Peripheral nerve conduits: technology update

The Potential Role of Adipose-Derived Stem Cells in Regeneration of Peripheral Nerves

Peripheral nerve injuries are common complications in surgical and dental practices, often resulting in functional deficiencies and reduced quality of life. Current treatment choices, such as autografts, have limitations, including donor site morbidity and suboptimal outcomes. Adipose-derived stem cells (ADSCs) have shown assuring regenerative potential due to their accessibility, ease of harvesting and propagation, and multipotent properties. This review investigates the therapeutic potential of ADSCs in peripheral nerve regeneration, focusing on their use in bioengineered nerve conduits and supportive microenvironments. The analysis is constructed on published case reports, organized reviews, and clinical trials from Phase I to Phase III that investigate ADSCs in managing nerve injuries, emphasizing both peripheral and orofacial applications. The findings highlight the advantages of ADSCs in promoting nerve regeneration, including their secretion of angiogenic and neurotrophic factors, support for cellular persistence, and supplementing scaffold-based tissue repair. The regenerative capabilities of ADSCs in peripheral nerve injuries offer a novel approach to augmenting nerve repair and functional recovery. The accessibility of adipose tissue and the minimally invasive nature of ADSC harvesting further encourage its prospective application as an autologous cell source in regenerative medicine. Future research is needed to ascertain standardized protocols and optimize clinical outcomes, paving the way for ADSCs to become a mainstay in nerve regeneration.

FRESH extrusion 3D printing of type-1 collagen hydrogels photocrosslinked using ruthenium

The extrusion bioprinting of collagen material has many applications relevant to tissue engineering and regenerative medicine. Freeform Reversible Embedding of Suspended Hydrogels (FRESH) technology is capable of 3D printing collagen material with the specifications and details needed for precise tissue guidance, a crucial requirement for effective tissue repair. While FRESH has shown repeated success and reliability for extrusion printing, the mechanical properties of completed collagen prints can be improved further by post-print crosslinking methodologies. Photoinitiator-based crosslinking methods are simple and have proven effective in strengthening protein-based materials. The ruthenium and sodium persulfate photoinitiator system (Ru(bpy)3/SPS) has been suggested as an effective crosslinking method for collagen materials. Herein, we describe the procedure our group has developed to combine extrusion-based 3D printing of type-1 collagen using FRESH technology with Ru(bpy)3/SPS photoinitiated crosslinking methods to improve the strength and stability of 3D printed collagen structures. Mechanical testing and cell biocompatibility assessments were performed to investigate the impact of Ru(bpy)3/SPS photoinitiated crosslinking and highlight the potential limitations of this method. These results demonstrate a significant improvement in the compressive strength of type-1 collagen samples as the Ru(bpy)3/SPS concentration increases. Additionally, type-1 collagen samples crosslinked with up to 1/10 mM Ru(bpy)3/SPS support PC12 cell viability over a period of 7 days. The primary limitations that were observed and described in detail in this protocol are: the FRESH slurry preparation, printing environment, extrusion printer hardware, and quality of the ruthenium reagent.

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

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

Enhancing facial nerve regeneration with scaffold-free conduits engineered using dental pulp stem cells and their endogenous, aligned extracellular matrix

Objective. Engineered nerve conduits must simultaneously enhance axon regeneration and orient axon extension to effectively restore function of severely injured peripheral nerves. The dental pulp contains a population of stem/progenitor cells that endogenously express neurotrophic factors (NTFs), growth factors known to induce axon repair. We have previously generated scaffold-free dental pulp stem/progenitor cell (DPSC) sheets comprising an aligned extracellular matrix (ECM). Through the intrinsic NTF expression of DPSCs and the topography of the aligned ECM, these sheets both induce and guide axon regeneration. Here, the capacity of bioactive conduits generated using these aligned DPSC sheets to restore function in critical-sized nerve injuries in rodents was evaluated.Approach. Scaffold-free nerve conduits were formed by culturing DPSCs on a substrate with aligned microgrooves, inducing the cells to align and deposit an aligned ECM. The sheets were then detached from the substrate and assembled into scaffold-free cylindrical tissues.Main results. In vitroanalyses confirmed that scaffold-free DPSC conduits maintained an aligned ECM and had uniformly distributed NTF expression. Implanting the aligned DPSC conduits across critical-sized defects in the buccal branch of rat facial nerves resulted in the regeneration of a fascicular nerve-like structure and myelinated axon extension across the injury site. Furthermore, compound muscle action potential and stimulated whisker movement measurements revealed that the DPSC conduit treatment promoted similar functional recovery compared to the clinical standard of care, autografts. Significance. This study demonstrates that scaffold-free aligned DPSC conduits supply trophic and guidance cues, key design elements needed to successfully promote and orient axon regeneration. Consequently, these conduits restore function in nerve injuries to similar levels as autograft treatments. These conduits offer a novel bioactive approach to nerve repair capable of improving clinical outcomes and patient quality of life.

Challenges and Advances in Peripheral Nerve Tissue Engineering Critical Factors Affecting Nerve Regeneration

Peripheral neuropathy is painful and can cause a considerable decline in quality of life. Surgery and autograft are the current approaches and clinical standards for restoring function after nerve damage. However, they usually result in unacceptable clinical results, so we need modern peripheral nerve defect treatment approaches. Tissue engineering techniques have been developed as a promising approach, but there are some considerations for translational application. Clinical application of novel tissue engineering methods is related to combining the appropriate cell and scaffold type to introduce safe and efficient bioscaffolds. Efficient nerve regeneration occurs by mimicking the extracellular matrix and combining topographical, biochemical, mechanical, and conductive signs via different cells, biomolecules, and polymers. In brief, ideal engineered biomaterial scaffolds will have to cover all characteristics of nerve tissue, such as nerve number, myelin, and axon thickness. Nerve regeneration has a highly sensitive response to its surrounding microenvironment. For designing a suitable construct, matching the regenerative potential of the autograft as the golden standard is essential. This review article examines the newest advancements in peripheral nerve tissue engineering. Specifically, the discussion will focus on incorporating innovative cues, biological modification, biomaterials, techniques, and concepts in this area of research.

A tacrolimus-eluting nerve guidance conduit enhances regeneration in a critical-sized peripheral nerve injury rat model

Critical-sized peripheral nerve injuries pose a significant clinical challenge and lead to functional loss and disability. Current regeneration strategies, including autografts, synthetic nerve conduits, and biologic treatments, encounter challenges such as limited availability, donor site morbidity, suboptimal recovery, potential immune responses, and sustained stability and bioactivity. An obstacle in peripheral nerve regeneration is the immune response that can lead to inflammation and scarring that impede the regenerative process. Addressing both the immunological and regenerative needs is crucial for successful nerve recovery. Here, we introduce a novel biodegradable tacrolimus-eluting nerve guidance conduit engineered from a blend of poly (L-lactide-co-caprolactone) to facilitate peripheral nerve regeneration and report the testing of this conduit in 15-mm critical-sized gaps in the sciatic nerve of rats. The conduit's diffusion holes enable the local release of tacrolimus, a potent immunosuppressant with neuro-regenerative properties, directly into the injury site. A series of in vitro experiments were conducted to assess the ability of the conduit to maintain a controlled tacrolimus release profile that could promote neurite outgrowth. Subsequent in vivo assessments in rat models of sciatic nerve injury revealed significant enhancements in nerve regeneration, as evidenced by improved axonal growth and functional recovery compared to controls using placebo conduits. These findings indicate the synergistic effects of combining a biodegradable conduit with localized, sustained delivery of tacrolimus, suggesting a promising approach for treating peripheral nerve injuries. Further optimization of the design and long-term efficacy studies and clinical trials are needed before the potential for clinical translation in humans can be considered.

