PEOT/PBT Guides Enhance Nerve Regeneration in Long Gap Defects

Organ Neuroprosthetics: Connecting Transplanted and Artificial Organs with the Nervous System

Implantable neural interfaces with the central and peripheral nervous systems are currently used to restore sensory, motor, and cognitive functions in disabled people with very promising results. They have also been used to modulate autonomic activities to treat diseases such as diabetes or hypertension. Here, this study proposes to extend the use of these technologies to (re-)establish the connection between new (transplanted or artificial) organs and the nervous system in order to increase the long-term efficacy and the effective biointegration of these solutions. In this perspective paper, some clinically relevant applications of this approach are briefly described. Then, the choices that neural engineers must implement about the type, implantation location, and closed-loop control algorithms to successfully realize this approach are highlighted. It is believed that these new "organ neuroprostheses" are going to become more and more valuable and very effective solutions in the years to come.

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

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

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

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

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.

Multifaceted polymeric nerve guidance conduits with distinctive double-layered architecture and plasma-induced inner chemistry gradient for the repair of critical-sized defects

Despite tissue engineering advances, current nerve guidance conduits (NGCs) are still failing in repairing critical-sized defects. This study aims, therefore, at tackling large nerve gaps (2 cm) by designing NGCs possessing refined physicochemical properties enhancing the activity of Schwann cells (SCs) that support nerve regeneration over long distances. As such, a combinatorial strategy adopting novel plasma-induced surface chemistry and architectural heterogeneity was considered. A mechanically suitable copolymer (Polyactive®) was electrospun to produce nanofibrous NGCs mimicking the extracellular matrix. An innovative seamless double-layered architecture consisting of an inner wall comprised of bundles of aligned fibers with intercalated random fibers and an outer wall fully composed of random fibers was conceived to synergistically provide cell guidance cues and sufficient nutrient inflow. NGCs were subjected to argon plasma treatments using a dielectric barrier discharge (DBD) and a plasma jet (PJ). Surface chemical changes were examined by advanced X-ray photoelectron spectroscopy (XPS) micro-mappings. The DBD homogeneously increased the surface oxygen content from 17 % to 28 % on the inner wall. The PJ created a gradient chemistry throughout the inner wall with an oxygen content gradually increasing from 21 % to 30 %. In vitro studies revealed enhanced primary SC adhesion, elongation and proliferation on plasma-treated NGCs. A cell gradient was observed on the PJ-treated NGCs thus underlining the favorable oxygen gradient in promoting cell chemotaxis. A gradual change from circular to highly elongated SC morphologies mimicking the bands of Büngner was visualized along the gradient. Overall, plasma-treated NGCs are promising candidates paving the way towards critical nerve gap repair.

PCL NGCs integrated with urolithin-A-loaded hydrogels for nerve regeneration

Inflammation and oxidative stress are among the leading causes of poor prognosis after peripheral nerve injury (PNI). Urolithin-A (UA), an intermediate product produced by the catabolism of ellagitannins in the gastrointestinal tract, has anti-inflammatory, antioxidant, and immunomodulatory properties for inflammation, oxidative damage, and aging-related diseases. Hence, we prepared UA-loaded hydrogels and embedded them in the lumen of PCL nerve guide conduits (NGCs). The hydrogels continuously released appropriate doses of UA into the microenvironment. Based on in vitro studies, UA facilitates cell proliferation and reduces oxidative damage. Besides, the experimental evaluation revealed good biocompatibility of the materials involved. We implanted NGCs into rat models to bridge the sciatic nerve defects in an in vivo study. The sciatic functional index of the PCL/collagen/UA group was comparable to that of the autograft group. Additionally, the consequences of electrophysiological, gastrocnemius muscle and nerve histology assessment of the PCL/collagen/UA group were better than those in the PCL and PCL/collagen groups and close to those in the autograft group. In this study, UA sustained release via the PCL/collagen/UA NGC was found to be an effective alternative treatment for PNI, validating our hypothesis that UA could promote regeneration of nerve tissue.

Multimodular Bio-Inspired Organized Structures Guiding Long-Distance Axonal Regeneration

Axonal bundles or axonal tracts have an aligned and unidirectional architecture present in many neural structures with different lengths. When peripheral nerve injury (PNI), spinal cord injury (SCI), traumatic brain injury (TBI), or neurodegenerative disease occur, the intricate architecture undergoes alterations leading to growth inhibition and loss of guidance through large distance. In order to overcome the limitations of long-distance axonal regeneration, here we combine a poly-L-lactide acid (PLA) fiber bundle in the common lumen of a sequence of hyaluronic acid (HA) conduits or modules and pre-cultured Schwann cells (SC) as cells supportive of axon extension. This multimodular preseeded conduit is then used to induce axon growth from a dorsal root ganglion (DRG) explant placed at one of its ends and left for 21 days to follow axon outgrowth. The multimodular conduit proved effective in promoting directed axon growth, and the results may thus be of interest for the regeneration of long tissue defects in the nervous system. Furthermore, the hybrid structure grown within the HA modules consisting in the PLA fibers and the SC can be extracted from the conduit and cultured independently. This “neural cord” proved to be viable outside its scaffold and opens the door to the generation of ex vivo living nerve in vitro for transplantation.

