Regenerative potential of silk conduits in repair of peripheral nerve injury in adult rats

Biomaterial-based mechanical regulation facilitates scarless wound healing with functional skin appendage regeneration

Scar formation resulting from burns or severe trauma can significantly compromise the structural integrity of skin and lead to permanent loss of skin appendages, ultimately impairing its normal physiological function. Accumulating evidence underscores the potential of targeted modulation of mechanical cues to enhance skin regeneration, promoting scarless repair by influencing the extracellular microenvironment and driving the phenotypic transitions. The field of skin repair and skin appendage regeneration has witnessed remarkable advancements in the utilization of biomaterials with distinct physical properties. However, a comprehensive understanding of the underlying mechanisms remains somewhat elusive, limiting the broader application of these innovations. In this review, we present two promising biomaterial-based mechanical approaches aimed at bolstering the regenerative capacity of compromised skin. The first approach involves leveraging biomaterials with specific biophysical properties to create an optimal scarless environment that supports cellular activities essential for regeneration. The second approach centers on harnessing mechanical forces exerted by biomaterials to enhance cellular plasticity, facilitating efficient cellular reprogramming and, consequently, promoting the regeneration of skin appendages. In summary, the manipulation of mechanical cues using biomaterial-based strategies holds significant promise as a supplementary approach for achieving scarless wound healing, coupled with the restoration of multiple skin appendage functions.

Enhancing Functional Recovery after Segmental Nerve Defect Using Nerve Allograft Treated with Plasma-Derived Exosome

BACKGROUND

Nerve injuries can result in detrimental functional outcomes. Currently, autologous nerve graft offers the best outcome for segmental peripheral nerve injury. Allografts are alternatives, but do not have comparable results. This study evaluated whether plasma-derived exosome can improve nerve regeneration and functional recovery when combined with decellularized nerve allografts.

METHODS

The effect of exosomes on Schwann cell proliferation and migration were evaluated. A rat model of sciatic nerve repair was used to evaluate the effect on nerve regeneration and functional recovery. A fibrin sealant was used as the scaffold for exosome. Eighty-four Lewis rats were divided into autograft, allograft, and allograft with exosome groups. Gene expression of nerve regeneration factors was analyzed on postoperative day 7. At 12 and 16 weeks, rats were subjected to maximum isometric tetanic force and compound muscle action potential. Nerve specimens were then analyzed by means of histology and immunohistochemistry.

RESULTS

Exosomes were readily taken up by Schwann cells that resulted in improved Schwann cell viability and migration. The treated allograft group had functional recovery (compound muscle action potential, isometric tetanic force) comparable to that of the autograft group. Similar results were observed in gene expression analysis of nerve regenerating factors. Histologic analysis showed no statistically significant differences between treated allograft and autograft groups in terms of axonal density, fascicular area, and myelin sheath thickness.

CONCLUSIONS

Plasma-derived exosome treatment of decellularized nerve allograft may provide comparable clinical outcomes to that of an autograft. This can be a promising strategy in the future as an alternative for segmental peripheral nerve repair.

CLINICAL RELEVANCE STATEMENT

Off-the-shelf exosomes may improve recovery in nerve allografts.

Enhanced peripheral nerve regeneration by mechano-electrical stimulation

To address limitations in current approaches for treating large peripheral nerve defects, the presented study evaluated the feasibility of functional material-mediated physical stimuli on peripheral nerve regeneration. Electrospun piezoelectric poly(vinylidene fluoride-trifluoroethylene) nanofibers were utilized to deliver mechanical actuation-activated electrical stimulation to nerve cells/tissues in a non-invasive manner. Using morphologically and piezoelectrically optimized nanofibers for neurite extension and Schwann cell maturation based on in vitro experiments, piezoelectric nerve conduits were synthesized and implanted in a rat sciatic nerve transection model to bridge a critical-sized sciatic nerve defect (15 mm). A therapeutic shockwave system was utilized to periodically activate the piezoelectric effect of the implanted nerve conduit on demand. The piezoelectric nerve conduit-mediated mechano-electrical stimulation (MES) induced enhanced peripheral nerve regeneration, resulting in full axon reconnection with myelin regeneration from the proximal to the distal ends over the critical-sized nerve gap. In comparison, a control group, in which the implanted piezoelectric conduits were not activated in vivo, failed to exhibit such nerve regeneration. In addition, at both proximal and distal ends of the implanted conduits, a decreased number of damaged myelination (ovoids), an increased number of myelinated nerves, and a larger axonal diameter were observed under the MES condition as compared to the control condition. Furthermore, unlike the control group, the MES condition exhibited a superior functional nerve recovery, assessed by walking track analysis and polarization-sensitive optical coherence tomography, demonstrating the significant potential of the piezoelectric conduit-based physical stimulation approach for the treatment of peripheral nerve injury.

Application of optogenetic glial cells to neuron-glial communication

Optogenetic techniques combine optics and genetics to enable cell-specific targeting and precise spatiotemporal control of excitable cells, and they are increasingly being employed. One of the most significant advantages of the optogenetic approach is that it allows for the modulation of nearby cells or circuits with millisecond precision, enabling researchers to gain a better understanding of the complex nervous system. Furthermore, optogenetic neuron activation permits the regulation of information processing in the brain, including synaptic activity and transmission, and also promotes nerve structure development. However, the optimal conditions remain unclear, and further research is required to identify the types of cells that can most effectively and precisely control nerve function. Recent studies have described optogenetic glial manipulation for coordinating the reciprocal communication between neurons and glia. Optogenetically stimulated glial cells can modulate information processing in the central nervous system and provide structural support for nerve fibers in the peripheral nervous system. These advances promote the effective use of optogenetics, although further experiments are needed. This review describes the critical role of glial cells in the nervous system and reviews the optogenetic applications of several types of glial cells, as well as their significance in neuron-glia interactions. Together, it briefly discusses the therapeutic potential and feasibility of optogenetics.

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.

Properties of Resorbable Conduits Based on Poly(L-Lactide) Nanofibers and Chitosan Fibers for Peripheral Nerve Regeneration

New tubular conduits have been developed for the regeneration of peripheral nerves and the repair of defects that are larger than 3 cm. The conduits consist of a combination of poly(L-lactide) nanofibers and chitosan composite fibers with chitin nanofibrils. In vitro studies were conducted to assess the biocompatibility of the conduits using human embryonic bone marrow stromal cells (FetMSCs). The studies revealed good adhesion and differentiation of the cells on the conduits just one day after cultivation. Furthermore, an in vivo study was carried out to evaluate motor-coordination disorders using the sciatic nerve functional index (SFI) assessment. The presence of chitosan monofibers and chitosan composite fibers with chitin nanofibrils in the conduit design increased the regeneration rate of the sciatic nerve, with an SFI value ranging from 76 to 83. The degree of recovery of nerve conduction was measured by the amplitude of M-response, which showed a 46% improvement. The conduit design imitates the oriented architecture of the nerve, facilitates electrical communication between the damaged nerve's ends, and promotes the direction of nerve growth, thereby increasing the regeneration rate.

