Effect of controlled delivery of neurotrophin-3 from fibrin on spinal cord injury in a long term model

Bibliometric analysis of stem cells for spinal cord injury: current status and emerging frontiers

Background: This study aimed to conduct a bibliometric analysis of the literature on stem cell therapy for spinal cord injury to visualize the research status, identify hotspots, and explore the development trends in this field. Methods: We searched the Web of Science Core Collection database using relevant keywords ("stem cells" and "spinal cord injury") and retrieved the published literature between 2000 and 2022. Data such as journal title, author information, institutional affiliation, country, and keywords were extracted. Afterwards, we performed bibliometric analysis of the retrieved data using Bibliometrix, VOSviewer, and CiteSpace. Results: A total of 5375 articles related to stem cell therapy for spinal cord injury were retrieved, and both the annual publication volume and the cumulative publication volume showed an upward trend. neural regeneration research was the journal with the most publications and the fastest cumulative publication growth (162 articles), Okano Hideyuki was the author with the highest number of publications and citations (114 articles), Sun Yat-sen University was the institution with the highest number of publications (420 articles), and China was the country with the highest number of publications (5357 articles). However, different authors, institutions, and countries need to enhance their cooperation in order to promote the generation of significant academic achievements. Current research in this field has focused on stem cell transplantation, neural regeneration, motor function recovery, exosomes, and tissue engineering. Meanwhile, future research directions are primarily concerned with the molecular mechanisms, safety, clinical trials, exosomes, scaffolds, hydrogels, and inflammatory responses of stem cell therapy for spinal cord injuries. Conclusion: In summary, this study provided a comprehensive analysis of the current research status and frontiers of stem cell therapy for spinal cord injury. The findings provide a foundation for future research and clinical translation efforts of stem cell therapy in this field.

Protein scaffolds in human clinics

Fundamental clinical areas such as drug delivery and regenerative medicine require biocompatible materials as mechanically stable scaffolds or as nanoscale drug carriers. Among the wide set of emerging biomaterials, polypeptides offer enticing properties over alternative polymers, including full biocompatibility, biodegradability, precise interactivity, structural stability and conformational and functional versatility, all of them tunable by conventional protein engineering. However, proteins from non-human sources elicit immunotoxicities that might bottleneck further development and narrow their clinical applicability. In this context, selecting human proteins or developing humanized protein versions as building blocks is a strict demand to design non-immunogenic protein materials. We review here the expanding catalogue of human or humanized proteins tailored to execute different levels of scaffolding functions and how they can be engineered as self-assembling materials in form of oligomers, polymers or complex networks. In particular, we emphasize those that are under clinical development, revising their fields of applicability and how they have been adapted to offer, apart from mere mechanical support, highly refined functions and precise molecular interactions.

Injectable, macroporous scaffolds for delivery of therapeutic genes to the injured spinal cord

Biomaterials are being developed as therapeutics for spinal cord injury (SCI) that can stabilize and bridge acute lesions and mediate the delivery of transgenes, providing a localized and sustained reservoir of regenerative factors. For clinical use, direct injection of biomaterial scaffolds is preferred to enable conformation to unique lesions and minimize tissue damage. While an interconnected network of cell-sized macropores is necessary for rapid host cell infiltration into-and thus integration of host tissue with-implanted scaffolds, injectable biomaterials have generally suffered from a lack of control over the macrostructure. As genetic vectors have short lifetimes in vivo, rapid host cell infiltration into scaffolds is a prerequisite for efficient biomaterial-mediated delivery of transgenes. We present scaffolds that can be injected and assembled in situ from hyaluronic acid (HA)-based, spherical microparticles to form scaffolds with a network of macropores (∼10 μm). The results demonstrate that addition of regularly sized macropores to traditional hydrogel scaffolds, which have nanopores (∼10 nm), significantly increases the expression of locally delivered transgene to the spinal cord after a thoracic injury. Maximal cell and axon infiltration into scaffolds was observed in scaffolds with more regularly sized macropores. The delivery of lentiviral vectors encoding the brain-derived neurotrophic factor (BDNF), but not neurotrophin-3, from these scaffolds further increased total numbers and myelination of infiltrating axons. Modest improvements to the hindlimb function were observed with BDNF delivery. The results demonstrate the utility of macroporous and injectable HA scaffolds as a platform for localized gene therapies after SCI.

Nanomaterial integration into the scaffolding materials for nerve tissue engineering: a review

The nervous system, which consists of a complex network of millions of neurons, is one of the most highly intricate systems in the body. This complex network is responsible for the physiological and cognitive functions of the human body. Following injuries or degenerative diseases, damage to the nervous system is overwhelming because of its complexity and its limited regeneration capacity. However, neural tissue engineering currently has some capacities for repairing nerve deficits and promoting neural regeneration, with more developments in the future. Nevertheless, controlling the guidance of stem cell proliferation and differentiation is a challenging step towards this goal. Nanomaterials have the potential for the guidance of the stem cells towards the neural lineage which can overcome the pitfalls of the classical methods since they provide a unique microenvironment that facilitates cell-matrix and cell-cell interaction, and they can manipulate the cell signaling mechanisms to control stem cells' fate. In this article, the suitable cell sources and microenvironment cues for neuronal tissue engineering were examined. Afterward, the nanomaterials that impact stem cell proliferation and differentiation towards neuronal lineage were reviewed.

Effect of bone marrow stromal cells in combination with biomaterials in early phases of distraction osteogenesis: An experimental study in a rabbit femur model

Acceleration of the consolidation of the distracted bone is a relevant medical need. As a platform to improve in vivo bone engineering, we developed a novel distraction osteogenesis (DO) model in a rabbit large bone (femur) and tested if the application of cultured bone marrow stromal cells (BMSCs) immediately after the osteotomy promotes the formation of bone. This report consists of two components, an animal study to evaluate the quality of the regenerate following different treatments and an in vitro study to evaluate osteogenic potential of BMSC cultures. To illuminate the mechanism of action of injected cells, we tested stem cell cultures enriched in osteogenic-BMSCs (O-BMSCs) as compared with cultures enriched in non-osteogenic BMSCs (NO-BMSCs). Finally, we included a group of animals treated with biomaterials (fibrin and ground cortical bone) in addition to cells. Injection of O-BMSCs promoted the maturity of distracted callus and decreased fibrosis. When combined with biomaterials, O-BMSCs modified the ossification pattern from endochondral to intramembranous type. The use of NO-BMSCs not only did not increase the maturity but also increased porosity of the bone. These preclinical results indicate that the BMSC cultures must be tested in vitro prior to clinical use, since a number of factors may influence their outcome in bone formation. We hypothesize that the use of osteogenic BMSCs and biomaterials could be clinically beneficial to shorten the consolidation period of the distraction and the total period of bone lengthening.