Development of an In Vitro Biomimetic Peripheral Neurovascular Platform

Nerves and blood vessels are present in most organs and are indispensable for their function and homeostasis. Within these organs, neurovascular (NV) tissue forms congruent patterns and establishes vital interactions. Several human pathologies, including diabetes type II, produce NV disruptions with serious consequences that are complicated to study using animal models. Complex in vitro organ platforms, with neural and vascular supply, allow the investigation of such interactions, whether in a normal or pathological context, in an affordable, simple, and direct manner. To date, a few in vitro models contain NV tissue, and most strategies report models with nonbiomimetic representations of the native environment. To this end, we have established here an NV platform that contains mature vasculature and neural tissue, composed of human microvascular endothelial cells (HMVECs), induced pluripotent stem cell (iPSCs)-derived sensory neurons, and primary rat Schwann cells (SCs) within a fibrin-embedded polymeric scaffold. First, we show that SCs can induce the formation of and stabilize vascular networks to the same degree as the traditional and more thoroughly studied human dermal fibroblasts (HDFs). We also show that through SC prepatterning, we are able to control vessel orientation. Using our NV platform, we demonstrate the concomitant formation of three-dimensional neural and vascular tissue, and the influence of different medium formulations and cell types on the NV tissue outcome. Finally, we propose a protocol to form mature NV tissue, via the integration of independent neural and vascular constituents. The platform described here provides a versatile and advanced model for in vitro research of the NV axis.

Micropatterns and peptide gradient on the inner surface of a guidance conduit synergistically promotes nerve regeneration in vivo

Both of the surface topographical features and distribution of biochemical cues can influence the cell-substrate interactions and thereby tissue regeneration in vivo. However, they have not been combined simultaneously onto a biodegradable scaffold to demonstrate the synergistic role so far. In this study, a proof-of-concept study is performed to prepare micropatterns and peptide gradient on the inner wall of a poly (D,L-lactide-co-caprolactone) (PLCL) guidance conduit and its advantages in regeneration of peripheral nerve in vivo. After linear ridges/grooves of 20/40 μm in width are created on the PLCL film, its surface is aminolyzed in a kinetically controlled manner to obtain the continuous gradient of amino groups, which are then transferred to CQAASIKVAV peptide density gradient via covalent coupling of glutaraldehyde. The Schwann cells are better aligned along with the stripes, and show a faster migration rate toward the region of higher peptide density. Implantation of the nerve guidance conduit made of the PLCL film having both the micropatterns and peptide gradient can significantly accelerate the regeneration of sciatic nerve in terms of rate, function recovery and microstructures, and reduction of fibrosis in muscle tissues. Moreover, this nerve conduit can also benefit the M2 polarization of macrophages and promote vascularization in vivo.

Graphene-Based Materials Prove to Be a Promising Candidate for Nerve Regeneration Following Peripheral Nerve Injury

Peripheral nerve injury is a common medical condition that has a great impact on patient quality of life. Currently, surgical management is considered to be a gold standard first-line treatment; however, is often not successful and requires further surgical procedures. Commercially available FDA- and CE-approved decellularized nerve conduits offer considerable benefits to patients suffering from a completely transected nerve but they fail to support neural regeneration in gaps > 30 mm. To address this unmet clinical need, current research is focused on biomaterial-based therapies to regenerate dysfunctional neural tissues, specifically damaged peripheral nerve, and spinal cord. Recently, attention has been paid to the capability of graphene-based materials (GBMs) to develop bifunctional scaffolds for promoting nerve regeneration, often via supporting enhanced neural differentiation. The unique features of GBMs have been applied to fabricate an electroactive conductive surface in order to direct stem cells and improve neural proliferation and differentiation. The use of GBMs for nerve tissue engineering (NTE) is considered an emerging technology bringing hope to peripheral nerve injury repair, with some products already in preclinical stages. This review assesses the last six years of research in the field of GBMs application in NTE, focusing on the fabrication and effects of GBMs for neurogenesis in various scaffold forms, including electrospun fibres, films, hydrogels, foams, 3D printing, and bioprinting.

3D culture platform of human iPSCs-derived nociceptors for peripheral nerve modelling and tissue innervation

Functional humanized in vitro nerve models are coveted as an alternative to animal models due to their ease of access, lower cost, clinical relevance and no need for recurrent animal sacrifice. To this end, we developed a sensory nerve model using induced pluripotent stem cells (iPSCs)-derived nociceptors that are electrically active and exhibit a functional response to noxious stimuli. The differentiated neurons were co-cultured with primary Schwann cells on an aligned microfibrous scaffold to produce biomimetic peripheral nerve tissue. Compared to glass coverslips, our scaffold enhances tissue development and stabilization. Using this model, we demonstrate that myelin damage can be induced from hyperglycemia exposure (glucose at 45 mM) and mitigated by epalrestat (1µM) supplementation. Through fibrin embedding of the platform, we were able to create 3D anisotropic myelinated tissue, reaching over 6.5 mm in length. Finally, as a proof-of-concept, we incorporated pancreatic pseudoislets and endometrial organoids into our nerve platform, to demonstrate the potential in generating nociceptor innervation models. In summary, we propose here an improved tool for neurobiology research with potential applications in pathology modelling, drug screening and target tissue innervation.

