Fabrication of a Highly Aligned Neural Scaffold via a Table Top Stereolithography 3D Printing and Electrospinning<sup/>

Evaluation of Random and Aligned Polycaprolactone Nanofibrous Electrospun Scaffold for Human Periodontal Ligament Engineering in Biohybrid Titanium Implants

Background: Stem cells are introduced to regenerate some living tissue to restore function and longevity. The study aims to isolate in vitro human periodontal ligament stem cells (hPDLSCs) and investigate their proliferation rate on plasma-treated aligned and random polycaprolactone (PCL) nanofibrous scaffolds made via an electrospinning technique to attempt periodontal-like tissue in dental implants. Materials and Methods: hPDLSCs were isolated from extracted human premolars and cultured on plasma-treated or untreated PCL-aligned and random scaffolds to enhance adhesion of periodontal ligament (PDL) cells as well as interaction and proliferation. Cell morphology, adhesion, and proliferation rate were evaluated using field emission scanning electron microscopy (FESEM) and the methyl tetrazolium (MTT) assay. The wettability of PCL scaffolds was tested using a goniometer. Results: The hydrophilicity of plasma-treated scaffolds was significantly increased (p ≤ 0.05) in both aligned and random nanofibers compared to the nontreated nanofibrous scaffold. Cells arranged in different directions on the random nanofiber scaffold, while for aligned scaffold nanofibers, the cells were arranged in a pattern that followed the direction of the aligned electrospun nanofibres. The rate of hPDLSC proliferation on an aligned PCL nanofiber scaffold was significantly higher than on a random PCL nanofibrous scaffold with a continuous, well-arranged monolayer of cells, as shown in FESEM. Conclusion: The aligned PCL nanofiber scaffold is superior to random PCL when used as an artificial scaffold for hPDLSC regeneration in PDL tissue engineering applications.

Biomimetic design and integrated biofabrication of an in-vitro three-dimensional multi-scale multilayer cortical model

The lack of accurate and reliable in vitro brain models hinders the development of brain science and research on brain diseases. Owing to the complex structure of the brain tissue and its highly nonlinear characteristics, the construction of brain-like in vitro tissue models remains one of the most challenging research fields in the construction of living tissues. This study proposes a multi-scale design of a brain-like model with a biomimetic cortical structure, which includes the macroscopic structural features of six layers of different cellular components, as well as micrometer-scale continuous fiber structures running through all layers vertically. To achieve integrated biomanufacturing of such a complex multi-scale brain-like model, a multi-material composite printing/culturing integrated bioprinting platform was developed in-house by integrating cell-laden hydrogel ink direct writing printing and electrohydrodynamic fiber 3D printing technologies. Through integrated bioprinting, multi-scale models with different cellular components and fiber structural parameters were prepared to study the effects of macroscopic and microscopic structural features on the directionality of neural cells, as well as the interaction between glial cells and neurons within the tissue model in a three-dimensional manner. The results revealed that the manufactured in vitro biomimetic cortical model achieved morphological connections between the layers of neurons, reflecting the structure and cellular morphology of the natural cortex. Micrometer-scale (10 μm) cross-layer fibers effectively guided and controlled the extension length and direction of the neurites of surrounding neural cells but had no significant effect on the migration of neurons. In contrast, glial cells significantly promoted the migration of surrounding PC12 cells towards the glial layer but did not contribute to the extension of neurites. This study provides a basis for the design and manufacture of accurate brain-like models for the functionalization of neuronal tissues.

Evaluating the efficacy of uniformly designed square mesh resin 3D printed scaffolds in directing the orientation of electrospun PCL nanofibers

