A radiopaque electrospun scaffold for engineering fibrous musculoskeletal tissues: Scaffold characterization and in vivo applications

Recent Progress in Electrospun Nanofiber-Based Degenerated Intervertebral Disc Repair

Annulus fibrosus fissure and fibrosis of nucleus pulposus are severe morphological characteristics of intervertebral disc degeneration. Currently, surgery or drugs are used to relieve pain in such cases. Tissue engineering is a new multidisciplinary strategy with great potential for use in joint replacement and organ regeneration. Based on the natural anatomy of intervertebral discs, intervertebral disc scaffolds are fabricated by exploiting the special arrangement of extracellular matrix fibers. Electrospun nanofibers possess clear advantages in repairing degenerated intervertebral discs. This article reviews and summarizes recently developed methods for improving and fabricating electrospun nanofiber annulus fibrosus scaffolds in terms of nanofiber alignment, material selection, loading additives, and the progress made in combining other advanced technologies with electrospun nanofibers. In addition, the improvement in mechanical properties and biocompatibility of nucleus pulposus scaffolds by electrospun nanofiber-reinforced hydrogels is discussed. Finally, complete intervertebral disc scaffolds can be fabricated using the disc-like angle-ply structure and other emerging fabrication methods. Taken together, electrospun nanofiber intervertebral disc scaffolds are promising for clinical applications.

Photo-Cross-Linkable Human Albumin Colloidal Gels Facilitate In Vivo Vascular Integration for Regenerative Medicine

Biodegradable cellular and acellular scaffolds have great potential to regenerate damaged tissues or organs by creating a proper extracellular matrix (ECM) capable of recruiting endogenous cells to support cellular ingrowth. However, since hydrogel-based scaffolds normally degrade through surface erosion, cell migration and ingrowth into scaffolds might be inhibited early in the implantation. This could result in insufficient de novo tissue formation in the injured area. To address these challenges, continuous and microsized strand-like networks could be incorporated into scaffolds to guide and recruit endogenous cells in rapid manner. Fabrication of such microarchitectures in scaffolds is often a laborious and time-consuming process and could compromise the structural integrity of the scaffold or impact cell viability. Here, we have developed a fast single-step approach to fabricate colloidal hydrogels, which are made up of randomly packed human serum albumin-based photo-cross-linkable microparticles with continuous internal networks of microscale voids. The human serum albumin conjugated with methacrylic groups were assembled to microsized aggregates for achieving unique porous structures inside the colloidal gels. The albumin hydrogels showed tunable mechanical properties such as elastic modulus, porosity, and biodegradability, providing a suitable ECM for various cells such as cardiomyoblasts and endothelial cells. In addition, the encapsulated cells within the hydrogel showed improved cell retention and increased survivability in vitro. Microporous structures of the colloidal gels can serve as a guide for the infiltration of host cells upon implantation, achieving rapid recruitment of hematopoietic cells and, ultimately, enhancing the tissue regeneration capacity of implanted scaffolds.

Three-Dimensional-Printed Flexible Scaffolds Have Tunable Biomimetic Mechanical Properties for Intervertebral Disc Tissue Engineering

The intervertebral disc (IVD) exhibits complex structure and biomechanical function, which supports the weight of the body and permits motion. Surgical treatments for IVD degeneration (e.g., lumbar fusion, disc replacement) often disrupt the mechanical environment of the spine which lead to adjacent segment disease. Alternatively, disc tissue engineering strategies, where cell-seeded hydrogels or fibrous biomaterials are cultured in vitro to promote matrix deposition, do not recapitulate the complex IVD mechanical properties. In this study, we use 3D printing of flexible polylactic acid (FPLA) to fabricate a viscoelastic scaffold with tunable biomimetic mechanics for whole spine motion segment applications. We optimized the mechanical properties of the scaffolds for equilibrium and dynamic moduli in compression and tension by varying fiber spacing or porosity, generating scaffolds with de novo mechanical properties within the physiological range of spine motion segments. The biodegradation analysis of the 3D printed scaffolds showed that FPLA exhibits lower degradation rate and thus has longer mechanical stability than standard PLA. FPLA scaffolds were biocompatible, supporting viability of nucleus pulposus (NP) cells in 2D and in FPLA+hydrogel composites. Composite scaffolds cultured with NP cells maintained baseline physiological mechanical properties and promoted matrix deposition up to 8 weeks in culture. Mesenchymal stromal cells (MSCs) cultured on FPLA adhered to the scaffold and exhibited fibrocartilaginous differentiation. These results demonstrate for the first time that 3D printed FPLA scaffolds have de novo viscoelastic mechanical properties that match the native IVD motion segment in both tension and compression and have the potential to be used as a mechanically stable and biocompatible biomaterial for engineered disc replacement.