Scaffold design considerations for peripheral nerve regeneration

Peripheral nerve injury (PNI) represents a serious clinical and public health problem due to its high incurrence and poor spontaneous recovery. Compared to autograft, which is still the best current practice for long-gap peripheral nerve defects in clinics, the use of polymer-based biodegradable nerve guidance conduits (NGCs) has been gaining momentum as an alternative to guide the repair of severe PNI without the need of secondary surgery and donor nerve tissue. However, simple hollow cylindrical tubes can barely outperform autograft in terms of the regenerative efficiency especially in critical sized PNI. With the rapid development of tissue engineering technology and materials science, various functionalized NGCs have emerged to enhance nerve regeneration over the past decades. From the aspect of scaffold design considerations, with a specific focus on biodegradable polymers, this review aims to summarize the recent advances in NGCs by addressing the onerous demands of biomaterial selections, structural designs, and manufacturing techniques that contributes to the biocompatibility, degradation rate, mechanical properties, drug encapsulation and release efficiency, immunomodulation, angiogenesis, and the overall nerve regeneration potential of NGCs. In addition, several commercially available NGCs along with their regulation pathways and clinical applications are compared and discussed. Lastly, we discuss the current challenges and future directions attempting to provide inspiration for the future design of ideal NGCs that can completely cure long-gap peripheral nerve defects.

Extrusion 3D-printing and characterization of poly(caprolactone fumarate) for bone regeneration applications

Polycaprolactone fumarate (PCLF) is a cross-linkable PCL derivative extensively considered for tissue engineering applications. Although injection molding has been widely used to develop PCLF scaffolds, platforms developed using such technique lack precise control on architecture, design, and porosity required to ensure adequate cellular and tissue responses. In particular, the scaffolds should provide a suitable surface for cell attachment and proliferation, and facilitate cell-cell communication and nutrient flow. 3D printing technologies have led to new architype for biomaterial development with micro-architecture mimicking native tissue. Here, we developed a method for 3D printing of PCLF structures using the extrusion printing technique. The crosslinking property of PCLF enabled the unique post-processing of 3D printed scaffolds resulting in highly porous and flexible PCLF scaffolds with compressive properties imitating natural features of cancellous bone. Generated scaffolds supported excellent attachment and proliferation of mesenchymal stem cells (MSC). The high porosity of PCLF scaffolds facilitated vascularized membrane formation demonstrable with the stringency of the ex ovo chicken chorioallantoic membrane (CAM) implantation. Furthermore, upon implantation to rat calvarium defects, PCLF scaffolds enabled an exceptional new bone formation with a bone mineral density of newly formed bone mirroring native bone tissue. These studies suggest that the 3D-printed highly porous PCLF scaffolds may serve as a suitable biomaterial platform to significantly expand the utility of the PCLF biomaterial for bone tissue engineering applications.

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

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

Comprehensive Development of a Cellulose Acetate and Soy Protein-Based Scaffold for Nerve Regeneration

BACKGROUND

The elaboration of biocompatible nerve guide conduits (NGCs) has been studied in recent years as a treatment for total nerve rupture lesions (axonotmesis). Different natural polymers have been used in these studies, including cellulose associated with soy protein. The purpose of this report was to describe manufacturing NGCs suitable for nerve regeneration using the method of dip coating and evaporation of solvent with cellulose acetate (CA) functionalized with soy protein acid hydrolysate (SPAH).

METHODS

The manufacturing method and bacterial control precautions for the CA/SPAH NGCs were described. The structure of the NGCs was analyzed under a scanning electron microscope (SEM); porosity was analyzed with a degassing method using a porosimeter. Schwann cell (SCL 4.1/F7) biocompatibility of cell-seeded nerve guide conduits was evaluated with the MTT assay.

RESULTS

The method employed allowed an easy elaboration and customization of NGCs, free of bacteria, with pores in the internal surface, and the uniform wall thickness allowed manipulation, which showed flexibility; additionally, the sample was suturable. The NGCs showed initial biocompatibility with Schwann cells, revealing cells adhered to the NGC structure after 5 days.

CONCLUSIONS

The fabricated CA/SPAH NGCs showed adequate features to be used for peripheral nerve regeneration studies. Future reports are necessary to discuss the ideal concentration of CA and SPAH and the mechanical and physicochemical properties of this biomaterial.

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

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

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

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

Supplementary Information

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

Nerve regeneration using decellularized tissues: challenges and opportunities

In tissue engineering, the decellularization of organs and tissues as a biological scaffold plays a critical role in the repair of neurodegenerative diseases. Various protocols for cell removal can distinguish the effects of treatment ability, tissue structure, and extracellular matrix (ECM) ability. Despite considerable progress in nerve regeneration and functional recovery, the slow regeneration and recovery potential of the central nervous system (CNS) remains a challenge. The success of neural tissue engineering is primarily influenced by composition, microstructure, and mechanical properties. The primary objective of restorative techniques is to guide existing axons properly toward the distal end of the damaged nerve and the target organs. However, due to the limitations of nerve autografts, researchers are seeking alternative methods with high therapeutic efficiency and without the limitations of autograft transplantation. Decellularization scaffolds, due to their lack of immunogenicity and the preservation of essential factors in the ECM and high angiogenic ability, provide a suitable three-dimensional (3D) substrate for the adhesion and growth of axons being repaired toward the target organs. This study focuses on mentioning the types of scaffolds used in nerve regeneration, and the methods of tissue decellularization, and specifically explores the use of decellularized nerve tissues (DNT) for nerve transplantation.

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.

Systematic Comparison of Commercial Hydrogels Revealed That a Synergy of Laminin and Strain-Stiffening Promotes Directed Migration of Neural Cells

Hydrogels have shown potential in replacing damaged nerve tissue, but the ideal hydrogel is yet to be found. In this study, various commercially available hydrogels were compared. Schwann cells, fibroblasts, and dorsal root ganglia neurons were seeded on the hydrogels, and their morphology, viability, proliferation, and migration were examined. Additionally, detailed analyses of the gels' rheological properties and topography were conducted. Our results demonstrate vast differences on cell elongation and directed migration on the hydrogels. Laminin was identified as the driver behind cell elongation and in combination with a porous, fibrous, and strain-stiffening matrix structure responsible for oriented cell motility. This study improves our understanding of cell-matrix interactions and thereby facilitates tailored fabrication of hydrogels in the future.

