A review of bioactive release from nerve conduits as a neurotherapeutic strategy for neuronal growth in peripheral nerve injury

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

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

Combining PLGA microspheres loaded with Liver X receptor agonist GW3965 with a chitosan nerve conduit can promote the healing and regeneration of the wounded sciatic nerve

Globally, peripheral nerve injury (PNI) is a common clinical issue. Successfully repairing severe PNIs has posed a major challenge for clinicians. GW3965 is a highly selective LXR agonist, and previous studies have demonstrated its positive protective effects in both central and peripheral nerve diseases. In this work, we examined the potential reparative effects of GW3965-loaded polylactic acid co-glycolic acid microspheres in conjunction with a chitosan nerve conduit for peripheral nerve damage. The experiment revealed that GW3965 promoted Schwann cell proliferation and neurotrophic factor release in vitro. In vivo experiments conducted on rats showed that GW3965 facilitated the restoration of motor function, promoted axon and myelin regeneration in the sciatic nerve, and enhanced the microenvironment of nerve regeneration. These results offer a novel therapeutic approach for the healing of nerve damage. Overall, this work provides valuable insights and presents a promising therapeutic strategy for addressing PNI.

Updates in peripheral nerve surgery of the upper extremity: diagnosis and treatment options

The loss of function resulting from peripheral nerve injuries confers a significant burden to the patient and society. The treatment of peripheral nerve injuries requires an accurate diagnosis and formulation of a functional reconstructive plan. Advances in peripheral nerve imaging complement electrodiagnostic studies, and provide us with detailed information regarding the status of nerve injury, repair, and regeneration in order to prognosticate recovery and determine the need for surgical intervention. When direct nerve repair is not possible, the methods for bridging a nerve gap are the nerve autograft, allograft and conduit. While current research supports the use of conduits and nerve allografts for shorter nerve gaps, the nerve autograft still remains the gold standard for bridging a nerve gap. When direct nerve repair or nerve grafting fails, or is anticipated to be insufficient, nerve transfers are an alternative for reconstruction. Knowledge of axonal counts, upper limb innervation patterns, location and clustering of upper limb peripheral nerves allows for the design of new nerve transfers. The options of nerve transfers for radial, ulnar and median nerve injuries are outlined, as well as their outcomes. Nerve transfers are an attractive option for restoring motor and sensory function while minimizing donor site morbidity. However, one must consider their limitations, and preserve donor sites for secondary tendon transfer options. This article presents the latest information regarding the imaging of peripheral nerves, methods to bridge a nerve gap, and nerve transfers to aid the peripheral nerve surgeon in choosing a reconstructive plan.

Development and preclinical evaluation of bioactive nerve conduits for peripheral nerve regeneration: A comparative study

In severe peripheral nerve injuries, nerve conduits (NCs) are good alternatives to autografts/allografts; however, the results the available devices guarantee for are still not fully satisfactory. Herein, differently bioactivated NCs based on the new polymer oxidized polyvinyl alcohol (OxPVA) are compared in a rat model of sciatic nerve neurotmesis (gap: 5 mm; end point: 6 weeks). Thirty Sprague Dawley rats are randomized to 6 groups: Reverse Autograft (RA); Reaxon®; OxPVA; OxPVA + EAK (self-assembling peptide, mechanical incorporation); OxPVA + EAK-YIGSR (mechanical incorporation); OxPVA + Nerve Growth Factor (NGF) (adsorption). Preliminarily, all OxPVA-based devices are comparable with Reaxon® in Sciatic Functional Index score and gait analysis; moreover, all conduits sustain nerve regeneration (S100, β-tubulin) without showing substantial inflammation (CD3, F4/80) evidences. Following morphometric analyses, OxPVA confirms its potential in PNI repair (comparable with Reaxon®) whereas OxPVA + EAK-YIGSR stands out for its myelinated axons total number and density, revealing promising in injury recovery and for future application in clinical practice.