Evaluation of Tissue Engineering Approaches for Intervertebral Disc Regeneration in Relevant Animal Models

Translation of tissue engineering strategies for the regeneration of intervertebral disc (IVD) requires a strong understanding of pathophysiology through the relevant animal model. There is no relevant animal model due to differences in disc anatomy, cellular composition, extracellular matrix components, disc physiology, and mechanical strength from humans. However, available animal models if used correctly could provide clinically relevant information for the translation into humans. In this review, we have investigated different types of strategies for the development of clinically relevant animal models to study biomaterials, cells, biomolecular or their combination in developing tissue engineering-based treatment strategies. Tissue engineering strategies that utilize various animal models for IVD regeneration are summarized and outcomes have been discussed. The understanding of animal models for the validation of regenerative approaches is employed to understand and treat the pathophysiology of degenerative disc disease (DDD) before proceeding for human trials. These animal models play an important role in building a therapeutic regime for IVD tissue regeneration, which can serve as a platform for clinical applications.

Intrinsically radiopaque biomaterial assortments: a short review on the physical principles, X-ray imageability, and state-of-the-art developments

X-ray attenuation ability, otherwise known as radiopacity of a material, could be indisputably tagged as the central and decisive parameter that produces contrast in an X-ray image. Radiopaque biomaterials are vital in the healthcare sector that helps clinicians to track them unambiguously during pre and post interventional radiological procedures. Medical imaging is one of the most powerful resources in the diagnostic sector that aids improved treatment outcomes for patients. Intrinsically radiopaque biomaterials enable themselves for visual targeting/positioning as well as to monitor their fate and further provide the radiologists with critical insights about the surgical site. Moreover, the emergence of advanced real-time imaging modalities is a boon to the contemporary healthcare systems that allow to perform minimally invasive surgical procedures and thereby reduce the healthcare costs and minimize patient trauma. X-ray based imaging is one such technologically upgraded diagnostic tool with many variants like digital X-ray, computed tomography, digital subtraction angiography, and fluoroscopy. In light of these facts, this review is aimed to briefly consolidate the physical principles of X-ray attenuation by a radiopaque material, measurement of radiopacity, classification of radiopaque biomaterials, and their recent advanced applications.

Advances in electrospun scaffolds for meniscus tissue engineering and regeneration

The meniscus plays a critical role in maintaining the homeostasis, biomechanics, and structural stability of the knee joint. Unfortunately, it is predisposed to damages either from sports-related trauma or age-related degeneration. The meniscus has an inherently limited capacity for tissue regeneration. Self-healing of injured adult menisci only occurs in the peripheral vascularized portion, while the spontaneous repair of the inner avascular region seems never happens. Repair, replacement, and regeneration of menisci through tissue engineering strategies are promising to address this problem. Recently, many scaffolds for meniscus tissue engineering have been proposed for both experimental and preclinical investigations. Electrospinning is a feasible and versatile technique to produce nano- to micro-scale fibers that mimic the microarchitecture of native extracellular matrix and is an effective approach to prepare nanofibrous scaffolds for constructing engineered meniscus. Electrospun scaffolds are reported to be capable of inducing colonization of meniscus cells by modulating local extracellular density and stimulating endogenous regeneration by driving reprogramming of meniscus wound microenvironment. Electrospun nanofibrous scaffolds with tunable mechanical properties, controllable anisotropy, and various porosities have shown promises for meniscus repair and regeneration and will undoubtedly inspire more efforts in exploring effective therapeutic approaches towards clinical applications. In this article, we review the current advances in the use of electrospun nanofibrous scaffolds for meniscus tissue engineering and repair and discuss prospects for future studies.

Rapid Fabrication of MgNH4PO4·H2O/SrHPO4 Porous Composite Scaffolds with Improved Radiopacity via 3D Printing Process

Although bone repair scaffolds are required to possess high radiopacity to be distinguished from natural bone tissues in clinical applications, the intrinsic radiopacity of them is usually insufficient. For improving the radiopacity, combining X-ray contrast agents with bone repair scaffolds is an effective method. In the present research, MgNH₄PO₄·H₂O/SrHPO₄ 3D porous composite scaffolds with improved radiopacity were fabricated via the 3D printing technique. Here, SrHPO₄ was firstly used as a radiopaque agent to improve the radiopacity of magnesium phosphate scaffolds. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) were used to characterize the phases, morphologies, and element compositions of the 3D porous composite scaffolds. The radiography image showed that greater SrHPO₄ contents corresponded to higher radiopacity. When the SrHPO₄ content reached 9.34%, the radiopacity of the composite scaffolds was equal to that of a 6.8 mm Al ladder. The porosity and in vitro degradation of the porous composite scaffolds were studied in detail. The results show that magnesium phosphate scaffolds with various Sr contents could sustainably degrade and release the Mg, Sr, and P elements during the experiment period of 28 days. In addition, the cytotoxicity on MC3T3-E1 osteoblast precursor cells was evaluated, and the results show that the porous composite scaffolds with a SrHPO₄ content of 9.34% possessed superior cytocompatibility compared to that of the pure MgNH₄PO₄·H₂O scaffolds when the extract concentration was 0.1 g/mL. Cell adhesion experiments showed that all of the scaffolds could support MC3T3-E1 cellular attachment well. This research indicates that MgNH₄PO₄·H₂O/SrHPO₄ porous composite scaffolds have potential applications in the bone repair fields.