Peripheral Nerve Regeneration: Opportunities and Challenges

Peripheral nerve injury has detrimental effects on the quality of life for patients and is a worldwide issue with high rates of morbidity. Research on the molecular mechanisms of nerve injury, microsurgical techniques, and advances in stem cell research have led to substantial progress in the field of translational neurophysiology. Current research on peripheral nerve regeneration aims to accelerate peripheral nerve development through pluripotent stem cells and potential use of smart exosomes, pharmacological agents, and bioengineering of nerve conduits. In this article critically reviewed and summarized various methods used for peripheral nerve regeneration and highlight the opportunities and challenges that come along with these strategies.

Biodegradable and Electrically Conductive Melanin-Poly (3-Hydroxybutyrate) 3D Fibrous Scaffolds for Neural Tissue Engineering Applications

Due to the severity of peripheral nerve injuries (PNI) and spinal cord injuries (SCI), treatment options for patients are limited. In this context, biomaterials designed to promote regeneration and reinstate the lost function are being explored. Such biomaterials should be able to mimic the biological, chemical, and physical cues of the extracellular matrix for maximum effectiveness as therapeutic agents. Development of biomaterials with desirable physical, chemical, and electrical properties, however, has proven challenging. Here a novel biomaterial formulation achieved by blending the pigment melanin and the natural polymer Poly-3-hydroxybutyrate (PHB) is proposed. Physio-chemical measurements of electrospun fibers reveal a feature rich surface nano-topography, a semiconducting-nature, and brain-tissue-like poroviscoelastic properties. Resulting fibers improve cell adhesion and growth of mouse sensory and motor neurons, without any observable toxicity. Further, the presence of polar functional groups positively affect the kinetics of fibers degradation at a pH (≈7.4) comparable to that of body fluids. Thus, melanin-PHB blended scaffolds are found to be physio-chemically, electrically, and biologically compatible with neural tissues and could be used as a regenerative modality for neural tissue injuries. A biomaterial for scaffolds intended to promote regeneration of nerve tissue after injury is developed. This biomaterial, obtained by mixing the pigment melanin and the natural polymer PHB, is biodegradable, electrically conductive, and beneficial to the growth of motor and sensory neurons. Thus, it is believed that this biomaterial can be used in the context of healthcare applications.

Hyaluronic Acid Scaffolds for Loco-Regional Therapy in Nervous System Related Disorders

Hyaluronic acid (HA) is a Glycosaminoglycan made of disaccharide units containing N-acetyl-D-glucosamine and glucuronic acid. Its molecular mass can reach 10 MDa and its physiological properties depend on its polymeric property, polyelectrolyte feature and viscous nature. HA is a ubiquitous compound found in almost all biological tissues and fluids. So far, HA grades are produced by biotechnology processes, while in the human organism it is a major component of the extracellular matrix (ECM) in brain tissue, synovial fluid, vitreous humor, cartilage and skin. Indeed, HA is capable of forming hydrogels, polymer crosslinked networks that are very hygroscopic. Based on these considerations, we propose an overview of HA-based scaffolds developed for brain cancer treatment, central and peripheral nervous systems, discuss their relevance and identify the most successful developed systems.

Surface-Modified Polypyrrole-Coated PLCL and PLGA Nerve Guide Conduits Fabricated by 3D Printing and Electrospinning

The efficiency of nerve guide conduits (NGCs) in repairing peripheral nerve injury is not high enough yet to be a substitute for autografts and is still insufficient for clinical use. To improve this efficiency, 3D electrospun scaffolds (3D/E) of poly(l-lactide-co-ε-caprolactone) (PLCL) and poly(l-lactide-co-glycolide) (PLGA) were designed and fabricated by the combination of 3D printing and electrospinning techniques, resulting in an ideal porous architecture for NGCs. Polypyrrole (PPy) was deposited on PLCL and PLGA scaffolds to enhance biocompatibility for nerve recovery. The designed pore architecture of these "PLCL-3D/E" and "PLGA-3D/E" scaffolds exhibited a combination of nano- and microscale structures. The mean pore size of PLCL-3D/E and PLGA-3D/E scaffolds were 289 ± 79 and 287 ± 95 nm, respectively, which meets the required pore size for NGCs. Furthermore, the addition of PPy on the surfaces of both PLCL-3D/E (PLCL-3D/E/PPy) and PLGA-3D/E (PLGA-3D/E/PPy) led to an increase in their hydrophilicity, conductivity, and noncytotoxicity compared to noncoated PPy scaffolds. Both PLCL-3D/E/PPy and PLGA-3D/E/PPy showed conductivity maintained at 12.40 ± 0.12 and 10.50 ± 0.08 Scm^-1 for up to 15 and 9 weeks, respectively, which are adequate for the electroconduction of neuron cells. Notably, the PLGA-3D/E/PPy scaffold showed superior cytocompatibility when compared with PLCL-3D/E/PPy, as evident via the viability assay, proliferation, and attachment of L929 and SC cells. Furthermore, analysis of cell health through membrane leakage and apoptotic indices showed that the 3D/E/PPy scaffolds displayed significant decreases in membrane leakage and reductions in necrotic tissue. Our finding suggests that these 3D/E/PPy scaffolds have a favorable design architecture and biocompatibility with potential for use in peripheral nerve regeneration applications.

Systematic review of the therapeutic use of Schwann cells in the repair of peripheral nerve injuries: Advancements from animal studies to clinical trials

Objective

To systematically evaluate the literature on the therapeutic use of Schwann cells (SC) in the repair of peripheral nerve injuries.

Methods

The Cochrane Library and PubMed databases were searched using terms [(“peripheral nerve injury” AND “Schwann cell” AND “regeneration”) OR (“peripheral nerve injuries”)]. Studies published from 2008 to 2022 were eligible for inclusion in the present study. Only studies presenting data from in-vivo investigations utilizing SCs in the repair of peripheral nerve injuries qualified for review. Studies attempting repair of a gap of ≥10 mm were included. Lastly, studies needed to have some measure of quantifiable regenerative outcome data such as histomorphometry, immunohistochemical, electrophysiology, or other functional outcomes.

Results

A search of the PubMed and Cochrane databases revealed 328 studies. After screening using the abstracts and methods, 17 studies were found to meet our inclusion criteria. Good SC adherence and survival in conduit tubes across various studies was observed. Improvement in morphological and functional outcomes with the use of SCs in long gap peripheral nerve injuries was observed in nearly all studies.

Conclusion

Based on contemporary literature, SCs have demonstrated clear potential in the repair of peripheral nerve injury in animal studies. It has yet to be determined which nerve conduit or graft will prove superior for delivery and retention of SCs for nerve regeneration. Recent developments in isolation and culturing techniques will enable further translational utilization of SCs in future clinical trials.

