A biomaterials approach to peripheral nerve regeneration: bridging the peripheral nerve gap and enhancing functional recovery

Influences of physical stimulations on the migration and differentiation of Schwann cells involved in peripheral nerve repair

Peripheral nerve injury repair has always been a research concern of scientists. At the tissue level, axonal regeneration has become a research spotlight in peripheral nerve repair. Through transplantation of autologous nerve grafts or other emerging biomaterials functional recovery after facial nerve injury is not ideal in clinical scenarios. Great strides have been made to improve facial nerve repair at the micro-cellular level. Physical stimulation techniques can trigger Schwann cells (SCs) to migrate and differentiate into cells required for peripheral nerve repair. Classified by the sources of physical stimulations, SCs repair peripheral nerves through galvanotaxis, magnetotaxis and durotaxis. This article summarized the activation, directional migration and differentiation of SCs induced by physical stimulations, thus providing new ideas for the research of peripheral nerve repair.

Silk-based nerve guidance conduits with macroscopic holes modulate the vascularization of regenerating rat sciatic nerve

JOURNAL/nrgr/04.03/01300535-202506000-00029/figure1/v/2024-08-05T133530Z/r/image-tiff Peripheral nerve injuries induce a severe motor and sensory deficit. Since the availability of autologous nerve transplants for nerve repair is very limited, alternative treatment strategies are sought, including the use of tubular nerve guidance conduits (tNGCs). However, the use of tNGCs results in poor functional recovery and central necrosis of the regenerating tissue, which limits their application to short nerve lesion defects (typically shorter than 3 cm). Given the importance of vascularization in nerve regeneration, we hypothesized that enabling the growth of blood vessels from the surrounding tissue into the regenerating nerve within the tNGC would help eliminate necrotic processes and lead to improved regeneration. In this study, we reported the application of macroscopic holes into the tubular walls of silk-based tNGCs and compared the various features of these improved silk+ tNGCs with the tubes without holes (silk- tNGCs) and autologous nerve transplants in an 8-mm sciatic nerve defect in rats. Using a combination of micro-computed tomography and histological analyses, we were able to prove that the use of silk+ tNGCs induced the growth of blood vessels from the adjacent tissue to the intraluminal neovascular formation. A significantly higher number of blood vessels in the silk+ group was found compared with autologous nerve transplants and silk-, accompanied by improved axon regeneration at the distal coaptation point compared with the silk- tNGCs at 7 weeks postoperatively. In the 15-mm (critical size) sciatic nerve defect model, we again observed a distinct ingrowth of blood vessels through the tubular walls of silk+ tNGCs, but without improved functional recovery at 12 weeks postoperatively. Our data proves that macroporous tNGCs increase the vascular supply of regenerating nerves and facilitate improved axonal regeneration in a short-defect model but not in a critical-size defect model. This study suggests that further optimization of the macroscopic holes silk+ tNGC approach containing macroscopic holes might result in improved grafting technology suitable for future clinical use.

Conductive hydrogel luminal filler for peripheral nerve regeneration

Peripheral nerve injuries impair quality of life due to pain and loss of sensory and motor functions. Current treatments like autografts and nerve guidance conduits (NGCs) have limitations in functional restoration. Luminal fillers can enhance the effectiveness of NGCs by providing beneficial intraneural environments. In this study, we devised a novel injectable conductive luminal filler that allows for electrically active environments and efficient electrical stimulation of nerves. We developed injectable conductive hydrogel as a luminal filler for NGCs, composed of pluronic-coated reduced graphene oxide (rGO) and gelatin-based polymers, that gels spontaneously under physiological conditions. This filler combines nerve-like softness (0.31 ± 0.02 kPa), appropriate conductivity (2.7 ± 0.3 mS/cm), quick gelation (<5 min), and in vivo degradability. In a rat peripheral nerve defect model, the conductive hydrogel filler with electrical stimulation showed promising results in nerve regrowth, myelination, and functional recovery, performing comparably to autografts. This study underscores the potential of conductive fillers in enhancing nerve regeneration therapies.

The effects of benzimidazole and electrical stimulation on peripheral nerve regeneration after short- and long-term injury

This research investigated the effects of benzimidazole (BZ) and electrical stimulation (ES) on peripheral nerve regeneration after short- and long-term injury and assessed functional recovery by means of stereological, histological, and electrophysiological analyses. Fifty-four male albino Wistar rats were divided into nine groups of six animals each. No treatment or surgery was applied to the control (CONT) group. The sciatic nerve was crushed for 5 s in the short-term injury (STI) and for 60 s in the long-term injury (LTI) groups. In the STI + BZ group and the LTI + BZ group, the rats received 25 mg/kg/day of BZ via oral gavage for 28 days. In the STI + ES and LTI + ES groups, a 3-V current was applied for 20 min daily for 28 days. In the STI + BZ + ES group and the LTI + BZ + ES groups, 3-V ES was applied for 20 min per day for 28 days following oral administration of BZ at 25 mg/kg/day for 28 days. All groups were subjected to electrophysiological, electron microscopic, stereological, and statistical analyses. The stereological analyses revealed a significant increases in the numbers of myelinated axons in the STI + ES groups compared with the STI (p < 0.01). BZ treatment yielded no significant differences in the numbers of myelinated axons in the groups (p > 0.05). Histological evaluation of the STI and LTI groups showed that ES and BZ treatment positively affect the histological structure of the nerve.

Novel genipin-crosslinked acellular biogenic conduits for tissue engineering applications

BACKGROUND

Collagen-based conduits have been generated in-vivo stimulating a fibrotic response through the implantation of a non-resorbable material in animal models, creating biogenic substitutes. However, they often exhibit clinical limitations due to prolonged generation times, exclusive autologous use and insufficient mechanical strength. Consequently, decellularization and cross-linking could solve the aforementioned drawbacks, providing a non-immunogenic and ready-to-use natural substitute with enhanced biomechanical properties. Nevertheless, these processes may alter microarchitecture and biocompatibility. Hence, this is the first study to characterize ex-vivo the biogenic conduits of 1-and 2-months maturation time which were subjected to decellularization and genipin (GP) cross-linking procedures performing histological, structural, biomechanical, biocompatibility, and immunological analyses to identify the most suitable option for peripheral nerve regeneration.

RESULTS

Histological examination indicated consistent uniformity of the biogenic conduits at both timepoints post-implantation, maintaining their overall structural integrity and collagen pattern following decellularization and GP crosslinking treatments. Furthermore, no evidence of nuclear debris was observed in the decellularized groups at either stage of maturation, confirming the decellularization protocol's efficiency. The substitutes with longer maturation time presented a generally higher preservation of ECM key components. In addition, the GP crosslinking significantly increased the resistance values of decellularized biogenic conduits, without drastically affecting the ex-vivo cell biocompatibility nor macrophage polarization rate phenotype.

