Engineering Silk Fibroin-Based Nerve Conduit with Neurotrophic Factors for Proximal Protection after Peripheral Nerve Injury

Engineering Injectable and Highly Interconnected Porous Silk Fibroin Microspheres for Tissue Regeneration

Injectable porous microspheres represent a promising therapeutic platform for cell delivery, drug delivery, and tissue regeneration. Yet, the engineering of silk fibroin microspheres with a highly interconnected porous structure remains an unsolved challenge. In this study, a simple and efficient method is developed that does not require the use of organic solvents to prepare silk fibroin microspheres with a predictable structure. Through extensive screening, the addition of glucose is found to direct the formation of a highly interconnected porous structure from the interior to the surface of silk fibroin microspheres. Compared to silk fibroin microspheres (SF microspheres) produced through a combination of electro-spray, cryopreservation, and freeze drying, silk fibroin-glucose microspheres (SF-Glu microspheres) demonstrates enhanced capabilities in promoting cell adhesion and proliferation in vitro. Both SF-Glu and SF microspheres exhibit the capacity to maintain the sustained release kinetics of the loaded model drug. Furthermore, SF-Glu microspheres facilitate the recruitment of endogenous cells, capillary migration, and macrophage phenotype switch following subcutaneous injection in the rats. This study opens a new avenue for the construction of porous silk fibroin microspheres, which could lead to a broader range of applications in regenerative medicine.

Synergic effects of core-shell nanospheres and magnetic field for sciatic nerve regeneration in decellularized artery conduits with Schwann cells

Numerous conduits have been developed to improve peripheral nerve regeneration. However, challenges remain, including remote control of conduit function, and programmed cell behaviors like orientation. We synthesized Fe3O4-MnO2@Zirconium-based Metal-organic frameworks@Retinoic acid (FMZMR) core-shell and assessed their impact on Schwann cell function and behavior within conduits made from decellularized human umbilical arteries (DHUCA) under magnetic field (MF). FMZMR core-shell, featuring a spherical porous structure and catalytic properties, effectively scavenges radicals and facilitates controlled drug release under MF. The histology of the DHUCA indicates effective decellularization with adequate tensile strength and Young's modulus for sciatic nerve regeneration. In-vitro results demonstrate that FMZMR core-shell is biocompatible and promotes Schwann cell proliferation through remotely controlled drug release. Furthermore, its synergy with MF enhances cell orientation and increases neurite length by ~ 1.93-fold. Functional and histological evaluations indicate that the FMZMR core-shell combined with MF promotes nerve regeneration, decreases muscle atrophy, and enhances new neuron growth and myelin formation, without negatively affecting vital tissues. This study suggests that the synergistic effect of FMZMR core-shell with MF can alleviate some of the treatment challenges.

Engineered bio-functional material-based nerve guide conduits for optic nerve regeneration: a view from the cellular perspective, challenges and the future outlook

Nerve injuries can be tantamount to severe impairment, standard treatment such as the use of autograft or surgery comes with complications and confers a shortened relief. The mechanism relevant to the regeneration of the optic nerve seems yet to be fully uncovered. The prevailing rate of vision loss as a result of direct or indirect insult on the optic nerve is alarming. Currently, the use of nerve guide conduits (NGC) to some extent has proven reliable especially in rodents and among the peripheral nervous system, a promising ground for regeneration and functional recovery, however in the optic nerve, this NGC function seems quite unfamous. The insufficient NGC application and the unabridged regeneration of the optic nerve could be a result of the limited information on cellular and molecular activities. This review seeks to tackle two major factors (i) the cellular and molecular activity involved in traumatic optic neuropathy and (ii) the NGC application for the optic nerve regeneration. The understanding of cellular and molecular concepts encompassed, ocular inflammation, extrinsic signaling and intrinsic signaling for axon growth, mobile zinc role, Ca2+ factor associated with the optic nerve, alternative therapies from nanotechnology based on the molecular information and finally the nanotechnological outlook encompassing applicable biomaterials and the use of NGC for regeneration. The challenges and future outlook regarding optic nerve regenerations are also discussed. Upon the many approaches used, the comprehensive role of the cellular and molecular mechanism may set grounds for the efficient application of the NGC for optic nerve regeneration.

