Multichanneled Collagen Conduits for Peripheral Nerve Regeneration: Design, Fabrication, and Characterization

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.

Preparation and performance study of multichannel PLA artificial nerve conduits

Compared with single-channel nerve conduits, multichannel artificial nerve conduits are more beneficial for repairing damaged peripheral nerves of long-distance nerve defects. Multichannel nerve conduits can be fabricated by the mold method and the electrospinning method but with disadvantages such as low strength and large differences in batches, while the braiding method can solve this problem. In this study, polylactic acid yarns were used as the braiding yarn, and the number of spindles during braiding was varied to achieve 4, 5, 6, 7 and 8 multichannel artificial nerve conduits. A mathematical model of the number of braiding yarn spindles required to meet certain size specification parameters of the multichannel conduit was established. The cross-sectional morphology and mechanical properties of the conduits were characterized by scanning electron microscopy observation and mechanical testing; the results showed that the multichannel structure was well constructed; the tensile strength of the multichannel conduit was more than 30 times that of the rabbit tibial nerve. The biocompatibility of the conduit was tested; thein vitrocell culture results proved that the braided multichannel nerve conduits were nontoxic to Schwann cells, and the cell adhesion and proliferation were optimal in the 4-channel conduit among the multichannel conduits, which was close to the single-channel conduit.

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.

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.

3D Printed Personalized Nerve Guide Conduits for Precision Repair of Peripheral Nerve Defects

The treatment of peripheral nerve defects has always been one of the most challenging clinical practices in neurosurgery. Currently, nerve autograft is the preferred treatment modality for peripheral nerve defects, while the therapy is constantly plagued by the limited donor, loss of donor function, formation of neuroma, nerve distortion or dislocation, and nerve diameter mismatch. To address these clinical issues, the emerged nerve guide conduits (NGCs) are expected to offer effective platforms to repair peripheral nerve defects, especially those with large or complex topological structures. Up to now, numerous technologies are developed for preparing diverse NGCs, such as solvent casting, gas foaming, phase separation, freeze-drying, melt molding, electrospinning, and three-dimensional (3D) printing. 3D printing shows great potential and advantages because it can quickly and accurately manufacture the required NGCs from various natural and synthetic materials. This review introduces the application of personalized 3D printed NGCs for the precision repair of peripheral nerve defects and predicts their future directions.

[Use of acellular allogeneic nerve transplants for nerve defects of the hand : Experience from a German hand surgery center]

Peripheral nerve injuries are often encountered in traumatological care. The aim of this manuscript is to provide initial data, experiences and performance reports from Germany in the implantation of acellular human nerve transplants in peripheral sensory nerve defects of the hand and to put these data in the context of a comprehensive review of the literature. Of the patients 4 (7 digital nerves) were examined 6 months postoperatively and 5 patients (6 digital nerves) were examined 1 year after the operation (3 were also at the 6‑month examination). All patients had a clinical improvement after nerve reconstruction (≥ S3 according to the classification of sensory recovery of the Medical Research Council modified by Mackinnon and Dellon). Disadvantages of our clinical study are the small number of patients, the inhomogeneity (primary and secondary nerve reconstruction) and the lack of comparison with other nerve reconstruction methods.

A ‘Relay’-Type Drug-Eluting Nerve Guide Conduit: Computational Fluid Dynamics Modeling of the Drug Eluting Efficiency of Various Drug Release Systems

Nerve guidance conduits (NGCs) are tubular scaffolds that act as a bridge between the proximal and distal ends of the native nerve to facilitate the nerve regeneration. The application of NGCs is mostly limited to nerve defects less than 3 mm due to the lack of sufficient cells in the lumen. The development of drug-release-system-embedded NGCs has the potential to improve the nerve regeneration performance by providing long-term release of growth factors. However, most of the past works only focused on one type of drug release system, limiting the variation in drug release system types and features. Therefore, in this study, computer-aided design (CAD) models were constructed and Computational Fluid Dynamics (CFD) simulations were carried out to investigate the effect of growth factor transporting efficiency on different drug release systems. To overcome the challenges posed by the current NGCs in treating long nerve gap injuries (>4 cm), a novel ‘relay’ NGC design is first proposed in this paper and has the potential to improve the nerve regeneration performance to next level. The intermediate cavities introduced along the length of the multi-channel NGCs act as a relay to further enhance the cell concentrations or growth factor delivery as well as the regeneration performance. Four different drug release systems, namely, a single-layer microsphere system, a double-layer microsphere system, bulk hydrogel, and hydrogel film, were chosen for the simulation. The results show that the double-layer microsphere system achieves the highest growth factor volume fraction among all the drug release systems. For the single-layer microsphere system, growth factor concentration can be significantly improved by increasing the microsphere quantities and decreasing the diameter and adjacent distance of microspheres. Bulk hydrogel systems hold the lowest growth factor release performance, and the growth factor concentration monotonically increased with the increase of film thickness in the hydrogel film system. Owing to the easy fabrication of hydrogel film and the even distribution of growth factors, the hydrogel film system can be regarded as a strong candidate in drug-eluting NGCs. The use of computational simulations can be regarded as a guideline for the design and application of drug release systems, as well as a promising tool for further nerve tissue engineering study.