Investigations into the effects of scaffold microstructure on slow-release system with bioactive factors for bone repair

In recent years, bone tissue engineering (BTE) has played an essential role in the repair of bone tissue defects. Although bioactive factors as one component of BTE have great potential to effectively promote cell differentiation and bone regeneration, they are usually not used alone due to their short effective half-lives, high concentrations, etc. The release rate of bioactive factors could be controlled by loading them into scaffolds, and the scaffold microstructure has been shown to significantly influence release rates of bioactive factors. Therefore, this review attempted to investigate how the scaffold microstructure affected the release rate of bioactive factors, in which the variables included pore size, pore shape and porosity. The loading nature and the releasing mechanism of bioactive factors were also summarized. The main conclusions were achieved as follows: i) The pore shapes in the scaffold may have had no apparent effect on the release of bioactive factors but significantly affected mechanical properties of the scaffolds; ii) The pore size of about 400 μm in the scaffold may be more conducive to controlling the release of bioactive factors to promote bone formation; iii) The porosity of scaffolds may be positively correlated with the release rate, and the porosity of 70%-80% may be better to control the release rate. This review indicates that a slow-release system with proper scaffold microstructure control could be a tremendous inspiration for developing new treatment strategies for bone disease. It is anticipated to eventually be developed into clinical applications to tackle treatment-related issues effectively.

Advances in Biomimetic Nerve Guidance Conduits for Peripheral Nerve Regeneration

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

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.

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

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

Preparation and mechanical optimization of a two-layer silk/magnesium wires braided porous artificial nerve guidance conduit

Peripheral nerve injures have long been a tricky problem in surgery and a feasible treatment is the transplantation of nerve guidance conduits (NGCs). This study presents a two-layer composite NGC with fair mechanical properties and good biocompatibility. The inner layer was made of degummed silk yarns/magnesium wires using braiding technology, and the outer layer was made from mixed solution of silk fibroin/chitosan (SF/CS) using freeze-drying treatment. Orthogonal experimental design was applied to rationally design the braided structural layer and obtain the optimal combination of technical process parameters. Meanwhile, the SF/CS porous outer layer was optimized from three concentrations of SF/CS solution. In vitro and in vivo study suggested that the textile-forming scaffold exhibited good biocompatibility and no toxicity. During 4 weeks' degradation, the skeleton of conduits retained its shape, and magnesium ions released from degraded magnesium wires contributed to sustainable release and uniform dispersion, proliferation and adhesion of Schwann cells, indicating potential approach in the development of NGCs.

Development of a regenerative porous PLCL nerve guidance conduit with swellable hydrogel-based microgrooved surface pattern via 3D printing

Peripheral nerve injury causes severe loss of motor and sensory functions, consequently increasing morbidity in affected patients. An autogenous nerve graft is considered the current gold standard for reconstructing nerve defects and recovering lost neurological functions; however, there are certain limitations to this method, such as limited donor nerve supply. With advances in regenerative medicine, recent research has focused on the fabrication of tissue-engineered nerve grafts as promising alternatives to the autogenous nerve grafts. In this study, we designed a nerve guidance conduit using an electrospun poly(lactide-co-ε-caprolactone) (PLCL) membrane with a visible light-crosslinked gelatin hydrogel. The PLCL nanoporous membrane with permeability served as a flexible and non-collapsible epineurium for the nerve conduit; the inner-aligned gelatin hydrogel paths were fabricated via 3D printing and a photocrosslinking system. The resultant gelatin hydrogel with microgrooved surface pattern was established as a conducting guidance path for the effective regeneration of axons and served as a reservoir that can incorporate and release bioactive molecules. From in vivo performance tests using a rat sciatic nerve defect model, our PLCL/gelatin conduit demonstrated successful axonal regeneration, remyelination capacities and facilitated functional recovery. Hence, the PLCL/gelatin conduit developed in this study is a promising substitute for autogenous nerve grafts. STATEMENT OF SIGNIFICANCE: Nerve guidance conduits (NGCs) are developed as promising recovery techniques for bridging peripheral nerve defects. However, there are still technological limitations including differences in the structures and components between natural peripheral nerve and NGCs. In this study, we designed a NGC composed of an electrospun poly(lactide-co-ε-caprolactone) (PLCL) membrane and 3D printed inner gelatin hydrogel to serve as a flexible and non-collapsible epineurium and a conducting guidance path, respectively, to mimic the fascicular structure of the peripheral nerve. In particular, in vitro cell tests clearly showed that gelatin hydrogel could guide the cells and function as a reservoir that incorporate and release nerve growth factor. From in vivo performance tests, our regenerative conduit successfully led to axonal regeneration with effective functional recovery.

