Functional Polymer‐Based Nerve Guide Conduits to Promote Peripheral Nerve Regeneration

A novel flexible nerve guidance conduit promotes nerve regeneration while providing excellent mechanical properties

JOURNAL/nrgr/04.03/01300535-202507000-00029/figure1/v/2024-09-09T124005Z/r/image-tiff Autografting is the gold standard for surgical repair of nerve defects > 5 mm in length; however, autografting is associated with potential complications at the nerve donor site. As an alternative, nerve guidance conduits may be used. The ideal conduit should be flexible, resistant to kinks and lumen collapse, and provide physical cues to guide nerve regeneration. We designed a novel flexible conduit using electrospinning technology to create fibers on the innermost surface of the nerve guidance conduit and employed melt spinning to align them. Subsequently, we prepared disordered electrospun fibers outside the aligned fibers and helical melt-spun fibers on the outer wall of the electrospun fiber lumen. The presence of aligned fibers on the inner surface can promote the extension of nerve cells along the fibers. The helical melt-spun fibers on the outer surface can enhance resistance to kinking and compression and provide stability. Our novel conduit promoted nerve regeneration and functional recovery in a rat sciatic nerve defect model, suggesting that it has potential for clinical use in human nerve injuries.

Recent perspectives on the synergy of mesenchymal stem cells with micro/nano strategies in peripheral nerve regeneration-a review

Despite the intrinsic repair of peripheral nerve injury (PNI), it is important to carefully monitor the process of peripheral nerve repair, as peripheral nerve regeneration is slow and incomplete in large traumatic lesions. Hence, mesenchymal stem cells (MSCs) with protective and regenerative functions are utilized in synergy with innovative micro/nano technologies to enhance the regeneration process of peripheral nerves. Nonetheless, as MSCs are assessed using standard regenerative criteria including sensory-motor indices, structural features, and morphology, it is challenging to differentiate between the protective and regenerative impacts of MSCs on neural tissue. This study aims to analyze the process of nerve regeneration, particularly the performance of MSCs with and without synergistic approaches. It also focuses on the paracrine secretions of MSCs and their conversion into neurons with functional properties that influence nerve regeneration after PNI. Furthermore, the study explores new ideas for nerve regeneration after PNI by considering the synergistic effect of MSCs and therapeutic compounds, neuronal cell derivatives, biological or polymeric conduits, organic/inorganic nanoparticles, and electrical stimulation. Finally, the study highlights the main obstacles to developing synergy in nerve regeneration after PNI and aims to open new windows based on recent advances in neural tissue regeneration.

Multifunctional Biodegradable Conductive Hydrogel Regulating Microenvironment for Stem Cell Therapy Enhances the Nerve Tissue Repair

The nerve guidance conduits incorporated with stem cells, which can differentiate into the Schwann cells (SCs) to facilitate myelination, shows great promise for repairing the severe peripheral nerve injury. The innovation of advanced hydrogel materials encapsulating stem cells, is highly demanded for generating supportive scaffolds and adaptive microenvironment for nerve regeneration. Herein, this work demonstrates a novel strategy in regulating regenerative microenvironment for peripheral nerve repair with a biodegradable conductive hydrogel scaffold, which can offer multifunctional capabilities in immune regulation, enhancing angiogenesis, driving SCs differentiation, and promoting axon regrowth. The biodegradable conductive hydrogel is constructed by incorporation of polydopamine-modified silicon phosphorus (SiP@PDA) nanosheets into a mixture of methacryloyl gelatin and decellularized extracellular matrix (GelMA/ECM). The biomimetic electrical microenvironment performs an efficacious strategy to facilitate macrophage polarization toward a pro-healing phenotype (M2), meanwhile the conductive hydrogel supports vascularization in regenerated tissue through sustained Si element release. Furthermore, the MSCs 3D-cultured in GelMA/ECM-SiP@PDA conductive hydrogel exhibits significantly increased expression of genes associated with SC-like cell differentiation, thus facilitating the myelination and axonal regeneration. Collectively, both the in vitro and in vivo studies demonstrates that the rationally designed biodegradable multifunctional hydrogel significantly enhances nerve tissues repair.

Magnetoactive Composite Conduits Based on Poly(3-hydroxybutyrate) and Magnetite Nanoparticles for Repair of Peripheral Nerve Injury

Peripheral nerve injury poses a threat to the mobility and sensitivity of a nerve, thereby leading to permanent function loss due to the low regenerative capacity of mature neurons. To date, the most widely clinically applied approach to bridging nerve injuries is autologous nerve grafting, which faces challenges such as donor site morbidity, donor shortages, and the necessity of a second surgery. An effective therapeutic strategy is urgently needed worldwide to overcome the current limitations. Herein, a magnetic nerve guidance conduit (NGC) based on biocompatible biodegradable poly(3-hydroxybutyrate) (PHB) and 8 wt % of magnetite nanoparticles modified by citric acid (Fe3O4-CA) was fabricated by electrospinning. The crystalline structure of NGCs was studied by X-ray diffraction, which indicated an enlarged β-phase of PHB in the composite conduit compared to a pure PHB conduit. Tensile tests revealed greater ductility of PHB/Fe3O4-CA: the composite conduit has Young's modulus of 221 ± 52 MPa and an elongation at break of 28.6 ± 2.9%, comparable to clinical materials. Saturation magnetization (σs) of Fe3O4-CA and PHB/Fe3O4-CA is 61.88 ± 0.29 and 7.44 ± 0.07 emu/g, respectively. The water contact angle of the PHB/Fe3O4-CA conduit is lower as compared to pure PHB, while surface free energy (σ) is significantly higher, which was attributed to higher surface roughness and an amorphous phase as well as possible PHB/Fe3O4-CA interface interactions. In vitro, the conduits supported the proliferation of rat mesenchymal stem cells (rMSCs) and SH-SY5Y cells in a low-frequency magnetic field (0.67 Hz, 68 mT). In vivo, the conduits were used to bridge damaged sciatic nerves in rats; pure PHB and composite PHB/Fe3O4-CA conduits did not cause acute inflammation and performed a barrier function, which promotes nerve regeneration. Thus, these conduits are promising as implants for the regeneration of peripheral nerves.

