Biomimetic Architectures for Peripheral Nerve Repair: A Review of Biofabrication Strategies

Advances in Exosome-Based Therapies for the Repair of Peripheral Nerve Injuries

Peripheral nerve injuries (PNIs) are the term used to describe injuries that occur to the nerve fibers of the peripheral nervous system (PNS). Such injuries may be caused by trauma, infection, or aberrant immunological response. Although the peripheral nervous system has a limited capacity for self-repair, in cases of severe damage, this process is either interrupted entirely or is only partially completed. The evaluation of variables that promote the repair of peripheral nerves has consistently been a focal point. Exosomes are a subtype of extracellular vesicles that originate from cellular sources and possess abundant proteins, lipids, and nucleic acids, play a critical role in facilitating intercellular communication. Due to their modifiable composition, they possess exceptional capabilities as carriers for therapeutic compounds, including but not limited to mRNAs or microRNAs. Exosome-based therapies have gained significant attention in the treatment of several nervous system diseases due to their advantageous properties, such as low toxicity, high stability, and limited immune system activation. The objective of this review article is to provide an overview of exosome-based treatments that have been developed in recent years for a range of PNIs, including nerve trauma, diabetic neuropathy, amyotrophic lateral sclerosis (ALS), glaucoma, and Guillain-Barre syndrome (GBS). It was concluded that exosomes could provide favorable results in the improvement of peripheral PNIs by facilitating the transfer of regenerative factors. The development of bioengineered exosome therapy for PNIs should be given more attention to enhance the efficacy of exosome treatment for PNIs.

Application of magnetism in tissue regeneration: recent progress and future prospects

Tissue regeneration is a hot topic in the field of biomedical research in this century. Material composition, surface topology, light, ultrasonic, electric field and magnetic fields (MFs) all have important effects on the regeneration process. Among them, MFs can provide nearly non-invasive signal transmission within biological tissues, and magnetic materials can convert MFs into a series of signals related to biological processes, such as mechanical force, magnetic heat, drug release, etc. By adjusting the MFs and magnetic materials, desired cellular or molecular-level responses can be achieved to promote better tissue regeneration. This review summarizes the definition, classification and latest progress of MFs and magnetic materials in tissue engineering. It also explores the differences and potential applications of MFs in different tissue cells, aiming to connect the applications of magnetism in various subfields of tissue engineering and provide new insights for the use of magnetism in tissue regeneration.

The effects of exosomes originating from different cell sources on the differentiation of bone marrow mesenchymal stem cells into schwann cells

BACKGROUND

Bone marrow mesenchymal stem cells (BMSCs) can differentiate into Schwann cells (SCs) during peripheral nerve injury; in our previous research, we showed that SC-derived exosomes (SC-exos) played a direct induction role while fibroblast-derived exosomes (Fb-exos) had no obvious induction role. The induction role of neural stem cell (NSC)-derived exosomes (NSC-exos) has also been widely confirmed. However, no studies have compared the induction effects of these three types of cells at the same time. Therefore, by investigating the effect of these three cell-derived exosomes upon the induction of BMSCs to differentiate into SCs, this study explored the role of different exosomes in promoting the differentiation of stem cells into SCs cells, and conducted a comparison between the two groups by RNA sequencing to further narrow the range of target genes and related gene pathways in order to study their related mechanisms.

MATERIALS AND METHODS

We extracted exosomes from SCs, fibroblasts (Fb) and neural stem cells (NSC) and then investigated the ability of these exosomes to induce differentiation into BMSCs under different culture conditions. The expression levels of key proteins and gene markers were detected in induced cells by fluorescence immunoassays, western blotting and polymerase chain reaction (PCR); then, we statistically compared the relative induction effects under different conditions. Finally, we analyzed the three types of exosomes by RNA-seq to predict target genes and related gene pathways.

RESULTS

BMSCs were cultured by three media: conventional (no induction), pre-induction or pre-induction + original induction medium (ODM) with exosomes of the same cell origin under different culture conditions. When adding the three different types of exosomes separately, the overall induction of BMSCs to differentiate into SCs was significantly increased (P < 0.05). The induction ability was ranked as follows: pre-induction + ODM + exosome group > pre-induction + exosome group > non-induction + exosome group. Using exosomes from different cell sources under the same culture conditions, we observed the following trends under the three culture conditions: RSC96-exos group ≥ NSC-exos group > Fb-exos group. The overall ability to induce BMSCs into SCs was significantly greater in the RSC96-exos group and the NSC-exos group. Although there was no significant difference in induction efficiency when comparing these two groups, the overall induction ability of the RSC96-exos group was slightly higher than that of the NSC-exos group. By combining the differentiation induction results with the RNA-seq data, the three types of exosomes were divided into three comparative groups: RSC vs. NSC, RSC vs. Fb and NSC vs. Fb. We identified 203 differentially expressed mRNA target genes in these three groups. Two differentially expressed genes were upregulated simultaneously, namely riboflavin kinase (RFK, ENSRNOG00000022273) and ribosomal RNA processing 36 (Rrp36, ENSRNOG00000017836). We did not identify any co-upregulated target genes for the miRNAs, but did identify one target gene of the lncRNAs, namely ENSRNOG00000065005. Analysis identified 90 GO terms related to nerves and axons in the mRNAs; in addition, KEGG enrichment and GASA analysis identified 13 common differential expression pathways in the three groups.

CONCLUSIONS

Our analysis found that pre-induction + ODM + RSC96/NSC-exos culture conditions were most conducive with regards to induction and differentiation. RSC96-exos and NSC-exos exhibited significantly greater differentiation efficiency of BMSCs into SCs. Although there was no statistical difference, the data indicated a trend for RSC96-exos to be advantageous We identified 203 differentially expressed mRNAs between the three groups and two differentially expressed target mRNAs were upregulated, namely riboflavin kinase (RFK, ENSRNOG00000022273) and ribosomal RNA processing 36 (Rrp36, ENSRNOG00000017836). 90 GO terms were related to nerves and axons. Finally, we identified 13 common differentially expressed pathways across our three types of exosomes. It is hoped that the efficiency of BMSCs induction differentiation into SCs can be improved, bringing hope to patients and more options for clinical treatment.

Perspectives on the Novel Multifunctional Nerve Guidance Conduits: From Specific Regenerative Procedures to Motor Function Rebuilding

Peripheral nerve injury potentially destroys the quality of life by inducing functional movement disorders and sensory capacity loss, which results in severe disability and substantial psychological, social, and financial burdens. Autologous nerve grafting has been commonly used as treatment in the clinic; however, its rare donor availability limits its application. A series of artificial nerve guidance conduits (NGCs) with advanced architectures are also proposed to promote injured peripheral nerve regeneration, which is a complicated process from axon sprouting to targeted muscle reinnervation. Therefore, exploring the interactions between sophisticated NGC complexes and versatile cells during each process including axon sprouting, Schwann cell dedifferentiation, nerve myelination, and muscle reinnervation is necessary. This review highlights the contribution of functional NGCs and the influence of microscale biomaterial architecture on biological processes of nerve repair. Progressive NGCs with chemical molecule induction, heterogenous topographical morphology, electroactive, anisotropic assembly microstructure, and self-powered electroactive and magnetic-sensitive NGCs are also collected, and they are expected to be pioneering features in future multifunctional and effective NGCs.