The role of exosomes in inter-cellular and inter-organ communication of the peripheral nervous system

Exosomes, nano-sized extracellular vesicles, are produced via the endosomal pathway and released in the extracellular space upon fusion of multivesicular bodies with the plasma membrane. Recent evidence shows that these extracellular vesicles play a key role in cell-to-cell communication. Exosomes transport bioactive proteins, messenger RNA (mRNAs) and microRNA (miRNAs) in an active form to adjacent cells or to distant organs. In this review, we focus on the role of exosomes in peripheral nerve maintenance and repair, as well as peripheral nerve/organ crosstalk, and discuss the potential benefits of exploiting exosomes for treating PNS injuries. In addition, we will highlight the emerging role of exosomes as new important vehicles for physiological systemic crosstalk failures, which could lead to organ dysfunction during neuroinflammation or aging.

Effect of 3D Printing Temperature on Bioactivity of Bone Morphogenetic Protein-2 Released from Polymeric Constructs

Growth factors such as bone morphogenetic protein-2 (BMP-2) are potent tools for tissue engineering. Three-dimensional (3D) printing offers a potential strategy for delivery of BMP-2 from polymeric constructs; however, these biomolecules are sensitive to inactivation by the elevated temperatures commonly employed during extrusion-based 3D printing. Therefore, we aimed to correlate printing temperature to the bioactivity of BMP-2 released from 3D printed constructs composed of a model polymer, poly(propylene fumarate). Following encapsulation of BMP-2 in poly(DL-lactic-co-glycolic acid) particles, growth factor-loaded fibers were fabricated at three different printing temperatures. Resulting constructs underwent 28 days of aqueous degradation for collection of released BMP-2. Supernatants were then assayed for the presence of bioactive BMP-2 using a cellular assay for alkaline phosphatase activity. Cumulative release profiles indicated that BMP-2 released from constructs that were 3D printed at physiologic and intermediate temperatures exhibited comparable total amounts of bioactive BMP-2 release as those encapsulated in non-printed particulate delivery vehicles. Meanwhile, the elevated printing temperature of 90 °C resulted in a decreased amount of total bioactive BMP-2 release from the fibers. These findings elucidate the effects of elevated printing temperatures on BMP-2 bioactivity during extrusion-based 3D printing, and enlighten polymeric material selection for 3D printing with growth factors.

Tissue engineered axon-based "living scaffolds" promote survival of spinal cord motor neurons following peripheral nerve repair

Peripheral nerve injury (PNI) impacts millions annually, often leaving debilitated patients with minimal repair options to improve functional recovery. Our group has previously developed tissue engineered nerve grafts (TENGs) featuring long, aligned axonal tracts from dorsal root ganglia (DRG) neurons that are fabricated in custom bioreactors using the process of axon "stretch-growth." We have shown that TENGs effectively serve as "living scaffolds" to promote regeneration across segmental nerve defects by exploiting the newfound mechanism of axon-facilitated axon regeneration, or "AFAR," by simultaneously providing haptic and neurotrophic support. To extend this work, the current study investigated the efficacy of living versus nonliving regenerative scaffolds in preserving host sensory and motor neuronal health following nerve repair. Rats were assigned across five groups: naïve, or repair using autograft, nerve guidance tube (NGT) with collagen, NGT + non-aligned DRG populations in collagen, or TENGs. We found that TENG repairs yielded equivalent regenerative capacity as autograft repairs based on preserved health of host spinal cord motor neurons and acute axonal regeneration, whereas NGT repairs or DRG neurons within an NGT exhibited reduced motor neuron preservation and diminished regenerative capacity. These acute regenerative benefits ultimately resulted in enhanced levels of functional recovery in animals receiving TENGs, at levels matching those attained by autografts. Our findings indicate that TENGs may preserve host spinal cord motor neuron health and regenerative capacity without sacrificing an otherwise uninjured nerve (as in the case of the autograft) and therefore represent a promising alternative strategy for neurosurgical repair following PNI.