The porcine accessory carpal bone as a model for biologic joint replacement for trapeziometacarpal osteoarthritis

Given its complex shape and relatively small size, the trapezium surface at the trapeziometacarpal (TMC) joint is a particularly attractive target for anatomic biologic joint resurfacing, especially given its propensity to develop osteoarthritis, and the limited and sub-optimal treatment options available. For this to advance to clinical translation, however, an appropriate large animal model is required. In this study, we explored the porcine accessory carpal bone (ACB) as a model for the human trapezium. We characterized ACB anatomy, geometry, joint and tissue-scale mechanics, and composition across multiple donors. We showed that the ACB is similar both in size, and in the saddle shape of the main articulating surface to the human trapezium, and that loads experienced across each joint are similar. Using this information, we then devised a fabrication method and workflow to produce patient-specific tissue-engineered replicas based on CT scans, and showed that when such replicas are implanted orthotopically in an ex vivo model, normal loading is restored. Data from this study establish the porcine ACB as a model system in which to evaluate function of engineered living joint resurfacing strategies. STATEMENT OF SIGNIFICANCE: Biologic joint resurfacing, or the replacement of a joint with living tissue as opposed to metal and plastic, is the holy grail of orthopaedic tissue engineering. However, despite marked advances in engineering native-like osteochondral tissues and in matching patient-specific anatomy, these technologies have not yet reached clinical translation. Given its propensity for developing osteoarthritis, as well as its small size and complex shape, the trapezial surface of the trapeziometacarpal joint at the base of the thumb presents a unique opportunity for pursuing a biologic joint resurfacing strategy. This work establishes the porcine accessory carpal bone as an animal model for the human trapezium and presents a viable test-bed for evaluating the function of engineered living joint resurfacing strategies.

Proper animal experimental designs for preclinical research of biomaterials for intervertebral disc regeneration

Low back pain is a vital musculoskeletal disease that impairs life quality, leads to disability and imposes heavy economic burden on the society, while it is greatly attributed to intervertebral disc degeneration (IDD). However, the existing treatments, such as medicines, chiropractic adjustments and surgery, cannot achieve ideal disc regeneration. Therefore, advanced bioactive therapies are implemented, including stem cells delivery, bioreagents administration, and implantation of biomaterials etc. Among these researches, few reported unsatisfying regenerative outcomes. However, these advanced therapies have barely achieved successful clinical translation. The main reason for the inconsistency between satisfying preclinical results and poor clinical translation may largely rely on the animal models that cannot actually simulate the human disc degeneration. The inappropriate animal model also leads to difficulties in comparing the efficacies among biomaterials in different reaches. Therefore, animal models that better simulate the clinical charateristics of human IDD should be acknowledged. In addition, in vivo regenerative outcomes should be carefully evaluated to obtain robust results. Nevertheless, many researches neglect certain critical characteristics, such as adhesive properties for biomaterials blocking annulus fibrosus defects and hyperalgesia that is closely related to the clinical manifestations, e.g., low back pain. Herein, in this review, we summarized the animal models established for IDD, and highlighted the proper models and parameters that may result in acknowledged IDD models. Then, we discussed the existing biomaterials for disc regeneration and the characteristics that should be considered for regenerating different parts of discs. Finally, well-established assays and parameters for in vivo disc regeneration are explored.

Design Considerations to Facilitate Clinical Radiological Evaluation of Implantable Biomedical Structures

Clinical effectiveness of implantable medical devices would be improved with in situ monitoring to ensure device positioning, determine subsequent damage, measure biodegradation, and follow healing. While standard clinical imaging protocols are appropriate for diagnosing disease and injury, these protocols have not been vetted for imaging devices. This study investigated how radiologists use clinical imaging to detect the location and integrity of implanted devices and whether embedding nanoparticle contrast agents into devices can improve assessment. To mimic the variety of devices available, phantoms from hydrophobic polymer films and hydrophilic gels were constructed, with and without computed tomography (CT)-visible TaOx and magnetic resonance imaging (MRI)-visible Fe3O4 nanoparticles. Some phantoms were purposely damaged by nick or transection. Phantoms were implanted in vitro into tissue and imaged with clinical CT, MRI, and ultrasound. In a blinded study, radiologists independently evaluated whether phantoms were present, assessed the type, and diagnosed whether phantoms were damaged or intact. Radiologists identified the location of phantoms 80% of the time. However, without incorporated nanoparticles, radiologists correctly assessed damage in only 54% of cases. With an incorporated imaging agent, the percentage jumped to 86%. The imaging technique which was most useful to radiologists varied with the properties of phantoms. With benefits and drawbacks to all three imaging modalities, future implanted devices should be engineered for visibility in the modality which best fits the treated tissue, the implanted device's physical location, and the type of required information. Imaging protocols should also be tailored to best exploit the properties of the imaging agents.

