A State-of-the-Art of Functional Scaffolds for 3D Nervous Tissue Regeneration

Transformative advances in modeling brain aging and longevity: Success, challenges and future directions

Research on brain aging is crucial for understanding age-related neurodegenerative disorders and developing several therapeutic interventions. Numerous models ranging from two-dimensional (2D) cell-based, invertebrate, vertebrate, and sophisticated three-dimensional (3D) models have been used to understand the process of brain aging. Invertebrate models are ideal for researching conserved aging processes because of their simplicity, short lifespans, and genetic tractability. Moreover, vertebrate models, including zebrafish and rodents, exhibit more complex nervous systems and behaviors, enabling the exploration of age-related neurodegeneration and cognitive decline. 2D cell culture models derived from primary cells or immortalized cell lines are widely used for mechanistic studies at the cellular level but lack the physiological complexity of brain tissue. Recent advancements have shifted focus to 3D models, which better recapitulate the brain's microenvironment. Organoids derived from induced pluripotent stem cells mimic human brain architecture and enable the study of cell-cell interactions and aging in a human-specific context. Brain-on-a-chip systems integrate microfluidics and 3D cultures to model blood-brain barrier dynamics and neuronal networks. Additionally, scaffold-based 3D cultures and spheroids provide intermediate complexity, allowing researchers to study extracellular matrix interactions and age-related changes in neuronal function. These 3D models bridge the gap between traditional 2D cultures and animal-based in vivo studies, offering unprecedented insights into brain aging mechanisms. By combining these diverse models, researchers can unravel the multifaceted processes of brain aging and accelerate the development of targeted therapies for age-related neurodegenerative disorders.

"Biological evaluation of 3D-Printed chitosan-based scaffolds for tissue engineering"

Chitosan is a natural polysaccharide with excellent biocompatibility, biodegradability, and antibacterial capabilities, making it a good candidate for tissue engineering. 3D printing has revolutionized biomaterial fabrication owing to its precision, customization, and ability to create complex structures. This article aims to provide an overview of the current advances in 3D printing techniques for fabricating scaffolds based on chitosan and its derivatives. It also describes various printing methods, including extrusion bioprinting technique, inkjet bioprinting, stereolithography, digital light processing, and indirect 3D printing for controlling porosity, mechanical strength, and biological characteristics of chitosan scaffolds for a variety of tissues, like bone, vascular, cardiac, cartilage, and skin. This review also examines the biological properties of 3D-printed chitosan scaffolds. The advancements in biological performance and 3D printing technology indicate a promising future for developing flexible, customized scaffolds consisting of chitosan.

A Roadmap of Peptide-Based Materials in Neural Regeneration

Injuries to the nervous system lead to irreversible damage and limited functional recovery. The peripheral nervous system (PNS) can self-regenerate to some extent for short nerve gaps. In contrast, the central nervous system (CNS) has an intrinsic limitation to self-repair owing to its convoluted neural microenvironment and inhibitory response. The primary phase of CNS injury, happening within 48 h, results from external impacts like mechanical stress. Afterward, the secondary phase of the injury occurs, originating from neuronal excitotoxicity, mitochondrial dysfunction, and neuroinflammation. No golden standard to treat injured neurons exists, and conventional medicine serves only as a protective approach to alleviating the symptoms of chronic injury. Synthetic peptides provide a promising approach for neural repair, either as soluble drugs or by using their intrinsic self-assembly propensity to serve as an extracellular matrix (ECM) mimic for cell adhesion and to incorporate bioactive epitopes. In this review, an overview of nerve injury models, common in vitro models, and peptide-based therapeutics such as ECM mimics is provided. Due to the complexity of treating neuronal injuries, a multidisciplinary collaboration between biologists, physicians, and material scientists is paramount. Together, scientists with complementary expertise will be required to formulate future therapeutic approaches for clinical use.

