Modern Trends for Peripheral Nerve Repair and Regeneration: Beyond the Hollow Nerve Guidance Conduit

miRNA-21-5p is an important contributor to the promotion of injured peripheral nerve regeneration using hypoxia-pretreated bone marrow-derived neural crest cells

JOURNAL/nrgr/04.03/01300535-202501000-00035/figure1/v/2024-05-14T021156Z/r/image-tiff Our previous study found that rat bone marrow-derived neural crest cells (acting as Schwann cell progenitors) have the potential to promote long-distance nerve repair. Cell-based therapy can enhance peripheral nerve repair and regeneration through paracrine bioactive factors and intercellular communication. Nevertheless, the complex contributions of various types of soluble cytokines and extracellular vesicle cargos to the secretome remain unclear. To investigate the role of the secretome and extracellular vesicles in repairing damaged peripheral nerves, we collected conditioned culture medium from hypoxia-pretreated neural crest cells, and found that it significantly promoted the repair of sensory neurons damaged by oxygen-glucose deprivation. The mRNA expression of trophic factors was highly expressed in hypoxia-pretreated neural crest cells. We performed RNA sequencing and bioinformatics analysis and found that miR-21-5p was enriched in hypoxia-pretreated extracellular vesicles of neural crest cells. Subsequently, to further clarify the role of hypoxia-pretreated neural crest cell extracellular vesicles rich in miR-21-5p in axonal growth and regeneration of sensory neurons, we used a microfluidic axonal dissociation model of sensory neurons in vitro, and found that hypoxia-pretreated neural crest cell extracellular vesicles promoted axonal growth and regeneration of sensory neurons, which was greatly dependent on loaded miR-21-5p. Finally, we constructed a miR-21-5p-loaded neural conduit to repair the sciatic nerve defect in rats and found that the motor and sensory functions of injured rat hind limb, as well as muscle tissue morphology of the hind limbs, were obviously restored. These findings suggest that hypoxia-pretreated neural crest extracellular vesicles are natural nanoparticles rich in miRNA-21-5p. miRNA-21-5p is one of the main contributors to promoting nerve regeneration by the neural crest cell secretome. This helps to explain the mechanism of action of the secretome and extracellular vesicles of neural crest cells in repairing damaged peripheral nerves, and also promotes the application of miR-21-5p in tissue engineering regeneration medicine.

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

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

Physical exercise to promote regeneration after peripheral nerve injury in animal models: A systematic review

Peripheral nerve injuries are common injuries that often have a drastic effect on patient's activities of daily living and physical function. While techniques for the surgical repair of these injuries have improved over time, rehabilitation methods following these repairs have been non-standardized and under researched. Electronic searches were conducted in Ovid/Medline and SCOPUS to identify articles that discuss rehabilitation and exercise following peripheral nerve injury in animal models and its effects on peripheral nerve regeneration and recovery of function. Thirty-eight articles met inclusion criteria; all were prospective studies in animal models. This systematic review suggests that exercise is a useful tool in returning autonomy to the individual and has beneficial effects in the recovery from peripheral nerve injury. It is beneficial to use rehabilitation exercises following the repair of peripheral nerve injuries to promote regeneration, and timing of that exercise may be just as important as the exercise prescribed. However, further studies with standardized models and outcome measures need to be conducted before translation to clinical trials.

BDNF-loaded chitosan-based mimetic mussel polymer conduits for repair of peripheral nerve injury

Care for patients with peripheral nerve injury is multifaceted, as traditional methods are not devoid of limitations. Although the utilization of neural conduits shows promise as a therapeutic modality for peripheral nerve injury, its efficacy as a standalone intervention is limited. Hence, there is a pressing need to investigate a composite multifunctional neural conduit as an alternative treatment for peripheral nerve injury. In this study, a BDNF-loaded chitosan-based mimetic mussel polymer conduit was prepared. Its unique adhesion characteristics allow it to be suture-free, improve the microenvironment of the injury site, and have good antibacterial properties. Researchers utilized a rat sciatic nerve injury model to evaluate the progression of nerve regeneration at the 12-week postoperative stage. The findings of this study indicate that the chitosan-based mimetic mussel polymer conduit loaded with BDNF had a substantial positive effect on myelination and axon outgrowth. The observed impact demonstrated a favorable outcome in terms of sciatic nerve regeneration and subsequent functional restoration in rats with a 15-mm gap. Hence, this approach is promising for nerve tissue regeneration during peripheral nerve injury.

Ficus carica L. (Fig) promotes nerve regeneration in a mouse model of sciatic nerve crush

Peripheral nerve injuries result in significant loss of motor and sensory function, and the slow rate of nerve regeneration can prolong recovery time. Thus, approaches that promote axonal regeneration are critical to improve the outcomes for patients with peripheral nerve injuries. In this study, we investigated the effects of Ficus carica L. (fig) and Vaccinium macrocarpon Ait. (cranberry), which are rich in phytochemicals with demonstrable and diverse medicinal properties, on nerve regeneration in a mouse model of sciatic nerve crush. Our investigation revealed that fig extract, but not cranberry extract, prevented the decline in muscle weight and nerve conduction velocity induced by nerve crush. The fig extract also mitigated motor function impairment, myelin thinning, and axon diameter reduction, indicating its potential to promote nerve regeneration. Furthermore, the fig extract enhanced macrophage infiltration into the nerve tissue, suggesting that it could ameliorate nerve injury by promoting tissue repair via increased macrophage infiltration. The study provides valuable insights into the potential of the fig extract as a novel agent promoting nerve regeneration. Further investigation into the mechanisms underlying the action of fig extracts is needed to translate these findings into clinical applications for patients with peripheral nerve injuries.

Harnessing the Potential of Self-Assembled Peptide Hydrogels for Neural Regeneration and Tissue Engineering

Spinal cord injury, traumatic brain injury, and neurosurgery procedures usually lead to neural tissue damage. Self-assembled peptide (SAP) hydrogels, a type of innovative hierarchical nanofiber-forming peptide sequences serving as hydrogelators, have emerged as a promising solution for repairing tissue defects and promoting neural tissue regeneration. SAPs possess numerous features, such as adaptable morphologies, biocompatibility, injectability, tunable mechanical stability, and mimicking of the native extracellular matrix. This review explores the capacity of neural cell regeneration and examines the critical aspects of SAPs in neuroregeneration, including their biochemical composition, topology, mechanical behavior, conductivity, and degradability. Additionally, it delves into the latest strategies involving SAPs for central or peripheral neural tissue engineering. Finally, the prospects of SAP hydrogel design and development in the realm of neuroregeneration are discussed.

Rat ventral caudal nerve as a model for long distance regeneration

In the rat, tail nerves are the longest peripheral nerves in their body. We suggest that ventral caudal nerve (VCN) may serve as a model for studying nerve injury and long distance regeneration. For this purpose, we have studied the anatomy and morphometry of the VCN in control animals. 10 cm long segment of the VCN was removed, and transversal sections were collected at 10 mm distances. The myelinated axons were counted, and the series of data were used to characterize the craniocaudal tapering of the nerve. In a separate group of animals, retrograde tracing with Fluorogold was used to localize and quantitate the spinal neurons projecting their axons into the VCN. After complete nerve transection, the time course of histopathological changes in the distal segment was studied. The primary goal was to define the time needed for axonal disintegration. In later periods, axonal debris removal and rearrangement of tissue elements was documented. After compression injury (axonotmesis), Wallerian degeneration was followed by spontaneous regeneration of axons. We show that the growing axons will span the 10 cm distance within 4-8 weeks. After different survival periods, the numbers of regenerating axons were counted at 10 mm distances. These data were used to characterize the dynamics of axonal regeneration during 4 months' survival period. In the present study we show that axonal regeneration across 10 cm distance can be studied and quantitatively analyzed in a small laboratory animal.