Electrospinning and 3D bioprinting for intervertebral disc tissue engineering

Intervertebral disc (IVD) degeneration is a major cause of low back pain and represents a massive socioeconomic burden. Current conservative and surgical treatments fail to restore native tissue architecture and functionality. Tissue engineering strategies, especially those based on 3D bioprinting and electrospinning, have emerged as possible alternatives by producing cell-seeded scaffolds that replicate the structure of the IVD extracellular matrix. In this review, we provide an overview of recent advancements and limitations of 3D bioprinting and electrospinning for the treatment of IVD degeneration, focusing on future areas of research that may contribute to their clinical translation.

Ex-vivo biomechanics of repaired rat intervertebral discs using genipin crosslinked fibrin adhesive hydrogel

Microdiscectomy is the current standard surgical treatment for intervertebral disc (IVD) herniation, however annulus fibrosus (AF) defects remain unrepaired which can alter IVD biomechanical properties and lead to reherniation, IVD degeneration and recurrent back pain. Genipin-crosslinked fibrin (FibGen) hydrogel is an injectable AF sealant previously shown to partially restore IVD motion segment biomechanical properties. A small animal model of herniation and repair is needed to evaluate repair potential for early-stage screening of IVD repair strategies prior to more costly large animal and eventual human studies. This study developed an ex-vivo rat caudal IVD herniation model and characterized torsional, axial tension-compression and stress relaxation biomechanical properties before and after herniation injury with or without repair using FibGen. Injury group involved an annular defect followed by removal of nucleus pulposus tissue to simulate a severe herniation while Repaired group involved FibGen injection. Injury significantly altered axial range of motion, neutral zone, torsional stiffness, torque range and stress-relaxation biomechanical parameters compared to Intact. FibGen repair restored the stress-relaxation parameters including effective hydraulic permeability indicating it effectively sealed the IVD defect, and there was a trend for improved tensile stiffness and axial neutral zone length. This study demonstrated a model for studying IVD herniation injury and repair strategies using rat caudal IVDs ex-vivo and demonstrated FibGen sealed IVDs to restore water retention and IVD pressurization. This ex-vivo small animal model may be modified for future in-vivo studies to screen IVD repair strategies using FibGen and other IVD repair biomaterials as an augment to additional large animal and human IVD testing.

Bioprinting of radiopaque constructs for tissue engineering and understanding degradation behavior by use of Micro-CT

Bioprinting has emerged as a potential technique to fabricate tissue engineering constructs and in vitro models directly using living cells as a raw material for fabrication, conforming to the heterogeneity and architectural complexity of the tissues. In several of tissue engineering and in vitro disease modelling or surgical planning applications, it is desirable to have radiopaque constructs for monitoring and evaluation. In the present work, enhanced radiopaque constructs are generated by substituting Calcium ions with Barium ions for crosslinking of alginate hydrogels. The constructs are characterized for their structural integrity and followed by cell culture studies to evaluate their biocompatibility. This was followed by the radiopacity evaluation. The radiological images obtained by micro-CT technique was further applied to investigate the degradation behavior of the scaffolds. In conclusion, it is observed that barium crosslinking can provide a convenient means to obtain radiopaque constructs with potential for multi-faceted applications.

Restoration of physiologic loading modulates engineered intervertebral disc structure and function in an in vivo model

Tissue-engineered whole disc replacements are an emerging treatment strategy for advanced intervertebral disc degeneration. A challenge facing the translation of tissue-engineered disc replacement to clinical use are the opposing needs of initial immobilization to advantage integration contrasted with physiologic loading and its anabolic effects. Here, we utilize our established rat tail model of tissue engineered disc replacement with external fixation to study the effects of remobilization at two time points postimplantation on engineered disc structure, composition, and function. Our results suggest that the restoration of mechanical loading following immobilization enhanced collagen and proteoglycan content within the nucleus pulposus and annulus fibrosus of the engineered discs, in addition to improving the integration of the endplate region of the construct with native bone. Despite these benefits, angulation of the vertebral bodies at the implanted level occurred following remobilization at both early and late time points, reducing tensile failure properties in the remobilized groups compared to the fixed group. These results demonstrate the necessity of restoring physiologic mechanical loading to engineered disc implants in vivo, and the need to transition toward their evaluation in larger animal models with more human-like anatomy and motion compared to the rat tail.

Intervertebral Disc Diseases PART 2: A Review of the Current Diagnostic and Treatment Strategies for Intervertebral Disc Disease

With an aging population, there is a proportional increase in the prevalence of intervertebral disc diseases. Intervertebral disc diseases are the leading cause of lower back pain and disability. With a high prevalence of asymptomatic intervertebral disc diseases, there is a need for accurate diagnosis, which is key to management. A thorough understanding of the pathophysiology and clinical manifestation aids in understanding the natural history of these conditions. Recent developments in radiological and biomarker investigations have potential to provide noninvasive alternatives to the gold standard, invasive discogram. There is a large volume of literature on the management of intervertebral disc diseases, which we categorized into five headings: (a) Relief of pain by conservative management, (b) restorative treatment by molecular therapy, (c) reconstructive treatment by percutaneous intervertebral disc techniques, (d) relieving compression and replacement surgery, and (e) rigid fusion surgery. This review article aims to provide an overview on various current diagnostic and treatment options and discuss the interplay between each arms of these scientific and treatment advancements, hence providing an outlook of their potential future developments and collaborations in the management of intervertebral disc diseases.