Scaffold design considerations for peripheral nerve regeneration

Peripheral nerve injury (PNI) represents a serious clinical and public health problem due to its high incurrence and poor spontaneous recovery. Compared to autograft, which is still the best current practice for long-gap peripheral nerve defects in clinics, the use of polymer-based biodegradable nerve guidance conduits (NGCs) has been gaining momentum as an alternative to guide the repair of severe PNI without the need of secondary surgery and donor nerve tissue. However, simple hollow cylindrical tubes can barely outperform autograft in terms of the regenerative efficiency especially in critical sized PNI. With the rapid development of tissue engineering technology and materials science, various functionalized NGCs have emerged to enhance nerve regeneration over the past decades. From the aspect of scaffold design considerations, with a specific focus on biodegradable polymers, this review aims to summarize the recent advances in NGCs by addressing the onerous demands of biomaterial selections, structural designs, and manufacturing techniques that contributes to the biocompatibility, degradation rate, mechanical properties, drug encapsulation and release efficiency, immunomodulation, angiogenesis, and the overall nerve regeneration potential of NGCs. In addition, several commercially available NGCs along with their regulation pathways and clinical applications are compared and discussed. Lastly, we discuss the current challenges and future directions attempting to provide inspiration for the future design of ideal NGCs that can completely cure long-gap peripheral nerve defects.

Electrical stimulation enhances sciatic nerve regeneration using a silk-based conductive scaffold beyond traditional nerve guide conduits

Despite recent advancements in peripheral nerve regeneration, the creation of nerve conduits with chemical and physical cues to enhance glial cell function and support axonal growth remains challenging. This study aimed to assess the impact of electrical stimulation (ES) using a conductive nerve conduit on sciatic nerve regeneration in a rat model with transection injury. The study involved the fabrication of conductive nerve conduits using silk fibroin and Au nanoparticles (AuNPs). Collagen hydrogel loaded with green fluorescent protein (GFP)-positive adipose-derived mesenchymal stem cells (ADSCs) served as the filling for the conduit. Both conductive and non-conductive conduits were applied with and without ES in rat models. Locomotor recovery was assessed using walking track analysis. Histological evaluations were performed using H&E, luxol fast blue staining and immunohistochemistry. Moreover, TEM analysis was conducted to distinguish various ultrastructural aspects of sciatic tissue. In the ES + conductive conduit group, higher S100 (p < 0.0001) and neurofilament (p < 0.001) expression was seen after 6 weeks. Ultrastructural evaluations showed that conductive scaffolds with ES minimized Wallerian degeneration. Furthermore, the conductive conduit with ES group demonstrated significantly increased myelin sheet thickness and decreased G. ratio compared to the autograft. Immunofluorescent images confirmed the presence of GFP-positive ADSCs by the 6th week. Locomotor recovery assessments revealed improved function in the conductive conduit with ES group compared to the control group and groups without ES. These results show that a Silk/AuNPs conduit filled with ADSC-seeded collagen hydrogel can function as a nerve conduit, aiding in the restoration of substantial gaps in the sciatic nerve with ES. Histological and locomotor evaluations indicated that ES had a greater impact on functional recovery compared to using a conductive conduit alone, although the use of conductive conduits did enhance the effects of ES.

Beyond the limiting gap length: peripheral nerve regeneration through implantable nerve guidance conduits

Peripheral nerve damage results in the loss of sensorimotor and autonomic functions, which is a significant burden to patients. Furthermore, nerve injuries greater than the limiting gap length require surgical repair. Although autografts are the preferred clinical choice, their usage is impeded by their limited availability, dimensional mismatch, and the sacrifice of another functional donor nerve. Accordingly, nerve guidance conduits, which are tubular scaffolds engineered to provide a biomimetic environment for nerve regeneration, have emerged as alternatives to autografts. Consequently, a few nerve guidance conduits have received clinical approval for the repair of short-mid nerve gaps but failed to regenerate limiting gap damage, which represents the bottleneck of this technology. Thus, it is still necessary to optimize the morphology and constituent materials of conduits. This review summarizes the recent advances in nerve conduit technology. Several manufacturing techniques and conduit designs are discussed, with emphasis on the structural improvement of simple hollow tubes, additive manufacturing techniques, and decellularized grafts. The main objective of this review is to provide a critical overview of nerve guidance conduit technology to support regeneration in long nerve defects, promote future developments, and speed up its clinical translation as a reliable alternative to autografts.

Bredigite-Based Bioactive Nerve Guidance Conduit for Pro-Healing Macrophage Polarization and Peripheral Nerve Regeneration