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

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

Emerging 4D fabrication of next-generation nerve guiding conduits: a critical perspective

The latest advancements in the field of manufacturing for biomedicine, digital health, targeted therapy, and personalized medicine have fuelled the fabrication of smart medical devices. Four-dimensional (4D) fabrication strategies, which combine the manufacturing of three-dimensional (3D) parts with smart materials and/or design, have proved beneficial in creating customized and self-fitting structures that change their properties on demand with time. These frontier techniques that yield dynamic implants can indeed alleviate various drawbacks of current clinical practices, such as the use of sutures and complex microsurgeries and associated inflammation, among others. Among various clinical applications, 4D fabrication has lately made remarkable progress in the development of next-generation nerve-guiding conduits for treating peripheral nerve injuries (PNIs) by improving the end-to-end co-aptation of transected nerve endings. The current perspective highlights the relevance of 4D fabrication in developing state-of-the-art technologies for the treatment of PNIs. Various 4D fabrication/bio-fabrication techniques for PNI treatment are summarized while identifying the challenges and opportunities for the future. Such advancements hold immense promise for improving the quality of life of patients suffering from nerve damage and the potential for extending the treatment of many other disorders. Although the techniques are being described for PNIs, they will lend themselves suitably to certain cases of cranial nerve injuries as well.

Review on electrically conductive smart nerve guide conduit for peripheral nerve regeneration

At present, peripheral nerve injuries (PNIs) are one of the leading causes of substantial impairment around the globe. Complete recovery of nerve function after an injury is challenging. Currently, autologous nerve grafts are being used as a treatment; however, this has several downsides, for example, donor site morbidity, shortage of donor sites, loss of sensation, inflammation, and neuroma development. The most promising alternative is the development of a nerve guide conduit (NGC) to direct the restoration and renewal of neuronal axons from the proximal to the distal end to facilitate nerve regeneration and maximize sensory and functional recovery. Alternatively, the response of nerve cells to electrical stimulation (ES) has a substantial regenerative effect. The incorporation of electrically conductive biomaterials in the fabrication of smart NGCs facilitates the function of ES throughout the active proliferation state. This article overviews the potency of the various categories of electroactive smart biomaterials, including conductive and piezoelectric nanomaterials, piezoelectric polymers, and organic conductive polymers that researchers have employed latterly to fabricate smart NGCs and their potentiality in future clinical application. It also summarizes a comprehensive analysis of the recent research and advancements in the application of ES in the field of NGC.

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.

Battery-Free, Wireless, Cuff-Type, Multimodal Physical Sensor for Continuous Temperature and Strain Monitoring of Nerve

Peripheral nerve injuries cause various disabilities related to loss of motor and sensory functions. The treatment of these injuries typically requires surgical operations for improving functional recovery of the nerve. However, capabilities for continuous nerve monitoring remain a challenge. Herein, a battery-free, wireless, cuff-type, implantable, multimodal physical sensing platform for continuous in vivo monitoring of temperature and strain from the injured nerve is introduced. The thin, soft temperature, and strain sensors wrapped around the nerve exhibit good sensitivity, excellent stability, high linearity, and minimum hysteresis in relevant ranges. In particular, the strain sensor integrated with circuits for temperature compensation provides reliable, accurate strain monitoring with negligible temperature dependence. The system enables power harvesting and data communication to wireless, multiple implanted devices wrapped around the nerve. Experimental evaluations, verified by numerical simulations, with animal tests, demonstrate the feasibility and stability of the sensor system, which has great potential for continuous in vivo nerve monitoring from an early stage to complete regeneration.

Ropivacaine microsphere-loaded electroconductive nerve dressings for long-acting analgesia and functional recovery following diabetic peripheral nerve injury

In recent years, electroconductive hydrogels (ECHs) have shown great potential in promoting nerve regeneration and motor function recovery following diabetic peripheral nerve injury (PNI), attributed to their similar electrical and mechanical characteristics to innate nervous tissue. It is well-established that PNI causes motor deficits and pain, especially in diabetics. Current evidence suggests that ropivacaine (ROP) encapsulated in poly lactic-co-glycolic acid (PLGA) microspheres (MSs) yield a sustained analgesic effect. In this study, an ECH electroconductive network loaded with MS/ROP (ECH-MS/ROP) was designed as a promising therapeutic approach for diabetic PNI to exert lasting analgesia and functional recovery. This dual delivery system allowed ROP's slow and sequential release, achieving sustained analgesia as demonstrated by our in vivo experiments. Meanwhile, this system was designed like a lamellar dressing, with desirable adhesive and self-curling properties, convenient for treating injured nerve tissues via automatically wrapping tube-like structures, facilitating the process of implantation. Our in vitro assays verified that ECH-MS/ROP was able to enhance the adhesion and motility of Schwann cells. Besides, both in vitro and in vivo studies substantiated that ECH-MS/ROP stimulated myelinated axon regeneration through the MEK/ERK signaling pathway, thereby improving muscular denervation atrophy and facilitating functional recovery. Therefore, this study suggests that the ECH-MS/ROP dressing provides a promising strategy for treating diabetic PNI to facilitate nerve regeneration, functional recovery and pain relief.