Anisotropic scaffolds for peripheral nerve and spinal cord regeneration

The treatment of long-gap (>10 mm) peripheral nerve injury (PNI) and spinal cord injury (SCI) remains a continuous challenge due to limited native tissue regeneration capabilities. The current clinical strategy of using autografts for PNI suffers from a source shortage, while the pharmacological treatment for SCI presents dissatisfactory results. Tissue engineering, as an alternative, is a promising approach for regenerating peripheral nerves and spinal cords. Through providing a beneficial environment, a scaffold is the primary element in tissue engineering. In particular, scaffolds with anisotropic structures resembling the native extracellular matrix (ECM) can effectively guide neural outgrowth and reconnection. In this review, the anatomy of peripheral nerves and spinal cords, as well as current clinical treatments for PNI and SCI, is first summarized. An overview of the critical components in peripheral nerve and spinal cord tissue engineering and the current status of regeneration approaches are also discussed. Recent advances in the fabrication of anisotropic surface patterns, aligned fibrous substrates, and 3D hydrogel scaffolds, as well as their in vitro and in vivo effects are highlighted. Finally, we summarize potential mechanisms underlying the anisotropic architectures in orienting axonal and glial cell growth, along with their challenges and prospects.

Biomimetic and hierarchical nerve conduits from multifunctional nanofibers for guided peripheral nerve regeneration

Development of a functional nerve conduit to replace autografts remains a significant challenge particularly considering the compositional complexity and structural hierarchy of native peripheral nerves. In the present study, a multiscale strategy was adopted to fabricate 3D biomimetic nerve conduit from Antheraea pernyi silk fibroin (ApF)/(Poly(L-lactic acid-co-caprolactone)) (PLCL)/graphene oxide (GO) (ApF/PLCL/GO) nanofibers via nanofiber dispersion, template-molding, freeze-drying and crosslinking. The resultant conduits exhibit parallel multichannels (ϕ = 125 µm) surrounded by biomimetic fibrous fragments with tailored degradation rate and improved mechanical properties in comparison with the scaffold without GO. In vitro studies showed that such 3D biomimetic nerve scaffolds had the ability to offer an effective guiding interface for neuronal cell growth. Furthermore, these conduits showed a similarity to autografts in vivo repairing sciatic nerve defects based on a series of analysis (walking track, triceps weight, morphogenesis, vascularization, axonal regrowth and myelination). The conduits almost completely degraded within 12 weeks. These findings demonstrate that the 3D hierarchical nerve guidance conduit (NGC) with fascicle-like structure have great potential for peripheral nerve repair.

Preparation of Bioscaffolds Delivering Stem Cells for Neural Regeneration

Bioscaffolds have been proven for their feasibility in neural repair. Neural conduits have been investigated in the repair of wounded peripheral nerve and spinal cord. These conduits support axonal growth by providing structural guidance. Induced pluripotent stem cells (iPSCs) that are induced from a patient's own somatic cells have demonstrated significant neural cell differentiation capability and can circumvent immune system rejection. The combinatorial implantation of neural conduits and iPSCs may significantly enhance neural regeneration. The repair of nerves and spinal cords using biodegradable multichannel collagen conduits has been reported in our previous studies. In this review, we describe a method to fabricate a collagen neural conduit containing iPSC-derived neural cells.

Preparation of ciprofloxacin loaded zein conduits with good mechanical properties and antibacterial activity

Conduit scaffolds have potential applications in tissue engineering as nerve conduits, urological stent and blood vessel graft. Zein is a well-reported biopolymer in tissue engineering and drug delivery systems. Herein, we prepared ciprofloxacin loaded zein conduits using a facile rolling method. Zein conduits (ZCs) were evaluated for physical structure, porosity, bending stiffness, degradation, drug release, in vitro and in vivo antibacterial efficacy and cells toxicity. ZCs showed porous structure with porosity > 60 % and good mechanical strength with bending stiffness of 28.54 N.mm². Slow enzymatic degradation (87 % in 30 days) was also observed for ZCs. Slow release of ciprofloxacin up to 42 days was observed that could assure prevention of post-implantation infection. In vitro and in vivo antibacterial study verified the short-time and long-time antibacterial efficacy of zein conduits on Gram-positive and Gram-negative bacteria. Live/dead measurement and CCK-8 assay on L929 cells demonstrated good cell compatibility for all zein conduits (>90 % cell viability and cells proliferation in 3 days). Overall, the rolling method could be exploited for preparation of ciprofloxacin loaded zein conduits, which had the potential for tissue engineering applications.

Effect of nanocomposite coating and biomolecule functionalization on silk fibroin based conducting 3D braided scaffolds for peripheral nerve tissue engineering

In this work, the effects of carbon nanofiber (CNF) dispersed poly-ε-caprolactone (PCL) nanocomposite coatings and biomolecules functionalization on silk fibroin based conducting braided nerve conduits were studied for enhancing Neuro 2a cellular activities. A unique combination of biomolecules (UCM) and varying concentrations of CNF (5, 7.5, 10% w/w) were dispersed in 10% (w/v) PCL solution for coating on degummed silk threads. The coated silk threads were braided to develop the scaffold structure. As the concentration of CNF increased in the coating, the electrical impedance decreased up to 400 Ω indicating better conductivity. The tensile and dynamic mechanical property analysis showed better mechanical properties in CNF coated samples. In vitro cytocompatibility analysis proved the non-toxicity of the developed braided conduits. Cell attachment, growth and proliferation were significantly enhanced on the biomolecule functionalized nanocomposite coated silk braided structure, exhibiting their potential for peripheral nerve regeneration and recovery.

