Influences of mechanical properties and permeability on chitosan nano/microfiber mesh tubes as a scaffold for nerve regeneration

Preparation and mechanical optimization of a two-layer silk/magnesium wires braided porous artificial nerve guidance conduit

Peripheral nerve injures have long been a tricky problem in surgery and a feasible treatment is the transplantation of nerve guidance conduits (NGCs). This study presents a two-layer composite NGC with fair mechanical properties and good biocompatibility. The inner layer was made of degummed silk yarns/magnesium wires using braiding technology, and the outer layer was made from mixed solution of silk fibroin/chitosan (SF/CS) using freeze-drying treatment. Orthogonal experimental design was applied to rationally design the braided structural layer and obtain the optimal combination of technical process parameters. Meanwhile, the SF/CS porous outer layer was optimized from three concentrations of SF/CS solution. In vitro and in vivo study suggested that the textile-forming scaffold exhibited good biocompatibility and no toxicity. During 4 weeks' degradation, the skeleton of conduits retained its shape, and magnesium ions released from degraded magnesium wires contributed to sustainable release and uniform dispersion, proliferation and adhesion of Schwann cells, indicating potential approach in the development of NGCs.

Asbestos fiber length and width comparison between manual and semi-automated measurements

The objective of the present study is to find a fast and accurate procedure to measure the length and width of asbestos fibers using images acquired by a scanning electron microscope (SEM), a phase-contrast microscope (PCM), and a polarized light microscope (PLM). The accuracy of the procedure was evaluated by comparing fiber length and width measurements to manual measurements. Four different types of images were used in the evaluation: (1) backscattered electron SEM images of fibrous tremolite, (2) secondary electron SEM images of fibrous grunerite, (3) PCM images of fibrous grunerite, and (4) PLM images of fibrous grunerite. Fiber length and width were measured with ImageJ (manual measurement) and Image-Pro software and were compared on an individual fiber basis and over the number-length and number-width distribution of each sample. The results of the comparison showed that the individual length and width measurements with ImageJ and Image-Pro software had a nearly 1:1 relationship except for the width measurement in PLM images (8% of the variance in ImageJ width measurements was not explained by Image-Pro width measurements). Similarly, the number-length distributions were not significantly different (p > 0.05) between ImageJ and Image-Pro, but the number-width distributions were significantly different (p < 0.05) for PLM and secondary electron SEM images. Although the image analysis procedure for measuring fiber length and width with Image-Pro is not a fully automated procedure and still requires some manual intervention, it can be a more efficient and equally accurate alternative to time-consuming manual fiber length and width measurements for well dispersed fibers with high aspect ratios.

Fabrication and Characterization of Nanofibrous Poly (L-Lactic Acid)/Chitosan-Based Scaffold by Liquid-Liquid Phase Separation Technique for Nerve Tissue Engineering

Fabrication method is one of the essential factors which directly affect on the properties of scaffold. Several techniques have been well established to fabricate nanofibrous scaffolds such as electrospinning. However, preparing a three-dimensional (3-D) interconnected macro-pore scaffold essential for transporting the cell metabolites and nutrients is difficult using the electrospinning method. The main aim of this study was developing a highly porous scaffold by poly (L-lactic acid) (PLLA)/chitosan blend using liquid-liquid phase separation (LLPS) technique, a fast and cost-benefit method, in order to use in nerve tissue engineering. In addition, the effect of different polymeric concentrations on morphology, mechanical properties, hydrophilicity, in vitro degradation rate and pH alteration of the scaffolds were evaluated. Moreover, cell attachment, cell viability and cell proliferation of scaffolds as candidates for nerve tissue engineering was investigated. PLLA/chitosan blend not only had desirable structural properties, porosity, hydrophilicity, mechanical properties, degradation rate and pH alteration but also provided a favorable environment for attachment, viability, and proliferation of human neuroblastoma cells, exhibiting significant potential for nerve tissue engineering applications. However, the polymeric concentration in blend fabrication had influence on both characteristics and cell responses. It concluded that PLLA/chitosan nanofibrous 3-D scaffold fabricated by LLPS method as a suitable candidate for nerve tissue engineering.