Electrical aligned polyurethane nerve guidance conduit modulates macrophage polarization and facilitates immunoregulatory peripheral nerve regeneration

Biomaterials can modulate the local immune microenvironments to promote peripheral nerve regeneration. Inspired by the spatial orderly distribution and endogenous electric field of nerve fibers, we aimed to investigate the synergistic effects of electrical and topological cues on immune microenvironments of peripheral nerve regeneration. Nerve guidance conduits (NGCs) with aligned electrospun nanofibers were fabricated using a polyurethane copolymer containing a conductive aniline trimer and degradable L-lysine (PUAT). In vitro experiments showed that the aligned PUAT (A-PUAT) membranes promoted the recruitment of macrophages and induced their polarization towards the pro-healing M2 phenotype, which subsequently facilitated the migration and myelination of Schwann cells. Furthermore, NGCs fabricated from A-PUAT increased the proportion of pro-healing macrophages and improved peripheral nerve regeneration in a rat model of sciatic nerve injury. In conclusion, this study demonstrated the potential application of NGCs in peripheral nerve regeneration from an immunomodulatory perspective and revealed A-PUAT as a clinically-actionable strategy for peripheral nerve injury.

Design and Development of a Polymeric-Based Curcumin Nanoparticle for Drug Delivery Enhancement and Potential Incorporation into Nerve Conduits

Peripheral nerve injuries (PNI) impact millions of individuals in the United States, prompting thousands of nerve repair procedures annually. Nerve conduits (NC) are commonly utilized to treat nerve injuries under 3 cm but larger gaps still pose a challenge for successful peripheral nerve regeneration (PNR) and functional recovery. This is partly attributed to the absence of bioactive agents such as stem cells or growth factors in FDA-approved conduits due to safety, harvesting, and reproducibility concerns. Therefore, curcumin, a bioactive phytochemical, has emerged as a promising alternative bioactive agent due to its ability to enhance PNR and overcome said challenges. However, its hydrophobicity and rapid degradation in aqueous solutions are considerable limitations. In this work, a nanoscale delivery platform with tannic acid (TA) and polyvinylpyrrolidone (PVP) was developed to encapsulate curcumin for increased colloidal and chemical stability. The curcumin nanoparticles (CurNPs) demonstrate significantly improved stability in water, reduced degradation rates, and controlled release kinetics when compared to free curcumin. Further, cell studies show that the CurNP is biocompatible when introduced to neuronal cells (SH-SY5Y), rat Schwann cells (RSC-S16), and murine macrophages (J774 A.1) at 5 μM, 5 μM, and 10 μM of curcumin, respectively. As a result of these improved physicochemical properties, confocal fluorescence microscopy revealed superior delivery of curcumin into these cells when in the form of CurNPs compared to its free form. A hydrogen peroxide-based oxidative stress study also demonstrated the CurNP's potential to protect J774 A.1 cells against excessive oxidative stress. Overall, this study provides evidence for the suitability of CurNPs to be used as a bioactive agent in NC applications.

Multichannel Conduits with Fascicular Complementation: Significance in Long Segmental Peripheral Nerve Injury

Despite the advances in tissue engineering approaches, reconstruction of long segmental peripheral nerve defects remains unsatisfactory. Although autologous grafts with proper fascicular complementation have shown meaningful functional recovery according to the Medical Research Council Classification (MRCC), the lack of donor nerve for such larger defect sizes (>30 mm) has been a serious clinical issue. Further clinical use of hollow nerve conduits is limited to bridging smaller segmental defects of denuded nerve ends (<30 mm). Recently, bioinspired multichannel nerve guidance conduits (NGCs) gained attention as autograft substitutes as they mimic the fascicular connective tissue microarchitecture in promoting aligned axonal outgrowth with desirable innervation for complete sensory and motor function restoration. This review outlines the hierarchical organization of nerve bundles and their significance in the sensory and motor functions of peripheral nerves. This review also emphasizes the major challenges in addressing the longer nerve defects with the role of fascicular arrangement in the multichannel nerve guidance conduits and the need for fascicular matching to accomplish complete functional restoration, especially in treating long segmental nerve defects. Further, currently available fabrication strategies in developing multichannel nerve conduits and their inconsistency in existing preclinical outcomes captured in this review would seed a new process in designing an ideal larger nerve conduit for peripheral nerve repair.

Vibrational spectroscopy and multiphoton microscopy for label-free visualization of nervous system degeneration and regeneration

Neurological disorders, including spinal cord injury, peripheral nerve injury, traumatic brain injury, and neurodegenerative diseases, pose significant challenges in terms of diagnosis, treatment, and understanding the underlying pathophysiological processes. Label-free multiphoton microscopy techniques, such as coherent Raman scattering, two-photon excited autofluorescence, and second and third harmonic generation microscopy, have emerged as powerful tools for visualizing nervous tissue with high resolution and without the need for exogenous labels. Coherent Raman scattering processes as well as third harmonic generation enable label-free visualization of myelin sheaths, while their combination with two-photon excited autofluorescence and second harmonic generation allows for a more comprehensive tissue visualization. They have shown promise in assessing the efficacy of therapeutic interventions and may have future applications in clinical diagnostics. In addition to multiphoton microscopy, vibrational spectroscopy methods such as infrared and Raman spectroscopy offer insights into the molecular signatures of injured nervous tissues and hold potential as diagnostic markers. This review summarizes the application of these label-free optical techniques in preclinical models and illustrates their potential in the diagnosis and treatment of neurological disorders with a special focus on injury, degeneration, and regeneration. Furthermore, it addresses current advancements and challenges for bridging the gap between research findings and their practical applications in a clinical setting.

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

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

Recent advancements in hydrogels as novel tissue engineering scaffolds for dental pulp regeneration

Although conventional root canal treatment offers an effective therapeutic solution, it negatively affects the viability of the affected tooth. In recent years, pulp regeneration technology has emerged as a novel method for treating irreversible pulpitis due to its ability to maintain tooth vitality. The successful implementation of this technique depends on scaffolds and transplantation of exogenous stem cells or recruitment of endogenous stem cells. Accordingly, the three-dimensional structure and viscoelastic characteristics of hydrogel scaffolds, which parallel those of the extracellular matrix, have generated considerable interest. Furthermore, hydrogels support the controlled release of regenerative drugs and to load a wide variety of bioactive molecules. By integrating antibacterial agents into the hydrogel matrix and stimulating an immune response, root canal disinfection can be significantly improved and the rate of pulp regeneration can be accelerated. This review aims to provide an overview of the clinical applications of hydrogels that have been reported in the last 5 years, and offer a comprehensive summary of the different approaches that have been utilized for the optimization of hydrogel scaffolds for pulp regeneration. Advancements and challenges in pulp regeneration using hydrogels treating aged teeth are discussed.