Novel spiral structured nerve guidance conduits with multichannels and inner longitudinally aligned nanofibers for peripheral nerve regeneration

Nerve guidance conduits (NGCs) are artificial substitutes for autografts, which serve as the gold standard in treating peripheral nerve injury. A recurring challenge in tissue engineered NGCs is optimizing the cross-sectional surface area to achieve a balance between allowing nerve infiltration while supporting maximum axonal extension from the proximal to distal stump. In this study, we address this issue by investigating the efficacy of an NGC with a higher cross-sectional surface composed of spiral structures and multi-channels, coupled with inner longitudinally aligned nanofibers and protein on aiding nerve repair in critical sized nerve defect. The NGCs were implanted into 15-mm-long rat sciatic nerve injury gaps for 4 weeks. Nerve regeneration was assessed using an established set of assays, including the walking track analysis, electrophysiological testing, pinch reflex assessment, gastrocnemius muscle measurement, and histological assessment. The results indicated that the novel NGC design yielded promising data in encouraging nerve regeneration within a relatively short recovery time. The performance of the novel NGC for nerve regeneration was superior to that of the control nerve conduits with tubular structures. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1410-1419, 2019.

Electrospinning and Electrospun Nanofibers: Methods, Materials, and Applications

Electrospinning is a versatile and viable technique for generating ultrathin fibers. Remarkable progress has been made with regard to the development of electrospinning methods and engineering of electrospun nanofibers to suit or enable various applications. We aim to provide a comprehensive overview of electrospinning, including the principle, methods, materials, and applications. We begin with a brief introduction to the early history of electrospinning, followed by discussion of its principle and typical apparatus. We then discuss its renaissance over the past two decades as a powerful technology for the production of nanofibers with diversified compositions, structures, and properties. Afterward, we discuss the applications of electrospun nanofibers, including their use as "smart" mats, filtration membranes, catalytic supports, energy harvesting/conversion/storage components, and photonic and electronic devices, as well as biomedical scaffolds. We highlight the most relevant and recent advances related to the applications of electrospun nanofibers by focusing on the most representative examples. We also offer perspectives on the challenges, opportunities, and new directions for future development. At the end, we discuss approaches to the scale-up production of electrospun nanofibers and briefly discuss various types of commercial products based on electrospun nanofibers that have found widespread use in our everyday life.

The mechanics of scaling-up multichannel scaffold technology for clinical nerve repair

Peripheral nerve injury remains a large clinical problem, with challenges to the successful translation of nerve repair devices. One promising technology is the multichannel scaffold, a conduit incorporating arrays of linear microchannels, which has high open lumen volume to guide regenerating nerves toward distal targets. To maximize open lumen volume, and scale-up scaffolds for translation, this study explored how mechanical properties were affected by 1) material choice (poly(lactide co-glycolide) (PLGA) and poly(caprolactone) (PCL)), 2) microstructure (porous and non-porous), and 3) channel architecture (200 µm and 300 µm diameter). After testing in transverse compression and bending, it was noted that introduction of porosity and increasing microchannel diameter increased scaffold compliance from 0.05 ± 0.1-2.75 ± 0.8 mm/N. Porosity also increased flexibility and eliminated kinking, which could potentially damage regenerating nerves. Material choice determined both scaffold deformation and mechanics. Porous PLGA scaffolds were stiffer than porous PCL, with greater deformation. Having demonstrated stability and flexibility, porous PCL multichannel scaffolds were scaled from 1.5 mm to 10 mm in diameter, a range applicable to the clinic. Even at 10 mm in diameter, the linear structure, high open lumen volume and compliance were retained. This demonstrates significant progress towards translation and brings multichannel technology closer to the clinic.

A multi-layered nerve guidance conduit design adapted to facilitate surgical implantation

BACKGROUND AND AIMS

The gold standard procedure after a severe nerve injury is the nerve autograft, yet this technique has drawbacks. In recent years, progress has been made in the development of artificial nerve guides to replace the autograft, but no device has been able to demonstrate superiority. The present study introduces an adaptable foundation design for peripheral nerve regeneration.

METHODS

Silk fibroin was electrospun, creating a tri-layered material with aligned fiber surfaces and a randomly deposited fiber interior. This material was rolled into a micro-channeled conduit, which was then enveloped by a jacket layer of the same tri-layered material.

RESULTS

The proposed implant design succeeds in incorporating various desirable aspects of synthetic nerve guides, while facilitating the surgical implantation process for medical application. The aligned fiber surfaces of the conduit support axon guidance, while the tri-layered architecture improves its structural integrity compared with a fully aligned fiber material. Moreover, the jacket layer creates a small niche on each end which facilitates surgical implantation. An in vivo study in rats showed that nerve regeneration using this device was comparable to results after direct suture.