An injectable and degradable heterogeneous microgel assembly capable of forming a "micro-nest group" for cell condensation and cartilage regeneration

Cell condensation, linking the migration and chondrogenic differentiation of MSCs, plays a crucial role in cartilage development. Current cartilage repair strategies are inadequately concerned with this process, leading to a suboptimal quality of regenerated cartilage. Inspired by the "nest flocks" structure of Social Weavers, a degradable heterogeneous microgel assembly (F/S-MA) is developed, which can release SDF-1, to form a "micro-nest group" structure and bond with HAV peptides to promote cell recruitment, condensation and chondrogenic differentiation. First, slow-degrading microgels (S-microgels) grafted with HAV peptides and fast-degrading microgels (F-microgels) loaded with SDF-1 are fabricated by an amidation reaction and Schiff base reaction, respectively. They employ sulfhydryl-modified gelatin as assembling agents to form F/S-MA through a thiol-ene reaction, exhibiting injectability, tissue adhesion, and microporosity. F-microgels undergo rapid degradation, leading to the release of SDF-1 and the formation of a "micro-nest group" in F/S-MA. Consequently, F/S-MA exhibits cell recruitment ability, meanwhile facilitating BMSC condensation through the synergistic effects of the "micro-nest group" and HAV peptides. In vitro experiments prove that F/S-MA enhances the expression of cell-condensation-related markers, ultimately upregulating the secretion of cartilage matrix. Animal experiments show that F/S-MA optimizes the quality of regenerated cartilage by improving cell recruitment and condensation. F/S-MA enhances cell condensation through structural and component design, which will provide new insights for cartilage regeneration.

Stem cell therapies for neurological disorders: current progress, challenges, and future perspectives

Stem cell-based therapies have emerged as a promising approach for treating various neurological disorders by harnessing the regenerative potential of stem cells to restore damaged neural tissue and circuitry. This comprehensive review provides an in-depth analysis of the current state of stem cell applications in primary neurological conditions, including Parkinson's disease (PD), Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), stroke, spinal cord injury (SCI), and other related disorders. The review begins with a detailed introduction to stem cell biology, discussing the types, sources, and mechanisms of action of stem cells in neurological therapies. It then critically examines the preclinical evidence from animal models and early human trials investigating the safety, feasibility, and efficacy of different stem cell types, such as embryonic stem cells (ESCs), mesenchymal stem cells (MSCs), neural stem cells (NSCs), and induced pluripotent stem cells (iPSCs). While ESCs have been studied extensively in preclinical models, clinical trials have primarily focused on adult stem cells such as MSCs and NSCs, as well as iPSCs and their derivatives. We critically assess the current state of research for each cell type, highlighting their potential applications and limitations in different neurological conditions. The review synthesizes key findings from recent, high-quality studies for each neurological condition, discussing cell manufacturing, delivery methods, and therapeutic outcomes. While the potential of stem cells to replace lost neurons and directly reconstruct neural circuits is highlighted, the review emphasizes the critical role of paracrine and immunomodulatory mechanisms in mediating the therapeutic effects of stem cells in most neurological disorders. The article also explores the challenges and limitations associated with translating stem cell therapies into clinical practice, including issues related to cell sourcing, scalability, safety, and regulatory considerations. Furthermore, it discusses future directions and opportunities for advancing stem cell-based treatments, such as gene editing, biomaterials, personalized iPSC-derived therapies, and novel delivery strategies. The review concludes by emphasizing the transformative potential of stem cell therapies in revolutionizing the treatment of neurological disorders while acknowledging the need for rigorous clinical trials, standardized protocols, and multidisciplinary collaboration to realize their full therapeutic promise.