Additive Manufactured Scaffolds for Bone Tissue Engineering: Physical Characterization of Thermoplastic Composites with Functional Fillers

Thermoplastic polymer-filler composites are excellent materials for bone tissue engineering (TE) scaffolds, combining the functionality of fillers with suitable load-bearing ability, biodegradability, and additive manufacturing (AM) compatibility of the polymer. Two key determinants of their utility are their rheological behavior in the molten state, determining AM processability and their mechanical load-bearing properties. We report here the characterization of both these physical properties for four bone TE relevant composite formulations with poly(ethylene oxide terephthalate)/poly(butylene terephthalate (PEOT/PBT) as a base polymer, which is often used to fabricate TE scaffolds. The fillers used were reduced graphene oxide (rGO), hydroxyapatite (HA), gentamicin intercalated in zirconium phosphate (ZrP-GTM) and ciprofloxacin intercalated in MgAl layered double hydroxide (MgAl-CFX). The rheological assessment showed that generally the viscous behavior dominated the elastic behavior (G″ > G') for the studied composites, at empirically determined extrusion temperatures. Coupled rheological-thermal characterization of ZrP-GTM and HA composites showed that the fillers increased the solidification temperatures of the polymer melts during cooling. Both these findings have implications for the required extrusion temperatures and bonding between layers. Mechanical tests showed that the fillers generally not only made the polymer stiffer but more brittle in proportion to the filler fractions. Furthermore, the elastic moduli of scaffolds did not directly correlate with the corresponding bulk material properties, implying composite-specific AM processing effects on the mechanical properties. Finally, we show computational models to predict multimaterial scaffold elastic moduli using measured single material scaffold and bulk moduli. The reported characterizations are essential for assessing the AM processability and ultimately the suitability of the manufactured scaffolds for the envisioned bone regeneration application.

Advanced Multi-Dimensional Cellular Models as Emerging Reality to Reproduce In Vitro the Human Body Complexity

A hot topic in biomedical science is the implementation of more predictive in vitro models of human tissues to significantly improve the knowledge of physiological or pathological process, drugs discovery and screening. Bidimensional (2D) culture systems still represent good high-throughput options for basic research. Unfortunately, these systems are not able to recapitulate the in vivo three-dimensional (3D) environment of native tissues, resulting in a poor in vitro–in vivo translation. In addition, intra-species differences limited the use of animal data for predicting human responses, increasing in vivo preclinical failures and ethical concerns. Dealing with these challenges, in vitro 3D technological approaches were recently bioengineered as promising platforms able to closely capture the complexity of in vivo normal/pathological tissues. Potentially, such systems could resemble tissue-specific extracellular matrix (ECM), cell–cell and cell–ECM interactions and specific cell biological responses to mechanical and physical/chemical properties of the matrix. In this context, this review presents the state of the art of the most advanced progresses of the last years. A special attention to the emerging technologies for the development of human 3D disease-relevant and physiological models, varying from cell self-assembly (i.e., multicellular spheroids and organoids) to the use of biomaterials and microfluidic devices has been given.

Electrospun Fiber Scaffolds for Engineering Glial Cell Behavior to Promote Neural Regeneration

Electrospinning is a fabrication technique used to produce nano- or micro- diameter fibers to generate biocompatible, biodegradable scaffolds for tissue engineering applications. Electrospun fiber scaffolds are advantageous for neural regeneration because they mimic the structure of the nervous system extracellular matrix and provide contact guidance for regenerating axons. Glia are non-neuronal regulatory cells that maintain homeostasis in the healthy nervous system and regulate regeneration in the injured nervous system. Electrospun fiber scaffolds offer a wide range of characteristics, such as fiber alignment, diameter, surface nanotopography, and surface chemistry that can be engineered to achieve a desired glial cell response to injury. Further, electrospun fibers can be loaded with drugs, nucleic acids, or proteins to provide the local, sustained release of such therapeutics to alter glial cell phenotype to better support regeneration. This review provides the first comprehensive overview of how electrospun fiber alignment, diameter, surface nanotopography, surface functionalization, and therapeutic delivery affect Schwann cells in the peripheral nervous system and astrocytes, oligodendrocytes, and microglia in the central nervous system both in vitro and in vivo. The information presented can be used to design and optimize electrospun fiber scaffolds to target glial cell response to mitigate nervous system injury and improve regeneration.