CONCLUSION

This proof-of-principle study, therefore, advances the development of tissue engineered nerve grafts by creating an optimized guidance conduit design capable of successful nerve regeneration.

Advances in Controlling Differentiation of Adult Stem Cells for Peripheral Nerve Regeneration

Adult stems cells, possessing the ability to grow, migrate, proliferate, and transdifferentiate into various specific phenotypes, constitute a great asset for peripheral nerve regeneration. Adult stem cells' ability to undergo transdifferentiation is sensitive to various cell-to-cell interactions and external stimuli involving interactions with physical, mechanical, and chemical cues within their microenvironment. Various studies have employed different techniques for transdifferentiating adult stem cells from distinct sources into specific lineages (e.g., glial cells and neurons). These techniques include chemical and/or electrical induction as well as cell-to-cell interactions via co-culture along with the use of various 3D conduit/scaffold designs. Such scaffolds consist of unique materials that possess controllable physical/mechanical properties mimicking cells' natural extracellular matrix. However, current limitations regarding non-scalable transdifferentiation protocols, fate commitment of transdifferentiated stem cells, and conduit/scaffold design have required new strategies for effective stem cells transdifferentiation and implantation. In this progress report, a comprehensive review of recent advances in the transdifferentiation of adult stem cells via different approaches along with multifunctional conduit/scaffolds designs is presented for peripheral nerve regeneration. Potential cellular mechanisms and signaling pathways associated with differentiation are also included. The discussion with current challenges in the field and an outlook toward future research directions is concluded.

Microstructure and in vivo characterization of multi-channel nerve guidance scaffolds

In a previous study, we demonstrated a novel manufacturing approach to fabricate multi-channel scaffolds (MCS) for use in spinal cord injuries (SCI). In the present study, we extended similar materials processing technology to fabricate significantly longer (5X) porous poly caprolactone (PCL) MCS and evaluated their efficacy in 1 cm sciatic peripheral nerve injury (PNI) model. Due to the increase in MCS dimensions and the challenges that may arise in a longer nerve gap model, microstructural characterization involved MCS wall permeability to assess nutrient flow, topography, and microstructural uniformity to evaluate the potential for homogeneous linear axon guidance. It was determined that the wall permeability dramatically varied from 0.02 ± 0.01 × 10^-13 to 21.7 ± 11.4 × 10^-13 m² for 50% and 70% porous PCL, respectively. Using interferometry, the porous PCL surface roughness was determined to be 10.7 ± 1.2 μm, which is believed to be sufficient to promote cell integration. Using micro computed tomography, the 3D MCS microstructure was determined to be uniform over 1 cm with an open lumen volume of 44.6% ± 3.6%. In vivo implantation, in the rat sciatic nerve model, over 4 weeks, demonstrated that MCS scaffolds maintained structural integrity, were biocompatible, and supported linear axon guidance and distal end egress over 1 cm. Taken together, this study demonstrated that MCS technology previously developed for the SCI is also relevant to longer nerve gap PNI.

Dual-component collagenous peptide/reactive oligomer hydrogels as potential nerve guidance materials - from characterization to functionalization

Toward a new generation of improved nerve guidance conduits (NGCs), novel biomaterials are required to address pressing clinical shortcomings in peripheral nerve regeneration (PNR) and to promote biological performance. A dual-component hydrogel system formed by cross-linking reaction between maleic anhydride groups in an oligomeric building block for cross-linking of free amine functionalities in partially hydrolyzed collagen is formulated for continuous processing and NGC fabrication. The influence of the gelation base is optimized for processing from a double syringe delivery system with a static mixer. A hydrophilic low-concentrated base was introduced to control network formation and to utilize highly reactive macromers for gelation. Cross-linking extent and building block conversion were improved and homogenous monoliths were fabricated. Chemically derivatized hydrogels were obtained by conversion of a fraction of anhydride groups in the oligomeric precursor with monovalent primary amine-containing grafting molecules prior to gelation. Network stability in functionalized hydrogels was maintained and cationic moieties were implement to the gel that promoted in vitro cell attachment and spreading irrespective of mechanical stiffness. A molding strategy was introduced that allowed for fabrication of flexible tubular conduits in tunable dimensions and with chemically patterned structures. These hydrogel-based conduits hold promise for the next generation NGCs with integrated chemical cues for PNR.

Decellularized sciatic nerve matrix as a biodegradable conduit for peripheral nerve regeneration

The use of autologous nerve grafts remains the gold standard for treating nerve defects, but current nerve repair techniques are limited by donor tissue availability and morbidity associated with tissue loss. Recently, the use of conduits in nerve injury repair, made possible by tissue engineering, has shown therapeutic potential. We manufactured a biodegradable, collagen-based nerve conduit containing decellularized sciatic nerve matrix and compared this with a silicone conduit for peripheral nerve regeneration using a rat model. The collagen-based conduit contains nerve growth factor, brain-derived neurotrophic factor, and laminin, as demonstrated by enzyme-linked immunosorbent assay. Scanning electron microscopy images showed that the collagen-based conduit had an outer wall to prevent scar tissue infiltration and a porous inner structure to allow axonal growth. Rats that were implanted with the collagen-based conduit to bridge a sciatic nerve defect experienced significantly improved motor and sensory nerve functions and greatly enhanced nerve regeneration compared with rats in the sham control group and the silicone conduit group. Our results suggest that the biodegradable collagen-based nerve conduit is more effective for peripheral nerve regeneration than the silicone conduit.