Pure polylysine-based foamy scaffolds and their interaction with MC3T3-E1 cells and osteogenesis

Polypeptide-derived copolymers have widely been exploited for drug/gene delivery due to their pendant functional groups and non-toxic degradation products. However, fabrication of polypeptide-based scaffolds for tissue engineering has seldom been reported. In this study, foamy poly(N _ε -benzyl formateoxycarbonyl-L-Lysine) (PZL) and poly(N _ε -benzyl formateoxycarbonyl-L-lysine-co-L-phenylalanine) (PZLP) scaffolds were successfully prepared by a combination of ring-opening polymerization of α-amino acid N-carboxyanhydride and negative porous NaCl templating approach. The physicochemical properties of these scaffolds including glass transition temperature, contact angle, compression modulus and degradation behavior were characterized. Both in vitro and in vivo biocompatibility of the scaffolds were evaluated by MC3T3-E1 cell culture and SD subcutaneous model, respectively. The results from live-dead staining, MTT and ALP activity assays indicated that PZL scaffolds were more conducive to the adhesion, proliferation and osteoblastic differentiation of MC3T3-E1 cells compared to PZLP scaffolds in the initial culture period due to their specific surface properties. While porous structure rather than surface properties of scaffolds played a decisive role in the later stage of cell culture. The results of in vivo studies including H&E, Masson's trichrome and CD34 staining further demonstrated that PZL scaffolds supported the ingrowth of microvessels than PZLP scaffolds due to their surface property difference. Collectively, PZL scaffolds displayed good biocompatibility and could be a promising candidate for tissue engineering application.

In Vivo Tracking of Tissue Engineered Constructs

To date, the fields of biomaterials science and tissue engineering have shown great promise in creating bioartificial tissues and organs for use in a variety of regenerative medicine applications. With the emergence of new technologies such as additive biomanufacturing and 3D bioprinting, increasingly complex tissue constructs are being fabricated to fulfill the desired patient-specific requirements. Fundamental to the further advancement of this field is the design and development of imaging modalities that can enable visualization of the bioengineered constructs following implantation, at adequate spatial and temporal resolution and high penetration depths. These in vivo tracking techniques should introduce minimum toxicity, disruption, and destruction to treated tissues, while generating clinically relevant signal-to-noise ratios. This article reviews the imaging techniques that are currently being adopted in both research and clinical studies to track tissue engineering scaffolds in vivo, with special attention to 3D bioprinted tissue constructs.

Biomimetic nanofibers can construct effective tissue-engineered intervertebral discs for therapeutic implantation

We present a total tissue engineered (TE) intervertebral disc (IVD) to address IVD degradation, which is a major cause of chronic neck and back pain. The TE IVD is comprised of an alginate hydrogel-based nucleus pulposus (NP) and hierarchically organized, concentric ring-aligned electrospun (ES) polycaprolactone (PCL)/poly (d,l-lactide-co-glycolide) (PLGA)/Collagen type I (PPC)-based annulus fibrosus (AF). The TE IVD exhibits excellent hydrophilicity to simulate highly hydrated native IVD. Long-term in vivo implantation assays demonstrate the excellent structural (shape maintenance, hydration, and integration with surrounding tissues) and functional (mechanical supporting and flexibility) performances of the TE IVD. Our study provides a novel approach for treating IVD degeneration.

Mechanical Considerations for Electrospun Nanofibers in Tendon and Ligament Repair

Electrospun nanofibers possess unique qualities such as nanodiameter, high surface area to volume ratio, biomimetic architecture, and tunable chemical and electrical properties. Numerous studies have demonstrated the potential of nanofibrous architecture to direct cell morphology, migration, and more complex biological processes such as differentiation and extracellular matrix (ECM) deposition through topographical guidance cues. These advantages have created great interest in electrospun fibers for biomedical applications, including tendon and ligament repair. Electrospun nanofibers, despite their nanoscale size, generally exhibit poor mechanical properties compared to larger conventionally manufactured polymer fiber materials. This invites the question of what role electrospun polymer nanofibers can play in tendon and ligament repair applications that have both biological and mechanical requirements. At first glance, the strength and stiffness of electrospun nanofiber grafts appear to be too low to fill the rigorous loading conditions of these tissues. However, there are a number of strategies to enhance and tune the mechanical properties of electrospun nanofiber grafts. As researchers design the next-generation electrospun tendon and ligament grafts, it is critical to consider numerous physiologically relevant mechanical criteria and to evaluate graft mechanical performance in conditions and loading environments that reflect in vivo conditions and surgical fixation methods.