Structural and functional healing of peripheral nerves damaged by trauma or chronic disease remain major clinical challenges, requiring the development of an effective nerve guidance conduit (NGC). The present study investigates a NGC fabrication strategy based on bredigite (BRT, Ca7MgSi4O16) bioceramic for the treatment of peripheral nerve injury. Here, BRT bioceramic shows good biocompatibility and sustainable release of Ca2+, Mg2+, and Si4+ ions. Both BRT extracts and BRT-incorporating electrospun membranes promote the proliferation and myelination potential of RSC96 cells, as well as accelerate vascular formation by human umbilical vein endothelial cells. Notably, BRT facilitates RAW 264.7 cell polarization to the pro-healing phenotype under LPS-induced inflammatory stimulation. More importantly, the macrophages activated by BRT in turn promote RSC96 cell migration and remyelination. In a rat sciatic nerve defect model, improved electrophysiological performance and alleviated gastrocnemius muscle atrophy are observed at 12 weeks post-implantation. Further experiments verify that BRT-loaded NGC facilitates axonal regrowth and revascularization with high M2-like macrophage infiltration. These findings support the beneficial effects of BRT for creating a pro-healing immune microenvironment and orchestrating multicellular processes associated with functional nerve regeneration, indicating the potential of rationally engineered bioceramics as safe, effective, and economical materials for peripheral nerve repair.

Effects of Physical Cues on Stem Cell-Derived Extracellular Vesicles toward Neuropathy Applications

The peripheral nervous system undergoes sufficient stress when affected by diabetic conditions, chemotherapeutic drugs, and personal injury. Consequently, peripheral neuropathy arises as the most common complication, leading to debilitating symptoms that significantly alter the quality and way of life. The resulting chronic pain requires a treatment approach that does not simply mask the accompanying symptoms but provides the necessary external environment and neurotrophic factors that will effectively facilitate nerve regeneration. Under normal conditions, the peripheral nervous system self-regenerates very slowly. The rate of progression is further hindered by the development of fibrosis and scar tissue formation, which does not allow sufficient neurite outgrowth to the target site. By incorporating scaffolding supplemented with secretome derived from human mesenchymal stem cells, it is hypothesized that neurotrophic factors and cellular signaling can facilitate the optimal microenvironment for nerve reinnervation. However, conventional methods of secretory vesicle production are low yield, thus requiring improved methods to enhance paracrine secretions. This report highlights the state-of-the-art methods of neuropathy treatment as well as methods to optimize the clinical application of stem cells and derived secretory vesicles for nerve regeneration.

One-Pot Assembly of Drug-Eluting Silk Coatings with Applications for Nerve Regeneration

Clinical use of polymeric scaffolds for tissue engineering often suffers from their inability to promote strong cellular interactions. Functionalization with biomolecules may improve outcomes; however, current functionalization approaches using covalent chemistry or physical adsorption can lead to loss of biomolecule bioactivity. Here, we demonstrate a novel bottom-up approach for enhancing the bioactivity of poly(l-lactic acid) electrospun scaffolds though interfacial coassembly of protein payloads with silk fibroin into nanothin coatings. In our approach, protein payloads are first added into an aqueous solution with Bombyx mori-derived silk fibroin. Phosphate anions are then added to trigger coassembly of the payload and silk fibroin, as well as noncovalent formation of a payload-silk fibroin coating at poly(l-lactic) acid fiber surfaces. Importantly, the coassembly process results in homogeneous distribution of protein payloads, with the loading quantity depending on payload concentration in solution and coating time. This coassembly process yields greater loading capacity than physical adsorption methods, and the payloads can be released over time in physiologically relevant conditions. We also demonstrate that the coating coassembly process can incorporate nerve growth factor and that coassembled coatings lead to significantly more neurite extension than loading via adsorption in a rat dorsal root ganglia explant culture model.

Neuro-bone tissue engineering: emerging mechanisms, potential strategies, and current challenges

The skeleton is a highly innervated organ in which nerve fibers interact with various skeletal cells. Peripheral nerve endings release neurogenic factors and sense skeletal signals, which mediate bone metabolism and skeletal pain. In recent years, bone tissue engineering has increasingly focused on the effects of the nervous system on bone regeneration. Simultaneous regeneration of bone and nerves through the use of materials or by the enhancement of endogenous neurogenic repair signals has been proven to promote functional bone regeneration. Additionally, emerging information on the mechanisms of skeletal interoception and the central nervous system regulation of bone homeostasis provide an opportunity for advancing biomaterials. However, comprehensive reviews of this topic are lacking. Therefore, this review provides an overview of the relationship between nerves and bone regeneration, focusing on tissue engineering applications. We discuss novel regulatory mechanisms and explore innovative approaches based on nerve-bone interactions for bone regeneration. Finally, the challenges and future prospects of this field are briefly discussed.