Oxidative stress in pancreatic alpha and beta cells as a selection criterion for biocompatible biomaterials

The clinical success rate of islet transplantation, namely independence from insulin injections, is limited by factors that lead to graft failure, including inflammation, acute ischemia, acute phase response, and insufficient vascularization. The ischemia and insufficient vascularization both lead to high levels of oxidative stress, which are further aggravated by islet encapsulation, inflammation, and undesirable cell-biomaterial interactions. To identify biomaterials that would not further increase damaging oxidative stress levels and that are also suitable for manufacturing a beta cell encapsulation device, we studied five clinically approved polymers for their effect on oxidative stress and islet (alpha and beta cell) function. We found that 300 poly(ethylene oxide terephthalate) 55/poly(butylene terephthalate) 45 (PEOT/PBT300) was more resistant to breakage and more elastic than other biomaterials, which is important for its immunoprotective function. In addition, it did not induce oxidative stress or reduce viability in the MIN6 beta cell line, and even promoted protective endogenous antioxidant expression over 7 days. Importantly, PEOT/PBT300 is one of the biomaterials we studied that did not interfere with insulin secretion in human islets.

In vitro degradation profiles and in vivo biomaterial-tissue interactions of microwell array delivery devices

To effectively apply microwell array cell delivery devices their biodegradation rate must be tailored towards their intended use and implantation location. Two microwell array devices with distinct degradation profiles, either suitable for the fabrication of retrievable systems in the case of slow degradation, or cell delivery systems capable of extensive remodeling using a fast degrading polymer, were compared in this study. Thin films of a poly(ethylene glycol)‐poly(butylene terephthalate) (PEOT‐PBT) and a poly(ester urethane) were evaluated for their in vitro degradation profiles over 34 weeks incubation in PBS at different pH values. The PEOT‐PBT films showed minimal in vitro degradation over time, while the poly(ester urethane) films showed extensive degradation and fragmentation over time. Subsequently, microwell array cell delivery devices were fabricated from these polymers and intraperitoneally implanted in Albino Oxford rats to study their biocompatibility over a 12‐week period. The PEOT‐PBT implants shown to be capable to maintain the microwell structure over time. Implants provoked a foreign body response resulting in multilayer fibrosis that integrated into the surrounding tissue. The poly(ester urethane) implants showed a loss of the microwell structures over time, as well as a fibrotic response until the onset of fragmentation, at least 4 weeks post implantation. It was concluded that the PEOT‐PBT implants could be used as retrievable cell delivery devices while the poly(ester urethane) implants could be used for cell delivery devices that require remodeling within a 4–12 week period.

A three-dimensional biomimetic peripheral nerve model for drug testing and disease modelling

In vitro peripheral nerve models provide valuable tools to study neurobiology questions and assess drug performance, in a regenerative or pathology context. To this end, we have developed a representative model of the peripheral nerve that displays three-dimensional (3D) neural anisotropy and myelination, which we showcase here as a simple and low-cost platform for drug screening. The model is composed of three main parts, including rat primary Schwann cells (SCs) seeded onto an electrospun scaffold to create bands of Büngner (BoB), primed PC12 cells as neuronal cell population, and a fibrin hydrogel to provide three-dimensionality. We also validated the use of primed PC12 as a neuron population by comparing it to rat dorsal root ganglions (DRGs) neurons. In both models we could obtain well aligned neurites and mature myelin segments. In short term cultures (7 days), we found that the addition of exogenous SCs enhanced neurite length and neurite growth area, compared to scaffolds with a laminin coating only. Addition of fibrin also lead to increased outgrowth of DRG and primed PC12 neurites, compared to 2D cultures. Moreover, neurite outgrowth in fibrin cultures was simultaneously multiplanar and anisotropic, suggesting that the SC-seeded scaffold can direct not only the growth of adjacent neurites, but also those growing above it. These results highlight the feasibility of the combination of a SC pre-seeded scaffold with a fibrin hydrogel, to direct and improve neurite growth in 3D. To demonstrate the model potential, we tested our platform at an immature (7 days in vitro) and mature state (28 days in vitro) of development. At the immature stage we could inhibit neurite growth through protein blocking (via antibody binding) and show suramin (200 μM) neurotoxicity on cells. At the mature stage, when myelin is compact, we exposed cells to hyperglycemic conditions (45 mM glucose) to mimic diabetic conditions and showed that myelin deforms consequently. Moreover, we demonstrated that by supplementing cultures with epalrestat (1 μM), myelin deformation can be partly prevented. In sum, we developed a biomimetic nerve platform using an affordable and accessible cell line as neuronal population, which displays similar results to primary neurons, but does not require recurrent animal sacrifice. This platform holds great promise as it can be used to conveniently and inexpensively perform drug screenings on peripheral nerve-like tissue, in a normal or pathological state.

