Axon-like aligned conductive CNT/GelMA hydrogel fibers combined with electrical stimulation for spinal cord injury recovery

Rehabilitation and regenerative medicine are two promising approaches for spinal cord injury (SCI) recovery, but their combination has been limited. Conductive biomaterials could bridge regenerative scaffolds with electrical stimulation by inducing axon regeneration and supporting physiological electrical signal transmission. Here, we developed aligned conductive hydrogel fibers by incorporating carbon nanotubes (CNTs) into methacrylate acylated gelatin (GelMA) hydrogel via rotating liquid bath electrospinning. The electrospun CNT/GelMA hydrogel fibers mimicked the micro-scale aligned structure, conductivity, and soft mechanical properties of neural axons. For in vitro studies, CNT/GelMA hydrogel fibers supported PC12 cell proliferation and aligned adhesion, which was enhanced by electrical stimulation (ES). Similarly, the combination of aligned CNT/GelMA hydrogel fibers and ES promoted neuronal differentiation and axon-like neurite sprouting in neural stem cells (NSCs). Furthermore, CNT/GelMA hydrogel fibers were transplanted into a T9 transection rat spinal cord injury model for in vivo studies. The results showed that the incorporating CNTs could remain at the injury site with the GelMA fibers biodegraded and improve the conductivity of regenerative tissue. The aligned structure of the hydrogel could induce the neural fibers regeneration, and the ES enhanced the remyelination and axonal regeneration. Behavioral assessments and electrophysiological results suggest that the combination of aligned CNT/GelMA hydrogel fibers and ES could significantly restore motor function in rats. This study demonstrates that conductive aligned CNT/GelMA hydrogel fibers can not only induce neural regeneration as a scaffold but also support ESto promote spinal cord injury recovery. The conductive hydrogel fibers enable merging regenerative medicine and rehabilitation, showing great potential for satisfactory locomotor recovery after SCI.

Advances in electroactive bioscaffolds for repairing spinal cord injury

Spinal cord injury (SCI) is a devastating neurological disorder, leading to loss of motor or somatosensory function, which is the most challenging worldwide medical problem. Re-establishment of intact neural circuits is the basis of spinal cord regeneration. Considering the crucial role of electrical signals in the nervous system, electroactive bioscaffolds have been widely developed for SCI repair. They can produce conductive pathways and a pro-regenerative microenvironment at the lesion site similar to that of the natural spinal cord, leading to neuronal regeneration and axonal growth, and functionally reactivating the damaged neural circuits. In this review, we first demonstrate the pathophysiological characteristics induced by SCI. Then, the crucial role of electrical signals in SCI repair is introduced. Based on a comprehensive analysis of these characteristics, recent advances in the electroactive bioscaffolds for SCI repair are summarized, focusing on both the conductive bioscaffolds and piezoelectric bioscaffolds, used independently or in combination with external electronic stimulation. Finally, thoughts on challenges and opportunities that may shape the future of bioscaffolds in SCI repair are concluded.

State of the Art and Current Challenges on Electroactive Biomaterials and Strategies for Neural Tissue Regeneration

The loss or failure of an organ/tissue stands as one of the healthcare system's most prevalent, devastating, and costly challenges. Strategies for neural tissue repair and regeneration have received significant attention due to their particularly strong impact on patients' well-being. Many research efforts are dedicated not only to control the disease symptoms but also to find solutions to repair the damaged tissues. Neural tissue engineering (TE) plays a key role in addressing this problem and significant efforts are being carried out to develop strategies for neural repair treatment. In the last years, active materials allowing to tune cell-materials interaction are being increasingly used, representing a recent paradigm in TE applications. Among the most important stimuli influencing cell behavior are the electrical and mechanical ones. In this way, materials with the ability to provide this kind of stimuli to the neural cells seem to be appropriate to support neural TE. In this scope, this review summarizes the different biomaterials types used for neural TE, highlighting the relevance of using active biomaterials and electrical stimulation. Furthermore, this review provides not only a compilation of the most relevant studies and results but also strategies for novel and more biomimetic approaches for neural TE.