Matrices, scaffolds & carriers for cell delivery in nerve regeneration

Nerve injuries can be life-long debilitating traumas that severely impact patients' quality of life. While many acellular neural scaffolds have been developed to aid the process of nerve regeneration, complete functional recovery is still very difficult to achieve, especially for long-gap peripheral nerve injury and most cases of spinal cord injury. Cell-based therapies have shown many promising results for improving nerve regeneration. With recent advances in neural tissue engineering, the integration of biomaterial scaffolds and cell transplantation are emerging as a more promising approach to enhance nerve regeneration. This review provides an overview of important considerations for designing cell-carrier biomaterial scaffolds. It also discusses current biomaterials used for scaffolds that provide permissive and instructive microenvironments for improved cell transplantation.

Functional Multichannel Poly(Propylene Fumarate)-Collagen Scaffold with Collagen-Binding Neurotrophic Factor 3 Promotes Neural Regeneration After Transected Spinal Cord Injury

Many factors contribute to the poor axonal regrowth and ineffective functional recovery after spinal cord injury (SCI). Biomaterials have been used for SCI repair by promoting bridge formation and reconnecting the neural tissue at the lesion site. The mechanical properties of biomaterials are critical for successful design to ensure the stable support as soon as possible when compressed by the surrounding spine and musculature. Poly(propylene fumarate) (PPF) scaffolds with high mechanical strength have been shown to provide firm spatial maintenance and to promote repair of tissue defects. A multichannel PPF scaffold is combined with collagen biomaterial to build a novel biocompatible delivery system coated with neurotrophin-3 containing an engineered collagen-binding domain (CBD-NT3). The parallel-aligned multichannel structure of PPF scaffolds guide the direction of neural tissue regeneration across the lesion site and promote reestablishment of bridge connectivity. The combinatorial treatment consisting of PPF and collagen loaded with CBD-NT3 improves the inhibitory microenvironment, facilitates axonal and neuronal regeneration, survival of various types of functional neurons and remyelination and synapse formation of regenerated axons following SCI. This novel treatment strategy for SCI repair effectively promotes neural tissue regeneration after transected spinal injury by providing a regrowth-supportive microenvironment and eventually induces functional improvement.

Experimental Models of Spinal Cord Injury in Laboratory Rats

Pathologies associated with spinal cord injury are some of the leading diseases in the world. The search for new therapeutic agents and 3D biodegradable materials for the recovery of spinal cord functions is a topical issue. In this review, we have summarized the literature data on the most common experimental models of spinal cord injury in laboratory rats and analyzed the experience of using 3D biodegradable materials (scaffolds) in experimental studies of spinal trauma. The advantages and disadvantages of the described models are systematically analyzed in this review.

Effect of hyaluronic acid hydrogels containing astrocyte-derived extracellular matrix and/or V2a interneurons on histologic outcomes following spinal cord injury

One reason for the lack of regeneration, and poor clinical outcomes, following central nervous system (CNS) injury is the formation of a glial scar that inhibits new axon growth. In addition to forming the glial scar, astrocytes have been shown to be important for spontaneous SCI recovery in rodents, suggesting some astrocyte populations are pro-regenerative, while others are inhibitory following injury. In this work, the effect of implanting hyaluronic acid (HA) hydrogels containing extracellular matrix (ECM) harvested from mouse embryonic stem cell (mESC)-derived astrocytes on histologic outcomes following SCI in rats was explored. In addition, the ability of HA hydrogels with and without ECM to support the transplantation of mESC-derived V2a interneurons was tested. The incorporation of ECM harvested from protoplasmic (grey matter) astrocytes, but not ECM harvested from fibrous (white matter) astrocytes, into hydrogels was found to reduce the size of the glial scar, increase axon penetration into the lesion, and reduce macrophage/microglia staining two weeks after implantation. HA hydrogels were also found to support transplantation of V2a interneurons and the presence of these cells caused an increase in neuronal processes both within the lesion and in the 500 μm surrounding the lesion. Overall, protoplasmic mESC-derived astrocyte ECM showed potential to treat CNS injury. In addition, ECM:HA hydrogels represent a novel scaffold with beneficial effects on histologic outcomes after SCI both with and without cells.

Improved 3D Hydrogel Cultures of Primary Glial Cells for In Vitro Modelling of Neuroinflammation

In the central nervous system, numerous acute injuries and neurodegenerative disorders, as well as implanted devices or biomaterials engineered to enhance function result in the same outcome: excess inflammation leads to gliosis, cytotoxicity, and/or formation of a glial scar that collectively exacerbate injury or prevent healthy recovery. With the intent of creating a system to model glial scar formation and study inflammatory processes, we have generated a 3D cell scaffold capable of housing primary cultured glial cells: microglia that regulate the foreign body response and initiate the inflammatory event, astrocytes that respond to form a fibrous scar, and oligodendrocytes that are typically vulnerable to inflammatory injury. The present work provides a detailed step-by-step method for the fabrication, culture, and microscopic characterization of a hyaluronic acid-based 3D hydrogel scaffold with encapsulated rat brain-derived glial cells. Further, protocols for characterization of cell encapsulation and the hydrogel scaffold by confocal immunofluorescence and scanning electron microscopy are demonstrated, as well as the capacity to modify the scaffold with bioactive substrates, with incorporation of a commercial basal lamina mixture to improved cell integration.