Additive manufacturing of peripheral nerve conduits - Fabrication methods, design considerations and clinical challenges

Tissue-engineered nerve guidance conduits (NGCs) are a viable clinical alternative to autografts and allografts and have been widely used to treat peripheral nerve injuries (PNIs). Although these NGCs are successful to some extent, they cannot aid in native regeneration by improving native-equivalent neural innervation or regrowth. Further, NGCs exhibit longer recovery period and high cost limiting their clinical applications. Additive manufacturing (AM) could be an alternative to the existing drawbacks of the conventional NGCs fabrication methods. The emergence of the AM technique has offered ease for developing personalized three-dimensional (3D) neural constructs with intricate features and higher accuracy on a larger scale, replicating the native feature of nerve tissue. This review introduces the structural organization of peripheral nerves, the classification of PNI, and limitations in clinical and conventional nerve scaffold fabrication strategies. The principles and advantages of AM-based techniques, including the combinatorial approaches utilized for manufacturing 3D nerve conduits, are briefly summarized. This review also outlines the crucial parameters, such as the choice of printable biomaterials, 3D microstructural design/model, conductivity, permeability, degradation, mechanical property, and sterilization required to fabricate large-scale additive-manufactured NGCs successfully. Finally, the challenges and future directions toward fabricating the 3D-printed/bioprinted NGCs for clinical translation are also discussed.

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

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

Functional Gait Assessment Using Manual, Semi-Automated and Deep Learning Approaches Following Standardized Models of Peripheral Nerve Injury in Mice

Objective: To develop a standardized model of stretch−crush sciatic nerve injury in mice, and to compare outcomes of crush and novel stretch−crush injuries using standard manual gait and sensory assays, and compare them to both semi-automated as well as deep-learning gait analysis methods. Methods: Initial studies in C57/Bl6 mice were used to develop crush and stretch−crush injury models followed by histologic analysis. In total, 12 eight-week-old 129S6/SvEvTac mice were used in a six-week behavioural study. Behavioral assessments using the von Frey monofilament test and gait analysis recorded on a DigiGait platform and analyzed through both Visual Gait Lab (VGL) deep learning and standardized sciatic functional index (SFI) measurements were evaluated weekly. At the termination of the study, neurophysiological nerve conduction velocities were recorded, calf muscle weight ratios measured and histological analyses performed. Results: Histological evidence confirmed more severe histomorphological injury in the stretch−crush injured group compared to the crush-only injured group at one week post-injury. Von Frey monofilament paw withdrawal was significant for both groups at week one compared to baseline (p < 0.05), but not between groups with return to baseline at week five. SFI showed hindered gait at week one and two for both groups, compared to baseline (p < 0.0001), with return to baseline at week five. Hind stance width (HSW) showed similar trends as von Frey monofilament test as well as SFI measurements, yet hind paw angle (HPA) peaked at week two. Nerve conduction velocity (NCV), measured six weeks post-injury, at the termination of the study, did not show any significant difference between the two groups; yet, calf muscle weight measurements were significantly different between the two, with the stretch−crush group demonstrating a lower (poorer) weight ratio relative to uninjured contralateral legs (p < 0.05). Conclusion: Stretch−crush injury achieved a more reproducible and constant injury compared to crush-only injuries, with at least a Sunderland grade 3 injury (perineurial interruption) in histological samples one week post-injury in the former. However, serial behavioral outcomes were comparable between the two crush groups, with similar kinetics of recovery by von Frey testing, SFI and certain VGL parameters, the latter reported for the first time in rodent peripheral nerve injury. Semi-automated and deep learning-based approaches for gait analysis are promising, but require further validation for evaluation in murine hind-limb nerve injuries.

Donors for nerve transplantation in craniofacial soft tissue injuries

Neural tissue is an important soft tissue; for instance, craniofacial nerves govern several aspects of human behavior, including the expression of speech, emotion transmission, sensation, and motor function. Therefore, nerve repair to promote functional recovery after craniofacial soft tissue injuries is indispensable. However, the repair and regeneration of craniofacial nerves are challenging due to their intricate anatomical and physiological characteristics. Currently, nerve transplantation is an irreplaceable treatment for segmental nerve defects. With the development of emerging technologies, transplantation donors have become more diverse. The present article reviews the traditional and emerging alternative materials aimed at advancing cutting-edge research on craniofacial nerve repair and facilitating the transition from the laboratory to the clinic. It also provides a reference for donor selection for nerve repair after clinical craniofacial soft tissue injuries. We found that autografts are still widely accepted as the first options for segmental nerve defects. However, allogeneic composite functional units have a strong advantage for nerve transplantation for nerve defects accompanied by several tissue damages or loss. As an alternative to autografts, decellularized tissue has attracted increasing attention because of its low immunogenicity. Nerve conduits have been developed from traditional autologous tissue to composite conduits based on various synthetic materials, with developments in tissue engineering technology. Nerve conduits have great potential to replace traditional donors because their structures are more consistent with the physiological microenvironment and show self-regulation performance with improvements in 3D technology. New materials, such as hydrogels and nanomaterials, have attracted increasing attention in the biomedical field. Their biocompatibility and stimuli-responsiveness have been gradually explored by researchers in the regeneration and regulation of neural networks.

A focused review on three-dimensional bioprinting technology for artificial organ fabrication

Three-dimensional (3D) bioprinting technology has attracted a great deal of interest because it can be easily adapted to many industries and research sectors, such as biomedical, manufacturing, education, and engineering. Specifically, 3D bioprinting has provided significant advances in the medical industry, since such technology has led to significant breakthroughs in the synthesis of biomaterials, cells, and accompanying elements to produce composite living tissues. 3D bioprinting technology could lead to the immense capability of replacing damaged or injured tissues or organs with newly dispensed cell biomaterials and functional tissues. Several types of bioprinting technology and different bio-inks can be used to replicate cells and generate supporting units as complex 3D living tissues. Bioprinting techniques have undergone great advancements in the field of regenerative medicine to provide 3D printed models for numerous artificial organs and transplantable tissues. This review paper aims to provide an overview of 3D-bioprinting technologies by elucidating the current advancements, recent progress, opportunities, and applications in this field. It highlights the most recent advancements in 3D-bioprinting technology, particularly in the area of artificial organ development and cancer research. Additionally, the paper speculates on the future progress in 3D-bioprinting as a versatile foundation for several biomedical applications.

