Channel density and porosity of degradable bridging scaffolds on axon growth after spinal injury

Development of BDNF/NGF/IKVAV Peptide Modified and Gold Nanoparticle Conductive PCL/PLGA Nerve Guidance Conduit for Regeneration of the Rat Spinal Cord Injury

Spinal cord injuries are very common worldwide, leading to permanent nerve function loss with devastating effects in the affected patients. The challenges and inadequate results in the current clinical treatments are leading scientists to innovative neural regenerative research. Advances in nanoscience and neural tissue engineering have opened new avenues for spinal cord injury (SCI) treatment. In order for designed nerve guidance conduit (NGC) to be functionally useful, it must have ideal scaffold properties and topographic features that promote the linear orientation of damaged axons. In this study, it is aimed to develop channeled polycaprolactone (PCL)/Poly-D,L-lactic-co-glycolic acid (PLGA) hybrid film scaffolds, modify their surfaces by IKVAV pentapeptide/gold nanoparticles (AuNPs) or polypyrrole (PPy) and investigate the behavior of motor neurons on the designed scaffold surfaces in vitro under static/bioreactor conditions. Their potential to promote neural regeneration after implantation into the rat SCI by shaping the film scaffolds modified with neural factors into a tubular form is also examined. It is shown that channeled groups decorated with AuNPs highly promote neurite orientation under bioreactor conditions and also the developed optimal NGC (PCL/PLGA G1-IKVAV/BDNF/NGF-AuNP50) highly regenerates SCI. The results indicate that the designed scaffold can be an ideal candidate for spinal cord regeneration.

Multichannel bridges and NSC synergize to enhance axon regeneration, myelination, synaptic reconnection, and recovery after SCI

Regeneration in the injured spinal cord is limited by physical and chemical barriers. Acute implantation of a multichannel poly(lactide-co-glycolide) (PLG) bridge mechanically stabilizes the injury, modulates inflammation, and provides a permissive environment for rapid cellularization and robust axonal regrowth through this otherwise inhibitory milieu. However, without additional intervention, regenerated axons remain largely unmyelinated (<10%), limiting functional repair. While transplanted human neural stem cells (hNSC) myelinate axons after spinal cord injury (SCI), hNSC fate is highly influenced by the SCI inflammatory microenvironment, also limiting functional repair. Accordingly, we investigated the combination of PLG scaffold bridges with hNSC to improve histological and functional outcome after SCI. In vitro, hNSC culture on a PLG scaffold increased oligodendroglial lineage selection after inflammatory challenge. In vivo, acute PLG bridge implantation followed by chronic hNSC transplantation demonstrated a robust capacity of donor human cells to migrate into PLG bridge channels along regenerating axons and integrate into the host spinal cord as myelinating oligodendrocytes and synaptically integrated neurons. Axons that regenerated through the PLG bridge formed synaptic circuits that connected the ipsilateral forelimb muscle to contralateral motor cortex. hNSC transplantation significantly enhanced the total number of regenerating and myelinated axons identified within the PLG bridge. Finally, the combination of acute bridge implantation and hNSC transplantation exhibited robust improvement in locomotor recovery. These data identify a successful strategy to enhance neurorepair through a temporally layered approach using acute bridge implantation and chronic cell transplantation to spare tissue, promote regeneration, and maximize the function of new axonal connections.

Building-Block Size Mediates Microporous Annealed Particle Hydrogel Tube Microenvironment Following Spinal Cord Injury

Spinal cord injury (SCI) is a life-altering event, which often results in loss of sensory and motor function below the level of trauma. Biomaterial therapies have been widely investigated in SCI to promote directional regeneration but are often limited by their pre-constructed size and shape. Herein, the design parameters of microporous annealed particles (MAPs) are investigated with tubular geometries that conform to the injury and direct axons across the defect to support functional recovery. MAP tubes prepared from 20-, 40-, and 60-micron polyethylene glycol (PEG) beads are generated and implanted in a T9-10 murine hemisection model of SCI. Tubes attenuate glial and fibrotic scarring, increase innate immune cell density, and reduce inflammatory phenotypes in a bead size-dependent manner. Tubes composed of 60-micron beads increase the cell density of the chronic macrophage response, while neutrophil infiltration and phenotypes do not deviate from those seen in controls. At 8 weeks postinjury, implantation of tubes composed of 60-micron beads results in enhanced locomotor function, robust axonal ingrowth, and remyelination through both lumens and the inter-tube space. Collectively, these studies demonstrate the importance of bead size in MAP construction and highlight PEG tubes as a biomaterial therapy to promote regeneration and functional recovery in SCI.

Biomimetic Strategies for Peripheral Nerve Injury Repair: An Exploration of Microarchitecture and Cellularization

Injuries to the nervous system present formidable challenges to scientists, clinicians, and patients. While regeneration within the central nervous system is minimal, peripheral nerves can regenerate, albeit with limitations. The regenerative mechanisms of the peripheral nervous system thus provide fertile ground for clinical and scientific advancement, and opportunities to learn fundamental lessons regarding nerve behavior in the context of regeneration, particularly the relationship of axons to their support cells and the extracellular matrix environment. However, few current interventions adequately address peripheral nerve injuries. This article aims to elucidate areas in which progress might be made toward developing better interventions, particularly using synthetic nerve grafts. The article first provides a thorough review of peripheral nerve anatomy, physiology, and the regenerative mechanisms that occur in response to injury. This is followed by a discussion of currently available interventions for peripheral nerve injuries. Promising biomaterial fabrication techniques which aim to recapitulate nerve architecture, along with approaches to enhancing these biomaterial scaffolds with growth factors and cellular components, are then described. The final section elucidates specific considerations when developing nerve grafts, including utilizing induced pluripotent stem cells, Schwann cells, nerve growth factors, and multilayered structures that mimic the architectures of the natural nerve.

Biomimetic engineered approaches for neural tissue engineering: Spinal cord injury

The healing process for spinal cord injuries is complex and presents many challenges. Current advances in nerve regeneration are based on promising tissue engineering techniques, However, the chances of success depend on better mimicking the extracellular matrix (ECM) of neural tissue and better supporting neurons in a three-dimensional environment. The ECM provides excellent biological conditions, including desirable morphological features, electrical conductivity, and chemical compositions for neuron attachment, proliferation and function. This review outlines the rationale for developing a construct for neuron regrowth in spinal cord injury using appropriate biomaterials and scaffolding techniques.