A Review on Chitosan's Uses as Biomaterial: Tissue Engineering, Drug Delivery Systems and Cancer Treatment

Chitosan, derived from chitin, is a biopolymer consisting of arbitrarily distributed β-(1-4)-linked D-glucosamine and N-acetyl-D-glucosamine that exhibits outstanding properties— biocompatibility, biodegradability, non-toxicity, antibacterial activity, the capacity to form films, and chelating of metal ions. Most of these peculiar properties are attributed to the presence of free protonable amino groups along the chitosan backbone, which also gives it solubility in acidic conditions. Moreover, this biopolymer can also be physically modified, thereby presenting a variety of forms to be developed. Consequently, this polysaccharide is used in various fields, such as tissue engineering, drug delivery systems, and cancer treatment. In this sense, this review aims to gather the state-of-the-art concerning this polysaccharide when used as a biomaterial, providing information about its characteristics, chemical modifications, and applications. We present the most relevant and new information about this polysaccharide-based biomaterial’s applications in distinct fields and also the ability of chitosan and its various derivatives to selectively permeate through the cancer cell membranes and exhibit anticancer activity, and the possibility of adding several therapeutic metal ions as a strategy to improve the therapeutic potential of this polymer.

Natural-Based Biomaterials for Peripheral Nerve Injury Repair

Peripheral nerve injury treatment is a relevant problem because of nerve lesion high incidence and because of unsatisfactory regeneration after severe injuries, thus resulting in a reduced patient's life quality. To repair severe nerve injuries characterized by substance loss and to improve the regeneration outcome at both motor and sensory level, different strategies have been investigated. Although autograft remains the gold standard technique, a growing number of research articles concerning nerve conduit use has been reported in the last years. Nerve conduits aim to overcome autograft disadvantages, but they must satisfy some requirements to be suitable for nerve repair. A universal ideal conduit does not exist, since conduit properties have to be evaluated case by case; nevertheless, because of their high biocompatibility and biodegradability, natural-based biomaterials have great potentiality to be used to produce nerve guides. Although they share many characteristics with synthetic biomaterials, natural-based biomaterials should also be preferable because of their extraction sources; indeed, these biomaterials are obtained from different renewable sources or food waste, thus reducing environmental impact and enhancing sustainability in comparison to synthetic ones. This review reports the strengths and weaknesses of natural-based biomaterials used for manufacturing peripheral nerve conduits, analyzing the interactions between natural-based biomaterials and biological environment. Particular attention was paid to the description of the preclinical outcome of nerve regeneration in injury repaired with the different natural-based conduits.

Induced neural stem cell differentiation on a drawn fiber scaffold-toward peripheral nerve regeneration

To achieve regeneration of long sections of damaged nerves, restoration methods such as direct suturing or autologous grafting can be inefficient. Solutions involving biohybrid implants, where neural stem cells are grown in vitro on an active support before implantation, have attracted attention. Using such an approach, combined with recent advancements in microfabrication technology, the chemical and physical environment of cells can be tailored in order to control their behaviors. Herein, a neural stem cell polycarbonate fiber scaffold, fabricated by 3D printing and thermal drawing, is presented. The combined effect of surface microstructure and chemical functionalization using poly-L-ornithine (PLO) and double-walled carbon nanotubes (DWCNTs) on the biocompatibility of the scaffold, induced differentiation of the neural stem cells (NSCs) and channeling of the neural cells was investigated. Upon treatment of the fiber scaffold with a suspension of DWCNTs in PLO (0.039 g l^-1) and without recombinants a high degree of differentiation of NSCs into neuronal cells was confirmed by using nestin, galactocerebroside and doublecortin immunoassays. These findings illuminate the potential use of this biohybrid approach for the realization of future nerve regenerative implants.

Combination of electrospinning with other techniques for the fabrication of 3D polymeric and composite nanofibrous scaffolds with improved cellular interactions

The development of three-dimensional (3D) scaffolds with physical and chemical topological cues at the macro-, micro-, and nanometer scale is urgently needed for successful tissue engineering applications. 3D scaffolds can be manufactured by a wide variety of techniques. Electrospinning technology has emerged as a powerful manufacturing technique to produce non-woven nanofibrous scaffolds with very interesting features for tissue engineering products. However, electrospun scaffolds have some inherent limitations that compromise the regeneration of thick and complex tissues. By integrating electrospinning and other fabrication technologies, multifunctional 3D fibrous assemblies with micro/nanotopographical features can be created. The proper combination of techniques leads to materials with nano and macro-structure, allowing an improvement in the biological performance of tissue-engineered constructs. In this review, we focus on the most relevant strategies to produce electrospun polymer/composite scaffolds with 3D architecture. A detailed description of procedures involving physical and chemical agents to create structures with large pores and 3D fiber assemblies is introduced. Finally, characterization and biological assays including in vitro and in vivo studies of structures intended for the regeneration of functional tissues are briefly presented and discussed.