Visual Prosthesis: Interfacing Stimulating Electrodes with Retinal Neurons to Restore Vision

The bypassing of degenerated photoreceptors using retinal neurostimulators is helping the blind to recover functional vision. Researchers are investigating new ways to improve visual percepts elicited by these means as the vision produced by these early devices remain rudimentary. However, several factors are hampering the progression of bionic technologies: the charge injection limits of metallic electrodes, the mechanical mismatch between excitable tissue and the stimulating elements, neural and electric crosstalk, the physical size of the implanted devices, and the inability to selectively activate different types of retinal neurons. Electrochemical and mechanical limitations are being addressed by the application of electromaterials such as conducting polymers, carbon nanotubes and nanocrystalline diamonds, among other biomaterials, to electrical neuromodulation. In addition, the use of synthetic hydrogels and cell-laden biomaterials is promising better interfaces, as it opens a door to establishing synaptic connections between the electrode material and the excitable cells. Finally, new electrostimulation approaches relying on the use of high-frequency stimulation and field overlapping techniques are being developed to better replicate the neural code of the retina. All these elements combined will bring bionic vision beyond its present state and into the realm of a viable, mainstream therapy for vision loss.

Biomaterials for Local, Controlled Drug Delivery to the Injured Spinal Cord

Affecting approximately 17,000 new people each year, spinal cord injury (SCI) is a devastating injury that leads to permanent paraplegia or tetraplegia. Current pharmacological approaches are limited in their ability to ameliorate this injury pathophysiology, as many are not delivered locally, for a sustained duration, or at the correct injury time point. With this review, we aim to communicate the importance of combinatorial biomaterial and pharmacological approaches that target certain aspects of the dynamically changing pathophysiology of SCI. After reviewing the pathophysiology timeline, we present experimental biomaterial approaches to provide local sustained doses of drug. In this review, we present studies using a variety of biomaterials, including hydrogels, particles, and fibers/conduits for drug delivery. Subsequently, we discuss how each may be manipulated to optimize drug release during a specific time frame following SCI. Developing polymer biomaterials that can effectively release drug to target specific aspects of SCI pathophysiology will result in more efficacious approaches leading to better regeneration and recovery following SCI.

Stem cells for spinal cord injury: Strategies to inform differentiation and transplantation

The complex pathology of spinal cord injury (SCI), involving a cascade of secondary events and the formation of inhibitory barriers, hampers regeneration across the lesion site and often results in irreversible loss of motor function. The limited regenerative capacity of endogenous cells after SCI has led to a focus on the development of cell therapies that can confer both neuroprotective and neuroregenerative benefits. Stem cells have emerged as a candidate cell source because of their ability to self-renew and differentiate into a multitude of specialized cell types. While ethical and safety concerns impeded the use of stem cells in the past, advances in isolation and differentiation methods have largely mitigated these issues. A confluence of work in stem cell biology, genetics, and developmental neurobiology has informed the directed differentiation of specific spinal cell types. After transplantation, these stem cell-derived populations can replace lost cells, provide trophic support, remyelinate surviving axons, and form relay circuits that contribute to functional recovery. Further refinement of stem cell differentiation and transplantation methods, including combinatorial strategies that involve biomaterial scaffolds and drug delivery, is critical as stem cell-based treatments enter clinical trials. Biotechnol. Bioeng. 2017;114: 245-259. © 2016 Wiley Periodicals, Inc.

Recombinant human bone morphogenetic protein-2 suspended in fibrin glue enhances bone formation during distraction osteogenesis in rabbits

INTRODUCTION

Bone morphogenetic protein-2 (BMP-2) has high potential for bone formation, but its in vivo effects are unpredictable due to the short life time. This study was designed to evaluate the effects of recombinant human (rh) BMP-2 suspended in fibrin on bone formation during distraction osteogenesis (DO) in rabbits.

MATERIAL AND METHODS

The in vitro release kinetics of rhBMP-2 suspended in fibrin was tested using an enzyme-linked immunosorbent assay. Unilateral tibial lengthening for 10 mm was achieved in 48 rabbits. At the completion of osteodistraction, vehicle, fibrin, rhBMP-2 or rhBMP-2 suspended in fibrin (rhBMP-2 + fibrin) was injected into the center of the lengthened gap, with 12 animals in each group. Eight weeks later, the distracted callus was examined by histology, micro-CT and biomechanical testing. Radiographs of the distracted tibiae were taken at both 4 and 8 weeks after drug treatment.

RESULTS

It was found that fibrin prolonged the life span of rhBMP-2 in vitro with sustained release during 17 days. The rhBMP-2 + fibrin treated animals showed the best results in bone mineral density, bone volume fraction, cortical bone thickness by micro-CT evaluation and mechanical properties by the three-point bending test when compared to the other groups (p < 0.05). In histological images, rhBMP-2 + fibrin treatment showed increased callus formation and better gap bridging compared to the other groups.

CONCLUSIONS

The results of this study suggest that fibrin holds promise to be a good carrier of rhBMP-2, and rhBMP-2 suspended in fibrin showed a stronger promoting effect on bone formation during DO in rabbits.

Biomaterials for Enhancing CNS Repair

The health of the central nervous system (CNS) does not only rely on the state of the neural cells but also on how various extracellular components organize cellular behaviors into proper tissue functions. Biomaterials have been valuable in restoring or augmenting the roles of extracellular components in the CNS in the event of injury and disease. In this review, we highlight how biomaterials have been enabling tools in important therapeutic strategies involving cell transplantation and drug/protein delivery. We further discuss advances in biomaterial design and applications that can potentially be translated into the CNS to provide unprecedented benefits.

Co-transplantation of autologous OM-MSCs and OM-OECs: a novel approach for spinal cord injury

Spinal cord injury (SCI) is a disastrous injury that leads to motor and sensory dysfunctions in patients. In recent years, co-transplantation has become an increasingly used therapeutic treatment for patients with SCI. Both mesenchymal stem cells (MSCs) and olfactory-ensheathing cells (OECs) have been adopted to ameliorate SCI, with promising outcomes. Remarkable effects on the rehabilitation of patients with SCI have been achieved using MSCs. Olfactory mucosa (OM) MSCs from human OM are one of the most ideal cell resources for auto-transplantation in clinical application owing to their a high proliferation rate and multipotent capability. In addition, OECs derived from OM have been used to improve functional recovery of SCI and resulted in promising functional recovery in years. Accordingly, co-transplantation of OM-MSCs coupled with OM-OECs has been adopted to improve the recovery of SCI. Here we reviewed the reported applications of OM-MSCs and OM-OECs for SCI treatment and proposed that a novel combined strategy using both autologous OM-MSCs and OM-OECs would achieve a better approach for the treatment of SCI.