Regeneration of sciatic nerves by transplanted microvesicles of human neural stem cells derived from embryonic stem cells

Injured nerves cannot regenerate on their own, and a lack of engraftable human nerves has been a major obstacle in cell-based therapies for regenerating damaged nerves. A monolayer culture approach to obtain adherent neural stem cells from human embryonic stem cells (hESC-NSCs) was established, and the greatest number of stemness characteristics were achieved by the eighth generation of hESC-NSCs (P8 hESC-NSCs). To overcome deficits in cell therapy, we used microvesicles secreted from P8 hESC-NSCs (hESC-NSC-MVs) instead of entire hESC-NSCs. To investigate the therapeutic efficacy of hESC-NSC-MVs in vitro, hESC-NSC-MVs were cocultured with dorsal root ganglia to determine the length of axons. In vivo, we transected the sciatic nerve in SD rats and created a 5-mm gap. A sciatic nerve defect was bridged using a silicone tube filled with hESC-NSC-MVs (45 μg) in the MVs group, P8 hESC-NSCs (1 × 106 single cells) in the cell group and PBS in the control group. The hESC-NSC-MVs group showed better morphological recovery and a significantly greater number of regenerated axons than the hESC-NSCs group 12 weeks after nerve injury. These results indicated that the hESC-NSC-MVs group had the greatest ability to repair and reconstruct nerve structure and function. As a result, hESC-NSC-MVs may have potential for applications in the field of nerve regenerative repair.

Tailoring synthetic polymeric biomaterials towards nerve tissue engineering: a review

The nervous system is known as a crucial part of the body and derangement in this system can cause potentially lethal consequences or serious side effects. Unfortunately, the nervous system is unable to rehabilitate damaged regions following seriously debilitating disorders such as stroke, spinal cord injury and brain trauma which, in turn, lead to the reduction of quality of life for the patient. Major challenges in restoring the damaged nervous system are low regenerative capacity and the complexity of physiology system. Synthetic polymeric biomaterials with outstanding properties such as excellent biocompatibility and non-immunogenicity find a wide range of applications in biomedical fields especially neural implants and nerve tissue engineering scaffolds. Despite these advancements, tailoring polymeric biomaterials for design of a desired scaffold is fundamental issue that needs tremendous attention to promote the therapeutic benefits and minimize adverse effects. This review aims to (i) describe the nervous system and related injuries. Then, (ii) nerve tissue engineering strategies are discussed and (iii) physiochemical properties of synthetic polymeric biomaterials systematically highlighted. Moreover, tailoring synthetic polymeric biomaterials for nerve tissue engineering is reviewed.

3D-printed nerve conduit with vascular networks to promote peripheral nerve regeneration

Peripheral nerve regeneration remains a challenge in tissue engineering and regenerative medicine. However, the existing approaches have limited regenerative capability. 3D-printed nerve conduits with well-defined properties are potent tools to facilitate peripheral nerve regeneration after injuries. Meanwhile, the vascular networks within the constructs can promote the exchange of oxygen, neurotrophic factors, and removal of waste products, thereby providing an advantageous microenvironment for tissue regeneration. It will be an interesting approach to integrate 3D-printed nerve conduit with vascular networks for the guidance of regenerated nerves. We hypothesize that 3D-printed vascularized nerve conduit will be an effective platform to promote nerve regeneration and functional restoration.

Ultraviolet Functionalization of Electrospun Scaffolds to Activate Fibrous Runways for Targeting Cell Adhesion

A critical challenge in scaffold design for tissue engineering is recapitulating the complex biochemical patterns that regulate cell behavior in vivo. In this work, we report the adaptation of a standard sterilization methodology-UV irradiation-for patterning the surfaces of two complementary polymeric electrospun scaffolds with oxygen cues able to efficiently immobilize biomolecules. Independently of the different polymer chain length of poly(ethylene oxide terephthalate)/poly(butylene terephthalate) (PEOT/PBT) copolymers and PEOT/PBT ratio, it was possible to easily functionalize specific regions of the scaffolds by inducing an optimized and spatially controlled adsorption of proteins capable of boosting the adhesion and spreading of cells along the activated fibrous runways. By allowing an efficient design of cell attachment patterns without inducing any noticeable change on cell morphology nor on the integrity of the electrospun fibers, this procedure offers an affordable and resourceful approach to generate complex biochemical patterns that can decisively complement the functionality of the next generation of tissue engineering scaffolds.

The Rheology of PEOT/PBT Block Copolymers in the Melt State and in the Thermally-Induced Sol/Gel Transition. Implications on the 3D-Printing Bio-Scaffold Process

Poly(ethyleneoxideterephthalate)/poly(butyleneterephthalate) (PEOT/PBT) segmented block copolymers are widely used for the manufacturing of 3D-printed bio-scaffolds, due to a combination of several properties, such as cell viability, bio-compatibility, and bio-degradability. Furthermore, they are characterized by a relatively low viscosity at high temperatures, which is desired during the injection stages of the printing process. At the same time, the microphase separated morphology generated by the demixing of hard and soft segments at intermediate temperatures allows for a quick transition from a liquid-like to a solid-like behavior, thus favoring the shaping and the dimensional stability of the scaffold. In this work, for the first time, the rheology of a commercial PEOT/PBT material is studied over a wide range of temperatures encompassing both the melt state and the phase transition regime. Non-isothermal viscoelastic measurements under oscillatory shear flow allow for a quantitative determination of the material processability in the melt state. Additionally, isothermal experiments below the order⁻disorder temperature are used to determine the temperature dependence of the phase transition kinetics. The importance of the rheological characterization when designing the 3D-printing scaffold process is also discussed.