Advances in 3D tissue models for neural engineering: self-assembled versus engineered tissue models

Neural tissue engineering has emerged as a promising field that aims to create functional neural tissue for therapeutic applications, drug screening, and disease modelling. It is becoming evident in the literature that this goal requires development of three-dimensional (3D) constructs that can mimic the complex microenvironment of native neural tissue, including its biochemical, mechanical, physical, and electrical properties. These 3D models can be broadly classified as self-assembled models, which include spheroids, organoids, and assembloids, and engineered models, such as those based on decellularized or polymeric scaffolds. Self-assembled models offer advantages such as the ability to recapitulate neural development and disease processes in vitro, and the capacity to study the behaviour and interactions of different cell types in a more realistic environment. However, self-assembled constructs have limitations such as lack of standardised protocols, inability to control the cellular microenvironment, difficulty in controlling structural characteristics, reproducibility, scalability, and lengthy developmental timeframes. Integrating biomimetic materials and advanced manufacturing approaches to present cells with relevant biochemical, mechanical, physical, and electrical cues in a controlled tissue architecture requires alternate engineering approaches. Engineered scaffolds, and specifically 3D hydrogel-based constructs, have desirable properties, lower cost, higher reproducibility, long-term stability, and they can be rapidly tailored to mimic the native microenvironment and structure. This review explores 3D models in neural tissue engineering, with a particular focus on analysing the benefits and limitations of self-assembled organoids compared with hydrogel-based engineered 3D models. Moreover, this paper will focus on hydrogel based engineered models and probe their biomaterial components, tuneable properties, and fabrication techniques that allow them to mimic native neural tissue structures and environment. Finally, the current challenges and future research prospects of 3D neural models for both self-assembled and engineered models in neural tissue engineering will be discussed.

Anisotropic hydrogel scaffold by flow-induced stereolithography 3D printing technique

Essential organs, such as the heart and liver, contain a unique porous network that allows oxygen and nutrients to be exchanged, with distinct random to ordered regions displaying varying degrees of strength. A novel technique, referred to here as flow-induced lithography, was developed. This technique generates tunable anisotropic three-dimensional (3D) structures. The ink for this bioprinting technique was made of titanium dioxide nanorods (Ti) and kaolinite nanoclay (KLT) dispersed in a GelMA/PEGDA polymeric suspension. By controlling the flow rate, aligned particle microstructures were achieved in the suspensions. The application of UV light to trigger the polymerization of the photoactive prepolymer freezes the oriented particles in the polymer network. Because the viability test was successful in shearing suspensions containing cells, the flow-induced lithography technique can be used with both acellular scaffolds and cell-laden structures. Fabricated hydrogels show outstanding mechanical properties resembling human tissues, as well as significant cell viability (> 95 %) over one week. As a result of this technique and the introduction of bio-ink, a novel approach has been pioneered for developing anisotropic tissue implants utilizing low-viscosity biomaterials.

Harnessing the Potential of Self-Assembled Peptide Hydrogels for Neural Regeneration and Tissue Engineering

Spinal cord injury, traumatic brain injury, and neurosurgery procedures usually lead to neural tissue damage. Self-assembled peptide (SAP) hydrogels, a type of innovative hierarchical nanofiber-forming peptide sequences serving as hydrogelators, have emerged as a promising solution for repairing tissue defects and promoting neural tissue regeneration. SAPs possess numerous features, such as adaptable morphologies, biocompatibility, injectability, tunable mechanical stability, and mimicking of the native extracellular matrix. This review explores the capacity of neural cell regeneration and examines the critical aspects of SAPs in neuroregeneration, including their biochemical composition, topology, mechanical behavior, conductivity, and degradability. Additionally, it delves into the latest strategies involving SAPs for central or peripheral neural tissue engineering. Finally, the prospects of SAP hydrogel design and development in the realm of neuroregeneration are discussed.

Biofabrication methods for reconstructing extracellular matrix mimetics

In the human body, almost all cells interact with extracellular matrices (ECMs), which have tissue and organ-specific compositions and architectures. These ECMs not only function as cellular scaffolds, providing structural support, but also play a crucial role in dynamically regulating various cellular functions. This comprehensive review delves into the examination of biofabrication strategies used to develop bioactive materials that accurately mimic one or more biophysical and biochemical properties of ECMs. We discuss the potential integration of these ECM-mimics into a range of physiological and pathological in vitro models, enhancing our understanding of cellular behavior and tissue organization. Lastly, we propose future research directions for ECM-mimics in the context of tissue engineering and organ-on-a-chip applications, offering potential advancements in therapeutic approaches and improved patient outcomes.