Human umbilical cord mesenchymal stem cell-derived exosomes loaded into a composite conduit promote functional recovery after peripheral nerve injury in rats

Complete transverse injury of peripheral nerves is challenging to treat. Exosomes secreted by human umbilical cord mesenchymal stem cells are considered to play an important role in intercellular communication and regulate tissue regeneration. In previous studies, a collagen/hyaluronic acid sponge was shown to provide a suitable regeneration environment for Schwann cell proliferation and to promote axonal regeneration. This three-dimensional (3D) composite conduit contains a collagen/hyaluronic acid inner sponge enclosed in an electrospun hollow poly (lactic-co-glycolic acid) tube. However, whether there is a synergy between the 3D composite conduit and exosomes in the repair of peripheral nerve injury remains unknown. In this study, we tested a comprehensive strategy for repairing long-gap (10 mm) peripheral nerve injury that combined the 3D composite conduit with human umbilical cord mesenchymal stem cell-derived exosomes. Repair effectiveness was evaluated by sciatic functional index, sciatic nerve compound muscle action potential recording, recovery of muscle mass, measuring the cross-sectional area of the muscle fiber, Masson trichrome staining, and transmission electron microscopy of the regenerated nerve in rats. The results showed that transplantation of the 3D composite conduit loaded with human umbilical cord mesenchymal stem cell-derived exosomes promoted peripheral nerve regeneration and restoration of motor function, similar to autograft transplantation. More CD31-positive endothelial cells were observed in the regenerated nerve after transplantation of the loaded conduit than after transplantation of the conduit without exosomes, which may have contributed to the observed increase in axon regeneration and distal nerve reconnection. Therefore, the use of a 3D composite conduit loaded with human umbilical cord mesenchymal stem cell-derived exosomes represents a promising cell-free therapeutic option for the treatment of peripheral nerve injury.

Construction of in vitro liver-on-a-chip models and application progress

The liver is the largest internal organ of the human body. It has a complex structure and function and plays a vital role in drug metabolism. In recent decades, extensive research has aimed to develop in vitro models that can simulate liver function to demonstrate changes in the physiological and pathological environment of the liver. Animal models and in vitro cell models are common, but the data obtained from animal models lack relevance when applied to humans, while cell models have limited predictive ability for metabolism and toxicity in humans. Recent advancements in tissue engineering, biomaterials, chip technology, and 3D bioprinting have provided opportunities for further research in in vitro models. Among them, liver-on-a-Chip (LOC) technology has made significant achievements in reproducing the in vivo behavior, physiological microenvironment, and metabolism of cells and organs. In this review, we discuss the development of LOC and its research progress in liver diseases, hepatotoxicity tests, and drug screening, as well as chip combinations. First, we review the structure and the physiological function of the liver. Then, we introduce the LOC technology, including general concepts, preparation materials, and methods. Finally, we review the application of LOC in disease modeling, hepatotoxicity tests, drug screening, and chip combinations, as well as the future challenges and directions of LOC.

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.

Sacrificial biomaterials in 3D fabrication of scaffolds for tissue engineering applications

Three-dimensional (3D) printing technology has progressed exceedingly in the area of tissue engineering. Despite the tremendous potential of 3D printing, building scaffolds with complex 3D structure, especially with soft materials, still exist as a challenge due to the low mechanical strength of the materials. Recently, sacrificial materials have emerged as a possible solution to address this issue, as they could serve as temporary support or templates to fabricate scaffolds with intricate geometries, porous structures, and interconnected channels without deformation or collapse. Here, we outline the various types of scaffold biomaterials with sacrificial materials, their pros and cons, and mechanisms behind the sacrificial material removal, compare the manufacturing methods such as salt leaching, electrospinning, injection-molding, bioprinting with advantages and disadvantages, and discuss how sacrificial materials could be applied in tissue-specific applications to achieve desired structures. We finally conclude with future challenges and potential research directions.

Electrospun Nanofibrous Conduit Filled with a Collagen-Based Matrix (ColM) for Nerve Regeneration

Traumatic nerve defects result in dysfunctions of sensory and motor nerves and are usually accompanied by pain. Nerve guidance conduits (NGCs) are widely applied to bridge large-gap nerve defects. However, few NGCs can truly replace autologous nerve grafts to achieve comprehensive neural regeneration and function recovery. Herein, a three-dimensional (3D) sponge-filled nanofibrous NGC (sf@NGC) resembling the structure of native peripheral nerves was developed. The conduit was fabricated by electrospinning a poly(L-lactide-co-glycolide) (PLGA) membrane, whereas the intraluminal filler was obtained by freeze-drying a collagen-based matrix (ColM) resembling the extracellular matrix. The effects of the electrospinning process and of the composition of ColM on the physicochemical performance of sf@NGC were investigated in detail. Furthermore, the biocompatibility of the PLGA sheath and ColM were evaluated. The continuous and homogeneous PLGA nanofiber membrane had high porosity and tensile strength. ColM was shown to exhibit an ECM-like architecture characterized by a multistage pore structure and a high porosity level of over 70%. The PLGA sheath and ColM were shown to possess stagewise degradability and good biocompatibility. In conclusion, sf@NGC may have a favorable potential for the treatment of nerve reconstruction.

Advances in Large Gap Peripheral Nerve Injury Repair and Regeneration with Bridging Nerve Guidance Conduits

Peripheral nerve injury is a common complication of accidents and diseases. The traditional autologous nerve graft approach remains the gold standard for the treatment of nerve injuries. While sources of autologous nerve grafts are very limited and difficult to obtain. Nerve guidance conduits are widely used in the treatment of peripheral nerve injuries as an alternative to nerve autografts and allografts. However, the development of nerve conduits does not meet the needs of large gap peripheral nerve injury. Functional nerve conduits can provide a good microenvironment for axon elongation and myelin regeneration. Herein, the manufacturing methods and different design types of functional bridging nerve conduits for nerve conduits combined with electrical or magnetic stimulation and loaded with Schwann cells, etc., are summarized. It summarizes the literature and finds that the technical solutions of functional nerve conduits with electrical stimulation, magnetic stimulation and nerve conduits combined with Schwann cells can be used as effective strategies for bridging large gap nerve injury and provide an effective way for the study of large gap nerve injury repair. In addition, functional nerve conduits provide a new way to construct delivery systems for drugs and growth factors in vivo.

The Porous Structure of Peripheral Nerve Guidance Conduits: Features, Fabrication, and Implications for Peripheral Nerve Regeneration

Porous structure is an important three-dimensional morphological feature of the peripheral nerve guidance conduit (NGC), which permits the infiltration of cells, nutrients, and molecular signals and the discharge of metabolic waste. Porous structures with precisely customized pore sizes, porosities, and connectivities are being used to construct fully permeable, semi-permeable, and asymmetric peripheral NGCs for the replacement of traditional nerve autografts in the treatment of long-segment peripheral nerve injury. In this review, the features of porous structures and the classification of NGCs based on these characteristics are discussed. Common methods for constructing 3D porous NGCs in current research are described, as well as the pore characteristics and the parameters used to tune the pores. The effects of the porous structure on the physical properties of NGCs, including biodegradation, mechanical performance, and permeability, were analyzed. Pore structure affects the biological behavior of Schwann cells, macrophages, fibroblasts, and vascular endothelial cells during peripheral nerve regeneration. The construction of ideal porous structures is a significant advancement in the regeneration of peripheral nerve tissue engineering materials. The purpose of this review is to generalize, summarize, and analyze methods for the preparation of porous NGCs and their biological functions in promoting peripheral nerve regeneration to guide the development of medical nerve repair materials.

Development of Organs-on-Chips and Their Impact on Precision Medicine and Advanced System Simulation

Drugs may undergo costly preclinical studies but still fail to demonstrate their efficacy in clinical trials, which makes it challenging to discover new drugs. Both in vitro and in vivo models are essential for disease research and therapeutic development. However, these models cannot simulate the physiological and pathological environment in the human body, resulting in limited drug detection and inaccurate disease modelling, failing to provide valid guidance for clinical application. Organs-on-chips (OCs) are devices that serve as a micro-physiological system or a tissue-on-a-chip; they provide accurate insights into certain functions and the pathophysiology of organs to precisely predict the safety and efficiency of drugs in the body. OCs are faster, more economical, and more precise. Thus, they are projected to become a crucial addition to, and a long-term replacement for, traditional preclinical cell cultures, animal studies, and even human clinical trials. This paper first outlines the nature of OCs and their significance, and then details their manufacturing-related materials and methodology. It also discusses applications of OCs in drug screening and disease modelling and treatment, and presents the future perspective of OCs.