Mimosa-Inspired Stimuli-Responsive Curling Bioadhesive Tape Promotes Peripheral Nerve Regeneration

Trauma often results in peripheral nerve injuries (PNIs). These injuries are particularly challenging therapeutically because of variable nerve diameters, slow axonal regeneration, infection of severed ends, fragility of the nerve tissue, and the intricacy of surgical intervention. Surgical suturing is likely to cause additional damage to peripheral nerves. Therefore, an ideal nerve scaffold should possess good biocompatibility, diameter adaptability, and a stable biological interface for seamless biointegration with tissues. Inspired by the curl of Mimosa pudica, this study aimed to design and develop a diameter-adaptable, suture-free, stimulated curling bioadhesive tape (SCT) hydrogel for repairing PNI. The hydrogel is fabricated from chitosan and acrylic acid-N-hydroxysuccinimide lipid via gradient crosslinking using glutaraldehyde. It closely matches the nerves of different individuals and regions, thereby providing a bionic scaffold for axonal regeneration. In addition, this hydrogel rapidly absorbs tissue fluid from the nerve surface achieving durable wet-interface adhesion. Furthermore, the chitosan-based SCT hydrogel loaded with insulin-like growth factor-I effectively promotes peripheral nerve regeneration with excellent bioactivity. This procedure for peripheral nerve injury repair using the SCT hydrogel is simple and reduces the difficulty and duration of surgery, thereby advancing adaptive biointerfaces and reliable materials for nerve repair.

Sciatic nerve injury regeneration in adult male rats using gelatin methacrylate (GelMA)/poly(2-ethy-2-oxazoline) (PEtOx) hydrogel containing 4-aminopyridine (4-AP)

One of the most important parts of the body is the peripheral nervous system, and any injuries in this system may result in potentially lethal consequences or severe side effects. The peripheral nervous system may not rehabilitate the harmed regions following disabling disorders, which reduce the quality of life of patients. Fortunately, in recent years, hydrogels have been proposed as exogenous alternatives to bridge damaged nerve stumps to create a useful microenvironment for advancing nerve recovery. However, hydrogel-based medicine in the therapy of peripheral nerve injury still needs a lot of improvement. In this study, GelMA/PEtOx hydrogel was used for the first time to deliver 4-Aminopyridine (4-AP) small molecules. 4-AP is a broad-spectrum potassium channel blocker, which has been demonstrated to increase neuromuscular function in patients with various demyelinating disorders. The prepared hydrogel showed a porosity of 92.2 ± 2.6% after 20 min, swelling ratio of 456.01 ± 2.0% after 180 min, weight loss of 81.7 ± 3.1% after 2 weeks, and good blood compatibility as well as sustainable drug release. MTT analysis was performed to assess the cell viability of the hydrogel and proved that the hydrogel is an appropriate substrate for the survival of cells. In vivo studies were performed for functional analysis and the sciatic functional index (SFI) as well as hot plate latency results showed that the use of GelMA/PEtOx+4-AP hydrogel enhances the regeneration compared to the GelMA/PEtOx hydrogel and the control group.

Thermoelectric Freeze-Casting of Biopolymer Blends: Fabrication and Characterization of Large-Size Scaffolds for Nerve Tissue Engineering Applications

Peripheral nerve injuries (PNIs) are detrimental to the quality of life of affected individuals. Patients are often left with life-long ailments that affect them physically and psychologically. Autologous nerve transplant is still the gold standard treatment for PNIs despite limited donor site and partial recovery of nerve functions. Nerve guidance conduits are used as a nerve graft substitute and are efficient for the repair of small nerve gaps but require further improvement for repairs exceeding 30 mm. Freeze-casting is an interesting fabrication method for the conception of scaffolds meant for nerve tissue engineering since the microstructure obtained comprises highly aligned micro-channels. The present work focuses on the fabrication and characterization of large scaffolds (35 mm length, 5 mm diameter) made of collagen/chitosan blends by freeze-casting via thermoelectric effect instead of traditional freezing solvents. As a freeze-casting microstructure reference, scaffolds made from pure collagen were used for comparison. Scaffolds were covalently crosslinked for better performance under load and laminins were further added to enhance cell interactions. Microstructural features of lamellar pores display an average aspect ratio of 0.67 ± 0.2 for all compositions. Longitudinally aligned micro-channels are reported as well as enhanced mechanical properties in traction under physiological-like conditions (37 °C, pH = 7.4) resulting from crosslinking treatment. Cell viability assays using a rat Schwann cell line derived from sciatic nerve (S16) indicate that scaffold cytocompatibility is similar between scaffolds made from collagen only and scaffolds made from collagen/chitosan blend with high collagen content. These results confirm that freeze-casting via thermoelectric effect is a reliable manufacturing strategy for the fabrication of biopolymer scaffolds for future peripheral nerve repair applications.