Machine intelligence for nerve conduit design and production

Nerve guidance conduits (NGCs) have emerged from recent advances within tissue engineering as a promising alternative to autografts for peripheral nerve repair. NGCs are tubular structures with engineered biomaterials, which guide axonal regeneration from the injured proximal nerve to the distal stump. NGC design can synergistically combine multiple properties to enhance proliferation of stem and neuronal cells, improve nerve migration, attenuate inflammation and reduce scar tissue formation. The aim of most laboratories fabricating NGCs is the development of an automated process that incorporates patient-specific features and complex tissue blueprints (e.g. neurovascular conduit) that serve as the basis for more complicated muscular and skin grafts. One of the major limitations for tissue engineering is lack of guidance for generating tissue blueprints and the absence of streamlined manufacturing processes. With the rapid expansion of machine intelligence, high dimensional image analysis, and computational scaffold design, optimized tissue templates for 3D bioprinting (3DBP) are feasible. In this review, we examine the translational challenges to peripheral nerve regeneration and where machine intelligence can innovate bottlenecks in neural tissue engineering.

Time-restricted release of multiple neurotrophic factors promotes axonal regeneration and functional recovery after peripheral nerve injury

Delivery of multiple neurotrophic factors (NTFs), especially with time-restricted release kinetics, holds great potential for nerve repair. In this study, we utilized the tetracycline-regulatable Tet-On 3G system to control the expression of c-Jun, which is a common regulator of multiple NTFs in Schwann cells (SCs). In vitro, Tet-On/c-Jun-modified SCs showed a tightly controllable secretion of multiple NTFs, including glial cell line-derived NTF, nerve growth factor, brain-derived NTF, and artemin, by the addition or removal of doxycycline (Dox). When Tet-On/c-Jun-transduced SCs were grafted in vivo, the expression of NTFs could also be regulated by oral administration or removal of Dox. Fluoro-Gold retrograde tracing results indicated that a biphasic NTF expression scheme (Dox+3/-9, NTFs were up-regulated for 3 wk and declined to physiologic levels for another 9 wk) achieved more axonal regeneration than continuous up-regulation of NTFs (Dox+12) or no NTF induction (Dox-12). More importantly, the Dox+3/-9-group animals showed much better functional recovery than the animals in the Dox+12 and Dox-12 groups. Our findings, for the first time, demonstrated drug-controllable expression of multiple NTFs in nerve repair cells both in vitro and in vivo. These findings provide new hope for developing an optimal therapeutic alternative for nerve repair through the time-restricted release of multiple NTFs using Tet-On/c-Jun-modified SCs.-Huang, L., Xia, B., Shi, X., Gao, J., Yang, Y., Xu, F., Qi, F., Liang, C., Huang, J., Luo, Z. Time-restricted release of multiple neurotrophic factors promotes axonal regeneration and functional recovery after peripheral nerve injury.

Tissue-engineered nerve grafts using a scaffold-independent and injectable drug delivery system: a novel design with translational advantages

OBJECTIVE

Currently commercially available nerve conduits have demonstrated suboptimal clinical efficacy in repairing peripheral nerve defects. Although tissue-engineered nerve grafts (TENGs) with sustained release of neurotrophic factors (NTFs) are experimentally proved to be more effective than these blank conduits, there remains a lack of clinical translation. NTFs are typically immobilized onto scaffold materials of the conduit via adsorption, specific binding or other incorporation techniques. These scaffold-based delivery strategies increase complexity and cost of conduit fabrication and lack flexibility in choosing different drugs. Therefore, to facilitate clinical translation and commercialization, we construct a TENG using a scaffold-independent drug delivery system (DDS).

APPROACH

This study adopted a scaffold-independent DDS based on methoxy-poly (ethylene glycol)-b-poly(γ-ethyl-L-glutamate) (mPEG-PELG) thermosensitive hydrogels that undergo sol-to-gel transition at body temperature. In addition, TENG, a chitosan scaffold filled with nerve growth factor (NGF)-loaded mPEG-PELG that gel in the lumen upon injection during surgery and function as a drug-releasing conduit-filler, was designed. Subsequently, the efficacy of DDS and therapeutic effects of TENG were assessed.