In Vitro and In Vivo Biocompatibility Assessment of a Thermosensitive Injectable Chitosan-Based Hydrogel for Musculoskeletal Tissue Engineering

Musculoskeletal impairments, especially cartilage and meniscus lesions, are some of the major contributors to disabilities. Thus, novel tissue engineering strategies are being developed to overcome these issues. In this study, the aim was to investigate the biocompatibility, in vitro and in vivo, of a thermosensitive, injectable chitosan-based hydrogel loaded with three different primary mesenchymal stromal cells. The cell types were human adipose-derived mesenchymal stromal cells (hASCs), human bone marrow stem cells (hBMSCs), and neonatal porcine infrapatellar fat-derived cells (IFPCs). For the in vitro study, the cells were encapsulated in sol-phase hydrogel, and then, analyzed via live/dead assay at 1, 4, 7, and 14 days to compare their capacity to survive in the hydrogel. To assess biocompatibility in vivo, cellularized scaffolds were subcutaneously implanted in the dorsal pouches of nude mice and analyzed at 4 and 12 weeks. Our data showed that all the different cell types survived (the live cell percentages were between 60 and 80 at all time points in vitro) and proliferated in the hydrogel (from very few at 4 weeks to up to 30% at 12 weeks in vivo); moreover, the cell-laden hydrogels did not trigger an immune response in vivo. Hence, our hydrogel formulation showed a favorable profile in terms of safety and biocompatibility, and it may be applied in tissue engineering strategies for cartilage and meniscus repair.

Effect of Electrical Stimulation on Nerve-Guided Facial Nerve Regeneration

This study aimed to investigate the effect of electrical stimulation on poly(d,l-lactide-co-ε-caprolactone) nerve guidance conduits (NGCs) in promoting the recovery of facial function and nerve regeneration after facial nerve (FN) injury in a rat model. In the experimental group, both the NGC and transcutaneous electrical nerve stimulation (ES) were used simultaneously; in the control group, only NGC was used. ES groups were divided into two groups, and direct current (DC) and charge-balanced pulse stimulation (Pulse) were applied. The ES groups showed significantly improved whisker movement than the NGC-only group. The number of myelinated neurons was higher in ES groups, and the myelin sheath was also thicker and more uniform. In addition, the expression of neurostructural proteins was also higher in ES groups than in the NGC-only group. This study revealed that FN regeneration and functional recovery occurred more efficiently when ES was applied in combination with NGCs.

Design of Novel Mechanically Resistant and Biodegradable Multichannel Platforms for the Treatment of Peripheral Nerve Injuries

Peripheral nerve injury is one of the most debilitating pathologies that severely impair patients' life. Although many efforts have been made to advance in the treatment of such a complex disorder, successful strategies to ensure full recovery are still scarce. The aim of the present work was to develop flexible and mechanically resistant platforms intended to act as a support and guide for neural cells during the regeneration process of peripheral nerve injury. For this purpose, poly(lactic-co-glycolic acid) (PLGA)/poly(d,l-lactic acid) (PDLLA)/poly(ethylene glycol) 400 (PEG)-multichannel-based scaffolds (MCs) were prepared through a multistep process involving electrospun microfibers coated with a polymer blend solution and used as a sacrificial mold. In particular, scaffolds characterized by random (MCR) and aligned (MCA) multichannel were obtained. A design of experiments approach (DoE) was employed to identify a scaffold-optimized composition. MCs were characterized for morphological and mechanical properties, suturability, degradability, cell colonization, and in vivo safety. A new biodegradable, biocompatible, and safe microscale multichannel scaffold was developed as the result of an easy multistep procedure.

Artificial PGA/Collagen-based Bilayer Conduit in Short Gap Interposition Setting Provides Comparable Regenerative Potential to Direct Suture

The aim of this study was to evaluate whether the Nerbridge, an artificial polyglycolic acid conduit with collagen matrix, is comparable to direct nerve suture in a rat sciatic nerve injury model in a short-gap interposition (SGI) setting.

Methods

Sixty-six female Lewis rats were randomly divided into the sham group (n = 13); no reconstruction (no-recon) group (n = 13; rat model with 10 mm sciatic nerve defect); direct group (n = 20; rat sciatic nerve injury directly connected by 10-0 Nylon); and SGI group (n = 20; sciatic nerve injury repaired using 5-mm Nerbridge). Motor function and histological recovery were evaluated. The sciatic nerve and gastrocnemius muscle were harvested for quantification of the degree of nerve regeneration and muscle atrophy.

Results

The SGI and direct groups achieved equal recovery in both functional and histological outcomes. At weeks 3 and 8 postsurgery, there was a significant improvement in the sciatic functional index of the SGI group when compared with that of the no-recon group (P < 0.05). Furthermore, the direct and SGI groups had less muscle atrophy at 4 and 8 weeks postsurgery compared with the no-recon group (P < 0.05). The axon density and diameter at the distal site in the SGI group were significantly higher than that in the no-recon group and comparable to that in the direct and sham groups.

Conclusion

An artificial nerve conduit has equal potential as direct suture in motor nerve reconstruction when used in the SGI setting.

Recent Advances in Polymeric Drug Delivery Systems for Peripheral Nerve Regeneration

When a traumatic event causes complete denervation, muscle functional recovery is highly compromised. A possible solution to this issue is the implantation of a biodegradable polymeric tubular scaffold, providing a biomimetic environment to support the nerve regeneration process. However, in the case of consistent peripheral nerve damage, the regeneration capabilities are poor. Hence, a crucial challenge in this field is the development of biodegradable micro- nanostructured polymeric carriers for controlled and sustained release of molecules to enhance nerve regeneration. The aim of these systems is to favor the cellular processes that support nerve regeneration to increase the functional recovery outcome. Drug delivery systems (DDSs) are interesting solutions in the nerve regeneration framework, due to the possibility of specifically targeting the active principle within the site of interest, maximizing its therapeutical efficacy. The scope of this review is to highlight the recent advances regarding the study of biodegradable polymeric DDS for nerve regeneration and to discuss their potential to enhance regenerative performance in those clinical scenarios characterized by severe nerve damage.

Chitosan and Pectin Hydrogels for Tissue Engineering and In Vitro Modeling

Hydrogels are fascinating biomaterials that can act as a support for cells, i.e., a scaffold, in which they can organize themselves spatially in a similar way to what occurs in vivo. Hydrogel use is therefore essential for the development of 3D systems and allows to recreate the cellular microenvironment in physiological and pathological conditions. This makes them ideal candidates for biological tissue analogues for application in the field of both tissue engineering and 3D in vitro models, as they have the ability to closely mimic the extracellular matrix (ECM) of a specific organ or tissue. Polysaccharide-based hydrogels, because of their remarkable biocompatibility related to their polymeric constituents, have the ability to interact beneficially with the cellular components. Although the growing interest in the use of polysaccharide-based hydrogels in the biomedical field is evidenced by a conspicuous number of reviews on the topic, none of them have focused on the combined use of two important polysaccharides, chitosan and pectin. Therefore, the present review will discuss the biomedical applications of polysaccharide-based hydrogels containing the two aforementioned natural polymers, chitosan and pectin, in the fields of tissue engineering and 3D in vitro modeling.

Current Status of Polysaccharides-Based Drug Delivery Systems for Nervous Tissue Injuries Repair

Neurological disorders affecting both CNS and PNS still represent one of the most critical and challenging pathologies, therefore many researchers have been focusing on this field in recent decades. Spinal cord injury (SCI) and peripheral nerve injury (PNI) are severely disabling diseases leading to dramatic and, in most cases, irreversible sensory, motor, and autonomic impairments. The challenging pathophysiologic consequences involved in SCI and PNI are demanding the development of more effective therapeutic strategies since, as yet, a therapeutic strategy that can effectively lead to a complete recovery from such pathologies is not available. Drug delivery systems (DDSs) based on polysaccharides have been receiving more and more attention for a wide range of applications, due to their outstanding physical-chemical properties. This review aims at providing an overview of the most studied polysaccharides used for the development of DDSs intended for the repair and regeneration of a damaged nervous system, with particular attention to spinal cord and peripheral nerve injury treatments. In particular, DDSs based on chitosan and their association with alginate, dextran, agarose, cellulose, and gellan were thoroughly revised.