Electroactive scaffolds of biodegradable polyurethane/polydopamine-functionalized graphene oxide regulating the inflammatory response and revitalizing the axonal growth cone for peripheral nerve regeneration

Long-gap peripheral nerve injury remains a major challenge in regenerative medicine and results in permanent sensory and motor dysfunction. Nerve guidance scaffolds (NGSs) are known as a promising alternative to autologous nerve grafting. The latter, the current "gold standard" in clinical practice, is frequently constrained by the limited availability of sources and the inevitable damage to the donor area. Given the electrophysiological properties of nerves, electroactive biomaterials are being intensively investigated in nerve tissue engineering. In this study, we engineered a conductive NGS compounded of biodegradable waterborne polyurethane (WPU) and polydopamine-reduced graphene oxide (pGO) for repairing impaired peripheral nerves. The incorporation of pGO at the optimal concentration (3 wt%) promoted in vitro spreading of Schwann cells (SCs) with high expression of the proliferation marker S100 protein. In an in vivo study of sciatic nerve transection injury, WPU/pGO NGSs were found to regulate the immune microenvironment by activating macrophage M2 polarization and upregulate growth-associated protein 43 (GAP43) to facilitate axonal elongation. Histological and motor function analysis demonstrated that WPU/pGO NGSs had a neuroprosthetic effect close to that of an autograft, which significantly promoted the regeneration of myelinated axons, reduced gastrocnemius atrophy, and enhanced hindlimb motor function. These findings together suggested that electroactive WPU/pGO NGSs may represent a safe and effective strategy to manage large nerve defects.

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.

Spatio-temporally deciphering peripheral nerve regeneration in vivo after extracellular vesicle therapy under NIR-II fluorescence imaging

Extracellular vesicles (EVs) show potential as a therapeutic tool for peripheral nerve injury (PNI), promoting neurological regeneration. However, there are limited data on the in vivo spatio-temporal trafficking and biodistribution of EVs. In this study, we introduce a new non-invasive near-infrared fluorescence imaging strategy based on glucose-conjugated quantum dot (QDs-Glu) labeling to target and track EVs in a sciatic nerve injury rat model in real-time. Our results demonstrate that the injected EVs migrated from the uninjured site to the injured site of the nerve, with an increase in fluorescence signals detected from 4 to 7 days post-injection, indicating the release of contents from the EVs with therapeutic effects. Immunofluorescence and behavioral tests revealed that the EV therapy promoted nerve regeneration and functional recovery at 28 days post-injection. We also found a relationship between functional recovery and the NIR-II fluorescence intensity change pattern, providing novel evidence for the therapeutic effects of EV therapy using real-time NIR-II imaging at the live animal level. This approach initiates a new path for monitoring EVs in treating PNI under in vivo NIR-II imaging, enhancing our understanding of the efficacy of EV therapy on peripheral nerve regeneration and its mechanisms.

Electrohydrodynamic Printing of Microfibrous Architectures with Cell-Scale Spacing for Improved Cellular Migration and Neurite Outgrowth

Electrohydrodynamic (EHD) printing provides unparalleled opportunities in fabricating microfibrous architectures to direct cellular orientation. However, it faces great challenges in depositing orderly microfibers with cell-scale spacing due to inherent fiber-fiber electrostatic interactions. Here a finite element method is established to analyze the electrostatic forces induced on the EHD-printed microfibers and the relationship between the fiber diameter and spacing for parallel deposition of EHD-printed microfibers is revealed theoretically and experimentally. It is found that uniform fiber arrangement can be achieved when the fiber spacing is five times larger than the fiber diameter. This finding enables the successful printing of parallel fibrous architectures with a fiber diameter of 4.9 ± 0.1 µm and a cell-scale fiber spacing of 25.6 ± 1.9 µm. The resultant microfibrous architectures exhibit unique capability to direct cellular alignment and enhance cellular density and migration as the fiber spacing decreases from 100 to 25 µm. The EHD-printed parallel microfibers with cell-scale spacing are found to improve the outgrowth length of neurites and accelerate the migration of Schwann cells from Dorsal Root Ganglion spheres, which facilitate the formation of densely-arranged and highly-aligned cellular constructs. The presented method is promising to produce biomimetic microfibrous architectures for functional nerve regeneration.

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.

Regulation of nerve cells using conductive nanofibrous scaffolds for controlled release of Lycium barbarum polysaccharides and nerve growth factor

Currently, more and more patients suffer from peripheral nerve injury due to trauma, tumor and other causes worldwide. Biomaterial-based nerve conduits are increasingly recognized as a potential alternative to nerve autografts for the treatment of peripheral nerve injury. However, an ideal nerve conduit must offer topological guidance and biochemical and electrical signal transduction mechanisms. In this work, aligned conductive nanofibrous scaffolds comprising polylactic-co-glycolic acid and multiwalled carbon nanotubes (MWCNTs) were fabricated via coaxial electrospinning, and nerve growth factor (NGF) and Lycium barbarum polysaccharides (LBP) purified from the wolfberry were loaded on the core and shell layers of the nanofibers, respectively. LBP were confirmed to accelerate long-distance axon regeneration after severe peripheral nerve injury. In addition, the synergistic promotion of LBP and NGF on nerve cell proliferation and neurite outgrowth was demonstrated. MWCNTs were introduced into the aligned fibers to further increase the electrical conductivity, which promoted the directional growth and neurite extension of neurons in vitro. Further, the combination of conductive fibrous scaffolds with electrical stimulation that mimics endogenous electric fields significantly promoted the differentiation of PC12 cells and the axon outgrowth of neurons. Based on robust cell-induced behaviors, conductive composite fibers with optimized fiber alignment may be used for the promotion of nerve recovery.