Anisotropic 3D scaffolds for spinal cord guided repair: Current concepts

A spinal cord injury (SCI) can be caused by unforeseen events such as a fall, a vehicle accident, a gunshot, or a malignant illness, which has a significant impact on the quality of life of the patient. Due to the limited regenerative potential of the central nervous system (CNS), SCI is one of the most daunting medical challenges of modern medicine. Great advances have been made in tissue engineering and regenerative medicine, which include the transition from two-dimensional (2D) to three-dimensional (3D) biomaterials. Combinatory treatments that use 3D scaffolds may significantly enhance the repair and regeneration of functional neural tissue. In an effort to mimic the chemical and physical properties of neural tissue, scientists are researching the development of the ideal scaffold made of synthetic and/or natural polymers. Moreover, in order to restore the architecture and function of neural networks, 3D scaffolds with anisotropic properties that replicate the native longitudinal orientation of spinal cord nerve fibres are being designed. In an effort to determine if scaffold anisotropy is a crucial property for neural tissue regeneration, this review focuses on the most current technological developments relevant to anisotropic scaffolds for SCI. Special consideration is given to the architectural characteristics of scaffolds containing axially oriented fibres, channels, and pores. By analysing neural cell behaviour in vitro and tissue integration and functional recovery in animal models of SCI, the therapeutic efficacy is evaluated for its successes and limitations.

Review on Bioinspired Design of ECM-Mimicking Scaffolds by Computer-Aided Assembly of Cell-Free and Cell Laden Micro-Modules

Tissue engineering needs bioactive drug delivery scaffolds capable of guiding cell biosynthesis and tissue morphogenesis in three dimensions. Several strategies have been developed to design and fabricate ECM-mimicking scaffolds suitable for directing in vitro cell/scaffold interaction, and controlling tissue morphogenesis in vivo. Among these strategies, emerging computer aided design and manufacturing processes, such as modular tissue unit patterning, promise to provide unprecedented control over the generation of biologically and biomechanically competent tissue analogues. This review discusses recent studies and highlights the role of scaffold microstructural properties and their drug release capability in cell fate control and tissue morphogenesis. Furthermore, the work highlights recent advances in the bottom-up fabrication of porous scaffolds and hybrid constructs through the computer-aided assembly of cell-free and/or cell-laden micro-modules. The advantages, current limitations, and future challenges of these strategies are described and discussed.

Current Concepts of Biomaterial Scaffolds and Regenerative Therapy for Spinal Cord Injury

Spinal cord injury (SCI) is a catastrophic condition associated with significant neurological deficit and social and financial burdens. It is currently being managed symptomatically, with no real therapeutic strategies available. In recent years, a number of innovative regenerative strategies have emerged and have been continuously investigated in preclinical research and clinical trials. In the near future, several more are expected to come down the translational pipeline. Among ongoing and completed trials are those reporting the use of biomaterial scaffolds. The advancements in biomaterial technology, combined with stem cell therapy or other regenerative therapy, can now accelerate the progress of promising novel therapeutic strategies from bench to bedside. Various types of approaches to regeneration therapy for SCI have been combined with the use of supportive biomaterial scaffolds as a drug and cell delivery system to facilitate favorable cell-material interactions and the supportive effect of neuroprotection. In this review, we summarize some of the most recent insights of preclinical and clinical studies using biomaterial scaffolds in regenerative therapy for SCI and summarized the biomaterial strategies for treatment with simplified results data. One hundred and sixty-eight articles were selected in the present review, in which we focused on biomaterial scaffolds. We conducted our search of articles using PubMed and Medline, a medical database. We used a combination of "Spinal cord injury" and ["Biomaterial", or "Scaffold"] as search terms and searched articles published up until 30 April 2022. Successful future therapies will require these biomaterial scaffolds and other synergistic approaches to address the persistent barriers to regeneration, including glial scarring, the loss of a structural framework, and biocompatibility. This database could serve as a benchmark to progress in future clinical trials for SCI using biomaterial scaffolds.

Oligo (Poly (Ethylene Glycol) Fumarate)-Based Multicomponent Cryogels for Neural Tissue Replacement

Synthetic hydrogels provide a promising platform to produce neural tissue analogs with improved control over structural, physical, and chemical properties. In this study, oligo (poly (ethylene glycol) fumarate) (OPF)-based macroporous cryogels were developed as a potential next-generation alternative to a non-porous OPF hydrogel previously proposed as an advanced biodegradable scaffold for spinal cord repair. A series of OPF cryogel conduits in combination with PEG diacrylate and 2-(methacryloyloxy) ethyl-trimethylammonium chloride (MAETAC) cationic monomers were synthesized and characterized. The contribution of each component to viscoelastic and hydration behaviors and porous structure was identified, and concentration relationships for these properties were revealed. The rheological properties of the materials corresponded to those of neural tissues and scaffolds, according to the reviewed data. A comparative assessment of adhesion, migration, and proliferation of neuronal cells in multicomponent cryogels was carried out to optimize cell-supporting characteristics. The results show that OPF-based cryogels can be used as a tunable synthetic scaffold for neural tissue repair with advantages over their hydrogel counterparts.

Anesthesia and analgesia for common research models of adult mice

Anesthesia and analgesia are major components of many interventional studies on laboratory animals. However, various studies have shown improper reporting or use of anesthetics/analgesics in research proposals and published articles. In many cases, it seems "anesthesia" and "analgesia" are used interchangeably, while they are referring to two different concepts. Not only this is an unethical practice, but also it may be one of the reasons for the proven suboptimal quality of many animal researches. This is a widespread problem among investigations on various species of animals. However, it could be imagined that it may be more prevalent for the most common species of laboratory animals, such as the laboratory mice. In this review, proper anesthetic/analgesic methods for routine procedures on laboratory mice are discussed. We considered the available literature and critically reviewed their anesthetic/analgesic methods. Detailed dosing and pharmacological information for the relevant drugs are provided and some of the drugs' side effects are discussed. This paper provides the necessary data for an informed choice of anesthetic/analgesic methods in some routine procedures on laboratory mice.

Silk-Elastin-like Polymers for Acute Intraparenchymal Treatment of the Traumatically Injured Spinal Cord: A First Systematic Experimental Approach

Despite the promising potential of hydrogel-based therapeutic approaches for spinal cord injury (SCI), the need for new biomaterials to design effective strategies for SCI treatment and the outstanding properties of silk-elastin-like polymers (SELP), the potential use of SELPs in SCI is currently unknown. In this context, we assessed the effects elicited by the in vivo acute intraparenchymal injection of an SELP named (EIS)₂-RGD6 in a clinically relevant model of SCI. After optimization of the injection system, the distribution, structure, biodegradability, and cell infiltration capacity of (EIS)₂-RGD6 were assessed. Finally, the effects exerted by the (EIS)₂-RGD6 injection-in terms of motor function, myelin preservation, astroglial and microglia/macrophage reactivity, and fibrosis-were evaluated. We found that (EIS)₂-RGD6 can be acutely injected in the lesioned spinal cord without inducing further damage, showing a widespread distribution covering all lesioned areas with a single injection and facilitating the formation of a slow-degrading porous scaffold at the lesion site that allows for the infiltration and/or proliferation of endogenous cells with no signs of collapse and without inducing further microglial and astroglial reactivity, as well as even reducing SCI-associated fibrosis. Altogether, these observations suggest that (EIS)₂-RGD6-and, by extension, SELPs-could be promising polymers for the design of therapeutic strategies for SCI treatment.