Nano-fibrous and ladder-like multi-channel nerve conduits: Degradation and modification by gelatin

We recently fabricated multi-channel PLLA nerve conduits (NCs, conduits diameter: ~3mm, channels diameter: ~200μm) with nano-fibrous microstructure (NNCs) and ladder-like microstructure (LNCs), and found the nanofibers in the NNCs promote differentiation of nerve stem cells (NSCs) into neurons. In the present study, we evaluated the degradation profile of NNCs and LNCs, and observed that NNCs degraded too fast to implant. To delay the degradation and retain the nano-scale effect of NNCs, we used gelatin to wrap (2% w/v gelatin) or embed (8% w/v gelatin) NNCs and LNCs via vacuum infusion and chemical cross-linking with genipin. NNCs-wrapped maintained their original nano-fibrous microstructure, but NNCs-embedded presented a porous morphology without nanofibers appearing. Incorporation of gelatin did not change their compressive moduli, but increased the creep recovery ratios of LNCs and NNCs. In vitro degradation revealed that integrity was maintained and the mass loss was <5% for NNCs-wrapped after 10weeks, in comparison with 15% mass loss and collapsed structure of the pure NNCs after 4weeks. Meanwhile, there were no obvious changes in the degradation of LNCs with modification. Nerve stem cells (NSCs) were then seeded onto the six NCs represented as: NNCs, NNCs-wrapped, NNCs-embedded, LNCs, LNCs-wrapped, and LNCs-embedded. Immunocytochemistry analysis demonstrated that gelatin coating enhanced the adhesion and proliferation of NSCs, and the NNCs-wrapped scaffold promoted the differentiation proportion of NSCs into neurons from 25.8% (on pure NNCs) to 53.4% after 14days of seeding. On the other hand, only 14.3% of neurons were derived from the differentiation of the seeded NSCs on the NNCs-embedded. NNCs-wrapped would be a good choice for future studies in nerve injury repair in vivo due to its appropriate degradation rate, flexibility, and nano-scale effect.

Functionalised collagen spheres reduce H2O2 mediated apoptosis by scavenging overexpressed ROS

Excess reactive oxygen species (ROS) has been implicated in numerous diseases including cancer, cardiovascular and neurodegenerative diseases. Overexpression of ROS can lead to oxidative stress and subsequently to H₂O₂-mediated cell apoptosis. In this study, it was demonstrated that biodegradable PLGA microspheres coated with collagen type I and decorated with MnO₂ nanoparticles acted as ROS scavengers controlling the H₂O₂-mediated apoptosis of cells undergoing oxidative stress. The results showed that the functionalized collagen spheres can protect cells even under very harsh conditions of oxidative stress.

Dual-Component Gelatinous Peptide/Reactive Oligomer Formulations as Conduit Material and Luminal Filler for Peripheral Nerve Regeneration

Toward the next generation of nerve guidance conduits (NGCs), novel biomaterials and functionalization concepts are required to address clinical demands in peripheral nerve regeneration (PNR). As a biological polymer with bioactive motifs, gelatinous peptides are promising building blocks. In combination with an anhydride-containing oligomer, a dual-component hydrogel system (cGEL) was established. First, hollow cGEL tubes were fabricated by a continuous dosing and templating process. Conduits were characterized concerning their mechanical strength, in vitro and in vivo degradation and biocompatibility. Second, cGEL was reformulated as injectable shear thinning filler for established NGCs, here tyrosine-derived polycarbonate-based braided conduits. Thereby, the formulation contained the small molecule LM11A-31. The biofunctionalized cGEL filler was assessed regarding building block integration, mechanical properties, in vitro cytotoxicity, and growth permissive effects on human adipose tissue-derived stem cells. A positive in vitro evaluation motivated further application of the filler material in a sciatic nerve defect. Compared to the empty conduit and pristine cGEL, the functionalization performed superior, though the autologous nerve graft remains the gold standard. In conclusion, LM11A-31 functionalized cGEL filler with extracellular matrix (ECM)-like characteristics and specific biochemical cues holds great potential to support PNR.

Fibrin glue as a stabilization strategy in peripheral nerve repair when using porous nerve guidance conduits

Porous conduits provide a protected pathway for nerve regeneration, while still allowing exchange of nutrients and wastes. However, pore sizes >30 µm may permit fibrous tissue infiltration into the conduit, which may impede axonal regeneration. Coating the conduit with Fibrin Glue (FG) is one option for controlling the conduit's porosity. FG is extensively used in clinical peripheral nerve repair, as a tissue sealant, filler and drug-delivery matrix. Here, we compared the performance of FG to an alternative, hyaluronic acid (HA) as a coating for porous conduits, using uncoated porous conduits and reverse autografts as control groups. The uncoated conduit walls had pores with a diameter of 60 to 70 µm that were uniformly covered by either FG or HA coatings. In vitro, FG coatings degraded twice as fast as HA coatings. In vivo studies in a 1 cm rat sciatic nerve model showed FG coating resulted in poor axonal density (993 ± 854 #/mm²), negligible fascicular area (0.03 ± 0.04 mm²), minimal percent wet muscle mass recovery (16 ± 1 in gastrocnemius and 15 ± 5 in tibialis anterior) and G-ratio (0.73 ± 0.01). Histology of FG-coated conduits showed excessive fibrous tissue infiltration inside the lumen, and fibrin capsule formation around the conduit. Although FG has been shown to promote nerve regeneration in non-porous conduits, we found that as a coating for porous conduits in vivo, FG encourages scar tissue infiltration that impedes nerve regeneration. This is a significant finding considering the widespread use of FG in peripheral nerve repair.