3D Printed Conductive Multiscale Nerve Guidance Conduit with Hierarchical Fibers for Peripheral Nerve Regeneration

Nerve guidance conduits (NGCs) have become a promising alternative for peripheral nerve regeneration; however, the outcome of nerve regeneration and functional recovery is greatly affected by the physical, chemical, and electrical properties of NGCs. In this study, a conductive multiscale filled NGC (MF-NGC) consisting of electrospun poly(lactide-co-caprolactone) (PCL)/collagen nanofibers as the sheath, reduced graphene oxide /PCL microfibers as the backbone, and PCL microfibers as the internal structure for peripheral nerve regeneration is developed. The printed MF-NGCs presented good permeability, mechanical stability, and electrical conductivity, which further promoted the elongation and growth of Schwann cells and neurite outgrowth of PC12 neuronal cells. Animal studies using a rat sciatic nerve injury model reveal that the MF-NGCs promote neovascularization and M2 transition through the rapid recruitment of vascular cells and macrophages. Histological and functional assessments of the regenerated nerves confirm that the conductive MF-NGCs significantly enhance peripheral nerve regeneration, as indicated by improved axon myelination, muscle weight increase, and sciatic nerve function index. This study demonstrates the feasibility of using 3D-printed conductive MF-NGCs with hierarchically oriented fibers as functional conduits that can significantly enhance peripheral nerve regeneration.

Development of double network polyurethane-chitosan composite bioinks for soft neural tissue engineering

Three-dimensional (3D) bioprinting is an emerging manufacturing technology to print materials with cells for tissue engineering applications. In this study, we prepared novel ternary soft segment-based biodegradable polyurethane (tPU) using waterborne processes. The ternary soft segment included poly(ε-caprolactone) (PCL), polylactide, and poly(3-hydroxybutyrate) (PHB). tPU2 with a soft segment of PCL, poly(D,L-lactide), and PHB in a molar ratio of 0.7 : 0.2 : 0.1 demonstrated lower stiffness (∼2.3 kPa) and a greater tan δ value (∼0.64) and maintained good vitality (91.3%) of neural stem cells (NSCs) among various tPUs. The bioprinted tPU2 constructs facilitated cell proliferation (∼200% in 7 days) and neural differentiation of NSCs. Meanwhile, tPU2 formed double network composite hydrogels with gelatin or agarose, and the composite hydrogels showed good biocompatibility and achieved high-resolution (∼80 μm nozzle) bioprinting. In addition, a new series of double network polyurethane-chitosan composite (PUC) hydrogels were developed by combining tPU2 with a self-healing chitosan hydrogel. The PUC hydrogel demonstrated self-healing properties and bioprintability without the need for a post-crosslinking process. The bioprinted PUC composite hydrogel promoted cell proliferation (∼300% in 7 days) and neural differentiation of NSCs better than the tPU2 bioink. This study revealed new formulae of a polyurethane bioink and a polyurethane-chitosan composite bioink for 3D bioprinting and tissue engineering applications.

Regeneration of sciatic nerve injury through nanofiber neural guidance channels containing collagen hydrogel and acetyl L carnitine: An in vitro and in vivo study

Unlike the central nervous system (CNS), peripheral nervous system (PNS) injuries are partially repairable. Nerve guidance channels (NGCs) have been shown to improve the level of nerve repair after injury. In the present study, we developed a nanofiber NGC for the delivery of acetyl L carnitine (ALC) in a rat model of sciatic nerve injury. NGCs were produced by electrospinning a polymer blend of polycaprolacton and gelatin. The physicochemical and biological properties of developed scaffolds were investigated using Scanning electron microscopy, surface hydrophilicity measurement, porosity measurement, tensile strength studies, cell viability assay, and cell attachment assay. ALC was included in the collagen hydrogels at three weight ratios of 1%, 3%, and 5%. Cell viability assay showed that the hydrogels containing 5% ALC demonstrated a more favorable effect on PC-12 metabolic activity. Therefore, this concentration was chosen to treat PNS injury. The NGCs were implanted in rats and then their lumen was filled with collagen hydrogel + 5%ALC. The results of histopathological examinations and functional recovery studies showed that NGCs filled with ALC containing hydrogel have significant recovery potential compared to NGCs loaded with collagen hydrogels without ALC. Our results support the potential use of ALC-delivering NGCs in the treatment of peripheral nerve injury in the clinic.

Chondroitin sulfate-based composites: a tour d'horizon of their biomedical applications

Chondroitin sulfate (CS), a natural anionic mucopolysaccharide, belonging to the glycosaminoglycan family, acts as the primary element of the extracellular matrix (ECM) of diverse organisms. It comprises repeating units of disaccharides possessing β-1,3-linked N-acetyl galactosamine (GalNAc), and β-1,4-linked D-glucuronic acid (GlcA), and exhibits antitumor, anti-inflammatory, anti-coagulant, anti-oxidant, and anti-thrombogenic activities. It is a naturally acquired bio-macromolecule with beneficial properties, such as biocompatibility, biodegradability, and immensely low toxicity, making it the center of attention in developing biomaterials for various biomedical applications. The authors have discussed the structure, unique properties, and extraction source of CS in the initial section of this review. Further, the current investigations on applications of CS-based composites in various biomedical fields, focusing on delivering active pharmaceutical compounds, tissue engineering, and wound healing, are discussed critically. In addition, the manuscript throws light on preclinical and clinical studies associated with CS composites. A short section on Chondroitinase ABC has also been canvassed. Finally, this review emphasizes the current challenges and prospects of CS in various biomedical fields.

Enhancing the Antioxidant Ability of Momordica grosvenorii Saponin to Resist Gastrointestinal Stresses via Microcapsules of Sodium Alginate and Chitosan and Its Application in Beverage

Momordica grosvenorii saponin (MGS), as a promising dietary supplement with remarkable biological properties, has poor stability under acidic conditions and thus hinders its application in functional foods. In this study, capsules of chitosan and sodium alginate were successfully prepared to enhance the stability of MGS. The optimized parameters for preparing MGS capsules were established. Sodium alginate of 20.8 mg/mL and triplication of MGS powder were added to chitosan of 4 mg/mL and calcium chloride of 10 mg/mL at a volume ratio of 3:1, stirring at 1000 r/min for 30 min to form the capsules. In this case, the fresh particles averaged 1687 μm with an encapsulation efficiency (EE) of 80.25% MGS. The capsule tolerated acidic environments better, and in vitro MGS could be controlled to release in a stimulated gastrointestinal tract system. The antioxidant activity and delayed release of MGS could be achieved by microencapsulation of chitosan/sodium alginate. Moreover, one drink containing 19 mg/mL MGS was successfully developed for the fruit.