Stem Cell Strategies in Promoting Neuronal Regeneration after Spinal Cord Injury: A Systematic Review

Spinal cord injury (SCI) is a devastating condition with a significant medical and socioeconomic impact. To date, no effective treatment is available that can enable neuronal regeneration and recovery of function at the damaged level. This is thought to be due to scar formation, axonal degeneration and a strong inflammatory response inducing a loss of neurons followed by a cascade of events that leads to further spinal cord damage. Many experimental studies demonstrate the therapeutic effect of stem cells in SCI due to their ability to differentiate into neuronal cells and release neurotrophic factors. Therefore, it appears to be a valid strategy to use in the field of regenerative medicine. This review aims to provide an up-to-date summary of the current research status, challenges, and future directions for stem cell therapy in SCI models, providing an overview of this constantly evolving and promising field.

Biomaterial-targeted precision nanoparticle delivery to the injured spinal cord

Drug delivery requires precision in timing, location, and dosage to achieve therapeutic benefits. Challenges in addressing all three of these critical criteria result in poor temporal dexterity, widespread accumulation and off-target effects, and high doses with the potential for toxicity. To address these challenges, we have developed the BiomatErial Accumulating Carriers for On-demand Nanotherapy (BEACON) platform that utilizes an implantable biomaterial to serve as a target for systemically delivered nanoparticles (NPs). With the BEACON system, administered NPs are conjugated with a ligand that has high affinity for a receptor in the implanted biomaterial. To test BEACON, an in vivo spinal cord injury (SCI) model was used as it provides an injury model where the three identified criteria can be tested as it is a dynamic and complicated injury model with no currently approved therapies. Through our work, we have demonstrated temporal dexterity in NP administration by injecting 6 days post-SCI, decreased off-target accumulation with a significant drop in liver accumulation, and retention of our NPs in the target biomaterial. The BEACON system can be applied broadly, beyond the nervous system, to improve systemically delivered NP accumulation at an implanted biomaterial target. STATEMENT OF SIGNIFICANCE: Targeted drug delivery approaches have the potential to improve therapeutic regimens for patients on a case-by-case basis. Improved localization of a therapeutic to site of interest can result in increased efficacy and limit the need for repeat dosing. Unfortunately, targeted strategies can fall short when receptors on cells or tissues are too widespread or change over the course of disease or injury progression. The BEACON system developed herein eliminates the need to target a cell or tissue receptor by targeting an implantable biomaterial with location-controllable accumulation and sustained presentation over time. The targeting paradigm presented by BEACON is widely applicable throughout tissue engineering and regenerative medicine without the need to retool for each new application.

Injectable decellularized nucleus pulposus tissue exhibits neuroinhibitory properties

Background

Chronic low back pain (LBP) is a leading cause of disability, but treatments for LBP are limited. Degeneration of the intervertebral disc due to loss of neuroinhibitory sulfated glycosaminoglycans (sGAGs) allows nerves from dorsal root ganglia to grow into the core of the disc. Treatment with a decellularized tissue hydrogel that contains sGAGs may inhibit nerve growth and prevent disc-associated LBP.

Methods

A protocol to decellularize porcine nucleus pulposus (NP) was adapted from previous methods. DNA, sGAG, α-gal antigen, and collagen content were analyzed before and after decellularization. The decellularized tissue was then enzymatically modified to be injectable and form a gel at 37°C. Following this, the mechanical properties, microstructure, cytotoxicity, and neuroinhibitory properties were analyzed.

Results

The decellularization process removed 99% of DNA and maintained 74% of sGAGs and 154% of collagen compared to the controls NPs. Rheology demonstrated that regelled NP exhibited properties similar to but slightly lower than collagen-matched controls. Culture of NP cells in the regelled NP demonstrated an increase in metabolic activity and DNA content over 7 days. The collagen content of the regelled NP stayed relatively constant over 7 days. Analysis of the neuroinhibitory properties demonstrated regelled NP significantly inhibited neuronal growth compared to collagen controls.

Conclusions

The decellularization process developed here for porcine NP tissue was able to remove the antigenic material while maintaining the sGAG and collagen. This decellularized tissue was then able to be modified into a thermally forming gel that maintained the viability of cells and demonstrated robust neuroinhibitory properties in vitro. This biomaterial holds promise as an NP supplement to prevent nerve growth into the native disc and NP in vivo.

Biomaterial and Therapeutic Approaches for the Manipulation of Macrophage Phenotype in Peripheral and Central Nerve Repair

Injury to the peripheral or central nervous systems often results in extensive loss of motor and sensory function that can greatly diminish quality of life. In both cases, macrophage infiltration into the injury site plays an integral role in the host tissue inflammatory response. In particular, the temporally related transition of macrophage phenotype between the M1/M2 inflammatory/repair states is critical for successful tissue repair. In recent years, biomaterial implants have emerged as a novel approach to bridge lesion sites and provide a growth-inductive environment for regenerating axons. This has more recently seen these two areas of research increasingly intersecting in the creation of 'immune-modulatory' biomaterials. These synthetic or naturally derived materials are fabricated to drive macrophages towards a pro-repair phenotype. This review considers the macrophage-mediated inflammatory events that occur following nervous tissue injury and outlines the latest developments in biomaterial-based strategies to influence macrophage phenotype and enhance repair.

Highly Permeable DNA Supramolecular Hydrogel Promotes Neurogenesis and Functional Recovery after Completely Transected Spinal Cord Injury

Regeneration after severe spinal cord injury cannot occur naturally in mammals. Transplanting stem cells to the injury site is a highly promising method, but it faces many challenges because it relies heavily on the microenvironment provided by both the lesion site and delivery material. Although mechanical properties, biocompatibility, and biodegradability of delivery materials have been extensively explored, their permeability has rarely been recognized. Here, a DNA hydrogel is designed with extremely high permeability to repair a 2 mm spinal cord gap in Sprague-Dawley rats. The rats recover basic hindlimb function with detectable motor-evoked potentials, and a renascent neural network is formed via the proliferation and differentiation of both implanted and endogenous stem cells. The signal at the lesion area is conveyed by, on average, 15 newly formed synapses. This hydrogel system offers great potential in clinical trials. Further, it should be easily adaptable to other tissue regeneration applications.

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.