Human IL12p80 Promotes Murine Oligodendrocyte Differentiation to Repair Nerve Injury

Nerve injury of the central nervous system and the peripheral nervous system still poses a major challenge in modern clinics. Understanding the roles of neurotrophic factors and their molecular mechanisms on neuro-regeneration will not only benefit patients with neural damage but could potentially treat neurodegenerative disorders, such as amyotrophic lateral sclerosis. In this study, we showed that human IL12 p40-p40 homodimer (hIL12p80) within PLA and PLGA conduits improved sciatic nerve regeneration in mice. As such, the group of conduits with NSCs and hIL12p80 (CNI) showed the best recovery among the groups in the sciatic functional index (SFI), compound muscle action potential (CMAP), and Rotarod performance analyses. In addition, the CNI group had a faster recovery and outperformed the other groups in SFI and Rotarod performance tests beginning in the fourth week post-surgery. Immunohistochemistry showed that the CNI group increased the diameter of the newly regenerated nerve by two-fold (p < 0.01). In vitro studies showed that hIL12p80 stimulated differentiation of mouse NSCs to oligodendrocyte lineages through phosphorylation of Stat3 at Y705 and S727. Furthermore, implantation using PLGA conduits (C2.0 and C2.1) showed better recovery in the Rotarod test and CMAP than using PLA conduits in FVB mice. In B6 mice, the group with C2.1 + NSCs + hIL12p80 (C2.1NI) not only promoted sciatic functional recovery but also reduced the rate of experimental autotomy. These results suggested that hIL12p80, combined with NSCs, enhanced the functional recovery and accelerated the regeneration of damaged nerves in the sciatic nerve injury mice. Our findings could further shed light on IL12′s application not only in damaged nerves but also in rectifying the oligodendrocytes’ defects in neurodegenerative diseases, such as amyotrophic lateral sclerosis and multiple sclerosis.

Implantable nerve guidance conduits: Material combinations, multi-functional strategies and advanced engineering innovations

Nerve guidance conduits (NGCs) have attracted much attention due to their great necessity and applicability in clinical use for the peripheral nerve repair. Great efforts in recent years have been devoted to the development of high-performance NGCs using various materials and strategies. The present review provides a comprehensive overview of progress in the material innovation, structural design, advanced engineering technologies and multi functionalization of state-of-the-art nerve guidance conduits NGCs. Abundant advanced engineering technologies including extrusion-based system, laser-based system, and novel textile forming techniques in terms of weaving, knitting, braiding, and electrospinning techniques were also analyzed in detail. Findings arising from this review indicate that the structural mimetic NGCs combined with natural and synthetic materials using advanced manufacturing technologies can make full use of their complementary advantages, acquiring better biomechanical properties, chemical stability and biocompatibility. Finally, the existing challenges and future opportunities of NGCs were put forward aiming for further research and applications of NGCs.

NANOCOMPOSITES BASED ON SINGLECOMPONENT AND MULTICOMPONENT POLYMER MATRICES FOR BIOMEDICAL APPLICATIONS

The review is devoted to analysis of the publications in the area of polymers of biomedical applications. Different types of the polymer matrices for drug delivery are analyzed, including polyurethanes, hydroxyacrylates, and multicomponent polymer matrices, which created by method of interpenetrating polymer networks. Particular attention is paid to description of synthesized and investigated nanocomposites based on polyurethane / poly (2-hydroxyethyl methacrylate) polymer matrix and nanooxides modified by biologically active compounds.

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

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

Conductive polypyrrole-coated electrospun chitosan nanoparticles/poly(D,L-lactide) fibrous mat: influence of drug delivery and Schwann cells proliferation

For nerve tissue engineering (NTE), scaffolds with the ability to release drugs under control and support the rapid proliferation of cells are very important for the repair of nerve defects. This study aimed to fabricate a conductive drug-loaded fiber mat by electrospinning and assess its potential as a scaffold for Schwann cells proliferation. The conductive polypyrrole (PPy) was coated on an electrospun poly (D, L-lactide) (PLA) fibrous mat, which was simultaneously embedded with protein-loaded chitosan nanoparticles and ibuprofen as a model small molecule drug. The fibrous mat shows suitable conductivity, mechanical properties, and hydrophilicity for NTE. For drug release and degradation studies, the fibrous mat can achieve sustained release of bovine serum albumin (BSA) and ibuprofen, and the PPy coating can increase the surface wettability and conductivity while slowing down the degradation of the fibrous mat. The application of electrical stimulation (ES) to the fibrous mat can accelerate the release of ibuprofen, but there was no significant effect on the release rate of the protein. The fibrous mat showed no cytotoxicityin vitro, and Schwann cells (SCs) can adhere, grow, and proliferate well on mats. At the 120th hour of culturein vitro, the relative growth rate of SCs on the conductive drug-loaded fibrous mat reached 198.22 ± 2.34%, which was an increase of 37.93% compared to the SCs on the drug-loaded fibrous mat with ES. The density and elongation of SCs on the conductive drug-loaded fibrous mat were greater than those on the PLA fibrous mat, indicating that the conductive polypyrrole-coated electrospun chitosan nanoparticles/PLA fibrous mat has good potential for application in nerve regeneration.

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

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

Tissue Engineered Neurovascularization Strategies for Craniofacial Tissue Regeneration

Craniofacial tissue injuries, diseases, and defects, including those within bone, dental, and periodontal tissues and salivary glands, impact an estimated 1 billion patients globally. Craniofacial tissue dysfunction significantly reduces quality of life, and successful repair of damaged tissues remains a significant challenge. Blood vessels and nerves are colocalized within craniofacial tissues and act synergistically during tissue regeneration. Therefore, the success of craniofacial regenerative approaches is predicated on successful recruitment, regeneration, or integration of both vascularization and innervation. Tissue engineering strategies have been widely used to encourage vascularization and, more recently, to improve innervation through host tissue recruitment or prevascularization/innervation of engineered tissues. However, current scaffold designs and cell or growth factor delivery approaches often fail to synergistically coordinate both vascularization and innervation to orchestrate successful tissue regeneration. Additionally, tissue engineering approaches are typically investigated separately for vascularization and innervation. Since both tissues act in concert to improve craniofacial tissue regeneration outcomes, a revised approach for development of engineered materials is required. This review aims to provide an overview of neurovascularization in craniofacial tissues and strategies to target either process thus far. Finally, key design principles are described for engineering approaches that will support both vascularization and innervation for successful craniofacial tissue regeneration.

Telocytes inside of the peripheral nervous system - a 3D endoneurial network and putative role in cell communication

In this paper, we developed the hypothesis concerning the reasons to assimilate endoneurial fibroblast-like dendritic phenotype [shortly termed endoneurial dendritic cells (EDCs)] to the endoneurial telocytes (TCs). We reviewed the literature concerning EDCs status and report our observations on ultrastructure and some immune electron microscopic aspects of the cutaneous peripheral nerves. Our data demonstrate that EDCs long time considered as fibroblasts or fibroblast-like, with an ovoidal nucleus and one or more moniliform cell extensions [telopodes (Tps)], which perform homocellular junctions, also able to shed extracellular microvesicles can be assimilated to TC phenotype. Sometimes, small profiles of basement membrane accompany to some extent Tps. Altogether data resulted from scientific literature and our results strength the conclusion EDCs are really TCs inside of the peripheral nervous system. The inner three-dimensional (3D) network of endoneurial TCs by their homo- and heterocellular communications appears as a genuine cell-to-cell communication system inside of each peripheral nerve.