Natural Polymer‐Derived Bioscaffolds for Peripheral Nerve Regeneration

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

The application of collagen in the repair of peripheral nerve defect

Collagen is a natural polymer expressed in the extracellular matrix of the peripheral nervous system. It has become increasingly crucial in peripheral nerve reconstruction as it was involved in regulating Schwann cell behaviors, maintaining peripheral nerve functions during peripheral nerve development, and being strongly upregulated after nerve injury to promote peripheral nerve regeneration. Moreover, its biological properties, such as low immunogenicity, excellent biocompatibility, and biodegradability make it a suitable biomaterial for peripheral nerve repair. Collagen provides a suitable microenvironment to support Schwann cells’ growth, proliferation, and migration, thereby improving the regeneration and functional recovery of peripheral nerves. This review aims to summarize the characteristics of collagen as a biomaterial, analyze its role in peripheral nerve regeneration, and provide a detailed overview of the recent advances concerning the optimization of collagen nerve conduits in terms of physical properties and structure, as well as the application of the combination with the bioactive component in peripheral nerve regeneration.

Electroceuticals for peripheral nerve regeneration

Electroceuticals provide promising opportunities for peripheral nerve regeneration, in terms of modulating the extensive endogenous tissue repair mechanisms between neural cell body, axons and target muscles. However, great challenges remain to deliver effective and controllable electroceuticals via bioelectronic implantable device. In this review, the modern fabrication methods of bioelectronic conduit for bridging critical nerve gaps after nerve injury are summarized, with regard to conductive materials and core manufacturing process. In addition, to deliver versatile electrical stimulation, the integration of implantable bioelectronic device is discussed, including wireless energy harvesters, actuators and sensors. Moreover, a comprehensive insight of beneficial mechanisms is presented, including up-to-date in vitro, in vivo and clinical evidence. By integrating conductive biomaterials, 3D engineering manufacturing process and bioelectronic platform to deliver versatile electroceuticals, the modern biofabrication enables comprehensive biomimetic therapies for neural tissue engineering and regeneration in the new era.

Gene delivery of chitosan-graft-polyethyleneimine vectors loaded on scaffolds for nerve regeneration

As an important transcription factor, c-Jun could upregulate growth factors expression in Schwann cells (SCs). Arginine-Glycine-Aspartate (RGD)-functionalized chitosan-graft-polyethyleneimine (RCP) gene vectors were prepared through the maleic anhydride & the carbodiimide methods, and electrostatically bound with c-Jun plasmids (pJUN), finally loaded on poly-L-lactic acid/silk fibroin parallel fiber films to fabricate nerve scaffold (RCP/pJUN-PSPF@PGA), which could locally deliver c-Jun plasmids into SCs via the mediation of RGD peptides, and upregulate the expression of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) in SCs. After the scaffold was bridged in sciatic nerve defect, the delivery of c-Jun plasmids from RCP/pJUN-PSPF@PGA facilitated SCs to sustain the expressions of NGF, BDNF and vascular endothelial growth factor in the injury field, promoting myelination, axonal growth and microvascular generation and nerve regeneration, muscle reinnervation and functional recovery. These results suggested that RCP/pDNA-PSPF@PGA, as an effective gene delivery platform, could provide a local gene therapy to improve nerve regeneration.

Collagen Modified Anisotropic PLA Scaffold as a base for Peripheral Nerve Regeneration

Reconstruction of damaged nerves remains a significant unmet challenge in clinical medicine. Topographical and mechanical stimulations play important roles to repair peripheral nerve injury. The synergistic effects of topography and mechanical rigidity may significantly accelerate nerve regeneration. In this work, we developed a nerve-guiding collagen/polylactic acid (PLA) electrospun scaffold to facilitate peripheral nerve repair. The obtained anisotropic PLA electrospun scaffolds simulated the directional arranged structure of nerve realistically and promoted axonal regeneration after sciatic nerve injury when compared with the isotropic PLA electrospun scaffolds. Moreover, the collagen-modified PLA electrospun scaffolds further provided sufficient mechanical support and favorable microenvironment for axon regeneration. In addition, we observed that collagen-modified PLA electrospun scaffolds facilitated the axon regeneration by regulating YAP molecular pathway. Taken together, we engineered collagen-modified anisotropic PLA electrospun scaffolds may be a potential candidate to combine topography and mechanical rigidity for peripheral nerve regeneration. This article is protected by copyright. All rights reserved.

Modulation of the Crosstalk between Schwann Cells and Macrophages for Nerve Regeneration: A Therapeutic Strategy Based on a Multifunctional Tetrahedral Framework Nucleic Acids System

Peripheral nerve injury (PNI) is currently recognized as one of the most significant public health issues and affects the general well-being of millions of individuals worldwide. Despite advances in nerve tissue engineering, nerve repair still cannot guarantee complete functional recovery. In the present study, an innovative approach is adopted to establish a multifunctional tetrahedral framework nucleic acids (tFNAs) system, denoted as MiDs, which can integrate the powerful programmability, permeability, and structural stability of tFNAs, with the nerve regeneration potential of microRNA-22 to enhance the communication between Schwann cells (SCs) and macrophages for more effective functional rehabilitation of peripheral nerves. Relevant results demonstrate that MiDs can amplify the ability of SCs to recruit macrophages and facilitate their polarization into the pro-healing M2 phenotype to reconstruct the post-injury microenvironment. Furthermore, MiDs can initiate the adaptive intracellular reprogramming of SCs within a short period to further promote axon regeneration and remyelination. MiDs represent a new possibility for enhancing nerve repair and may have critical clinical applications in the future.