Sacrificial gelatin of PAM-Alginate-BC hydrogel tube with tunable diameter as nerve conduit

Peripheral nerve regeneration is still one of the biggest challenges, autologous nerve transplantation is the 'gold standard' for evaluating its alternative therapy. However, the source of autologous nerves is limited, it is imminent to synthesize a nerve-guiding catheter material with a controllable diameter of a small orifice. Sacrificial gelatin polyacrylamide-Alginate-Bacterial cellulose (PAM-Alginate-BC) hydrogel overcomes poor mechanical properties with tunable diameter. In addition, the PAM-Alginate-BC hydrogel possesses the beneficial properties required for cell scaffolding with surface adhesion ability. The PAM-Alginate-BC materials options have potential applications in peripheral nerve regeneration.

Tissue Engineering in Neuroscience: Applications and Perspectives

Neurological disorders have always been a threat to human physical and mental health nowadays, which are closely related to the nonregeneration of neurons in the nervous system (NS). The damage to the NS is currently difficult to repair using conventional therapies, such as surgery and medication. Therefore, repairing the damaged NS has always been a vast challenge in the area of neurology. Tissue engineering (TE), which integrates the cell biology and materials science to reconstruct or repair organs and tissues, has widespread applications in bone, periodontal tissue defects, skin repairs, and corneal transplantation. Recently, tremendous advances have been made in TE regarding neuroscience. In this review, we summarize TE's recent progress in neuroscience, including pathological mechanisms of various neurological disorders, the concepts and classification of TE, and the most recent development of TE in neuroscience. Lastly, we prospect the future directions and unresolved problems of TE in neuroscience.

Nerve ECM and PLA-PCL based electrospun bilayer nerve conduit for nerve regeneration

Introduction: The porcine nerve-derived extracellular matrix (ECM) fabricated as films has good performance in peripheral nerve regeneration. However, when constructed as conduits to bridge nerve defects, ECM lacks sufficient mechanical strength. Methods: In this study, a novel electrospun bilayer-structured nerve conduit (BNC) with outer poly (L-lactic acid-co-ε-caprolactone) (PLA-PCL) and inner ECM was fabricated for nerve regeneration. The composition, structure, and mechanical strength of BNC were characterized. Then BNC biosafety was evaluated by cytotoxicity, subcutaneous implantation, and cell affinity tests. Furthermore, BNC was used to bridge 10-mm rat sciatic nerve defect, and nerve functional recovery was assessed by walking track, electrophysiology, and histomorphology analyses. Results: Our results demonstrate that BNC has a network of nanofibers and retains some bioactive molecules, including collagen I, collagen IV, laminin, fibronectin, glycosaminoglycans, nerve growth factor, and brain-derived neurotrophic factor. Biomechanical analysis proves that PLA-PCL improves the BNC mechanical properties, compared with single ECM conduit (ENC). The functional evaluation of in vivo results indicated that BNC is more effective in nerve regeneration than PLA-PCL conduit or ENC. Discussion: In conclusion, BNC not only retains the good biocompatibility and bioactivity of ECM, but also obtains the appropriate mechanical strength from PLA-PCL, which has great potential for clinical repair of nerve defects.

An experimental and numerical study of the microstructural and biomechanical properties of human peripheral nerve endoneurium for the design of tissue scaffolds

Biomimetic design of scaffold architectures represents a promising strategy to enable the repair of tissue defects. Natural endoneurium extracellular matrix (eECM) exhibits a sophisticated microstructure and remarkable microenvironments conducive for guiding neurite regeneration. Therefore, the analysis of eECM is helpful to the design of bionic scaffold. Unfortunately, a fundamental lack of understanding of the microstructural characteristics and biomechanical properties of the human peripheral nerve eECM exists. In this study, we used microscopic computed tomography (micro-CT) to reconstruct a three-dimensional (3D) eECM model sourced from mixed nerves. The tensile strength and effective modulus of human fresh nerve fascicles were characterized experimentally. Permeability was calculated from a computational fluid dynamic (CFD) simulation of the 3D eECM model. Fluid flow of acellular nerve fascicles was tested experimentally to validate the permeability results obtained from CFD simulations. The key microstructural parameters, such as porosity is 35.5 ± 1.7%, tortuosity in endoneurium (X axis is 1.26 ± 0.028, Y axis is 1.26 ± 0.020 and Z axis is 1.17 ± 0.03, respectively), tortuosity in pore (X axis is 1.50 ± 0.09, Y axis is 1.44 ± 0.06 and Z axis is 1.13 ± 0.04, respectively), surface area-to-volume ratio (SAVR) is 0.165 ± 0.007 μm^-1 and pore size is 11.8 ± 2.8 μm, respectively. These were characterized from the 3D eECM model and may exert different effects on the stiffness and permeability. The 3D microstructure of natural peripheral nerve eECM exhibits relatively lower permeability (3.10 m² × 10^-12 m²) than other soft tissues. These key microstructural and biomechanical parameters may play an important role in the design and fabrication of intraluminal guidance scaffolds to replace natural eECM. Our findings can aid the development of regenerative therapies and help improve scaffold design.

3D Gradient and Linearly Aligned Magnetic Microcapsules in Nerve Guidance Conduits with Remotely Spatiotemporally Controlled Release to Enhance Peripheral Nerve Repair

Although numerous strategies have been implemented to develop nerve guidance conduits (NGCs) to treat peripheral nerve injury (PNI), functionalization of an NGC to make it remotely controllable for providing spatiotemporal modulation on in situ nerve tissues remains a challenge. In this study, a gelatin/silk (GS) hydrogel was used to develop an NGC based on its self-owned reversible thermoresponsive sol-to-gel phase transformation ability that permitted rapid three-dimensional (3D) micropatterning of the incorporated nerve growth factor (NGF)-loaded magnetic poly(lactic-co-glycolic acid) (PLGA) microcapsules (called NGF@MPs) via multiple magnetic guidance. The thermally controllable viscosity of GS enabled the rapid formation of a 3D gradient and linearly aligned distribution of NGF@MPs, leading to magnetically controlled 3D gradient release of NGF to enhance topographical nerve guidance and wound healing in PNIs. Particularly, the as-formed micropatterned hydrogel, called NGF@MPs-GS, showed corrugation topography with a pattern height H of 15 μm, which resulted in the linear axon alignment of more than 90% of cells. In addition, by an external magnetic field, spatiotemporal controllability of NGF release was obtained and permitted neurite elongation that was almost 2-fold longer than that in the group with external addition of NGF. Finally, an NGC prototype was fabricated and implanted into the injured sciatic nerve. The patterned implant, assisted by magnetic stimulation, demonstrated accelerated restoration of motor function within 14 days after implantation. It further contributed to the enhancement of axon outgrowth and remyelination after 28 days. This NGC, with controllable mechanical, biochemical, and topographical cues, is a promising platform for the enhancement of nerve regeneration.

Development of alginate-spermidine micro/nanogels as potential antioxidant and anti-inflammatory tool in peripheral nerve injuries. Formulation studies and physico-chemical characterization

The development of a successful strategy to ensure a full recovery in patients affected by peripheral nerve injury (PNI), one of the most debilitating pathologies, is, still today, a major clinical challenge. Herein, spermidine (SP), an endogenous polyamine, is employed with a dual role: as cross-linking agent for alginate (ALG) and as antioxidant and anti-inflammatory compound. In particular, micro/nanogels based on the ionic interaction between ALG and SP were obtained via ionotropic gelation. Different ALG concentrations and viscosity grades and different SP concentrations were considered. The influence of such variables on micro/nanogels size was investigated by means of a Design of Experiments (DoE) approach (full factorial design). The formation of micro/nanogels was proved by Scanning Electron Microscope (SEM) analysis and by rheological and profilometry measurements. Fourier Transform Infrared (FTIR) measurements performed on nanogels of optimal composition confirmed SP-ALG interaction. The addition of trehalose as cryoprotectant agent to nanogel dispersion was considered in view of the employment of freeze-drying process to obtain a stable product. Moreover, in vitro studies on Schwann cells proved the ability of SP of expressing antioxidant and anti-inflammatory properties, even if involved in the formation of nanogels.