Nanomaterial payload delivery to central nervous system glia for neural protection and repair

Central nervous system (CNS) glia, including astrocytes, microglia, and oligodendrocytes, play prominent roles in traumatic injury and degenerative disorders. Due to their importance, active pharmaceutical ingredients (APIs) are being developed to modulate CNS glia in order to improve outcomes in traumatic injury and disease. While many of these APIs show promise in vitro, the majority of APIs that are systemically delivered show little penetration through the blood-brain barrier (BBB) or blood-spinal cord barrier (BSCB) and into the CNS, rendering them ineffective. Novel nanomaterials are being developed to deliver APIs into the CNS to modulate glial responses and improve outcomes in injury and disease. Nanomaterials are attractive options as therapies for central nervous system protection and repair in degenerative disorders and traumatic injury due to their intrinsic capabilities in API delivery. Nanomaterials can improve API accumulation in the CNS by increasing permeation through the BBB of systemically delivered APIs, extending the timeline of API release, and interacting biophysically with CNS cell populations due to their mechanical properties and nanoscale architectures. In this review, we present the recent advances in the fields of both locally implanted nanomaterials and systemically administered nanoparticles developed for the delivery of APIs to the CNS that modulate glial activity as a strategy to improve outcomes in traumatic injury and disease. We identify current research gaps and discuss potential developments in the field that will continue to translate the use of glia-targeting nanomaterials to the clinic.

Conducting polymer-based nanostructured materials for brain-machine interfaces

As scientists discovered that raw neurological signals could translate into bioelectric information, brain-machine interfaces (BMI) for experimental and clinical studies have experienced massive growth. Developing suitable materials for bioelectronic devices to be used for real-time recording and data digitalizing has three important necessitates which should be covered. Biocompatibility, electrical conductivity, and having mechanical properties similar to soft brain tissue to decrease mechanical mismatch should be adopted for all materials. In this review, inorganic nanoparticles and intrinsically conducting polymers are discussed to impart electrical conductivity to systems, where soft materials such as hydrogels can offer reliable mechanical properties and a biocompatible substrate. Interpenetrating hydrogel networks offer more mechanical stability and provide a path for incorporating polymers with desired properties into one strong network. Promising fabrication methods, like electrospinning and additive manufacturing, allow scientists to customize designs for each application and reach the maximum potential for the system. In the near future, it is desired to fabricate biohybrid conducting polymer-based interfaces loaded with cells, giving the opportunity for simultaneous stimulation and regeneration. Developing multi-modal BMIs, Using artificial intelligence and machine learning to design advanced materials are among the future goals for this field. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Neurological Disease.

Regulation of nerve cells using conductive nanofibrous scaffolds for controlled release of Lycium barbarum polysaccharides and nerve growth factor

Currently, more and more patients suffer from peripheral nerve injury due to trauma, tumor and other causes worldwide. Biomaterial-based nerve conduits are increasingly recognized as a potential alternative to nerve autografts for the treatment of peripheral nerve injury. However, an ideal nerve conduit must offer topological guidance and biochemical and electrical signal transduction mechanisms. In this work, aligned conductive nanofibrous scaffolds comprising polylactic-co-glycolic acid and multiwalled carbon nanotubes (MWCNTs) were fabricated via coaxial electrospinning, and nerve growth factor (NGF) and Lycium barbarum polysaccharides (LBP) purified from the wolfberry were loaded on the core and shell layers of the nanofibers, respectively. LBP were confirmed to accelerate long-distance axon regeneration after severe peripheral nerve injury. In addition, the synergistic promotion of LBP and NGF on nerve cell proliferation and neurite outgrowth was demonstrated. MWCNTs were introduced into the aligned fibers to further increase the electrical conductivity, which promoted the directional growth and neurite extension of neurons in vitro. Further, the combination of conductive fibrous scaffolds with electrical stimulation that mimics endogenous electric fields significantly promoted the differentiation of PC12 cells and the axon outgrowth of neurons. Based on robust cell-induced behaviors, conductive composite fibers with optimized fiber alignment may be used for the promotion of nerve recovery.

