The reparative response to cross-linked collagen-based scaffolds in a rat spinal cord gap model

Biomaterials in Traumatic Brain Injury: Perspectives and Challenges

Traumatic brain injury (TBI) is a leading cause of mortality and long-term impairment globally. TBI has a dynamic pathology, encompassing a variety of metabolic and molecular events that occur in two phases: primary and secondary. A forceful external blow to the brain initiates the primary phase, followed by a secondary phase that involves the release of calcium ions (Ca2+) and the initiation of a cascade of inflammatory processes, including mitochondrial dysfunction, a rise in oxidative stress, activation of glial cells, and damage to the blood-brain barrier (BBB), resulting in paracellular leakage. Currently, there are no FDA-approved drugs for TBI, but existing approaches rely on delivering micro- and macromolecular treatments, which are constrained by the BBB, poor retention, off-target toxicity, and the complex pathology of TBI. Therefore, there is a demand for innovative and alternative therapeutics with effective delivery tactics for the diagnosis and treatment of TBI. Tissue engineering, which includes the use of biomaterials, is one such alternative approach. Biomaterials, such as hydrogels, including self-assembling peptides and electrospun nanofibers, can be used alone or in combination with neuronal stem cells to induce neurite outgrowth, the differentiation of human neural stem cells, and nerve gap bridging in TBI. This review examines the inclusion of biomaterials as potential treatments for TBI, including their types, synthesis, and mechanisms of action. This review also discusses the challenges faced by the use of biomaterials in TBI, including the development of biodegradable, biocompatible, and mechanically flexible biomaterials and, if combined with stem cells, the survival rate of the transplanted stem cells. A better understanding of the mechanisms and drawbacks of these novel therapeutic approaches will help to guide the design of future TBI therapies.

Encapsulation of Human Spinal Cord Progenitor Cells in Hyaluronan-Gelatin Hydrogel for Spinal Cord Injury Treatment

Transplanting human induced pluripotent stem cells (iPSCs)-derived spinal cord progenitor cells (SCPCs) is a promising approach to treat spinal cord injuries. However, stem cell therapies face challenges in cell survival, cell localization to the targeted site, and the control of cell differentiation. Here, we encapsulated SCPCs in thiol-modified hyaluronan-gelatin hydrogels and optimized scaffold mechanical properties and cell encapsulation density to promote cell viability and neuronal differentiation in vitro and in vivo. Different compositions of hyaluronan-gelatin hydrogels formulated by varying concentrations of poly(ethylene glycol) diacrylate were mechanically characterized by using atomic force microscopy. In vitro SCPC encapsulation study showed higher cell viability and proliferation with lower substrate Young's modulus (200 Pa vs 580 Pa) and cell density. Moreover, the soft hydrogels facilitated a higher degree of neuronal differentiation with extended filament structures in contrast to clumped cellular morphologies obtained in stiff hydrogels (p < 0.01). When transplanted in vivo, the optimized SCPC-encapsulated hydrogels resulted in higher cell survival and localization at the transplanted region as compared to cell delivery without hydrogel encapsulation at 2 weeks postimplantation within the rat spinal cord (p < 0.01). Notably, immunostaining demonstrated that the hydrogel-encapsulated SCPCs differentiated along the neuronal and oligodendroglial lineages in vivo. The lack of pluripotency and proliferation also supported the safety of the SCPC transplantation approach. Overall, the injectable hyaluronan-gelatin hydrogel shows promise in supporting the survival and neural differentiation of human SCPCs after transplantation into the spinal cord.

Application of Conductive Hydrogels on Spinal Cord Injury Repair: A Review

Spinal cord injury (SCI) causes severe motor or sensory damage that leads to long-term disabilities due to disruption of electrical conduction in neuronal pathways. Despite current clinical therapies being used to limit the propagation of cell or tissue damage, the need for neuroregenerative therapies remains. Conductive hydrogels have been considered a promising neuroregenerative therapy due to their ability to provide a pro-regenerative microenvironment and flexible structure, which conforms to a complex SCI lesion. Furthermore, their conductivity can be utilized for noninvasive electrical signaling in dictating neuronal cell behavior. However, the ability of hydrogels to guide directional axon growth to reach the distal end for complete nerve reconnection remains a critical challenge. In this Review, we highlight recent advances in conductive hydrogels, including the incorporation of conductive materials, fabrication techniques, and cross-linking interactions. We also discuss important characteristics for designing conductive hydrogels for directional growth and regenerative therapy. We propose insights into electrical conductivity properties in a hydrogel that could be implemented as guidance for directional cell growth for SCI applications. Specifically, we highlight the practical implications of recent findings in the field, including the potential for conductive hydrogels to be used in clinical applications. We conclude that conductive hydrogels are a promising neuroregenerative therapy for SCI and that further research is needed to optimize their design and application.

Pharmacological Functions, Synthesis, and Delivery Progress for Collagen as Biodrug and Biomaterial

Collagen has been widely applied as a functional biomaterial in regulating tissue regeneration and drug delivery by participating in cell proliferation, differentiation, migration, intercellular signal transmission, tissue formation, and blood coagulation. However, traditional extraction of collagen from animals potentially induces immunogenicity and requires complicated material treatment and purification steps. Although semi-synthesis strategies such as utilizing recombinant E. coli or yeast expression systems have been explored as alternative methods, the influence of unwanted by-products, foreign substances, and immature synthetic processes have limited its industrial production and clinical applications. Meanwhile, macromolecule collagen products encounter a bottleneck in delivery and absorption by conventional oral and injection vehicles, which promotes the studies of transdermal and topical delivery strategies and implant methods. This review illustrates the physiological and therapeutic effects, synthesis strategies, and delivery technologies of collagen to provide a reference and outlook for the research and development of collagen as a biodrug and biomaterial.