Novel Bioinspired Nerve Scaffold with High Synchrony between Biodegradation and Nerve Regeneration for Repair of Peripheral Nerve Injury

The morphological structure reconstruction and functional recovery of long-distance peripheral nerve injury (PNI) are global medical challenges. Biodegradable nerve scaffolds that provide mechanical support for the growth and extension of neurites are a desired way to repair long-distance PNI. However, the synchrony of scaffold degradation and nerve regeneration is still challenging. Here, a novel bioinspired multichannel nerve guide conduit (MNGC) with topographical cues based on silk fibroin and ε-polylysine modification was constructed. This conduit (SF(A) + PLL MNGC) exhibited sufficient mechanical strength, excellent degradability, and favorable promotion of cell growth. Peripheral nerve repairing was evaluated by an in vivo 10 mm rat sciatic model. In vivo evidence demonstrated that SF(A) + PLL MNGC was completely biodegraded in the body within 4 weeks after providing sufficient physical support and guide for neurite extension, and a 10 mm sciatic nerve defect was effectively repaired without scar formation, indicating a high synchronous effect of scaffold biodegradation and nerve regeneration. More importantly, the regenerated nerve of the SF(A) + PLL MNGC group showed comparable morphological reconstruction and functional recovery to that of autologous nerve transplantation. This work proved that the designed SF(A) + PLL MNGC has potential for application in long-distance PNI repair in the clinic.

Nerve guide conduits, nerve transfers, and local and free muscle transfer in facial nerve palsy

PURPOSE OF REVIEW

To highlight the recent literature on reinnervation options in the management of facial nerve paralysis using nerve conduits, and nerve and muscle transfers.

RECENT FINDINGS

Engineering of natural and synthetic nerve conduits has progressed and many of these products are now available on the market. The use of the masseter nerve has become more popular recently as a choice in nerve transfer procedures due to various unique advantages. Various authors have recently described mimetic muscle reinnervation using more than one nerve transfer, as well as dual and triple innervation of free muscle transfer.

SUMMARY

The ideal nerve conduit continues to be elusive, however significant progress has been made with many natural and synthetic materials and designs tested and introduced on the market. Many authors have modified the classic approaches in motor nerve transfer, as well as local and free muscle transfer, and described new ones, that aim to combine their advantages, particularly the simplification to a single stage and use of multiple reinnervation to the mimetic muscles. These advances are valuable to the reconstructive surgeon as powerful tools that can be tailored to the unique challenges of patients with facial nerve palsy looking for dynamic reanimation options.

A brief review on the mechanisms and approaches of silk spinning-inspired biofabrication

Silk spinning, observed in spiders and insects, exhibits a remarkable biological source of inspiration for advanced polymer fabrications. Because of the systems design, silk spinning represents a holistic and circular approach to sustainable polymer fabrication, characterized by renewable resources, ambient and aqueous processing conditions, and fully recyclable "wastes." Also, silk spinning results in structures that are characterized by the combination of monolithic proteinaceous composition and mechanical strength, as well as demonstrate tunable degradation profiles and minimal immunogenicity, thus making it a viable alternative to most synthetic polymers for the development of advanced biomedical devices. However, the fundamental mechanisms of silk spinning remain incompletely understood, thus impeding the efforts to harness the advantageous properties of silk spinning. Here, we present a concise and timely review of several essential features of silk spinning, including the molecular designs of silk proteins and the solvent cues along the spinning apparatus. The solvent cues, including salt ions, pH, and water content, are suggested to direct the hierarchical assembly of silk proteins and thus play a central role in silk spinning. We also discuss several hypotheses on the roles of solvent cues to provide a relatively comprehensive analysis and to identify the current knowledge gap. We then review the state-of-the-art bioinspired fabrications with silk proteins, including fiber spinning and additive approaches/three-dimensional (3D) printing. An emphasis throughout the article is placed on the universal characteristics of silk spinning developed through millions of years of individual evolution pathways in spiders and silkworms. This review serves as a stepping stone for future research endeavors, facilitating the in vitro recapitulation of silk spinning and advancing the field of bioinspired polymer fabrication.

Silk fibroin carriers with sustained release capacity for treating neurological diseases

Neurological diseases such as traumatic brain injury, cerebral ischemia, Parkinson's, and Alzheimer's disease usually occur in the central and peripheral nervous system and result in nervous dysfunction, such as cognitive impairment and motor dysfunction. Long-term clinical intervention is necessary for neurological diseases where neural stem cell transplantation has made substantial progress. However, many risks remain for cell therapy, such as puncture bleeding, postoperative infection, low transplantation success rate, and tumor formation. Sustained drug delivery, which aims to maintain the desired steady-state drug concentrations in plasma or local injection sites, is considered as a feasible option to help overcome side effects and improve the therapeutic efficiency of drugs on neurological diseases. Natural polymers such as silk fibroin have excellent biocompatibility, which can be prepared for various end-use material formats, such as microsphere, gel, coating/film, scaffold/conduit, microneedle, and enables the dynamic release of loaded drugs to achieve a desired therapeutic response. Sustained-release drug delivery systems are based on the mechanism of diffusion and degradation by altering the structures of silk fibroin and drugs, factors, and cells, which can induce nerve recovery and restore the function of the nervous system in a slow and persistent manner. Based on these desirable properties of silk fibroin as a carrier with sustained-release capacity, this paper discusses the role of various forms of silk fibroin-based drug delivery materials in treating neurological diseases in recent years.