Bio-Scaffolds as Cell or Exosome Carriers for Nerve Injury Repair

Central and peripheral nerve injuries can lead to permanent paralysis and organ dysfunction. In recent years, many cell and exosome implantation techniques have been developed in an attempt to restore function after nerve injury with promising but generally unsatisfactory clinical results. Clinical outcome may be enhanced by bio-scaffolds specifically fabricated to provide the appropriate three-dimensional (3D) conduit, growth-permissive substrate, and trophic factor support required for cell survival and regeneration. In rodents, these scaffolds have been shown to promote axonal regrowth and restore limb motor function following experimental spinal cord or sciatic nerve injury. Combining the appropriate cell/exosome and scaffold type may thus achieve tissue repair and regeneration with safety and efficacy sufficient for routine clinical application. In this review, we describe the efficacies of bio-scaffolds composed of various natural polysaccharides (alginate, chitin, chitosan, and hyaluronic acid), protein polymers (gelatin, collagen, silk fibroin, fibrin, and keratin), and self-assembling peptides for repair of nerve injury. In addition, we review the capacities of these constructs for supporting in vitro cell-adhesion, mechano-transduction, proliferation, and differentiation as well as the in vivo properties critical for a successful clinical outcome, including controlled degradation and re-absorption. Finally, we describe recent advances in 3D bio-printing for nerve regeneration.

Anisotropic scaffolds for peripheral nerve and spinal cord regeneration

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

Development of a conduit of PLGA-gelatin aligned nanofibers produced by electrospinning for peripheral nerve regeneration

A promising alternative to conventional nerve grafting is the use of artificial grafts made from biodegradable and biocompatible materials and support cells. The aim of this study has been to produce a biodegradable nerve conduit and investigate the cytocompatibility with stem cells and its regeneration promoting properties in a rat animal model. A poly (lactic-co-glycolic acid) (PLGA) conduit of aligned nanofibers was produced by the electrospinning method, functionalized with gelatin and seeded either with mouse embryonic stem cells (mESCs) or with human mesenchymal stem cells (SHED). The cell proliferation and viability were analyzed in vitro. The conduits were implanted in a rat model of sciatic nerve lesion by transection. The functional recovery was monitored for 8 weeks using the Sciatic Functional Index (SFI) and histological analyses were used to assess the nerve regeneration. Scaffolds of aligned PLGA fibers with an average diameter of 0.90 ± 0.36 μm and an alignment coefficient of 0.817 ± 0.07 were produced. The treatment with gelatin increased the fiber diameter to 1.05 ± 0.32 μm, reduced the alignment coefficient to 0.655 ± 0.045 and made the scaffold very hydrophilic. The cell viability and Live/dead assay showed that the stem cells remained viable and proliferated after 7 days in culture. Confocal images of phalloidin/DAPI staining showed that the cells adhered and proliferated widely, in fully adaptation with the biomaterial. The SFI values of the group that received the conduit were similar to the values of the control lesioned group. In conclusion, conduits composed of PLGA-gelatin nanofibers were produced and promoted a very good interaction with the stem cells. Although in vitro studies have shown this biomaterial to be a promising biomaterial for the regeneration of nerve tissue, in vivo studies of this graft have not shown significant improvements in nerve regeneration.

Facial nerve regeneration with bioabsorbable collagen conduits filled with collagen filaments: An experimental study

Introduction

A bioabsorbable collagen conduit (Renerve™) filled with collagen filaments is currently approved as an artificial nerve conduit in Japan and is mainly used for connecting and repairing peripheral nerves after traumatic nerve injury. However, there are few reports on its applications for reconstructing and repairing the facial nerve. The present study evaluated the efficacy of the conduit on promoting nerve regeneration in a murine model with a nerve defect at the buccal branch of the facial nerve.

Methods

Under inhalational anesthesia and microscopic guidance, the buccal branch of the left facial nerve in an 8-week-old Lewis rat was exposed, and a 7 mm gap was created in the nerve. The gap was then connected with either the nerve conduits (NC group) or an autologous nerve graft (the autograft group). At 13 weeks after the procedure, we compared the histological and physiological regenerations in the both groups.

Results

We found compound muscle action potential amplitude is significantly larger in the autograft group (2.8 ± 1.4 mV) than in NC group (1.3 ± 0.5 mV) (p < 0.05). The number of myelinated fibers of the autograft group was higher (3634 ± 1645) than that of NC group (1112 ± 490) (p < 0.01). The fiber diameter of the autograft group (4.8 ± 1.9 μm) was larger than that of NC group (3.8 ± 1.4 μm) (p < 0.05). The myelin thickness of the autograft group was thicker than that of NC group (0.6 ± 0.3 μm vs. 0.4 ± 0.1 μm) (p < 0.05). G-ratio of the autograft group (0.74 ± 0.19) was lower than that of NC group (0.79 ± 0.10) (p < 0.05).

Conclusion

This study demonstrated the efficacy of collagen nerve conduit for facial nerve reconstruction following nerve injury. However, the effectiveness of the conduit on the promotion of nerve regeneration was inferior to that of the autograft.

Assessing the combination of magnetic field stimulation, iron oxide nanoparticles, and aligned electrospun fibers for promoting neurite outgrowth from dorsal root ganglia in vitro

Magnetic fiber composites combining superparamagnetic iron oxide nanoparticles (SPIONs) and electrospun fibers have shown promise in tissue engineering fields. Controlled grafting of SPIONs to the fibers post-electrospinning generates biocompatible magnetic composites without altering desired fiber morphology. Here, for the first time, we assess the potential of SPION-grafted scaffolds combined with magnetic fields to promote neurite outgrowth by providing contact guidance from the aligned fibers and mechanical stimulation from the SPIONs in the magnetic field. Neurite outgrowth from primary rat dorsal root ganglia (DRG) was assessed from explants cultured on aligned control and SPION-grafted electrospun fibers as well as on non-grafted fibers with SPIONs dispersed in the culture media. To determine the optimal magnetic field stimulation to promote neurite outgrowth, we generated a static, alternating, and linearly moving magnet and simulated the magnetic flux density at different areas of the scaffold over time. The alternating magnetic field increased neurite length by 40% on control fibers compared to a static magnetic field. Additionally, stimulation with an alternating magnetic field resulted in a 30% increase in neurite length and 62% increase in neurite area on SPION-grafted fibers compared to DRG cultured on PLLA fibers with untethered SPIONs added to the culture media. These findings demonstrate that SPION-grafted fiber composites in combination with magnetic fields are more beneficial for stimulating neurite outgrowth on electrospun fibers than dispersed SPIONs. STATEMENT OF SIGNIFICANCE: Aligned electrospun fibers improve axonal regeneration by acting as a passive guidance cue but do not actively interact with cells, while magnetic nanoparticles can be remotely manipulated to interact with neurons and elicit neurite outgrowth. Here, for the first time, we examine the combination of magnetic fields, magnetic nanoparticles, and aligned electrospun fibers to enhance neurite outgrowth. We show an alternating magnetic field alone increases neurite outgrowth on aligned electrospun fibers. However, combining the alternating field with magnetic nanoparticle-grafted fibers does not affect neurite outgrowth compared to control fibers but improves outgrowth compared to freely dispersed magnetic nanoparticles. This study provides the groundwork for utilizing magnetic electrospun fibers and magnetic fields as a method for promoting axonal growth.