Current Concepts in Meniscus Tissue Engineering and Repair

The meniscus is the most commonly injured structure in the human knee. Meniscus deficiency has been shown to lead to advanced osteoarthritis (OA) due to abnormal mechanical forces, and replacement strategies for this structure have lagged behind other tissue engineering endeavors. The challenges include the complex 3D structure with individualized size parameters, the significant compressive, tensile and shear loads encountered, and the poor blood supply. In this progress report, a review of the current clinical treatments for different types of meniscal injury is provided. The state-of-the-art research in cellular therapies and novel cell sources for these therapies is discussed. The clinically available cell-free biomaterial implants and the current progress on cell-free biomaterial implants are reviewed. Cell-based tissue engineering strategies for the repair and replacement of meniscus are presented, and the current challenges are identified. Tissue-engineered meniscal biocomposite implants may provide an alternative solution for the treatment of meniscal injury to prevent OA in the long run, because of the limitations of the existing therapies.

A compound scaffold with uniform longitudinally oriented guidance cues and a porous sheath promotes peripheral nerve regeneration in vivo

Scaffolds with inner fillers that convey directional guidance cues represent promising candidates for nerve repair. However, incorrect positioning or non-uniform distribution of intraluminal fillers might result in regeneration failure. In addition, proper porosity (to enhance nutrient and oxygen exchange but prevent fibroblast infiltration) and mechanical properties (to ensure fixation and to protect regenerating axons from compression) of the outer sheath are also highly important for constructing advanced nerve scaffolds. In this study, we constructed a compound scaffold using a stage-wise strategy, including directionally freezing orientated collagen-chitosan (O-CCH) filler, electrospinning poly(ε-caprolactone) (PCL) sheaths and assembling O-CCH/PCL scaffolds. Based on scanning electron microscopy (SEM) and mechanical tests, a blend of collagen/chitosan (1:1) was selected for filler fabrication, and a wall thickness of 400 μm was selected for PCL sheath production. SEM and three-dimensional (3D) reconstruction further revealed that the O-CCH filler exhibited a uniform, longitudinally oriented microstructure (over 85% of pores were 20-50 μm in diameter). The electrospun PCL porous sheath with pore sizes of 6.5 ± 3.3 μm prevented fibroblast invasion. The PCL sheath exhibited comparable mechanical properties to commercially available nerve conduits, and the O-CCH filler showed a physiologically relevant substrate stiffness of 2.0 ± 0.4 kPa. The differential degradation time of the filler and sheath allows the O-CCH/PCL scaffold to protect regenerating axons from compression stress while providing enough space for regenerating nerves. In vitro and in vivo studies indicated that the O-CCH/PCL scaffolds could promote axonal regeneration and Schwann cell migration. More importantly, functional results indicated that the CCH/PCL compound scaffold induced comparable functional recovery to that of the autograft group at the end of the study. Our findings demonstrated that the O-CCH/PCL scaffold with uniform longitudinal guidance filler and a porous sheath exhibits favorable properties for clinical use and promotes nerve regeneration and functional recovery. The O-CCH/PCL scaffold provides a promising new path for developing an optimal therapeutic alternative for peripheral nerve reconstruction.

STATEMENT OF SIGNIFICANCE

Scaffolds with inner fillers displaying directional guidance cues represent a promising candidate for nerve repair. However, further clinical translation should pay attention to the problem of non-uniform distribution of inner fillers, the porosity and mechanical properties of the outer sheath and the morphological design facilitating operation. In this study, a stage-wise fabrication strategy was used, which made it possible to develop an O-CCH/PCL compound scaffold with a uniform longitudinally oriented inner filler and a porous outer sheath. The uniform distribution of the pores in the O-CCH/PCL scaffold provides a solution to resolve the problem of non-uniform distribution of inner fillers, which impede the clinical translation of scaffolds with longitudinal microstructured fillers, especially for aligned-fiber-based scaffolds. In vitro and in vivo studies indicated that the O-CCH/PCL scaffolds could provide topographical cues for axonal regeneration and SC migration, which were not found for random scaffolds (with random microstructure resemble sponge-based scaffolds). The electrospun porous PCL sheath of the O-CCH/PCL scaffold not only prevented fibroblast infiltration, but also satisfied the mechanical requirements for clinical use, paving the way for clinical translation. The differential degradation time of the O-CCH filler and the PCL sheath makes O-CCH/PCL scaffold able to provide long protection for regenerating axons from compression stress, but enough space for regenerating nerve. These findings highlight the possibility of developing an optimal therapeutic alternative for nerve defects using the O-CCH/PCL scaffold.

Current strategies for treatment of intervertebral disc degeneration: substitution and regeneration possibilities

Background

Intervertebral disc degeneration has an annual worldwide socioeconomic impact masked as low back pain of over 70 billion euros. This disease has a high prevalence over the working age class, which raises the socioeconomic impact over the years. Acute physical trauma or prolonged intervertebral disc mistreatment triggers a biochemical negative tendency of catabolic-anabolic balance that progress to a chronic degeneration disease. Current biomedical treatments are not only ineffective in the long-run, but can also cause degeneration to spread to adjacent intervertebral discs. Regenerative strategies are desperately needed in the clinics, such as: minimal invasive nucleus pulposus or annulus fibrosus treatments, total disc replacement, and cartilaginous endplates decalcification.