Chitosan films for regenerative medicine: fabrication methods and mechanical characterization of nanostructured chitosan films

Regenerative medicine is continuously facing new challenges and it is searching for new biocompatible, green/natural polymer materials, possibly biodegradable and non-immunogenic. Moreover, the critical importance of the nano/microstructuring of surfaces is overall accepted for their full biocompatibility and in vitro/in vivo performances. Chitosan is emerging as a promising biopolymer for tissue engineering and its application can be further improved by exploiting its nano/microstructuration. Here, we report the state of the art of chitosan films and scaffolds nano/micro-structuration. We show that it is possible to obtain, by solvent casting, chitosan thin films with good mechanical properties and to structure them at the microscale and even nanoscale level, with resolutions down to 100 nm.

Relevance and Recent Developments of Chitosan in Peripheral Nerve Surgery

Developments in tissue engineering yield biomaterials with different supporting strategies to promote nerve regeneration. One promising material is the naturally occurring chitin derivate chitosan. Chitosan has become increasingly important in various tissue engineering approaches for peripheral nerve reconstruction, as it has demonstrated its potential to interact with regeneration associated cells and the neural microenvironment, leading to improved axonal regeneration and less neuroma formation. Moreover, the physiological properties of its polysaccharide structure provide safe biodegradation behavior in the absence of negative side effects or toxic metabolites. Beneficial interactions with Schwann cells (SC), inducing differentiation of mesenchymal stromal cells to SC-like cells or creating supportive conditions during axonal recovery are only a small part of the effects of chitosan. As a result, an extensive body of literature addresses a variety of experimental strategies for the different types of nerve lesions. The different concepts include chitosan nanofibers, hydrogels, hollow nerve tubes, nerve conduits with an inner chitosan layer as well as hybrid architectures containing collagen or polyglycolic acid nerve conduits. Furthermore, various cell seeding concepts have been introduced in the preclinical setting. First translational concepts with hollow tubes following nerve surgery already transferred the promising experimental approach into clinical practice. However, conclusive analyses of the available data and the proposed impact on the recovery process following nerve surgery are currently lacking. This review aims to give an overview on the physiologic properties of chitosan, to evaluate its effect on peripheral nerve regeneration and discuss the future translation into clinical practice.

Neuromuscular degenerative effects of Ankaferd Blood Stopper® in mouse sciatic nerve model

PURPOSE

Ankaferd Blood Stopper^® (ABS), a licenced medicinal herbal extract, is commonly used as an effective topical haemostatic agent. This study is designed to investigate whether topical ABS application may cause peripheral nerve degeneration and neuromuscular dysfunction in a mouse sciatic nerve model.

METHODS

Twenty mice were randomly divided into two groups; an ABS treated experimental group and a saline-treated control group. Left sciatic nerves were treated with 0.3 ml of ABS in the experimental group and 0.3 ml of sterile saline in the control group for 5 min. Peripheral nerve degeneration and neuromuscular dysfunction were evaluated by behavioural tests, electrophysiological analysis and weight ratio comparison of target muscles.

RESULTS

The motor function, assessed by the sciatic function index, was significantly impaired in ABS-treated animals as compared to the animals treated with saline. Motor coordination, evaluated with the rotarod test, was significantly decreased (-42%) in ABS-treated animals compared to the saline-treated animals. The degree of pain, assessed by the reaction latency to thermal stimuli (hot-plate test), was significantly prolonged (313%) in ABS-treated mice when compared to the saline-treated mice. ABS-treated mice showed a significant reduction in motor nerve conduction velocity (MNCV) (-52%) and the compound muscle action potential (CMAP) (-47%); however, it significantly prolonged onset latency (23%). The gastrocnemius muscles weight ratio of the ABS group was considerably lower than that of the control group.

CONCLUSIONS

These findings demonstrate that ABS triggers peripheral nerve degeneration and functional impairment and, thus promotes a deterioration of sciatic nerves.