Anisotropic 3D scaffolds for spinal cord guided repair: Current concepts

A spinal cord injury (SCI) can be caused by unforeseen events such as a fall, a vehicle accident, a gunshot, or a malignant illness, which has a significant impact on the quality of life of the patient. Due to the limited regenerative potential of the central nervous system (CNS), SCI is one of the most daunting medical challenges of modern medicine. Great advances have been made in tissue engineering and regenerative medicine, which include the transition from two-dimensional (2D) to three-dimensional (3D) biomaterials. Combinatory treatments that use 3D scaffolds may significantly enhance the repair and regeneration of functional neural tissue. In an effort to mimic the chemical and physical properties of neural tissue, scientists are researching the development of the ideal scaffold made of synthetic and/or natural polymers. Moreover, in order to restore the architecture and function of neural networks, 3D scaffolds with anisotropic properties that replicate the native longitudinal orientation of spinal cord nerve fibres are being designed. In an effort to determine if scaffold anisotropy is a crucial property for neural tissue regeneration, this review focuses on the most current technological developments relevant to anisotropic scaffolds for SCI. Special consideration is given to the architectural characteristics of scaffolds containing axially oriented fibres, channels, and pores. By analysing neural cell behaviour in vitro and tissue integration and functional recovery in animal models of SCI, the therapeutic efficacy is evaluated for its successes and limitations.

Smart biomaterials and their potential applications in tissue engineering

Smart biomaterials have been rapidly advancing ever since the concept of tissue engineering was proposed. Interacting with human cells, smart biomaterials can play a key role in novel tissue morphogenesis. Various aspects of biomaterials utilized in or being sought for the goal of encouraging bone regeneration, skin graft engineering, and nerve conduits are discussed in this review. Beginning with bone, this study summarizes all the available bioceramics and materials along with their properties used singly or in conjunction with each other to create scaffolds for bone tissue engineering. A quick overview of the skin-based nanocomposite biomaterials possessing antibacterial properties for wound healing is outlined along with skin regeneration therapies using infrared radiation, electrospinning, and piezoelectricity, which aid in wound healing. Furthermore, a brief overview of bioengineered artificial skin grafts made of various natural and synthetic polymers has been presented. Finally, by examining the interactions between natural and synthetic-based biomaterials and the biological environment, their strengths and drawbacks for constructing peripheral nerve conduits are highlighted. The description of the preclinical outcome of nerve regeneration in injury healed with various natural-based conduits receives special attention. The organic and synthetic worlds collide at the interface of nanomaterials and biological systems, producing a new scientific field including nanomaterial design for tissue engineering.

Characterization of a Novel Aspect of Tissue Scarring Following Experimental Spinal Cord Injury and the Implantation of Bioengineered Type-I Collagen Scaffolds in the Adult Rat: Involvement of Perineurial-like Cells?

Numerous intervention strategies have been developed to promote functional tissue repair following experimental spinal cord injury (SCI), including the bridging of lesion-induced cystic cavities with bioengineered scaffolds. Integration between such implanted scaffolds and the lesioned host spinal cord is critical for supporting regenerative growth, but only moderate-to-low degrees of success have been reported. Light and electron microscopy were employed to better characterise the fibroadhesive scarring process taking place after implantation of a longitudinally microstructured type-I collagen scaffold into unilateral mid-cervical resection injuries of the adult rat spinal cord. At long survival times (10 weeks post-surgery), sheets of tightly packed cells (of uniform morphology) could be seen lining the inner surface of the repaired dura mater of lesion-only control animals, as well as forming a barrier along the implant–host interface of the scaffold-implanted animals. The highly uniform ultrastructural features of these scarring cells and their anatomical continuity with the local, reactive spinal nerve roots strongly suggest their identity to be perineurial-like cells. This novel aspect of the cellular composition of reactive spinal cord tissue highlights the increasingly complex nature of fibroadhesive scarring involved in traumatic injury, and particularly in response to the implantation of bioengineered collagen scaffolds.

Biomaterial and Therapeutic Approaches for the Manipulation of Macrophage Phenotype in Peripheral and Central Nerve Repair

Injury to the peripheral or central nervous systems often results in extensive loss of motor and sensory function that can greatly diminish quality of life. In both cases, macrophage infiltration into the injury site plays an integral role in the host tissue inflammatory response. In particular, the temporally related transition of macrophage phenotype between the M1/M2 inflammatory/repair states is critical for successful tissue repair. In recent years, biomaterial implants have emerged as a novel approach to bridge lesion sites and provide a growth-inductive environment for regenerating axons. This has more recently seen these two areas of research increasingly intersecting in the creation of 'immune-modulatory' biomaterials. These synthetic or naturally derived materials are fabricated to drive macrophages towards a pro-repair phenotype. This review considers the macrophage-mediated inflammatory events that occur following nervous tissue injury and outlines the latest developments in biomaterial-based strategies to influence macrophage phenotype and enhance repair.

Scar tissue removal-activated endogenous neural stem cells aid Taxol-modified collagen scaffolds in repairing chronic long-distance transected spinal cord injury

Scar tissue removal combined with biomaterial implantation is considered an effective measure to repair chronic transected spinal cord injury (SCI). However, whether more scar tissue removal surgeries could affect the treatment effects of biomaterial implantation still needs to be explored. In this study, we performed the first scar tissue removal surgery in the 3^rd month and the second in the 6^th month after completely removing 1 cm of spinal tissue in canines. We found that Taxol-modified linear ordered collagen scaffold (LOCS + Taxol) implantation could promote axonal regeneration, neurogenesis, and electrophysiological and functional recovery only in canines at the first scar tissue removal surgery, but not in canines at the second scar tissue removal surgery. Interestingly, we found that more endogenous neural stem cells (NSCs) around the injured site could be activated in canines with the first rather than the second scar tissue removal. Furthermore, we demonstrated that Taxol could promote the neuronal differentiation of NSCs in the myelin inhibition microenvironment through the p38 MAPK signaling pathway in vitro. Therefore, we speculated that endogenous NSC activation by the first scar tissue removal surgery and its further differentiation into neurons induced by Taxol may contribute to functional recovery in canines. Together, LOCS + Taxol implantation in combination with the first scar tissue removal provides a promising therapy for chronic long-distance transected SCI repair with the help of scar tissue removal activated endogenous NSCs.