MAIN RESULTS

The results demonstrated that NGF-loaded mPEG-PELG controllably and sustainably released bioactive NGF for 28 d. When bridging a 10 mm rat sciatic nerve gap, the morphological, electrophysiological, and functional analyses revealed that NGF-releasing TENG (Scaffold  +  NGF/mPEG-PELG) achieved superior regenerative outcomes compared to plain scaffolds and those combined with systemic delivery of NGF (daily intramuscular injection (IM)), and its effects were relatively similar to autografts.

SIGNIFICANCE

This study has proposed a TENG using thermosensitive hydrogels as an injectable implant to controllably release NGF, which has promising therapeutic potential and translatability. Such TENGs obviate the need for conduit modification, complex preloading or binding mediators, therefore they allow the ease of drug switching in clinical practice and greatly simplify the manufacturing process due to the independent preparation of drug delivery system.

Sericin Nerve Guidance Conduit Delivering Therapeutically Repurposed Clobetasol for Functional and Structural Regeneration of Transected Peripheral Nerves

Peripheral nerve injury often causes significant function loss. Autologous nerve grafting as a gold-standard repair strategy for treating such an injury is limited by donor nerve supply. Tissue-engineered nerve guidance conduits (TENGCs) as a promising alternative for autografting are challenged by large nerve gaps. Herein, we fabricate a glutaraldehyde-cross-linked sericin nerve guidance conduit (GSC) incorporated with clobetasol, a glucocorticoid receptor agonist, for repairing a 10 mm long sciatic nerve gap in a rat model. The GSC exhibits biocompatibility and regeneration-favorable physicochemical properties. GSC's degradation products promote the secretion of neurotrophic factors in Schwann cells. By repurposing clobetasol for peripheral nerve regeneration, our work uncovers clobetasol's previously unknown functions in promoting Schwann cell proliferation and upregulating the expression of myelin-related genes. Importantly, the implantation of this clobetasol-loaded GSC in vivo leads to successful regeneration of the transected sciatic nerve. Strikingly, the regeneration outcome is functionally comparable to that of autologous nerve grafting (evidenced by three parameters). Specifically, the static sciatic index (SSI), relative reaction time (RRT) and nerve conduction velocity (NCV) in Clobetasol/GSC group are -74.55, 1.30, and 46.4 mm/s at Week 12, respectively, while these parameters are -64.53, 1.23, and 49.8 mm/s in Autograft group. Thus, this work represents the first report unveiling clobetasol's potential in peripheral nerve regeneration, reveals the feasibility of applying a sericin conduit for repairing a large nerve defect, and demonstrates the effectiveness of the clobetasol-loaded-GSC based strategy in transected nerves' regeneration.

Current concepts in peripheral nerve surgery

The injuries of the peripheral nerves are relatively frequent. Some of them may lead to defects which cannot be repaired with direct end-to-end repair without tension. These injuries may cause function loss to the patient, and they consist a challenge for the treating microsurgeon. Autologous nerve grafts remain the gold standard for bridging the peripheral nerve defects. Nevertheless, there are selected cases where alternative types of nerve reconstruction can be performed in order to cover the peripheral nerve defects. In all these types of reconstruction, the basic principles of microsurgery are necessary and the surgeon should be aware of them in order to achieve a successful reconstruction. The purpose of the present review was to present the most current data concerning the surgical options available for bridging such defects.

Drug-delivering nerve conduit improves regeneration in a critical-sized gap

Autologous nerve grafts are the current "gold standard" for repairing large nerve gaps. However, they cause morbidity at the donor nerve site and only a limited amount of nerve can be harvested. Nerve conduits are a promising alternative to autografts and can act as guidance cues for the regenerating axons, without the need to harvest donor nerve. Separately, it has been shown that localized delivery of GDNF can enhance axon growth and motor recovery. FK506, an FDA approved small molecule, has also been shown to enhance peripheral nerve regeneration. This paper describes the design of a novel hole-based drug delivery apparatus integrated with a polytetrafluoroethylene (PTFE) nerve conduit for controlled local delivery of a protein such as GDNF or a small molecule such as FK506. The PTFE devices were tested in a diffusion chamber, and the bioactivity of the released media was evaluated by measuring neurite growth of dorsal root ganglions (DRGs) exposed to the released drugs. The drug delivering nerve guide was able to release bioactive concentrations of FK506 or GDNF. Following these tests, optimized drug releasing nerve conduits were implanted across 10 mm sciatic nerve gaps in a BL6 yellow fluorescent protein (YFP) mouse model, where they demonstrated significant improvement in muscle mass, compound muscle action potential, and axon myelination in vivo as compared with nerve conduits without the drug. The drug delivery nerve guide could release drug for extended periods of time and enhance axon growth in vitro and in vivo.