Biomimetic nerve guidance conduit containing engineered exosomes of adipose-derived stem cells promotes peripheral nerve regeneration

BACKGROUND

Efficient and stable delivery of neurotrophic factors (NTFs) is crucial to provide suitable microenvironment for peripheral nerve regeneration. Neurotrophin-3 (NT-3) is an important NTF during peripheral nerve regeneration which is scarce in the first few weeks of nerve defect. Exosomes are nanovesicles and have been served as promising candidate for biocarrier. In this work, NT-3 mRNA was encapsulated in adipose-derived stem cell (ADSC)-derived exosomes (Exo^NT-3). These engineered exosomes were applied as NT-3 mRNA carrier and then were loaded in nerve guidance conduit (Exo^NT-3-NGC) to bridge rat sciatic nerve defect.

METHOD

NT-3 mRNA was encapsulated in exosomes by forcedly expression of NT-3 mRNA in the donor ADSCs. Exo^NT-3 were co-cultured with SCs in vitro; after 24 h of culture, the efficiency of NT-3 mRNA delivery was evaluated by qPCR, western blotting and ELISA. Then, Exo^NT-3 were loaded in alginate hydrogel to construct the nerve guidance conduits (Exo^NT-3-NGC). Exo^NT-3-NGC were implanted in vivo to reconstruct 10 mm rat sciatic nerve defect. The expression of NT-3 was measured 2 weeks after the implantation operation. The sciatic nerve functional index (SFI) was examined at 2 and 8 weeks after the operation. Moreover, the therapeutic effect of Exo^NT-3-NGC was also evaluated by morphology assay, immunofluorescence staining of regenerated nerves, function evaluation of gastrocnemius muscles after 8 weeks of implantation.

RESULTS

The engineered exosomes could deliver NT-3 mRNA to the recipient cells efficiently and translated into functional protein. The constructed NGC could realize stable release of exosomes at least for 2 weeks. After NGC implantation in vivo, Exo^NT-3-NGC group significantly promote nerve regeneration and improve the function recovery of gastrocnemius muscles compared with control exosomes (Exo^empty-NGC) group.

CONCLUSION

In this work, NGC was constructed to allow exosome-mediated NT-3 mRNA delivery. After Exo^NT-3-NGC implantation in vivo, the level of NT-3 could restore which enhance the nerve regeneration. Our study provide a potential approach to improve nerve regeneration.

Nerve Guidance Conduits with Hierarchical Anisotropic Architecture for Peripheral Nerve Regeneration

Nerve guidance conduits with multifunctional features could offer microenvironments for improved nerve regeneration and functional recovery. However, the challenge remains to optimize multiple cues in nerve conduit systems due to the interplay of these factors during fabrication. Here, a modular assembly for the fabrication of nerve conduits is utilized to address the goal of incorporating multifunctional guidance cues for nerve regeneration. Silk-based hollow conduits with suitable size and mechanical properties, along with silk nanofiber fillers with tunable hierarchical anisotropic architectures and microporous structures, are developed and assembled into conduits. These conduits supported improves nerve regeneration in terms of cell proliferation (Schwann and PC12 cells) and growth factor secretion (BDNF, brain-derived neurotrophic factor) in vitro, and the in vivo repair and functional recovery of rat sciatic nerve defects. Nerve regeneration using these new conduit designs is comparable to autografts, providing a path towards future clinical impact.

AK, Olaiya NG, Iqbal MO, Jummaat F, A.K. AS, Adnan AS. Insights into the Role of Biopolymer Aerogel Scaffolds in Tissue Engineering and Regenerative Medicine

The global transplantation market size was valued at USD 8.4 billion in 2020 and is expected to grow at a compound annual growth rate of 11.5% over the forecast period. The increasing demand for tissue transplantation has inspired researchers to find alternative approaches for making artificial tissues and organs function. The unique physicochemical and biological properties of biopolymers and the attractive structural characteristics of aerogels such as extremely high porosity, ultra low-density, and high surface area make combining these materials of great interest in tissue scaffolding and regenerative medicine applications. Numerous biopolymer aerogel scaffolds have been used to regenerate skin, cartilage, bone, and even heart valves and blood vessels by growing desired cells together with the growth factor in tissue engineering scaffolds. This review focuses on the principle of tissue engineering and regenerative medicine and the role of biopolymer aerogel scaffolds in this field, going through the properties and the desirable characteristics of biopolymers and biopolymer tissue scaffolds in tissue engineering applications. The recent advances of using biopolymer aerogel scaffolds in the regeneration of skin, cartilage, bone, and heart valves are also discussed in the present review. Finally, we highlight the main challenges of biopolymer-based scaffolds and the prospects of using these materials in regenerative medicine.

Biomedical applications of environmental friendly poly-hydroxyalkanoates

Biological polyesters of hydroxyacids are known as polyhydroxyalkanoates (PHA). They have proved to be an alternative, environmentally friendly and attractive candidate for the replacement of petroleum-based plastics in many applications. Many bacteria synthesize these compounds as an intracellular carbon and energy compound usually under unbalanced growth conditions. Biodegradability and biocompatibility of different PHA has been studied in cell culture systems or in an animal host during the last few decades. Such investigations have proposed that PHA can be used as biomaterials for applications in conventional medical devices such as sutures, patches, meshes, implants, and tissue engineering scaffolds as well. Moreover, findings related to encapsulation capability and degradation kinetics of some PHA polymers has paved their way for development of controlled drug delivery systems. The present review discusses about bio-plastics, their characteristics, examines the key findings and recent advances highlighting the usage of bio-plastics in different medical devices. The patents concerning to PHA application in biomedical field have been also enlisted that will provide a brief overview of the status of research in bio-plastic. This would help medical researchers and practitioners to replace the synthetic plastics aids that are currently being used. Simultaneously, it could also prove to be a strong step in reducing the plastic pollution that surged abruptly due to the COVID-19 medical waste.

Foreign Body Reaction to Implanted Biomaterials and Its Impact in Nerve Neuroprosthetics

The implantation of any foreign material into the body leads to the development of an inflammatory and fibrotic process-the foreign body reaction (FBR). Upon implantation into a tissue, cells of the immune system become attracted to the foreign material and attempt to degrade it. If this degradation fails, fibroblasts envelop the material and form a physical barrier to isolate it from the rest of the body. Long-term implantation of medical devices faces a great challenge presented by FBR, as the cellular response disrupts the interface between implant and its target tissue. This is particularly true for nerve neuroprosthetic implants-devices implanted into nerves to address conditions such as sensory loss, muscle paralysis, chronic pain, and epilepsy. Nerve neuroprosthetics rely on tight interfacing between nerve tissue and electrodes to detect the tiny electrical signals carried by axons, and/or electrically stimulate small subsets of axons within a nerve. Moreover, as advances in microfabrication drive the field to increasingly miniaturized nerve implants, the need for a stable, intimate implant-tissue interface is likely to quickly become a limiting factor for the development of new neuroprosthetic implant technologies. Here, we provide an overview of the material-cell interactions leading to the development of FBR. We review current nerve neuroprosthetic technologies (cuff, penetrating, and regenerative interfaces) and how long-term function of these is limited by FBR. Finally, we discuss how material properties (such as stiffness and size), pharmacological therapies, or use of biodegradable materials may be exploited to minimize FBR to nerve neuroprosthetic implants and improve their long-term stability.