Silk-in-Silk Nerve Guidance Conduits Enhance Regeneration in a Rat Sciatic Nerve Injury Model

Advanced nerve guidance conduits can provide an off-the-shelf alternative to autografts for the rehabilitation of segmental peripheral nerve injuries. In this study, the excellent processing ability of silk fibroin and the outstanding cell adhesion quality of spider dragline silk are combined to generate a silk-in-silk conduit for nerve repair. Fibroin-based silk conduits (SC) are characterized, and Schwann cells are seeded on the conduits and spider silk. Rat sciatic nerve (10 mm) defects are treated with an autograft (A), an empty SC, or a SC filled with longitudinally aligned spider silk fibers (SSC) for 14 weeks. Functional recovery, axonal re-growth, and re-myelination are assessed. The material characterizations determine a porous nature of the conduit. Schwann cells accept the conduit and spider silk as growth substrate. The in vivo results show a significantly faster functional regeneration of the A and SSC group compared to the SC group. In line with the functional results, the histomorphometrical analysis determines a comparable axon density of the A and SSC groups, which is significantly higher than the SC group. These findings demonstrate that the here introduced silk-in-silk nerve conduit achieves a similar regenerative performance as autografts largely due to the favorable guiding properties of spider dragline silk.

Transection injury differentially alters the proteome of the human sural nerve

Regeneration after severe peripheral nerve injury is often poor. Knowledge of human nerve regeneration and the growth microenvironment is greatly lacking. We aimed to identify the regenerative proteins in human peripheral nerve by comparing the proteome before and after a transection injury. In a unique study design, we collected closely matched samples of naïve and injured sural nerve. Naïve and injured (two weeks after injury) samples were analyzed using mass spectrometry and immunoassays. We found significantly altered levels following the nerve injury. Mass spectrometry revealed that injury samples had 568 proteins significantly upregulated and 471 significantly downregulated compared to naïve samples (q-value ≤ 0.05 and Z ≥ |2| (log2)). We used Gene Ontology (GO) pathway overrepresentation analysis to highlight groups of proteins that were significantly upregulated or downregulated with injury-induced degeneration and regeneration. Significant protein changes in key pathways were identified including growth factor levels, Schwann cell de-differentiation, myelination downregulation, epithelial-mesenchymal transition (EMT), and axonal regeneration pathways. The proteomes of the uninjured nerve compared to the degenerating/regenerating nerve may reveal biomarkers to aid in the development of repair strategies such as infusing supplemental trophic factors and in monitoring neural tissue regeneration.

The success of biomaterial-based tissue engineering strategies for peripheral nerve regeneration

Peripheral nerve injury is a clinically common injury that causes sensory dysfunction and locomotor system degeneration, which seriously affects the quality of the patients' daily life. Long gapped defects in large nerve are difficult to repair via surgery and limited donor source of autologous nerve greatly challenges the successful nerve repair by transplantation. Significantly, remarkable progress has been made in repairing the peripheral nerve injury using artificial nerve grafts and a variety of products for peripheral nerve repair have emerged been approved globally in recent years. The raw materials of these commercial products includes natural/synthetic polymers, extracellular matrix. Despite a lot of effort, the desirable functional recovery still remains great challenges in long gapped nerve defects. Thus this review discusses the recent development of tissue engineering products for peripheral nerve repair and the design of bionic grafts improving the local microenvironment for accelerating nerve regeneration against locomotor disorder, which may provide potential strategies for the repair of long gaps or thick nerve defects by multifunctional biomaterials.

A Review on the Technological Advances and Future Perspectives of Axon Guidance and Regeneration in Peripheral Nerve Repair

Despite a significant advance in the pathophysiological understanding of peripheral nerve damage, the successful treatment of large nerve defects remains an unmet medical need. In this article, axon growth guidance for peripheral nerve regeneration was systematically reviewed and discussed mainly from the engineering perspective. In addition, the common approaches to surgery, bioengineering approaches to emerging technologies such as optogenetic stimulation and magnetic stimulation for functional recovery were discussed, along with their pros and cons. Additionally, clear future perspectives of axon guidance and nerve regeneration were addressed.