Lentiviral Interleukin-10 Gene Therapy Preserves Fine Motor Circuitry and Function After a Cervical Spinal Cord Injury in Male and Female Mice

In mammals, spinal cord injuries often result in muscle paralysis through the apoptosis of lower motor neurons and denervation of neuromuscular junctions. Previous research shows that the inflammatory response to a spinal cord injury can cause additional tissue damage after the initial trauma. To modulate this inflammatory response, we delivered lentiviral anti-inflammatory interleukin-10, via loading onto an implantable biomaterial scaffold, into a left-sided hemisection at the C5 vertebra in mice. We hypothesized that improved behavioral outcomes associated with anti-inflammatory treatment are due to the sparing of fine motor circuit components. We examined behavioral recovery using a ladder beam, tissue sparing using histology, and electromyogram recordings using intraspinal optogenetic stimulation at 2 weeks post-injury. Ladder beam analysis shows interleukin-10 treatment results in significant improvement of behavioral recovery at 2 and 12 weeks post-injury when compared to mice treated with a control virus. Histology shows interleukin-10 results in greater numbers of lower motor neurons, axons, and muscle innervation at 2 weeks post-injury. Furthermore, electromyogram recordings suggest that interleukin-10-treated animals have signal-to-noise ratios and peak-to-peak amplitudes more similar to that of uninjured controls than to that of control injured animals at 2 weeks post-injury. These data show that gene therapy using anti-inflammatory interleukin-10 can significantly reduce tissue damage and subsequent motor deficits after a spinal cord injury. Together, these results suggest that early modulation of the injury response can preserve muscle function with long-lasting benefits.

Endocrine Therapy for the Functional Recovery of Spinal Cord Injury

Spinal cord injury (SCI) is a major cause of physical disability and leads to patient dissatisfaction with their quality of life. Patients with SCI usually exhibit severe clinical symptoms, including sensory and motor dysfunction below the injured levels, paraplegia, quadriplegia and urinary retention, which can exacerbate the substantial medical and social burdens. The major pathological change observed in SCI is inflammatory reaction, which induces demyelination, axonal degeneration, and the apoptosis and necrosis of neurons. Traditional medical treatments are mainly focused on the recovery of motor function and prevention of complications. To date, numerous studies have been conducted to explore the cellular and molecular mechanism of SCI and have proposed lots of effective treatments, but the clinical applications are still limited due to the complex pathogenesis and poor prognosis after SCI. Endocrine hormones are kinds of molecules that are synthesized by specialized endocrine organs and can participate in the regulation of multiple physiological activities, and their protective effects on several disorders have been widely discussed. In addition, many studies have identified that endocrine hormones can promote nerve regeneration and functional recovery in individuals with central nervous system diseases. Therefore, studies investigating the clinical applications of endocrine hormones as treatments for SCI are necessary. In this review, we described the neuroprotective roles of several endocrine hormones in SCI; endocrine hormone administration reduces cell death and promotes functional repair after SCI. We also proposed novel therapies for SCI.

Multichannel Bioactive Silk Nanofiber Conduits Direct and Enhance Axonal Regeneration after Spinal Cord Injury

After a spinal cord injury, axonal regeneration over long distances is challenging due to the lack of physical guidance cues and bioactive signals. In this study, a multichannel bioactive silk fibroin nanofiber conduit was fabricated to improve spinal cord injury repair by enhancing axonal regeneration. The conduit was composed of longitudinally oriented silk fibroin nanofibers and then functionalized with laminin. In vitro, the bioactive conduits could promote neuron-like development and directional neurite extension of PC12 cells by providing a bioactive stimulus and physical guidance. In a spinal cord injury model in Sprague-Dawley rats, the biofunctionalized conduits displayed superior integration with the host tissue due to enhanced cell infiltration and tissue ingrowth. The glial scar was significantly reduced, allowing axonal ingrowth along with the channel direction. Compared to a single-channel conduit, the multichannel conduit improved spinal cord regeneration by boosting tissue ingrowth and axonal regeneration, indicating that the conduit architectures play critical roles in spinal cord regeneration. These silk fibroin conduits, along with the multichannel architecture, nanoscale cues, and the ability to bind bioactive compounds, represent promising candidates for spinal cord regeneration.

Biofunctional scaffolds with high packing density of aligned electrospun fibers support neural regeneration

Neurons of the central nervous system do not regenerate spontaneously after injury. As such, biofunctional tissue scaffolds have been explored to provide a growth-promoting environment to enhance neural regeneration. In this regard, aligned electrospun fibers have proven invaluable for regeneration by offering guidance for axons to cross the injury site. However, a high fiber density could potentially limit axonal ingrowth into the scaffold. Here, we explore which fiber density provides the optimal environment for neurons to regenerate. By changing fiber electrospinning time, we generated scaffolds with different fiber densities and implanted these in a rat model of spinal cord injury (SCI). We found that neurons were able to grow efficiently into scaffolds with high fiber density, even if the gaps between fiber bundles were very small (<1 μm). Scaffolds with high fiber density showed good host-implant integration. Cell infiltration was not affected by fiber density. Efficient blood vessel ingrowth likely requires larger gaps between fibers or faster degrading fibers. We conclude that scaffolds with high fiber densities, and thus a large number of small gaps in between fiber bundles, provide the preferred environment for nerve regeneration after SCI.

Rapid prototyping fabrication of soft and oriented polyester scaffolds for axonal guidance

Biodegradable polyesters have been extensively used for preparation of nerve guidance scaffolds, due to their high biocompatibility and defined degradation periods. However, conventional methods for fabrication of porous polyester scaffolds provide limited control over shape and micro-architecture. Here, a fabrication procedure based on 3D printing was developed to generate highly ordered and anatomically personalized, polyester scaffolds for soft tissue regeneration. Scaffolds composed of Poly-lactic-glycolic acid (PLGA) and poly-L-lactic acid (PLLA) were specifically customized for nerve injuries. This was obtained by using an oriented multi-layer printing pattern which established a linear structure in the fabricated scaffolds to match the aligned topography of nerve tissues. The oriented scaffold was shown to guide regenerating axons to linear conformations and support growth of induced pluripotent stem cell-derived neurons in vitro and in vivo in a model of spinal cord injury. The described scaffolds may advance the field of nerve regeneration. Furthermore, modifications could be integrated to generate soft implants for various types of tissues.

Directional axonal regrowth induced by an aligned fibrin nanofiber hydrogel contributes to improved motor function recovery in canine L2 spinal cord injury

Spinal cord injuries (SCI) normally disrupt the long axonal tracts of the spinal cord and cause permanent neurological deficits, for which there is currently a lack of effective therapeutic methods. Biomaterial-based regenerative medicine is a pivotal strategy to induce axonal regeneration through delivery of biophysical and/or biochemical regulatory cues by biomaterials. We previously fabricated a hierarchically aligned fibrin hydrogel (AFG) that could promote neurogenic differentiation of stem cells in vitro and has been successfully applied for peripheral nerve and spinal cord regeneration in rats. In this study, AFG was used to repair a canine lumbar segment 2 hemisection spinal cord injury, and the consistency of histological, imageological and behavioral results was compared. AFG was used to construct an aligned fiber bridge that supported cell adhesion in vitro and rapidly facilitated tissue invasion along the long axis of fibers in vivo, Moreover, in vivo results demonstrated regrowth of axons in an oriented pattern connecting the rostral and caudal stumps. Consistent results were confirmed by diffusion tensor imaging, which allowed successful tracing of reconnected nerve fibers across the defect. As a result, directional axonal regrowth contributed to significantly improved recovery of motor functional behavior of SCI canines with AFG implantation. Our results suggest that AFG has great promise for rapidly directing axonal regrowth for nerve regeneration.