Laminin-coated nerve guidance conduits based on poly(l-lactide-co-glycolide) fibers and yarns for promoting Schwann cells’ proliferation and migration

A laminin-coated and yarn-encapsulated PLGA nerve guidance conduit for Schwann cells’ proliferation and migration.

3D-engineering of Cellularized Conduits for Peripheral Nerve Regeneration

Tissue engineered conduits have great promise for bridging peripheral nerve defects by providing physical guiding and biological cues. A flexible method for integrating support cells into a conduit with desired architectures is wanted. Here, a 3D-printing technology is adopted to prepare a bio-conduit with designer structures for peripheral nerve regeneration. This bio-conduit is consisted of a cryopolymerized gelatin methacryloyl (cryoGelMA) gel cellularized with adipose-derived stem cells (ASCs). By modeling using 3D-printed “lock and key” moulds, the cryoGelMA gel is structured into conduits with different geometries, such as the designed multichannel or bifurcating and the personalized structures. The cryoGelMA conduit is degradable and could be completely degraded in 2-4 months in vivo. The cryoGelMA scaffold supports the attachment, proliferation and survival of the seeded ASCs, and up-regulates the expression of their neurotrophic factors mRNA in vitro. After implanted in a rat model, the bio-conduit is capable of supporting the re-innervation across a 10 mm sciatic nerve gap, with results close to that of the autografts in terms of functional and histological assessments. The study describes an indirect 3D-printing technology for fabricating cellularized designer conduits for peripheral nerve regeneration, and could lead to the development of future nerve bio-conduits for clinical use.

Design of barrier coatings on kink-resistant peripheral nerve conduits

Here, we report on the design of braided peripheral nerve conduits with barrier coatings. Braiding of extruded polymer fibers generates nerve conduits with excellent mechanical properties, high flexibility, and significant kink-resistance. However, braiding also results in variable levels of porosity in the conduit wall, which can lead to the infiltration of fibrous tissue into the interior of the conduit. This problem can be controlled by the application of secondary barrier coatings. Using a critical size defect in a rat sciatic nerve model, the importance of controlling the porosity of the nerve conduit walls was explored. Braided conduits without barrier coatings allowed cellular infiltration that limited nerve recovery. Several types of secondary barrier coatings were tested in animal studies, including (1) electrospinning a layer of polymer fibers onto the surface of the conduit and (2) coating the conduit with a cross-linked hyaluronic acid-based hydrogel. Sixteen weeks after implantation, hyaluronic acid-coated conduits had higher axonal density, displayed higher muscle weight, and better electrophysiological signal recovery than uncoated conduits or conduits having an electrospun layer of polymer fibers. This study indicates that braiding is a promising method of fabrication to improve the mechanical properties of peripheral nerve conduits and demonstrates the need to control the porosity of the conduit wall to optimize functional nerve recovery.

Dual-factor loaded functional silk fibroin scaffolds for peripheral nerve regeneration with the aid of neovascularization

Dual-factor loaded functional silk fibroin scaffolds enhanced peripheral nerve regeneration with the aid of neovascularization.

Scaffold and scaffold-free self-assembled systems in regenerative medicine

Self-assembly in tissue engineering refers to the spontaneous chemical or biological association of components to form a distinct functional construct, reminiscent of native tissue. Such self-assembled systems have been widely used to develop platforms for the delivery of therapeutic and/or bioactive molecules and various cell populations. Tissue morphology and functional characteristics have been recapitulated in several self-assembled constructs, designed to incorporate stimuli responsiveness and controlled architecture through spatial confinement or field manipulation. In parallel, owing to substantial functional properties, scaffold-free cell-assembled devices have aided in the development of functional neotissues for various clinical targets. Herein, we discuss recent advancements and future aspirations in scaffold and scaffold-free self-assembled devices for regenerative medicine purposes. Biotechnol. Bioeng. 2016;113: 1155-1163. © 2015 Wiley Periodicals, Inc.

Collagen microsphere serving as a cell carrier supports oligodendrocyte progenitor cell growth and differentiation for neurite myelination in vitro

Introduction

Microspheres fabricated from natural materials serve as a promising biodegradable and biocompatible carrier in a small volume for efficient cell delivery to the lesion of the injured neural tissue to generate biological functions. As the major component of extracellular matrix and due to its natural abundance within the body, collagen may be fabricated into microspheres and improve the ability of pre-seeded cells on the microspheres to encounter the hostile micro-environment in the lesion.