Conductive Polyaniline Particles Regulating In Vitro Hydrolytic Degradation and Erosion of Hydroxyapatite/Poly(lactide-co-glycolide) Porous Scaffolds for Bone Tissue Engineering

In addition to biocompatibility and bioactivity, scaffolds with superior bone tissue regenerative capacity should possess excellent functionality (e.g., electroactivity and conductivity) and biodegradability matching with the rate of bone reconstruction. However, current conductive scaffolds display a reduced biodegradability rate and weakened biocompatibility. In this study, injectable conductive porous scaffolds were fabricated, incorporating camphor sulfonic acid-doped polyaniline (PANI) into hydroxyapatite/poly(lactide-co-glycolide) (HA/PLGA) scaffolds, using solvent-casting/particulate-leaching methodology. These scaffolds demonstrated excellent electroactivity, conductivity, hydrophilicity, thermodynamic properties, antibacterial properties, and biocompatibility. Their degradation behavior was explored by regulating the PANI content. The results demonstrated that adding an appropriate content of PANI would increase the pore size, porosity, and water absorption of the conductive scaffold and promote the formation of filamentous fiber byproducts with acidic hydrolysates, which accelerated the degradation rate of the scaffold. Owing to π-π stacking and hydrogen bonding, the conductive scaffold with 10 wt % PANI efficiently retarded the decrease in the thermal and mechanical properties of the scaffolds during a 16 week degradation. Thus, better regulation of degradation behavior and correlation would allow conductive porous scaffolds, such as bone implants, to achieve better bone ingrowth and restoration.

Surface charge and dynamic mechanoelectrical stimuli improves adhesion, proliferation and differentiation of neuron-like cells

Neuronal diseases and trauma are among the current major health-care problems. Patients frequently develop an irreversible state of neuronal disfunction that lacks treatment, strongly reducing life quality and expectancy. Novel strategies are thus necessary and tissue engineering research is struggling to provide alternatives to current treatments, making use of biomaterials capable to provide cell supports and active stimuli to develop permissive environments for neural regeneration. As neuronal cells are naturally found in electrical microenvironments, the electrically active materials can pave the way for new and effective neuroregenerative therapies. In this work the influence of piezoelectric poly(vinylidene fluoride) with different surface charges and dynamic mechanoelectrical stimuli on neuron-like cells adhesion, proliferation and differentiation was addressed. It is successfully demonstrated that both surface charge and electrically active dynamic microenvironments can be suitable to improve neuron-like cells adhesion, proliferation, and differentiation. These findings provide new knowledge to develop effective approaches for preclinical applications.

Electroactive aniline tetramer-spider silks with conductive and electrochromic functionality

Electroactive aniline tetramer-spider silk composite fibers with high conductivity and mechanical strength were developed using a dip coating method. The fabricated spider silk composite fibers retain the high mechanical strength (0.92 GPa) and unique reversible relaxation-contraction behavior of spider dragline silks. The aniline tetramer modified on the silk surface imparted electroactive properties to the composite fibers. The color of aniline tetramer/spider silk composite fibers could be controlled by applying different pH values and voltages. Furthermore, the composite fiber's resistivity could reach 186 Ω m which can conduct electrical current to light LEDs. This study could provide a valuable guideline for developing highly-conductive electrochromic spider silks for use in E-textiles.

Conductive Collagen-Based Hydrogel Combined With Electrical Stimulation to Promote Neural Stem Cell Proliferation and Differentiation

Injectable biomimetic hydrogels are a promising strategy for enhancing tissue repair after spinal cord injury (SCI) by restoring electrical signals and increasing stem cell differentiation. However, fabricating hydrogels that simultaneously exhibit high electrical conductivities, excellent mechanical properties, and biocompatibility remains a great challenge. In the present study, a collagen-based self-assembling cross-linking polymer network (SCPN) hydrogel containing poly-pyrrole (PPy), which imparted electroconductive properties, is developed for potential application in SCI repair. The prepared collagen/polypyrrole (Col/PPy)-based hydrogel exhibited a continuous and porous structure with pore sizes ranging from 50 to 200 μm. Mechanical test results indicated that the Young’s moduli of the prepared hydrogels were remarkably enhanced with PPy content in the range 0–40 mM. The conductivity of Col/PPy40 hydrogel was 0.176 ± 0.07 S/cm, which was beneficial for mediating electrical signals between tissues and accelerating the rate of nerve repair. The investigations of swelling and degradation of the hydrogels indicated that PPy chains interpenetrated and entangled with the collagen, thereby tightening the network structure of the hydrogel and improving its stability. The cell count kit-8 (CCK-8) assay and live/dead staining assay demonstrated that Col/PPy40 coupled with electrical simulation promoted the proliferation and survival of neural stem cells (NSCs). Compared with the other groups, the immunocytochemical analysis, qPCR, and Western blot studies suggested that Col/PPy40 coupled with ES maximally induced the differentiation of NSCs into neurons and inhibited the differentiation of NSCs into astrocytes. The results also indicated that the neurons in ES-treated Col/PPy40 hydrogel have longer neurites (170.8 ± 37.2 μm) and greater numbers of branch points (4.7 ± 1.2). Therefore, the prepared hydrogel system coupled with ES has potential prospects in the field of SCI treatment.