Therapeutic potential of carbohydrates as regulators of macrophage activation

It is well established for a broad range of disease states, including cancer and Mycobacterium tuberculosis infection, that pathogenesis is bolstered by polarisation of macrophages towards an anti-inflammatory phenotype, known as M2. As these innate immune cells are relatively long-lived, their re-polarisation to pro-inflammatory, phagocytic and bactericidal "classically activated" M1 macrophages is an attractive therapeutic approach. On the other hand, there are scenarios where the resolving inflammation, wound healing and tissue remodelling properties of M2 macrophages are beneficial - for example the successful introduction of biomedical implants. Although there are numerous endogenous and exogenous factors that have an impact on the macrophage polarisation spectrum, this review will focus specifically on prominent macrophage-modulating carbohydrate motifs with a view towards highlighting structure-function relationships and therapeutic potential.

Comparison of nerve regenerative efficacy between decellularized nerve graft and nonwoven chitosan conduit

BACKGROUND

Recently decellularized nerves with various methods are reported as highly functional nerve grafts for the treatment of nerve defects.

OBJECTIVE

To evaluate the efficacy of decellularized allogeneic nerve, compared with oriented chitosan mesh tube, and an autologous nerve.

METHODS

Sciatic nerves harvested from Sprague-Dawley (SD) rats were decellularized in combination with Sodium dodecyl sulfate and Triton X-100. A graft into the sciatic nerve in Wistar rats was performed with the decellularized SD rat sciatic nerves or oriented chitosan nonwoven nanofiber mesh tubes (15 mm in length, N=5 in each group). A portion of sciatic nerve of Wistar rat was cut, reversed and re-sutured in-situ as a control. Nerve functional and histological evaluations were performed 25 weeks postoperatively.

RESULTS

It was revealed that functional, electrophysiological and histological recoveries in the decellularized nerve group match those in the autograft group. Recovery of sensory function and nerve maturation in the decellularized nerve group were superior to those in the chitosan mesh tube group.

CONCLUSIONS

Nerve regeneration in the decellularized nerves could match that in the autografts and is somehow superior to artificial chitosan mesh tube. Detergents wash of SDS and Triton X-100 could obtain highly functional nerve grafts from allografts.

Differentiation of Rat Bone Marrow Mesenchymal Stem Cells Into Neuron-Like Cells In Vitro and Co-Cultured with Biological Scaffold as Transplantation Carrier

BACKGROUND Autograft and allograft transplantation are used to prompt the regeneration of axons after nerve injury. However, the poor self-regeneration caused by the glial scar and growth inhibitory factors after neuronal necrosis limit the efficacy of these methods. The purpose of this study was to develop a new chitosan porous scaffold for cell seeding. MATERIAL AND METHODS The bone marrow mesenchymal stem cells (BMSCs) and tissue-engineered biomaterial scaffold compound were constructed and co-cultured in vitro with the differentiated BMSCs of Wistar rats and chitosan scaffold in a 3D environment. The purity of the third-generation BMSCs culture was identified using flow cytometry and assessment of induced neuronal differentiation. The scaffolds were prepared by the freeze-drying method. The internal structure of scaffolds and the change of cells' growth and morphology were observed under a scanning electron microscope. The proliferation of cells was detected with the MTT method. RESULTS On day 5 there was a significant difference in the absorbance value of the experimental group (0.549±0.0256) and the control group (0.487±0.0357) (P>0.05); but on day 7 there was no significant difference in the proliferation of the experimental group (0.751±0.011) and the control group and (0.78±0.017) (P>0.05). CONCLUSIONS Tissue engineering technology can provide a carrier for cells seeding and is expected to become an effective method for the regeneration and repair of nerve cells. Our study showed that chitosan porous scaffolds can be used for such purposes.