Magnesium-Encapsulated Injectable Hydrogel and 3D-Engineered Polycaprolactone Conduit Facilitate Peripheral Nerve Regeneration

Peripheral nerve injury is a challenging orthopedic condition that can be treated by autograft transplantation, a gold standard treatment in the current clinical setting. Nevertheless, limited availability of autografts and potential morbidities in donors hampers its widespread application. Bioactive scaffold-based tissue engineering is a promising strategy to promote nerve regeneration. Additionally, magnesium (Mg) ions enhance nerve regeneration; however, an effectively controlled delivery vehicle is necessary to optimize their in vivo therapeutic effects. Herein, a bisphosphonate-based injectable hydrogel exhibiting sustained Mg2+ delivery for peripheral nerve regeneration is developed. It is observed that Mg2+ promoted neurite outgrowth in a concentration-dependent manner by activating the PI3K/Akt signaling pathway and Sema5b. Moreover, implantation of polycaprolactone (PCL) conduits filled with Mg2+ -releasing hydrogel in 10 mm nerve defects in rats significantly enhanced axon regeneration and remyelination at 12 weeks post-operation compared to the controls (blank conduits or conduits filled with Mg2+ -absent hydrogel). Functional recovery analysis reveals enhanced reinnervation in the animals treated with the Mg2+ -releasing hydrogel compared to that in the control groups. In summary, the Mg2+ -releasing hydrogel combined with the 3D-engineered PCL conduit promotes peripheral nerve regeneration and functional recovery. Thus, a new strategy to facilitate the repair of challenging peripheral nerve injuries is proposed.

Improvement of sciatic nerve regeneration by multichannel nanofibrous membrane-embedded electro-conductive conduits functionalized with laminin

Multichannel structures in the design of nerve conduits offer potential advantages for regeneration of damaged nerves. However, lack of biochemical cues and electrical stimulation could hamper satisfactory nerve regeneration. The aim of this study was to simultaneously evaluate the effects of topographical, biological, and electrical cues on sciatic nerve regeneration. Accordingly, a series of multichannel nerve conduit was made using longitudinally-aligned laminin-coated poly (lactic-co-glycolic acid) (PLGA)/carbon nanotubes (CNT) nanofibers (NF, mean diameter: 455 ± 362 nm) in the lumen and randomly-oriented polycaprolactone (PCL) NF (mean diameter: 340 ± 200 nm) on the outer surface. In vitro studies revealed that the materials were nontoxic and able to promote cell attachment and proliferation on nanofibers and on fibrin gel. To determine the influence of laminin as biological and CNT as electrical cues on nerve regeneration, either of hollow PCL conduits, PLGA NF-embedded, PLGA/CNT NF-embedded or laminin-coated PLGA/CNT NF-embedded PCL conduits were implanted in rats. A new surgery method was utilized and results were compared with an autograft. The results of motor and sensory tests in addition to histopathological examination of the regenerated nerves demonstrated the formation of nerve fibers in laminin-coated PLGA/CNT NF-embedded PCL conduits. Results suggested that these conduits have the potential to improve sciatic nerve regeneration. Graphical abstract.

Enhanced Nerve Regeneration by Bionic Conductive Nerve Scaffold Under Electrical Stimulation

Repair of peripheral nerve defect (PND) with a poor prognosis is hard to deal with. Neural conduit applied to nerve defect at present could not achieve the effect of autologous nerve transplantation. We prepared bionic conductive neural scaffolds to provide a new strategy for the treatment of PNDs. The highly aligned poly (L-lactic acid) (PLLA) fiber mats and the multi-microchannel conductive scaffolds were combined into bionic conductive nerve scaffolds, which were implanted into rats with sciatic nerve defects. The experimental animals were divided into the scaffold group (S), scaffold with electrical stimulation (ES) group (S&E), and autologous nerve transplantation group (AT). The regenerative effect of bionic conductive nerve scaffolds was analyzed. Compared with aligned PLLA fiber mats (APFMs), highly aligned fiber mats had a higher fiber orientation and did not change the tensile strength, Young’s modulus, degradation rate, elongation at break of the fiber membrane, and biocompatibility. The bionic conductive nerve scaffolds were well matched with the rat sciatic nerve. The evaluations of the sciatic nerve in Group S&E were close to those in Group AT and better than those in Group S. Immunohistochemical results showed that the expression levels of neurofilament heavy polypeptide (NF-H) and protein S100-B (S100-β) in Group S&E were higher than those in Group S, and the expression levels of low-density lipoprotein receptor-related protein 4 (LRP4), mitogen-activated protein kinase (MAPK) p38, extracellular signal-regulated kinase (ERK), and mitogen-activated protein kinase kinase (MEK) in Group AT were higher than those in Group S. Bionic conductive nerve scaffolds combined with ES could enhance peripheral nerve regeneration and achieve satisfactory nerve regeneration close to autologous nerve grafts. ERK, p38 MAPK, MEK, and LRP4 may be involved in peripheral nerve regeneration under ES.

Novel Tissue-Engineered Multimodular Hyaluronic Acid-Polylactic Acid Conduits for the Regeneration of Sciatic Nerve Defect

The gold standard for the treatment of peripheral nerve injuries, the autograft, presents several drawbacks, and engineered constructs are currently suitable only for short gaps or small diameter nerves. Here, we study a novel tissue-engineered multimodular nerve guidance conduit for the treatment of large nerve damages based in a polylactic acid (PLA) microfibrillar structure inserted inside several co-linear hyaluronic acid (HA) conduits. The highly aligned PLA microfibers provide a topographical cue that guides axonal growth, and the HA conduits play the role of an epineurium and retain the pre-seeded auxiliary cells. The multimodular design increases the flexibility of the device. Its performance for the regeneration of a critical-size (15 mm) rabbit sciatic nerve defect was studied and, after six months, very good nerve regeneration was observed. The multimodular approach contributed to a better vascularization through the micrometrical gaps between HA conduits, and the pre-seeded Schwann cells increased axonal growth. Six months after surgery, a cross-sectional available area occupied by myelinated nerve fibers above 65% at the central and distal portions was obtained when the multimodular device with pre-seeded Schwann cells was employed. The results validate the multi-module approach for the regeneration of large nerve defects and open new possibilities for surgical solutions in this field.

The Therapeutic Potential of Exosomes in Soft Tissue Repair and Regeneration

Soft tissue defects are common following trauma and tumor extirpation. These injuries can result in poor functional recovery and lead to a diminished quality of life. The healing of skin and muscle is a complex process that, at present, leads to incomplete recovery and scarring. Regenerative medicine may offer the opportunity to improve the healing process and functional outcomes. Barriers to regenerative strategies have included cost, regulatory hurdles, and the need for cell-based therapies. In recent years, exosomes, or extracellular vesicles, have gained tremendous attention in the field of soft tissue repair and regeneration. These nanosized extracellular particles (30-140 nm) can break the cellular boundaries, as well as facilitate intracellular signal delivery in various regenerative physiologic and pathologic processes. Existing studies have established the potential of exosomes in regenerating tendons, skeletal muscles, and peripheral nerves through different mechanisms, including promoting myogenesis, increasing tenocyte differentiation and enhancing neurite outgrowth, and the proliferation of Schwann cells. These exosomes can be stored for immediate use in the operating room, and can be produced cost efficiently. In this article, we critically review the current advances of exosomes in soft tissue (tendons, skeletal muscles, and peripheral nerves) healing. Additionally, new directions for clinical applications in the future will be discussed.

Application of Hybrid Electrically Conductive Hydrogels Promotes Peripheral Nerve Regeneration

Peripheral nerve injury (PNI) occurs frequently, and the prognosis is unsatisfactory. As the gold standard of treatment, autologous nerve grafting has several disadvantages, such as lack of donors and complications. The use of functional biomaterials to simulate the natural microenvironment of the nervous system and the combination of different biomaterials are considered to be encouraging alternative methods for effective tissue regeneration and functional restoration of injured nerves. Considering the inherent presence of an electric field in the nervous system, electrically conductive biomaterials have been used to promote nerve regeneration. Due to their singular physical properties, hydrogels can provide a three-dimensional hydrated network that can be integrated into diverse sizes and shapes and stimulate the natural functions of nerve tissue. Therefore, conductive hydrogels have become the most effective biological material to simulate human nervous tissue's biological and electrical characteristics. The principal merits of conductive hydrogels include their physical properties and their electrical peculiarities sufficient to effectively transmit electrical signals to cells. This review summarizes the recent applications of conductive hydrogels to enhance peripheral nerve regeneration.