Fabrication of PEOT/PBT Nanofibers by Atmospheric Pressure Plasma Jet Treatment of Electrospinning Solutions for Tissue Engineering

This study focuses on the enhanced electrospinning of 300-Polyethylene oxide-polyethylene oxide terephthalate/polybutylene terephthalate (PEOT/PBT). An atmospheric pressure plasma jet for liquid treatment is applied to a solution with 9 w/v% PEOT/PBT dissolved in either chloroform (CHCl₃ ), CHCl₃  + N,N-dimethylformamide (DMF), CHCl₃  + methanol (MeOH), or CHCl₃  + hexafluoroisopropanol (HFIP). For all conditions, the plasma-treated samples present better-quality fibers: less or no-beads and uniform fiber diameter distribution. Except for CHCl₃  + DMF, no significant changes to the material bulk are detected, as shown with size exclusion chromatography (SEC). X-ray photoelectron spectroscopy (XPS) spectra performed on nanofibers record an increase in C-C bonds for the CHCl₃  + DMF combination upon plasma modification, while a shift and slight increase in oxygen-containing bonds is found for the CHCl₃  + HFIP and CHCl₃  + MeOH mixtures. MTT assay shows no-cytotoxic effects for CHCl₃  + DMF, while a better cellular adhesion is found on nanofibers from CHCl₃  + MeOH and CHCl₃  + HFIP. Among the examined additives, MeOH is preferable as it produces beadless electrospun nanofibers with an average diameter of 290 ± 100 nm without causing significant changes to the final nanofiber surface properties.

Rapid prototyping of flexible intrafascicular electrode arrays by picosecond laser structuring

OBJECTIVE

Interfacing the peripheral nervous system can be performed with a large variety of electrode arrays. However, stimulating and recording a nerve while having a reasonable amount of channels limits the number of available systems. Translational research towards human clinical trial requires device safety and biocompatibility but would benefit from design flexibility in the development process to individualize probes.

APPROACH

We selected established medical grade implant materials like precious metals and Parylene C to develop a rapid prototyping process for novel intrafascicular electrode arrays using a picosecond laser structuring. A design for a rodent animal model was developed in conjunction with an intrafascicular implantation strategy. Electrode characterization and optimization was performed first in saline solution in vitro before performance and biocompatibility were validated in sciatic nerves of rats in chronic implantation.

MAIN RESULTS

The novel fabrication process proved to be suitable for prototyping and building intrafascicular electrode arrays. Electrochemical properties of the electrode sites were enhanced and tested for long-term stability. Chronic implantation in the sciatic nerve of rats showed good biocompatibility, selectivity and stable stimulation thresholds.

SIGNIFICANCE

Established medical grade materials can be used for intrafascicular nerve electrode arrays when laser structuring defines structure size in the micro-scale. Design flexibility reduces re-design cycle time and material certificates are beneficial support for safety studies on the way to clinical trials.

Electrohydrodynamic Jet 3D Printed Nerve Guide Conduits (NGCs) for Peripheral Nerve Injury Repair

The prevalence of peripheral nerve injuries resulting in loss of motor function, sensory function, or both, is on the rise. Artificial Nerve Guide Conduits (NGCs) are considered an effective alternative treatment for autologous nerve grafts, which is the current gold-standard for treating peripheral nerve injuries. In this study, Polycaprolactone-based three-dimensional porous NGCs are fabricated using Electrohydrodynamic jet 3D printing (EHD-jetting) for the first time. The main advantage of this technique is that all the scaffold properties, namely fibre diameter, pore size, porosity, and fibre alignment, can be controlled by tuning the process parameters. In addition, EHD-jetting has the advantages of customizability, repeatability, and scalability. Scaffolds with five different pore sizes (125 to 550 μm) and porosities (65 to 88%) are fabricated and the effect of pore size on the mechanical properties is evaluated. In vitro degradation studies are carried out to investigate the degradation profile of the scaffolds and determine the influence of pore size on the degradation rate and mechanical properties at various degradation time points. Scaffolds with a pore size of 125 ± 15 μm meet the requirements of an optimal NGC structure with a porosity greater than 60%, mechanical properties closer to those of the native peripheral nerves, and an optimal degradation rate matching the nerve regeneration rate post-injury. The in vitro neural differentiation studies also corroborate the same results. Cell proliferation was highest in the scaffolds with a pore size of 125 ± 15 μm assessed by the PrestoBlue assay. The Reverse Transcription-Polymerase Chain Reaction (RT-PCR) results involving the three most important genes concerning neural differentiation, namely β3-tubulin, NF-H, and GAP-43, confirm that the scaffolds with a pore size of 125 ± 15 μm have the highest gene expression of all the other pore sizes and also outperform the electrospun Polycaprolactone (PCL) scaffold. The immunocytochemistry results, expressing the two important nerve proteins β3-tubulin and NF200, showed directional alignment of the neurite growth along the fibre direction in EHD-jet 3D printed scaffolds.