Replicating the architecture of extracellular matrices (ECM) is crucial in tissue engineering to support tissues' natural structure and functionality. The ECM's structure plays a significant role in directing cell alignment. Electrospinning is an effective technique for fabricating nanofibrous substrates that mimic the architecture of extracellular matrices (ECM). This study aims to evaluate the efficacy of resin 3D-printed scaffolds made from a low-conductivity material (i.e., a resin composed of methacrylated oligomers, monomers, and photoinitiators) in directing the alignment of electrospun polycaprolactone (PCL) nanofibers. Six 3D-printed scaffolds were fabricated using stereolithography (SLA) technology and strategically positioned on an aluminum foil collector plate during electrospinning. The structured geometry of the scaffolds, rather than the local electric field distribution, is hypothesized to guide nanofiber alignment. Images acquired through the scanning electron microscopy (SEM) were used to analyze and statistically quantify the nanofibrous scaffolds to evaluate the alignment of nanofibers over the scaffolds compared to a set of randomly deposited control nanofiber samples in the absence of the 3D printed scaffolds. SEM images also showed significant alignment of nanofibers within the pores of scaffolds, using histograms as a means for indicating the distribution of orientation angles. Statistical analysis revealed that this distribution deviates from normality due to the deviations in the tails and the existence of relatively smaller peaks at angles relative to 0°, particularly within a range of ± 50° and ± 40°. It is further found that the average peak orientation angle relative to 0° had a maximum probability of 0.014. Furthermore, the statistical analysis confirmed the distribution and significant differences in orientation between test samples with 3D-printed scaffolds and control samples. These findings demonstrate the effectiveness of resin 3D-printed scaffolds, particularly their geometric filtering effect, leading to controlled nanofiber alignment, which is proposed to be beneficial for enhancing cell adhesion, proliferation, and cell migration in tissue engineering applications.

4D Printing for Biomedical Applications

4D (bio-)printing endows 3D printed (bio-)materials with multiple functionalities and dynamic properties. 4D printed materials have been recently used in biomedical engineering for the design and fabrication of biomedical devices, such as stents, occluders, microneedles, smart 3D-cell engineered microenvironments, drug delivery systems, wound closures, and implantable medical devices. However, the success of 4D printing relies on the rational design of 4D printed objects, the selection of smart materials, and the availability of appropriate types of external (multi-)stimuli. Here, this work first highlights the different types of smart materials, external stimuli, and design strategies used in 4D (bio-)printing. Then, it presents a critical review of the biomedical applications of 4D printing and discusses the future directions of biomedical research in this exciting area, including in vivo tissue regeneration studies, the implementation of multiple materials with reversible shape memory behaviors, the creation of fast shape-transformation responses, the ability to operate at the microscale, untethered activation and control, and the application of (machine learning-based) modeling approaches to predict the structure-property and design-shape transformation relationships of 4D (bio)printed constructs.

Manufacturing 3D Biomimetic Tissue: A Strategy Involving the Integration of Electrospun Nanofibers with a 3D-Printed Framework for Enhanced Tissue Regeneration

3D printing and electrospinning are versatile techniques employed to produce 3D structures, such as scaffolds and ultrathin fibers, facilitating the creation of a cellular microenvironment in vitro. These two approaches operate on distinct working principles and utilize different polymeric materials to generate the desired structure. This review provides an extensive overview of these techniques and their potential roles in biomedical applications. Despite their potential role in fabricating complex structures, each technique has its own limitations. Electrospun fibers may have ambiguous geometry, while 3D-printed constructs may exhibit poor resolution with limited mechanical complexity. Consequently, the integration of electrospinning and 3D-printing methods may be explored to maximize the benefits and overcome the individual limitations of these techniques. This review highlights recent advancements in combined techniques for generating structures with controlled porosities on the micro-nano scale, leading to improved mechanical structural integrity. Collectively, these techniques also allow the fabrication of nature-inspired structures, contributing to a paradigm shift in research and technology. Finally, the review concludes by examining the advantages, disadvantages, and future outlooks of existing technologies in addressing challenges and exploring potential opportunities.

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.

Applications, advancements, and challenges of 3D bioprinting in organ transplantation

To date, organ transplantation remains an effective method for treating end-stage diseases of various organs. In recent years, despite the continuous development of organ transplantation technology, a variety of problems restricting its progress have emerged one after another, and the shortage of donors is at the top of the list. Bioprinting is a very useful tool that has huge application potential in many fields of life science and biotechnology, among which its use in medicine occupies a large area. With the development of bioprinting, advances in medicine have focused on printing cells and tissues for tissue regeneration and reconstruction of viable human organs, such as the heart, kidneys, and bones. In recent years, with the development of organ transplantation, three-dimensional (3D) bioprinting has played an increasingly important role in this field, giving rise to many unsolved problems, including a shortage of organ donors. This review respectively introduces the development of 3D bioprinting as well as its working principles and main applications in the medical field, especially in the applications, and advancements and challenges of 3D bioprinting in organ transplantation. With the continuous update and progress of printing technology and its deeper integration with the medical field, many obstacles will have new solutions, including tissue repair and regeneration, organ reconstruction, etc., especially in the field of organ transplantation. 3D printing technology will provide a better solution to the problem of donor shortage.