Two-Dimensional Nanomaterials for Peripheral Nerve Engineering: Recent Advances and Potential Mechanisms

Peripheral nerve tissues possess the ability to regenerate within artificial nerve scaffolds, however, despite the advance of biomaterials that support nerve regeneration, the functional nerve recovery remains unsatisfactory. Importantly, the incorporation of two-dimensional nanomaterials has shown to significantly improve the therapeutic effect of conventional nerve scaffolds. In this review, we examine whether two-dimensional nanomaterials facilitate angiogenesis and thereby promote peripheral nerve regeneration. First, we summarize the major events occurring after peripheral nerve injury. Second, we discuss that the application of two-dimensional nanomaterials for peripheral nerve regeneration strategies by facilitating the formation of new vessels. Then, we analyze the mechanism that the newly-formed capillaries directionally and metabolically support neuronal regeneration. Finally, we prospect that the two-dimensional nanomaterials should be a potential solution to long range peripheral nerve defect. To further enhance the therapeutic effects of two-dimensional nanomaterial, strategies which help remedy the energy deficiency after peripheral nerve injury could be a viable solution.

3D culture platform of human iPSCs-derived nociceptors for peripheral nerve modelling and tissue innervation

Functional humanized in vitro nerve models are coveted as an alternative to animal models due to their ease of access, lower cost, clinical relevance and no need for recurrent animal sacrifice. To this end, we developed a sensory nerve model using induced pluripotent stem cells (iPSCs)-derived nociceptors that are electrically active and exhibit a functional response to noxious stimuli. The differentiated neurons were co-cultured with primary Schwann cells on an aligned microfibrous scaffold to produce biomimetic peripheral nerve tissue. Compared to glass coverslips, our scaffold enhances tissue development and stabilization. Using this model, we demonstrate that myelin damage can be induced from hyperglycemia exposure (glucose at 45 mM) and mitigated by epalrestat (1µM) supplementation. Through fibrin embedding of the platform, we were able to create 3D anisotropic myelinated tissue, reaching over 6.5 mm in length. Finally, as a proof-of-concept, we incorporated pancreatic pseudoislets and endometrial organoids into our nerve platform, to demonstrate the potential in generating nociceptor innervation models. In summary, we propose here an improved tool for neurobiology research with potential applications in pathology modelling, drug screening and target tissue innervation.

Polylysine-decorated macroporous microcarriers laden with adipose-derived stem cells promote nerve regeneration in vivo

Cell transplantation is an effective strategy to improve the repair effect of nerve guide conduits (NGCs). However, problems such as low loading efficiency and cell anoikis undermine the outcomes. Microcarriers are efficient 3D cell culture scaffolds, which can also prevent cell anoikis by providing substrate for adhesion during transplantation. Here, we demonstrate for the first time microcarrier-based cell transplantation in peripheral nerve repair. We first prepared macroporous chitosan microcarriers (CSMCs) by the emulsion-phase separation method, and then decorated the CSMCs with polylysine (pl-CSMCs) to improve cell affinity. We then loaded the pl-CSMCs with adipose-derived stem cells (ADSCs) and injected them into electrospun polycaprolactone/chitosan NGCs to repair rat sciatic nerve defects. The ADSCs-laden pl-CSMCs effectively improved nerve regeneration as demonstrated by evaluation of histology, motor function recovery, electrophysiology, and gastrocnemius recovery. With efficient cell transplantation, convenient operation, and the multiple merits of ADSCs, the ADSCs-laden pl-CSMCs hold good potential in peripheral nerve repair.

In Situ Fabrication of Nerve Growth Factor Encapsulated Chitosan Nanoparticles in Oxidized Bacterial Nanocellulose for Rat Sciatic Nerve Regeneration

Autograft is currently the gold standard in the clinical treatment of peripheral nerve injury (PNI), which, however, is limited by the availability of a donor nerve and secondary injuries. Nerve guidance conduits (NGC) provide a suitable microenvironment to promote the regeneration of injured nerves, which could be the substitutes for autografts. In this study, nerve growth factor (NGF) encapsulated chitosan nanoparticles (CSNPs) were first constructed in situ in an oxidized bacterial cellulose (OBC) conduit using the ion gel method after the introduction of a CS/NGF solution under pressure to enable a sustainable release of NGF. A novel NGF@CSNPs/OBC nanocomposite with antibacterial activity, biodegradability, and porous microstructure was successfully developed. In vitro experiments showed that the nanocomposite promoted the adhesion and proliferation of Schwann cells. When the nanocomposite was applied as NGC to repair the sciatic nerve defect of rats, a successful repair of the 10 mm nerve defect was observed after 4 weeks. At week 9, the diameter, morphology, histology, and functional recovery of the regenerated nerve was comparable to the autografts, indicating that the NGC effectively promoted the regeneration and function recovery of the nerve. In summary, the NGF@CSNPs/OBC as a novel NGC provides great potential in the treatment of PNI.

Anisotropic scaffolds for peripheral nerve and spinal cord regeneration

The treatment of long-gap (>10 mm) peripheral nerve injury (PNI) and spinal cord injury (SCI) remains a continuous challenge due to limited native tissue regeneration capabilities. The current clinical strategy of using autografts for PNI suffers from a source shortage, while the pharmacological treatment for SCI presents dissatisfactory results. Tissue engineering, as an alternative, is a promising approach for regenerating peripheral nerves and spinal cords. Through providing a beneficial environment, a scaffold is the primary element in tissue engineering. In particular, scaffolds with anisotropic structures resembling the native extracellular matrix (ECM) can effectively guide neural outgrowth and reconnection. In this review, the anatomy of peripheral nerves and spinal cords, as well as current clinical treatments for PNI and SCI, is first summarized. An overview of the critical components in peripheral nerve and spinal cord tissue engineering and the current status of regeneration approaches are also discussed. Recent advances in the fabrication of anisotropic surface patterns, aligned fibrous substrates, and 3D hydrogel scaffolds, as well as their in vitro and in vivo effects are highlighted. Finally, we summarize potential mechanisms underlying the anisotropic architectures in orienting axonal and glial cell growth, along with their challenges and prospects.

PVDF and P(VDF-TrFE) Electrospun Scaffolds for Nerve Graft Engineering: A Comparative Study on Piezoelectric and Structural Properties, and In Vitro Biocompatibility

Polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene (P(VDF-TrFE)) are considered as promising biomaterials for supporting nerve regeneration because of their proven biocompatibility and piezoelectric properties that could stimulate cell ingrowth due to their electrical activity upon mechanical deformation. For the first time, this study reports on the comparative analysis of PVDF and P(VDF-TrFE) electrospun scaffolds in terms of structural and piezoelectric properties as well as their in vitro performance. A dynamic impact test machine was developed, validated, and utilised, to evaluate the generation of an electrical voltage upon the application of an impact load (varying load magnitude and frequency) onto the electrospun PVDF (15-20 wt%) and P(VDF-TrFE) (10-20 wt%) scaffolds. The cytotoxicity and in vitro performance of the scaffolds was evaluated with neonatal rat (nrSCs) and adult human Schwann cells (ahSCs). The neurite outgrowth behaviour from sensory rat dorsal root ganglion neurons cultured on the scaffolds was analysed qualitatively. The results showed (i) a significant increase of the β-phase content in the PVDF after electrospinning as well as a zeta potential similar to P(VDF-TrFE), (ii) a non-constant behaviour of the longitudinal piezoelectric strain constant d33, depending on the load and the load frequency, and (iii) biocompatibility with cultured Schwann cells and guiding properties for sensory neurite outgrowth. In summary, the electrospun PVDF-based scaffolds, representing piezoelectric activity, can be considered as promising materials for the development of artificial nerve conduits for the peripheral nerve injury repair.