CONCLUSIONS

These findings indicate the suitability of our decellularization protocol for biogenic conduits, and subsequent crosslinking with GP improves their biomechanical properties without altering their biocompatibility or immunological profile, suggesting their potential as a ready-to-use tubular substitute for nerve and other tissue engineering applications.

PREVENTION OF SYMPTOMATIC NEUROMA BY USING SYNTHETIC CONDUITS IN FINGER AMPUTATION STUMPS

Objective

Compare the formation of symptomatic neuromas in patients submitted to digital amputations, with and without nerve conduits (Neurolac® ), and sensitivity return.

Methods

Prospective, case-control study, including 14 patients with digital amputations (total of 17 fingers) whose conduits were used on the ulnar or radial side, while the contralateral side was used in the same patients as control. The Tinel test, Semmes-Weinstein monofilament, and two-point discrimination tests were evaluated at one week, two weeks, one month, three months, and six months postoperatively.

Results

Using nerve conduits (Neurolac®) in digital nerve amputation stumps had statistical significance (p = 0.04) in preventing pain due to symptomatic neuroma at the end of six months after digital regularization.

Conclusion

There is a favorable trend towards using conduits as prophylaxis of symptomatic neuroma formation since the nerves in which they were used showed fewer clinical signs of neuroma formation six months after surgery. Level of evidence II, Prospective comparative study.

Engineering the Future of Restorative Clinical Peripheral Nerve Surgery

Peripheral nerve injury is a significant clinical challenge, often leading to permanent functional deficits. Standard interventions, such as autologous nerve grafts or distal nerve transfers, require sacrificing healthy nerve tissue and typically result in limited motor or sensory recovery. Nerve regeneration is complex and influenced by several factors: 1) the regenerative capacity of proximal neurons, 2) the ability of axons and support cells to bridge the injury, 3) the capacity of Schwann cells to maintain a supportive environment, and 4) the readiness of target muscles or sensory organs for reinnervation. Emerging bioengineering solutions, including biomaterials, drug delivery systems, fusogens, electrical stimulation devices, and tissue-engineered products, aim to address these challenges. Effective translation of these therapies requires a deep understanding of the physiology and pathology of nerve injury. This article proposes a comprehensive framework for developing restorative strategies that address all four major physiological responses in nerve repair. By implementing this framework, we envision a paradigm shift that could potentially enable full functional recovery for patients, where current approaches offer minimal hope.

E-jet 3D printed aligned nerve guidance conduits incorporated with decellularized extracellular matrix hydrogel encapsulating extracellular vesicles for peripheral nerve repair

Peripheral nerve injury (PNI) as a common clinical issue that presents significant challenges for repair. Factors such as donor site morbidity from autologous transplantation, slow recovery of long-distance nerve damage, and deficiencies in local cytokines and extracellular matrix contribute to the complexity of effective PNI treatment. It is extremely urgent to develop functional nerve guidance conduits (NGCs) as substitutes for nerve autografts. We fabricate an aligned topological scaffold by combining the E-jet 3D printing and electrospinning to exert synergistic topographical cue for peripheral nerve regeneration. To address the limitation of NGCs with hollow lumens in repairing long-distance nerve defects, we modified the internal microenvironment by filling the lumen with umbilical cord-derived decellularized extracellular matrix (dECM) hydrogels and extracellular vesicles (EVs). This approach led to the development of a functional HE-NGC. Herein, the HE-NGCs provided obvious guidance and proliferation to SCs and PC12 in vitro due to the sustained-release effect of dECM hydrogels and the outstanding proliferation-promoting role of EVs. The HE-NGCs was surgically implanted in vivo to bridge 12-mm gap sciatic nerve defect in rats and it had a satisfactory effect in reestablishment of the sciatic nerve, including the recovery of motor functions and the myelination. Further studies revealed that HE-NGCs might promoted axon growth by activating the PI3K/Akt/mTOR and inhibiting the MAPK signaling pathways. These findings indicate that HE-NGCs effectively promote nerve regeneration, offering a promising strategy for applications in peripheral nerve repair. STATEMENT OF SIGNIFICANCE: This study introduces an approach using an E-jet 3D printing system to fabricate three-dimensional aligned scaffolds with varying gap sizes, optimizing the structure for Schwann cells migration. We present, for the first time, a comprehensive investigation into the effects of EVs derived from umbilical cord mesenchymal stem cells on Schwann cells behavior. By leveraging the natural extracellular matrix (ECM), we significantly enhanced the efficacy and longevity of EVs encapsulated within a dECM hydrogel. Our provided strategy involves utilizing EVs to support nerve cell migration and proliferation along aligned NGCs. As the dECM hydrogel degrades, EVs are gradually released, facilitating the deposition of new ECM and enabling the repair of nerve defects up to 12-mm in length.

3D Porous Polycaprolactone with Chitosan-Graft-PCL Modified Surface for In Situ Tissue Engineering

Tissue engineering has emerged as a promising approach for improved regeneration of native tissue and could increase the quality of life of many patients. However, the treatment of injured tissue transitions is still in its early stages, relying primarily on a purely physical approach in medical surgery. A biodegradable implant with a modified surface that is capable of biological active protein delivery via a nanoparticulate release system could advance the field of musculoskeletal disorder treatments enormously. In this study, interconnected 3D macroporous scaffolds based on Polycaprolactone (PCL) were fabricated in a successive process of blending, annealing and leaching. Blending with varying parts of Polyethylene oxide (PEO), NaCl and (powdered) sucrose and altering processing conditions yielded scaffolds with a huge variety of morphologies. The resulting unmodified hydrophobic scaffolds were modified using two graft polymers (CS-g-PCLx) with x = 29 and 56 (x = PCL units per chitosan unit). Due to the chitosan backbone hydrophilicity was increased and a platform for a versatile nanoparticulate release system was introduced. The graft polymers were synthesized via ring opening polymerization (ROP) of ε-Caprolactone using hydroxy groups of the chitosan backbone as initiators (grafting from). The suspected impact on biocompatibility of the modification was investigated by in vitro cell testing. In addition, the CS-g-PCL modification opened up the possibility of Layer by Layer (LbL) coating with alginate (ALG) and TGF-β3-loaded chitosan tripolyphosphate (CS-TGF-β3-TPP) nanoparticles. The subsequent release study showed promising amounts of growth factor released regarding successful in vitro cell differentiation and therefore could have a possible therapeutic impact.