Natural Polymer‐Derived Bioscaffolds for Peripheral Nerve Regeneration

In recent decades, artificial nerve scaffolds have become a promising substitute for peripheral nerve repair. Considerable efforts have been devoted to improving the therapeutic effectiveness of artificial scaffolds. Among numerous biomaterials for tissue engineering scaffolds fabrication, natural polymers are considered as tremendous candidates because of their excellent biocompatibility, low toxicity, high cell affinity, wide source, and environmental protection. With the development of engineering technology, a variety of natural polymer‐derived nerve scaffolds have emerged, which are endowed with biological properties and appropriate physicochemical performances to gradually adapt to the needs of nerve regeneration. Significantly, the intergradation of exogenous biomolecules onto the artificial scaffolds is able to avoid low stability, rapid degradation, and redistribution of direct therapeutic drugs in vivo, thereby enhancing nerve regeneration and functional reconstruction. Here, the development of nerve scaffolds derived from natural polymers, and their applications in continuous administration and peripheral nerve regeneration are comprehensively and carefully reviewed, providing an advanced perspective of the field.

In Situ Forming Hydrogel as a Tracer and Degradable Lacrimal Plug for Dry Eye Treatment

Lacrimal plug is an effective and widely therapeutic strategy to treat dry eye. However, almost all commercialized plugs are fixed in a certain design and associated with many complications, such as spontaneous plug extrusion, epiphora, and granuloma and cannot be traced in the long-term. Herein, a simple in situ forming hydrogel is developed as a tracer and degradable lacrimal plug to achieve the best match with the irregular lacrimal passages. In this strategy, methacrylate-modified silk fibroin (SFMA) is served as a network, and a self-assembled indocyanine green fluorescence tracer nanoparticle (FTN) is embedded as an indicator to develop the hydrogel plug using visible photo-crosslinking. This SFMA/FTN hydrogel plug has excellent biocompatibility and biodegradability, which can be noninvasively monitored by near-infrared light. In vivo tests based on dry eye rabbits show that the SFMA/FTN hydrogel plug can completely block the lacrimal passages and greatly improve the various clinical indicators of dry eye. These results demonstrate that the SFMA/FTN hydrogel is suitable as an injectable and degradable lacrimal plug with a long-term tracking function. The work offers a new approach to the development of absorbable plugs for the treatment of dry eye.

The Role of Metals in the Neuroregenerative Action of BDNF, GDNF, NGF and Other Neurotrophic Factors

Mature neurotrophic factors and their propeptides play key roles ranging from the regulation of neuronal growth and differentiation to prominent participation in neuronal survival and recovery after injury. Their signaling pathways sculpture neuronal circuits during brain development and regulate adaptive neuroplasticity. In addition, neurotrophic factors provide trophic support for damaged neurons, giving them a greater capacity to survive and maintain their potential to regenerate their axons. Therefore, the modulation of these factors can be a valuable target for treating or preventing neurologic disorders and age-dependent cognitive decline. Neuroregenerative medicine can take great advantage by the deepening of our knowledge on the molecular mechanisms underlying the properties of neurotrophic factors. It is indeed an intriguing topic that a significant interplay between neurotrophic factors and various metals can modulate the outcome of neuronal recovery. This review is particularly focused on the roles of GDNF, BDNF and NGF in motoneuron survival and recovery from injuries and evaluates the therapeutic potential of various neurotrophic factors in neuronal regeneration. The key role of metal homeostasis/dyshomeostasis and metal interaction with neurotrophic factors on neuronal pathophysiology is also highlighted as a novel mechanism and potential target for neuronal recovery. The progress in mechanistic studies in the field of neurotrophic factor-mediated neuroprotection and neural regeneration, aiming at a complete understanding of integrated pathways, offers possibilities for the development of novel neuroregenerative therapeutic approaches.

Implantable nerve guidance conduits: Material combinations, multi-functional strategies and advanced engineering innovations

Nerve guidance conduits (NGCs) have attracted much attention due to their great necessity and applicability in clinical use for the peripheral nerve repair. Great efforts in recent years have been devoted to the development of high-performance NGCs using various materials and strategies. The present review provides a comprehensive overview of progress in the material innovation, structural design, advanced engineering technologies and multi functionalization of state-of-the-art nerve guidance conduits NGCs. Abundant advanced engineering technologies including extrusion-based system, laser-based system, and novel textile forming techniques in terms of weaving, knitting, braiding, and electrospinning techniques were also analyzed in detail. Findings arising from this review indicate that the structural mimetic NGCs combined with natural and synthetic materials using advanced manufacturing technologies can make full use of their complementary advantages, acquiring better biomechanical properties, chemical stability and biocompatibility. Finally, the existing challenges and future opportunities of NGCs were put forward aiming for further research and applications of NGCs.