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.

PEDOT-Coated PLA Fibers Electrospun from Solutions Incorporating Fe(III)Tosylate in Different Solvents by Vapor-Phase Polymerization for Neural Regeneration

Therapeutic solutions for injuries in the peripheral nervous system are limited and not existing in the case of the central nervous system. The electrical stimulation of cells through a cell-supporting conductive scaffold may contribute to new therapeutic solutions for nerve regeneration. In this work, biocompatible Polylactic acid (PLA) fibrous scaffolds incorporating Fe(III)Tosylate (FeTos) were produced by electrospinning a mixture of PLA/FeTos solutions towards a rotating cylinder, inducing fiber alignment. Fibers were coated with the conductive polymer Poly(3,4 ethylenedioxythiophene) (PEDOT) formed by vapor-phase polymerization of EDOT at 70 °C for 2 h. Different solvents (ETH, DMF and THF) were used as FeTos solvents to investigate the impact on the scaffold's conductivity. Scaffold conductivity was estimated to be as high as 1.50 × 10-1 S/cm when FeTos was dissolved in DMF. In vitro tests were performed to evaluate possible scaffold cytotoxicity, following ISO 10993-5, revealing no cytotoxic effects. Differentiation and growth of cells from the neural cell line SH-SY5Y seeded on the scaffolds were also assessed, with neuritic extensions observed in cells differentiated in neurons with retinoic acid. These extensions tended to follow the preferential alignment of the scaffold fibers.

A Bioinspired Injectable, Adhesive, and Self-Healing Hydrogel with Dual Hybrid Network for Neural Regeneration after Spinal Cord Injury

Hydrogel-based regenerated scaffolds show promise as a platform for neural regeneration following spinal cord injury (SCI). Nevertheless, the persistent problem of poor mechanical strength and limited integration with the host tissue still exists. In this study, a bioinspired hydrogel with highly sophisticated features for neural regeneration after SCI is developed. The hydrogel is composed of dihydroxyphenylalanine (DOPA)-grafted chitosan and a designer peptide, offering a unique set of qualities such as being injectable, having self-healing abilities, and adhering to tissues. Compared to conventional hydrogels, this hydrogel ensures a significant promotion of immune response modulation and axon regrowth while featuring synapse formation of various neurotransmitters and myelin regeneration. Subsequently, functional recoveries are enhanced, including motor function, sensory function, and particularly bladder defect repair. These positive findings demonstrate that the hydrogel has great potential as a strategy for repairing SCI. Moreover, the versatility of this strategy goes beyond neural regeneration and holds promise for tissue regeneration in other contexts. Overall, this proposed hydrogel represents an innovative and multifaceted tool for engineering structures in the biomedical field.

Hydrogel-based 3D human iPSC-derived neuronal culture for the study of rabies virus infection

Background

Rabies is a highly fatal infectious disease that poses a significant threat to human health in developing countries. In vitro study-based understanding of pathogenesis and tropism of different strains of rabies virus (RABV) in the central nervous system (CNS) is limited due to the lack of suitable culture models that recapitulate the complex communication pathways among host cells, extracellular matrices, and viruses. Therefore, a three-dimensional (3D) cell culture that mimics cell-matrix interactions, resembling in vivo microenvironment, is necessary to discover relevant underlying mechanisms of RABV infection and host responses.

Methods

The 3D collagen-Matrigel hydrogel encapsulating hiPSC-derived neurons for RABV infection was developed and characterized based on cell viability, morphology, and gene expression analysis of neuronal markers. The replication kinetics of two different strains of RABV [wild-type Thai (TH) and Challenge Virus Standard (CVS)-11 strains] in both 2D and 3D neuronal cultures were examined. Differential gene expression analysis (DEG) of the neuropathological pathway of RABV-infected 2D and 3D models was also investigated via NanoString analysis.