Neuroprotection on ischemic brain injury by Mg2+/H2 released from endovascular Mg implant

Most acute ischemic stroke patients with large vessel occlusion require stent implantation for complete recanalization. Yet, due to ischemia-reperfusion injury, over half of these patients still experience poor prognoses. Thus, neuroprotective treatment is imperative to alleviate the ischemic brain injury, and a proof-of-concept study was conducted on "biodegradable neuroprotective stent". This concept is premised on the hypothesis that locally released Mg2+/H2 from Mg metal within the bloodstream could offer synergistic neuroprotection against reperfusion injury in distant cerebral ischemic tissues. Initially, the study evaluated pure Mg's neuroactive potential using oxygen-glucose deprivation/reoxygenation (OGD/R) injured neuron cells. Subsequently, a pure Mg wire was implanted into the common carotid artery of the transient middle cerebral artery occlusion (MCAO) rat model to simulate human brain ischemia/reperfusion injury. In vitro analyses revealed that pure Mg extract aided mouse hippocampal neuronal cell (HT-22) in defending against OGD/R injury. Additionally, the protective effects of the Mg wire on behavioral abnormalities, neural injury, blood-brain barrier disruption, and cerebral blood flow reduction in MCAO rats were verified. Conclusively, Mg-based biodegradable neuroprotective implants could serve as an effective local Mg2+/H2 delivery system for treating distant cerebral ischemic diseases.

Engineered bio-functional material-based nerve guide conduits for optic nerve regeneration: a view from the cellular perspective, challenges and the future outlook

Nerve injuries can be tantamount to severe impairment, standard treatment such as the use of autograft or surgery comes with complications and confers a shortened relief. The mechanism relevant to the regeneration of the optic nerve seems yet to be fully uncovered. The prevailing rate of vision loss as a result of direct or indirect insult on the optic nerve is alarming. Currently, the use of nerve guide conduits (NGC) to some extent has proven reliable especially in rodents and among the peripheral nervous system, a promising ground for regeneration and functional recovery, however in the optic nerve, this NGC function seems quite unfamous. The insufficient NGC application and the unabridged regeneration of the optic nerve could be a result of the limited information on cellular and molecular activities. This review seeks to tackle two major factors (i) the cellular and molecular activity involved in traumatic optic neuropathy and (ii) the NGC application for the optic nerve regeneration. The understanding of cellular and molecular concepts encompassed, ocular inflammation, extrinsic signaling and intrinsic signaling for axon growth, mobile zinc role, Ca2+ factor associated with the optic nerve, alternative therapies from nanotechnology based on the molecular information and finally the nanotechnological outlook encompassing applicable biomaterials and the use of NGC for regeneration. The challenges and future outlook regarding optic nerve regenerations are also discussed. Upon the many approaches used, the comprehensive role of the cellular and molecular mechanism may set grounds for the efficient application of the NGC for optic nerve regeneration.

[Research progress of silk-based biomaterials for peripheral nerve regeneration]

Objective

To describe the research progress of silk-based biomaterials in peripheral nerve repair and provide useful ideals to accelerate the regeneration of large-size peripheral nerve injury.

Methods

The relative documents about silk-based biomaterials used in peripheral nerve regeneration were reviewed and the different strategies that could accelerate peripheral nerve regeneration through building bioactive microenvironment with silk fibroin were discussed.

Results

Many silk fibroin tissue engineered nerve conduits have been developed to provide multiple biomimetic microstructures, and different microstructures have different mechanisms of promoting nerve repair. Biomimetic porous structures favor the nutrient exchange at wound sites and inhibit the invasion of scar tissue. The aligned structures can induce the directional growth of nerve tissue, while the multiple channels promote the axon elongation. When the fillers are introduced to the conduits, better growth, migration, and differentiation of nerve cells can be achieved. Besides biomimetic structures, different nerve growth factors and bioactive drugs can be loaded on silk carriers and released slowly at nerve wounds, providing suitable biochemical cues. Both the biomimetic structures and the loaded bioactive ingredients optimize the niches of peripheral nerves, resulting in quicker and better nerve repair. With silk biomaterials as a platform, fusing multiple ways to achieve the multidimensional regulation of nerve microenvironments is becoming a critical strategy in repairing large-size peripheral nerve injury.

Conclusion

Silk-based biomaterials are useful platforms to achieve the design of biomimetic hierarchical microstructures and the co-loading of various bioactive ingredients. Silk fibroin nerve conduits provide suitable microenvironment to accelerate functional recovery of peripheral nerves. Different optimizing strategies are available for silk fibroin biomaterials to favor the nerve regeneration, which would satisfy the needs of various nerve tissue repair. Bioactive silk conduits have promising future in large-size peripheral nerve regeneration.

Development of Biomaterials for Addressing Upper Extremity Peripheral Nerve Gaps

Peripheral nerve injuries within the upper extremities can lead to impaired function and reduced quality of life. Although autografts have traditionally served as the primary therapeutic approach to bridge nerve gaps, these present challenges related to donor site morbidity. This review delves into the realm of biomaterials tailored for addressing nerve gaps. Biomaterials, whether natural or synthetically derived, offer the potential not only to act as scaffolds for nerve regeneration but also to be enhanced with growth factors and agents that promote nerve recovery. The historical progression of these biomaterials as well as their current applications, advantages, inherent challenges, and future impact in the arena of regenerative medicine are discussed. By providing a comprehensive overview, we aim to shed light on the transformative potential of biomaterials in peripheral nerve repair and the path toward refining their efficacy in clinical settings.

Bioelectronics for electrical stimulation: materials, devices and biomedical applications

Bioelectronics is a hot research topic, yet an important tool, as it facilitates the creation of advanced medical devices that interact with biological systems to effectively diagnose, monitor and treat a broad spectrum of health conditions. Electrical stimulation (ES) is a pivotal technique in bioelectronics, offering a precise, non-pharmacological means to modulate and control biological processes across molecular, cellular, tissue, and organ levels. This method holds the potential to restore or enhance physiological functions compromised by diseases or injuries by integrating sophisticated electrical signals, device interfaces, and designs tailored to specific biological mechanisms. This review explains the mechanisms by which ES influences cellular behaviors, introduces the essential stimulation principles, discusses the performance requirements for optimal ES systems, and highlights the representative applications. From this review, we can realize the potential of ES based bioelectronics in therapy, regenerative medicine and rehabilitation engineering technologies, ranging from tissue engineering to neurological technologies, and the modulation of cardiovascular and cognitive functions. This review underscores the versatility of ES in various biomedical contexts and emphasizes the need to adapt to complex biological and clinical landscapes it addresses.

Advancements in stimulation therapies for peripheral nerve regeneration

Soft-tissue injuries affecting muscles, nerves, vasculature, tendons, and ligaments often diminish the quality of life due to pain, loss of function, and financial burdens. Both natural healing and surgical interventions can result in scarring, which potentially may impede functional recovery and lead to persistent pain. Scar tissue, characterized by a highly disorganized fibrotic extracellular matrix, may serve as a physical barrier to regeneration and drug delivery. While approaches such as drugs, biomaterials, cells, external stimulation, and other physical forces show promise in mitigating scarring and promoting regenerative healing, their implementation remains limited and challenging. Ultrasound, laser, electrical, and magnetic forms of external stimulation have been utilized to promote soft tissue as well as neural tissue regeneration. After stimulation, neural tissues experience increased proliferation of Schwann cells, secretion of neurotropic factors, production of myelin, and growth of vasculature, all aimed at supporting axon regeneration and innervation. Yet, the outcomes of healing vary depending on the pathophysiology of the damaged nerve, the timing of stimulation following injury, and the specific parameters of stimulation employed. Increased treatment intensity and duration have been noted to hinder the healing process by inducing tissue damage. These stimulation modalities, either alone or in combination with nerve guidance conduits and scaffolds, have been demonstrated to promote healing. However, the literature currently lacks a detailed understanding of the stimulation parameters used for nerve healing applications. In this article, we aim to address this gap by summarizing existing reports and providing an overview of stimulation parameters alongside their associated healing outcomes.

