and osteogenic differentiation of human mesenchymal stem cells (hMSCs). These strategies can be combined with 3D printing, which has emerged as a useful tool to produce tunable, continuous scaffolds functionalized with bioactive cues. However, functionalization often includes one or more post-fabrication processing steps, which can lead to unwanted side effects and often produce biomaterials with homogeneously distributed chemistries. To address these challenges, surface functionalization can be achieved in a single step by solvent-cast 3D printing peptide-functionalized polymers. Peptide-poly(caprolactone) (PCL) conjugates were synthesized bearing hyaluronic acid (HA)-binding (HAbind-PCL) or mineralizing (E3-PCL) peptides, which have been shown to promote hMSC chondrogenesis or osteogenesis, respectively. This 3D printing strategy enables unprecedented control of surface peptide presentation and spatial organization within a continuous construct. Scaffolds presenting both cartilage-promoting and bone-promoting peptides had a synergistic effect that enhanced hMSC chondrogenic and osteogenic differentiation in the absence of differentiation factors compared to scaffolds without peptides or only one peptide. Furthermore, multi-peptide organization significantly influenced hMSC response. Scaffolds presenting HAbind and E3 peptides in discrete opposing zones promoted hMSC osteogenic behavior. In contrast, presenting both peptides homogeneously throughout the scaffolds drove hMSC differentiation towards a mixed population of articular and hypertrophic chondrocytes. These significant results indicated that hMSC behavior was driven by dual-peptide presentation and organization. The downstream potential of this platform is the ability to fabricate biomaterials with spatially controlled biochemical cues to guide functional tissue regeneration without the need for differentiation factors.

Engineering Musculoskeletal Grafts for Multi-Tissue Unit Repair: Lessons From Developmental Biology and Wound Healing

In the musculoskeletal system, bone, tendon, and skeletal muscle integrate and act coordinately as a single multi-tissue unit to facilitate body movement. The development, integration, and maturation of these essential components and their response to injury are vital for conferring efficient locomotion. The highly integrated nature of these components is evident under disease conditions, where rotator cuff tears at the bone-tendon interface have been reported to be associated with distal pathological alterations such as skeletal muscle degeneration and bone loss. To successfully treat musculoskeletal injuries and diseases, it is important to gain deep understanding of the development, integration and maturation of these musculoskeletal tissues along with their interfaces as well as the impact of inflammation on musculoskeletal healing and graft integration. This review highlights the current knowledge of developmental biology and wound healing in the bone-tendon-muscle multi-tissue unit and perspectives of what can be learnt from these biological and pathological processes within the context of musculoskeletal tissue engineering and regenerative medicine. Integrating these knowledge and perspectives can serve as guiding principles to inform the development and engineering of musculoskeletal grafts and other tissue engineering strategies to address challenging musculoskeletal injuries and diseases.

Osteochondral Tissue Engineering: The Potential of Electrospinning and Additive Manufacturing

The socioeconomic impact of osteochondral (OC) damage has been increasing steadily over time in the global population, and the promise of tissue engineering in generating biomimetic tissues replicating the physiological OC environment and architecture has been falling short of its projected potential. The most recent advances in OC tissue engineering are summarised in this work, with a focus on electrospun and 3D printed biomaterials combined with stem cells and biochemical stimuli, to identify what is causing this pitfall between the bench and the patients' bedside. Even though significant progress has been achieved in electrospinning, 3D-(bio)printing, and induced pluripotent stem cell (iPSC) technologies, it is still challenging to artificially emulate the OC interface and achieve complete regeneration of bone and cartilage tissues. Their intricate architecture and the need for tight spatiotemporal control of cellular and biochemical cues hinder the attainment of long-term functional integration of tissue-engineered constructs. Moreover, this complexity and the high variability in experimental conditions used in different studies undermine the scalability and reproducibility of prospective regenerative medicine solutions. It is clear that further development of standardised, integrative, and economically viable methods regarding scaffold production, cell selection, and additional biochemical and biomechanical stimulation is likely to be the key to accelerate the clinical translation and fill the gap in OC treatment.

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.

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.

Ectopic models recapitulating morphological and functional features of articular cartilage

BACKGROUND

Articular cartilage is an extremely specialized connective tissue which covers all diarthrodial joints. Implantation of chondrogenic cells without or with additional biomaterial scaffolds in ectopic locationsin vivo generates substitutes of cartilage with structural and functional characteristics that are used in fundamental investigations while also serving as a basis for translational studies.

METHODS

Literature search in Pubmed.

RESULTS AND DISCUSSION

This narrative review summarizes the most relevant ectopic models, among which subcutaneous, intramuscular, and kidney capsule transplantation and elaborates on implanted cells and biomaterial scaffolds and on their use to recapitulate morphological and functional features of articular cartilage. Although the absence of a physiological joint environment and biomechanical stimuli is the major limiting factor, ectopic models are an established component for articular cartilage research aiming to generate a bridge between in vitro data and the clinically more relevant translational orthotopic in vivo models when their limitations are considered.