Effect of Electrical and Electromechanical Stimulation on PC12 Cell Proliferation and Axon Outgrowth

Peripheral nerve injuries have become a common clinical disease with poor prognosis and complicated treatments. The development of tissue engineering pointed a promising direction to produce nerve conduits for nerve regeneration. Electrical and mechanical stimulations have been incorporated with tissue engineering, since such external stimulations could promote nerve cell proliferation, migration and differentiation. However, the combination of electrical and mechanical stimulations (electromechanical stimulation) and its effects on neuron proliferation and axon outgrowth have been rarely investigated. Herein, silver nanowires (AgNWs) embedded polydimethylsiloxane (PDMS) electrodes were developed to study the effects of electromechanical stimulation on rat pheochromocytoma cells (PC12 cells) behaviors. AgNWs/PDMS electrodes demonstrated good biocompatibility and established a stable electric field during mechanical stretching. PC12 cells showed enhanced proliferation rate and axon outgrowth under electrical stimulation alone, and the cell number significantly increased with higher electrical stimulation intensity. The involvement of mechanical stretching in electrical stimulation reduced the cell proliferation rate and axon outgrowth, compared with the case of electrical stimulation alone. Interestingly, the cellular axons outgrowth was found to depend on the stretching direction, where the axons prefer to align perpendicularly to the stretch direction. These results suggested that AgNWs/PDMS electrodes provide an in vitro platform to investigate the effects of electromechanical stimulation on nerve cell behaviors and can be potentially used for nerve regeneration in the future.

Preparation and Characterization of Salt-Mediated Injectable Thermosensitive Chitosan/Pectin Hydrogels for Cell Embedding and Culturing

In recent years, growing attention has been directed to the development of 3D in vitro tissue models for the study of the physiopathological mechanisms behind organ functioning and diseases. Hydrogels, acting as 3D supporting architectures, allow cells to organize spatially more closely to what they physiologically experience in vivo. In this scenario, natural polymer hybrid hydrogels display marked biocompatibility and versatility, representing valid biomaterials for 3D in vitro studies. Here, thermosensitive injectable hydrogels constituted by chitosan and pectin were designed. We exploited the feature of chitosan to thermally undergo sol-gel transition upon the addition of salts, forming a compound that incorporates pectin into a semi-interpenetrating polymer network (semi-IPN). Three salt solutions were tested, namely, beta-glycerophosphate (βGP), phosphate buffer (PB) and sodium hydrogen carbonate (SHC). The hydrogel formulations (i) were injectable at room temperature, (ii) gelled at 37 °C and (iii) presented a physiological pH, suitable for cell encapsulation. Hydrogels were stable in culture conditions, were able to retain a high water amount and displayed an open and highly interconnected porosity and suitable mechanical properties, with Young's modulus values in the range of soft biological tissues. The developed chitosan/pectin system can be successfully used as a 3D in vitro platform for studying tissue physiopathology.

Hydroxypropyl chitosan/soy isolate protein conduits promote peripheral nerve regeneration

Designing scaffolds, with optimized micro-structure and function for promoting the release of neuro-related factors, is significant in peripheral nerve regeneration. Herein, a series of hydroxypropyl chitosan/soy protein isolate composite sponges (HCSS) were fabricated by a freeze-drying technique. The physicochemical properties of the resultant HCSS were examined by a Fourier infrared spectrometer, X-ray diffractometer, scanning electron microscope, water absorption assay, water retention assay, and compressive strength assay. The results indicated that HCSS exhibited an interconnected porous micro-structure and a high water retention ratio with the increase in SPI content. The biological characterization found that the HCSS-50 containing 50% SPI content profoundly promoted the proliferation of RSC96 cells and the secretion of neuro-related factors without excessive ROS production. In addition, HCSS-50 could significantly promote the expression of neuro-related factors; for example, the expression of TGF-β was 3 times higher than that of the control group. Finally, an optimized HCSS-based conduit was fabricated from HCSS-50 to repair sciatic nerve injury in rats with the combination of BMSCs or BMSC-derived Schwann cells. The results suggested that the constructed HCSS-based conduit accompanying BMSC-derived Schwann cells could effectively promote axonal regeneration and upregulate expression of neuro-related factors such as Krox20, Zeb2, and GAP43. Collectively, a newly engineered nerve conduit system was developed by incorporating HCSS-50 and BMSC-derived Schwann cells, which could be an alternative candidate for peripheral nerve regeneration.

Natural Flammulina velutipes-Based Nerve Guidance Conduit as a Potential Biomaterial for Peripheral Nerve Regeneration: In Vitro and In Vivo Studies

The treatment and repair of serious peripheral nerve injuries remain challenging in the clinical practice, while the application of multifunctional nerve guidance conduits (NGCs) based on naturally derived polymers has attracted much attention in recent years because of their excellent physicochemical properties and biological characteristics. Flammulina velutipes (Curt. ex FV) is a popular edible mushroom characterized by hollow tubular structures, antibacterial activities, and high nutritional properties. In this study, FV is utilized to construct NGCs (labeled FVC) via a freeze-drying technique without chemical modifications. The morphology, physical properties, cellular biocompatibility, antibacterial properties, and nerve regeneration capacity of FVC were assessed both in vitro and in vivo. FVC is composed of hollow tubes and evenly irregular interconnected micropores with 73.8 ± 5.5% porosity and 476.1 ± 12.9 μm hollow tube diameter. The inner surface of the FVC presents multiple microgrooves elongated parallel to the long axis. Moreover, FVC possessed strong antibacterial activity and could inhibit Gram-positive Staphylococcus aureus growth by up to 96.0% and Gram-negative Escherichia coli growth by up to 94.8% in vitro. FVC exhibited excellent biocompatibility and effectively promoted PC-12 cell proliferation and elongation in vitro. When applied to repair critical-sized sciatic nerve defects, FVC could effectively stimulate nerve functional recovery and axonal outgrowth in a rat model. Interestingly, Western blot analysis indicated that growth-associated protein 43 (GAP-43) had increased expression levels in the FVC group compared with the autograft group. This result suggested that by activating the Janus activated kinase2 (JAK2)/Phosphorylation ofsignal transducer and activator of transcription-3 (STAT3) signaling pathway, FVC upregulated Phosphorylation of signal transducer and activator of transcription-3 (P-STAT3) in vivo, resulting in the secretion of GAP-43. Collectively, a natural NGC FVC was fabricated based on FV without chemical modifications. The morphology, physical properties, cellular biocompatibility, antibacterial properties, and nerve regeneration capacity of FVC provide new insights for its further optimization and application in the field of nerve tissue engineering.

IL-17F depletion accelerates chitosan conduit guided peripheral nerve regeneration

Peripheral nerve injury is a serious health problem and repairing long nerve deficits remains a clinical challenge nowadays. Nerve guidance conduit (NGC) serves as the most promising alternative therapy strategy to autografts but its repairing efficiency needs improvement. In this study, we investigated whether modulating the immune microenvironment by Interleukin-17F (IL-17F) could promote NGC mediated peripheral nerve repair. Chitosan conduits were used to bridge sciatic nerve defect in IL-17F knockout mice and wild-type mice with autografts as controls. Our data revealed that IL-17F knockout mice had improved functional recovery and axonal regeneration of sciatic nerve bridged by chitosan conduits comparing to the wild-type mice. Notably, IL-17F knockout mice had enhanced anti-inflammatory macrophages in the NGC repairing microenvironment. In vitro data revealed that IL-17F knockout peritoneal and bone marrow derived macrophages had increased anti-inflammatory markers after treatment with the extracts from chitosan conduits, while higher pro-inflammatory markers were detected in the Raw264.7 macrophage cell line, wild-type peritoneal and bone marrow derived macrophages after the same treatment. The biased anti-inflammatory phenotype of macrophages by IL-17F knockout probably contributed to the improved chitosan conduit guided sciatic nerve regeneration. Additionally, IL-17F could enhance pro-inflammatory factors production in Raw264.7 cells and wild-type peritoneal macrophages. Altogether, IL-17F may partially mediate chitosan conduit induced pro-inflammatory polarization of macrophages during nerve repair. These results not only revealed a role of IL-17F in macrophage function, but also provided a unique and promising target, IL-17F, to modulate the microenvironment and enhance the peripheral nerve regeneration.