Electrospun and woven silk fibroin/poly(lactic-co-glycolic acid) nerve guidance conduits for repairing peripheral nerve injury

We have designed a novel nerve guidance conduit (NGC) made from silk fibroin and poly(lactic-co-glycolic acid) through electrospinning and weaving (ESP-NGCs). Several physical and biological properties of the ESP-NGCs were assessed in order to evaluate their biocompatibility. The physical properties, including thickness, tensile stiffness, infrared spectroscopy, porosity, and water absorption were determined in vitro. To assess the biological properties, Schwann cells were cultured in ESP-NGC extracts and were assessed by morphological observation, the MTT assay, and immunohistochemistry. In addition, ESP-NGCs were subcutaneously implanted in the backs of rabbits to evaluate their biocompatibility in vivo. The results showed that ESP-NGCs have high porosity, strong hydrophilicity, and strong tensile stiffness. Schwann cells cultured in the ESP-NGC extract fluids showed no significant differences compared to control cells in their morphology or viability. Histological evaluation of the ESP-NGCs implanted in vivo indicated a mild inflammatory reaction and high biocompatibility. Together, these data suggest that these novel ESP-NGCs are biocompatible, and may thus provide a reliable scaffold for peripheral nerve repair in clinical application.

Chitosan tubes can restore the function of resected phrenic nerves

OBJECTIVES

We previously reported that the phrenic nerve could be morphologically repaired by implantation of a chitosan nanofibre tube (C-tube). In the current study, we investigated whether implantation of C-tubes could improve the function of an injured phrenic nerve using a beagle dog model.

METHODS

Seven beagle dogs underwent right thoracotomy under general anaesthesia. An approximately 5 mm length of the right phrenic nerve was resected. Five dogs had a C-tube implantation (C-tube group) and other two dogs did not have the C-tube implantation (control group). Diaphragm movements were longitudinally measured by X-ray fluoroscopy before surgery, immediately after the surgery, and 3, 6 and 12 months after the surgery. The diaphragm movement was determined by diaphragm levels at inspiration and expiration phases, and the excursion difference between them was calculated. At 12 months after the surgery, rethoracotomy was performed to examine electrical phrenic nerve conduction. The C-tube and phrenic nerve were then excised for histological assessment of nerve regeneration.

RESULTS

Three of the five animals of the C-tube group showed improvement of diaphragm movement with time. In these three animals, slow phrenic nerve conduction was observed. Histological assessment showed that the injured nerve was connected by newly regenerating nerve fibres surrounded by granulation tissue within the C-tube. On the other hand, the animals in the control group and two animals of the C-tube group showed neither improved diaphragm movement, nor electrical conduction to the diaphragm. No nerve fibre regeneration was found by histology.

CONCLUSIONS

Our results suggest that, in addition to morphological improvement, C-tube implantation can functionally improve the injured phrenic nerve by promoting phrenic nerve regeneration.

Preparation and Properties of 3D Chitosan Microtubes

The preparation of 3D chitosan microtubes from polymer solutions in citric and lactic acids by the wet and dry molding methods is described. The mechanism of formation of the insoluble polymeric layer constructing the walls of these microtubes is characterized. The microtubes obtained from chitosan solutions in citric acid are found to have a fragile porous inner layer. For those obtained from chitosan solutions in lactic acid the morphology, elastic-deformation properties, physicomechanical properties, and biocompatibility were assessed. These samples have smooth outer and inner surfaces with no visible defects and high values of elongation at break. The strength of the microtubes obtained by the dry method is much higher than in the case of the wet one. A high adhesion and high proliferative activity of the epithelial-like MA-104 cellular culture on the surface of our microtubular substrates in model in vitro experiments were revealed. Prospects of using chitosan microtubes as vascular prostheses are suggested.

A comparison of the performance of mono- and bi-component electrospun conduits in a rat sciatic model

Synthetic nerve conduits represent a promising strategy to enhance functional recovery in peripheral nerve injury repair. However, the efficiency of synthetic nerve conduits is often compromised by the lack of molecular factors to create an enriched microenvironment for nerve regeneration. Here, we investigate the in vivo response of mono (MC) and bi-component (BC) fibrous conduits obtained by processing via electrospinning poly(ε-caprolactone) (PCL) and gelatin solutions. In vitro studies demonstrate that the inclusion of gelatin leads to uniform electrospun fiber size and positively influences the response of Dorsal Root Ganglia (DRGs) neurons as confirmed by the preferential extensions of neurites from DRG bodies. This behavior can be attributed to gelatin as a bioactive cue for the cultured DRG and to the reduced fibers size. However, in vivo studies in rat sciatic nerve defect model show an opposite response: MC conduits stimulate superior nerve regeneration than gelatin containing PCL conduits as confirmed by electrophysiology, muscle weight and histology. The G-ratio, 0.71 ± 0.07 for MC and 0.66 ± 0.05 for autograft, is close to 0.6, the value measured in healthy nerves. In contrast, BC implants elicited a strong host response and infiltrating tissue occluded the conduits preventing the formation of myelinated axons. Therefore, although gelatin promotes in vitro nerve regeneration, we conclude that bi-component electrospun conduits are not satisfactory in vivo due to intrinsic limits to their mechanical performance and degradation kinetics, which are essential to peripheral nerve regeneration in vivo.