On the use of Parylene C polymer as substrate for peripheral nerve electrodes

Parylene C is a highly flexible polymer used in several biomedical implants. Since previous studies have reported valuable biocompatible and manufacturing characteristics for brain and intraneural implants, we tested its suitability as a substrate for peripheral nerve electrodes. We evaluated 1-year-aged in vitro samples, where no chemical differences were observed and only a slight deviation on Young’s modulus was found. The foreign body reaction (FBR) to longitudinal Parylene C devices implanted in the rat sciatic nerve for 8 months was characterized. After 2 weeks, a capsule was formed around the device, which continued increasing up to 16 and 32 weeks. Histological analyses revealed two cell types implicated in the FBR: macrophages, in contact with the device, and fibroblasts, localized in the outermost zone after 8 weeks. Molecular analysis of implanted nerves comparing Parylene C and polyimide devices revealed a peak of inflammatory cytokines after 1 day of implant, returning to low levels thereafter. Only an increase of CCL2 and CCL3 was found at chronic time-points for both materials. Although no molecular differences in the FBR to both polymers were found, the thick tissue capsule formed around Parylene C puts some concern on its use as a scaffold for intraneural electrodes.

Biomimetic Architectures for Peripheral Nerve Repair: A Review of Biofabrication Strategies

Biofabrication techniques have endeavored to improve the regeneration of the peripheral nervous system (PNS), but nothing has surpassed the performance of current clinical practices. However, these current approaches have intrinsic limitations that compromise patient care. The "gold standard" autograft provides the best outcomes but requires suitable donor material, while implantable hollow nerve guide conduits (NGCs) can only repair small nerve defects. This review places emphasis on approaches that create structural cues within a hollow NGC lumen in order to match or exceed the regenerative performance of the autograft. An overview of the PNS and nerve regeneration is provided. This is followed by an assessment of reported devices, divided into three major categories: isotropic hydrogel fillers, acting as unstructured interluminal support for regenerating nerves; fibrous interluminal fillers, presenting neurites with topographical guidance within the lumen; and patterned interluminal scaffolds, providing 3D support for nerve growth via structures that mimic native PNS tissue. Also presented is a critical framework to evaluate the impact of reported outcomes. While a universal and versatile nerve repair strategy remains elusive, outlined here is a roadmap of past, present, and emerging fabrication techniques to inform and motivate new developments in the field of peripheral nerve regeneration.

3D Fabrication with Integration Molding of a Graphene Oxide/Polycaprolactone Nanoscaffold for Neurite Regeneration and Angiogenesis

Treating peripheral nerve injury faces major challenges and may benefit from bioactive scaffolds due to the limited autograft resources. Graphene oxide (GO) has emerged as a promising nanomaterial with excellent physical and chemical properties. GO has functional groups that confer biocompatibility that is better than that of graphene. Here, GO/polycaprolactone (PCL) nanoscaffolds are fabricated using an integration molding method. The nanoscaffolds exhibit many merits, including even GO nanoparticle distribution, macroporous structure, and strong mechanical support. Additionally, the process enables excellent quality control. In vitro studies confirm the advantages of the GO/PCL nanoscaffolds in terms of Schwann cell proliferation, viability, and attachment, as well as neural characteristics maintenance. This is the first study to evaluate the in vivo performance of GO-based nanoscaffolds in this context. GO release and PCL biodegradation is analyzed after long-term in vivo study. It is also found that the GO/PCL nerve guidance conduit could successfully repair a 15 mm sciatic nerve defect. The pro-angiogenic characteristic of GO is evaluated in vivo using immunohistochemistry. In addition, the AKT-endothelial nitric oxide synthase (eNOS)-vascular endothelial growth factor (VEGF) signaling pathway might play a major role in the angiogenic process. These findings demonstrate that the GO/PCL nanoscaffold efficiently promotes functional and morphological recovery in peripheral nerve regeneration, indicating its promise for tissue engineering applications.

Segregation of motor and sensory axons regenerating through bicompartmental tubes by combining extracellular matrix components with neurotrophic factors

Segregation of regenerating motor and sensory axons may be a good strategy to improve selective functionality of regenerative interfaces to provide closed-loop commands. Provided that extracellular matrix components and neurotrophic factors exert guidance effects on different neuronal populations, we assessed in vivo the potential of separating sensory and motor axons regenerating in a bicompartmental Y-type tube, with each branch prefilled with an adequate combination of extracellular matrix and neurotrophic factors. The severed rat sciatic nerve was repaired using a bicompartmental tube filled with a collagen matrix enriched with fibronectin (FN) and brain-derived neurotrophic factor (BDNF) encapsulated in poly-lactic co-glycolic acid microspheres (FN + MP.BDNF) in one compartment to preferentially attract motor axons and collagen enriched with laminin (LM) and nerve growth factor (NGF) and neurotrophin-3 (NT-3) in microspheres (LM + MP.NGF/NT-3) in the other compartment for promoting sensory axons regeneration. Control animals were implanted with the same Y-tube with a collagen matrix with microspheres (MP) containing PBS (Col + MP.PBS). By using retrotracer labelling, we found that LM + MP.NGF/NT-3 did not attract higher number of regenerated sensory axons compared with controls, and no differences were observed in sensory functional recovery. However, FN + MP.BDNF guided a higher number of regenerating motor axons compared with controls, improving also motor recovery. A small proportion of sensory axons with large soma size, likely proprioceptive neurons, was also attracted to the FN + MP.BDNF compartment. These results demonstrate that muscular axonal guidance can be modulated in vivo by the addition of fibronectin and BDNF.