Polycistronic Delivery of IL-10 and NT-3 Promotes Oligodendrocyte Myelination and Functional Recovery in a Mouse Spinal Cord Injury Model

One million estimated cases of spinal cord injury (SCI) have been reported in the United States and repairing an injury has constituted a difficult clinical challenge. The complex, dynamic, inhibitory microenvironment postinjury, which is characterized by proinflammatory signaling from invading leukocytes and lack of sufficient factors that promote axonal survival and elongation, limits regeneration. Herein, we investigated the delivery of polycistronic vectors, which have the potential to coexpress factors that target distinct barriers to regeneration, from a multiple channel poly(lactide-co-glycolide) (PLG) bridge to enhance spinal cord regeneration. In this study, we investigated polycistronic delivery of IL-10 that targets proinflammatory signaling, and NT-3 that targets axonal survival and elongation. A significant increase was observed in the density of regenerative macrophages for IL-10+NT-3 condition relative to conditions without IL-10. Furthermore, combined delivery of IL-10+NT-3 produced a significant increase of axonal density and notably myelinated axons compared with all other conditions. A significant increase in functional recovery was observed for IL-10+NT-3 delivery at 12 weeks postinjury that was positively correlated to oligodendrocyte myelinated axon density, suggesting oligodendrocyte-mediated myelination as an important target to improve functional recovery. These results further support the use of multiple channel PLG bridges as a growth supportive substrate and platform to deliver bioactive agents to modulate the SCI microenvironment and promote regeneration and functional recovery. Impact statement Spinal cord injury (SCI) results in a complex microenvironment that contains multiple barriers to regeneration and functional recovery. Multiple factors are necessary to address these barriers to regeneration, and polycistronic lentiviral gene therapy represents a strategy to locally express multiple factors simultaneously. A bicistronic vector encoding IL-10 and NT-3 was delivered from a poly(lactide-co-glycolide) bridge, which provides structural support that guides regeneration, resulting in increased axonal growth, myelination, and subsequent functional recovery. These results demonstrate the opportunity of targeting multiple barriers to SCI regeneration for additive effects.

Recent advances in nanotherapeutic strategies for spinal cord injury repair

Spinal cord injury (SCI) is a devastating and complicated condition with no cure available. The initial mechanical trauma is followed by a secondary injury characterized by inflammatory cell infiltration and inhibitory glial scar formation. Due to the limitations posed by the blood-spinal cord barrier, systemic delivery of therapeutics is challenging. Recent development of various nanoscale strategies provides exciting and promising new means of treating SCI by crossing the blood-spinal cord barrier and delivering therapeutics. As such, we discuss different nanomaterial fabrication methods and provide an overview of recent studies where nanomaterials were developed to modulate inflammatory signals, target inhibitory factors in the lesion, and promote axonal regeneration after SCI. We also review emerging areas of research such as optogenetics, immunotherapy and CRISPR-mediated genome editing where nanomaterials can provide synergistic effects in developing novel SCI therapy regimens, as well as current efforts and barriers to clinical translation of nanomaterials.

Emerging Trends in Information-Driven Engineering of Complex Biological Systems

Synthetic biological systems are used for a myriad of applications, including tissue engineered constructs for in vivo use and microengineered devices for in vitro testing. Recent advances in engineering complex biological systems have been fueled by opportunities arising from the combination of bioinspired materials with biological and computational tools. Driven by the availability of large datasets in the "omics" era of biology, the design of the next generation of tissue equivalents will have to integrate information from single-cell behavior to whole organ architecture. Herein, recent trends in combining multiscale processes to enable the design of the next generation of biomaterials are discussed. Any successful microprocessing pipeline must be able to integrate hierarchical sets of information to capture key aspects of functional tissue equivalents. Micro- and biofabrication techniques that facilitate hierarchical control as well as emerging polymer candidates used in these technologies are also reviewed.

Polymer scaffolds facilitate spinal cord injury repair

During the past decades, improving patient neurological recovery following spinal cord injury (SCI) has remained a challenge. An effective treatment for SCI would not only reduce fractured elements and isolate developing local glial scars to promote axonal regeneration but also ameliorate secondary effects, including inflammation, apoptosis, and necrosis. Three-dimensional (3D) scaffolds provide a platform in which these mechanisms can be addressed in a controlled manner. Polymer scaffolds with favorable biocompatibility and appropriate mechanical properties have been engineered to minimize cicatrization, customize drug release, and ensure an unobstructed space to promote cell growth and differentiation. These properties make polymer scaffolds an important potential therapeutic platform. This review highlights the recent developments in polymer scaffolds for SCI engineering. STATEMENT OF SIGNIFICANCE: How to improve the efficacy of neurological recovery after spinal cord injury (SCI) is always a challenge. Tissue engineering provides a promising strategy for SCI repair, and scaffolds are one of the most important elements in addition to cells and inducing factors. The review highlights recent development and future prospects in polymer scaffolds for SCI therapy. The review will guide future studies by outlining the requirements and characteristics of polymer scaffold technologies employed against SCI. Additionally, the peculiar properties of polymer materials used in the therapeutic process of SCI also have guiding significance to other tissue engineering approaches.

Devising micro/nano-architectures in multi-channel nerve conduits towards a pro-regenerative matrix for the repair of spinal cord injury

Multi-channel nerve conduits have shown significant advantages in guidance of axonal growth and functional restoration after spinal cord injury (SCI). It was realized that the micro/nano-architectures of these implanted conduits can effectively tune the lesion-induced biological responses, including inflammation and scar formation. In this work, two PLLA multi-channel conduits were fabricated with ladder-like porous channel wall (labelled as LNCs) and nano-fibrous channel wall (labelled as NNCs), respectively, and transferred into complete spinal cord transected injury model in rats. The implantation of such two scaffolds significantly alleviated the infiltration of macrophages/microglia and accumulation of astrocyte and collagen scar, especially in the NNCs group. Meanwhile, recruitment of endogenous stem cells and axonal growth was observed in both of the multi-channel conduits. Compared to the LNCs, the extracellular matrix (ECM) - mimicry nanostructures in the NNCs promoted directional nerve fiber growth within the channels. Moreover, a relatively denser nano-architecture in the channel wall confined the nerve fiber extension within the channels. These results from in vivo evaluations suggested that the NNCs implants possess a great potential in future application for SCI treatment and nerve regeneration. STATEMENTS OF SIGNIFICANCE: The implantation of biocompatible and degradable polymeric scaffolds holds great potential in clinical treatment and tissue regeneration after spinal cord injury (SCI). In this work, the ladder-like nerve conduits (LNCs) and nano-fibrous nerve conduits (NNCs) were fabricated and implanted into completely spinal cord transected rats, respectively. In vivo characteristics showed significant reduction in post-injury inflammation and scar formation, with elevated nerve stem cells (NSCs) recruitment and nerve fiber growth, hence both conduits resulted in significant functional restoration after implantation. Remarkably, we noticed that not only the multi-channels in the conduits can guide nerve fiber regeneration, their micro-/nano-structured walls also played a critical role in modulating the post-implantation biological responses.