Nanoparticle Assembly: From Self-Organization to Controlled Micropatterning for Enhanced Functionalities

Nanoparticles form long-range micropatterns via self-assembly or directed self-assembly with superior mechanical, electrical, optical, magnetic, chemical, and other functional properties for broad applications, such as structural supports, thermal exchangers, optoelectronics, microelectronics, and robotics. The precisely defined particle assembly at the nanoscale with simultaneously scalable patterning at the microscale is indispensable for enabling functionality and improving the performance of devices. This article provides a comprehensive review of nanoparticle assembly formed primarily via the balance of forces at the nanoscale (e.g., van der Waals, colloidal, capillary, convection, and chemical forces) and nanoparticle-template interactions (e.g., physical confinement, chemical functionalization, additive layer-upon-layer). The review commences with a general overview of nanoparticle self-assembly, with the state-of-the-art literature review and motivation. It subsequently reviews the recent progress in nanoparticle assembly without the presence of surface templates. Manufacturing techniques for surface template fabrication and their influence on nanoparticle assembly efficiency and effectiveness are then explored. The primary focus is the spatial organization and orientational preference of nanoparticles on non-templated and pre-templated surfaces in a controlled manner. Moreover, the article discusses broad applications of micropatterned surfaces, encompassing various fields. Finally, the review concludes with a summary of manufacturing methods, their limitations, and future trends in nanoparticle assembly.

Convergence of 3D Bioprinting and Nanotechnology in Tissue Engineering Scaffolds

Three-dimensional (3D) bioprinting has emerged as a promising scaffold fabrication strategy for tissue engineering with excellent control over scaffold geometry and microstructure. Nanobiomaterials as bioinks play a key role in manipulating the cellular microenvironment to alter its growth and development. This review first introduces the commonly used nanomaterials in tissue engineering scaffolds, including natural polymers, synthetic polymers, and polymer derivatives, and reveals the improvement of nanomaterials on scaffold performance. Second, the 3D bioprinting technologies of inkjet-based bioprinting, extrusion-based bioprinting, laser-assisted bioprinting, and stereolithography bioprinting are comprehensively itemized, and the advantages and underlying mechanisms are revealed. Then the convergence of 3D bioprinting and nanotechnology applications in tissue engineering scaffolds, such as bone, nerve, blood vessel, tendon, and internal organs, are discussed. Finally, the challenges and perspectives of convergence of 3D bioprinting and nanotechnology are proposed. This review will provide scientific guidance to develop 3D bioprinting tissue engineering scaffolds by nanotechnology.

Effects of surface patterning and topography on the cellular functions of tissue engineered scaffolds with special reference to 3D bioprinting

The extracellular matrix (ECM) of the tissue organ exhibits a topography from the nano to micrometer range, and the design of scaffolds has been inspired by the host environment. Modern bioprinting aims to replicate the host tissue environment to mimic the native physiological functions. A detailed discussion on the topographical features controlling cell attachment, proliferation, migration, differentiation, and the effect of geometrical design on the wettability and mechanical properties of the scaffold are presented in this review. Moreover, geometrical pattern-mediated stiffness and pore arrangement variations for guiding cell functions have also been discussed. This review also covers the application of designed patterns, gradients, or topographic modulation on 3D bioprinted structures in fabricating the anisotropic features. Finally, this review accounts for the tissue-specific requirements that can be adopted for topography-motivated enhancement of cellular functions during the fabrication process with a special thrust on bioprinting.

Manufacture of Soft-Hard Implants from Electrospun Filaments Embedded in 3D Printed Structures

Rotator cuff tendon tears are common injuries of the musculoskeletal system that often require surgical repair. However, re-tearing following repair is a significant clinical problem, with a failure rate of up to 40%, notably at the transition from bone to tendon. The development of biphasic materials consisting of soft and hard components, which can mimic this interface, is therefore promising. Here, a simple manufacturing approach is proposed that combines electrospun filaments and 3D printing to achieve scaffolds made of a soft polydioxanone cuff embedded in a porous polycaprolactone block. The insertion area of the cuff is based on the supraspinatus tendon footprint and the size of the cuff is scaled up from 9 to 270 electrospun filaments to reach a clinically relevant strength of 227N on average. The biological evaluation shows that the biphasic scaffold components are noncytotoxic, and that tendon and bone cells can be grown on the cuff and block, respectively. Overall, these results indicate that combining electrospinning and 3D printing is a feasible and promising approach to create soft-to-hard biphasic scaffolds that can improve the outcomes of rotator cuff repair.