Methods

In this study, collagen microspheres were fabricated using the water-in-oil emulsion technique and cross-linked with 1-ethyl-3-(3-dimethylaminopropryl) carbodiimide. Oligodendrocyte progenitor cells isolated from postnatal day P1 to 2 rats were cultured and differentiated on the microspheres. The microspheres carrying the oligodendrocyte progenitor cells were co-cultured with dorsal root ganglions from 15-day-old rat embryos. The myelination formation was studied for the co-culture of oligodendrocyte progenitor cells and dorsal root ganglions.

Results

We showed that the viability of oligodendrocyte progenitor cells, B104 cells and PC12 cells grown on microspheres was not significantly different with those in cell culture plates. Oligodendrocyte progenitor cells differentiated into oligodendrocytes on collagen microspheres. The oligodendrocytes grown on microspheres extended processes that wrapped the axons of dorsal root ganglion neurons and the formation of myelin sheath was observed in the co-culture.

Conclusions

This study demonstrates the feasibility of collagen microspheres in further applications for the delivery of neural progenitor cells for neural regeneration.

Enhancing the stiffness of collagen hydrogels for delivery of encapsulated chondrocytes to articular lesions for cartilage regeneration

This study investigated a dual crosslinking paradigm, consisting of (a) photocrosslinking with Rose Bengal (RB) and green light followed by (b) chemical crosslinking with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), and N-hydroxysuccinimide (NHS) to enhance collagen gel stiffness. In group 1, 50 μL collagen constructs of 2% (w/v) type I collagen containing 10 μM RB were allowed to gel spontaneously at 37 °C. In group 2, the spontaneous gels were exposed to green light (532 nm). In group 3, the photochemically crosslinked gels were subsequently treated with a 1-h exposure to 33 mM EDC/6 mM NHS. Samples (n = 18) were subjected to 0.08% (w/v) collagenase digestion, and the storage modulus of samples was measured by rheometry. Viability of encapsulated chondrocytes was measured by live/dead assay. Chondrocytes were ≥ 95% viable in all constructs at 10 days in vitro. Resistance to collagenase digestion increased as; spontaneous gels (2 h) < photochemical gels (3-4 h) < dual crosslinked gels (>24 h). The storage modulus of dual-crosslinked constructs was increased 5-fold over both photocrosslinked and spontaneous gels. As the dual crosslinking paradigm did not reduce encapsulated chondrocyte viability, these crosslinked collagen hydrogels could be a useful tool for the practical delivery of encapsulated chondrocytes to articular defects.

Neural tissue engineering options for peripheral nerve regeneration

Tissue engineered nerve grafts (TENGs) have emerged as a potential alternative to autologous nerve grafts, the gold standard for peripheral nerve repair. Typically, TENGs are composed of a biomaterial-based template that incorporates biochemical cues. A number of TENGs have been used experimentally to bridge long peripheral nerve gaps in various animal models, where the desired outcome is nerve tissue regeneration and functional recovery. So far, the translation of TENGs to the clinic for use in humans has met with a certain degree of success. In order to optimize the TENG design and further approach the matching of TENGs with autologous nerve grafts, many new cues, beyond the traditional ones, will have to be integrated into TENGs. Furthermore, there is a strong requirement for monitoring the real-time dynamic information related to the construction of TENGs. The aim of this opinion paper is to specifically and critically describe the latest advances in the field of neural tissue engineering for peripheral nerve regeneration. Here we delineate new attempts in the design of template (or scaffold) materials, especially in the context of biocompatibility, the choice and handling of support cells, and growth factor release systems. We further discuss the significance of RNAi for peripheral nerve regeneration, anticipate the potential application of RNAi reagents for TENGs, and speculate on the possible contributions of additional elements, including angiogenesis, electrical stimulation, molecular inflammatory mediators, bioactive peptides, antioxidant reagents, and cultured biological constructs, to TENGs. Finally, we consider that a diverse array of physicochemical and biological cues must be orchestrated within a TENG to create a self-consistent coordinated system with a close proximity to the regenerative microenvironment of the peripheral nervous system.

Fabrication and evaluation of PLLA multichannel conduits with nanofibrous microstructure for the differentiation of NSCs in vitro

Nerve conduits (NCs) with multiple longitudinally aligned channels, being mimicking the natural nerves anatomical structure, have been attracted more and more attentions. However, some specific structural parameters of a conduit that would be beneficial for further improvement of neural tissue regeneration were not comprehensively considered. Using a systematized device and combining low-pressure injection molding and thermal-induced phase separation, we fabricated 33-channel NCs (outer diameter 3.5 mm, channel diameter 200 μm) with different well-defined microscopic features, including NCs with a nano-fibrous microstructure (NNC), NCs with microspherical pores and nano-fibrous pore walls (MNC), and NCs with a ladder-like microstructure (LNC). The porosities of these NCs were ∼90% and were independent of the fine microstructures, whereas the pore size distributions were clearly distinct. The adsorption of bovine serum albumin for the NNC was a result of having the highest specific surface area, which was 3.5 times that of the LNC. But the mechanical strength of NNC was lower than that of two groups because of a relative high crystallinity and brittle characteristics. In vitro nerve stem cells (NSCs) incubation revealed that 14 days after seeding the NSCs, 31.32% cells were Map2 positive in the NNC group, as opposed to 15.76% in the LNC group and 23.29% in the MNC group. Addition of NGF into the culture medium, being distinctive specific surface area and a high adsorption of proteon for NNC, 81.11% of neurons derived from the differentiation of the seeded NSCs was obtained. As a result of imitating the physical structure of the basement membrane of the neural matrix, the nanofibrous structure of the NCs has facilitated the differentiation of NSCs into neurons.