Aligned hydrogel tubes guide regeneration following spinal cord injury

Directing the organization of cells into a tissue with defined architectures is one use of biomaterials for regenerative medicine. To this end, hydrogels are widely investigated as they have mechanical properties similar to native soft tissues and can be formed in situ to conform to a defect. Herein, we describe the development of porous hydrogel tubes fabricated through a two-step polymerization process with an intermediate microsphere phase that provides macroscale porosity (66.5%) for cell infiltration. These tubes were investigated in a spinal cord injury model, with the tubes assembled to conform to the injury and to provide an orientation that guides axons through the injury. Implanted tubes had good apposition and were integrated with the host tissue due to cell infiltration, with a transient increase in immune cell infiltration at 1 week that resolved by 2 weeks post injury compared to a gelfoam control. The glial scar was significantly reduced relative to control, which enabled robust axon growth along the inner and outer surface of the tubes. Axon density within the hydrogel tubes (1744 axons/mm²) was significantly increased more than 3-fold compared to the control (456 axons/mm²), with approximately 30% of axons within the tube myelinated. Furthermore, implantation of hydrogel tubes enhanced functional recovery relative to control. This modular assembly of porous tubes to fill a defect and directionally orient tissue growth could be extended beyond spinal cord injury to other tissues, such as vascular or musculoskeletal tissue. STATEMENT OF SIGNIFICANCE: Tissue engineering approaches that mimic the native architecture of healthy tissue are needed following injury. Traditionally, pre-molded scaffolds have been implemented but require a priori knowledge of wound geometries. Conversely, hydrogels can conform to any injury, but do not guide bi-directional regeneration. In this work, we investigate the feasibility of a system of modular hydrogel tubes to promote bi-directional regeneration after spinal cord injury. This system allows for tubes to be cut to size during surgery and implanted one-by-one to fill any injury, while providing bi-directional guidance. Moreover, this system of tubes can be broadly applied to tissue engineering approaches that require a modular guidance system, such as repair to vascular or musculoskeletal tissues.

Combinatorial lentiviral gene delivery of pro-oligodendrogenic factors for improving myelination of regenerating axons after spinal cord injury

Spinal cord injury (SCI) results in paralysis below the injury and strategies are being developed that support axonal regrowth, yet recovery lags, in part, because many axons are not remyelinated. Herein, we investigated strategies to increase myelination of regenerating axons by overexpression of platelet-derived growth factor (PDGF)-AA and noggin either alone or in combination in a mouse SCI model. Noggin and PDGF-AA have been identified as factors that enhance recruitment and differentiation of endogenous progenitors to promote myelination. Lentivirus encoding for these factors was delivered from a multichannel bridge, which we have previously shown creates a permissive environment and supports robust axonal growth through channels. The combination of noggin+PDGF enhanced total myelination of regenerating axons relative to either factor alone, and importantly, enhanced functional recovery relative to the control condition. The increase in myelination was consistent with an increase in oligodendrocyte-derived myelin, which was also associated with a greater density of cells of an oligodendroglial lineage relative to each factor individually and control conditions. These results suggest enhanced myelination of regenerating axons by noggin+PDGF that act on oligodendrocyte-lineage cells post-SCI, which ultimately led to improved functional outcomes.

3D Bioprinting Human Induced Pluripotent Stem Cell-Derived Neural Tissues Using a Novel Lab-on-a-Printer Technology

Most neurological diseases and disorders lack true cures, including spinal cord injury (SCI). Accordingly, current treatments only alleviate the symptoms of these neurological diseases and disorders. Engineered neural tissues derived from human induced pluripotent stem cells (hiPSCs) can serve as powerful tools to identify drug targets for treating such diseases and disorders. In this work, we demonstrate how hiPSC-derived neural progenitor cells (NPCs) can be bioprinted into defined structures using Aspect Biosystems’ novel RX1 bioprinter in combination with our unique fibrin-based bioink in rapid fashion as it takes under 5 min to print four tissues. This printing process preserves high levels of cell viability (>81%) and their differentiation capacity in comparison to less sophisticated bioprinting methods. These bioprinted neural tissues expressed the neuronal marker, βT-III (45 ± 20.9%), after 15 days of culture and markers associated with spinal cord (SC) motor neurons (MNs), such as Olig2 (68.8 ± 6.9%), and HB9 (99.6 ± 0.4%) as indicated by flow cytometry. The bioprinted neural tissues expressed the mature MN marker, ChaT, after 30 days of culture as indicated by immunocytochemistry. In conclusion, we have presented a novel method for high throughput production of mature hiPSC-derived neural tissues with defined structures that resemble those found in the SC.

Spinal Progenitor-Laden Bridges Support Earlier Axon Regeneration Following Spinal Cord Injury

IMPACT STATEMENT

Spinal cord injury (SCI) results in loss of tissue innervation below the injury. Spinal progenitors have a greater ability to repair the damage and can be injected into the injury, but their regenerative potential is hampered by their poor survival after transplantation. Biomaterials can create a cell delivery platform and generate a more hospitable microenvironment for the progenitors within the injury. In this work, polymeric bridges are used to deliver embryonic spinal progenitors to the injury, resulting in increased progenitor survival and subsequent regeneration and functional recovery, thus demonstrating the importance of combined therapeutic approaches for SCI.

Hepatocyte culture on 3D porous scaffolds of PCL/PMCL

The development of three-dimensional (3D) porous scaffolds for soft tissue engineering mainly focused on manipulation of scaffold properties with cell behaviors. By emulsion freeze-drying method, four types of porous scaffolds were prepared from amorphous poly(4-methy-ε-caprolactone) (PMCL) and semi-crystalline poly(ε-caprolactone) (PCL) at different weight ratios, named as PMCL0, PMCL30, PMCL50 and PMCL70, respectively. Visual observation on cross-sectional images of the scaffolds appeared as sponge-like materials with three-dimensional and highly porous morphologies. However, the pore size, porosity and wettability of blends were not decreased linearly with increasing amorphous PMCL. Distinguished from PMCL30 or PMCL70, PMCL50 preserved intact PCL crystals distributed in amorphous matrix, resulting in the lowest Young's modulus (E) and relatively high wettability. From in vitro cell culture, it was observed that PMCL50 scaffold supported human induced hepatocytes (hiHeps) proliferation and function preservation best among all scaffolds. hiHeps on PMCL50 inclined to adopt fibroblastic morphology, whereas formed spheroidal morphology on PMCL0. It was suggested that our bare scaffolds with tailored properties have shown remarkable capability towards hiHep proliferation and function expression.