A Gelatin/Alginate Double Network Hydrogel Nerve Guidance Conduit Fabricated by a Chemical-Free Gamma Radiation for Peripheral Nerve Regeneration

Nerve guidance conduits (NGCs) are widely developed using various materials for the functional repair of injured or diseased peripheral nerves. Especially, hydrogels are considered highly suitable for the fabrication of NGCs due to their beneficial tissue-mimicking characteristics (e.g., high water content, softness, and porosity). However, the practical applications of hydrogel-based NGCs are hindered due to their poor mechanical properties and complicated fabrication processes. To bridge this gap, a novel double-network (DN) hydrogel using alginate and gelatin by a two-step crosslinking process involving chemical-free gamma irradiation and ionic crosslinking, is developed. DN hydrogels (1% alginate and 15% gelatin), crosslinked with 30 kGy gamma irradiation and barium ions, exhibit substantially improved mechanical properties, including tensile strength, elastic modulus, and fracture stain, compared to single network (SN) gelatin hydrogels. Additionally, the DN hydrogel NGC exhibits excellent kink resistance, mechanical stability to successive compression, suture retention, and enzymatic degradability. In vivo studies with a sciatic defect rat model indicate substantially improved nerve function recovery with the DN hydrogel NGC compared to SN gelatin and commercial silicone NGCs, as confirm footprint analysis, electromyography, and muscle weight measurement. Histological examination reveals that, in the DN NGC group, the expression of Schwann cell and neuronal markers, myelin sheath, and exon diameter are superior to the other controls. Furthermore, the DN NGC group demonstrates increased muscle fiber formation and reduced fibrotic scarring. These findings suggest that the mechanically robust, degradable, and biocompatible DN hydrogel NGC can serve as a novel platform for peripheral nerve regeneration and other biomedical applications, such as implantable tissue constructs.

Biohacking Nerve Repair: Novel Biomaterials, Local Drug Delivery, Electrical Stimulation, and Allografts to Aid Surgical Repair

The regenerative capacity of the peripheral nervous system is limited, and peripheral nerve injuries often result in incomplete healing and poor outcomes even after repair. Transection injuries that induce a nerve gap necessitate microsurgical intervention; however, even the current gold standard of repair, autologous nerve graft, frequently results in poor functional recovery. Several interventions have been developed to augment the surgical repair of peripheral nerves, and the application of functional biomaterials, local delivery of bioactive substances, electrical stimulation, and allografts are among the most promising approaches to enhance innate healing across a nerve gap. Biocompatible polymers with optimized degradation rates, topographic features, and other functions provided by their composition have been incorporated into novel nerve conduits (NCs). Many of these allow for the delivery of drugs, neurotrophic factors, and whole cells locally to nerve repair sites, mitigating adverse effects that limit their systemic use. The electrical stimulation of repaired nerves in the perioperative period has shown benefits to healing and recovery in human trials, and novel biomaterials to enhance these effects show promise in preclinical models. The use of acellular nerve allografts (ANAs) circumvents the morbidity of donor nerve harvest necessitated by the use of autografts, and improvements in tissue-processing techniques may allow for more readily available and cost-effective options. Each of these interventions aid in neural regeneration after repair when applied independently, and their differing forms, benefits, and methods of application present ample opportunity for synergistic effects when applied in combination.

Advances in 3D tissue models for neural engineering: self-assembled versus engineered tissue models

Neural tissue engineering has emerged as a promising field that aims to create functional neural tissue for therapeutic applications, drug screening, and disease modelling. It is becoming evident in the literature that this goal requires development of three-dimensional (3D) constructs that can mimic the complex microenvironment of native neural tissue, including its biochemical, mechanical, physical, and electrical properties. These 3D models can be broadly classified as self-assembled models, which include spheroids, organoids, and assembloids, and engineered models, such as those based on decellularized or polymeric scaffolds. Self-assembled models offer advantages such as the ability to recapitulate neural development and disease processes in vitro, and the capacity to study the behaviour and interactions of different cell types in a more realistic environment. However, self-assembled constructs have limitations such as lack of standardised protocols, inability to control the cellular microenvironment, difficulty in controlling structural characteristics, reproducibility, scalability, and lengthy developmental timeframes. Integrating biomimetic materials and advanced manufacturing approaches to present cells with relevant biochemical, mechanical, physical, and electrical cues in a controlled tissue architecture requires alternate engineering approaches. Engineered scaffolds, and specifically 3D hydrogel-based constructs, have desirable properties, lower cost, higher reproducibility, long-term stability, and they can be rapidly tailored to mimic the native microenvironment and structure. This review explores 3D models in neural tissue engineering, with a particular focus on analysing the benefits and limitations of self-assembled organoids compared with hydrogel-based engineered 3D models. Moreover, this paper will focus on hydrogel based engineered models and probe their biomaterial components, tuneable properties, and fabrication techniques that allow them to mimic native neural tissue structures and environment. Finally, the current challenges and future research prospects of 3D neural models for both self-assembled and engineered models in neural tissue engineering will be discussed.

Chiral MoS2@BC fibrous membranes selectively promote peripheral nerve regeneration

BACKGROUND

Molybdenum disulfide (MoS2) has excellent physical and chemical properties. Further, chiral MoS2 (CMS) exhibits excellent chiroptical and enantioselective effects, and the enantioselective properties of CMS have been studied for the treatment of neurodegenerative diseases. Intriguingly, left- and right-handed materials have different effects on promoting the differentiation of neural stem cells into neurons. However, the effect of the enantioselectivity of chiral materials on peripheral nerve regeneration remains unclear.

METHODS

In this study, CMS@bacterial cellulose (BC) scaffolds were fabricated using a hydrothermal approach. The CMS@BC films synthesized with L-2-amino-3-phenyl-1-propanol was defined as L-CMS. The CMS@BC films synthesized with D-2-amino-3-phenyl-1-propanol was defined as D-CMS. The biocompatibility of CMS@BC scaffolds and their effect on Schwann cells (SCs) were validated by cellular experiments. In addition, these scaffolds were implanted in rat sciatic nerve defect sites for three months.