Influence of Magnesium Degradation on Schwannoma Cell Responses to Nerve Injury Using an In Vitro Injury Model

Nerve guidance conduits for peripheral nerve injuries can be improved using bioactive materials such as magnesium (Mg) and its alloys, which could provide both structural and trophic support. Therefore, we investigated whether exposure to Mg and Mg-1.6wt%Li thin films (Mg/Mg-1.6Li) would alter acute Schwann cell responses to injury. Using the RT4-D6P2T Schwannoma cell line (SCs), we tested extracts from freeze-killed cells (FKC) and nerves (FKN) as in vitro injury stimulants. Both FKC and FKN induced SC release of the macrophage chemoattractant protein 1 (MCP-1), a marker of the repair SC phenotype after injury. Next, FKC-stimulated cells exposed to Mg/Mg-1.6Li reduced MCP-1 release by 30%, suggesting that these materials could have anti-inflammatory effects. Exposing FKC-treated cells to Mg/Mg-1.6Li reduced the gene expression of the nerve growth factor (NGF), glial cell line-derived neurotrophic factor (GDNF), and myelin protein zero (MPZ), but not the p75 neurotrophin receptor. In the absence of FKC, Mg/Mg-1.6Li treatment increased the expression of NGF, p75, and MPZ, which can be beneficial to nerve regeneration. Thus, the presence of Mg can differentially alter SCs, depending on the microenvironment. These results demonstrate the applicability of this in vitro nerve injury model, and that Mg has wide-ranging effects on the repair SC phenotype.

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

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

Advancing Peripheral Nerve Regeneration: 3D Bioprinting of GelMA-Based Cell-Laden Electroactive Bioinks for Nerve Conduits

Peripheral nerve injuries often result in substantial impairment of the neurostimulatory organs. While the autograft is still largely used as the "gold standard" clinical treatment option, nerve guidance conduits (NGCs) are currently considered a promising approach for promoting peripheral nerve regeneration. While several attempts have been made to construct NGCs using various biomaterial combinations, a comprehensive exploration of the process science associated with three-dimensional (3D) extrusion printing of NGCs with clinically relevant sizes (length: 20 mm; diameter: 2-8 mm), while focusing on tunable buildability using electroactive biomaterial inks, remains unexplored. In addressing this gap, we present here the results of the viscoelastic properties of a range of a multifunctional gelatin methacrylate (GelMA)/poly(ethylene glycol) diacrylate (PEGDA)/carbon nanofiber (CNF)/gellan gum (GG) hydrogel bioink formulations and printability assessment using experiments and quantitative models. Our results clearly established the positive impact of the gellan gum on the enhancement of the rheological properties. Interestingly, the strategic incorporation of PEGDA as a secondary cross-linker led to a remarkable enhancement in the strength and modulus by 3 and 8-fold, respectively. Moreover, conductive CNF addition resulted in a 4-fold improvement in measured electrical conductivity. The use of four-component electroactive biomaterial ink allowed us to obtain high neural cell viability in 3D bioprinted constructs. While the conventionally cast scaffolds can support the differentiation of neuro-2a cells, the most important result has been the excellent cell viability of neural cells in 3D encapsulated structures. Taken together, our findings demonstrate the potential of 3D bioprinting and multimodal biophysical cues in developing functional yet critical-sized nerve conduits for peripheral nerve tissue regeneration.

Advanced Hollow Cathode Discharge Plasma Treatment of Unique Bilayered Fibrous Nerve Guidance Conduits for Enhanced/Oriented Neurite Outgrowth

Despite all recent progresses in nerve tissue engineering, critical-sized nerve defects are still extremely challenging to repair. Therefore, this study targets the bridging of critical nerve defects and promoting an oriented neuronal outgrowth by engineering innovative nerve guidance conduits (NGCs) synergistically possessing exclusive topographical, chemical, and mechanical cues. To do so, a mechanically adequate mixture of polycaprolactone (PCL) and polylactic-co-glycolic acid (PLGA) was first carefully selected as base material to electrospin nanofibrous NGCs simulating the extracellular matrix. The electrospinning process was performed using a newly designed 2-pole air gap collector that leads to a one-step deposition of seamless NGCs having a bilayered architecture with an inner wall composed of highly aligned fibers and an outer wall consisting of randomly oriented fibers. This architecture is envisaged to afford guidance cues for the extension of long neurites on the underlying inner fiber alignment and to concurrently provide a sufficient nutrient supply through the pores of the outer random fibers. The surface chemistry of the NGCs was then modified making use of a hollow cathode discharge (HCD) plasma reactor purposely designed to allow an effective penetration of the reactive species into the NGCs to eventually treat their inner wall. X-ray photoelectron spectroscopy (XPS) results have indeed revealed a successful O2 plasma modification of the inner wall that exhibited a significantly increased oxygen content (24 → 28%), which led to an enhanced surface wettability. The treatment increased the surface nanoroughness of the fibers forming the NGCs as a result of an etching effect. This effect reduced the ultimate tensile strength of the NGCs while preserving their high flexibility. Finally, pheochromocytoma (PC12) cells were cultured on the NGCs to monitor their ability to extend neurites which is the base of a good nerve regeneration. In addition to remarkably improved cell adhesion and proliferation on the plasma-treated NGCs, an outstanding neural differentiation occurred. In fact, PC12 cells seeded on the treated samples extended numerous long neurites eventually establishing a neural network-like morphology with an overall neurite direction following the alignment of the underlying fibers. Overall, PCL/PLGA NGCs electrospun using the 2-pole air gap collector and O2 plasma-treated using an HCD reactor are promising candidates toward a full repair of critical nerve damage.

An Adaptable Drug Delivery System Facilitates Peripheral Nerve Repair by Remodeling the Microenvironment

The multifaceted process of nerve regeneration following damage remains a significant clinical issue, due to the lack of a favorable regenerative microenvironment and insufficient endogenous biochemical signaling. However, the current nerve grafts have limitations in functionality, as they require a greater capacity to effectively regulate the intricate microenvironment associated with nerve regeneration. In this regard, we proposed the construction of a functional artificial scaffold based on a "two-pronged" approach. The whole system was developed by encapsulating Tazarotene within nanomicelles formed through self-assembly of reactive oxygen species (ROS)-responsive amphiphilic triblock copolymer, all of which were further loaded into a thermosensitive injectable hydrogel. Notably, the hydrogel exhibits obvious temperature sensitivity at a concentration of 6 wt %, and the nanoparticles possess concentration-dependent H2O2-response capability with a controlled release profile in 48 h. The combined strategy promoted the repair of injured peripheral nerves, attributed to the dual role of the materials, which mainly involved providing structural support, modulating the immune microenvironment, and enhancing angiogenesis. Overall, this study opens up intriguing prospects in tissue engineering.

Comparative Analysis of Various Spider Silks in Regard to Nerve Regeneration: Material Properties and Schwann Cell Response

Peripheral nerve reconstruction through the employment of nerve guidance conduits with Trichonephila dragline silk as a luminal filling has emerged as an outstanding preclinical alternative to avoid nerve autografts. Yet, it remains unknown whether the outcome is similar for silk fibers harvested from other spider species. This study compares the regenerative potential of dragline silk from two orb-weaving spiders, Trichonephila inaurata and Nuctenea umbratica, as well as the silk of the jumping spider Phidippus regius. Proliferation, migration, and transcriptomic state of Schwann cells seeded on these silks are investigated. In addition, fiber morphology, primary protein structure, and mechanical properties are studied. The results demonstrate that the increased velocity of Schwann cells on Phidippus regius fibers can be primarily attributed to the interplay between the silk's primary protein structure and its mechanical properties. Furthermore, the capacity of silk fibers to trigger cells toward a gene expression profile of a myelinating Schwann cell phenotype is shown. The findings for the first time allow an in-depth comparison of the specific cellular response to various native spider silks and a correlation with the fibers' material properties. This knowledge is essential to open up possibilities for targeted manufacturing of synthetic nervous tissue replacement.