Inhibitor of PI3K/Akt Signaling Pathway Small Molecule Promotes Motor Neuron Differentiation of Human Endometrial Stem Cells Cultured on Electrospun Biocomposite Polycaprolactone/Collagen Scaffolds

Small molecules as useful chemical tools can affect cell differentiation and even change cell fate. It is demonstrated that LY294002, a small molecule inhibitor of phosphatidylinositol 3-kinase (PI3K)/Akt signal pathway, can inhibit proliferation and promote neuronal differentiation of mesenchymal stem cells (MSCs). The purpose of this study was to investigate the differentiation effect of Ly294002 small molecule on the human endometrial stem cells (hEnSCs) into motor neuron-like cells on polycaprolactone (PCL)/collagen scaffolds. hEnSCs were cultured in a neurogenic inductive medium containing 1 μM LY294002 on the surface of PCL/collagen electrospun fibrous scaffolds. Cell attachment and viability of cells on scaffolds were characterized by scanning electron microscope (SEM) and 3-(4,5-dimethylthiazoyl-2-yl)2,5-diphenyltetrazolium bromide (MTT) assay. The expression of neuron-specific markers was assayed by real-time PCR and immunocytochemistry analysis after 15 days post induction. Results showed that attachment and differentiation of hEnSCs into motor neuron-like cells on the scaffolds with Ly294002 small molecule were higher than that of the cells on tissue culture plates as control group. In conclusion, PCL/collagen electrospun scaffolds with Ly294002 have potential for being used in neural tissue engineering because of its bioactive and three-dimensional structure which enhances viability and differentiation of hEnSCs into neurons through inhibition of the PI3K/Akt pathway. Thus, manipulation of this pathway by small molecules can enhance neural differentiation.

The formation of human auricular cartilage from microtic tissue: An in vivo study

OBJECTIVES

This study aimed to isolate, culture-expand and characterize the chondrocytes isolated from microtic cartilage and evaluate its potential as a cell source for ear cartilage reconstruction. Specific attention was to construct the auricular cartilage tissue by using fibrin as scaffold.

STUDY DESIGN

Cell culture experiment with the use of microtic chondrocytes.

DESIGN

Cell culture experiment with the use of microtic chondrocytes.

METHODS

After ear reconstructive surgery at the Universiti Kebangsaan Malaysia Medical Center, chondrocytes were isolated from microtic cartilage. Chondrocytes isolated from the tissue were cultured expanded until passage 4 (P4). Upon confluency at P4, chondrocytes were harvested and tissue engineered constructs were made with human plasma polymerized to fibrin. Constructs formed later is implanted at the dorsal part of nude mice for 8 weeks, followed by post-implantation evaluation with histology staining (Hematoxylin and Eosin (H&E) and Safranin O), immunohistochemistry and RT-PCR for chondrogenic associated genes expression level.

RESULTS

Under gross assessment, the construct after 8 weeks of implantation showed similar physical characteristics that of cartilage. Histological staining showed abundant lacunae cells embedded in extracellular matrix similar to that of native cartilage. Safranin O staining showed positive staining which indicates the presence of proteoglycan-rich matrix. Immunohistochemistry analysis showed the strong positive staining for collagen type II, the specific collagen type in the cartilage. Gene expression quantification showed no significant differences in the expression of chondrogenic gene used which is collagen type I, collagen type II, aggrecan core protein (ACP), elastin and sox9 genes when compared to construct formed from normal auricular tissue.

CONCLUSION

Chondrocytes isolated from microtia cartilage has the potential to be used as an alternative cell source for external ear reconstruction in future clinical application.

Biomaterial Approaches to Enhancing Neurorestoration after Spinal Cord Injury: Strategies for Overcoming Inherent Biological Obstacles

While advances in technology and medicine have improved both longevity and quality of life in patients living with a spinal cord injury, restoration of full motor function is not often achieved. This is due to the failure of repair and regeneration of neuronal connections in the spinal cord after injury. In this review, the complicated nature of spinal cord injury is described, noting the numerous cellular and molecular events that occur in the central nervous system following a traumatic lesion. In short, postinjury tissue changes create a complex and dynamic environment that is highly inhibitory to the process of neural regeneration. Strategies for repair are outlined with a particular focus on the important role of biomaterials in designing a therapeutic treatment that can overcome this inhibitory environment. The importance of considering the inherent biological response of the central nervous system to both injury and subsequent therapeutic interventions is highlighted as a key consideration for all attempts at improving functional recovery.

Design of growth factor sequestering biomaterials

Growth factors (GFs) are major regulatory proteins that can govern cell fate, migration, and organization. Numerous aspects of the cell milieu can modulate cell responses to GFs, and GF regulation is often achieved by the native extracellular matrix (ECM). For example, the ECM can sequester GFs and thereby control GF bioavailability. In addition, GFs can exert distinct effects depending on whether they are sequestered in solution, at two-dimensional interfaces, or within three-dimensional matrices. Understanding how the context of GF sequestering impacts cell function in the native ECM can instruct the design of soluble or insoluble GF sequestering moieties, which can then be used in a variety of bioengineering applications. This Feature Article provides an overview of the natural mechanisms of GF sequestering in the cell milieu, and reviews the recent bioengineering approaches that have sequestered GFs to modulate cell function. Results to date demonstrate that the cell response to GF sequestering depends on the affinity of the sequestering interaction, the spatial proximity of sequestering in relation to cells, the source of the GF (supplemented or endogenous), and the phase of the sequestering moiety (soluble or insoluble). We highlight the importance of context for the future design of biomaterials that can leverage endogenous molecules in the cell milieu and mitigate the need for supplemented factors.

Repair of injured spinal cord using biomaterial scaffolds and stem cells

The loss of neurons and degeneration of axons after spinal cord injury result in the loss of sensory and motor functions. A bridging biomaterial construct that allows the axons to grow through has been investigated for the repair of injured spinal cord. Due to the hostility of the microenvironment in the lesion, multiple conditions need to be fulfilled to achieve improved functional recovery. A scaffold has been applied to bridge the gap of the lesion as contact guidance for axonal growth and to act as a vehicle to deliver stem cells in order to modify the microenvironment. Stem cells may improve functional recovery of the injured spinal cord by providing trophic support or directly replacing neurons and their support cells. Neural stem cells and mesenchymal stem cells have been seeded into biomaterial scaffolds and investigated for spinal cord regeneration. Both natural and synthetic biomaterials have increased stem cell survival in vivo by providing the cells with a controlled microenvironment in which cell growth and differentiation are facilitated. This optimal multi‒disciplinary approach of combining biomaterials, stem cells, and biomolecules offers a promising treatment for the injured spinal cord.