Nerve Guidance Conduits with Hierarchical Anisotropic Architecture for Peripheral Nerve Regeneration

Nerve guidance conduits with multifunctional features could offer microenvironments for improved nerve regeneration and functional recovery. However, the challenge remains to optimize multiple cues in nerve conduit systems due to the interplay of these factors during fabrication. Here, a modular assembly for the fabrication of nerve conduits is utilized to address the goal of incorporating multifunctional guidance cues for nerve regeneration. Silk-based hollow conduits with suitable size and mechanical properties, along with silk nanofiber fillers with tunable hierarchical anisotropic architectures and microporous structures, are developed and assembled into conduits. These conduits supported improves nerve regeneration in terms of cell proliferation (Schwann and PC12 cells) and growth factor secretion (BDNF, brain-derived neurotrophic factor) in vitro, and the in vivo repair and functional recovery of rat sciatic nerve defects. Nerve regeneration using these new conduit designs is comparable to autografts, providing a path towards future clinical impact.

Biomimicry in 3D printing design: implications for peripheral nerve regeneration

Nerve guide conduits (NGCs) connect dissected nerve stumps and effectively repair short-range peripheral nerve defects. However, for long-range defects, autografts show better therapeutic effects, despite intrinsic limitations. Recent evidence shows that biomimetic design is essential for high-performance NGCs, and 3D printing is a promising fabricating technique. The current work includes a brief review of the challenges for peripheral nerve regeneration. The authors propose a potential solution using biomimetic 3D-printed NGCs as alternative therapies. The assessment of biomimetic designs includes microarchitecture, mechanical property, electrical conductivity and biologics inclusion. The applications of 3D printing in preparing NGCs and present strategies to improve therapeutic effects are also discussed.

Modified Hyaluronic Acid-Laminin-Hydrogel as Luminal Filler for Clinically Approved Hollow Nerve Guides in a Rat Critical Defect Size Model

Hollow nerve guidance conduits are approved for clinical use for defect lengths of up to 3 cm. This is because also in pre-clinical evaluation they are less effective in the support of nerve regeneration over critical defect lengths. Hydrogel luminal fillers are thought to improve the regeneration outcome by providing an optimized matrix inside bioartificial nerve grafts. We evaluated here a modified hyaluronic acid-laminin-hydrogel (M-HAL) as luminal filler for two clinically approved hollow nerve guides. Collagen-based and chitosan-based nerve guides were filled with M-HAL in two different concentrations and the regeneration outcome comprehensively studied in the acute repair rat sciatic nerve 15 mm critical defect size model. Autologous nerve graft (ANG) repair served as gold-standard control. At 120 days post-surgery, all ANG rats demonstrated electrodiagnostically detectable motor recovery. Both concentrations of the hydrogel luminal filler induced improved regeneration outcome over empty nerve guides. However, neither combination with collagen- nor chitosan-based nerve guides resulted in functional recovery comparable to the ANG repair. In contrast to our previous studies, we demonstrate here that M-HAL slightly improved the overall performance of either empty nerve guide type in the critical defect size model.

Design of Suspended Melt Electrowritten Fiber Arrays for Schwann Cell Migration and Neurite Outgrowth

In this study, well-defined, 3D arrays of air-suspended melt electrowritten fibers are made from medical grade poly(ɛ-caprolactone) (PCL). Low processing temperatures, lower voltages, lower ambient temperature, increased collector distance, and high collector speeds all aid to direct-write suspended fibers that can span gaps of several millimeters between support structures. Such processing parameters are quantitatively determined using a "wedge-design" melt electrowritten test frame to identify the conditions that increase the suspension probability of long-distance fibers. All the measured parameters impact the probability that a fiber is suspended over multimillimeter distances. The height of the suspended fibers can be controlled by a concurrently fabricated fiber wall and the 3D suspended PCL fiber arrays investigated with early post-natal mouse dorsal root ganglion explants. The resulting Schwann cell and neurite outgrowth extends substantial distances by 21 d, following the orientation of the suspended fibers and the supporting walls, often generating circular whorls of high density Schwann cells between the suspended fibers. This research provides a design perspective and the fundamental parametric basis for suspending individual melt electrowritten fibers into a form that facilitates cell culture.

Biopolymer-nanotube nerve guidance conduit drug delivery for peripheral nerve regeneration: In vivo structural and functional assessment

Peripheral nerve injuries account for roughly 3% of all trauma patients with over 900,000 repair procedures annually in the US. Of all extremity peripheral nerve injuries, 51% require nerve repair with a transected gap. The current gold-standard treatment for peripheral nerve injuries, autograft repair, has several shortcomings. Engineered constructs are currently only suitable for short gaps or small diameter nerves. Here, we investigate novel nerve guidance conduits with aligned microchannel porosity that deliver sustained-release of neurogenic 4-aminopyridine (4-AP) for peripheral nerve regeneration in a critical-size (15 mm) rat sciatic nerve transection model. The results of functional walking track analysis, morphometric evaluations of myelin development, and histological assessments of various markers confirmed the equivalency of our drug-conduit with autograft controls. Repaired nerves showed formation of thick myelin, presence of S100 and neurofilament markers, and promising functional recovery. The conduit's aligned microchannel architecture may play a vital role in physically guiding axons for distal target reinnervation, while the sustained release of 4-AP may increase nerve conduction, and in turn synaptic neurotransmitter release and upregulation of critical Schwann cell neurotrophic factors. Overall, our nerve construct design facilitates efficient and efficacious peripheral nerve regeneration via a drug delivery system that is feasible for clinical applications.

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Nerve guidance conduit platform with tunable scaffold properties for repair and regeneration of large-gap nerve injuries.

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Sustained 4-aminopyridine release amplifies neurotrophic factor release by Schwann cells to promote axon regeneration.

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Longitudinally aligned scaffold pores and controllable physicochemical properties provide guidance for axon regeneration.

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Critical-size rat sciatic nerve defect healing both structurally and functionally resembled autograft control treatment.

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Innovative and transformative scaffold technology imbued with structural and functional features for tissue regeneration.

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Scaffold enable tailorable release profiles for small molecules proteins and electrical stimulation for tissue regeneration.

Gellan-Based Composite System as a Potential Tool for the Treatment of Nervous Tissue Injuries: Cross-Linked Electrospun Nanofibers Embedded in a RC-33-Loaded Freeze-Dried Matrix

Injuries to the nervous system affect more than one billion people worldwide, and dramatically impact on the patient’s quality of life. The present work aimed to design and develop a gellan gum (GG)-based composite system for the local delivery of the neuroprotective sigma-1 receptor agonist, 1-[3-(1,1′-biphen)-4-yl] butylpiperidine (RC-33), as a potential tool for the treatment of tissue nervous injuries. The system, consisting of cross-linked electrospun nanofibers embedded in a RC-33-loaded freeze-dried matrix, was designed to bridge the lesion gap, control drug delivery and enhance axonal regrowth. The gradual matrix degradation should ensure the progressive interaction between the inner fibrous mat and the surrounding cellular environment. Nanofibers, prepared by electrospinning polymeric solutions containing GG, two different grades of poly (ethylene oxide) and poloxamer, were cross-linked with calcium ions. GG-based matrices, loaded with different amounts of RC-33, were prepared by freeze-drying. Dialysis studies and solid-state characterization pointed out the formation of an interaction product between GG and RC-33. RC-33-loaded freeze-dried matrices were characterized by the capability to absorb a high buffer content, forming a gel with marked viscoelastic properties, and by RC-33 controlled release properties. The presence of cross-linked nanofibers increased matrix mechanical resistance.

Advances of microfluidic intestine-on-a-chip for analyzing anti-inflammation of food

Microfluidic intestine-on-a-chip enables novel means of emulating human intestinal pathophysiology in vitro, which can potentially reduce animal testing and substitute simple 2D culture system. Though a great deal of work has been done in the development of microfluidic platforms for intestinal disease modeling and drug screening, potential investigation of the effect of bioactive food compounds on intestinal inflammation remains largely unexplored. In this review, different biomaterials and chip designs have been explored in the fabrication of intestine-on-a-chip. Other key parameters must be carefully controlled and selected, including shear stress, cell type and cell co-culture spatial configuration, etc. Appropriate techniques to quantify the barrier integrity including trans-epithelial electric resistance, specific tight junction markers and permeability measurements should be standardized and compared with in vivo data. Integration of the gut microbiome and the provision of intestinal-specific environment are the key parameters to realize the in vivo intestinal model simulation and accelerate the screening efficiency of bioactive food compounds.