Nanostructured guidance for peripheral nerve injuries: a review with a perspective in the oral and maxillofacial area

Injury to peripheral nerves can occur as a result of various surgical procedures, including oral and maxillofacial surgery. In the case of nerve transaction, the gold standard treatment is the end-to-end reconnection of the two nerve stumps. When it cannot be performed, the actual strategies consist of the positioning of a nerve graft between the two stumps. Guided nerve regeneration using nano-structured scaffolds is a promising strategy to promote axon regeneration. Biodegradable electrospun conduits composed of aligned nanofibers is a new class of devices used to improve neurite extension and axon outgrowth. Self assembled peptide nanofibrous scaffolds (SAPNSs) demonstrated promising results in animal models for central nervous system injuries, and, more recently, for peripheral nerve injury. Aims of this work are (1) to review electrospun and self-assembled nanofibrous scaffolds use in vitro and in vivo for peripheral nerve regeneration; and (2) its application in peripheral nerve injuries treatment. The review focused on nanofibrous scaffolds with a diameter of less than approximately 250 nm. The conjugation in a nano scale of a natural bioactive factor with a resorbable synthetic or natural material may represent the best compromise providing both biological and mechanical cues for guided nerve regeneration. Injured peripheral nerves, such as trigeminal and facial, may benefit from these treatments.

The use of chitosan-based scaffolds to enhance regeneration in the nervous system

Various biomaterials have been proposed to build up scaffolds for promoting neural repair. Among them, chitosan, a derivative of chitin, has been raising more and more interest among basic and clinical scientists. A number of studies with neuronal and glial cell cultures have shown that this biomaterial has biomimetic properties, which make it a good candidate for developing innovative devices for neural repair. Yet, in vivo experimental studies have shown that chitosan can be successfully used to create scaffolds that promote regeneration both in the central and in the peripheral nervous system. In this review, the relevant literature on the use of chitosan in the nervous tissue, either alone or in combination with other components, is overviewed. Altogether, the promising in vitro and in vivo experimental results make it possible to foresee that time for clinical trials with chitosan-based nerve regeneration-promoting devices is approaching quickly.

Nanofibrous nerve conduit-enhanced peripheral nerve regeneration

Fibre structures represent a potential class of materials for the formation of synthetic nerve conduits due to their biomimicking architecture. Although the advantages of fibres in enhancing nerve regeneration have been demonstrated, in vivo evaluation of fibre size effect on nerve regeneration remains limited. In this study, we analyzed the effects of fibre diameter of electrospun conduits on peripheral nerve regeneration across a 15-mm critical defect gap in a rat sciatic nerve injury model. By using an electrospinning technique, fibrous conduits comprised of aligned electrospun poly (ε-caprolactone) (PCL) microfibers (981 ± 83 nm, Microfiber) or nanofibers (251 ± 32 nm, Nanofiber) were obtained. At three months post implantation, axons regenerated across the defect gap in all animals that received fibrous conduits. In contrast, complete nerve regeneration was not observed in the control group that received empty, non-porous PCL film conduits (Film). Nanofiber conduits resulted in significantly higher total number of myelinated axons and thicker myelin sheaths compared to Microfiber and Film conduits. Retrograde labeling revealed a significant increase in number of regenerated dorsal root ganglion sensory neurons in the presence of Nanofiber conduits (1.93 ± 0.71 × 10(3) vs. 0.98 ± 0.30 × 10(3) in Microfiber, p < 0.01). In addition, the compound muscle action potential (CMAP) amplitudes were higher and distal motor latency values were lower in the Nanofiber conduit group compared to the Microfiber group. This study demonstrated the impact of fibre size on peripheral nerve regeneration. These results could provide useful insights for future nerve guide designs.