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

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

Near-Field Electrospinning: Crucial Parameters, Challenges, and Applications

Near-field electrospinning (NFES) is a micro- or nanofiber production technology based on jetting molten polymer or polymer solution. Thanks to the programmable collector and nozzle movement, it can generate designed patterns in the presence of an electric field. Despite a few shortcomings of NFES, its high resolution, simplicity, precision, high throughput, reproducibility, and low costs have convinced researchers to employ it for various purposes. Furthermore, as the paradigm of fiber-based structures shifts from random textures toward delicate designs, NFES can bridge the gap between existing inefficient processes and aspired technologies for precise patterning. NFES facilitates the production of ultrafine nanofibers because it can be used to fabricate them in every laboratory. These robust fibers are convenient tools for small and additive manufacturing. As such, NFES is considered a potent additive fabrication technology that facilitates the production of complicated patterns as well. It is suggested that near-field electrospun fibers exhibit outstanding results in various applications, owing to their precise and controllable positioning. Meanwhile, the ongoing development of NFES has yet to reach its climax, making it attractive for further research. In this review, the basic principles of NFES, derivatives, limitations, and applications in nanomanufacturing, tissue engineering, microscale electronics, biosensors, and optics are presented.

A photoelectric effect integrated scaffold for wireless regulation of nerve cellular behaviors

Electrical signal is regarded as a key factor to promote nerve cell neurogenesis. However, the usually used exogenous electrical stimulus mode needs additional equipment sources and complicated wirings, which is...

Research advances of biomaterials-based microenvironment-regulation therapies for repair and regeneration of spinal cord injury

Traumatic spinal cord injury (SCI) usually results in restricted behaviour recovery and even life-changing paralysis, accompanied with numerous complications. Pathologically, the initial injuries trigger a series of secondary injuries, leading to an expansion of lesion site, a mass of neuron loss, and eventual failure of endogenous axon regeneration. As the advances rapidly spring up in regenerative medicine and tissue engineering biomaterials, regulation of these secondary injuries becomes possible, shedding a light on normal functional restoration. The successful tissue regeneration lies in proper regulation of the inflammatory microenvironment, including the inflammatory immune cells and inflammatory factors that lead to oxidative stress, inhibitory glial scar and neuroexcitatory toxicity. Specifically, the approaches based on microenvironment-regulating biomaterials have shown great promise in the repair and regeneration of SCI. In this review, the pathological inflammatory microenvironments of SCI are discussed, followed by the introduction of microenvironment-regulating biomaterials in terms of their impressive therapeutic effect in attenuation of secondary inflammation and promotion of axon regrowth. With the emphasis on regulating secondary events, the biomaterials for SCI treatment will become promising for clinical applications.

Novel Electrode Designs for Neurostimulation in Regenerative Medicine: Activation of Stem Cells

Neural stem and progenitor cells (i.e., neural precursors) are found within specific regions in the central nervous system and have great regenerative capacity. These cells are electrosensitive and their behavior can be regulated by the presence of electric fields (EFs). Electrical stimulation is currently used to treat neurological disorders in a clinical setting. Herein we propose that electrical stimulation can be used to enhance neural repair by regulating neural precursor cell (NPC) kinetics and promoting their migration to sites of injury or disease. We discuss how intrinsic and extrinsic factors can affect NPC migration in the presence of an EF and how this impacts electrode design with the goal of enhancing tissue regeneration. We conclude with an outlook on future clinical applications of electrical stimulation and highlight technological advances that would greatly support these applications.