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.

Physical Stimulation Combined with Biomaterials Promotes Peripheral Nerve Injury Repair

Peripheral nerve injury (PNI) is a clinical problem with high morbidity that can cause severe damage. Surgical suturing or implants are usually required due to the slow speed and numerous factors affecting repair after PNI. An autologous nerve graft is the gold standard for PNI repair among implants. However, there is a potential problem of the functional loss of the donor site. Therefore, tissue-engineered nerve biomaterials are often used to bridge the gap between nerve defects, but the therapeutic effect is insufficient. In order to enhance the repair effect of nerve biomaterials for PNI, researchers are seeking to combine various stimulation elements, such as the addition of biological factors such as nerve growth factors or physical factors such as internal microstructural modifications of catheters and their combined application with physical stimulation therapy. Physical stimulation therapy is safer, is more convenient, and has more practical features than other additive factors. Its feasibility and convenience, when combined with nerve biomaterials, provide broader application prospects for PNI repair, and has therefore become a research hot spot. This paper will review the combined application of physical stimulation and biomaterials in PNI repair in recent years to provide new therapeutic ideas for the future use of physical stimulation in PNI repair.

Preliminary application of native Nephila edulis spider silk and fibrin implant causes granulomatous foreign body reaction in vivo in rat's spinal cord

After spinal cord injury, gliomesenchymal scaring inhibits axonal regeneration as a physical barrier. In peripheral nerve injuries, native spider silk was shown to be an effective scaffold to facilitate axonal re-growth and nerve regeneration. This study tested a two-composite scaffold made of longitudinally oriented native spider silk containing a Haemocomplettan fibrin sheath to bridge lesions in the spinal cord and enhance axonal sprouting. In vitro cultivation of neuronal cells on spider silk and fibrin revealed no cytotoxicity of the scaffold components. When spinal cord tissue was cultured on spider silk that was reeled around a metal frame, migration of different cell types, including neurons and neural stem cells, was observed. The scaffold was implanted into spinal cord lesions of four Wistar rats to evaluate the physical stress caused on the animals and examine the bridging potential for axonal sprouting and spinal cord regeneration. However, the implantation in-vivo resulted in a granulomatous foreign body reaction. Spider silk might be responsible for the strong immune response. Thus, the immune response to native spider silk seems to be stronger in the central nervous system than it is known to be in the peripheral body complicating the application of native spider silk in spinal cord injury treatment.

Exosomes derived from platelet-rich plasma administration in site mediate cartilage protection in subtalar osteoarthritis

Subtalar osteoarthritis (STOA) is often secondary to chronic ankle sprains, which seriously affects the quality of life of patients. Due to its etiology and pathogenesis was not studied equivocally yet, there is currently a lack of effective conservative treatments. Although they have been used for tissue repair, platelet-rich plasma-derived exosomes (PRP-Exo) have the disadvantage of low retention and short-lived therapeutic effects. This study aimed to determine whether incorporation of PRP-Exo in thermosensitive hydrogel (Gel) increased their retention in the joint and thereby playing a therapeutic role on STOA due to chronic mechanical instability established by transecting lateral ligaments (anterior talofibular ligament (ATFL)/calcaneal fibular ligament (CFL)). PRP-Exo incorporated Gel (Exo-Gel) system, composed of Poloxamer-407 and 188 mixture-based thermoresponsive hydrogel matrix in an optimal ratio, was determined by its release ability of Exo and rheology of Gel response to different temperature. The biological activity of Exo-Gel was evaluated in vitro, and the therapeutic effect of Exo-Gel on STOA was evaluated in vivo. Exo released from Exo-Gel continuously for 28 days could promote the proliferation and migration of mouse bone mesenchymal stem cells (mBMSCs) and chondrocytes, at the same time enhance the chondrogenic differentiation of mBMSCs, and inhibit inflammation-induced chondrocyte degeneration. In vivo experiments confirmed that Exo-Gel increased the local retention of Exo, inhibited the apoptosis and hypertrophy of chondrocytes, enhanced their proliferation, and potentially played the role in stem cell recruitment to delay the development of STOA. Thus, Delivery of PRP-Exo incorporated in thermosensitive Gel provides a novel approach of cell-free therapy and has therapeutic effect on STOA.

Graphical Abstract

A review on advances in the applications of spider silk in biomedical issues

Spider silk, as one of the hardest natural and biocompatible substances with extraordinary strength and flexibility, have become an ideal option in various areas of science and have made their path onto the biomedical industry. Despite its growing popularity, the difficulties in the extraction of silks from spiders and farming them have made it unaffordable and almost impossible for industrial scale. Biotechnology helped production of spider silks recombinantly in different hosts and obtaining diverse morphologies out of them based on different processing and assembly procedures. Herein, the characteristics of these morphologies and their advantages and disadvantages are summarized. A detailed view about applications of recombinant silks in skin regeneration and cartilage, tendon, bone, teeth, cardiovascular, and neural tissues engineering are brought out, where there is a need for strong scaffolds to support cell growth. Likewise, spider silk proteins have applications as conduit constructs, medical sutures, and 3D printer bioinks. Other characteristics of spider silks, such as low immunogenicity, hydrophobicity, homogeneity, and adjustability, have attracted much attention in drug and gene delivery. Finally, the challenges and obstacles ahead for industrializing the production of spider silk proteins in sufficient quantities in biomedicine, along with solutions to overcome these barriers, are discussed.

The balanced microenvironment regulated by the degradants of appropriate PLGA scaffolds and chitosan conduit promotes peripheral nerve regeneration

Tissue-engineered nerve grafts (TENGs) are the most promising way for repairing long-distance peripheral nerve defects. Chitosan and poly (lactic-co-glycolic acid) (PLGA) scaffolds are considered as the promising materials in the pharmaceutical and biomedical fields especially in the field of tissue engineering. To further clarify the effects of a chitosan conduit inserted with various quantity of poly (lactic-co-glycolic acid) (PLGA) scaffolds, and their degrades on the peripheral nerve regeneration, the chitosan nerve conduit inserted with different amounts of PLGA scaffolds were used to repair rat sciatic nerve defects. The peripheral nerve regeneration at the different time points was dynamically and comprehensively evaluated. Moreover, the influence of different amounts of PLGA scaffolds on the regeneration microenvironment including inflammatory response and cell state were also revealed. The modest abundance of PLGA is more instrumental to the success of nerve regeneration, which is demonstrated in terms of the structure of the regenerated nerve, reinnervation of the target muscle, nerve impulse conduction, and overall function. The PLGA scaffolds aid the migration and maturation of Schwann cells. Furthermore, the PLGA and chitosan degradation products in a correct ratio neutralize, reducing the inflammatory response and enhancing the regeneration microenvironment. The balanced microenvironment regulated by the degradants of appropriate PLGA scaffolds and chitosan conduit promotes peripheral nerve regeneration. The findings represent a further step towards programming TENGs construction, applying polyester materials in regenerative medicine, and understanding the neural regeneration microenvironment.