Collagen-based scaffolds: An auspicious tool to support repair, recovery, and regeneration post spinal cord injury

Spinal cord injury (SCI) is a perplexing traumatic disease that habitually gives ride to permanent disability, motor, and sensory impairment. Despite the existence of several therapeutic approaches for the injured motor or sensory neurons, they can't promote axonal regeneration. Whether prepared by conventional or rapid prototyping techniques, scaffolds can be applied to refurbish the continuity of the injured site, by creating a suitable environment for tissue repair, axonal regeneration, and vascularization. Collagen is a multi-sourced protein, found in animals skin, tendons, cartilage, bones, and human placenta, in addition to marine biomass. Collagen is highly abundant in the extracellular matrix and is known for its biocompatibility, biodegradability, porous structure, good permeability, low immunogenicity and thus is extensively applied in the pharmaceutical, cosmetic, and food industries as well as the tissue engineering field. Collagen in scaffolds is usually functionalized with different ligands and factors such as, stem cells, embryonic or human cells to augment its binding specificity and activity. The review summarizes the significance of collagen-based scaffolds and their influence on regeneration, repair and recovery of spinal cord injuries.

A Bio-inspired Multifunctionalized Silk Fibroin

A bio-inspired multifunctionalized silk fibroin (BMS) was synthesized in order to mimic the interaction of nidogen with the type IV collagen and laminin of basement membranes. The designed BMS consists of a motif of laminin α-chain-derived, called IK peptide, and type IV collagen covalently bound to the silk fibroin (SF) by using EDC/NHS coupling and a Cu-free click chemistry reaction, respectively. Silk fibroin was chosen as the main component of the BMS because it is versatile and biocompatible, induces an in vivo favorable bioresponse, and moreover can be functionalized with different methods. The chemical structure of BMS was analyzed by using X-ray photoelectron spectroscopy, attenuated total reflection-Fourier transform infrared, cross-polarization magic angle spinning nuclear magnetic resonance techniques, and colorimetric assay. The SF and BMS solutions were cross-linked by sonication to form hydrogels or casted to make films in order to evaluate and compare the early adhesion and viability of MRC5 cells. BMS hydrogels were also characterized by rheological and thermal analyses.

A combinatorial approach for spinal cord injury repair using multifunctional collagen-based matrices: development, characterization and impact on cell adhesion and axonal growth

Spinal cord injury is a devastating condition of the central nervous system, in which traditional treatments are largely ineffective due to the complex nature of the injured tissue. Therefore, biomaterial-based systems have been developed as possible alternative strategies to repair the damaged tissue. In the present study, we aimed to design bioactive agent loaded scaffolds composed of two layers with distinct physical properties to improve tissue regeneration. An electrospun layer with aligned nanofibers was made of collagen (Col) Type-I, poly(lactide-co-glycolide) (PLGA) and laminin to promote cell attachment of mesenchymal-like stem cells towards the direction of fibers, while a Col-based second layer was fabricated by plastic compression to act as a releasing system for NT-3 and chondroitinase ABC, so that axonal growth could be stimulated. Results showed that a source of mesenchymal stem cell (MSC)-like cells, adipose tissue-derived stem cells cultured on the fibrous layer of the matrices were able to adhere and proliferate, where the aligned fibers promoted the cell growth in an organized way. Furthermore, the bilayered matrices also promoted dorsal root ganglion neurite outgrowth. The bilayered matrice with Col/PLGA + laminin top layer appears to promote higher neurite growth. Collectively, the designed constructs show promising structural properties and biological performance for being employed as a scaffold for engineering the spinal cord tissue.

Neural stem cell delivery via porous collagen scaffolds promotes neuronal differentiation and locomotion recovery in spinal cord injury

Neural stem cell (NSC) grafts have demonstrated significant effects in animal models of spinal cord injury (SCI), yet their clinical translation remains challenging. Significant evidence suggests that the supporting matrix of NSC grafts has a crucial role in regulating NSC effects. Here we demonstrate that grafts based on porous collagen-based scaffolds (PCSs), similar to biomaterials utilized clinically in induced regeneration, can deliver and protect embryonic NSCs at SCI sites, leading to significant improvement in locomotion recovery in an experimental mouse SCI model, so that 12 weeks post-injury locomotion performance of implanted animals does not statistically differ from that of uninjured control animals. NSC-seeded PCS grafts can modulate key processes required to induce regeneration in SCI lesions including enhancing NSC neuronal differentiation and functional integration in vivo, enabling robust axonal elongation, and reducing astrogliosis. Our findings suggest that the efficacy and translational potential of emerging NSC-based SCI therapies could be enhanced by delivering NSC via scaffolds derived from well-characterized clinically proven PCS.

Collagen Type I: A Versatile Biomaterial

Collagen type I is the most abundant matrix protein in the human body and is highly demanded in tissue engineering, regenerative medicine, and pharmaceutical applications. To meet the uprising demand in biomedical applications, collagen type I has been isolated from mammalians (bovine, porcine, goat and rat) and non-mammalians (fish, amphibian, and sea plant) source using various extraction techniques. Recent advancement enables fabrication of collagen scaffolds in multiple forms such as film, sponge, and hydrogel, with or without other biomaterials. The scaffolds are extensively used to develop tissue substitutes in regenerating or repairing diseased or damaged tissues. The 3D scaffolds are also used to develop in vitro model and as a vehicle for delivering drugs or active compounds.

Spinal cord injury: pathophysiology, treatment strategies, associated challenges, and future implications

Axonal regeneration and formation of tripartite (axo-glial) junctions at damaged sites is a prerequisite for early repair of injured spinal cord. Transplantation of stem cells at such sites of damage which can generate both neuronal and glial population has gained impact in terms of recuperation upon infliction with spinal cord injury. In spite of the fact that a copious number of pre-clinical studies using different stem/progenitor cells have shown promising results at acute and subacute stages, at the chronic stages of injury their recovery rates have shown a drastic decline. Therefore, developing novel therapeutic strategies are the need of the hour in order to assuage secondary morbidity and effectuate improvement of the spinal cord injury (SCI)-afflicted patients' quality of life. The present review aims at providing an overview of the current treatment strategies and also gives an insight into the potential cell-based therapies for the treatment of SCI.