The Impacts of Intervertebral Disc Degeneration of the Spine, Alcohol Consumption, Smoking Tobacco Products, and Glycemic Disorders on the Expression Profiles of Neurotrophins-3 and -4

In the etiology of discogenic pain, attention is paid to the role of neurotrophic factors, which include classic neurotrophins (NTs). This study aimed to assess changes in the concentrations of NT-3 and NT-4 in the intervertebral discs (IVDs) of the lumbosacral (L/S) spine depending on the advancement of degenerative changes, pain severity, habits, and comorbidities. The study group included 113 patients who underwent microdiscectomy due to degenerative IVD disease of the L/S spine. The severity of degenerative IVD changes was assessed using the five-point Pfirrmann scale, and the pain intensity was assessed according to the visual analog scale (VAS). In turn, the control group included 81 participants from whom IVDs of the L/S section of the spine were collected post-mortem during forensic autopsy or organ donation. At the mRNA level, we noted NT-3 overexpression in the test samples compared with the controls (fold change (FC) = 9.12 ± 0.56; p < 0.05), while NT-4 transcriptional activity was decreased in the test samples compared with the controls (FC = 0.33 ± 0.07; p < 0.05). However, at the protein level, the concentrations of NT-3 (134 ± 5.78 pg/mL vs. 6.78 ± 1.17 pg/mL; p < 0.05) and NT-4 (316.77 ± 8.19 pg/mL vs. 76.92 ± 4.82 pg/mL; p < 0.05) were significantly higher in the test samples compared with the control samples. Nevertheless, the concentration of both proteins did not statistically significantly change depending on the advancement of degenerative changes and the pain intensity (p > 0.05). In addition, higher levels of NT-3 and NT-4 were noted in IVD samples from patients who consumed alcohol, smoked tobacco, were overweight/obese, or had comorbid diabetes compared with patients without these risk factors (p < 0.05). Our analysis confirmed that differences in the degenerative process of IVD, energy metabolism, and lifestyle are related to changes in the concentration profiles of NT-3 and NT-4.

Understanding exosomes: Part 2-Emerging leaders in regenerative medicine

Exosomes are the smallest subset of extracellular signaling vesicles secreted by most cells with the ability to communicate with other tissues and cell types over long distances. Their use in regenerative medicine has gained tremendous momentum recently due to their ability to be utilized as therapeutic options for a wide array of diseases/conditions. Over 5000 publications are currently being published yearly on this topic, and this number is only expected to dramatically increase as novel therapeutic strategies continue to be developed. Today exosomes have been applied in numerous contexts including neurodegenerative disorders (Alzheimer's disease, central nervous system, depression, multiple sclerosis, Parkinson's disease, post-traumatic stress disorders, traumatic brain injury, peripheral nerve injury), damaged organs (heart, kidney, liver, stroke, myocardial infarctions, myocardial infarctions, ovaries), degenerative processes (atherosclerosis, diabetes, hematology disorders, musculoskeletal degeneration, osteoradionecrosis, respiratory disease), infectious diseases (COVID-19, hepatitis), regenerative procedures (antiaging, bone regeneration, cartilage/joint regeneration, osteoarthritis, cutaneous wounds, dental regeneration, dermatology/skin regeneration, erectile dysfunction, hair regrowth, intervertebral disc repair, spinal cord injury, vascular regeneration), and cancer therapy (breast, colorectal, gastric cancer and osteosarcomas), immune function (allergy, autoimmune disorders, immune regulation, inflammatory diseases, lupus, rheumatoid arthritis). This scoping review is a first of its kind aimed at summarizing the extensive regenerative potential of exosomes over a broad range of diseases and disorders.

State of the Art and Current Challenges on Electroactive Biomaterials and Strategies for Neural Tissue Regeneration

The loss or failure of an organ/tissue stands as one of the healthcare system's most prevalent, devastating, and costly challenges. Strategies for neural tissue repair and regeneration have received significant attention due to their particularly strong impact on patients' well-being. Many research efforts are dedicated not only to control the disease symptoms but also to find solutions to repair the damaged tissues. Neural tissue engineering (TE) plays a key role in addressing this problem and significant efforts are being carried out to develop strategies for neural repair treatment. In the last years, active materials allowing to tune cell-materials interaction are being increasingly used, representing a recent paradigm in TE applications. Among the most important stimuli influencing cell behavior are the electrical and mechanical ones. In this way, materials with the ability to provide this kind of stimuli to the neural cells seem to be appropriate to support neural TE. In this scope, this review summarizes the different biomaterials types used for neural TE, highlighting the relevance of using active biomaterials and electrical stimulation. Furthermore, this review provides not only a compilation of the most relevant studies and results but also strategies for novel and more biomimetic approaches for neural TE.

Strain-induced bands of Büngner formation promotes axon growth in 3D tissue-engineered constructs

Treatment of peripheral nerve lesions remains a major challenge due to poor functional recovery; hence, ongoing research efforts strive to enhance peripheral nerve repair. In this study, we aimed to establish three-dimensional tissue-engineered bands of Büngner constructs by subjecting Schwann cells (SCs) embedded in fibrin hydrogels to mechanical stimulation. We show for the first time that the application of strain induces (i) longitudinal alignment of SCs resembling bands of Büngner, and (ii) the expression of a pronounced repair SC phenotype as evidenced by upregulation of BDNF, NGF, and p75NTR. Furthermore, we show that mechanically aligned SCs provide physical guidance for migrating axons over several millimeters in vitro in a co-culture model with rat dorsal root ganglion explants. Consequently, these constructs hold great therapeutic potential for transplantation into patients and might also provide a physiologically relevant in vitro peripheral nerve model for drug screening or investigation of pathologic or regenerative processes.

Cost-effectiveness analysis of Avance® allograft for the treatment of peripheral nerve injuries in the USA

Aim: Peripheral nerve injury (PNI) is a debilitating condition with significant associated morbidity, and which places a substantial socioeconomic burden on healthcare systems worldwide. Recently, allograft has emerged as a viable surgical alternative to autograft for the treatment of PNI. This study evaluated the cost effectiveness of allograft (Avance® Nerve Graft) compared with autograft for the peripheral nerve repair, from a US payer perspective. Methods: A Markov cohort model was developed to consider the treatment pathways followed by a patient population undergoing a single transected nerve repair with either allograft, or autograft. The marginal difference in meaningful recovery (MR) (effectiveness), and costs, between the two groups were estimated over a lifetime horizon. Deterministic and probabilistic sensitivity analyses (PSA) were performed to consider the uncertainty surrounding the base-case input parameter values and their effect on the overall incremental cost-effectiveness ratio (ICER). Results: The base-case analysis indicates that there is a small difference in the average probability of MR between the two groups (75.15% vs 70.46%; +4.69% with allograft). Allograft also results in cost savings ($12,677 vs $14,023; -$-1346 with allograft) compared with autograft. Deterministic sensitivity analysis shows that the costs of the initial surgical procedures are the main drivers of incremental cost, but that the intervention is likely to be cost saving compared with autograft regardless of the parameter variations made. Conclusion: The use of allograft with the Avance Nerve Graft has the potential to be a cost-effective alternative to autograft for the surgical treatment of PNI in the USA.

Advances in nerve guidance conduits for peripheral nerve repair and regeneration

Peripheral nerve injury (PNI) can cause partial or total motor and sensory nerve function, leading to physical disability and nerve pain that severely affects patients' quality of life. Autologous nerve transplantation is currently the clinically recognized gold standard, but due to its inherent limitations, researchers have been searching for alternative treatments. Nerve guidance conduits (NGCs) have attracted much attention as a favorable alternative to promote the repair and regeneration of damaged peripheral nerves. In this review, we provide an overview of the anatomy of peripheral nerves, peripheral nerve injury and repair, and current treatment methods. Importantly, different design strategies of NGCs used for the treatment of PNI and their applications in PNI repair are highlighted. Finally, an outlook on the future development and challenges of NGCs is presented.