Sciatic nerve regeneration in rats by a promising electrospun collagen/poly(ε-caprolactone) nerve conduit with tailored degradation rate

Background

To cope with the limitations faced by autograft acquisitions particularly for multiple nerve injuries, artificial nerve conduit has been introduced by researchers as a substitute for autologous nerve graft for the easy specification and availability for mass production. In order to best mimic the structures and components of autologous nerve, great efforts have been made to improve the designation of nerve conduits either from materials or fabrication techniques. Electrospinning is an easy and versatile technique that has recently been used to fabricate fibrous tissue-engineered scaffolds which have great similarity to the extracellular matrix on fiber structure.

Results

In this study we fabricated a collagen/poly(ε-caprolactone) (collagen/PCL) fibrous scaffold by electrospinning and explored its application as nerve guide substrate or conduit in vitro and in vivo. Material characterizations showed this electrospun composite material which was made of submicron fibers possessed good hydrophilicity and flexibility. In vitro study indicated electrospun collagen/PCL fibrous meshes promoted Schwann cell adhesion, elongation and proliferation. In vivo test showed electrospun collagen/PCL porous nerve conduits successfully supported nerve regeneration through an 8 mm sciatic nerve gap in adult rats, achieving similar electrophysiological and muscle reinnervation results as autografts. Although regenerated nerve fibers were still in a pre-mature stage 4 months postoperatively, the implanted collagen/PCL nerve conduits facilitated more axons regenerating through the conduit lumen and gradually degraded which well matched the nerve regeneration rate.

Conclusions

All the results demonstrated this collagen/PCL nerve conduit with tailored degradation rate fabricated by electrospinning could be an efficient alternative to autograft for peripheral nerve regeneration research. Due to its advantage of high surface area for cell attachment, it is believed that this electrospun nerve conduit could find more application in cell therapy for nerve regeneration in future, to further improve functional regeneration outcome especially for longer nerve defect restoration.

Chitin-based materials in tissue engineering: applications in soft tissue and epithelial organ

Chitin-based materials and their derivatives are receiving increased attention in tissue engineering because of their unique and appealing biological properties. In this review, we summarize the biomedical potential of chitin-based materials, specifically focusing on chitosan, in tissue engineering approaches for epithelial and soft tissues. Both types of tissues play an important role in supporting anatomical structures and physiological functions. Because of the attractive features of chitin-based materials, many characteristics beneficial to tissue regeneration including the preservation of cellular phenotype, binding and enhancement of bioactive factors, control of gene expression, and synthesis and deposition of tissue-specific extracellular matrix are well-regulated by chitin-based scaffolds. These scaffolds can be used in repairing body surface linings, reconstructing tissue structures, regenerating connective tissue, and supporting nerve and vascular growth and connection. The novel use of these scaffolds in promoting the regeneration of various tissues originating from the epithelium and soft tissue demonstrates that these chitin-based materials have versatile properties and functionality and serve as promising substrates for a great number of future applications.

Engineering nanoassemblies of polysaccharides

Polysaccharides offer a wealth of biochemical and biomechanical functionality that can be used to develop new biomaterials. In mammalian tissues, polysaccharides often exhibit a hierarchy of structure, which includes assembly at the nanometer length scale. Furthermore, their biochemical function is determined by their nanoscale organization. These biological nanostructures provide the inspiration for developing techniques to tune the assembly of polysaccharides at the nanoscale. These new polysaccharide nanostructures are being used for the stabilization and delivery of drugs, proteins, and genes, the engineering of cells and tissues, and as new platforms on which to study biochemistry. In biological systems polysaccharide nanostructures are assembled via bottom-up processes. Many biologically derived polysaccharides behave as polyelectrolytes, and their polyelectrolyte nature can be used to tune their bottom-up assembly. New techniques designed to tune the structure and composition of polysaccharides at the nanoscale are enabling researchers to study in detail the emergent biological properties that arise from the nanoassembly of these important biological macromolecules.