RESULTS

These chiral scaffolds displayed high hydrophilicity, good mechanical properties, and low cytotoxicity. Further, we found that the L-CMS scaffolds were superior to the D-CMS scaffolds in promoting SCs proliferation. After three months, the scaffolds showed good biocompatibility in vivo, and the nerve conducting velocities of the L-CMS and D-CMS scaffolds were 51.2 m/s and 26.8 m/s, respectively. The L-CMS scaffolds showed a better regenerative effect than the D-CMS scaffolds. Similarly, the sciatic nerve function index and effects on the motor and electrophysiological functions were higher for the L-CMS scaffolds than the D-CMS scaffolds. Finally, the axon diameter and myelin sheath thickness of the regenerated nerves were improved in the L-CMS group.

CONCLUSION

We found that the CMS@BC can promote peripheral nerve regeneration, and in general, the L-CMS group exhibited superior repair performance. Overall, the findings of this study reveal that CMS@BC can be used as a chiral nanomaterial nerve scaffold for peripheral nerve repair.

ADSCs encapsulated in Gelatin methacrylate substrate promotes the repair of peripheral nerve injury by SIRT6/PGC-1α pathway

Peripheral nerve injury is a prevalent disease but the spontaneous recovery of nerve function is protracted and incomplete. Given the damaging of stem cells and fragile of intra-neural structures in the course of stem cell transplantation, our study tried to investigate whether encapsulating adipose derived mesenchymal stem cells (ADSCs) with GelMA could achieve better repair in peripheral nerve injury. PC-12 cells were cultured on the surface of GelMA encapsulating ADSCs and 3D co-culture system was constructed. CCK-8, Real-Time PCR, ELISA, Immunofluorescent Assay and Western Blot were used to evaluate the functionality of this system. Ultimately, nerve conduit containing the 3D co-culture system was linked between the two ends of an injured nerve. ADSCs encapsulated in 5% GeIMA had a better activity than 10% GeIMA. Furthermore, the viability of PC-12 cells was also better in this 3D co-culture system than in co-culture system with ADSCs without GeIMA. The expression of SIRT6 and PGC-1α in PC-12 cells were prominently promoted, and the entry to nuclear of PGC-1α was more obvious in this 3D co-culture system. After silencing of SIRT6, the protein expression level of PGC-1α was inhibited, and the activity of PC-12 cells was significantly reduced, suggesting that ADSCs encapsulated in GelMA upregulated the expression of SIRT6 to induce the level of PGC-1α protein, thereby achieving an impact on the activity of PC-12 cells. In vivo, nerve conduit containing the 3D co-culture system significantly promoted the repair of damaged peripheral nerves. In conclusion, our study demonstrated that 5% GelMA enhanced ADSCs activity, thereby promoting the activity of nerve cells and repair of damaged peripheral nerves by SIRT6/PGC-1α pathway.

Cellular and Transcriptional Response of Human Astrocytes to Hybrid Protein Materials

Collagen is a major component of the tissue matrix, and soybean can regulate the tissue immune response. Both materials have been used to fabricate biomaterials for tissue repair. In this study, adult and fetal human astrocytes were grown in a soy protein isolate (SPI)-collagen hybrid gel or on the surface of a cross-linked SPI-collagen membrane. Hybrid materials reduced the cell proliferation rate compared to materials generated by collagen alone. However, the hybrid materials did not significantly change the cell motility compared to the control collagen material. RNA-sequencing (RNA-Seq) analysis showed downregulated genes in the cell cycle pathway, including CCNA2, CCNB1, CCNB2, CCND1, CCND2, and CDK1, which may explain lower cell proliferation in the hybrid material. This study also revealed the downregulation of genes encoding extracellular matrix (ECM) components, including HSPG2, LUM, SDC2, COL4A1, COL4A5, COL4A6, and FN1, as well as genes encoding chemokines, including CCL2, CXCL1, CXCL2, CX3CL1, CXCL3, and LIF, for adult human astrocytes grown on the hybrid membrane compared with those grown on the control collagen membrane. The study explored the cellular and transcriptional responses of human astrocytes to the hybrid material and indicated a potential beneficial function of the material in the application of neural repair.

Material matters: Degradation products affect regenerating Schwann cells

Devices to treat peripheral nerve injury (PNI) must balance many considerations to effectively guide regenerating nerves across a gap and achieve functional recovery. To enhance efficacy, design features like luminal fillers have been explored extensively. Material choice for PNI devices is also critical, as the determining factor of device mechanics, and degradation rate and has increasingly been found to directly impact biological response. This study investigated the ways in which synthetic polymer materials impact the differentiation state and myelination potential of Schwann cells, peripheral nerve glia. Microporous substrates of polycaprolactone (PCL), poly(lactide-co-glycolide) (PLGA) 85:15, or PLGA 50:50 were chosen, as materials already used in nerve repair devices, representing a wide range of mechanics and degradation profiles. Schwann cells co-cultured with dorsal root ganglion (DRG) neurons on the substrates expressed more mature myelination proteins (MPZ) on PLGA substrates compared to PCL. Changes to myelination and differentiation state of glia were reflected in adhesion proteins expressed by glia, including β-dystroglycan and integrin α6, both laminin binding proteins. Importantly, degradation products of the polymers affected glial expression independently of direct attachment. Fast degrading PLGA 50:50 substrates released measurable amounts of degradation products (lactic acid) within the culture period, which may push Schwann cells towards glycolytic metabolism, decreasing expression of early transcription factors like sox10. This study shows the importance of understanding not only material effects on attachment, but also on cellular metabolism which drives myelination responses.

Innovations in Peripheral Nerve Regeneration

The field of peripheral nerve regeneration is a dynamic and rapidly evolving area of research that continues to captivate the attention of neuroscientists worldwide. The quest for effective treatments and therapies to enhance the healing of peripheral nerves has gained significant momentum in recent years, as evidenced by the substantial increase in publications dedicated to this field. This surge in interest reflects the growing recognition of the importance of peripheral nerve recovery and the urgent need to develop innovative strategies to address nerve injuries. In this context, this article aims to contribute to the existing knowledge by providing a comprehensive review that encompasses both biomaterial and clinical perspectives. By exploring the utilization of nerve guidance conduits and pharmacotherapy, this article seeks to shed light on the remarkable advancements made in the field of peripheral nerve regeneration. Nerve guidance conduits, which act as artificial channels to guide regenerating nerves, have shown promising results in facilitating nerve regrowth and functional recovery. Additionally, pharmacotherapy approaches have emerged as potential avenues for promoting nerve regeneration, with various therapeutic agents being investigated for their neuroprotective and regenerative properties. The pursuit of advancing the field of peripheral nerve regeneration necessitates persistent investment in research and development. Continued exploration of innovative treatments, coupled with a deeper understanding of the intricate processes involved in nerve regeneration, holds the promise of unlocking the complete potential of these groundbreaking interventions. By fostering collaboration among scientists, clinicians, and industry partners, we can accelerate progress in this field, bringing us closer to the realization of transformative therapies that restore function and quality of life for individuals affected by peripheral nerve injuries.