Bone regeneration materials and their application over 20 years: A bibliometric study and systematic review

Bone regeneration materials (BRMs) bring us new sights into the clinical management bone defects. With advances in BRMs technologies, new strategies are emerging to promote bone regeneration. The aim of this study was to comprehensively assess the existing research and recent progress on BRMs, thus providing useful insights into contemporary research, as well as to explore potential future directions within the scope of bone regeneration therapy. A comprehensive literature review using formal data mining procedures was performed to explore the global trends of selected areas of research for the past 20 years. The study applied bibliometric methods and knowledge visualization techniques to identify and investigate publications based on the publication year (between 2002 and 2021), document type, language, country, institution, author, journal, keywords, and citation number. The most productive countries were China, United States, and Italy. The most prolific journal in the BRM field was Acta Biomaterialia, closely followed by Biomaterials. Moreover, recent investigations have been focused on extracellular matrices (ECMs) (370 publications), hydrogel materials (286 publications), and drug delivery systems (220 publications). Research hotspots related to BRMs and extracellular matrices from 2002 to 2011 were growth factor, bone morphogenetic protein (BMP)-2, and mesenchymal stem cell (MSC), whereas after 2012 were composite scaffolds. Between 2002 and 2011, studies related to BRMs and hydrogels were focused on BMP-2, in vivo, and in vitro investigations, whereas it turned to the exploration of MSCs, mechanical properties, and osteogenic differentiation after 2012. Research hotspots related to BRM and drug delivery were fibroblast growth factor, mesoporous materials, and controlled release during 2002–2011, and electrospinning, antibacterial activity, and in vitro bioactivity after 2012. Overall, composite scaffolds, 3D printing technology, and antibacterial activity were found to have an important intersection within BRM investigations, representing relevant research fields for the future. Taken together, this extensive analysis highlights the existing literature and findings that advance scientific insights into bone tissue engineering and its subsequent applications.

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

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

Recent progresses in novel in vitro models of primary neurons: A biomaterial perspective

Central nervous system (CNS) diseases have been a growing threat to the health of humanity, emphasizing the urgent need of exploring the pathogenesis and therapeutic approaches of various CNS diseases. Primary neurons are directly obtained from animals or humans, which have wide applications including disease modeling, mechanism exploration and drug development. However, traditional two-dimensional (2D) monoculture cannot resemble the native microenvironment of CNS. With the increasing understanding of the complexity of the CNS and the remarkable development of novel biomaterials, in vitro models have experienced great innovation from 2D monoculture toward three-dimensional (3D) multicellular culture. The scope of this review includes the progress of various in vitro models of primary neurons in recent years to provide a holistic view of the modalities and applications of primary neuron models and how they have been connected with the revolution of biofabrication techniques. Special attention has been paid to the interaction between primary neurons and biomaterials. First, a brief introduction on the history of CNS modeling and primary neuron culture was conducted. Next, detailed progress in novel in vitro models were discussed ranging from 2D culture, ex vivo model, spheroid, scaffold-based model, 3D bioprinting model, and microfluidic chip. Modalities, applications, advantages, and limitations of the aforementioned models were described separately. Finally, we explored future prospects, providing new insights into how basic science research methodologies have advanced our understanding of the CNS, and highlighted some future directions of primary neuron culture in the next few decades.

Chitosan-based high-strength supramolecular hydrogels for 3D bioprinting

The loss of tissues and organs is a major challenge for biomedicine, and the emerging 3D bioprinting technology has brought the dawn for the development of tissue engineering and regenerative medicine. Chitosan-based supramolecular hydrogels, as novel biomaterials, are considered as ideal materials for 3D bioprinting due to their unique dynamic reversibility and fantastic biological properties. Although chitosan-based supramolecular hydrogels have wonderful biological properties, the mechanical properties are still under early exploration. This paper aims to provide some inspirations for researchers to further explore. In this review, common 3D bioprinting techniques and the properties required for bioink for 3D bioprinting are firstly described. Then, several strategies to enhance the mechanical properties of chitosan hydrogels are introduced from the perspectives of both materials and supramolecular binding motifs. Finally, current challenges and future opportunities in this field are discussed. The combination of chitosan-based supramolecular hydrogels and 3D bioprinting will hold promise for developing novel biomedical implants.