Results

The 3D hiPSC-derived neurons revealed a more physiologically interconnected neuronal network as well as more robust and prolonged maturation and differentiation than the conventional 2D monolayer model. TH and CVS-11 exhibited distinct growth kinetics in 3D neuronal model. Additionally, gene expression analysis of the neuropathological pathway observed during RABV infection demonstrated a vast number of differentially expressed genes (DEGs) in 3D model. Unlike 2D neuronal model, 3D model displayed more pronounced cellular responses upon infection with CVS-11 when compared to the TH-infected group, highlighting the influence of the cell environment on RABV-host interactions. Gene ontology (GO) enrichment of DEGs in the infected 3D neuronal culture showed alterations of genes associated with the inflammatory response, apoptotic signaling pathway, glutamatergic synapse, and trans-synaptic signaling which did not significantly change in 2D culture.

Conclusion

We demonstrated the use of a hydrogel-based 3D hiPSC-derived neuronal model, a highly promising technology, to study RABV infection in a more physiological environment, which will broaden our understanding of RABV-host interactions in the CNS.

Spinal cord tissue engineering via covalent interaction between biomaterials and cells

Noncovalent interactions between cells and environmental cues have been recognized as fundamental physiological interactions that regulate cell behavior. However, the effects of the covalent interactions between cells and biomaterials on cell behavior have not been examined. Here, we demonstrate a combined strategy based on covalent conjugation between biomaterials (collagen fibers/lipid nanoparticles) and various cells (exogenous neural progenitor cells/astrocytes/endogenous tissue-resident cells) to promote neural regeneration after spinal cord injury (SCI). We found that metabolic azido-labeled human neural progenitor cells conjugated on dibenzocyclooctyne-modified collagen fibers significantly promoted cell adhesion, spreading, and differentiation compared with noncovalent adhesion. In addition, dibenzocyclooctyne-modified lipid nanoparticles containing edaravone, a well-known ROS scavenger, could target azide-labeled spinal cord tissues or transplanted azide-modified astrocytes to improve the SCI microenvironment. The combined application of these covalent conjugation strategies in a rat SCI model boosted neural regeneration, suggesting that the covalent interactions between cells and biomaterials have great potential for tissue regeneration.

Effect of porosity on mechanical and biological properties of bioprinted scaffolds

Treatment of tissue defects commonly represents a major problem in clinics due to difficulties involving a shortage of donors, inappropriate sizes, abnormal shapes, and immunological rejection. While many scaffold parameters such as pore shape, porosity percentage, and pore connectivity could be adjusted to achieve desired mechanical and biological properties. These parameters are crucial scaffold parameters that can be accurately produced by 3D bioprinting technology based on the damaged tissue. In the present research, the effect of porosity percentage (40%, 50%, and 60%) and different pore shapes (square, star, and gyroid) on the mechanical (e.g., stiffness, compressive and tensile behavior) and biological (e.g., biodegradation, and cell viability) properties of porous polycaprolactone (PCL) scaffolds coated with gelatin have been investigated. Moreover, human foreskin fibroblast cells were cultured on the scaffolds in the in-vitro procedures. MTT assay (4, 7, and 14 days) was utilized to determine the cytotoxicity of the porous scaffolds. It is revealed that the porous scaffolds produced by the bioprinter did not produce a cytotoxic effect. Among all the porous scaffolds, scaffolds with a pore size of about 500 μm and porosity of 50% showed the best cell proliferation compared to the controls after 14 days. The results demonstrated that the pore shape, porosity percentage, and pore connectivity have an important role in improving the mechanical and biological properties of porous scaffolds. These 3D bioprinted biodegradable scaffolds exhibit potential for future application as polymeric scaffolds in hard tissue engineering applications.