Organic electrode coatings for next-generation neural interfaces

Traditional neuronal interfaces utilize metallic electrodes which in recent years have reached a plateau in terms of the ability to provide safe stimulation at high resolution or rather with high densities of microelectrodes with improved spatial selectivity. To achieve higher resolution it has become clear that reducing the size of electrodes is required to enable higher electrode counts from the implant device. The limitations of interfacing electrodes including low charge injection limits, mechanical mismatch and foreign body response can be addressed through the use of organic electrode coatings which typically provide a softer, more roughened surface to enable both improved charge transfer and lower mechanical mismatch with neural tissue. Coating electrodes with conductive polymers or carbon nanotubes offers a substantial increase in charge transfer area compared to conventional platinum electrodes. These organic conductors provide safe electrical stimulation of tissue while avoiding undesirable chemical reactions and cell damage. However, the mechanical properties of conductive polymers are not ideal, as they are quite brittle. Hydrogel polymers present a versatile coating option for electrodes as they can be chemically modified to provide a soft and conductive scaffold. However, the in vivo chronic inflammatory response of these conductive hydrogels remains unknown. A more recent approach proposes tissue engineering the electrode interface through the use of encapsulated neurons within hydrogel coatings. This approach may provide a method for activating tissue at the cellular scale, however, several technological challenges must be addressed to demonstrate feasibility of this innovative idea. The review focuses on the various organic coatings which have been investigated to improve neural interface electrodes.

Fibrinogen and fibrin based micro and nano scaffolds incorporated with drugs, proteins, cells and genes for therapeutic biomedical applications

Over the past two decades, many types of natural and synthetic polymer-based micro- and nanocarriers, with exciting properties and applications, have been developed for application in various types of tissue regeneration, including bone, cartilage, nerve, blood vessels, and skin. The development of suitable polymers scaffold designs to aid the repair of specific cell types have created diverse and important potentials in tissue restoration. Fibrinogen (Fbg)- and fibrin (Fbn)-based micro- and nanostructures can provide suitable natural matrix environments. Since these primary materials are abundantly available in blood as the main coagulation proteins, they can easily interact with damaged tissues and cells through native biochemical interactions. Fbg- and Fbn-based micro and nanostructures can also be consecutively furnished/or encapsulated and specifically delivered, with multiple growth factors, proteins, and stem cells, in structures designed to aid in specific phases of the tissue regeneration process. The present review has been carried out to demonstrate the progress made with micro and nanoscaffold applications and features a number of applications of Fbg- and Fbn-based carriers in the field of biomaterials, including the delivery of drugs, active biomolecules, cells, and genes, that have been effectively used in tissue engineering and regenerative medicine.

Scaffolds to promote spinal cord regeneration

Substantial research effort in the spinal cord injury (SCI) field is directed towards reduction of secondary injury changes and enhancement of tissue sparing. However, pathway repair after complete transections, large lesions, or after chronic injury may require the implantation of some form of oriented bridging structure to restore tissue continuity across a trauma zone. These matrices or scaffolds should be biocompatible and create an environment that facilitates tissue growth and vascularization, and allow axons to regenerate through and beyond the implant in order to reconnect with "normal" tissue distal to the injury. The myelination of regrown axons is another important requirement. In this chapter, we describe recent advances in biomaterial technology designed to provide a terrain for regenerating axons to grow across the site of injury and/or create an environment for endogenous repair. Many different types of scaffold are under investigation; they can be biodegradable or nondegradable, natural or synthetic. Scaffolds can be designed to incorporate immobilized signaling molecules and/or used as devices for controlled release of therapeutic agents, including growth factors. These bridging structures can also be infiltrated with specific cell types deemed suitable for spinal cord repair.

Fibrin scaffolds containing ectomesenchymal stem cells enhance behavioral and histological improvement in a rat model of spinal cord injury

Fibrin has been widely used in wound healing. However, its benefit for spinal cord injury (SCI) is limited. In this study, we investigated the impact of fibrin scaffolds containing ectomesenchymal stem cells (EMSCs) on histological and behavioral recovery after SCI and compared it with fibrin alone. To achieve this, EMSCs derived from adult rat nasal respiratory mucosa were cultured, characterized and transfected with green fluorescent protein adenovirus before transplantation. Then, Sprague-Dawley host rats were randomly assigned into four groups: the control group (laminectomy); the SCI group (laminectomy and transection of spinal cords); the fibrin group (fibrin was transplanted immediately after SCI), and the fibrin cell (FC) group (fibrin scaffolds containing EMSCs were transplanted after SCI). Three days after the operation, a TUNEL assay indicated less apoptotic cells in the FC group than in the fibrin group. Two weeks after SCI, fluorescence staining demonstrated not only the survival and migration of EMSCs into the lesion sites, but also a higher number of nerve fibers in the FC group than in the fibrin group. Histological examination including immunohistochemistry and transmission electron microscopy 12 weeks after the operation showed more nerve fibers and a thicker myelin sheath in the FC group compared to the fibrin group. Western blotting confirmed these morphological results. Consistent with the histological results, Basso, Beattie and Bresnahan locomotor scores of the FC group were higher than those of the fibrin group. These results suggest that fibrin scaffolds containing EMSCs can improve the behavioral and histological recovery after SCI better than fibrin alone.

Using extracellular matrix for regenerative medicine in the spinal cord

Regeneration within the mammalian central nervous system (CNS) is limited, and traumatic injury often leads to permanent functional motor and sensory loss. The lack of regeneration following spinal cord injury (SCI) is mainly caused by the presence of glial scarring, cystic cavitation and a hostile environment to axonal growth at the lesion site. The more prominent experimental treatment strategies focus mainly on drug and cell therapies, however recent interest in biomaterial-based strategies are increasing in number and breadth. Outside the spinal cord, approaches that utilize the extracellular matrix (ECM) to promote tissue repair show tremendous potential for various application including vascular, skin, bone, cartilage, liver, lung, heart and peripheral nerve tissue engineering (TE). Experimentally, it is unknown if these approaches can be successfully translated to the CNS, either alone or in combination with synthetic biomaterial scaffolds. In this review we outline the first attempts to apply the potential of ECM-based biomaterials and combining cell-derived ECM with synthetic scaffolds.