Implantable Biomaterials for Peripheral Nerve Regeneration-Technology Trends and Translational Tribulations

The use of autografted nerve in surgical repair of peripheral nerve injuries (PNI) is severely limited due to donor site morbidity and restricted tissue availability. As an alternative, synthetic nerve guidance channels (NGCs) are available on the market for surgical nerve repair, but they fail to promote nerve regeneration across larger critical gap nerve injuries. Therefore, such injuries remain unaddressed, result in poor healing outcomes and are a limiting factor in limb reconstruction and transplantation. On the other hand, a myriad of advanced biomaterial strategies to address critical nerve injuries are proposed in preclinical literature but only few of those have found their way into clinical practice. The design of synthetic nerve grafts should follow rational criteria and make use of a combination of bioinstructive cues to actively promote nerve regeneration. To identify the most promising NGC designs for translation into applicable products, thorough mode of action studies, standardized readouts and validation in large animals are needed. We identify design criteria for NGC fabrication according to the current state of research, give a broad overview of bioactive and functionalized biomaterials and highlight emerging composite implant strategies using therapeutic cells, soluble factors, structural features and intrinsically conductive substrates. Finally, we discuss translational progress in bioartificial conduits for nerve repair from the surgeon's perspective and give an outlook toward future challenges in the field.

Design and Fabrication of Polymeric Hydrogel Carrier for Nerve Repair

Nerve regeneration and repair still remain a huge challenge for both central nervous and peripheral nervous system. Although some therapeutic substances, including neuroprotective agents, clinical drugs and stem cells, as well as various growth factors, are found to be effective to promote nerve repair, a carrier system that possesses a sustainable release behavior, in order to ensure high on-site concentration during the whole repair and regeneration process, and high bioavailability is still highly desirable. Hydrogel, as an ideal delivery system, has an excellent loading capacity and sustainable release behavior, as well as tunable physical and chemical properties to adapt to various biomedical scenarios; thus, it is thought to be a suitable carrier system for nerve repair. This paper reviews the structure and classification of hydrogels and summarizes the fabrication and processing methods that can prepare a suitable hydrogel carrier with specific physical and chemical properties. Furthermore, the modulation of the physical and chemical properties of hydrogels is also discussed in detail in order to obtain a better therapeutic effect to promote nerve repair. Finally, the future perspectives of hydrogel microsphere carriers for stroke rehabilitation are highlighted.

Nerve Growth Factor-Laden Anisotropic Silk Nanofiber Hydrogels to Regulate Neuronal/Astroglial Differentiation for Scarless Spinal Cord Repair

Scarless spinal cord regeneration remains a challenge due to the complicated microenvironment at lesion sites. In this study, the nerve growth factor (NGF) was immobilized in silk protein nanofiber hydrogels with hierarchical anisotropic microstructures to fabricate bioactive systems that provide multiple physical and biological cues to address spinal cord injury (SCI). The NGF maintained bioactivity inside the hydrogels and regulated the neuronal/astroglial differentiation of neural stem cells. The aligned microstructures facilitated the migration and orientation of cells, which further stimulated angiogenesis and neuron extensions both in vitro and in vivo. In a severe rat long-span hemisection SCI model, these hydrogel matrices reduced scar formation and achieved the scarless repair of the spinal cord and effective recovery of motor functions. Histological analysis confirmed the directional regenerated neuronal tissues, with a similar morphology to that of the normal spinal cord. The in vitro and in vivo results showed promising utility for these NGF-laden silk hydrogels for spinal cord regeneration while also demonstrating the feasibility of cell-free bioactive matrices with multiple cues to regulate endogenous cell responses.

Peripheral Nerve Injury Treatments and Advances: One Health Perspective

Peripheral nerve injuries (PNI) can have several etiologies, such as trauma and iatrogenic interventions, that can lead to the loss of structure and/or function impairment. These changes can cause partial or complete loss of motor and sensory functions, physical disability, and neuropathic pain, which in turn can affect the quality of life. This review aims to revisit the concepts associated with the PNI and the anatomy of the peripheral nerve is detailed to explain the different types of injury. Then, some of the available therapeutic strategies are explained, including surgical methods, pharmacological therapies, and the use of cell-based therapies alone or in combination with biomaterials in the form of tube guides. Nevertheless, even with the various available treatments, it is difficult to achieve a perfect outcome with complete functional recovery. This review aims to enhance the importance of new therapies, especially in severe lesions, to overcome limitations and achieve better outcomes. The urge for new approaches and the understanding of the different methods to evaluate nerve regeneration is fundamental from a One Health perspective. In vitro models followed by in vivo models are very important to be able to translate the achievements to human medicine.