Comparison of morphological and functional outcomes of mouse sciatic nerve repair with three biodegradable polymer conduits containing poly(lactic acid)

Poly(lactic acid) (PLA)-containing nerve guidance conduits (NGCs) are currently being investigated for nerve repair as an alternative to autograft, which leads to permanent functional impairment in the territory innervated by the removed nerve. Combination of polymers modifies the physical properties of the conduits, altering their nerve-guidance properties. Conduits made from PLA-only or combined with other polymers have been used successfully for nerve repair, but their efficiency has not been compared. We compared the morphological and functional outcomes of peripheral nerve repair by using NGCs made of poly(lactic acid) and combined or not with polycaprolactone (PLA/PCL) or polyvinylpyrrolidone (PLA/PVP). To assess the functional recovery, we employed a mechanical hyperalgesia analysis, sciatic functional index (SFI), and electroneuromyography. The mechanical hyperalgesia analysis showed that the PLA group improved more rapidly than the PLA/PVP and PLA/PCL groups; similarly, in the electroneuromyography assay, the PLA group exhibited higher amplitude than the PLA/PCL and PLA/PVP groups. However, the SFI improvement rates did not differ among the groups. Morphologically, the PLA group showed more vascularization, while the nerve fiber regeneration did not differ among the groups. In conclusion, the PLA-only conduits were superior to the other NGCs tested for nerve repair.

Time course study of long-term biocompatibility and foreign body reaction to intraneural polyimide-based implants

The foreign body reaction (FBR) against an implanted device is characterized by the formation of a fibrotic tissue around the implant. In the case of interfaces for peripheral nerves, used to stimulate specific group of axons and to record different nerve signals, the FBR induces a matrix deposition around the implant creating a physical separation between nerve fibers and the interface that may reduce its functionality over time. In order to understand how the FBR to intraneural interfaces evolves, polyimide non-functional devices were implanted in rat peripheral nerve. Functional tests (electrophysiological, pain and locomotion) and histological evaluation demonstrated that implanted devices did not cause any alteration in nerve function, in myelinated axons or in nerve architecture. The inflammatory response due to the surgical implantation decreased after 2 weeks. In contrast, inflammation was higher and more prolonged in the device implanted nerves with a peak after 2 weeks. With regard to tissue deposition, a tissue capsule appeared soon around the devices, acquiring maximal thickness at 2 weeks and being remodeled subsequently. Immunohistochemical analysis revealed two different cell types implicated in the FBR in the nerve: macrophages as the first cells in contact with the interface and fibroblasts that appear later at the edge of the capsule. Our results describe how the FBR against a polyimide implant in the peripheral nerve occurs and which are the main cellular players. Increasing knowledge of these responses will help to improve strategies to decrease the FBR against intraneural implants and to extend their usability. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 746-757, 2018.

Preferential Enhancement of Sensory and Motor Axon Regeneration by Combining Extracellular Matrix Components with Neurotrophic Factors

After peripheral nerve injury, motor and sensory axons are able to regenerate but inaccuracy of target reinnervation leads to poor functional recovery. Extracellular matrix (ECM) components and neurotrophic factors (NTFs) exert their effect on different neuronal populations creating a suitable environment to promote axonal growth. Here, we assessed in vitro and in vivo the selective effects of combining different ECM components with NTFs on motor and sensory axons regeneration and target reinnervation. Organotypic cultures with collagen, laminin and nerve growth factor (NGF)/neurotrophin-3 (NT3) or collagen, fibronectin and brain-derived neurotrophic factor (BDNF) selectively enhanced sensory neurite outgrowth of DRG neurons and motor neurite outgrowth from spinal cord slices respectively. For in vivo studies, the rat sciatic nerve was transected and repaired with a silicone tube filled with a collagen and laminin matrix with NGF/NT3 encapsulated in poly(lactic-co-glycolic acid) (PLGA) microspheres (MP) (LM + MP.NGF/NT3), or a collagen and fibronectin matrix with BDNF in PLGA MPs (FN + MP.BDNF). Retrograde labeling and functional tests showed that LM + MP.NGF/NT3 increased the number of regenerated sensory neurons and improved sensory functional recovery, whereas FN + MP.BDNF preferentially increased regenerated motoneurons and enhanced motor functional recovery. Therefore, combination of ECM molecules with NTFs may be a good approach to selectively enhance motor and sensory axons regeneration and promote appropriate target reinnervation.