Biomaterials and cells for neural tissue engineering: Current choices

The treatment of nerve injuries has taken a new dimension with the development of tissue engineering techniques. Prior to tissue engineering, suturing and surgery were the only options for effective treatment. With the advent of tissue engineering, it is now possible to design a scaffold that matches the exact biological and mechanical properties of the tissue. This has led to substantial reduction in the complications posed by surgeries and suturing to the patients. New synthetic and natural polymers are being applied to test their efficiency in generating an ideal scaffold. Along with these, cells and growth factors are also being incorporated to increase the efficiency of a scaffold. Efforts are being made to devise a scaffold that is biodegradable, biocompatible, conducting and immunologically inert. The ultimate goal is to exactly mimic the extracellular matrix in our body, and to elicit a combination of biochemical, topographical and electrical cues via various polymers, cells and growth factors, using which nerve regeneration can efficiently occur.

PEOT/PBT Guides Enhance Nerve Regeneration in Long Gap Defects

Development of new nerve guides is required for replacing autologous nerve grafts for the repair of long gap defects after nerve injury. A nerve guide comprised only of electrospun fibers able to bridge a critical (15 mm) nerve gap in a rat animal model is reported for the first time. The nerve conduits are made of poly(ethylene oxide terephthalate) and poly(butylene terephthalate) (PEOT/PBT), a biocompatible copolymer composed of alternating amorphous, hydrophilic poly(ethylene oxide terephthalate), and crystalline, hydrophobic poly(butylene terephthalate) segments. These guides show suitable mechanical properties, high porosity, and fibers aligned in the longitudinal axis of the guide. In vitro studies show that both neurites and Schwann cells exhibit growth alignment with PA fibers. In vivo studies reveal that, after rat sciatic nerve transection and repair with PEOT/PBT guides, axons grow occupying a larger area compared to silicone tubes. Moreover, after repair of limiting (10 mm) and critical (15 mm) nerve gaps, PEOT/PBT guides significantly increase the percentage of regenerated nerves, the number of regenerated myelinated axons, and improve motor, sensory, and autonomic reinnervation in both gaps. This nerve conduit design combines the properties of PEOT/PBT with electrospun structure, demonstrating that nerve regeneration through long gaps can be achieved through the design of instructive biomaterial constructs.

Reconstruction of Heel Soft Tissue Defects Using Medial Plantar Artery Island Pedicle Flap: Clinical Experience and Outcomes Analysis

We report our clinical experience with 11 cases of heel soft tissue defects treated with a medial plantar artery island pedicle flap. We also evaluated the functional outcomes of reconstruction. We enrolled 11 patients (10 [90.9%] males and 1 [9.1%] female), who received a medial plantar artery island pedicle flap for heel soft tissue defects from August 2010 to July 2014. The cause of the defects included trauma (post-traumatic heel skin necrosis) in 2 (18.2%) patients, infection (calcaneal osteomyelitis with overlying soft tissue infection) in 2 (18.2%) patients, and tumor (stage IA melanoma) in 7 (63.6%) patients. The mean average size of the defect was 3.7 cm × 4.4 cm (range 3.1 cm × 4.0 cm to 4.5  cm × 5.5  cm). The mean follow-up period was 19.6 (range 8 to 35) months, and all flaps survived within that period. The mean static 2-point discrimination was 34.4 (range 29 to 40) mm at the heel and 17.2 mm at the distal sole. The mean visual analog scale score for the aesthetic appearance of the reconstructed heel was 9 (range 8 to 9.5). The average revised Foot Function Index score was 25.8 (range 21 to 37). The medial plantar artery island pedicle flap is a versatile and effective method for reconstructing heel soft tissue defects.