Fibroadhesive scarring of grafted collagen scaffolds interferes with implant-host neural tissue integration and bridging in experimental spinal cord injury

Severe traumatic spinal cord injury (SCI) results in a devastating and permanent loss of function, and is currently an incurable condition. It is generally accepted that future intervention strategies will require combinational approaches, including bioengineered scaffolds, to support axon growth across tissue scarring and cystic cavitation. Previously, we demonstrated that implantation of a microporous type-I collagen scaffold into an experimental model of SCI was capable of supporting functional recovery in the absence of extensive implant–host neural tissue integration. Here, we demonstrate the reactive host cellular responses that may be detrimental to neural tissue integration after implantation of collagen scaffolds into unilateral resection injuries of the adult rat spinal cord. Immunohistochemistry demonstrated scattered fibroblast-like cell infiltration throughout the scaffolds as well as the presence of variable layers of densely packed cells, the fine processes of which extended along the graft–host interface. Few reactive astroglial or regenerating axonal profiles could be seen traversing this layer. Such encapsulation-type behaviour around bioengineered scaffolds impedes the integration of host neural tissues and reduces the intended bridging role of the implant. Characterization of the cellular and molecular mechanisms underpinning this behaviour will be pivotal in the future design of collagen-based bridging scaffolds intended for regenerative medicine.

Biomaterials for revascularization and immunomodulation after spinal cord injury

Spinal cord injury (SCI) causes immediate damage to the nervous tissue accompanied by loss of motor and sensory function. The limited self-repair competence of injured nervous tissue underscores the need for reparative interventions to recover function after SCI. The vasculature of the spinal cord plays a crucial role in SCI and repair. Ruptured and sheared blood vessels in the injury epicenter and blood vessels with a breached blood-spinal cord barrier (BSCB) in the surrounding tissue cause bleeding and inflammation, which contribute to the overall tissue damage. The insufficient formation of new functional vasculature in and near the injury impedes endogenous tissue repair and limits the prospect of repair approaches. Limiting the loss of blood vessels, stabilizing the BSCB, and promoting the formation of new blood vessels are therapeutic targets for spinal cord repair. Inflammation is an integral part of injury-mediated vascular damage, which has deleterious and reparative consequences. Inflammation and the formation of new blood vessels are intricately interwoven. Biomaterials can be effectively used for promoting and guiding blood vessel formation or modulating the inflammatory response after SCI, thereby governing the extent of damage and the success of reparative interventions. This review deals with the vasculature after SCI, the reciprocal interactions between inflammation and blood vessel formation, and the potential of biomaterials to support revascularization and immunomodulation in damaged spinal cord nervous tissue.

Brain and spinal cord injury repair by implantation of human neural progenitor cells seeded onto polymer scaffolds

Hypoxic-ischemic (HI) brain injury and spinal cord injury (SCI) lead to extensive tissue loss and axonal degeneration. The combined application of the polymer scaffold and neural progenitor cells (NPCs) has been reported to enhance neural repair, protection and regeneration through multiple modes of action following neural injury. This study investigated the reparative ability and therapeutic potentials of biological bridges composed of human fetal brain-derived NPCs seeded upon poly(glycolic acid)-based scaffold implanted into the infarction cavity of a neonatal HI brain injury or the hemisection cavity in an adult SCI. Implantation of human NPC (hNPC)–scaffold complex reduced the lesion volume, induced survival, engraftment, and differentiation of grafted cells, increased neovascularization, inhibited glial scar formation, altered the microglial/macrophage response, promoted neurite outgrowth and axonal extension within the lesion site, and facilitated the connection of damaged neural circuits. Tract tracing demonstrated that hNPC–scaffold grafts appear to reform the connections between neurons and their targets in both cerebral hemispheres in HI brain injury and protect some injured corticospinal fibers in SCI. Finally, the hNPC–scaffold complex grafts significantly improved motosensory function and attenuated neuropathic pain over that of the controls. These findings suggest that, with further investigation, this optimized multidisciplinary approach of combining hNPCs with biomaterial scaffolds provides a more versatile treatment for brain injury and SCI.

Biodegradable scaffolds seeded with human fetal brain cells can help repair neurological injuries in rodents. A team led by Kook In Park and Il-Shin Lee from the Yonsei University College of Medicine in Seoul, South Korea, created a mesh of plastic fibers that they bathed in neural progenitor cells. Over the course of several days, these cells differentiated into different types of brain cells, including neurons and glia. The researchers implanted these cell-scaffold complexes into the sites of injury in two rodent models: newborn mice with oxygen deprivation to the brain, and adult rats with severed spinal cords. In both cases, the treatment helped the injured tissues heal and improved the neurological or motor function of the animals. The authors suggest these tissue-engineered structures could also help people with brain or spine injuries.

Dendrite regeneration of adult Drosophila sensory neurons diminishes with aging and is inhibited by epidermal-derived matrix metalloproteinase 2

DeVault et al. show that the capacity for regeneration was present in adult neurons but diminished as the animal aged. The regenerated dendrites showed preferential alignment with the extracellular matrix, and inhibition of matrix metalloproteinase 2 led to increased dendrite regeneration.

Dendrites possess distinct structural and functional properties that enable neurons to receive information from the environment as well as other neurons. Despite their key role in neuronal function, current understanding of the ability of neurons to regenerate dendrites is lacking. This study characterizes the structural and functional capacity for dendrite regeneration in vivo in adult animals and examines the effect of neuronal maturation on dendrite regeneration. We focused on the class IV dendritic arborization (c4da) neuron of the Drosophila sensory system, which has a dendritic arbor that undergoes dramatic remodeling during the first 3 d of adult life and then maintains a relatively stable morphology thereafter. Using a laser severing paradigm, we monitored regeneration after acute and spatially restricted injury. We found that the capacity for regeneration was present in adult neurons but diminished as the animal aged. Regenerated dendrites recovered receptive function. Furthermore, we found that the regenerated dendrites show preferential alignment with the extracellular matrix (ECM). Finally, inhibition of ECM degradation by inhibition of matrix metalloproteinase 2 (Mmp2) to preserve the extracellular environment characteristics of young adults led to increased dendrite regeneration. These results demonstrate that dendrites retain regenerative potential throughout adulthood and that regenerative capacity decreases with aging.