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Guide scaffolds are necessary for construction of TENGs to benefeat Schwann cell migration and maturation.

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A large number of acid degradation products of PLGA scaffolds adversely affect cell proliferation, migration and apoptosis.

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Appropriate amount of PLGA scaffolds balance positive cell guidance and negative degradation inflammation.

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Dosage of PLGA and its combination with complementary biomaterials are key factors that affect regeneration effects.

Preclinical study of peripheral nerve regeneration using nerve guidance conduits based on polyhydroxyalkanaotes

Nerve guidance conduits (NGCs) are used as an alternative to the "gold standard" nerve autografting, preventing the need for surgical intervention required to harvest autologous nerves. However, the regeneration outcomes achieved with the current NGCs are only comparable with autografting when the gap is short (less than 10 mm). In the present study, we have developed NGCs made from a blend of polyhydroxyalkanoates, a family of natural resorbable polymers. Hollow NGCs made from a 75:25 poly(3-hydroxyoctanoate)/poly(3-hydroxybutyrate) blend (PHA-NGCs) were manufactured using dip-molding. These PHA-NGCs showed appropriate flexibility for peripheral nerve regeneration. In vitro cell studies performed using RT4-D6P2T rat Schwann cell line confirmed that the material is capable of sustaining cell proliferation and adhesion. PHA-NGCs were then implanted in vivo to repair 10 mm gaps of the median nerve of female Wistar rats for 12 weeks. Functional evaluation of the regenerated nerve using the grasping test showed that PHA-NGCs displayed similar motor recovery as the autograft, starting from week 7. Additionally, nerve cross-sectional area, density and number of myelinated cells, as well as axon diameter, fiber diameter, myelin thickness and g-ratio obtained using the PHA-NGCs were found comparable to an autograft. This preclinical data confirmed that the PHA-NGCs are indeed highly promising candidates for peripheral nerve regeneration.

The Restorative Effect of Gallic Acid on the Experimental Sciatic Nerve Damage Model

Objective

Peripheral nerve injuries occur mostly as a result of mechanical trauma. Due to the microvascular deterioration in peripheral nerve damage, it becomes challenging to remove free oxygen radicals. Gallic acid is a powerful antioxidant with anti-inflammatory effects and a free radical scavenger. The purpose of the study is to show that gallic acid contributes to the restorative effect in mechanical nerve damage, considering its antioxidant and anti-inflammatory effects.

Methods

Thirty male Sprague Dawley albino mature rats were included in the study. Ten of them constituted the control group, 10 out of 20 rats for which sciatic nerve damage was caused, constituted the saline group, and 10 formed the gallic acid group. Post-treatment motor functions, histological, immunohistochemical, and biochemical parameters of the rats were evaluated.

Results

Compared to the surgery+saline group, lower compound muscle action potential (CMAP) latency, higher CMAP amplitude, and higher inclined plane test values were found in the surgery+gallic acid group. Similarly, a higher nerve growth factor (NGF) percentage, a higher number of axons, and a lower percentage of fibrosis scores were observed in the surgery+gallic acid group. Finally, lower tissue malondialdehyde (MDA) and higher heat shock protein-70 (HSP-70) values were determined in the surgery+gallic acid group.

Conclusion

Gallic acid positively affects peripheral nerve injury healing due to its anti-inflammatory and antioxidant effects. It has been thought that gallic acid can be used as a supportive treatment in peripheral nerve damage.

Administration of Purified Exosome Product in a Rat Sciatic Serve Reverse Autograft Model

BACKGROUND

The nerve autograft remains the gold standard when reconstructing peripheral nerve defects. However, although autograft repair can result in useful functional recovery, poor outcomes are common, and better treatments are needed. The purpose of this study was to evaluate the effect of purified exosome product on functional motor recovery and nerve-related gene expression in a rat sciatic nerve reverse autograft model.

METHODS

Ninety-six Sprague-Dawley rats were divided into three experimental groups. In each group, a unilateral 10-mm sciatic nerve defect was created. The excised nerve was reversed and used to reconstruct the defect. Group I animals received the reversed autograft alone, group II animals received the reversed autograft with fibrin glue, and group III animals received the reversed autograft with purified exosome product suspended in the fibrin glue. The animals were killed at 3 and 7 days and 12 and 16 weeks after surgery. Evaluation included compound muscle action potentials, isometric tetanic force, tibialis anterior muscle wet weight, nerve regeneration-related gene expression, and nerve histomorphometry.

RESULTS

At 16 weeks, isometric tetanic force was significantly better in group III (p = 0.03). The average axon diameter of the peroneal nerve was significantly larger in group III at both 12 and 16 weeks (p = 0.015 at 12 weeks; p < 0.01 at 16 weeks). GAP43 and S100b gene expression was significantly up-regulated by purified exosome product.

CONCLUSIONS

Local administration of purified exosome product demonstrated improved nerve regeneration profiles in the reverse sciatic nerve autograft rat model. Thus, purified exosome product may have beneficial effects on nerve regeneration, gene profiles, and motor outcomes.

The corticospinal tract structure of collagen/silk fibroin scaffold implants using 3D printing promotes functional recovery after complete spinal cord transection in rats

No effective treatment has been established for nerve dysfunction caused by spinal cord injury (SCI). Orderly axonal growth at the site of spinal cord transection and creation of an appropriate biological microenvironment are important for functional recovery. To axially guiding axonal growth, designing a collagen/silk fibroin scaffold fabricated with 3D printing technology (3D-C/SF) emulated the corticospinal tract. The normal collagen/silk fibroin scaffold with freeze-drying technology (C/SF) or 3D-C/SF scaffold were implanted into rats with completely transected SCI to evaluate its effect on nerve repair during an 8-week observation period. Electrophysiological analysis and locomotor performance showed that the 3D-C/SF implants contributed to significant improvements in the neurogolical function of rats compared to C/SF group. By magnetic resonance imaging, 3D-C/SF implants promoted a striking degree of axonal regeneration and connection between the proximal and distal SCI sites. Compared with C/SF group, rats with 3D-C/SF scaffold exhibited fewer lesions and disordered structures in histological analysis and more GAP43-positive profiles at the lesion site. The above results indicated that the corticospinal tract structure of 3D printing collagen/silk fibroin scaffold improved axonal regeneration and promoted orderly connections within the neural network, which could provided a promising and innovative approach for tissue repair after SCI.