From molecular to nanotechnology strategies for delivery of neurotrophins: emphasis on brain-derived neurotrophic factor (BDNF)

Neurodegenerative diseases represent a major public health problem, but beneficial clinical treatment with neurotrophic factors has not been established yet. The therapeutic use of neurotrophins has been restrained by their instability and rapid degradation in biological medium. A variety of strategies has been proposed for the administration of these leading therapeutic candidates, which are essential for the development, survival and function of human neurons. In this review, we describe the existing approaches for delivery of brain-derived neurotrophic factor (BDNF), which is the most abundant neurotrophin in the mammalian central nervous system (CNS). Biomimetic peptides of BDNF have emerged as a promising therapy against neurodegenerative disorders. Polymer-based carriers have provided sustained neurotrophin delivery, whereas lipid-based particles have contributed also to potentiation of the BDNF action. Nanotechnology offers new possibilities for the design of vehicles for neuroprotection and neuroregeneration. Recent developments in nanoscale carriers for encapsulation and transport of BDNF are highlighted.

Engineering the regenerative microenvironment with biomaterials

Modern synthetic biomaterials are being designed to integrate bioactive ligands within hydrogel scaffolds for cells to respond and assimilate within the matrix. These advanced biomaterials are only beginning to be used to simulate the complex spatio-temporal control of the natural healing microenvironment. With increasing understanding of the role of growth factors and cytokines and their interactions with components of the extracellular matrix, novel biomaterials are being developed that more closely mimic the natural healing environments of tissues, resulting in increased efficacy in applications of tissue repair and regeneration. Herein, the important aspects of the healing microenvironment, and how these features can be incorporated within innovative hydrogel scaffolds, are presented.

Rodent Models and Behavioral Outcomes of Cervical Spinal Cord Injury

Rodent spinal cord injury (SCI) models have been developed to examine functional and physiological deficits after spinal cord injury with the hope that these models will elucidate information about human SCI. Models are needed to examine possible treatments and to understand histopathology after SCI; however, they should be considered carefully and chosen based on the goals of the study being performed. Contusion, compression, transection, and other models exist and have the potential to reveal important information about SCI that may be related to human SCI and the outcomes of treatment and timing of intervention.

Biomimetic hydrogels for controlled biomolecule delivery to augment bone regeneration

The regeneration of large bone defects caused by trauma or disease remains a significant clinical problem. Although osteoinductive growth factors such as bone morphogenetic proteins have entered clinics, transplantation of autologous bone remains the gold standard to treat bone defects. The effective treatment of bone defects by protein therapeutics in humans requires quantities that exceed the physiological doses by several orders of magnitude. This not only results in very high treatment costs but also bears considerable risks for adverse side effects. These issues have motivated the development of biomaterials technologies allowing to better control biomolecule delivery from the solid phase. Here we review recent approaches to immobilize biomolecules by affinity binding or by covalent grafting to biomaterial matrices. We focus on biomaterials concepts that are inspired by extracellular matrix (ECM) biology and in particular the dynamic interaction of growth factors with the ECM. We highlight the value of synthetic, ECM-mimicking matrices for future technologies to study bone biology and develop the next generation of 'smart' implants.

Regenerative medicine as applied to general surgery

The present review illustrates the state of the art of regenerative medicine (RM) as applied to surgical diseases and demonstrates that this field has the potential to address some of the unmet needs in surgery. RM is a multidisciplinary field whose purpose is to regenerate in vivo or ex vivo human cells, tissues, or organs to restore or establish normal function through exploitation of the potential to regenerate, which is intrinsic to human cells, tissues, and organs. RM uses cells and/or specially designed biomaterials to reach its goals and RM-based therapies are already in use in several clinical trials in most fields of surgery. The main challenges for investigators are threefold: Creation of an appropriate microenvironment ex vivo that is able to sustain cell physiology and function in order to generate the desired cells or body parts; identification and appropriate manipulation of cells that have the potential to generate parenchymal, stromal and vascular components on demand, both in vivo and ex vivo; and production of smart materials that are able to drive cell fate.

Biodegradable biomatrices and bridging the injured spinal cord: the corticospinal tract as a proof of principle

Important advances in the development of smart biodegradable implants for axonal regeneration after spinal cord injury have recently been reported. These advances are evaluated in this review with special emphasis on the regeneration of the corticospinal tract. The corticospinal tract is often considered the ultimate challenge in demonstrating whether a repair strategy has been successful in the regeneration of the injured mammalian spinal cord. The extensive know-how of factors and cells involved in the development of the corticospinal tract, and the advances made in material science and tissue engineering technology, have provided the foundations for the optimization of the biomatrices needed for repair. Based on the findings summarized in this review, the future development of smart biodegradable bridges for CST regrowth and regeneration in the injured spinal cord is discussed.