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

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

Biodegradable Electrospun Conduit with Aligned Fibers Based on Poly(lactic-co-glycolic Acid) (PLGA)/Carbon Nanotubes and Choline Bitartrate Ionic Liquid

Functionally active aligned fibers are a promising approach to enhance neuro adhesion and guide the extension of neurons for peripheral nerve regeneration. Therefore, the present study developed poly(lactic-co-glycolic acid) (PLGA)-aligned electrospun mats and investigated the synergic effect with carbon nanotubes (CNTs) and Choline Bitartrate ionic liquid (Bio-IL) on PLGA fibers. Morphology, thermal, and mechanical performances were determined as well as the hydrolytic degradation and the cytotoxicity. Results revealed that electrospun mats are composed of highly aligned fibers, and CNTs were aligned and homogeneously distributed into the fibers. Bio-IL changed thermal transition behavior, reduced glass transition temperature (Tg), and favored crystal phase formation. The mechanical properties increased in the presence of CNTs and slightly decreased in the presence of the Bio-IL. The results demonstrated a decrease in the degradation rate in the presence of CNTs, whereas the use of Bio-IL led to an increase in the degradation rate. Cytotoxicity results showed that all the electrospun mats display metabolic activity above 70%, which demonstrates that they are biocompatible. Moreover, superior biocompatibility was observed for the electrospun containing Bio-IL combined with higher amounts of CNTs, showing a high potential to be used in nerve tissue engineering.

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.

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.

Combination of stem cells and nerve guide conduit for the treatment of peripheral nerve injury: A meta-analysis

INTRODUCTION/AIMS

Many small-sized, single-center preclinical studies have investigated the benefits of introducing stem cells into the interior of nerve conduit. The aims of this meta-analysis are to review and contrast the effects of various types of stem cells in in vivo models used to reconstruct peripheral nerve injuries (PNIs) and to assess the reliability and stability of the available evidence.

METHODS

A systematic search was conducted using Cochrane Library, Embase, PubMed, and Web of Science to identify studies conducted from January 1, 2000, to September 21, 2022, and investigate stem cell therapy in peripheral nerve reconstruction animal models. Studies that met the relevant criteria were deemed eligible for this meta-analysis.

RESULTS

Fifty-five preclinical studies with a total of 1234 animals were incorporated. Stem cells demonstrated a positive impact on peripheral nerve regeneration at different follow-up times in the forest plots of five outcome indicators: compound muscle action potential (CMAP) amplitude, latency, muscle mass ratio, nerve conduction velocity, and sciatic functional index (SFI). In most comparisons, stem cell groups showed substantial differences compared with the control groups. The superior performance of adipose-derived stem cells (ADSCs) in terms of SFI, CMAP amplitude, and latency (p < .001) was identified.

DISCUSSION

The findings consistently demonstrated a favorable outcome in the reconstruction process when utilizing different groups of stem cells, as opposed to control groups where stem cells were not employed.

The effectiveness of acellular nerve allografts compared to autografts in animal models: A systematic review and meta-analysis

BACKGROUND

Treatment of nerve injuries proves to be a worldwide clinical challenge. Acellular nerve allografts are suggested to be a promising alternative for bridging a nerve gap to the current gold standard, an autologous nerve graft.

OBJECTIVE

To systematically review the efficacy of the acellular nerve allograft, its difference from the gold standard (the nerve autograft) and to discuss its possible indications.

MATERIAL AND METHODS

PubMed, Embase and Web of Science were systematically searched until the 4th of January 2022. Original peer reviewed paper that presented 1) distinctive data; 2) a clear comparison between not immunologically processed acellular allografts and autologous nerve transfers; 3) was performed in laboratory animals of all species and sex. Meta analyses and subgroup analyses (for graft length and species) were conducted for muscle weight, sciatic function index, ankle angle, nerve conduction velocity, axon count diameter, tetanic contraction and amplitude using a Random effects model. Subgroup analyses were conducted on graft length and species.

RESULTS

Fifty articles were included in this review and all were included in the meta-analyses. An acellular allograft resulted in a significantly lower muscle weight, sciatic function index, ankle angle, nerve conduction velocity, axon count and smaller diameter, tetanic contraction compared to an autologous nerve graft. No difference was found in amplitude between acellular allografts and autologous nerve transfers. Post hoc subgroup analyses of graft length showed a significant reduced muscle weight in long grafts versus small and medium length grafts. All included studies showed a large variance in methodological design.

CONCLUSION

Our review shows that the included studies, investigating the use of acellular allografts, showed a large variance in methodological design and are as a consequence difficult to compare. Nevertheless, our results indicate that treating a nerve gap with an allograft results in an inferior nerve recovery compared to an autograft in seven out of eight outcomes assessed in experimental animals. In addition, based on our preliminary post hoc subgroup analyses we suggest that when an allograft is being used an allograft in short and medium (0-1cm, > 1-2cm) nerve gaps is preferred over an allograft in long (> 2cm) nerve gaps.

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.

A multi-channel collagen conduit with aligned Schwann cells and endothelial cells for enhanced neuronal regeneration in spinal cord injury

Traumatic spinal cord injury (SCI) leads to Wallerian degeneration and the accompanying disruption of vasculature leads to ischemia, which damages motor and sensory function. Therefore, understanding the biological environment during regeneration is essential to promote neuronal regeneration and overcome this phenomenon. The band of Büngner is a structure of an aligned Schwann cell (SC) band that guides axon elongation providing a natural recovery environment. During axon elongation, SCs promote axon elongation while migrating along neovessels (endothelial cells [ECs]). To model this, we used extrusion 3D bioprinting to develop a multi-channel conduit (MCC) using collagen for the matrix region and sacrificial alginate to make the channel. The MCC was fabricated with a structure in which SCs and ECs were longitudinally aligned to mimic the sophisticated recovering SCI conditions. Also, we produced an MCC with different numbers of channels. The aligned SCs and ECs in the 9-channel conduit (9MCC-SE) were more biocompatible and led to more proliferation than the 5-channel conduit (5MCC-SE) in vitro. Also, the 9MCC-SE resulted in a greater healing effect than the 5MCC-SE with respect to neuronal regeneration, remyelination, inflammation, and angiogenesis in vivo. The above tissue recovery results led to motor function repair. Our results show that our 9MCC-SE model represents a new therapeutic strategy for SCI.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

CLINICAL RELEVANCE STATEMENT

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

Longitudinally aligned inner-patterned silk fibroin conduits for peripheral nerve regeneration

Peripheral nerve injuries represent a major clinical challenge, if nerve ends retract, there is no spontaneous regeneration, and grafts are required to proximate the nerve ends and give continuity to the nerve. The nerve guidance conduits (NGCs) presented in this work are silk fibroin (SF)-based, which is biocompatible and very versatile. The formation of conduits is obtained by forming a covalently cross-linked hydrogel in two concentric moulds, and the inner longitudinally aligned pattern of the SF NGCs is obtained through the use of a patterned inner mould. SF NGCs with two wall thicknesses of ~ 200 to ~ 400 μm are synthesized. Their physicochemical and mechanical characteristics have shown improved properties when the wall thickness is thicker such as resistance to kinking, which is of special importance as conduits might also be used to substitute nerves in flexible body parts. The Young modulus is higher for conduits with inner pattern, and none of the conduits has shown any salt deposition in presence of simulated body fluid, meaning they do not calcify; thus, the regeneration does not get impaired when conduits have contact with body fluids. In vitro studies demonstrated the biocompatibility of the SF NGCs; proliferation is enhanced when iSCs are cultured on top of conduits with longitudinally aligned pattern. BJ fibroblasts cannot infiltrate through the SF wall, avoiding scar tissue formation on the lumen of the graft when used in vivo. These conduits have been demonstrated to be very versatile and fulfil with the requirements for their use in PNR.