Comparison of the Efficacy of Optogenetic Stimulation of Glia versus Neurons in Myelination

Increasing evidence demonstrates that optogenetics contributes to the regulation of brain behavior, cognition, and physiology, particularly during myelination, potentially allowing for the bidirectional modulation of specific cell lines with spatiotemporal accuracy. However, the type of cell to be targeted, namely, glia vs neurons, and the degree to which optogenetically induced cell activity can regulate myelination during the development of the peripheral nervous system (PNS) are still underexplored. Herein, we report the comparison of optogenetic stimulation (OS) of Schwann cells (SCs) and motor neurons (MNs) for activation of myelination in the PNS. Capitalizing on these optogenetic tools, we confirmed that the formation of the myelin sheath was initially promoted more by OS of calcium translocating channelrhodopsin (CatCh)-transfected SCs than by OS of transfected MNs at 7 days in vitro (DIV). Additionally, the level of myelination was substantially enhanced even until 14 DIV. Surprisingly, after OS of SCs, > 91.1% ± 5.9% of cells expressed myelin basic protein, while that of MNs was 67.8% ± 6.1%. The potent effect of OS of SCs was revealed by the increased thickness of the myelin sheath at 14 DIV. Thus, the OS of SCs could highly accelerate myelination, while the OS of MNs only somewhat promoted myelination, indicating a clear direction for the optogenetic application of unique cell types for initiating and promoting myelination. Together, our findings support the importance of precise cell type selection for use in optogenetics, which in turn can be broadly applied to overcome the limitations of optogenetics after injury.

Advances and clinical challenges for translating nerve conduit technology from bench to bed side for peripheral nerve repair

Injuries to the peripheral nervous system remain a large-scale clinical problem. These injuries often lead to loss of motor and/or sensory function that significantly affects patients' quality of life. The current neurosurgical approach for peripheral nerve repair involves autologous nerve transplantation, which often leads to clinical complications. The most pressing need is to increase the regenerative capacity of existing tubular constructs in the repair of large nerve gaps through development of tissue-engineered approaches that can surpass the performance of autografts. To fully realize the clinical potential of nerve conduit technology, there is a need to reconsider design strategies, biomaterial selection, fabrication techniques and the various potential modifications to optimize a conduit microenvironment that can best mimic the natural process of regeneration. In recent years, a significant progress has been made in the designing and functionality of bioengineered nerve conduits to bridge long peripheral nerve gaps in various animal models. However, translation of this work from lab to commercial scale has not been achieve. The current review summarizes recent advances in the development of tissue engineered nerve guidance conduits (NGCs) with regard to choice of material, novel fabrication methods, surface modifications and regenerative cues such as stem cells and growth factors to improve regeneration performance. Also, the current clinical potential and future perspectives to achieve therapeutic benefits of NGCs will be discussed in context of peripheral nerve regeneration.

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.

Bioactive Elastic Scaffolds Loaded with Neural Stem Cells Promote Rapid Spinal Cord Regeneration

Despite decades of research, spinal cord injury (SCI) still causes irreparable damage to the human body. Key challenges that hinder the regeneration and extension of neurons following SCI must be overcome, including the overexpressed glial scar formation and strong inflammatory responses in lesion tissue. Transplantation of neural stem cells (NSCs) represents a promising therapeutic method due to its beneficial roles like growth factor secretion and anti-inflammation. However, NSCs usually differentiate into astrocytes, which is considered as one potential limitation of current NSC therapy. Herein, we fabricate an elastic poly(sebacoyl diglyceride) (PSeD) scaffold to mimic the mechanical properties of the natural spinal cord. The PSeD scaffold is coated with poly(sebacoyl diglyceride)-isoleucine-lysine-valine-alanine-valine-serine (PSeD-IKVAVS) to create a bioactive interface. The core point of this topic is divided into two parts. First, PSeD is a bioelastomer and its mechanical properties are similar to those of the natural spinal cord. This feature reduces the direct stimulation to the spinal cord tissue by the elastomer and then reduces the immune response or resistance caused by the host spinal cord tissue. Second, the IKVAVS peptide modifies PSeD to create a bioactive interface to support NSC growth and differentiation. In the in vivo study, the number of CD68-positive macrophages decreased in the PSeD-IKVAVS/NSC group compared to that in the SCI group (20% vs 60%). The low inflammation induced by the scaffold was beneficial to NSCs, resulting in increased locomotor recovery, as indicated by the increased Basso-Beattie-Bresnahan score (5, the average score in the PSeD-IKVAVS/NSC group, vs 2, the average score in the SCI group). Based on the above two characteristics, a PSeD-IKVAVS bioelastomer is fabricated, which provides a beneficial and bioactive microenvironment for NSCs after transplantation.

Motor and sensory Schwann cell phenotype commitment is diminished by extracorporeal shockwave treatment in vitro

The gold standard for peripheral nerve regeneration uses a sensory autograft to bridge a motor/sensory defect site. For motor nerves to regenerate, Schwann cells (SC) myelinate the newly grown axon. Sensory SCs have a reduced ability to produce myelin, partially explaining low success rates of autografts. This issue is masked in pre-clinical research by the excessive use of the rat sciatic nerve defect model, utilizing a mixed nerve with motor and sensory SCs. Aim of this study was to utilize extracorporeal shockwave treatment as a novel tool to influence SC phenotype. SCs were isolated from motor, sensory and mixed rat nerves and in vitro differences between them were assessed concerning initial cell number, proliferation rate, neurite outgrowth as well as ability to express myelin. We verified the inferior capacity of sensory SCs to promote neurite outgrowth and express myelin-associated proteins. Motor Schwann cells demonstrated low proliferation rates, but strongly reacted to pro-myelination stimuli. It is noteworthy for pre-clinical research that sciatic SCs are a strongly mixed culture, not representing one or the other. Extracorporeal shockwave treatment (ESWT), induced in motor SCs an increased proliferation profile, while sensory SCs gained the ability to promote neurite outgrowth and express myelin-associated markers. We demonstrate a strong phenotype commitment of sciatic, motor, and sensory SCs in vitro, proposing the experimental use of SCs from pure cultures to better mimic clinical situations. Furthermore we provide arguments for using ESWT on autografts to improve the regenerative capacity of sensory SCs.