PCL-based 3D nanofibrous structure with well-designed morphology and enhanced specific surface area for tissue engineering application

Tissue engineering opens a new horizon for biological tissue replacement applications. Scaffolds, appropriate cells, and signaling induction are the main three determinant parameters in any tissue engineering applications. Designing a suitable scaffold which can mimic the cellular inherent and natural habitation is of great importance for cellular growth and proliferation. Just like a natural extracellular matrix (ECM), scaffolds provide the cells with an environment for performing biological functions. Accordingly, vast surface area and three-dimensional nanofibrous structures are among the pivotal characteristics of functional scaffolds in tissue engineering, and enhancement of their properties is the main purpose of the present research. In our previous study, a patterned structure composed of continuous nanofibers and microparticles was introduced. In this work, a new modification is applied for adjustment of the surface area of an electrospun/electrosprayed scaffold. For this purpose, at predetermined stages during electrospinning/electrospraying, the nitrogen gas is flushed through the mesh holes of the collector in the opposite direction of the jet movement. This method has led to the formation of very thin nanofibrous layers at nitrogen flush intervals by providing a cooling effect of the sweeping nitrogen. As a consequence, a straticulated structure has been fabricated which possesses extremely high surface/volume ratio. The porosity, water absorption, and morphological analysis were conducted on the obtained scaffold. In vitro cytocompatibility assessments as well as histological analysis demonstrated that the fabricated scaffold provides a proper substrate for cellular attachment, proliferation and infiltration. These findings can be advantageous in three-dimensional tissue engineering such as bone tissue engineering applications. Furthermore, according to the advanced microstructure and vast surface area of the fabricated samples, they can be applied in many other applications, such as membrane, filtration, etc.

TEGylated Double-Walled Carbon Nanotubes as Platforms to Engineer Neuronal Networks

In the past two decades, important results have been obtained on the application of carbon nanotubes (CNTs) as components of smart interfaces promoting neuronal growth and differentiation. Different forms of CNTs have been employed as scaffolds, including raw CNTs and functionalized CNTs, characterized by a different number of walls, mainly single-walled CNTs (SWCNTs) or multiwalled CNTs (MWCNTs). However, double-walled carbon nanotubes (DWCNTs), which present interesting electronic and transport properties, have barely been studied in the field. Apart from the electrical conductivity, the morphology, shape, porosity, and corresponding mechanical properties of the scaffold material are important parameters when dealing with neuronal cells. Thus, the presence of open porous and interconnected networks is essential for cell growth and differentiation. Here, we present an easy methodology to prepare porous self-standing and electrically conductive DWCNT-based scaffolds and study the growth of neuro/glial networks and their synaptic activity. A cross-linking approach with triethylene glycol (TEG) derivatives is applied to improve the tensile performance of the scaffolds while neuronal growth and differentiation are promoted. By testing different DWCNT-based constructs, we confirm that the manufactured structures guarantee a biocompatible scaffold, while favoring the design of artificial networks with high complexity.

Filamented Light (FLight) Biofabrication of Highly Aligned Tissue-Engineered Constructs

Cell-laden hydrogels used in tissue engineering generally lack sufficient 3D topographical guidance for cells to mature into aligned tissues. A new strategy called filamented light (FLight) biofabrication rapidly creates hydrogels composed of unidirectional microfilament networks, with diameters on the length scale of single cells. Due to optical modulation instability, a light beam is divided optically into FLight beams. Local polymerization of a photoactive resin is triggered, leading to local increase in refractive index, which itself creates self-focusing waveguides and further polymerization of photoresin into long hydrogel microfilaments. Diameter and spacing of the microfilaments can be tuned from 2 to 30 µm by changing the coherence length of the light beam. Microfilaments show outstanding cell instructive properties with fibroblasts, tenocytes, endothelial cells, and myoblasts, influencing cell alignment, nuclear deformation, and extracellular matrix deposition. FLight is compatible with multiple types of photoresins and allows for biofabrication of centimeter-scale hydrogel constructs with excellent cell viability within seconds (<10 s per construct). Multidirectional microfilaments are achievable within a single hydrogel construct by changing the direction of FLight projection, and complex multimaterial/multicellular tissue-engineered constructs are possible by sequentially exchanging the cell-laden photoresin. FLight offers a transformational approach to developing anisotropic tissues using photo-crosslinkable biomaterials.