Main body

Herein, it is reviewed the state-of-the-art of intervertebral disc regeneration strategies from the perspective of cells, scaffolds, or constructs, including both popular and unique tissue engineering approaches. The premises for cell type and origin selection or even absence of cells is being explored. Choice of several raw materials and scaffold fabrication methods are evaluated. Extensive studies have been developed for fully regeneration of the annulus fibrosus and nucleus pulposus, together or separately, with a long set of different rationales already reported. Recent works show promising biomaterials and processing methods applied to intervertebral disc substitutive or regenerative strategies. Facing the abundance of studies presented in the literature aiming intervertebral disc regeneration it is interesting to observe how cartilaginous endplates have been extensively neglected, being this a major source of nutrients and water supply for the whole disc.

Conclusion

Several innovative avenues for tackling intervertebral disc degeneration are being reported – from acellular to cellular approaches, but the cartilaginous endplates regeneration strategies remain unaddressed. Interestingly, patient-specific approaches show great promise in respecting patient anatomy and thus allow quicker translation to the clinics in the near future.

Translation of an injectable triple-interpenetrating-network hydrogel for intervertebral disc regeneration in a goat model

Degeneration of the intervertebral discs is a progressive cascade of cellular, compositional and structural changes that is frequently associated with low back pain. As the first signs of disc degeneration typically arise in the disc's central nucleus pulposus (NP), augmentation of the NP via hydrogel injection represents a promising strategy to treat early to mid-stage degeneration. The purpose of this study was to establish the translational feasibility of a triple interpenetrating network hydrogel composed of dextran, chitosan, and teleostean (DCT) for augmentation of the degenerative NP in a preclinical goat model. Ex vivo injection of the DCT hydrogel into degenerated goat lumbar motion segments restored range of motion and neutral zone modulus towards physiologic values. To facilitate non-invasive assessment of hydrogel delivery and distribution, zirconia nanoparticles were added to make the hydrogel radiopaque. Importantly, the addition of zirconia did not negatively impact viability or matrix producing capacity of goat mesenchymal stem cells or NP cells seeded within the hydrogel in vitro. In vivo studies demonstrated that the radiopaque DCT hydrogel was successfully delivered to degenerated goat lumbar intervertebral discs, where it was distributed throughout both the NP and annulus fibrosus, and that the hydrogel remained contained within the disc space for two weeks without evidence of extrusion. These results demonstrate the translational potential of this hydrogel for functional regeneration of degenerate intervertebral discs.

STATEMENT OF SIGNIFICANCE

The results of this work demonstrate that a radiopaque hydrogel is capable of normalizing the mechanical function of the degenerative disc, is supportive of disc cell and mesenchymal stem cell viability and matrix production, and can be maintained in the disc space without extrusion following intradiscal delivery in a preclinical large animal model. These results support evaluation of this hydrogel as a minimally invasive disc therapeutic in long-term preclinical studies as a precursor to future clinical application in patients with disc degeneration and low back pain.

Initial investigation of individual and combined annulus fibrosus and nucleus pulposus repair ex vivo

Novel tissue engineered and biomaterial approaches to treat intervertebral disc (IVD) degeneration focus on single aspects of the progressive disease and hence are insufficient repair strategies. In this study, annulus fibrosus (AF) and nucleus pulposus (NP) biomaterial repair strategies were used individually and combined to treat IVD degeneration modeled in ex vivo rat-tail motion segments by annulotomy and nucleotomy. An injectable riboflavin cross-linked high-density collagen gel patched defects in the AF, while NP repair consisted of injections of a modified hyaluronic acid (HA) hydrogel. Qualitative imaging showed the annulotomy and nucleotomy successfully herniated NP material, while the HA NP injections restored intact NP morphology and the collagen AF patches sealed AF defects. Assessed by quantitative T2 magnetic resonance imaging, combined repair treatments yielded disc hydration not significantly different than intact hydration, while AF and NP repairs alone only restored ∼1/3 of intact hydration. Mechanical testing showed NP injections alone recovered on average ∼35% and ∼40% of the effective instantaneous and equilibrium moduli. The combined treatment comprising biomaterial AF and NP repair was effective at increasing NP hydration from NP repair alone, however HA injections alone are sufficient to improve mechanical properties.

STATEMENT OF SIGNIFICANCE

Intervertebral disc degeneration affects an estimated 90% of individuals throughout their life, and is a candidate pathology for tissue engineered repair. The current standard of clinical care reduces spinal articulation and leads to further degeneration along the spine, hence great interest in a regenerative medicine therapy. Literature studies focused on biomaterial repair strategies for treating degenerated discs have partially restored native disc function, however no studies have reported the use of combined therapies to address multiple aspects of disc degeneration. This initial investigation screened injectable biomaterial repair strategies ex vivo, and through complementary outcome measures showed a combined therapy restores disc function better than individual approaches. This study is the first of its kind to address multiple aspects of disc degeneration, using clinically-oriented biomaterials in a well-established animal model.