Aligned chitosan nanofiber hydrogel grafted with peptides mimicking bioactive brain-derived neurotrophic factor and vascular endothelial growth factor repair long-distance sciatic nerve defects in rats

Autologous nerve transplantation, which is the gold standard for clinical treatment of peripheral nerve injury, still has many limitations. In this study, aligned chitosan fiber hydrogel (ACG) grafted with a bioactive peptide mixture consisting of RGI (Ac-RGIDKRHWNSQGG) and KLT (Ac-KLTWQELYQLKYKGIGG), designated as ACG-RGI/KLT, was used as nerve conduit filler to repair sciatic nerve defects in rats. Methods: Chitosan nanofiber hydrogel was prepared by a combination of electrospinning and mechanical stretching methods, and was then grafted with RGI and KLT, which are peptides mimicking brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF), respectively. The physicochemical properties of ACG-RGI/KLT were fully characterized. In vitro, the distribution, proliferation, and secretory activity of Schwann cells were analyzed. Next, the in vivo repair potential for 15-mm rat sciatic nerve defects was examined. The recovery of regenerated nerve, muscle, and motor function was evaluated by neuromuscular histology, electrophysiology, and catwalk gait analysis. Results: We first constructed directionally aligned chitosan nanofiber hydrogel grafted with RGI/KLT peptide mixture (ACG-RGI/KLT). ACG-RGI/KLT oriented the Schwann cells, and promoted the proliferation and secretion of neurotrophic factors by Schwann cells. At an early injury stage, ACG-RGI/KLT not only enhanced nerve regeneration, but also promoted vascular penetration. At 12 weeks, ACG-RGI/KLT facilitated nerve regeneration and functional recovery in rats. Conclusions: Aligned chitosan nanofiber hydrogel grafted with RGI/KLT peptide provides an effective means of repairing sciatic nerve defects and shows great potential for clinical application.

Platelet-Rich Plasma Combined with Low-Dose Ultrashort Wave Therapy Accelerates Peripheral Nerve Regeneration

Finding treatments that accelerate peripheral nerve regeneration, prolongation, and functional recovery remains a challenging task. Platelet-rich plasma (PRP) contains numerous growth factors and active proteins, and low-dose ultrashort waves (USWs) stimulate the formation of nerve-nourishing vessels, which are powerful for nerve regeneration. The goal of this study was to evaluate the synergistic effects of serial ultrasound-guided PRP injections combined with low-dose USWs radiation on peripheral nerve regeneration in a crush injury model. Fifty rabbits were equally and randomly divided into normal control, model, USW, PRP, and PRP+USW groups. The neurological function, electrophysiological recovery, and histological and morphological evaluation of regenerated nerves, as well as a targeted muscle recovery assessment, were performed to investigate the regenerative effect of PRP combined with USW therapy. Our results showed that the PRP+USW group had the better early axonal regeneration and displayed the earliest positive compound muscle action potential among the treatment groups. At postintervention week 12, a histological evaluation showed similar expression of the S-100 protein in the PRP+USW and normal control groups. Moreover, the morphological assessment revealed a significant increase in the myelinated nerve fiber density and diameter and myelin sheath thickness compared with the USW and PRP groups. The morphometry of the target muscles indicated the lowest reduction in the percent volume in the PRP+USW group, and an ultrasound examination of the targeted muscle showed the best improvement in stiffness and perfusion parameters at 12 weeks after crush injury. Thus, serial ultrasound-guided PRP injections combined with low-dose USW radiation exert a synergistic effect on accelerating functional axon recovery and decreasing atrophy of the target muscles in a crush injury model. Impact Statement This research describes that the application of platelet-rich plasma combined with low-dose ultrashort waves treatment exert a synergistic effect on accelerating peripheral nerve regeneration. With the extensive use of platelet-rich plasma and physical factors in regenerative medicine or clinical rehabilitation medicine, our findings may help establish effective strategies for repairing peripheral nerve injury.

Peripheral Nerve Conduit: Materials and Structures

Peripheral nerve injuries (PNIs) are the most common injury types to affect the nervous system. Restoration of nerve function after PNI is a challenging medical issue. Extended gaps in transected peripheral nerves are only repaired using autologous nerve grafting. This technique, however, in which nerve tissue is harvested from a donor site and grafted onto a recipient site in the same body, has many limitations and disadvantages. Recent studies have revealed artificial nerve conduits as a promising alternative technique to substitute autologous nerves. This Review summarizes different types of artificial nerve grafts used to repair peripheral nerve injuries. These include synthetic and natural polymers with biological factors. Then, desirable properties of nerve guides are discussed based on their functionality and effectiveness. In the final part of this Review, fabrication methods and commercially available nerve guides are described.

Small-Molecule SB216763-Loaded Microspheres Repair Peripheral Nerve Injury in Small Gap Tubulization

Peripheral nerve injury has yet to be fully resolved because of its complicated pathological process. SB216763 is a small molecular compound that can enhance the remyelination of peripheral nerves by inhibiting glycogen synthase kinase-3β (GSK3β). GSK-3β inhibitor stimulates myelin gene expression and restores the myelin structure. Herein, we presented the effect of integrating small gap tubulization with SB216763-loaded microspheres by using a chitosan conduit. In vitro, SB216763 could promote neurite growth of dorsal root ganglia. In vivo studies showed that SB216763 increased the number of myelinated axons and the thickness of myelin sheaths. Electrophysiological examination and sciatic functional index results also indirectly indicated the role of SB216763 in repairing peripheral nerve injury. SB216763 promoted the recovery of muscle function. Therefore, combining SB216763-loaded PLGA microspheres with conduit small gap tubulization shows potential for applications in repairing peripheral nerve injury.

Spatiotemporal Differences in Gene Expression Between Motor and Sensory Autografts and Their Effect on Femoral Nerve Regeneration in the Rat

To improve the outcome after autologous nerve grafting in the clinic, it is important to understand the limiting variables such as distinct phenotypes of motor and sensory Schwann cells. This study investigated the properties of phenotypically different autografts in a 6 mm femoral nerve defect model in the rat, where the respective femoral branches distally of the inguinal bifurcation served as homotopic, or heterotopic autografts. Axonal regeneration and target reinnervation was analyzed by gait analysis, electrophysiology, and wet muscle mass analysis. We evaluated regeneration-associated gene expression between 5 days and 10 weeks after repair, in the autografts as well as the proximal, and distal segments of the femoral nerve using qRT-PCR. Furthermore we investigated expression patterns of phenotypically pure ventral and dorsal roots. We identified highly significant differences in gene expression of a variety of regeneration-associated genes along the central – peripheral axis in healthy femoral nerves. Phenotypically mismatched grafting resulted in altered spatiotemporal expression of neurotrophic factor BDNF, GDNF receptor GFRα1, cell adhesion molecules Cadm3, Cadm4, L1CAM, and proliferation associated Ki67. Although significantly higher quadriceps muscle mass following homotopic nerve grafting was measured, we did not observe differences in gait analysis, and electrophysiological parameters between treatment paradigms. Our study provides evidence for phenotypic commitment of autologous nerve grafts after injury and gives a conclusive overview of temporal expression of several important regeneration-associated genes after repair with sensory or motor graft.