Tailoring conductive inverse opal films with anisotropic elliptical porous patterns for nerve cell orientation

Background

The nervous system is critical to the operation of various organs and systems, while novel methods with designable neural induction remain to exploit.

Results

Here, we present a conductive inverse opal film with anisotropic elliptical porous patterns for nerve orientation induction. The films are fabricated based on polystyrene (PS) inverse opal scaffolds with periodical elliptical porous structure and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) mixed polyacrylamide (PAAm) polymers fillers. It is demonstrated that the anisotropic elliptical surface topography allows the nerve cells to be induced into orientation connected with the stretching direction. Because of the anisotropic features of the film which can be stretched into different directions, nerve cells can be induced to grow in one or two directions, forming a neural network and promoting the connection of nerve cells. It is worth mentioning that the PEDOT:PSS-doped PAAm hydrogels endow the film with conductive properties, which makes the composite films be a suitable candidate for neurites growth and differentiation.

Conclusions

All these features of the conductive and anisotropic inverse opal films imply their great prospects in biomedical applications.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01340-w.

Physiologically Self-Regulated, Fully Implantable, Battery-Free System for Peripheral Nerve Restoration

The long-segment peripheral nerve injury (PNI) represents a global medical challenge, leading to incomplete nerve tissue recovery and unsatisfactory functional reconstruction. However, the current electrical stimulation (ES) apparatuses fail perfect nerve repair due to their inability of the variable synchronous self-regulated function with physiological states. It is urgent to develop an implantable ES platform with physiologically adaptive function to provide instantaneous and nerve-preferred ES. Here, a physiologically self-regulated electrical signal is generated by integrating a novel tribo/piezoelectric hybrid nanogenerator with a nanoporous nerve guide conduit to construct a fully implantable neural electrical stimulation (FI-NES) system. The optimal neural ES parameters completely originate from the body itself and are highly self-responsive to different physiological states. The morphological evaluation, representative protein expression level, and functional reconstruction of the regenerated nerves are conducted to assess the PNI recovery process. Evidence shows that the recovery effect of 15 mm length nerve defects under the guidance of the FI-NES system is significantly close to the autograft. The designed FI-NES system provides an effective method for long-term accelerating the recovery of PNI in vivo and is also appropriate for other tissue injury or neurodegenerative diseases.

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

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

Conductive conduit based on electrospun poly (l-lactide-co-D, l-lactide) nanofibers containing 4-aminopyridine-loaded molecularly imprinted poly (methacrylic acid) nanoparticles used for peripheral nerve regeneration

Using biocompatible polymer nanofibrous conduits with a controlled drug delivery have attracted much attention for peripheral nerve regeneration. This work was aimed at preparing electrospun poly (l-lactide-co-D, l-lactide) (PLDLLA) containing multi-walled carbon nanotubes (MWCNTs) and 4-aminopyridine (4-AP)-loaded molecularly imprinted nanoparticles (MIP_4-AP) as well as evaluating their performance in in vitro and in vivo assessments. After synthesis of MIP_4-AP based on poly (methacrylic acid) with imprinting factor of 1.78, it was incorporated into the PLDLLA/MWCNTs nanofibers to optimize. By adjusting the process variables, the average diameter and electrical conductivity of the nanofibrous sample were 92 nm and 2870 × 10^-7 S cm^-1, respectively. Afterward, 4-AP release of the optimum sample showed the presence of MIP_4-AP leading to initial burst release decrease and plateau level postpone up to 96 h. Moreover, the culture results of PC12 as neuroblastoma cell line on optimal PLDLLA/MWCNTs/MIP_4-AP nanofibrous sample revealed the highest cell proliferation without cytotoxicity compared to neat nanofibers. Eventually, the animal model experiment exhibited that the conductive conduit based on the optimum sample was able to repair the rat's sciatic nerve after four weeks in accordance with sciatic function index and histological studies.

(4-Aminopyridine)-PLGA-PEG as a Novel Thermosensitive and Locally Injectable Treatment for Acute Peripheral Nerve Injury

Traumatic peripheral nerve injury (TPNI) represents a major medical problem that results in loss of motor and sensory function, and in severe cases, limb paralysis and amputation. To date, there are no effective treatments beyond surgery in selective cases. In repurposing studies, we found that daily systemic administration of the FDA-approved drug 4-aminopyridine (4-AP) enhanced functional recovery after acute peripheral nerve injury. This study was aimed at constructing a novel local delivery system of 4-AP using thermogelling polymers. We optimized a thermosensitive (4-AP)-poly(lactide-co-glycolide)-b-poly(ethylene glycol)-b-poly(lactide-co-glycolide) (PLGA-PEG-PLGA) block copolymer formulation. (4-AP)-PLGA-PEG exhibited controlled release of 4-AP both in vitro and in vivo for approximately 3 weeks, with clinically relevant safe serum levels in animals. Rheological investigation showed that (4-AP)-PLGA-PEG underwent a solution to gel transition at 32 °C, a physiologically relevant temperature, allowing us to administer it to an injured limb while subsequently forming an in situ gel. A single local administration of (4-AP)-PLGA-PEG remarkably enhanced motor and sensory functional recovery on post-sciatic nerve crush injury days 1, 3, 7, 14, and 21. Moreover, immunohistochemical studies of injured nerves treated with (4-AP)-PLGA-PEG demonstrated an increased expression of neurofilament heavy chain (NF-H) and myelin protein zero (MPZ) proteins, two major markers of nerve regeneration. These findings demonstrate that (4-AP)-PLGA-PEG may be a promising long-acting local therapeutic agent in TPNI, for which no pharmacologic treatment exists.