From Conservative Measures to Surgical Interventions, Treatment Approaches for Cubital Tunnel Syndrome: A Comprehensive Review

Cubital tunnel syndrome (CuTS) is a neuropathic condition characterized by the compression or irritation of the ulnar nerve at the elbow, resulting in a wide spectrum of symptoms ranging from pain and numbness to muscle weakness and impaired hand function. This comprehensive review delves into the diverse landscape of CuTS treatment approaches, emphasizing the importance of early intervention. The review explores how these strategies aim to alleviate symptoms and enhance patient well-being by beginning with conservative measures encompassing rest, splinting, medications, physical therapy, and lifestyle adjustments. Non-surgical medical interventions, including nerve gliding exercises, ultrasound-guided nerve injections, and orthotic devices, are considered alternative therapies for symptom relief. Surgical interventions, such as decompression procedures and emerging techniques, are discussed in detail, highlighting their indications and expected outcomes. Throughout this review, the critical role of patient-centered care is underscored, emphasizing the need for tailored treatment plans that respect individual preferences and goals. Recognizing the unique nature of each CuTS case, shared decision-making between patients and healthcare providers is advocated, ensuring that interventions align with specific patient needs. As research advances, promising developments in diagnosis, surgical techniques, and drug therapies offer hope for more effective management of CuTS, paving the way for improved symptom relief and enhanced nerve function.

Exosomes derived from human umbilical cord mesenchymal stem cells loaded with RVG-Lamp2b and Netrin-1 promotes Schwann cell invasion and migration

BACKGROUND

Netrin-1 has a neuroprotective effect by regulating angiogenesis, autophagy, apoptosis, and neuroinflammation. This study investigated the effects of netrin-1 delivery to mouse Schwann cells and vascular endothelial cells using exosomes modified with rabies virus glycoprotein (RVG) peptides.

MATERIALS AND METHODS

RVG-Lamp2b and/or Netrin-1 were overexpressed in human umbilical cord mesenchymal stem cells to obtain exosomes modified with RVG-Lamp2b and/or loaded with Netrin-1. Then, exosomes were labeled with carboxyfluorescein diacetate succinimidyl ester and co-cultured with mouse Schwann cells and endothelial cells. Netrin-1 expression in Schwann cells and endothelial cells was measured using quantitative polymerase chain reaction and immunoblotting. Moreover, methyl thiazolyl tetrazolium assays and Transwell assays were used to detect proliferation, migration, and invasion of Schwann cells and endothelial cells.

RESULTS

Exosomes with RVG-Lamp2b entered Schwann cells more readily compared with the exosomes without RVG-Lamp2b. Meanwhile, this was not the case in endothelial cells. Netrin-1-loaded exosomes significantly promoted Netrin-1 expression, cell proliferation, migration, invasion, and epithelial-mesenchymal transition in Schwann cells and endothelial cells. These effects were further enhanced by Netrin-1-loaded exosomes modified with RVG-Lamp2b in Schwann cells, but not in endothelial cells.

CONCLUSION

HucMSC-derived exosomes loaded with RVG-Lamp2b and Netrin-1 promote proliferation, migration, and invasion of Schwann cells.

Neuro-Inspired Biomimetic Microreactor for Sensory Recovery and Hair Follicle Neogenesis under Skin Burns

Deep burns are one of the most severe skin wounds, with typical symptoms being a contradiction between initial severe pain and a subsequent loss of sensation. Although it has long been known that sensory nerves promote skin regeneration and modulate skin function, no proven burn management strategies target sensory nerves. Here, a neuro-inspired biomimetic microreactor is designed based on the immune escape outer membrane of neuroblastoma cells and neural-associated intracellular proteins. The microreactor is constructed on a metal-organic framework (MOF) with a neuroblastoma membrane coating the surface and intracellular proteins loaded inside, called Neuro-MOF. It is loaded into a therapeutic hydrogel and triggers the release of its content proteins upon excitation by near-infrared light. The proteins compensate the skin microenvironment for permanent neurological damage after burns to initiate peripheral nerve regeneration and hair follicle niche formation. In addition, the neuroblastoma cell membrane is displayed on the surface of the Neuro-MOF microreactor, decreasing its immunogenicity and suppressing local inflammation. In a mouse model of deep skin burns, the Neuro-MOF microreactor exhibited significant functional skin regeneration effects, particularly sensory recovery and hair follicle neogenesis.

Nerve Regeneration with a Scaffold Incorporating an Absorbable Zinc-2% Iron Alloy Filament to Improve Axonal Guidance

Peripheral nerve damage that results in lost segments requires surgery, but currently available hollow scaffolds have limitations that could be overcome by adding internal guidance support. A novel solution is to use filaments of absorbable metals to supply physical support and guidance for nerve regeneration that then safely disappear from the body. Previously, we showed that thin filaments of magnesium metal (Mg) would support nerve regeneration. Here, we tested another absorbable metal, zinc (Zn), using a proprietary zinc alloy with 2% iron (Zn-2%Fe) that was designed to overcome the limitations of both Mg and pure Zn metal. Non-critical-sized gaps in adult rat sciatic nerves were repaired with silicone conduits plus single filaments of Zn-2%Fe, Mg, or no metal, with autografts as controls. After seventeen weeks, all groups showed equal recovery of function and axonal density at the distal end of the conduit. The Zn alloy group showed some improvements in early rat health and recovery of function. The alloy had a greater local accumulation of degradation products and inflammatory cells than Mg; however, both metals had an equally thin capsule (no difference in tissue irritation) and no toxicity or inflammation in neighboring nerve tissues. Therefore, Zn-2%Fe, like Mg, is biocompatible and has great potential for use in nervous tissue regeneration and repair.

Potential Application of Orofacial MSCs in Tissue Engineering Nerve Guidance for Peripheral Nerve Injury Repair

Injury to the peripheral nerve causes potential loss of sensory and motor functions, and peripheral nerve repair (PNR) remains a challenging endeavor. The current clinical methods of nerve repair, such as direct suture, autografts, and acellular nerve grafts (ANGs), exhibit their respective disadvantages like nerve tension, donor site morbidity, size mismatch, and immunogenicity. Even though commercially available nerve guidance conduits (NGCs) have demonstrated some clinical successes, the overall clinical outcome is still suboptimal, especially for nerve injuries with a large gap (≥ 3 cm) due to the lack of biologics. In the last two decades, the combination of advanced tissue engineering technologies, stem cell biology, and biomaterial science has significantly advanced the generation of a new generation of NGCs incorporated with biological factors or supportive cells, including mesenchymal stem cells (MSCs), which hold great promise to enhance peripheral nerve repair/regeneration (PNR). Orofacial MSCs are emerging as a unique source of MSCs for PNR due to their neural crest-origin and easy accessibility. In this narrative review, we have provided an update on the pathophysiology of peripheral nerve injury and the properties and biological functions of orofacial MSCs. Then we have highlighted the application of orofacial MSCs in tissue engineering nerve guidance for PNR in various preclinical models and the potential challenges and future directions in this field.