Misdirection and guidance of regenerating axons after experimental nerve injury and repair

Misdirection of regenerating axons is one of the factors that can explain the limited results often found after nerve injury and repair. In the repair of mixed nerves innervating different distal targets (skin and muscle), misdirection may, for example, lead to motor axons projecting toward skin, and vice versa—that is, sensory axons projecting toward muscle. In the repair of motor nerves innervating different distal targets, misdirection may result in reinnervation of the wrong target muscle, which might function antagonistically. In sensory nerve repair, misdirection might give an increased perceptual territory. After median nerve repair, for example, this might lead to a dysfunctional hand.

Different factors may be involved in the misdirection of regenerating axons, and there may be various mechanisms that can later correct for misdirection. In this review the authors discuss these different factors and mechanisms that act along the pathway of the regenerating axon. The authors review recently developed evaluation methods that can be used to investigate the accuracy of regeneration after nerve injury and repair (including the use of transgenic fluorescent mice, retrograde tracing techniques, and motion analysis). In addition, the authors discuss new strategies that can improve in vivo guidance of regenerating axons (including physical guidance with multichannel nerve tubes and biological guidance accomplished using gene therapy).

The behavior of neuronal cells on tendon-derived collagen sheets as potential substrates for nerve regeneration

Peripheral nervous system injuries result in a decreased quality of life, and generally require surgical intervention for repair. Currently, the gold standard of nerve autografting, based on the use of host tissue such as sensory nerves is suboptimal as it results in donor-site loss of function and requires a secondary surgery. Nerve guidance conduits fabricated from natural polymers such as collagen are a common alternative to bridge nerve defects. In the present work, tendon sections derived through a process named bioskiving were studied for their potential for use as a substrate to fabricate nerve guidance conduits. We show that cells such as rat Schwann cells adhere, proliferate, and align along the fibrous tendon substrate which has been shown to result in a more mature phenotype. Additionally we demonstrate that chick dorsal root ganglia explants cultured on the tendon grow to similar lengths compared to dorsal root ganglia cultured on collagen gels, but also grow in a more oriented manner on the tendon sections. These results show that tendon sections produced through bioskiving can support directional nerve growth and may be of use as a substrate for the fabrication of nerve guidance conduits.

The role of peripheral nerve ECM components in the tissue engineering nerve construction

The extracellular matrix (ECM) is the naturally occurring substrate that provides a support structure and an attachment site for cells. It also produces a biological signal, which plays an important role in and has significant impact on cell adhesion, migration, proliferation, differentiation, and gene expression. Peripheral nerve repair is a complicated process involving Schwann cell proliferation and migration, 'bands of Büngner' formation, and newborn nerve extension. In the ECM of peripheral nerves, macromolecules are deposited among cells; these constitute the microenvironment of Schwann cell growth. Such macromolecules include collagen (I, III, IV, V), laminin, fibronectin, chondroitin sulfate proteoglycans (CSPGs), and other nerve factors. Collagen, the main component of ECM, provides structural support and guides newborn neurofilament extension. Laminin, fibronectin, CSPGs, and neurotrophic factors, are promoters or inhibitors, playing different roles in nerve repair after injury. By a chemical decellularization process, acellular nerve allografting eliminates the antigens responsible for allograft rejection and maintains most of the ECM components, which can effectively guide and enhance nerve regeneration. Thus, the composition and features of peripheral nerve ECM suggest its superiority as nerve repair material. This review focuses on the structure, function, and application in the tissue engineering nerve construction of the peripheral nerve ECM components.

The role of immunophilin ligands in nerve regeneration

Tacrolimus (FK506) is a widely used immunosuppressant in organ transplantation. However, it also has neurotrophic activity that occurs independently of its immunosuppressive effects. Other neurotrophic immunophilin ligands that do not exhibit immunosuppression have subsequently been developed and studied in various models of nerve injury. This article reviews the literature on the use of tacrolimus and other immunophilin ligands in peripheral nerve, cranial nerve and spinal cord injuries. The most convincing evidence of enhanced nerve regeneration is seen with systemic administration of tacrolimus in peripheral nerve injury, although clinical use is limited due to its immunosuppressive side effects. Local tacrolimus delivery to the site of nerve repair in peripheral and cranial nerve injury is less effective but requires further investigation. Tacrolimus can enhance outcomes in nerve allograft reconstruction and accelerates reinnervation of complex functional allograft transplants. Other non-immunosuppressive immunophilins ligands such as V-10367 and FK1706 demonstrate enhanced neuroregeneration in the peripheral nervous system and CNS. Mixed results are found in the application of immunophilin ligands to treat spinal cord injury. Immunophilin ligands have great potential in the treatment of nerve injury, but further preclinical studies are necessary to permit translation into clinical trials.