Gold nanorods reinforced silk fibroin nanocomposite for peripheral nerve tissue engineering applications

Nowadays, regenerating peripheral nerves injuries (PNIs) remain a major clinical challenge, which has gained a great attention between scientists. Here, we represent a nanocomposite based on silk fibroin reinforced gold nanorods (SF/GNRs) to evaluate the proliferation and attachment of PC12 cells. The morphological characterization of nanocomposites with transmission electron microscopy (TEM) and Scanning electron microscopy (SEM) showed that the fabricated scaffolds have porous structure with interconnected pores that is suitable for cell adhesion and growth. GNRs significantly improved the poor electrical conductivity of bulk silk fibroin scaffold. Evaluating the morphology of PC12 cells on the scaffold also confirmed the normal morphology of cells with good rate of adhesion. SF/GNRs nanocomposites showed better cellular attachment, growth and proliferation without any toxicity compared with bulk SF scaffold. Moreover, immunostaining studies represented the overexpression of neural specific proteins like nestin and neuron specific enolase (NSE) in the cells cultured on SF/GNRs nanocomposites in comparison to neat SF scaffolds.

Sericin silk protein in peripheral nervous repair associated with the physical exercise of swimming in Wistar rats

OBJECTIVE

To verify the action of sericin associated to swim exercise with overload, on sciatic nerve repair in Wistar rats, after 22 days of nerve compression.

METHODS

Forty animals been composed of five groups: control, injury, injury-sericin, injury-swimming and injury-sericin-swimming. During the lesion procedure, sericin, in hydrolyzed form, applied directly to the injury in the injury-sericin and injury-sericin-swimming groups. Injury-swimming and injury-sericin-swimming groups underwent to 5 days per week for 3 weeks, with a 10% overload of the animal's body weight, and a weekly progressive evolution of swimming time, lasting 15, 20 and 25 min/day. Pre and throughout the treatment period the animals performed evaluation of sciatic functional index and pressure pain threshold with digital von Frey filament. Euthanasia was performed on the 22nd postoperative day, and two fragments of the nerve were collected and prepared for descriptive and quantitative analysis.

RESULTS

The sciatic functional index assessment showed significant differences in the motor function of the control until the 14th day. Regarding the allodynia, there was revealed a significant improvement in injury-swimming performance relative to injury, injury-sericin and injury-sericin-swimming, and the number of viable and non-viable nerve fibers smaller than 4 μm in diameter was significantly higher in the injury-sericin-swimming.

CONCLUSION

swimming showed a better evolution of the nociceptive threshold and allodynia. Sericin treatment had exacerbated pro-inflammatory characteristics. On the other hand, the association of sericine and swimming showed a possible regulatory effect by resting swimming exercise, with a significant increase of fibers of smaller diameter.

Therapeutic strategies for peripheral nerve injury: decellularized nerve conduits and Schwann cell transplantation

In recent years, the use of Schwann cell transplantation to repair peripheral nerve injury has attracted much attention. Animal-based studies show that the transplantation of Schwann cells in combination with nerve scaffolds promotes the repair of injured peripheral nerves. Autologous Schwann cell transplantation in humans has been reported recently. This article reviews current methods for removing the extracellular matrix and analyzes its composition and function. The development and secretory products of Schwann cells are also reviewed. The methods for the repair of peripheral nerve injuries that use myelin and Schwann cell transplantation are assessed. This survey of the literature data shows that using a decellularized nerve conduit combined with Schwann cells represents an effective strategy for the treatment of peripheral nerve injury. This analysis provides a comprehensive basis on which to make clinical decisions for the repair of peripheral nerve injury.

Layers and Multilayers of Self-Assembled Polymers: Tunable Engineered Extracellular Matrix Coatings for Neural Cell Growth

Growing primary cells and tissue in long-term cultures, such as primary neural cell culture, presents many challenges. A critical component of any environment that supports neural cell growth in vivo is an appropriate 2-D surface or 3-D scaffold, typically in the form of a thin polymer layer that coats an underlying plastic or glass substrate and aims to mimic critical aspects of the extracellular matrix. A fundamental challenge to mimicking a hydrophilic, soft natural cell environment is that materials with these properties are typically fragile and are difficult to adhere to and stabilize on an underlying plastic or glass cell culture substrate. In this review, we highlight the current state of the art and overview recent developments of new artificial extracellular matrix (ECM) surfaces for in vitro neural cell culture. Notably, these materials aim to strike a balance between being hydrophilic and soft while also being thick, stable, robust, and bound well to the underlying surface to provide an effective surface to support long-term cell growth. We focus on improved surface and scaffold coating systems that can mimic the natural physicochemical properties that enhance neuronal survival and growth, applied as soft hydrophilic polymer coatings for both in vitro cell culture and for implantable neural probes and 3-D matrixes that aim to enhance stability and longevity to promote neural biocompatibility in vivo. With respect to future developments, we outline four emerging principles that serve to guide the development of polymer assemblies that function well as artificial ECMs: (a) design inspired by biological systems and (b) the employment of principles of aqueous soft bonding and self-assembly to achieve (c) a high-water-content gel-like coating that is stable over time in a biological environment and possesses (d) a low modulus to more closely mimic soft, compliant real biological tissue. We then highlight two emerging classes of thick material coatings that have successfully captured these guiding principles: layer-by-layer deposited water-soluble polymers (LbL) and silk fibroin (SF) materials. Both materials can be deposited from aqueous solution yet transition to a water-insoluble coating for long-term stability while retaining a softness and water content similar to those of biological materials. These materials hold great promise as next-generation biocompatible coatings for tissue engineers and for chemists and biologists within the biomedical field.