Tissue Engineering for the Insertions of Tendons and Ligaments: An Overview of Electrospun Biomaterials and Structures

The musculoskeletal system is composed by hard and soft tissue. These tissues are characterized by a wide range of mechanical properties that cause a progressive transition from one to the other. These material gradients are mandatory to reduce stress concentrations at the junction site. Nature has answered to this topic developing optimized interfaces, which enable a physiological transmission of load in a wide area over the junction. The interfaces connecting tendons and ligaments to bones are called entheses, while the ones between tendons and muscles are named myotendinous junctions. Several injuries can affect muscles, bones, tendons, or ligaments, and they often occur at the junction sites. For this reason, the main aim of the innovative field of the interfacial tissue engineering is to produce scaffolds with biomaterial gradients and mechanical properties to guide the cell growth and differentiation. Among the several strategies explored to mimic these tissues, the electrospinning technique is one of the most promising, allowing to generate polymeric nanofibers similar to the musculoskeletal extracellular matrix. Thanks to its extreme versatility, electrospinning has allowed the production of sophisticated scaffolds suitable for the regeneration of both the entheses and the myotendinous junctions. The aim of this review is to analyze the most relevant studies that applied electrospinning to produce scaffolds for the regeneration of the enthesis and the myotendinous junction, giving a comprehensive overview on the progress made in the field, in particular focusing on the electrospinning strategies to produce these scaffolds and their mechanical, in vitro, and in vivo outcomes.

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.

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.

Osteochondral Injury, Management and Tissue Engineering Approaches

Osteochondral lesions (OL) are a common clinical problem for orthopedic surgeons worldwide and are associated with multiple clinical scenarios ranging from trauma to osteonecrosis. OL vary from chondral lesions in that they involve the subchondral bone and chondral surface, making their management more complex than an isolated chondral injury. Subchondral bone involvement allows for a natural healing response from the body as marrow elements are able to come into contact with the defect site. However, this repair is inadequate resulting in fibrous scar tissue. The second differentiating feature of OL is that damage to the subchondral bone has deleterious effects on the mechanical strength and nutritive capabilities to the chondral joint surface. The clinical solution must, therefore, address both the articular cartilage as well as the subchondral bone beneath it to restore and preserve joint health. Both cartilage and subchondral bone have distinctive functional requirements and therefore their physical and biological characteristics are very much dissimilar, yet they must work together as one unit for ideal joint functioning. In the past, the obvious solution was autologous graft transfer, where an osteochondral bone plug was harvested from a non-weight bearing portion of the joint and implanted into the defect site. Allografts have been utilized similarly to eliminate the donor site morbidity associated with autologous techniques and overall results have been good but both techniques have their drawbacks and limitations. Tissue engineering has thus been an attractive option to create multiphasic scaffolds and implants. Biphasic and triphasic implants have been under explored and have both a chondral and subchondral component with an interface between the two to deliver an implant which is biocompatible and emulates the osteochondral unit as a whole. It has been a challenge to develop such implants and many manufacturing techniques have been utilized to bring together two unalike materials and combine them with cellular therapies. We summarize the functions of the osteochondral unit and describe the currently available management techniques under study.

Regeneration of Damaged Tendon-Bone Junctions (Entheses)-TAK1 as a Potential Node Factor

Musculoskeletal dysfunctions are highly prevalent due to increasing life expectancy. Consequently, novel solutions to optimize treatment of patients are required. The current major research focus is to develop innovative concepts for single tissues. However, interest is also emerging to generate applications for tissue transitions where highly divergent properties need to work together, as in bone-cartilage or bone-tendon transitions. Finding medical solutions for dysfunctions of such tissue transitions presents an added challenge, both in research and in clinics. This review aims to provide an overview of the anatomical structure of healthy adult entheses and their development during embryogenesis. Subsequently, important scientific progress in restoration of damaged entheses is presented. With respect to enthesis dysfunction, the review further focuses on inflammation. Although molecular, cellular and tissue mechanisms during inflammation are well understood, tissue regeneration in context of inflammation still presents an unmet clinical need and goes along with unresolved biological questions. Furthermore, this review gives particular attention to the potential role of a signaling mediator protein, transforming growth factor beta-activated kinase-1 (TAK1), which is at the node of regenerative and inflammatory signaling and is one example for a less regarded aspect and potential important link between tissue regeneration and inflammation.

Transforming Nanofiber Mats into Hierarchical Scaffolds with Graded Changes in Porosity and/or Nanofiber Alignment

A method for generating hierarchical scaffolds with graded changes in porosity and/or fiber alignment through solution-masked, vapor-induced welding of electrospun poly(lactic-co-glycolic acid) (PLGA) nanofibers is reported. The success of this method relies on the fact that the PLGA nanofibers are swollen and welded more slowly by ethanol when immersed in its aqueous solution relative to direct exposure to its vapor. For a mat composed of random nanofibers, the treatment generates a gradation in porosity (both surface and bulk), with the over-welded region evolving from a highly porous mat into a dense film. If uniaxially aligned nanofibers are involved, however, graded changes are observed in both surface porosity and fiber alignment. When bone marrow stem cells are cultured on such a scaffold, they exhibit highly organized and random morphologies on the regions of uniaxially aligned nanofibers and dense film, respectively, with gradual changes in between. Such a scaffold shows promise in mimicking the connective tissue, such as the tendon-to-bone insertion, that relies on a graded transition in cell morphology from uniaxially aligned to random.

Specialty Tough Hydrogels and Their Biomedical Applications

Hydrogels have long been explored as attractive materials for biomedical applications given their outstanding biocompatibility, high water content, and versatile fabrication platforms into materials with different physiochemical properties and geometries. Nonetheless, conventional hydrogels suffer from weak mechanical properties, restricting their use in persistent load-bearing applications often required of materials used in medical settings. Thus, the fabrication of mechanically robust hydrogels that can prolong the lifetime of clinically suitable materials under uncompromising in vivo conditions is of great interest. This review focuses on design considerations and strategies to construct such tough hydrogels. Several promising advances in the proposed use of specialty tough hydrogels for soft actuators, drug delivery vehicles, adhesives, coatings, and in tissue engineering settings are highlighted. While challenges remain before these specialty tough hydrogels will be deemed translationally acceptable for clinical applications, promising preliminary results undoubtedly spur great hope in the potential impact this embryonic research field can have on the biomedical community.