Supplementary Information

The online version contains supplementary material available at 10.1007/s44164-023-00050-3.

Aminosilane Functionalized Aligned Fiber PCL Scaffolds for Peripheral Nerve Repair

Silane modification is a simple and cost-effective tool to modify existing biomaterials for tissue engineering applications. Aminosilane layer deposition has previously been shown to control NG108-15 neuronal cell and primary Schwann cell adhesion and differentiation by controlling deposition of ─NH2 groups at the submicron scale across the entirety of a surface by varying silane chain length. This is the first study toreport depositing 11-aminoundecyltriethoxysilane (CL11) onto aligned Polycaprolactone (PCL) scaffolds for peripheral nerve regeneration. Fibers are manufactured via electrospinning and characterized using water contact angle measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Confirmed modified fibers are investigated using in vitro cell culture of NG108-15 neuronal cells and primary Schwann cells to determine cell viability, cell differentiation, and phenotype. CL11-modified fibers significantly support NG108-15 neuronal cell and Schwann cell viability. NG108-15 neuronal cell differentiation maintains Schwann cell phenotype compared to unmodified PCL fiber scaffolds. 3D ex vivo culture of Dorsal root ganglion explants (DRGs) confirms further Schwann cell migration and longer neurite outgrowth from DRG explants cultured on CL11 fiber scaffolds compared to unmodified scaffolds. Thus, a reproducible and cost-effective tool is reported to modify biomaterials with functional amine groups that can significantly improve nerve guidance devices and enhance nerve regeneration.

Fabrication of Multi-Channel Nerve Guidance Conduits Containing Schwann Cells Based on Multi-Material 3D Bioprinting

Nerve guidance conduits (NGCs) are an essential solution for peripheral nerve repair and regeneration in tissue engineering and medicine. However, the ability of current NGCs is limited to repairing longer nerve gap (i.e., >20 mm) because it cannot meet the following two conditions simultaneously: (1) directional guidance of the axial high-density channels and (2) regenerative stimulation of the extracellular matrix secreted by Schwann cells (SCs). Therefore, we propose a multi-material 3D bioprinting process to fabricate multi-channel nerve guide conduits (MNGCs) containing SCs. In the article, cell-laden methacrylate gelatin (GelMA) was used as the bulk material of MNGCs. To improve the printing accuracy of the axial channels and the survival rate of SCs, we systematically optimized the printing temperature parameter based on hydrogel printability analysis. The multi-material bioprinting technology was used to realize the alternate printing of supporting gelatin and cell-laden GelMA. Then, the high-accuracy channels were fabricated through the UV cross-linking of GelMA and the dissolving technique of gelatin. The SCs distributed around the channels with a high survival rate, and the cell survival rate maintained above 90%. In general, the study on multi-material 3D printing was carried out from the fabricating technology and material analysis, which will provide a potential solution for the fabrication of MNGCs containing SCs.

Cell-free therapy based on extracellular vesicles: a promising therapeutic strategy for peripheral nerve injury

Peripheral nerve injury (PNI) is one of the public health concerns that can result in a loss of sensory or motor function in the areas in which injured and non-injured nerves come together. Up until now, there has been no optimized therapy for complete nerve regeneration after PNI. Exosome-based therapies are an emerging and effective therapeutic strategy for promoting nerve regeneration and functional recovery. Exosomes, as natural extracellular vesicles, contain bioactive molecules for intracellular communications and nervous tissue function, which could overcome the challenges of cell-based therapies. Furthermore, the bioactivity and ability of exosomes to deliver various types of agents, such as proteins and microRNA, have made exosomes a potential approach for neurotherapeutics. However, the type of cell origin, dosage, and targeted delivery of exosomes still pose challenges for the clinical translation of exosome therapeutics. In this review, we have focused on Schwann cell and mesenchymal stem cell (MSC)-derived exosomes in nerve tissue regeneration. Also, we expressed the current understanding of MSC-derived exosomes related to nerve regeneration and provided insights for developing a cell-free MSC therapeutic strategy for nerve injury.

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

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

Recent advances and future directions of 3D to 6D printing in brain cancer treatment and neural tissue engineering

The field of neural tissue engineering has undergone a revolution due to advancements in three-dimensional (3D) printing technology. This technology now enables the creation of intricate neural tissue constructs with precise geometries, topologies, and mechanical properties. Currently, there are various 3D printing techniques available, such as stereolithography and digital light processing, and a wide range of materials can be utilized, including hydrogels, biopolymers, and synthetic materials. Furthermore, the development of four-dimensional (4D) printing has gained traction, allowing for the fabrication of structures that can change shape over time using techniques such as shape-memory polymers. These innovations have the potential to facilitate neural regeneration, drug screening, disease modeling, and hold tremendous promise for personalized diagnostics, precise therapeutic strategies against brain cancers. This review paper provides a comprehensive overview of the current state-of-the-art techniques and materials for 3D printing in neural tissue engineering and brain cancer. It focuses on the exciting possibilities that lie ahead, including the emerging field of 4D printing. Additionally, the paper discusses the potential applications of five-dimensional and six-dimensional printing, which integrate time and biological functions into the printing process, in the fields of neuroscience.

Electroceuticals for Regeneration of Long Nerve Gap Using Biodegradable Conductive Conduits and Implantable Wireless Stimulator

Regeneration of over 10 mm long peripheral nerve defects remains a challenge due to the failure of regeneration by prolonged axotomy and denervation occurring in long-term recovery. Recent studies reveal that conductive conduits and electrical stimulation accelerate the regeneration of long nerve defects. In this study, an electroceutical platform combining a fully biodegradable conductive nerve conduit and a wireless electrical stimulator is proposed to maximize the therapeutic effect on nerve regeneration. Fully biodegradable nerve conduit fabricated using molybdenum (Mo) microparticles and polycaprolactone (PCL) can eliminate the unwanted effects of non-degradable implants, which occupy nerve paths and need to be removed through surgery increasing the risk of complications. The electrical and mechanical properties of Mo/PCL conduits are optimized by controlling the amounts of Mo and tetraglycol lubricant. The dissolution behavior and electrical conductivity of biodegradable nerve conduits in the biomimetic solutions are also evaluated. In in vivo experiments, the integrated strategy of a conductive Mo/PCL conduit with controlled therapeutic electrical stimulation shows accelerated axon regeneration for long sciatic nerve defects in rats compared to the use of the Mo/PCL conduit without stimulation and has a significant therapeutic effect based on the results obtained from the functional recovery test.