Biocompatible Chitin Hydrogel Incorporated with PEDOT Nanoparticles for Peripheral Nerve Repair

The nerve guidance conduit (NGC) is a promising clinical strategy for regenerating the critical-sized peripheral nerve injury. In this study, the polysaccharide chitin is used to fabricate the hydrogel film for inducing the impaired sciatic nerve regeneration through incorporating the conductive poly(3,4-ethylenedioxythiophene) nanoparticles (PEDOT NPs) and modifying with cell adhesive tetrapeptide Cys-Arg-Gly-Asp (CRGD) (ChT-PEDOT-p). The partial deacetylation process of chitin for exposing the amino groups is performed to (i) improve the electrostatic interaction between chitin and the negatively charged PEDOT for enhancing the composite hydrogel strength and (ii) offer the active sites for peptide modification. The as-prepared hydrogel remarkably promotes the in vitro RSC-96 cell adhesion and proliferation, as well as the Schwann cell activity-related gene S100, NF-200, and myelin basic protein (MBP) expression. Function of gastrocnemius muscle and thickness of myelinated axon in chitin/PEDOT groups are analogous to the autograft in 10 mm rat sciatic nerve defect. Immunofluorescence, immunohistochemistry, western blotting, and toluidine blue staining analyses on the regenerated sciatic nerve explain that the attachment and proliferation enhancement of Schwann cells and angiogenesis are the vital factors for the chitin/PEDOT composite to facilitate the nerve regeneration. This work provides an applicable chitin-based NGC material for accelerating the peripheral nerve restoration.

Spider silk nerve graft promotes axonal regeneration on long distance nerve defect in a sheep model

Peripheral nerve injuries with substantial tissue loss require autologous nerve transplantation or alternatively reconstruction with nerve conduits. Axonal elongation after nerve transection is about 1 mm/day. The precise time course of axonal regeneration on an ultrastructural level in nerve gap repair using either autologous or artificial implants has not been described. As peripheral nerve regeneration is a highly time critical process due to deterioration of the neuromuscular junction, this in vivo examination in a large animal model was performed in order to investigate axonal elongation rates and spider silk material degradation in a narrowly delimited time series (20, 30, 40, 50, 90, 120, 150 and 180 days) by using a novel spider silk based artificial nerve graft as a critical prerequisite for clinical translation. Autologous nerves or artificial nerve conduits based on spider silk of the spider species Trichonephila edulis were transplanted in a 6.0 cm nerve defect model in the black headed mutton. At each of the post-implant time point, electrophysiology recordings were performed to assess functional reinnervation of axonal fibers into the implants. Samples were analyzed by histology and immunofluorescence in order to verify the timeline of axonal regeneration including axonal regeneration rates of the spider silk implant and the autologous transplant groups. Spider silk was degraded within 3 month by a light immune response mainly mediated by Langhans Giant cells. In conjunction with behavioral analysis and electrophysiological measurements, the results indicate that the spider silk nerve implant supported an axonal regeneration comparable to an autologous nerve graft which is the current gold standard in nerve repair surgery. These findings indicate that a biomaterial based spider silk nerve conduit is as effective as autologous nerve implants and may be an important approach for long nerve defects.

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.

Natural-Based Biomaterials for Peripheral Nerve Injury Repair

Peripheral nerve injury treatment is a relevant problem because of nerve lesion high incidence and because of unsatisfactory regeneration after severe injuries, thus resulting in a reduced patient's life quality. To repair severe nerve injuries characterized by substance loss and to improve the regeneration outcome at both motor and sensory level, different strategies have been investigated. Although autograft remains the gold standard technique, a growing number of research articles concerning nerve conduit use has been reported in the last years. Nerve conduits aim to overcome autograft disadvantages, but they must satisfy some requirements to be suitable for nerve repair. A universal ideal conduit does not exist, since conduit properties have to be evaluated case by case; nevertheless, because of their high biocompatibility and biodegradability, natural-based biomaterials have great potentiality to be used to produce nerve guides. Although they share many characteristics with synthetic biomaterials, natural-based biomaterials should also be preferable because of their extraction sources; indeed, these biomaterials are obtained from different renewable sources or food waste, thus reducing environmental impact and enhancing sustainability in comparison to synthetic ones. This review reports the strengths and weaknesses of natural-based biomaterials used for manufacturing peripheral nerve conduits, analyzing the interactions between natural-based biomaterials and biological environment. Particular attention was paid to the description of the preclinical outcome of nerve regeneration in injury repaired with the different natural-based conduits.

Effect of Ethylene Oxide Sterilization on Polyvinyl Alcohol Hydrogel Compared with Gamma Radiation

This study investigated the effects of terminal sterilization of polyvinyl alcohol (PVA) biomaterials using clinically translatable techniques, specifically ethylene oxide (EtO) and gamma (γ) irradiation. While a few studies have reported the possibility of sterilizing PVA with γ-radiation, the use of EtO sterilization of PVA requires additional study. PVA solutions were chemically crosslinked with trisodium trimetaphosphate and sodium hydroxide. The three experimental groups included untreated control, EtO, and γ-irradiation, which were tested for the degree of swelling and water content, and mechanical properties such as radial compliance, longitudinal tensile, minimum bend radius, burst pressure, and suture retention strength. In addition, samples were characterized with scanning electron microscopy, differential scanning calorimetry, X-ray photoelectron spectroscopy, and water contact angle measurements. Cell attachment was assessed using the endothelial cell line EA.hy926, and the sterilized PVA cytotoxicity was studied with a live/dead stain. Platelet and fibrin accumulation was measured using an ex vivo shunt baboon model. Finally, the immune responses of PVA implants were analyzed after a 21-day subcutaneous implantation in rats and a 30-day implantation in baboon. EtO sterilization reduced the PVA graft wall thickness, its degree of swelling, and water content compared with both γ-irradiated and untreated PVA. Moreover, EtO sterilization significantly reduced the radial compliance and increased Young's modulus. EtO did not change PVA hydrophilicity, while γ-irradiation increased the water contact angle of the PVA. Consequently, endothelial cell attachment on the EtO-sterilized PVA showed similar results to the untreated PVA, while cell attachment significantly improved on the γ-irradiated PVA. When exposing the PVA grafts to circulating whole blood, fibrin accumulation of EtO-sterilized PVA was found to be significantly lower than γ-irradiated PVA. The immune responses of γ-irradiated PVA, EtO-treated PVA, and untreated PVA were compared. Implanted EtO-treated PVA showed the least MAC387 reaction. The terminal sterilization methods in this study changed PVA hydrogel properties; nevertheless, based on the characterizations performed, both sterilization methods were suitable for sterilizing PVA. We concluded that EtO can be used as an alternative method to sterilize PVA hydrogel material. Impact statement Polyvinyl alcohol (PVA) hydrogels have been used for a variety of tissue replacements, including neural, cardiac, meniscal, cartilage, muscle, pancreatic, and ocular applications. In addition, PVA can be made into a tubular shape and used as a small-diameter vascular graft. Ethylene oxide (EtO) is one of the Food and Drug Administration-approved methods for sterilization, but its effect on PVA has not been studied extensively. The outcome of this study provides the effects of EtO and γ-irradiation of PVA grafts on both the material properties and the in vivo responses, particularly for vascular applications. Knowledge of these effects may ultimately improve the success rate of PVA vascular grafts.