Chitosan Tubes Prefilled with Aligned Fibrin Nanofiber Hydrogel Enhance Facial Nerve Regeneration in Rabbits

Facial nerves are fragile and easily injured, for example, by traffic accidents or operations. Facial nerve injury drastically reduces the quality of life in affected patients, and its treatment presents clinical challenges. A promising therapeutic strategy includes nerve conduits with appropriate fillers capable of guiding nerve regeneration. In this study, a three-dimensional hierarchically aligned fibrin nanofiber hydrogel (AFG) assembled via electrospinning and molecular self-assembly was first used to mimic the architecture of the native fibrin cable, which is similar to the nerve extracellular matrix (ECM). AFG as a substrate in chitosan tubes (CST) was used to bridge a 7 mm-long gap in a rabbit buccal branch facial nerve defect model. The results showed that AFG and CST showed good compatibility to support the adhesion, activity, and proliferation of Schwann cells (SCs). Further morphological, histological, and functional analyses demonstrated that the regenerative outcome of AFG-prefilled CST was close to that of autologous nerve grafts and superior to that of CST alone or CSTs prefilled with random fibrin nanofiber hydrogel (RFG), which indicated that AFG-prefilled CST markedly improved axonal regeneration with enhanced remyelination and functional recovery, thus showing great potential for clinical application for facial nerve regeneration treatments.

Aligned fibrin/functionalized self-assembling peptide interpenetrating nanofiber hydrogel presenting multi-cues promotes peripheral nerve functional recovery

Nerve guidance conduits with hollow lumen fail to regenerate critical-sized peripheral nerve defects (15 mm in rats and 25 mm in humans), which can be improved by a beneficial intraluminal microenvironment. However, individual cues provided by intraluminal filling materials are inadequate to eliminate the functional gap between regenerated nerves and normal nerves. Herein, an aligned fibrin/functionalized self-assembling peptide (AFG/fSAP) interpenetrating nanofiber hydrogel that exerting synergistic topographical and biochemical cues for peripheral nerve regeneration is constructed via electrospinning and molecular self-assembly. The hydrogel possesses an aligned structure, high water content, appropriate mechanical properties and suitable biodegradation capabilities for nerve repair, which enhances the alignment and neurotrophin secretion of primary Schwann cells (SCs) in vitro, and successfully bridges a 15-mm sciatic nerve gap in rats in vivo. The rats transplanted with the AFG/fSAP hydrogel exhibit satisfactory morphological and functional recovery in myelinated nerve fibers and innervated muscles. The motor function recovery facilitated by the AFG/fSAP hydrogel is comparable with that of autografts. Moreover, the AFG/fSAP hydrogel upregulates the regeneration-associated gene expression and activates the PI3K/Akt and MAPK signaling pathways in the regenerated nerve. Altogether, the AFG/fSAP hydrogel represents a promising approach for peripheral nerve repair through an integration of structural guidance and biochemical stimulation.

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A novel aligned interpenetrating nanofiber hydrogel is first constructed for peripheral nerve regeneration.

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The aligned hydrogel presents synergistic topographical and biochemical cues for peripheral nerve regeneration.

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Nerve conduits filled with the aligned hydrogel can repair the long-distance sciatic nerve defects in 12 weeks.

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The function recovery facilitated by the aligned hydrogel is comparable with that of autografts.

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The aligned hydrogel upregulates regeneration-related genes and activates the PI3K/Akt and MAPK signaling pathways.

Fabrication of a dual-layer cell-laden tubular scaffold for nerve regeneration and bile duct reconstruction

Tubular scaffolds serve as a controllable extracellular environment to guide the repair and regeneration of tissues. But it is still a challenge to achieve both excellent mechanical properties and cell compatibility of artificial scaffolds for long-term structural and biological stability. In this study, a four-step solution casting method was developed to fabricate dual-layer cell-laden tubular scaffolds for nerve and bile duct regeneration. The dual-layer tubular scaffold consisted of a bone marrow mesenchymal stem cells (BMSCs)-laden hydrogel inner layer and an outer layer of gelatin methacrylate (GelMA)/polyethylene glycol diacrylate. While the inner layer had a good biocompatibility, the outer layer had desired mechanical properties. The interfacial toughness, Young's modulus, maximum tensile strain, and compressive modulus of dual-layer tubular scaffolds were 65 J m^-2, 122.37 ± 23.21 kPa, 100.87 ± 40.10%, and 39.14 ± 18.56 N m^-1, respectively. More importantly, the fabrication procedure was very cell-friendly, since the BMSC viability encapsulated in the inner layer of 10% (w/v) GelMA reached 94.68 ± 0.43% after 5 d of culture. Then, a preliminary evaluation of the potential application of dual-layer tubular scaffolds as nerve conduits and biliary scaffolds was performed, and demonstrated that the cell-laden dual-layer tubular scaffolds proposed in this work are expected to extend the application of tubular scaffolds in tissue engineering.