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.

Engineered hydrogels for peripheral nerve repair

Peripheral nerve injury (PNI) is a complex disease that often appears in young adults. It is characterized by a high incidence, limited treatment options, and poor clinical outcomes. This disease not only causes dysfunction and psychological disorders in patients but also brings a heavy burden to the society. Currently, autologous nerve grafting is the gold standard in clinical treatment, but complications, such as the limited source of donor tissue and scar tissue formation, often further limit the therapeutic effect. Recently, a growing number of studies have used tissue-engineered materials to create a natural microenvironment similar to the nervous system and thus promote the regeneration of neural tissue and the recovery of impaired neural function with promising results. Hydrogels are often used as materials for the culture and differentiation of neurogenic cells due to their unique physical and chemical properties. Hydrogels can provide three-dimensional hydration networks that can be integrated into a variety of sizes and shapes to suit the morphology of neural tissues. In this review, we discuss the recent advances of engineered hydrogels for peripheral nerve repair and analyze the role of several different therapeutic strategies of hydrogels in PNI through the application characteristics of hydrogels in nerve tissue engineering (NTE). Furthermore, the prospects and challenges of the application of hydrogels in the treatment of PNI are also discussed.

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.

Submicron-Grooved Films Modulate the Directional Alignment and Biological Function of Schwann Cells

Topographical cues on material surfaces are crucial for guiding the behavior of nerve cells and facilitating the repair of peripheral nerve defects. Previously, micron-grooved surfaces have shown great potential in controlling nerve cell alignment for studying the behavior and functions of those cells and peripheral nerve regeneration. However, the effects of smaller-sized topographical cues, such as those in the submicron- and nano-scales, on Schwann cell behavior remain poorly understood. In this study, four different submicron-grooved polystyrene films (800/400, 800/100, 400/400, and 400/100) were fabricated to study the behavior, gene expression, and membrane potential of Schwann cells. The results showed that all submicron-grooved films could guide the cell alignment and cytoskeleton in a groove depth-dependent manner. Cell proliferation and cell cycle assays revealed that there was no significant difference between the submicron groove samples and the flat control. However, the submicron grooves can direct the migration of cells and upregulate the expression of critical genes in axon regeneration and myelination (e.g., MBP and Smad6). Finally, the membrane potential of the Schwann cells was significantly altered on the grooved sample. In conclusion, this study sheds light on the role of submicron-grooved patterns in regulating the behavior and function of Schwann cells, which provides unique insights for the development of implants for peripheral nerve regeneration.

Conductive fibers for biomedical applications

Bioelectricity has been stated as a key factor in regulating cell activity and tissue function in electroactive tissues. Thus, various biomedical electronic constructs have been developed to interfere with cell behaviors to promote tissue regeneration, or to interface with cells or tissue/organ surfaces to acquire physiological status via electrical signals. Benefiting from the outstanding advantages of flexibility, structural diversity, customizable mechanical properties, and tunable distribution of conductive components, conductive fibers are able to avoid the damage-inducing mechanical mismatch between the construct and the biological environment, in return to ensure stable functioning of such constructs during physiological deformation. Herein, this review starts by presenting current fabrication technologies of conductive fibers including wet spinning, microfluidic spinning, electrospinning and 3D printing as well as surface modification on fibers and fiber assemblies. To provide an update on the biomedical applications of conductive fibers and fiber assemblies, we further elaborate conductive fibrous constructs utilized in tissue engineering and regeneration, implantable healthcare bioelectronics, and wearable healthcare bioelectronics. To conclude, current challenges and future perspectives of biomedical electronic constructs built by conductive fibers are discussed.

Aligned Polyhydroxyalkanoate Blend Electrospun Fibers as Intraluminal Guidance Scaffolds for Peripheral Nerve Repair

The use of nerve guidance conduits (NGCs) to treat peripheral nerve injuries is a favorable approach to the current "gold standard" of autografting. However, as simple hollow tubes, they lack specific topographical and mechanical guidance cues present in nerve grafts and therefore are not suitable for treating large gap injuries (30-50 mm). The incorporation of intraluminal guidance scaffolds, such as aligned fibers, has been shown to increase neuronal cell neurite outgrowth and Schwann cell migration distances. A novel blend of PHAs, P(3HO)/P(3HB) (50:50), was investigated for its potential as an intraluminal aligned fiber guidance scaffold. Aligned fibers of 5 and 8 μm diameter were manufactured by electrospinning and characterized using SEM. Fibers were investigated for their effect on neuronal cell differentiation, Schwann cell phenotype, and cell viability in vitro. Overall, P(3HO)/P(3HB) (50:50) fibers supported higher neuronal and Schwann cell adhesion compared to PCL fibers. The 5 μm PHA blend fibers also supported significantly higher DRG neurite outgrowth and Schwann cell migration distance using a 3D ex vivo nerve injury model.

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

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

Methods

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

Results

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

Conclusion

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

Recent Advances in Polymeric Drug Delivery Systems for Peripheral Nerve Regeneration

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

Effects of Altering Magnesium Metal Surfaces on Degradation In Vitro and In Vivo during Peripheral Nerve Regeneration

In vivo use of biodegradable magnesium (Mg) metal can be plagued by too rapid a degradation rate that removes metal support before physiological function is repaired. To advance the use of Mg biomedical implants, the degradation rate may need to be adjusted. We previously demonstrated that pure Mg filaments used in a nerve repair scaffold were compatible with regenerating peripheral nerve tissues, reduced inflammation, and improved axonal numbers across a short-but not long-gap in sciatic nerves in rats. To determine if the repair of longer gaps would be improved by a slower Mg degradation rate, we tested, in vitro and in vivo, the effects of Mg filament polishing followed by anodization using plasma electrolytic oxidation (PEO) with non-toxic electrolytes. Polishing removed oxidation products from the surface of as-received (unpolished) filaments, exposed more Mg on the surface, produced a smoother surface, slowed in vitro Mg degradation over four weeks after immersion in a physiological solution, and improved attachment of cultured epithelial cells. In vivo, treated Mg filaments were used to repair longer (15 mm) injury gaps in adult rat sciatic nerves after placement inside hollow poly (caprolactone) nerve conduits. The addition of single Mg or control titanium filaments was compared to empty conduits (negative control) and isografts (nerves from donor rats, positive control). After six weeks in vivo, live animal imaging with micro computed tomography (micro-CT) showed that Mg metal degradation rates were slowed by polishing vs. as-received Mg, but not by anodization, which introduced greater variability. After 14 weeks in vivo, functional return was seen only with isograft controls. However, within Mg filament groups, the amount of axonal growth across the injury site was improved with slower Mg degradation rates. Thus, anodization slowed degradation in vitro but not in vivo, and degradation rates do affect nerve regeneration.