The role of mechanobiology on the Schwann cell response: A tissue engineering perspective

Schwann cells (SCs), the glial cells of the peripheral nervous system (PNS), do not only form myelin sheaths thereby insulating the electrical signal propagated by the axons, but also play an essential role in the regeneration of injured axons. SCs are inextricably connected with their extracellular environment and the mechanical stimuli that are received determine their response during development, myelination and injuries. To this end, the mechanobiological response of SCs is being actively researched, as it can determine the suitability of fabricated scaffolds for tissue engineering and regenerative medicine applications. There is growing evidence that SCs are sensitive to changes in the mechanical properties of the surrounding environment (such as the type of material, its elasticity and stiffness), different topographical features provided by the environment, as well as shear stress. In this review, we explore how different mechanical stimuli affect SC behaviour and highlight the importance of exploring many different avenues when designing scaffolds for the repair of PNS injuries.

Traction of 3D and 4D Printing in the Healthcare Industry: From Drug Delivery and Analysis to Regenerative Medicine

Three-dimensional (3D) printing and 3D bioprinting are promising technologies for a broad range of healthcare applications from frontier regenerative medicine and tissue engineering therapies to pharmaceutical advancements yet must overcome the challenges of biocompatibility and resolution. Through comparison of traditional biofabrication methods with 3D (bio)printing, this review highlights the promise of 3D printing for the production of on-demand, personalized, and complex products that enhance the accessibility, effectiveness, and safety of drug therapies and delivery systems. In addition, this review describes the capacity of 3D bioprinting to fabricate patient-specific tissues and living cell systems (e.g., vascular networks, organs, muscles, and skeletal systems) as well as its applications in the delivery of cells and genes, microfluidics, and organ-on-chip constructs. This review summarizes how tailoring selected parameters (i.e., accurately selecting the appropriate printing method, materials, and printing parameters based on the desired application and behavior) can better facilitate the development of optimized 3D-printed products and how dynamic 4D-printed strategies (printing materials designed to change with time or stimulus) may be deployed to overcome many of the inherent limitations of conventional 3D-printed technologies. Comprehensive insights into a critical perspective of the future of 4D bioprinting, crucial requirements for 4D printing including the programmability of a material, multimaterial printing methods, and precise designs for meticulous transformations or even clinical applications are also given.

Decellularization of Full Heart—Optimizing the Classical Sodium-Dodecyl-Sulfate-Based Decellularization Protocol

Compared to cell therapy, where cells are injected into a defect region, the treatment of heart infarction with cells seeded in a vascularized scaffold bears advantages, such as an immediate nutrient supply or a controllable and persistent localization of cells. For this purpose, decellularized native tissues are a preferable choice as they provide an in vivo-like microenvironment. However, the quality of such scaffolds strongly depends on the decellularization process. Therefore, two protocols based on sodium dodecyl sulfate or sodium deoxycholate were tailored and optimized for the decellularization of a porcine heart. The obtained scaffolds were tested for their applicability to generate vascularized cardiac patches. Decellularization with sodium dodecyl sulfate was found to be more suitable and resulted in scaffolds with a low amount of DNA, a highly preserved extracellular matrix composition, and structure shown by GAG quantification and immunohistochemistry. After seeding human endothelial cells into the vasculature, a coagulation assay demonstrated the functionality of the endothelial cells to minimize the clotting of blood. Human-induced pluripotent-stem-cell-derived cardiomyocytes in co-culture with fibroblasts and mesenchymal stem cells transferred the scaffold into a vascularized cardiac patch spontaneously contracting with a frequency of 25.61 ± 5.99 beats/min for over 16 weeks. The customized decellularization protocol based on sodium dodecyl sulfate renders a step towards a preclinical evaluation of the scaffolds.