Increased vascularization promotes functional recovery in the transected spinal cord rats by implanted vascular endothelial growth factor-targeting collagen scaffold

Spinal cord injury (SCI) is global health concern. The effective strategies for SCI are relevant to the improvement on nerve regeneration microenvironment. Vascular endothelial growth factor (VEGF) is an important cytokine for inducing angiogenesis and accelerating nerve system function recovery from injury. We proposed that VEGF could improve nerve regeneration in SCI. However, an uncontrolled delivery system target to injury site not only decreases the therapeutic efficacy but also increases the risk of tumor information. We implanted collagen scaffold (CS) targeted with a constructed protein, collagen-binding VEGF (CBD-VEGF), to bridge transected spine cord gap in a rat transected SCI model. Functional and histological examinations were conducted to assess the repair capacity of the delivery system CS/CBD-VEGF. The results indicated that the implantation of CS/CBD-VEGF into the model rats improved the survival rate and exerted beneficial effect on functional recovery. The controlled intervention improved the microenvironment, guided axon growth, and promoted neovascularization at the injury site. Therefore, the delivery system with stable binding of VEGF potentially provides a better therapeutic option for SCI. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1024-1034, 2018.

Assessment of the characteristics and biocompatibility of gelatin sponge scaffolds prepared by various crosslinking methods

This comparative study aims to identify a biocompatible and effective crosslinker for preparing gelatin sponges. Glutaraldehyde (GTA), genipin (GP), 1-ethyl-3-(3-dimethyl aminopropyl)carbodiimide (EDC), and microbial transglutaminase (mTG) were used as crosslinking agents. The physical properties of the prepared samples were characterized, and material degradation was studied in vitro with various proteases and in vivo through subcutaneous implantation of the sponges in rats. Adipose-derived stromal stem cells (ADSCs) were cultured and inoculated onto the scaffolds to compare the cellular biocompatibility of the sponges. Cellular seeding efficiency and digestion time of the sponges were also evaluated. Cellular viability and proliferation in scaffolds were analyzed by fluorescence staining and MTT assay. All the samples exhibited high porosity, good swelling ratio, and hydrolysis properties; however, material strength, hydrolysis, and enzymolytic properties varied among the samples. GTA–sponge and GP–sponge possessed high compressive moduli, and EDC–sponge exhibited fast degradation performance. GTA and GP sponge implants exerted strong in vivo rejections, and the former showed poor cell growth. mTG–sponge exhibited the optimal comprehensive performance, with good porosity, compressive modulus, anti-degradation ability, and good biocompatibility. Hence, mTG–sponge can be used as a scaffold material for tissue engineering applications.

3D scaffolds for brain tissue regeneration: architectural challenges

Critical analysis of experimental studies on 3D scaffolds for brain tissue engineering.

Covalent growth factor tethering to direct neural stem cell differentiation and self-organization

Tethered growth factors offer exciting new possibilities for guiding stem cell behavior. However, many of the current methods present substantial drawbacks which can limit their application and confound results. In this work, we developed a new method for the site-specific covalent immobilization of azide-tagged growth factors and investigated its utility in a model system for guiding neural stem cell (NSC) behavior. An engineered interferon-γ (IFN-γ) fusion protein was tagged with an N-terminal azide group, and immobilized to two different dibenzocyclooctyne-functionalized biomimetic polysaccharides (chitosan and hyaluronan). We successfully immobilized azide-tagged IFN-γ under a wide variety of reaction conditions, both in solution and to bulk hydrogels. To understand the interplay between surface chemistry and protein immobilization, we cultured primary rat NSCs on both materials and showed pronounced biological effects. Expectedly, immobilized IFN-γ increased neuronal differentiation on both materials. Expression of other lineage markers varied depending on the material, suggesting that the interplay of surface chemistry and protein immobilization plays a large role in nuanced cell behavior. We also investigated the bioactivity of immobilized IFN-γ in a 3D environment in vivo and found that it sparked the robust formation of neural tube-like structures from encapsulated NSCs. These findings support a wide range of potential uses for this approach and provide further evidence that adult NSCs are capable of self-organization when exposed to the proper microenvironment.

STATEMENT OF SIGNIFICANCE

For stem cells to be used effectively in regenerative medicine applications, they must be provided with the appropriate cues and microenvironment so that they integrate with existing tissue. This study explores a new method for guiding stem cell behavior: covalent growth factor tethering. We found that adding an N-terminal azide-tag to interferon-γ enabled stable and robust Cu-free 'click' immobilization under a variety of physiologic conditions. We showed that the tagged growth factors retained their bioactivity when immobilized and were able to guide neural stem cell lineage commitment in vitro. We also showed self-organization and neurulation from neural stem cells in vivo. This approach will provide another tool for the orchestration of the complex signaling events required to guide stem cell integration.

Transplantation of hUC-MSCs seeded collagen scaffolds reduces scar formation and promotes functional recovery in canines with chronic spinal cord injury

Spinal cord injury (SCI) can lead to locomotor deficits, and the repair of chronic SCI is considered one of the most challenging clinical problems. Although extensive studies have evaluated treatments for acute SCI in small animals, comparatively fewer studies have been conducted on large-animal SCI in the chronic phase, which is more clinically relevant. Here, we used a collagen-based biomaterial, named the NeuroRegen scaffold, loaded with human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) in a canine chronic SCI model. To generate chronic SCI, the T8 spinal cord segment was removed by complete transection of the spinal cord. Two months later, glial scar tissue was removed and a NeuroRegen scaffold was transplanted into the lesion area. Functionalized NeuroRegen scaffold implantation promoted both locomotor recovery and endogenous neurogenesis in the lesion area. Moreover, some newly generated neurons successfully matured into 5-HT-positive neurons at 1 year post-injury. In addition, many regenerated axon fibers in the lesion area exhibited remyelination and synapse formation at 1 year post-injury in the functionalized NeuroRegen scaffold group. In conclusion, the NeuroRegen scaffold functionalized with hUC-MSCs is a promising potential therapeutic approach to chronic SCI that promotes neuronal regeneration, reduces glial scar formation, and ultimately improves locomotor recovery.