Micro/nano-patterns for enhancing differentiation of human neural stem cells and fabrication of nerve conduits via soft lithography and 3D printing

Topographical cues on materials can manipulate cellular fate, particularly for neural cells that respond well to such cues. Utilizing biomaterial surfaces with topographical features can effectively influence neuronal differentiation and promote neurite outgrowth. This is crucial for improving the regeneration of damaged neural tissue after injury. Here, we utilized groove patterns to create neural conduits that promote neural differentiation and axonal growth. We investigated the differentiation of human neural stem cells (NSCs) on silicon dioxide groove patterns with varying height-to-width/spacing ratios. We hypothesize that NSCs can sense the microgrooves with nanoscale depth on different aspect ratio substrates and exhibit different morphologies and differentiation fate. A comprehensive approach was employed, analyzing cell morphology, neurite length, and cell-specific markers. These aspects provided insights into the behavior of the investigated NSCs and their response to the topographical cues. Three groove-pattern models were designed with varying height-to-width/spacing ratios of 80, 42, and 30 for groove pattern widths of 1 μm, 5 μm, and 10 μm and nanoheights of 80 nm, 210 nm, and 280 nm. Smaller groove patterns led to longer neurites and more effective differentiation towards neurons, whereas larger patterns promoted multidimensional differentiation towards both neurons and glia. We transferred these cues onto patterned polycaprolactone (PCL) and PCL-graphene oxide (PCL-GO) composite 'stamps' using simple soft lithography and reproducible extrusion 3D printing methods. The patterned scaffolds elicited a response from NSCs comparable to that of silicon dioxide groove patterns. The smallest pattern stimulated the highest neurite outgrowth, while the middle-sized grooves of PCL-GO induced effective synaptogenesis. We demonstrated the potential for such structures to be wrapped into tubes and used as grafts for peripheral nerve regeneration. Grooved PCL and PCL-GO conduits could be a promising alternative to nerve grafting.

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.

Physical stimuli-emitting scaffolds: The role of piezoelectricity in tissue regeneration

The imbalance between life expectancy and quality of life is increasing due to the raising prevalence of chronic diseases. Musculoskeletal disorders and chronic wounds affect a growing percentage of people and demand more efficient tools for regenerative medicine. Scaffolds that can better mimic the natural physical stimuli that tissues receive under healthy conditions and during healing may significantly aid the regeneration process. Shape, mechanical properties, pore size and interconnectivity have already been demonstrated to be relevant scaffold features that can determine cell adhesion and differentiation. Much less attention has been paid to scaffolds that can deliver more dynamic physical stimuli, such as electrical signals. Recent developments in the precise measurement of electrical fields in vivo have revealed their key role in cell movement (galvanotaxis), growth, activation of secondary cascades, and differentiation to different lineages in a variety of tissues, not just neural. Piezoelectric scaffolds can mimic the natural bioelectric potentials and gradients in an autonomous way by generating the electric stimuli themselves when subjected to mechanical loads or, if the patient or the tissue lacks mobility, ultrasound irradiation. This review provides an analysis on endogenous bioelectrical signals, recent developments on piezoelectric scaffolds for bone, cartilage, tendon and nerve regeneration, and their main outcomes in vivo. Wound healing with piezoelectric dressings is addressed in the last section with relevant examples of performance in animal models. Results evidence that a fine adjustment of material composition and processing (electrospinning, corona poling, 3D printing, annealing) provides scaffolds that act as true emitters of electrical stimuli that activate endogenous signaling pathways for more efficient and long-term tissue repair.

Cell-directed assembly of luminal nanofibril fillers in nerve conduits for peripheral nerve repair

Graphene and its derivatives, graphene oxide (GO) and reduced graphene oxide (rGO), have attracted significant attention in the field of tissue engineering, particularly in nerve and muscle regeneration, owing to their excellent electrical conductivity. This paper reports the fabrication of cell-mixable rGO-decorated polycaprolactone (PCL) nanofibrils (NFs) to promote peripheral nerve repair with the assistant of electron transmission by rGO and cytokine paracrine by stem cells. Oxidized GO (GO-COOH) and branched polyethylenimine are layer-by-layer coated on hydrolyzed PCL NFs via electrostatic interaction, and the number of layering is manipulated to adjust the GO-COOH coating amount. The decorated GO-COOH is reduced in situ to rGO for electrical conductivity retrieval. PC12 cells cultivated with rGO-coated NF demonstrate spontaneous cell sheet assembly, and neurogenic differentiation is observed upon electrical stimulation. When transplant nerve guidance conduit containing the assembly of rGO-coated NF and adipose-derived stem cell to the site of neurotmesis injury of a sciatic nerve, animal movement is enhanced and autotomy is ameliorated for 8 weeks compared to transplanting the hollow conduit only. Histological analysis results reveal higher levels of muscle mass and lower levels of collagen deposition in the triceps surae muscle of the rGO-coated NF-treated legs. Therefore, the rGO-layered NF can be tailored to repair peripheral nerve injuries in combination with stem cell therapy.

Neuro-regenerative behavior of adipose-derived stem cells in aligned collagen I hydrogels

Peripheral nerve injuries persist as a major clinical issue facing the US population and can be caused by stretch, laceration, or crush injuries. Small nerve gaps are simple to treat, and the nerve stumps can be reattached with sutures. In longer nerve gaps, traditional treatment options consist of autografts, hollow nerve guidance conduits, and, more recently, manufactured fibrous scaffolds. These manufactured scaffolds often incorporate stem cells, growth factors, and/or extracellular matrix (ECM) proteins to better mimic the native environment but can have issues with homogenous cell distribution or uniformly oriented neurite outgrowth in scaffolds without fibrous alignment. Here, we utilize a custom device to fabricate collagen I hydrogels with aligned fibers and encapsulated adipose-derived mesenchymal stem cells (ASCs) for potential use as a peripheral nerve repair graft. Initial results of our scaffold system revealed significantly less cell viability in higher collagen gel concentrations; 3 mg/mL gels showed 84.8 ± 7.3% viable cells, compared to 6 mg/mL gels viability of 76.7 ± 9.5%. Mechanical testing of the 3 mg/mL gels showed a Young's modulus of 6.5 ± 0.8 kPa nearly matching 7.45 kPa known to support Schwann cell migration. Further analysis of scaffolds coupled with stretching in vitro revealed heightened angiogenic and factor secretion, ECM deposition, fiber alignment, and dorsal root ganglia (DRG) neurite outgrowth along the axis of fiber alignment. Our platform serves as an in vitro testbed to assess neuro-regenerative potential of ASCs in aligned collagen fiber scaffolds and may provide guidance on next-generation nerve repair scaffold design.

Li-Mg-Si bioceramics provide a dynamic immuno-modulatory and repair-supportive microenvironment for peripheral nerve regeneration

Biomaterials can modulate the local immune and repair-supportive microenvironments to promote peripheral nerve regeneration. Inorganic bioceramics have been widely used for regulating tissue regeneration and local immune response. However, little is known on whether inorganic bioceramics can have potential for enhancing peripheral nerve regeneration and what are the mechanisms underlying their actions. Here, the inorganic lithium-magnesium-silicon (Li-Mg-Si, LMS) bioceramics containing scaffolds are fabricated and characterized. The LMS-containing scaffolds had no cytotoxicity against rat Schwann cells (SCs), but promoted their migration and differentiation towards a remyelination state by up-regulating the expression of neurotrophic factors in a β-catenin-dependent manner. Furthermore, using single cell-sequencing, we showed that LMS-containing scaffolds promoted macrophage polarization towards the pro-regenerative M2-like cells, which subsequently facilitated the migration and differentiation of SCs. Moreover, implantation with the LMS-containing nerve guidance conduits (NGCs) increased the frequency of M2-like macrophage infiltration and enhanced nerve regeneration and motor functional recovery in a rat model of sciatic nerve injury. Collectively, these findings indicated that the inorganic LMS bioceramics offered a potential strategy for enhancing peripheral nerve regeneration by modulating the immune microenvironment and promoting SCs remyelination.