Pharmaceutical Intermediate-Modified Gold Nanoparticles: Against Multidrug-Resistant Bacteria and Wound-Healing Application via an Electrospun Scaffold

Remedying a multidrug-resistant (MDR) bacteria wound infection is a major challenge due to the inability of conventional antibiotics to treat such infections against MDR bacteria. Thus, developing wound dressings for wound care, particularly against MDR bacteria, is in huge demand. Here, we present a strategy in designing wound dressings: we use a small molecule (6-aminopenicillanic acid, APA)-coated gold nanoparticles (AuNPs) to inhibit MDR bacteria. We dope the AuNPs into electrospun fibers of poly(ε-caprolactone) (PCL)/gelatin to yield materials that guard against wound infection by MDR bacteria. We systematically evaluate the bactericidal activity of the AuNPs and wound-healing capability via the electrospun scaffold. APA-modified AuNPs (Au_APA) exhibit remarkable antibacterial activity even when confronted with MDR bacteria. Meanwhile, Au_APA has outstanding biocompatibility. Moreover, an in vivo bacteria-infected wound-healing experiment indicates that it has a striking ability to remedy a MDR bacteria wound infection. This wound scaffold can assist the wound care for bacterial infections.

In vivo performance of an acellular disc-like angle ply structure (DAPS) for total disc replacement in a small animal model

Total intervertebral disc replacement with a biologic engineered disc may be an alternative to spinal fusion for treating end-stage disc disease. In previous work, we developed disc-like angle ply structures (DAPS) that replicate the structure and function of the native disc and a rat tail model to evaluate DAPS in vivo. Here, we evaluated a strategy in which, after in vivo implantation, endogenous cells could colonize the acellular DAPS and form an extracellular matrix organized by the DAPS topographical template. To do so, acellular DAPS were implanted into the caudal spines of rats and evaluated over 12 weeks by mechanical testing, histology, and microcomputed tomography. An external fixation device was used to stabilize the implant site and various control groups were included to evaluate the effect of immobilization. There was robust tissue formation within the DAPS after implantation and compressive mechanical properties of the implant matched that of the native motion segment. Immobilization provided a stable site for fibrous tissue formation after either a discectomy or a DAPS implantation, but bony fusion eventually resulted, with segments showing intervertebral bridging after long-term implantation, a process that was accelerated by the implanted DAPS. Thus, while compressive mechanical properties were replicated after DAPS implantation, methods to actively prevent fusion must be developed. Future work will focus on limiting fusion by remobilizing the motion segment after a period of integration, delivering pro-chondrogenic factors, and pre-seeding DAPS with cells prior to implantation. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:23-31, 2017.

Zein/Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) electrospun blend fiber scaffolds: Preparation, characterization and cytocompatibility

In the present work, a series of Zein/Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) blend fiber scaffolds have been prepared by electrospinning method. The electrospun fibers showed a circular and uniform morphology with random distribution. The blend fiber scaffolds possessed well interconnected porous fibrous network structure with high porosity and large aspect surface areas. The FTIR and XPS spectra of Zein/P(3HB-co-4HB) blend fibers demonstrated the same characteristics to that of pure Zein and P(3HB-co-4HB) electrospun fibers. However, Zein might hinder the crystallization of P(3HB-co-4HB) owing to the formation of weak intermolecular interactions, which can affect the preferential orientation of P(3HB-co-4HB) molecules. Only one glass transition temperature (Tg) can be detected for electrospun Zein/P(3HB-co-4HB) blend fiber scaffolds implying the miscibility of Zein and P(3HB-co-4HB) in the blend fibers. The Zein/P(3HB-co-4HB) blend fiber scaffolds showed about 50% of improvement in tensile strength and 400% of increase in elongation at break by increasing P(3HB-co-4HB) content from 20% to 80%. The cytocompatibility of the Zein/P(3HB-co-4HB) blend fiber scaffolds was preliminarily evaluated by cell culture in vitro. The as-prepared electrospun Zein/P(3HB-co-4HB) blend fiber scaffolds with the characteristics of good biocompatibility, excellent pore characteristic as well as sufficient mechanical properties should be more promising for applications as tissue engineering scaffold.

Strategies in regenerative medicine for intervertebral disc repair using mesenchymal stem cells and bioscaffolds

The intervertebral disc (IVD) is a major weight bearing structure that undergoes degenerative changes with aging limiting its ability to dissipate axial spinal loading in an efficient manner resulting in the generation of low back pain. Low back pain is a number one global musculoskeletal disorder with massive socioeconomic impact. The WHO has nominated development of mesenchymal stem cells and bioscaffolds to promote IVD repair as primary research objectives. There is a clear imperative for the development of strategies to effectively treat IVD defects. Early preclinical studies with mesenchymal stem cells in canine and ovine models have yielded impressive results in IVD repair. Combinatorial therapeutic approaches encompassing biomaterial and cell-based therapies promise significant breakthroughs in IVD repair in the near future.