Functional recovery not correlated with axon regeneration through olfactory ensheathing cell-seeded scaffolds in a model of acute spinal cord injury

The implantation of bioengineered scaffolds into lesion-induced gaps of the spinal cord is a promising strategy for promoting functional tissue repair because it can be combined with other intervention strategies. Our previous investigations showed that functional improvement following the implantation of a longitudinally microstructured collagen scaffold into unilateral mid-cervical spinal cord resection injuries of adult Lewis rats was associated with only poor axon regeneration within the scaffold. In an attempt to improve graft-host integration as well as functional recovery, scaffolds were seeded with highly enriched populations of syngeneic, olfactory bulb-derived ensheathing cells (OECs) prior to implantation into the same lesion model. Regenerating neurofilament-positive axons closely followed the trajectory of the donor OECs, as well as that of the migrating host cells within the scaffold. However, there was only a trend for increased numbers of regenerating axons above that supported by non-seeded scaffolds or in the untreated lesions. Nonetheless, significant functional recovery in skilled forelimb motor function was observed following the implantation of both seeded and non-seeded scaffolds which could not be correlated to the extent of axon regeneration within the scaffold. Mechanisms other than simple bridging of axon regeneration across the lesion must be responsible for the improved motor function.

Sterilization of collagen scaffolds designed for peripheral nerve regeneration: Effect on microstructure, degradation and cellular colonization

In this study we investigated the impact of three different sterilization methods, dry heat (DHS), ethylene oxide (EtO) and electron beam radiation (β), on the properties of cylindrical collagen scaffolds with longitudinally oriented pore channels, specifically designed for peripheral nerve regeneration. Scanning electron microscopy, mechanical testing, quantification of primary amines, differential scanning calorimetry and enzymatic degradation were performed to analyze possible structural and chemical changes induced by the sterilization. Moreover, in vitro proliferation and infiltration of the rat Schwann cell line RSC96 within the scaffolds was evaluated, up to 10days of culture. No major differences in morphology and compressive stiffness were observed among scaffolds sterilized by the different methods, as all samples showed approximately the same structure and stiffness as the unsterilized control. Proliferation, infiltration, distribution and morphology of RSC96 cells within the scaffolds were also comparable throughout the duration of the cell culture study, regardless of the sterilization treatment. However, we found a slight increase of chemical crosslinking upon sterilization (EtO<DHS<β), together with an enhanced resistance to denaturation of the EtO treated scaffolds and a significantly accelerated enzymatic degradation of the β sterilized scaffolds. The results demonstrated that β irradiation impaired the scaffold properties to a greater extent, whereas EtO exposure appeared as the most suitable method for the sterilization of the proposed scaffolds.

Engineering Biomaterials to Influence Oligodendroglial Growth, Maturation, and Myelin Production

Millions of people suffer from damage or disease to the nervous system that results in a loss of myelin, such as through a spinal cord injury or multiple sclerosis. Diminished myelin levels lead to further cell death in which unmyelinated neurons die. In the central nervous system, a loss of myelin is especially detrimental because of its poor ability to regenerate. Cell therapies such as stem or precursor cell injection have been investigated as stem cells are able to grow and differentiate into the damaged cells; however, stem cell injection alone has been unsuccessful in many areas of neural regeneration. Therefore, researchers have begun exploring combined therapies with biomaterials that promote cell growth and differentiation while localizing cells in the injured area. The regrowth of myelinating oligodendrocytes from neural stem cells through a biomaterials approach may prove to be a beneficial strategy following the onset of demyelination. This article reviews recent advancements in biomaterial strategies for the differentiation of neural stem cells into oligodendrocytes, and presents new data indicating appropriate properties for oligodendrocyte precursor cell growth. In some cases, an increase in oligodendrocyte differentiation alongside neurons is further highlighted for functional improvements where the biomaterial was then tested for increased myelination both in vitro and in vivo.

Extracellular matrix components as therapeutics for spinal cord injury

There is no treatment for people with spinal cord injury that leads to significant functional improvements. The extracellular matrix is an intricate, 3-dimensional, structural framework that defines the environment for cells in the central nervous system. The components of extracellular matrix have signaling and regulatory roles in the fate and function of neuronal and non-neuronal cells in the central nervous system. This review discusses the therapeutic potential of extracellular matrix components for spinal cord repair.

Immunomodulatory and angiogenic responses induced by graphene oxide scaffolds in chronic spinal hemisected rats

Attractive physic-chemical features of graphene oxide (GO) and promising results in vitro with neural cells encourage its exploration for biomedical applications including neural regeneration. Fueled by previous findings at the subacute state, we herein investigate for the first time chronic tissue responses (at 30 days) to 3D scaffolds composed of partially reduced GO (rGO) when implanted in the injured rat spinal cord. These studies aim to define fibrotic, inflammatory and angiogenic changes at the lesion site induced by the chronic implantation of these porous structures. Injured animals receiving no scaffolds show badly structured lesion zones and more cavities than those carrying rGO materials, thus pointing out a significant role of the scaffolds in injury stabilization and sealing. Notably, GFAP(+) cells and pro-regenerative macrophages are evident at their interface. Moreover, rGO scaffolds support angiogenesis around and, more importantly, inside their structure, with abundant and functional new blood vessels in whose proximities inside the scaffolds some regenerated neuronal axons are found. On the contrary, lesion areas without rGO scaffolds show a diminished quantity of blood vessels and no axons at all. These findings provide a foundation for the usefulness of graphene-based materials in the design of novel biomaterials for spinal cord repair and encourage further investigation for the understanding of neural tissue responses to this kind of materials in vivo.

A novel composite type I collagen scaffold with micropatterned porosity regulates the entrance of phagocytes in a severe model of spinal cord injury

Traumatic spinal cord injury (SCI) is a damage to the spinal cord that results in loss or impaired motor and/or sensory function. SCI is a sudden and unexpected event characterized by high morbidity and mortality rate during both acute and chronic stages, and it can be devastating in human, social and economical terms. Despite significant progresses in the clinical management of SCI, there remain no effective treatments to improve neurological outcomes. Among experimental strategies, bioengineered scaffolds have the potential to support and guide injured axons contributing to neural repair. The major aim of this study was to investigate a novel composite type I collagen scaffold with micropatterned porosity in a rodent model of severe spinal cord injury. After segment resection of the thoracic spinal cord we implanted the scaffold in female Sprague-Dawley rats. Controls were injured without receiving implantation. Behavioral analysis of the locomotor performance was monitored up to 55 days postinjury. Two months after injury histopathological analysis were performed to evaluate the extent of scar and demyelination, the presence of connective tissue and axonal regrowth through the scaffold and to evaluate inflammatory cell infiltration at the injured site. We provided evidence that the new collagen scaffold was well integrated with the host tissue, slightly ameliorated locomotor function, and limited the robust recruitment of the inflammatory cells at the injury site during both the acute and chronic stage in spinal cord injured rats. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1040-1053, 2017.