Nanofibrous collagen nerve conduits for spinal cord repair

Nerve regeneration in an injured spinal cord is often restricted, contributing to the devastating outcome of neurologic impairment below the site of injury. Although implantation of tissue-engineered scaffolds has evolved as a potential treatment method, the outcomes remain sub-optimal. One possible reason may be the lack of topographical signals from these constructs to provide contact guidance to invading cells or regrowing axons. Nanofibers mimic the natural extracellular matrix architecturally and may therefore promote physiologically relevant cellular phenotypes. In this study, the potential application of electrospun collagen nanofibers (diameter=208.2±90.4 nm) for spinal cord injury (SCI) treatment was evaluated in vitro and in vivo. Primary rat astrocytes and dorsal root ganglias (DRGs) were seeded on collagen-coated glass cover slips (two-dimensional [2D] substrate controls), and randomly oriented or aligned collagen fibers to evaluate scaffold topographical effects on astrocyte behavior and neurite outgrowth, respectively. When cultured on collagen nanofibers, astrocyte proliferation and expression of glial fibrillary acidic protein (GFAP) were suppressed as compared to cells on 2D controls at days 3 (p<0.05) and 7 (p<0.01). Aligned fibers resulted in elongated astrocytes (elongation factor >4, p<0.01) and directed the orientation of neurite outgrowth from DRGs along fiber axes. In the contrast, neurites emanated radially on randomly oriented collagen fibers. By forming collagen scaffolds into spiral tubular structures, we demonstrated the feasibility of using electrospun nanofibers for the treatment of acute SCI using a rat hemi-section model. At days 10 and 30 postimplantation, extensive cellular penetration into the constructs was observed regardless of fiber orientation. However, scaffolds with aligned fibers appeared more structurally intact at day 30. ED1 immunofluorescent staining revealed macrophage invasion by day 10, which decreased significantly by day 30. Neural fiber sprouting as evaluated by neurofilament staining was observed as early as day 10. In addition, GFAP immunostained astrocytes were found only at the boundary of the lesion site, and no astrocyte accumulation was observed in the implantation area at any time point. These findings indicate the feasibility of fabricating 3D spiral constructs using electrospun collagen fibers and demonstrated the potential of these scaffolds for SCI repair.

Fabrication and characterization of tunable polysaccharide hydrogel blends for neural repair

Hydrogels are an important class of biomaterials that have the potential to be used as three-dimensional tissue engineering scaffolds for regenerative medicine. This is especially true in the central nervous system, where neurons do not have the ability to regenerate due to the prohibitory local environment following injury. Hydrogels can fill an injury site, replacing the growth-prohibiting environment with a more growth-permissive one. In this study, dextran and chitosan were incorporated into a methylcellulose and agarose hydrogel blend. This created several thermally sensitive polysaccharide hydrogel blends that had tunable mechanical and surface charge properties. Cortical neurons were cultured on the hydrogels to determine the blend that had the greatest neuron compatibility. Our results show that softer, more positively charged polysaccharide hydrogel blends allow for greater neuron attachment and neurite extension, showing their promise as CNS regeneration scaffolds.

Fibrin degradation enhances vascular smooth muscle cell proliferation and matrix deposition in fibrin-based tissue constructs fabricated in vitro

Completely biological tissue replacements can be fabricated by entrapping cells in a molded fibrin gel. Over time, the fibrin is degraded and replaced with cell-produced extracellular matrix. However, the relationship between fibrin degradation and matrix deposition has not been elucidated. We developed techniques to quantify fibrin degradation products (FDP) and examine plasmin activity in the conditioned medium from fibrin-based constructs. Fibrin-based tissue constructs fabricated with vascular smooth muscle cells (vSMC) were cultured for 5 weeks in the presence of varied concentrations of the fibrinolysis inhibitor -aminocaproic acid and cellularity, and deposited collagen and elastin were measured weekly. These data revealed that increasing concentrations of -aminocaproic acid led to delayed and diminished FDP production, lower vSMC proliferation, and decreased collagen and elastin deposition. FDP were shown to have a direct biological effect on vSMC cultures and vSMC within the fibrin-based constructs. Supplementing construct cultures with 250 or 500μg/mL FDP led to 30% higher collagen deposition than the untreated controls. FDP concentrations as high as 250μg/mL were estimated to exist within the constructs, indicating that FDP generation during remodeling of the fibrin-based constructs exerted direct biological activity. These results help explain many of the positive outcomes reported with fibrin-based tissue constructs in the literature, as well as demonstrate the importance of regulating plasmin activity during their fabrication.

Peptide amphiphiles and porous biodegradable scaffolds for tissue regeneration in the brain and spinal cord

Many promising strategies have been developed for controlling the release of drugs from scaffolds, yet there are still challenges that need to be addressed in order for these scaffolds to serve as successful treatments. The RADA4 self-assembling peptide spontaneously forms nanofibers, creating a scaffold-like tissue-bridging structure that provides a three-dimensional environment for the migration of living cells. We have found that RADA4: (1) facilitates the regeneration of axons in the brain of young and adult hamsters, leading to functional return of behavior and (2) demonstrates robust migration of host cells and growth of blood vessels and axons, leading to the repair of injured spinal cords in rats.

Clinical and experimental advances in regeneration of spinal cord injury

Spinal cord injury (SCI) is one of the major disabilities dealt with in clinical rehabilitation settings and is multifactorial in that the patients suffer from motor and sensory impairments as well as many other complications throughout their lifetimes. Many clinical trials have been documented during the last two decades to restore damaged spinal cords. However, only a few pharmacological therapies used in clinical settings which still have only limited effects on the regeneration, recovery speed, or retraining of the spinal cord. In this paper, we will introduce recent clinical trials, which performed pharmacological treatments and cell transplantations for patients with SCI, and evaluate recent in vivo studies for the regeneration of injured spinal cord, including stem-cell transplantation, application of neurotrophic factors and suppressor of inhibiting factors, development of biomaterial scaffolds and delivery systems, rehabilitation, and the combinations of these therapies to evaluate what can be appropriately applied in the future to the patients with SCI.

Tissue-engineered fibrin scaffolds containing neural progenitors enhance functional recovery in a subacute model of SCI

Two recurring problems with stem/neural progenitor cell (NPC) transplantation therapies for spinal cord injury (SCI) are poor cell survival and uncontrolled cell differentiation. The current study evaluated the viability and differentiation of embryonic stem cell-derived neural progenitor cells (ESNPCs) transplanted within fibrin scaffolds containing growth factors (GFs) and a heparin-binding delivery system (HBDS) to enhance cell survival and direct differentiation into neurons. Mouse ESNPCs were generated from mouse embryonic stem cells (ESCs) using a 4-/4+ retinoic acid (RA) induction protocol that resulted in a population of cells that was 70% nestin positive NPCs. The ESNPCs were transplanted directly into a rat subacute dorsal hemisection lesion SCI model. ESNPCs were either encapsulated in a fibrin scaffold; encapsulated in fibrin containing the HBDS, neurotrophin-3 (NT-3) and platelet derived growth factor (PDGF-AA); or encapsulated in fibrin scaffolds with NT-3 and PDGF-AA without the HBDS. We report that the combination of GFs and fibrin scaffold (without HBDS) enhanced the total number of ESNPCs present in the treated spinal cords and increased the number of ESNPC-derived NeuN positive neurons 8 weeks after transplantation. All experimental groups treated with ESNPCs exhibited an increase in behavioral function 4 weeks after transplantation. In a subset of animals, the ESNPCs over-proliferated as evidenced by SSEA-1 positive/Ki67 positive ESCs found at 4 and 8 weeks. These results demonstrate the potential of tissue-engineered fibrin scaffolds to enhance the survival of NPCs and highlight the need to purify cell populations used in therapies for SCI.