Application of bioactive hydrogels combined with dental pulp stem cells for the repair of large gap peripheral nerve injuries

Due to the limitations in autogenous nerve grafting or Schwann cell transplantation, large gap peripheral nerve injuries require a bridging strategy supported by nerve conduit. Cell based therapies provide a novel treatment for peripheral nerve injuries. In this study, we first experimented an optimal scaffold material synthesis protocol, from where we selected the 10% GFD formula (10% GelMA hydrogel, recombinant human basic fibroblast growth factor and dental pulp stem cells (DPSCs)) to fill a cellulose/soy protein isolate composite membrane (CSM) tube to construct a third generation of nerve regeneration conduit, CSM-GFD. Then this CSM-GFD conduit was applied to repair a 15-mm long defect of sciatic nerve in a rat model. After 12 week post implant surgery, at histologic level, we found CSM-GFD conduit could regenerate nerve tissue like neuron and Schwann like nerve cells and myelinated nerve fibers. At physical level, CSM-GFD achieved functional recovery assessed by a sciatic functional index study. In both levels, CSM-GFD performed like what gold standard, the nerve autograft, could do. Further, we unveiled that almost all newly formed nerve tissue at defect site was originated from the direct differentiation of exogeneous DPSCs in CSM-GFD. In conclusion, we claimed that this third-generation nerve regeneration conduit, CSM-GFD, could be a promising tissue engineering approach to replace the conventional nerve autograft to treat the large gap defect in peripheral nerve injuries.

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A novel 3^rd generation nerve conduit was successfully constructed and applied for repairing peripheral nerve injuries (PNI).

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Dental pulp stem cells (DPSCs) was optimized as an ideal seeding cells for nerve regeneration.

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A bioactive system combining GelMA with human bFGF and DPSCs could reconstruct the long gap PNI within 12 weeks in vivo.

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Our system could achieve the same outcome in nerve repair as that of autografting, a routine treatment for PNI.

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The proposed bioactive system may trigger an evolutional change into the current clinical practice in managing PNI.

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The majority of the regenerated nerve tissue was originated from the donor’s dental pulp stem cells.

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.

Aligned Fingolimod-Releasing Electrospun Fibers Increase Dorsal Root Ganglia Neurite Extension and Decrease Schwann Cell Expression of Promyelinating Factors

Researchers are investigating the use of biomaterials with aligned guidance cues, like those provided by aligned electrospun fibers, to facilitate axonal growth across critical-length peripheral nerve defects. To enhance the regenerative outcomes further, these aligned fibers can be designed to provide local, sustained release of therapeutics. The drug fingolimod improved peripheral nerve regeneration in preclinical rodent models by stimulating a pro-regenerative Schwann cell phenotype and axonal growth. However, the systemic delivery of fingolimod for nerve repair can lead to adverse effects, so it is necessary to develop a means of providing sustained delivery of fingolimod local to the injury. Here we created aligned fingolimod-releasing electrospun fibers that provide directional guidance cues in combination with the local, sustained release of fingolimod to enhance neurite outgrowth and stimulate a pro-regenerative Schwann cell phenotype. Electrospun fiber scaffolds were created by blending fingolimod into poly(lactic-co-glycolic acid) (PLGA) at a w/w% (drug/polymer) of 0.0004, 0.02, or 0.04%. We examined the effectiveness of these scaffolds to stimulate neurite extension in vitro by measuring neurite outgrowth from whole and dissociated dorsal root ganglia (DRG). Subsequently, we characterized Schwann cell migration and gene expression in vitro. The results show that drug-loaded PLGA fibers released fingolimod for 28 days, which is the longest reported release of fingolimod from electrospun fibers. Furthermore, the 0.02% fingolimod-loaded fibers enhanced neurite outgrowth from whole and dissociated DRG neurons, increased Schwann cell migration, and reduced the Schwann cell expression of promyelinating factors. The in vitro findings show the potential of the aligned fingolimod-releasing electrospun fibers to enhance peripheral nerve regeneration and serve as a basis for future in vivo studies.

Cellular Mechanisms of Rejection of Optic and Sciatic Nerve Transplants: An Observational Study

Background.

Organ transplantation is a standard therapeutic strategy for irreversible organ damage, but the utility of nerve transplantation remains generally unexplored, despite its potential benefit to a large patient population. Here, we aimed to establish a feasible preclinical mouse model for understanding the cellular mechanisms behind the rejection of peripheral and optic nerves.

Methods.

We performed syngenic and allogenic transplantation of optic and sciatic nerves in mice by inserting the nerve grafts inside the kidney capsule, and we assessed the allografts for signs of rejection through 14 d following transplantation. Then, we assessed the efficacy of CTLA4 Ig, Rapamycin, and anti-CD3 antibody in suppressing immune cell infiltration of the nerve allografts.

Results.

By 3 d posttransplantation, both sciatic and optic nerves transplanted from BALB/c mice into C57BL/6J recipients contained immune cell infiltrates, which included more CD11b⁺ macrophages than CD3⁺ T cells or B220⁺ B cells. Ex vivo immunogenicity assays demonstrated that sciatic nerves demonstrated higher alloreactivity in comparison with optic nerves. Interestingly, optic nerves contained higher populations of anti-inflammatory PD-L1⁺ cells than sciatic nerves. Treatment with anti-CD3 antibody reduced immune cell infiltrates in the optic nerve allograft, but exerted no significant effect in the sciatic nerve allograft.

Conclusions.

These findings establish the feasibility of a preclinical allogenic nerve transplantation model and provide the basis for future testing of directed, high-intensity immunosuppression in these mice.

3D Printing and Bioprinting Nerve Conduits for Neural Tissue Engineering

Fabrication of nerve conduits for perfectly repairing or replacing damaged peripheral nerve is an urgent demand worldwide, but it is also a formidable clinical challenge. In the last decade, with the rapid development of manufacture technologies, 3D printing and bioprinting have been becoming remarkable stars in the field of neural engineering. In this review, we explore that the biomaterial inks (hydrogels, thermoplastic, and thermoset polyesters and composite) and bioinks have been selected for 3D printing and bioprinting of peripheral nerve conduits. This review covers 3D manufacturing technologies, including extrusion printing, inkjet printing, stereolithography, and bioprinting with inclusion of cells, bioactive molecules, and drugs. Finally, an outlook on the future directions of 3D printing and 4D printing in customizable nerve therapies is presented.