Ultrasound Landmarks in the Approach to the Common Peroneal Nerve in a Sheep Model-Application in Peripheral Nerve Regeneration

Peripheral nerve injury (PNI) remains a medical challenge with no easy resolution. Over the last few decades, significant advances have been achieved in promoting peripheral nerve regeneration, and new assessment tools have been developed, both functional and imaging, to quantify the proportion and quality of nerve recovery. The exploration of new animal models, larger, more complex, and with more similarities to humans, has made it possible to reduce the gap between the results obtained in classic animal models, such as rodents, and the application of new therapies in humans and animals of clinical interest. Ultrasonography is an imaging technique recurrently used in clinical practice to assess the peripheral nerves, allowing for its anatomical and topographic characterization, aiding in the administration of anesthesia, and in the performance of nerve blocks. The use of this technique in animal models is scarce, but it could be a useful tool in monitoring the progression of nerve regeneration after the induction of controlled experimental lesions. In this work, sheep, a promising animal model in the area of peripheral nerve regeneration, were subjected to an ultrasonographic study of three peripheral nerves of the hind limb, the common peroneal, and tibial and sciatic nerves. The main aim was to establish values of dimensions and ultrasound appearance in healthy nerves and landmarks for their identification, as well as to perform an ultrasound evaluation of the cranial tibial muscle, an effector muscle of the common peroneal nerve, and to establish normal values for its ultrasound appearance and dimensions. The results obtained will allow the use of these data as control values in future work exploring new therapeutic options for nerve regeneration after induction of common peroneal nerve injuries in sheep.

Advances in Biomimetic Nerve Guidance Conduits for Peripheral Nerve Regeneration

Injuries to the peripheral nervous system are a common clinical issue, causing dysfunctions of the motor and sensory systems. Surgical interventions such as nerve autografting are necessary to repair damaged nerves. Even with autografting, i.e., the gold standard, malfunctioning and mismatches between the injured and donor nerves often lead to unwanted failure. Thus, there is an urgent need for a new intervention in clinical practice to achieve full functional recovery. Nerve guidance conduits (NGCs), providing physicochemical cues to guide neural regeneration, have great potential for the clinical regeneration of peripheral nerves. Typically, NGCs are tubular structures with various configurations to create a microenvironment that induces the oriented and accelerated growth of axons and promotes neuron cell migration and tissue maturation within the injured tissue. Once the native neural environment is better understood, ideal NGCs should maximally recapitulate those key physiological attributes for better neural regeneration. Indeed, NGC design has evolved from solely physical guidance to biochemical stimulation. NGC fabrication requires fundamental considerations of distinct nerve structures, the associated extracellular compositions (extracellular matrices, growth factors, and cytokines), cellular components, and advanced fabrication technologies that can mimic the structure and morphology of native extracellular matrices. Thus, this review mainly summarizes the recent advances in the state-of-the-art NGCs in terms of biomaterial innovations, structural design, and advanced fabrication technologies and provides an in-depth discussion of cellular responses (adhesion, spreading, and alignment) to such biomimetic cues for neural regeneration and repair.

Natural polysaccharides and their derivatives as potential medical materials and drug delivery systems for the treatment of peripheral nerve injuries

Peripheral nerve repair following injury is one of the most serious problems in neurosurgery. Clinical outcomes are often unsatisfactory and associated with a huge socioeconomic burden. Several studies have revealed the great potential of biodegradable polysaccharides for improving nerve regeneration. We review here the promising therapeutic strategies involving different types of polysaccharides and their bio-active composites for promoting nerve regeneration. Within this context, polysaccharide materials widely used for nerve repair in different forms are highlighted, including nerve guidance conduits, hydrogels, nanofibers and films. While nerve guidance conduits and hydrogels were used as main structural scaffolds, the other forms including nanofibers and films were generally used as additional supporting materials. We also discuss the issues of ease of therapeutic implementation, drug release properties and therapeutic outcomes, together with potential future directions of research.

Clinical evaluation of the perception of post-trauma paresthesia in the mandible, using a biomimetic material: A preliminary study in humans

There is a great effort from numerous research groups in the development of materials and therapeutic strategies for the functional recovery of patients who have suffered peripheral nerve injuries (PNI). In an article in vivo, the formation of a nerve bridge was observed, reconnecting the distal and proximal stumps, in the sciatic nerve of rats, indicating the effective participation of the biomaterial in the recovery of peripheral nerve injuries. For the current pilot study, 15 cases of multiple fractures of the mandible, with involvement of the inferior alveolar nerve (IAN) were selected and studied: JC (control cases) n = 6 with conventional treatment, and JT (treated cases) n = 9, with the use of biomimetic biomaterial. The evaluation of the return to sensitivity was measured through a self-assessment, where the patients assigned scores from 0 to 10, where zero (0) represented the complete absence of sensitivity and ten (10) the normality of the perception of local sensitivity. Patients were evaluated from the preoperative period to the 360th day. The statistical results obtained by the t-Student, Shapiro-Wilk normality and non-parametric One-Way ANOVA tests indicated statistically significant differences (p < 0.005; 0.005 e 0.5 respectively), between the two treatments, which were reflected in the clinical results observed, we also calculate the size of the effect represented by ϵ², calculated by Cohen's d. The results indicate a great difference between the treatments performed,ϵ² = 1.00. In the 6 cases followed up in the JC group, four remained with a significant deficit until the end of the evaluations and two indicated the remission of the lack of sensitivity in this period. In the JT group, in 28 days, all cases indicated complete remission of the lack of sensitivity with healing concentration. In one of the cases where there was a complete rupture of the mental nerve, the (score-10) was observed in 60 days. The observed results indicate the existence of a statistical significance between the groups and an important relationship when using the biomimetic biomaterial during the recovery of the perception of sensitivity in polytraumatized patients, compatible with the results observed in laboratory animals, which may indicate its clinical feasibility in the reduction of sequelae in PNI.

A scoping review of current and emerging techniques for evaluation of peripheral nerve health, degeneration, and regeneration: part 1, neurophysiology

Peripheral neuroregeneration research and therapeutic options are expanding exponentially. With this expansion comes an increasing need to reliably evaluate and quantify nerve health. Valid and responsive measures that can serve as biomarkers of the nerve status are essential for both clinical and research purposes for diagnosis, longitudinal follow-up, and monitoring the impact of any intervention. Furthermore, such biomarkers can elucidate regeneration mechanisms and open new avenues for research. Without these measures, clinical decision-making falls short, and research becomes more costly, time-consuming, and sometimes infeasible. As a companion to Part 2, which is focused on non-invasive imaging, Part 1 of this two-part scoping review systematically identifies and critically examines many current and emerging neurophysiological techniques that have the potential to evaluate peripheral nerve health, particularly from the perspective of regenerative therapies and research.

Medium chain length polyhydroxyalkanoates as potential matrix materials for peripheral nerve regeneration

Polyhydroxyalkanoates are natural, biodegradable, thermoplastic and sustainable polymers with a huge potential in fabrication of bioresorbable implantable devices for tissue engineering. We describe a comparative evaluation of three medium chain length polyhydroxyalkanoates (mcl-PHAs), namely poly(3-hydroxyoctanoate), poly(3-hydroxyoctanoate-co-3-hydoxydecanoate) and poly(3-hydroxyoctanoate-co-3-hydroxydecanoate-co-3-hydroxydodecanoate), one short chain length polyhydroxyalkanoate, poly(3-hydroxybutyrate), P(3HB) and synthetic aliphatic polyesters (polycaprolactone and polylactide) with a specific focus on nerve regeneration, due to mechanical properties of mcl-PHAs closely matching nerve tissues. In vitro biological studies with NG108-15 neuronal cell and primary Schwann cells did not show a cytotoxic effect of the materials on both cell types. All mcl-PHAs supported cell adhesion and viability. Among the three mcl-PHAs, P(3HO-co-3HD) exhibited superior properties with regards to numbers of cells adhered and viable cells for both cell types, number of neurite extensions from NG108-15 cells, average length of neurite extensions and Schwann cells. Although, similar characteristics were observed for flat P(3HB) surfaces, high rigidity of this biomaterial, and FDA-approved polymers such as PLLA, limits their applications in peripheral nerve regeneration. Therefore, we have designed, synthesized and evaluated these materials for nerve tissue engineering and regenerative medicine, the interaction of mcl-PHAs with neuronal and Schwann cells, identifying mcl-PHAs as excellent materials to enhance nerve regeneration and potentially their clinical application in peripheral nerve repair.