Scaffolds for peripheral nerve repair and reconstruction

Trauma-associated peripheral nerve defect is a widespread clinical problem. Autologous nerve grafting, the current gold standard technique for the treatment of peripheral nerve injury, has many internal disadvantages. Emerging studies showed that tissue engineered nerve graft is an effective substitute to autologous nerves. Tissue engineered nerve graft is generally composed of neural scaffolds and incorporating cells and molecules. A variety of biomaterials have been used to construct neural scaffolds, the main component of tissue engineered nerve graft. Synthetic polymers (e.g. silicone, polyglycolic acid, and poly(lactic-co-glycolic acid)) and natural materials (e.g. chitosan, silk fibroin, and extracellular matrix components) are commonly used along or together to build neural scaffolds. Many other materials, including the extracellular matrix, glass fabrics, ceramics, and metallic materials, have also been used to construct neural scaffolds. These biomaterials are fabricated to create specific structures and surface features. Seeding supporting cells and/or incorporating neurotrophic factors to neural scaffolds further improve restoration effects. Preliminary studies demonstrate that clinical applications of these neural scaffolds achieve satisfactory functional recovery. Therefore, tissue engineered nerve graft provides a good alternative to autologous nerve graft and represents a promising frontier in neural tissue engineering.

Biomimetic neural scaffolds: a crucial step towards optimal peripheral nerve regeneration

Peripheral nerve injury is a common disease that affects more than 20 million people in the United States alone and remains a major burden to society. The current gold standard treatment for critical-sized nerve defects is autologous nerve graft transplantation; however, this method is limited in many ways and does not always lead to satisfactory outcomes. The limitations of autografts have prompted investigations into artificial neural scaffolds as replacements, and some neural scaffold devices have progressed to widespread clinical use; scaffold technology overall has yet to be shown to be consistently on a par with or superior to autografts. Recent advances in biomimetic scaffold technologies have opened up many new and exciting opportunities, and novel improvements in material, fabrication technique, scaffold architecture, and lumen surface modifications that better reflect biological anatomy and physiology have independently been shown to benefit overall nerve regeneration. Furthermore, biomimetic features of neural scaffolds have also been shown to work synergistically with other nerve regeneration therapy strategies such as growth factor supplementation, stem cell transplantation, and cell surface glycoengineering. This review summarizes the current state of neural scaffolds, highlights major advances in biomimetic technologies, and discusses future opportunities in the field of peripheral nerve regeneration.

The role of precisely matching fascicles in the quick recovery of nerve function in long peripheral nerve defects

Peripheral nerve injury therapy in the clinic remains less than satisfactory. The gold standard of treatment for long peripheral nerve defects is autologous nerve grafts; however, numerous clinical complications are associated with this treatment. As tissue engineering has developed, tissue-engineered nerve grafts (TENGs) have shown potential applications as alternatives to autologous nerve grafts. To verify the important role of the biomimetic pathway of fascicle design in TENGs, we designed an animal model to study the role of the precise matching of fascicles in the effectiveness of nerve function recovery. 24 Sprague-Dawley rats were divided randomly into three groups (eight/group) that corresponded to 100% fascicle matching (100%FM), 50%FM and 0%FM. We selected Sprague–Dawley rat long-gap (15 mm) sciatic nerve defects. In the 6 weeks after surgery, we found that the 100%FM group showed the most effective functional recovery among the three groups. The 100%FM group showed better functional recovery on the basis of the sciatic functional index than the 50%FM and 0%FM groups. According to histological evaluation, the 100%FM group showed more regenerating nerve fibres. Moreover, in terms of the prevention of muscle atrophy, the 100%FM group showed excellent physiological outcomes. The 100%FM as tissue-engineered scaffolds can enhance nerve regeneration and effective functional recovery after the repair of large nerve defects. The results of this study provide a theoretical basis for future TENG designs including biomimetic fascicle pathways for repairing long nerve defects.

3D Fabrication with Integration Molding of a Graphene Oxide/Polycaprolactone Nanoscaffold for Neurite Regeneration and Angiogenesis

Treating peripheral nerve injury faces major challenges and may benefit from bioactive scaffolds due to the limited autograft resources. Graphene oxide (GO) has emerged as a promising nanomaterial with excellent physical and chemical properties. GO has functional groups that confer biocompatibility that is better than that of graphene. Here, GO/polycaprolactone (PCL) nanoscaffolds are fabricated using an integration molding method. The nanoscaffolds exhibit many merits, including even GO nanoparticle distribution, macroporous structure, and strong mechanical support. Additionally, the process enables excellent quality control. In vitro studies confirm the advantages of the GO/PCL nanoscaffolds in terms of Schwann cell proliferation, viability, and attachment, as well as neural characteristics maintenance. This is the first study to evaluate the in vivo performance of GO-based nanoscaffolds in this context. GO release and PCL biodegradation is analyzed after long-term in vivo study. It is also found that the GO/PCL nerve guidance conduit could successfully repair a 15 mm sciatic nerve defect. The pro-angiogenic characteristic of GO is evaluated in vivo using immunohistochemistry. In addition, the AKT-endothelial nitric oxide synthase (eNOS)-vascular endothelial growth factor (VEGF) signaling pathway might play a major role in the angiogenic process. These findings demonstrate that the GO/PCL nanoscaffold efficiently promotes functional and morphological recovery in peripheral nerve regeneration, indicating its promise for tissue engineering applications.

A conductive sodium alginate and carboxymethyl chitosan hydrogel doped with polypyrrole for peripheral nerve regeneration

Polymer materials with electrically conductive properties have good applications in their respective fields because of their special properties. However, they usually exhibited poor mechanical properties and biocompatibility. In this work, we present a simple approach to prepare conductive sodium alginate (SA) and carboxymethyl chitosan (CMCS) polymer hydrogels (SA/CMCS/PPy) that can provide sufficient help for peripheral nerve regeneration. SA/CMCS hydrogel was cross-linked by calcium ions provided by the sustained release system consisting of d-glucono-δ-lactone (GDL) and superfine calcium carbonate (CaCO₃), and the conductivity of the hydrogel was provided by doped with polypyrrole (PPy). Gelation time, swelling ratio, porosity and Young's modulus of the conductive SA/CMCS/PPy hydrogel were adjusted by polypyrrole content, and the conductivity of it was within 2.41 × 10⁻⁵ to 8.03 × 10⁻³ S cm⁻¹. The advantages of conductive hydrogels in cell growth were verified by controlling electrical stimulation of cell experiments, and the hydrogels were also used as a filling material for the nerve conduit in animal experiments. The SA/CMCS/PPy conductive hydrogel showed good biocompatibility and repair features as a bioactive biomaterial, we expect this conductive hydrogel will have a good potential in the neural tissue engineering.

Polymer materials with electrically conductive properties have good applications in their respective fields because of their special properties.