Local Immunomodulation with Anti-inflammatory Cytokine-Encoding Lentivirus Enhances Functional Recovery after Spinal Cord Injury

Trauma to the spinal cord and associated secondary inflammation can lead to permanent loss of sensory and motor function below the injury level, with the resulting environment serving as a barrier that limits regeneration. In this study, we investigate the localized expression of anti-inflammatory cytokines IL-10 and IL-4 via lentiviral transduction in multichannel bridges. Porous multichannel bridges provide physical guidance for axonal outgrowth with the cytokines hypothesized to modulate the neuroinflammatory microenvironment and enhance axonal regeneration. Gene expression analyses indicated that induced IL-10 and IL-4 expression decreased expression of pro-inflammatory genes and increased pro-regenerative genes relative to control. Moreover, these factors led to increased numbers of axons and myelination, with approximately 45% of axons myelinated and the number of oligodendrocyte myelinated axons significantly increased by 3- to 4-fold. Furthermore, the combination of a bridge with IL-10 and IL-4 expression improved locomotor function after injury to an average score of 6 relative to an average score of 3 for injury alone. Collectively, these studies highlight the potential for localized immunomodulation to decrease secondary inflammation and enhance regeneration that may have numerous applications.

Micropatterned biodegradable polyesters clicked with CQAASIKVAV promote cell alignment, directional migration, and neurite outgrowth

The interplay of microstructures and biological cues is critical to regulate the behaviors of Schwann cells (SCs) in terms of cellular spatial arrangement and directional migration as well as neurite orientation for bridging the proximal and distal stumps of the injured peripheral nervous system. In this study, stripe micropatterns having ridges/grooves of width 20/20 and 20/40 μm were fabricated on the surface of maleimide-functionalized biodegradable poly(ester carbonate) (P(LLA-MTMC)) films by the polydimethylsiloxane mold-pressing method, respectively. The laminin-derived CQAASIKVAV peptides end-capped with an SH group were then grafted by the thiol-ene click reaction under mild conditions to obtain micropatterned and peptide-grafted films. SCs cultured on these films, especially on the 20/40-μm film, displayed faster and aligned adhesion as well as a larger number of elongated cells with a higher length-to-width (L/W) ratio along the stripe direction than those on the flat-pep film. The migration rate of SCs was significantly enhanced in parallel to the stripe direction with a large net displacement. The micropatterned and peptide-grafted films, especially the 20/40-μm film, could promote SC proliferation and nerve growth factor (NGF) secretion in a manner similar to that of the peptide-grafted planar film. Moreover, the neurites of rat pheochromocytoma 12 (PC12) cells sprouted along the ridges with a longer average length on the micropatterned and peptide-grafted films. The synergistic effect of physical patterns and biological cues was evaluated by considering the results of cell adhesion force; immunofluorescence staining of vinculin; fluorescence staining of F-actin and the nucleus; as well as gene expression of neural cadherin (NCAD), neurocan (NCAN), and myelin protein zero (P0).

STATEMENT OF SIGNIFICANCE

The interplay of microstructures and biological cues is critical to regulate the behaviors of Schwann cells (SCs) and nerve cells, and thereby the regeneration of peripheral nerve system. In this study, the combined micropatterning and CQAASIKVAV grafting endowed the modified P(LLA-MTMC) films with both contact guidance and bioactive chemical cues to enhance cell proliferation, directional alignment and migration, longer net displacement and larger NGF secretion, and stronger neurite outgrowth of SCs and PC12 cells. Hence, the integration of physical micropatterns and bioactive molecules is an effective way to obtain featured biomaterials for the regeneration of nerves and other types of tissues.

Feasibility study on mouse live imaging after spinal cord injury and poly(lactide-co-glycolide) bridge implantation

Spinal cord injury (SCI) causes permanent paralysis below the damaged area. SCI is linked to neuronal death, demyelination, and limited ability of neuronal fibers to regenerate. Regeneration capacity is limited by the presence of many inhibitory factors in the spinal cord environment. The use of poly(lactide-co-glycolide) (PLG) bridges has demonstrated the ability to sustain long-term regeneration after SCI in a cervical hemisection mouse model. Critically, imaging of regenerating fibers and the myelination status of these neuronal filaments is a severe limitation to progress in SCI research. We used a transgenic mouse model that selectively expresses fluorescent reporters (eGFP) in the neuronal fibers of the spinal cord. We implanted a PLG bridge at C5 vertebra after hemisection and evaluated in live animals' neuronal fibers at the bridge interface and within the bridge 8 weeks postimplant. These in vivo observations were correlated with in situ evaluation 12 weeks postimplantation. We sectioned the spinal cords and performed fluorescent bioimaging on the sections to observe neuronal fibers going through the bridge. In parallel, to visualize myelination of regenerated axons, we exploited the characteristics of the third-harmonic generation arising from the myelin structure in these fixed sections.

Hierarchically aligned fibrin nanofiber hydrogel accelerated axonal regrowth and locomotor function recovery in rat spinal cord injury

BACKGROUND

Designing novel biomaterials that incorporate or mimic the functions of extracellular matrix to deliver precise regulatory signals for tissue regeneration is the focus of current intensive research efforts in tissue engineering and regenerative medicine.

METHODS AND RESULTS

To mimic the natural environment of the spinal cord tissue, a three-dimensional hierarchically aligned fibrin hydrogel (AFG) with oriented topography and soft stiffness has been fabricated by electrospinning and a concurrent molecular self-assembling process. In this study, the AFG was implanted into a rat dorsal hemisected spinal cord injury model to bridge the lesion site. Host cells invaded promptly along the aligned fibrin hydrogels to form aligned tissue cables in the first week, and then were followed by axonal regrowth. At 4 weeks after the surgery, neurofilament (NF)-positive staining fibers were detected near the rostral end as well as the middle site of defect, which aligned along the tissue cables. Abundant NF- and GAP-43-positive staining indicated new axon regrowth in the oriented tissue cables, which penetrated throughout the lesion site in 8 weeks. Additionally, the abundant blood vessels marked with RECA-1 had reconstructed within the lesion site at 4 weeks after surgery. Basso-Beattie-Bresnahan scoring showed that the locomotor performance of the AFG group recovered much faster than that of blank control group or the random fibrin hydrogel (RFG) group from 2 weeks after surgery. Furthermore, diffusion tensor imaging tractography of MRI confirmed the optimal axon fiber reconstruction compared with the RFG and control groups.

CONCLUSION

Taken together, our results suggested that the AFG scaffold provided an inductive matrix for accelerating directional host cell invasion, vascular system reconstruction, and axonal regrowth, which could promote and support extensive aligned axonal regrowth and locomotor function recovery.