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.

Physical processing for decellularized nerve xenograft in peripheral nerve regeneration

In severe or complex cases of peripheral nerve injuries, autologous nerve grafts are the gold standard yielding promising results, but limited availability and donor site morbidity are some of its disadvantages. Although biological or synthetic substitutes are commonly used, clinical outcomes are inconsistent. Biomimetic alternatives derived from allogenic or xenogenic sources offer an attractive off-the-shelf supply, and the key to successful peripheral nerve regeneration focuses on an effective decellularization process. In addition to chemical and enzymatic decellularization protocols, physical processes might offer identical efficiency. In this comprehensive minireview, we summarize recent advances in the physical methods for decellularized nerve xenograft, focusing on the effects of cellular debris clearance and stability of the native architecture of a xenograft. Furthermore, we compare and summarize the advantages and disadvantages, indicating the future challenges and opportunities in developing multidisciplinary processes for decellularized nerve xenograft.

IPSC-Derived Sensory Neurons Directing Fate Commitment of Human BMSC-Derived Schwann Cells: Applications in Traumatic Neural Injuries

The in vitro derivation of Schwann cells from human bone marrow stromal cells (hBMSCs) opens avenues for autologous transplantation to achieve remyelination therapy for post-traumatic neural regeneration. Towards this end, we exploited human induced pluripotent stem-cell-derived sensory neurons to direct Schwann-cell-like cells derived from among the hBMSC-neurosphere cells into lineage-committed Schwann cells (hBMSC-dSCs). These cells were seeded into synthetic conduits for bridging critical gaps in a rat model of sciatic nerve injury. With improvement in gait by 12-week post-bridging, evoked signals were also detectable across the bridged nerve. Confocal microscopy revealed axially aligned axons in association with MBP-positive myelin layers across the bridge in contrast to null in non-seeded controls. Myelinating hBMSC-dSCs within the conduit were positive for both MBP and human nucleus marker HuN. We then implanted hBMSC-dSCs into the contused thoracic cord of rats. By 12-week post-implantation, significant improvement in hindlimb motor function was detectable if chondroitinase ABC was co-delivered to the injured site; such cord segments showed axons myelinated by hBMSC-dSCs. Results support translation into a protocol by which lineage-committed hBMSC-dSCs become available for motor function recovery after traumatic injury to both peripheral and central nervous systems.

Bridging Gaps in Peripheral Nerves: From Current Strategies to Future Perspectives in Conduit Design

In peripheral nerve injuries (PNI) with substance loss, where tensionless end-to-end suture is not achievable, the positioning of a graft is required. Available options include autografts (e.g., sural nerve, medial and lateral antebrachial cutaneous nerves, superficial branch of the radial nerve), allografts (Avance^®; human origin), and hollow nerve conduits. There are eleven commercial hollow conduits approved for clinical, and they consist of devices made of a non-biodegradable synthetic polymer (polyvinyl alcohol), biodegradable synthetic polymers (poly(DL-lactide-ε-caprolactone); polyglycolic acid), and biodegradable natural polymers (collagen type I with/without glycosaminoglycan; chitosan; porcine small intestinal submucosa); different resorption times are available for resorbable guides, ranging from three months to four years. Unfortunately, anatomical/functional nerve regeneration requirements are not satisfied by any of the possible alternatives; to date, focusing on wall and/or inner lumen organization/functionalization seems to be the most promising strategy for next-generation device fabrication. Porous or grooved walls as well as multichannel lumens and luminal fillers are the most intriguing options, eventually also including the addition of cells (Schwann cells, bone marrow-derived, and adipose tissue derived stem cells) to support nerve regeneration. This review aims to describe common alternatives for severe PNI recovery with a highlight of future conduits.

Effect of Electrical Stimulation on Nerve-Guided Facial Nerve Regeneration

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

Procedure Costs of Peripheral Nerve Graft Reconstruction

Peripheral nerve injuries not repaired in an effective and timely manner may lead to permanent functional loss and/or pain. For gaps greater than 5 mm, autograft has been the gold standard. Allograft has recently emerged as an attractive alternative, delivering comparable functional recovery without risk of second surgical site morbidities. Cost is an important factor when considering surgical options, and with a paucity of nerve repair cost data, this study aimed to compare allograft and autograft procedure costs.

Methods

A retrospective cross-sectional observational study using the US all-payer PINC AI Healthcare Database examined facility procedure costs and cost drivers in patients undergoing allograft or autograft repair of an isolated single peripheral nerve injury between January 2018 and August 2020. Inpatient repairs were limited to nerve-specific DRGs. Multivariable regression evaluated risk-adjusted procedure cost differences.

Results

Peripheral nerve graft repairs (n = 1363) were more frequent in the outpatient setting, and more than half involved the use of allograft nerve. Procedure costs for allograft and autograft repair were not significantly different in the outpatient (P = 0.43) or inpatient (P = 0.71) setting even after controlling for other risk factors. Operating room cost was significantly higher for autograft in outpatient (P < 0.0001) but not inpatient (P = 0.46), whereas allograft implant cost was significantly higher in both settings (P < 0.0001).

Conclusions

No significant differences in procedure costs for autograft and allograft repair in inpatient and outpatient settings were found using real-world data. Future research should explore longer-term costs.

Artificial Nerve Conduit for Recurrent Laryngeal Nerve Reconstruction in Thyroid Surgery

OBJECTIVE

Oncological reconstruction of the recurrent laryngeal nerve (RLN) is sometimes necessary for RLN invaded by thyroid cancer. There have been no case reports of RLN reconstruction using artificial nerve conduits, which are often used for peripheral nerves. In this study, we retrospectively evaluate the feasibility, safety, and efficacy of a collagen conduit with collagen filaments for RLN reconstruction cases at our hospital.

METHODS

Artificial nerve conduits were used in seven cases of RLN reconstruction. Two patients had preoperative unilateral vocal cord paralysis with severe vocal cord atrophy, and two had vocal cord paresis without atrophy. The remaining three patients had functional vocal cords before surgery that had to be resected via surgery due to thyroid cancer infiltration of the RLN. Reconstruction was performed using RENERVE®, which is a collagen conduit. Voice examination and laryngeal endoscopy were performed 1, 3, and 12 months after surgery.

RESULTS

There was no improvement in the phonetics of the two patients with vocal cord atrophy before surgery. In the remaining five cases, three with functional vocal cords improved to preoperative values, and two with vocal cord paresis improved to greater than preoperative values.

CONCLUSION

We report the first case series using an artificial nerve conduit for human RLN reconstruction. In cases of RLN resection when the patient has good voice quality pre-operatively, reconstruction of the RLN using an artificial nerve may be a favorable option in cases where direct anastomosis or ansa cervicalis to RLN anastomosis cannot be performed.

LEVEL OF EVIDENCE

Level 4 Laryngoscope, 2023.