Intrinsic and extrinsic determinants of central nervous system axon outgrowth into alginate-based anisotropic hydrogels

Appropriate target reinnervation and functional recovery after spinal cord injury depend on longitudinally directed regrowth of injured axons. Anisotropic alginate-based capillary hydrogels (ACH) support peripheral nervous system derived axon growth, which is accompanied by glial supporting cell migration into the ACH. The aim of the present study was to analyze central nervous system (CNS) derived (entorhinal cortex, spinal cord slice cultures) axon regrowth into ACH containing linearly aligned capillaries of defined capillary sizes without and with gelatin constituent. Anisotropic ACH were prepared by ionotropic gel formation using Ba(2+), Cu(2+), Sr(2+), or Zn(2+) ions resulting in gels with average capillary diameters of 11, 13, 29, and 89μm, respectively. Postnatal rat entorhinal cortex or spinal cord slice cultures were placed on top of 500μm thick ACH. Seven days later axon growth and astroglial migration into the ACH were determined. Axon density within capillaries correlated positively with increasing capillary diameters, whereas longitudinally oriented axon outgrowth diminished with increasing capillary diameter. Axons growing into the hydrogels were always accompanied by astrocytes strongly suggesting that respective cells are required to mediate CNS axon elongation into ACH. Overall, midsize capillary diameter ACH appeared to be the best compromise between axon density and orientation. Taken together, ACH promote CNS axon ingrowth, which is determined by the capillary diameter and migration of slice culture derived astroglia into the hydrogel.

STATEMENT OF SIGNIFICANCE

Biomaterials are investigated as therapeutic tools to bridge irreversible lesions following traumatic spinal cord injury. The goal is to develop biomaterials, which promote longitudinally oriented regeneration of as many injured axons as possible as prerequisite for substantial functional recovery. Optimal parameters of the biomaterial have yet to be defined. In the present study we show that increasing capillary diameters within such hydrogels enhanced central nervous system axon regeneration at the expense of longitudinal orientation. Axon ingrowth into the hydrogels was only observed in the presence of glial supporting cells, namely astrocytes. This suggests that alginate-based hydrogels need to be colonized with respective cells in order to facilitate axon ingrowth.

Bridging the lesion-engineering a permissive substrate for nerve regeneration

Biomaterial-based strategies to restore connectivity after lesion at the spinal cord are focused on bridging the lesion and providing an favourable substrate and a path for axonal re-growth. Following spinal cord injury (SCI) a hostile environment for neuronal cell growth is established by the activation of multiple inhibitory mechanisms that hamper regeneration to occur. Implantable scaffolds can provide mechanical support and physical guidance for axon re-growth and, at the same time, contribute to alleviate the hostile environment by the in situ delivery of therapeutic molecules and/or relevant cells. Basic research on SCI has been contributing with the description of inhibitory mechanisms for regeneration as well as identifying drugs/molecules that can target inhibition. This knowledge is the background for the development of combined strategies with biomaterials. Additionally, scaffold design is significantly evolving. From the early simple hollow conduits, scaffolds with complex architectures that can modulate cell fate are currently being tested. A number of promising pre-clinical studies combining scaffolds, cells, drugs and/or nucleic acids are reported in the open literature. Overall, it is considered that to address the multi-factorial inhibitory environment of a SCI, a multifaceted therapeutic approach is imperative. The progress in the identification of molecules that target inhibition after SCI and its combination with scaffolds and/or cells are described and discussed in this review.

Advances in regenerative therapies for spinal cord injury: a biomaterials approach

Spinal cord injury results in the permanent loss of function, causing enormous personal, social and economic problems. Even though neural regeneration has been proven to be a natural mechanism, central nervous system repair mechanisms are ineffective due to the imbalance of the inhibitory and excitatory factors implicated in neuroregeneration. Therefore, there is growing research interest on discovering a novel therapeutic strategy for effective spinal cord injury repair. To this direction, cell-based delivery strategies, biomolecule delivery strategies as well as scaffold-based therapeutic strategies have been developed with a tendency to seek for the answer to a combinatorial approach of all the above. Here we review the recent advances on regenerative/neural engineering therapies for spinal cord injury, aiming at providing an insight to the most promising repair strategies, in order to facilitate future research conduction.

New trends in spinal cord tissue engineering

ABSTRACT 

Restoration of lost neuronal function after spinal cord injury still remains a considerable challenge for current medicine. Over the last decade, regenerative medicine has recorded rapid and promising advancements in stem cell research, genetic engineering and the progression of new sophisticated biomaterials as well as nanotechnology. This advancement has also been reflected in neural tissue engineering, where, along with the development of a new generation of well-designed biopolymer scaffolds, multifactorial therapeutic strategies are being validated in order to determine the greatest possible repair efficacy of the complex CNS pathophysiology. Much attention is currently focused on the designing of multifunctional polymer scaffolds as systems for targeted drug or gene delivery, electrical stimulation or as substrates creating a special micro-environment, promoting the growth and desired differentiation of various cell lines. In this review, the latest advances in biomaterial technology together with various combinatorial strategies designed to treat spinal cord injury treatment are summarized and discussed.

Astrocytes alignment and reactivity on collagen hydrogels patterned with ECM proteins

To modulate the surface properties of collagen and subsequent cell-surface interactions, a method was developed to transfer protein patterns from glass coverslips to collagen type I hydrogel surfaces. Two proteins and one proteoglycan found in central nervous system extracellular matrix as well as fibrinogen were patterned in stripes onto collagen hydrogel and astrocytes were cultured on these surfaces. The addition of the stripe protein patterns to hydrogels created astrocyte layers in which cells were aligned with underlying patterns and had reduced chondroitin sulfate expression compared to the cells grown on collagen alone. Protein patterns were covalently cross-linked to the collagen and stable over four days in culture with no visible cellular modifications. The present method can be adapted to transfer other types of protein patterns from glass coverslips to collagen hydrogels.