Regenerating the Injured Spinal Cord at the Chronic Phase by Engineered iPSCs-Derived 3D Neuronal Networks

Cell therapy using induced pluripotent stem cell-derived neurons is considered a promising approach to regenerate the injured spinal cord (SC). However, the scar formed at the chronic phase is not a permissive microenvironment for cell or biomaterial engraftment or for tissue assembly. Engineering of a functional human neuronal network is now reported by mimicking the embryonic development of the SC in a 3D dynamic biomaterial-based microenvironment. Throughout the in vitro cultivation stage, the system's components have a synergistic effect, providing appropriate cues for SC neurogenesis. While the initial biomaterial supported efficient cell differentiation in 3D, the cells remodeled it to provide an inductive microenvironment for the assembly of functional SC implants. The engineered tissues are characterized for morphology and function, and their therapeutic potential is investigated, revealing improved structural and functional outcomes after acute and chronic SC injuries. Such technology is envisioned to be translated to the clinic to rewire human injured SC.

An Update on Graphene-Based Nanomaterials for Neural Growth and Central Nervous System Regeneration

Thanks to their reduced size, great surface area, and capacity to interact with cells and tissues, nanomaterials present some attractive biological and chemical characteristics with potential uses in the field of biomedical applications. In this context, graphene and its chemical derivatives have been extensively used in many biomedical research areas from drug delivery to bioelectronics and tissue engineering. Graphene-based nanomaterials show excellent optical, mechanical, and biological properties. They can be used as a substrate in the field of tissue engineering due to their conductivity, allowing to study, and educate neural connections, and guide neural growth and differentiation; thus, graphene-based nanomaterials represent an emerging aspect in regenerative medicine. Moreover, there is now an urgent need to develop multifunctional and functionalized nanomaterials able to arrive at neuronal cells through the blood-brain barrier, to manage a specific drug delivery system. In this review, we will focus on the recent applications of graphene-based nanomaterials in vitro and in vivo, also combining graphene with other smart materials to achieve the best benefits in the fields of nervous tissue engineering and neural regenerative medicine. We will then highlight the potential use of these graphene-based materials to construct graphene 3D scaffolds able to stimulate neural growth and regeneration in vivo for clinical applications.

Human Neural Stem Cells for Cell-Based Medicinal Products

Neural stem cells represent an attractive tool for the development of regenerative therapies and are being tested in clinical trials for several neurological disorders. Human neural stem cells can be isolated from the central nervous system or can be derived in vitro from pluripotent stem cells. Embryonic sources are ethically controversial and other sources are less well characterized and/or inefficient. Recently, isolation of NSC from the cerebrospinal fluid of patients with spina bifida and with intracerebroventricular hemorrhage has been reported. Direct reprogramming may become another alternative if genetic and phenotypic stability of the reprogrammed cells is ensured. Here, we discuss the advantages and disadvantages of available sources of neural stem cells for the production of cell-based therapies for clinical applications. We review available safety and efficacy clinical data and discuss scalability and quality control considerations for manufacturing clinical grade cell products for successful clinical application.

Highly Ordered 3D Tissue Engineering Scaffolds as a Versatile Culture Platform for Nerve Cells Growth

Tissue engineering scaffolds provide an encouraging alternative for nerve injuries due to their biological support for nerve cell growth, which can be used for neuronal repair. Nerve cells have been reported to be mostly cultured on 2D scaffolds that cannot mimic the native extracellular matrix. Herein, highly ordered 3D scaffolds are fabricated for nerve cell culture by melt electrospinning writing, the microstructures and geometries of the scaffolds could be well modulated. An effective strategy for scaffold surface modification to promote nerve cell growth is proposed. The effects of scaffolds with different surface modifications, viz., plasma treatment, single poly-D-lysine (PDL) coating after plasma treatment, single laminin (LM) coating after plasma treatment, double PDL and LM coatings after plasma treatment, on PC12 cell growth are evaluated. Experiments show the scaffold modified with double PDL and LM coatings after plasma treatment facilitated the growth of PC12 cells most effectively, indicating the synergistic effect of PDL and LM on the growth of nerve cells. This is the first systematic and quantitative study of the effects of different scaffold surface modifications on nerve cell growth. The above results provide a versatile culture platform for growing nerve cells, and for recovery from peripheral nerve injury.