Biomaterial design strategies for the treatment of spinal cord injuries

The highly debilitating nature of spinal cord injuries has provided much inspiration for the design of novel biomaterials that can stimulate cellular regeneration and functional recovery. Many experts agree that the greatest hope for treatment of spinal cord injuries will involve a combinatorial approach that integrates biomaterial scaffolds, cell transplantation, and molecule delivery. This manuscript presents a comprehensive review of biomaterial-scaffold design strategies currently being applied to the development of nerve guidance channels and hydrogels that more effectively stimulate spinal cord tissue regeneration. To enhance the regenerative capacity of these two scaffold types, researchers are focusing on optimizing the mechanical properties, cell-adhesivity, biodegradability, electrical activity, and topography of synthetic and natural materials, and are developing mechanisms to use these scaffolds to deliver cells and biomolecules. Developing scaffolds that address several of these key design parameters will lead to more successful therapies for the regeneration of spinal cord tissue.

The effect of the dosage of NT-3/chitosan carriers on the proliferation and differentiation of neural stem cells

This study aimed to determine the optimal dosage range of NT-3 in the soluble form or loaded with chitosan carriers by using NT-3/chitosan carriers to support the survival and proliferation of neural stem cells (NSCs) and induce them to differentiate into desired phenotypes. NSCs were co-cultured with chitosan carriers loaded with different doses of NT-3. As the control, NSCs were cultured in the defined medium, into which were added different doses of NT-3 in the soluble form every day. The ELISA kit was used to study the NT-3 releasing kinetics, which showed that, in the initial co-culture stage from 1 h to 14 weeks, the chitosan carriers loaded with different doses of NT-3 released NT-3 stably and constantly. The 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay was conducted to measure the cell viability, and the immunocytochemical methods were adopted to quantitatively analyze the phenotypes differentiating from the NSCs. Compared to the 100 ng NT-3 daily addition group (1400 ng over 14 days), the 25 ng, 50 ng and 200 ng NT-3 daily addition group showed dramatically shorter processes length and much lower differentiation percentage from NSCs into neurons. By contrast, the NT-3 (25 ng)-chitosan carriers group had not only higher cell viability, but also similar processes length and differentiation percentage from NSCs into neurons to the 100 ng NT-3 daily addition group. The method developed in this study significantly reduced the NT-3 amount required to support the survival, proliferation and differentiation of NSCs in vitro. Meanwhile, the chitosan carriers used here provided an ideal 3-dimensional scaffold for the adhesion, migration, proliferation and differentiation of NSC and the differentiated cells. Therefore, this method may open a new field for the large-scaled culture and amplification of NSCs in vitro to replace the lost neural cells, meanwhile lower the consumption of neurotrophic factors in the cell transplantation therapy of brain and spinal injury.

Controlled release of neurotrophin-3 from fibrin-based tissue engineering scaffolds enhances neural fiber sprouting following subacute spinal cord injury

This study investigated whether delayed treatment of spinal cord injury with controlled release of neurotrophin-3 (NT-3) from fibrin scaffolds can stimulate enhanced neural fiber sprouting. Long Evans rats received a T9 dorsal hemisection spinal cord injury. Two weeks later, the injury site was re-exposed, and either a fibrin scaffold alone, a fibrin scaffold containing a heparin-based delivery system with different concentrations of NT-3 (500 and 1,000 ng/mL), or a fibrin scaffold containing 1,000 ng/mL of NT-3 (no delivery system) was implanted into the injury site. The injured spinal cords were evaluated for morphological differences using markers for neurons, astrocytes, and chondroitin sulfate proteoglycans 2 weeks after treatment. The addition of 500 ng/mL of NT-3 with the delivery system resulted in an increase in neural fiber density compared to fibrin alone. These results demonstrate that the controlled release of NT-3 from fibrin scaffolds can enhance neural fiber sprouting even when treatment is delayed 2 weeks following injury.

Nucleic acid therapeutic carriers with on-demand triggered release

Biohybrid platforms such as synthetic polymer networks engineered from artificial and natural materials hold immense potential as drug and gene delivery vehicles. Here, we report the synthesis and characterization of novel polymer networks that release oligonucleotide sequences via enzymatic and physical triggers. Chemical monomers and acrylated oligonucleotides were copolymerized into networks, and phosphoimaging revealed that 70% of the oligonucleotides were incorporated into the networks. We observed that the immobilized oligonucleotides were readily cleaved when the networks were incubated with the type II restriction enzyme BamHI. The diffusion of the cleaved fragments through the macromolecular chains resulted in relatively constant release profiles very close to zero-order. To our knowledge, this is the first study which harnesses the sequence-specificity of restriction endonucleases as triggering agents for the cleavage and release of oligonucleotide sequences from a synthetic polymer network. The polymer networks exhibited an oligonucleotide diffusion coefficient of 5.6 x 10(-8) cm(2)/s and a diffusional exponent of 0.92. Sigmoidal temperature responsive characteristics of the networks matched the theoretical melting temperature of the oligonucleotides and indicated a cooperative melting transition of the oligonucleotides. The networks were also triggered to release a RNA-cleaving deoxyribozyme, which degraded a HIV-1 mRNA transcript in vitro. To tailor release profiles of the oligonucleotides, we controlled the structure of the macromolecular architecture of the networks by varying their cross-linking content. When incubated with DNase I, networks of cross-linking content 0.15%, 0.22%, and 0.45% exhibited oligonucleotide diffusion coefficients of 1.67 x 10(-8), 7.65 x 10(-9), and 2.7 x 10(-9) cm(2)/s, and diffusional exponents of 0.55, 0.8, and 0.8, respectively. The modular nature of our platform promises to open new avenues for the creation and optimization of a rich toolbox of novel drug and gene delivery platforms. We anticipate further inquiry into nucleic acid based programmable on-demand switches and modulatory devices of exquisite sensitivity and control.