Functional improvement following implantation of a microstructured, type-I collagen scaffold into experimental injuries of the adult rat spinal cord

The formation of cystic cavitation following severe spinal cord injury (SCI) constitutes one of the major barriers to successful axonal regeneration and tissue repair. The development of bioengineered scaffolds that assist in the bridging of such lesion-induced gaps may contribute to the formulation of combination strategies aimed at promoting functional tissue repair. Our previous in vitro investigations have demonstrated the directed axon regeneration and glial migration supporting properties of microstructured collagen scaffold that had been engineered to possess mechanical properties similar to those of spinal cord tissues. Here, the effect of implanting the longitudinally orientated scaffold into unilateral resection injuries (2mm long) of the mid-cervical lateral funiculus of adult rats has been investigated using behavioural and correlative morphological techniques. The resection injuries caused an immediate and long lasting (up to 12 weeks post injury) deficit of food pellet retrieval by the ipsilateral forepaw. Implantation of the orientated collagen scaffold promoted a significant improvement in pellet retrieval by the ipsilateral forepaw at 6 weeks which continued to improve up to 12 weeks post injury. In contrast, implantation of a non-orientated gelatine scaffold did not result in significant functional improvement. Surprisingly, the improved motor performance was not correlated with the regeneration of lesioned axons through the implanted scaffold. This observation supports the notion that biomaterials may support functional recovery by mechanisms other than simple bridging of the lesion site, such as the local sprouting of injured, or even non-injured fibres.

Repair of injured spinal cord using biomaterial scaffolds and stem cells

The loss of neurons and degeneration of axons after spinal cord injury result in the loss of sensory and motor functions. A bridging biomaterial construct that allows the axons to grow through has been investigated for the repair of injured spinal cord. Due to the hostility of the microenvironment in the lesion, multiple conditions need to be fulfilled to achieve improved functional recovery. A scaffold has been applied to bridge the gap of the lesion as contact guidance for axonal growth and to act as a vehicle to deliver stem cells in order to modify the microenvironment. Stem cells may improve functional recovery of the injured spinal cord by providing trophic support or directly replacing neurons and their support cells. Neural stem cells and mesenchymal stem cells have been seeded into biomaterial scaffolds and investigated for spinal cord regeneration. Both natural and synthetic biomaterials have increased stem cell survival in vivo by providing the cells with a controlled microenvironment in which cell growth and differentiation are facilitated. This optimal multi‒disciplinary approach of combining biomaterials, stem cells, and biomolecules offers a promising treatment for the injured spinal cord.

Improvements in biomaterial matrices for neural precursor cell transplantation

Progress is being made in developing neuroprotective strategies for traumatic brain injuries; however, there will never be a therapy that will fully preserve neurons that are injured from moderate to severe head injuries. Therefore, to restore neurological function, regenerative strategies will be required. Given the limited regenerative capacity of the resident neural precursors of the CNS, many investigators have evaluated the regenerative potential of transplanted precursors. Unfortunately, these precursors do not thrive when engrafted without a biomaterial scaffold. In this article we review the types of natural and synthetic materials that are being used in brain tissue engineering applications for traumatic brain injury and stroke. We also analyze modifications of the scaffolds including immobilizing drugs, growth factors and extracellular matrix molecules to improve CNS regeneration and functional recovery. We conclude with a discussion of some of the challenges that remain to be solved towards repairing and regenerating the brain.

Carriers in cell-based therapies for neurological disorders

There is a pressing need for long-term neuroprotective and neuroregenerative therapies to promote full function recovery of injuries in the human nervous system resulting from trauma, stroke or degenerative diseases. Although cell-based therapies are promising in supporting repair and regeneration, direct introduction to the injury site is plagued by problems such as low transplanted cell survival rate, limited graft integration, immunorejection, and tumor formation. Neural tissue engineering offers an integrative and multifaceted approach to tackle these complex neurological disorders. Synergistic therapeutic effects can be obtained from combining customized biomaterial scaffolds with cell-based therapies. Current scaffold-facilitated cell transplantation strategies aim to achieve structural and functional rescue via offering a three-dimensional permissive and instructive environment for sustainable neuroactive factor production for prolonged periods and/or cell replacement at the target site. In this review, we intend to highlight important considerations in biomaterial selection and to review major biodegradable or non-biodegradable scaffolds used for cell transplantation to the central and peripheral nervous system in preclinical and clinical trials. Expanded knowledge in biomaterial properties and their prolonged interaction with transplanted and host cells have greatly expanded the possibilities for designing suitable carrier systems and the potential of cell therapies in the nervous system.

Molecular self-assembly guides the fabrication of peptide nanofiber scaffolds for nerve repair

The particular features render ionic self-complementary peptide-formed and peptide amphiphile-formed nanofiber scaffolds to be compelling biomaterial substrates for nerve repair.

Long-term characterization of axon regeneration and matrix changes using multiple channel bridges for spinal cord regeneration

Spinal cord injury (SCI) results in loss of sensory and motor function below the level of injury and has limited available therapies. The host response to SCI is typified by limited endogenous repair, and biomaterial bridges offer the potential to alter the microenvironment to promote regeneration. Porous multiple channel bridges implanted into the injury provide stability to limit secondary damage and support cell infiltration that limits cavity formation. At the same time, the channels provide a path that physically directs axon growth across the injury. Using a rat spinal cord hemisection injury model, we investigated the dynamics of axon growth, myelination, and scar formation within and around the bridge in vivo for 6 months, at which time the bridge has fully degraded. Axons grew into and through the channels, and the density increased overtime, resulting in the greatest axon density at 6 months postimplantation, despite complete degradation of the bridge by that time point. Furthermore, the persistence of these axons contrasts with reports of axonal dieback in other models and is consistent with axon stability resulting from some degree of connectivity. Immunostaining of axons revealed both motor and sensory origins of the axons found in the channels of the bridge. Extensive myelination was observed throughout the bridge at 6 months, with centrally located and peripheral channels seemingly myelinated by oligodendrocytes and Schwann cells, respectively. Chondroitin sulfate proteoglycan deposition was restricted to the edges of the bridge, was greatest at 1 week, and significantly decreased by 6 weeks. The dynamics of collagen I and IV, laminin, and fibronectin deposition varied with time. These studies demonstrate that the bridge structure can support substantial long-term axon growth and myelination with limited scar formation.