Effect Of combining FK506 and neurotrophins on neurite branching and elongation

INTRODUCTION

There is a clinical need to improve the outcomes of peripheral nerve regeneration and repair after injury. In addition to its immunosuppressive effects, FK506 (tacrolimus) has been shown to have neuroregenerative properties. To determine biologically relevant local FK506 and growth factor concentrations, we performed an in vitro bioassay using dorsal root ganglion (DRG) from chicken embryos.

METHODS

Neurite elongation and neurite branching were analyzed microscopically after addition of FK506, glial cell line-derived neurotrophic factor (GDNF), and nerve growth factor (NGF), each alone and in combination.

RESULTS

FK506 induced modest neurite elongation (∼500-800 µm) without improving neurite branching significantly. The combination of FK506 with NGF, GDNF, or both, exerted a potentiating or competitive effect on neurite elongation (∼700-1100 µm) based on dosage and competitive effect on neurite branching (∼0.2-0.4).

CONCLUSIONS

These results strongly suggest that the interaction of FK506 with GDNF and NGF mediates distinct enhancement of neurite growth. Muscle Nerve 55: 570-581, 2017.

Gelatin/nanoceria nanocomposite fibers as antioxidant scaffolds for neuronal regeneration

BACKGROUND

The design of efficient nerve conduits able to sustain the axonal outgrowth and its guidance towards appropriate targets is of paramount importance in nerve tissue engineering.

METHODS

In this work, we propose the preparation of highly aligned nanocomposite fibers of gelatin/cerium oxide nanoparticles (nanoceria), prepared by electrospinning. Nanoceria are powerful self-regenerative antioxidant nanomaterials, that behave as strong reactive oxygen species scavengers, and among various beneficial effects, they have been proven to inhibit the cell senescence and to promote the neurite sprouting.

RESULTS

After a detailed characterization of the developed substrates, they have been tested on neuron-like SH-SY5Y cells, demonstrating strong antioxidant properties and beneficial multi-cue effects in terms of neurite development and alignment.

CONCLUSIONS

Obtained findings suggest efficiency of the proposed substrates in providing combined topographical stimuli and antioxidant effects to cultured cells.

GENERAL SIGNIFICANCE

Proposed nanocomposite scaffolds represent a promising approach for nerve tissue engineering and regenerative medicine.

Approaches to Peripheral Nerve Repair: Generations of Biomaterial Conduits Yielding to Replacing Autologous Nerve Grafts in Craniomaxillofacial Surgery

Peripheral nerve injury is a common clinical entity, which may arise due to traumatic, tumorous, or even iatrogenic injury in craniomaxillofacial surgery. Despite advances in biomaterials and techniques over the past several decades, reconstruction of nerve gaps remains a challenge. Autografts are the gold standard for nerve reconstruction. Using autografts, there is donor site morbidity, subsequent sensory deficit, and potential for neuroma development and infection. Moreover, the need for a second surgical site and limited availability of donor nerves remain a challenge. Thus, increasing efforts have been directed to develop artificial nerve guidance conduits (ANCs) as new methods to replace autografts in the future. Various synthetic conduit materials have been tested in vitro and in vivo, and several first- and second-generation conduits are FDA approved and available for purchase, while third-generation conduits still remain in experimental stages. This paper reviews the current treatment options, summarizes the published literature, and assesses future prospects for the repair of peripheral nerve injury in craniomaxillofacial surgery with a particular focus on facial nerve regeneration.

Advances in the neurological and neurosurgical management of peripheral nerve trauma

Peripheral nerve trauma frequently affects younger people and may result in significant and long-lasting functional disability. Currently, diagnosis and monitoring of peripheral nerve injury relies on clinical and electrodiagnostic information, supplemented by intraoperative electrophysiological studies. However, in a significant proportion of nerve injuries, the likelihood of spontaneous regeneration resulting in good functional outcome remains uncertain and unnecessary delays to treatment may be faced while monitoring for recovery. Advances in non-invasive imaging techniques to diagnose and monitor nerve injury and regeneration are being developed, and have the potential to streamline the decision-making process. In addition, advances in operative and non-operative treatment strategies may provide more effective ways to maximise functional outcomes following severe peripheral nerve trauma. This review discusses these advances in light of the current state of the art of management of peripheral nerve trauma.