Polymer nanofibrous structures: Fabrication, biofunctionalization, and cell interactions

Extracellular matrix fibers (ECM) such as collagen, elastin, and keratin provide biological and physical support for cell attachment, proliferation, migration, differentiation and ultimately cell fate. Therefore, ECM fibers are an important component in tissue and organ development and regeneration. Meanwhile, polymer nanofibers could play the same critical role in tissue regeneration process. Fibrous structures can be fabricated from a variety of materials and methods with diameters ranging throughout the size scale where cells can sense individual fibers (several nanometers to several microns). Polymer nanofiber scaffolds can be designed in a way that predictably modulates a variety of important cell behaviors towards a desired overall function. The nanofibrous topography itself, independent of the fiber material, has demonstrated the potential to modulate cell behaviors desirable in tissue engineering such as: unidirectional alignment; increased viability, attachment, and ECM production; guided migration; and controlled differentiation. The versatility of polymer nanofibers for functionalization with biomolecules opens the door to vast opportunities for the design of tissue engineering scaffolds with even greater control over cell incorporation and function. Despite the promise of polymer nanofibers as tissue engineering scaffolds there have been few clinically relevant successes because no single fabrication technique currently combines control over structural arrangement, material composition, and biofunctionalization, while maintaining reasonable cost and yield. Promising strategies are currently being investigated to allow for the fabrication of optimal polymer nanofiber tissue engineering scaffolds with the goal of treating damaged and degenerated tissues in a clinical setting.

Electrospun nanofibers for neural tissue engineering

Biodegradable nanofibers produced by electrospinning represent a new class of promising scaffolds to support nerve regeneration. We begin with a brief discussion on the electrospinning of nanofibers and methods for controlling the structure, porosity, and alignment of the electrospun nanofibers. The methods include control of the nanoscale morphology and microscale alignment of the nanofibers, as well as the fabrication of macroscale, three-dimensional tubular structures. We then highlight recent studies that utilize electrospun nanofibers to manipulate biological processes relevant to nervous tissue regeneration, including stem cell differentiation, guidance of neurite extension, and peripheral nerve injury treatments. The main objective of this feature article is to provide valuable insights into methods for investigating the mechanisms of neurite growth on novel nanofibrous scaffolds and optimization of the nanofiber scaffolds and conduits for repairing peripheral nerve injuries.

Electrospinning of polysaccharides for regenerative medicine

Electrospinning techniques enable the production of continuous fibers with dimensions on the scale of nanometers from a wide range of natural and synthetic polymers. The number of recent studies regarding electrospun polysaccharides and their derivatives, which are potentially useful for regenerative medicine, is increasing dramatically. However, difficulties regarding the processibility of the polysaccharides (e.g., poor solubility and high surface tension) have limited their application. In this review, we summarize the characteristics of various polysaccharides such as alginate, cellulose, chitin, chitosan, hyaluronic acid, starch, dextran, and heparin, which are either currently being used or have potential to be used for electrospinning. The recent progress of nanofiber matrices electrospun from polysaccharides and their biomedical applications in tissue engineering, wound dressings, drug delivery, and enzyme immobilization are discussed.

The application of nanofibrous scaffolds in neural tissue engineering

The repairing process in the nervous system is complicated and brings great challenges to researchers. Tissue engineering scaffolds provide an alternative approach for neural regeneration. Sub-micron and nano-scale fibrous scaffolds which mimic the topography of natural extracellular matrix (ECM) can be potential scaffold candidates for neural tissue engineering. Two fiber-fabrication methods have been explored in the field of nerve regeneration: electrospinning and self-assembly. Electrospinning produces fibers with diameters ranging from several micrometers to hundreds of nanometers. The fibrous nerve conduits can be introduced at lesion sites by implantation. Self-assembly fibers have diameters of tens of nanometers and can be injected for central nervous system (CNS) injury repair. Both fibrous scaffolds would enhance neurite extension and axon regrowth. These functional nanofibrous scaffolds can serve as powerful tools for neural tissue engineering.

Current applications and future perspectives of artificial nerve conduits

Artificial nerve guide conduits have the advantage over autografts in terms of their availability and ease of fabrication. However, clinical outcomes associated with the use of artificial nerve conduits are often inferior to that of autografts, particularly over long lesion gaps. There have been significant advances in the designs of artificial nerve conduits over the years. In terms of materials selection and design, a wide variety of new synthetic polymers and biopolymers have been evaluated. The inclusion of nerve conduit lumen fillers has also been demonstrated as essential to enable nerve regeneration across large defect gaps. These lumen filler designs have involved the integration of physical cues for contact guidance and biochemical signals to control cellular function and differentiation. Novel conduit architectural designs using porous and fibrous substrates have also been developed. This review highlights the recent advances in synthetic nerve guide designs for peripheral nerve regeneration, and the in vivo applicability and future prospects of these nerve guide conduits.