Conductive Hydrogel for a Photothermal-Responsive Stretchable Artificial Nerve and Coalescing with a Damaged Peripheral Nerve

Modern development of flexible electronics has made use of bioelectronic materials as artificial tissue in vivo. As hydrogels are more similar to nerve tissue, functional hydrogels have become a promising candidate for bioelectronics. Meanwhile, interfacing functional hydrogels and living tissues is at the forefront of bioelectronics. The peripheral nerve injury often leads to paralysis, chronic pain, neurologic disorders, and even disability, because it has affected the bioelectrical signal transmission between the brain and the rest of body. Here, a kind of light-stimuli-responsive and stretchable conducting polymer hydrogel (CPH) is developed to explore artificial nerve. The conductivity of CPH can be enhanced when illuminated by near-infrared light, which can promote the conduction of the bioelectrical signal. When CPH is mechanically elongated, it still has high durability of conductivity and, thus, can accommodate unexpected strain of nerve tissues in motion. Thereby, CPH can better serve as an implant of the serious peripheral nerve injury in vivo, especially in the case that the length of the missing nerve exceeds 10 mm.

High-resolution combinatorial 3D printing of gelatin-based biomimetic triple-layered conduits for nerve tissue engineering

Peripheral nerve injury is a common clinical problem often requiring surgical nerve reconstruction. To this end, tissue-engineered conduit has been proved to be crucial for nerve reconstruction. Despite its progress in recent years, the design and fabrication of translational biomimetic nerve conduits is highly challenging. Therefore, this study aims to design and fabricate mechanically-tunable nerve conduits with biomimetic structural features of the human nerve suitable for nerve tissue engineering. Herein, we employed combinatorial approach comprising of electrohydrodynamic (EHD) jet printing, dip-coating, and electrospinning techniques for fabricating triple-layered conduits. The intricate structural details were achieved via high-resolution EHD jet printed PCL filaments with tunable directionality, as the innermost layer; followed by dip coating of gelatin hydrogels to form the middle layer, and lastly, wrapped with electrospun PCL nanofibers as an outer layer of the conduits. The mechanical properties, porosity, and biocompatibility of the fabricated conduits were studied and compared with control. The results of this study confirmed that the combinatorial approach has greater potential to fabricate mechanically-tunable triple-layered conduits with favorable neuronal precursor and vascular cell compatibility.

Design, Bioanalytical, and Biomedical Applications of Aptamer-Based Hydrogels

Aptamers are special types of single-stranded DNA generated by a process called systematic evolution of ligands by exponential enrichment (SELEX). Due to significant advances in the chemical synthesis and biotechnological production, aptamers have gained considerable attention as versatile building blocks for the next generation of soft materials. Hydrogels are high water-retainable materials with a three-dimensional (3D) polymeric network. Aptamers, as a vital element, have greatly expanded the applications of hydrogels. Due to their biocompatibility, selective binding, and molecular recognition, aptamer-based hydrogels can be utilized for bioanalytical and biomedical applications. In this review, we focus on the latest strategies of aptamer-based hydrogels in bioanalytical and biomedical applications. We begin this review with an overview of the underlying design principles for the construction of aptamer-based hydrogels. Next, we will discuss some bioanalytical and biomedical applications of aptamer-based hydrogel including biosensing, target capture and release, logic devices, gene and cancer therapy. Finally, the recent progress of aptamer-based hydrogels is discussed, along with challenges and future perspectives.

Polymeric Guide Conduits for Peripheral Nerve Tissue Engineering

Peripheral nerve injuries (PNIs) are usually caused by trauma, immune diseases, and genetic factors. Peripheral nerve injury (PNI) may lead to limb numbness, muscle atrophy, and loss of neurological function. Although an abundance of theories have been proposed, very few treatments can effectively lead to complete recovery of neurological function. Autologous nerve transplantation is currently the gold standard. Nevertheless, only 50% of all patients were successfully cured using this method. In addition, it causes inevitable damage to the donor site, and available donor sites in humans are very limited. Tissue engineering has become a research hotspot aimed at achieving a better therapeutic effect from peripheral nerve regeneration. Nerve guide conduits (NGCs) show great potential in the treatment of PNI. An increasing number of scaffold materials, including natural and synthetic polymers, have been applied to fabricate NGCs for peripheral nerve regeneration. This review focuses on recent nerve guide conduit (NGC) composite scaffold materials that are applied for nerve tissue engineering. Furthermore, the development tendency of NGCs and future areas of interest are comprehensively discussed.

Additive Manufacturing of Nerve Guidance Conduits for Regeneration of Injured Peripheral Nerves

As a common and frequent clinical disease, peripheral nerve defect has caused a serious social burden, which is characterized by poor curative effect, long course of treatment and high cost. Nerve autografting is first-line treatment of peripheral nerve injuries (PNIs) but can result in loss of function of the donor site, neuroma formation, and prolonged operative time. Nerve guidance conduit (NGC) serves as the most promising alternative to autologous transplantation, but its production process is complicated and it is difficult to effectively combine growth factors and bioactive substances. In recent years, additive manufacturing of NGCs has effectively solved the above problems due to its simple and efficient manufacturing method, and it can be used as the carrier of bioactive substances. This review examines recent advances in additive manufacture of NGCs for PNIs as well as insight into how these approaches could be improved in future studies.