Past, Present, and Future of Nerve Conduits in the Treatment of Peripheral Nerve Injury

With significant advances in the research and application of nerve conduits, they have been used to repair peripheral nerve injury for several decades. Nerve conduits range from biological tubes to synthetic tubes, and from nondegradable tubes to biodegradable tubes. Researchers have explored hollow tubes, tubes filled with scaffolds containing neurotrophic factors, and those seeded with Schwann cells or stem cells. The therapeutic effect of nerve conduits is improving with increasing choice of conduit material, new construction of conduits, and the inclusion of neurotrophic factors and support cells in the conduits. Improvements in functional outcomes are expected when these are optimized for use in clinical practice.

Nerve autografts and tissue-engineered materials for the repair of peripheral nerve injuries: a 5-year bibliometric analysis

With advances in biomedical methods, tissue-engineered materials have developed rapidly as an alternative to nerve autografts for the repair of peripheral nerve injuries. However, the materials selected for use in the repair of peripheral nerve injuries, in particular multiple injuries and large-gap defects, must be chosen carefully. Various methods and materials for protecting the healthy tissue and repairing peripheral nerve injuries have been described, and each method or material has advantages and disadvantages. Recently, a large amount of research has been focused on tissue-engineered materials for the repair of peripheral nerve injuries. Using the keywords “pe-ripheral nerve injury”, “autotransplant”, “nerve graft”, and “biomaterial”, we retrieved publications using tissue-engineered materials for the repair of peripheral nerve injuries appearing in the Web of Science from 2010 to 2014. The country with the most total publications was the USA. The institutions that were the most productive in this field include Hannover Medical School (Germany), Washington University (USA), and Nantong University (China). The total number of publications using tissue-engineered materials for the repair of peripheral nerve injuries grad-ually increased over time, as did the number of Chinese publications, suggesting that China has made many scientific contributions to this field of research.

Injured Nerve Regeneration using Cell-Based Therapies: Current Challenges

This paper reviews the recent research progress in the past several years on promoting peripheral nerve recovery using stem and progenitory cells. The emphasis is placed on studies aimed at assessing various stem cells capable of expressing neurotrophic and growth factors and surviving after implantation in the nerve or a conduit. Approaches to improving nerve conduit design are summarized. The contribution of stem cells to axonal regeneration and neural repair is discussed. The side effects associated with cell-based treatment are highlighted. From the studies reviewed, it is concluded that the fate of transplanted stem cells needs further elucidation in a microenvironment-dependent manner.

Carbon-nanotube-interfaced glass fiber scaffold for regeneration of transected sciatic nerve

Carbon nanotubes (CNTs), with their unique and unprecedented properties, have become very popular for the repair of tissues, particularly for those requiring electrical stimuli. Whilst most reports have demonstrated in vitro neural cell responses of the CNTs, few studies have been performed on the in vivo efficacy of CNT-interfaced biomaterials in the repair and regeneration of neural tissues. Thus, we report here for the first time the in vivo functions of CNT-interfaced nerve conduits in the regeneration of transected rat sciatic nerve. Aminated CNTs were chemically tethered onto the surface of aligned phosphate glass microfibers (PGFs) and CNT-interfaced PGFs (CNT-PGFs) were successfully placed into three-dimensional poly(L/D-lactic acid) (PLDLA) tubes. An in vitro study confirmed that neurites of dorsal root ganglion outgrew actively along the aligned CNT-PGFs and that the CNT interfacing significantly increased the maximal neurite length. Sixteen weeks after implantation of a CNT-PGF nerve conduit into the 10 mm gap of a transected rat sciatic nerve, the number of regenerating axons crossing the scaffold, the cross-sectional area of the re-innervated muscles and the electrophysiological findings were all significantly improved by the interfacing with CNTs. This first in vivo effect of using a CNT-interfaced scaffold in the regeneration process of a transected rat sciatic nerve strongly supports the potential use of CNT-interfaced PGFs at the interface between the nerve conduit and peripheral neural tissues.