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

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

Functional and Biomimetic Materials for Engineering of the Three-Dimensional Cell Microenvironment

The cell microenvironment has emerged as a key determinant of cell behavior and function in development, physiology, and pathophysiology. The extracellular matrix (ECM) within the cell microenvironment serves not only as a structural foundation for cells but also as a source of three-dimensional (3D) biochemical and biophysical cues that trigger and regulate cell behaviors. Increasing evidence suggests that the 3D character of the microenvironment is required for development of many critical cell responses observed in vivo, fueling a surge in the development of functional and biomimetic materials for engineering the 3D cell microenvironment. Progress in the design of such materials has improved control of cell behaviors in 3D and advanced the fields of tissue regeneration, in vitro tissue models, large-scale cell differentiation, immunotherapy, and gene therapy. However, the field is still in its infancy, and discoveries about the nature of cell-microenvironment interactions continue to overturn much early progress in the field. Key challenges continue to be dissecting the roles of chemistry, structure, mechanics, and electrophysiology in the cell microenvironment, and understanding and harnessing the roles of periodicity and drift in these factors. This review encapsulates where recent advances appear to leave the ever-shifting state of the art, and it highlights areas in which substantial potential and uncertainty remain.

Peripheral nerve growth within a hydrogel microchannel scaffold supported by a kink-resistant conduit

Nerve repair in several mm-long nerve gaps often requires an interventional technology. Microchannel scaffolds have proven effective for bridging nerve gaps and guiding axons in the peripheral nervous system (PNS). Nonetheless, fabricating microchannel scaffolds at this length scale remains a challenge and/or is time consuming and cumbersome. In this work, a simple computer-aided microdrilling technique was used to fabricate 10 mm-long agarose scaffolds consisting of 300 µm-microchannels and 85 µm-thick walls in less than an hour. The agarose scaffolds alone, however, did not exhibit adequate stiffness and integrity to withstand the mechanical stresses during implantation and suturing. To provide mechanical support and enable suturing, poly caprolactone (PCL) conduits were fabricated and agarose scaffolds were placed inside. A modified salt-leaching technique was developed to introduce interconnected porosity in PCL conduits to allow for tuning of the mechanical properties such as elastic modulus and strain to failure. It was shown that the PCL conduits were effective in stabilizing the agarose scaffolds in 10 mm-long sciatic nerve gaps of rats for at least 8 weeks. Robust axon ingress and Schwann cell penetration were observed within the microchannel scaffolds without using growth factors. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 3392-3399, 2017.

Implantable porous gelatin microspheres sustained release of bFGF and improved its neuroprotective effect on rats after spinal cord injury

In this study, porous gelatin microspheres (GMSs) were constructed to improve the neuroprotective effect of basic fibroblast growth factor (bFGF) on spinal cord injury. GMSs were prepared by a W/O emulsion template, followed by cross-linking, washing and drying. The particle sizes and surface porosity of the blank GMSs were carefully characterized by scan electronic microscopy. The blank GMSs have a mean particle size of 35μm and theirs surface was coarse and porous. bFGF was easily encapsulated inside the bulk GMSs through diffusion along the porous channel. 200μg of bFGF was completely encapsulated in 100mg of GMSs. The bFGF-loaded GMSs displayed a continuous drug release pattern without an obvious burst release over two weeks in vitro. Moreover, the therapeutic effects of bFGF-loaded GMSs were also evaluated in spinal cord injury rat model. After implantation of bFGF-loaded GMSs, the recovery of the motor function of SCI rats were evaluated by behavioral score and foot print experiment. The motor function of SCI rats treated with bFGF-loaded GMSs was more obvious than that treated with free bFGF solution (P<0.05). At the 28^th days after treatment, rats were sacrificed and the injured spinal were removed for histopathological and apoptosis examination. Compared with treatment with free bFGF solution, treatment with bFGF-loaded GMSs resulted in a less necrosis, less infiltration of leukocytes, and a reduced the cavity ratio and less apoptotic cells in injured spinal(P<0.01), indicating its better therapeutic effect. Implantable porous GMSs may be a potential carrier to deliver bFGF for therapy of spinal cord injury.

Three-dimensional aligned nanofibers-hydrogel scaffold for controlled non-viral drug/gene delivery to direct axon regeneration in spinal cord injury treatment

Spinal cord injuries (SCI) often lead to persistent neurological dysfunction due to failure in axon regeneration. Unfortunately, currently established treatments, such as direct drug administration, do not effectively treat SCI due to rapid drug clearance from our bodies. Here, we introduce a three-dimensional aligned nanofibers-hydrogel scaffold as a bio-functionalized platform to provide sustained non-viral delivery of proteins and nucleic acid therapeutics (small non-coding RNAs), along with synergistic contact guidance for nerve injury treatment. A hemi-incision model at cervical level 5 in the rat spinal cord was chosen to evaluate the efficacy of this scaffold design. Specifically, aligned axon regeneration was observed as early as one week post-injury. In addition, no excessive inflammatory response and scar tissue formation was triggered. Taken together, our results demonstrate the potential of our scaffold for neural tissue engineering applications.

Tissue Engineering Approaches to Modulate the Inflammatory Milieu following Spinal Cord Injury

Tissue engineering strategies have shown promise in promoting healing and regeneration after spinal cord injury (SCI); however, these strategies are limited by inflammation and the immune response. Infiltration of cells of the innate and adaptive immune responses and the inflammation that follows cause secondary damage adjacent to the injury, increased scarring, and a potently inhibitory environment for the regeneration of damaged neurons. While the inflammation that ensues is typically associated with limited regeneration, the immune response is a crucial element in the closing of the blood-brain barrier, minimizing the spread of injury, and initiating healing. This review summarizes the strategies that have been developed to modulate the immune response towards an anti-inflammatory environment that is permissive to the regeneration of neurons, glia, and parenchyma. We focus on the use of biomaterials, biologically active molecules, gene therapy, nanoparticles, and stem cells to modulate the immune response, and illustrate concepts for future therapies. Current clinical treatments for SCI are limited to systemic hypothermia or methylprednisolone, which both act by systemically mitigating the effects of immune response but have marginal efficacy. Herein, we discuss emerging research strategies to further enhance these clinical treatments by directly targeting specific aspects of the immune response.

Injectable Hydrogels for Spinal Cord Repair: A Focus on Swelling and Intraspinal Pressure

Spinal cord injury (SCI) is a devastating condition that leaves patients with limited motor and sensory function at and below the injury site, with little to no hope of a meaningful recovery. Because of their ability to mimic multiple features of central nervous system (CNS) tissues, injectable hydrogels are being developed that can participate as therapeutic agents in reducing secondary injury and in the regeneration of spinal cord tissue. Injectable biomaterials can provide a supportive substrate for tissue regeneration, deliver therapeutic factors, and regulate local tissue physiology. Recent reports of increasing intraspinal pressure after SCI suggest that this physiological change can contribute to injury expansion, also known as secondary injury. Hydrogels contain high water content similar to native tissue, and many hydrogels absorb water and swell after formation. In the case of injectable hydrogels for the spinal cord, this process often occurs in or around the spinal cord tissue, and thus may affect intraspinal pressure. In the future, predictable swelling properties of hydrogels may be leveraged to control intraspinal pressure after injury. Here, we review the physiology of SCI, with special attention to the current clinical and experimental literature, underscoring the importance of controlling intraspinal pressure after SCI. We then discuss how hydrogel fabrication, injection, and swelling can impact intraspinal pressure in the context of developing injectable biomaterials for SCI treatment.