Development and Application of Three-Dimensional Bioprinting Scaffold in the Repair of Spinal Cord Injury

Spinal cord injury (SCI) is a disabling and destructive central nervous system injury that has not yet been successfully treated at this stage. Three-dimensional (3D) bioprinting has become a promising method to produce more biologically complex microstructures, which fabricate living neural constructs with anatomically accurate complex geometries and spatial distributions of neural stem cells, and this is critical in the treatment of SCI. With the development of 3D printing technology and the deepening of research, neural tissue engineering research using different printing methods, bio-inks, and cells to repair SCI has achieved certain results. Although satisfactory results have not yet been achieved, they have provided novel ideas for the clinical treatment of SCI. Considering the potential impact of 3D bioprinting technology on neural studies, this review focuses on 3D bioprinting methods widely used in SCI neural tissue engineering, and the latest technological applications of bioprinting of nerve tissues for the repair of SCI are discussed. In addition to introducing the recent progress, this work also describes the existing limitations and highlights emerging possibilities and future prospects in this field.

Emerging 4D Printing Strategies for Next-Generation Tissue Regeneration and Medical Devices

The rapid development of 3D printing has led to considerable progress in the field of biomedical engineering. Notably, 4D printing provides a potential strategy to achieve a time-dependent physical change within tissue scaffolds or replicate the dynamic biological behaviors of native tissues for smart tissue regeneration and the fabrication of medical devices. The fabricated stimulus-responsive structures can offer dynamic, reprogrammable deformation or actuation to mimic complex physical, biochemical, and mechanical processes of native tissues. Although there is notable progress made in the development of the 4D printing approach for various biomedical applications, its more broad-scale adoption for clinical use and tissue engineering purposes is complicated by a notable limitation of printable smart materials and the simplistic nature of achievable responses possible with current sources of stimulation. In this review, the recent progress made in the field of 4D printing by discussing the various printing mechanisms that are achieved with great emphasis on smart ink mechanisms of 4D actuation, construct structural design, and printing technologies, is highlighted. Recent 4D printing studies which focus on the applications of tissue/organ regeneration and medical devices are then summarized. Finally, the current challenges and future perspectives of 4D printing are also discussed.

Characterization of interconnectivity of gelatin methacrylate hydrogels using photoacoustic imaging

Hydrogels can provide a three-dimensional microenvironment for cells and thus serve as an extracellular matrix in a biofabrication process. The properties of hydrogels, such as their porosity and mechanical properties, significantly influence the cell growth. However, there is still a lack of effective methods for characterizing the hydrogel structure noninvasively. Herein, a photoacoustic (PA) imaging-based method is proposed for the characterization of gelatin methacrylate (GelMA) hydrogels. Owing to their high PA contrast, red blood cells (RBCs) are included as mediators in the GelMA hydrogel to analyze its pore distribution. The interconnectivity of the pores is further analyzed through the lysis of RBCs. The diffusion of the RBC lysis buffer in the GelMA is consistent with the trend observed in simulations. The analyzed vitality of HEK293 cells in different GelMA hydrogels reveals that understanding the diffusion of solutes (i.e., nutrients) is a potential strategy to optimize the hydrogel parameters during biofabrication.

Tissue Engineered Neurovascularization Strategies for Craniofacial Tissue Regeneration

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

Directing Axonal Growth: A Review on the Fabrication of Fibrous Scaffolds That Promotes the Orientation of Axons

Tissues are commonly defined as groups of cells that have similar structure and uniformly perform a specialized function. A lesser-known fact is that the placement of these cells within these tissues plays an important role in executing its functions, especially for neuronal cells. Hence, the design of a functional neural scaffold has to mirror these cell organizations, which are brought about by the configuration of natural extracellular matrix (ECM) structural proteins. In this review, we will briefly discuss the various characteristics considered when making neural scaffolds. We will then focus on the cellular orientation and axonal alignment of neural cells within their ECM and elaborate on the mechanisms involved in this process. A better understanding of these mechanisms could shed more light onto the rationale of fabricating the scaffolds for this specific functionality. Finally, we will discuss the scaffolds used in neural tissue engineering (NTE) and the methods used to fabricate these well-defined constructs.

Fabrication of a 3D Printed PCL Nerve Guide: In Vitro and In Vivo Testing

Nerve guides are medical devices designed to guide proximal and distal ends of injured peripheral nerves in order to assist regeneration of the damaged nerves. A 3D-printed polycaprolactone (PCL) nerve guide using an aligned gelatin-poly(3-hydroxybutyrate-co-3-hydroxyvalerate) electrospun mat, seeded with PC12 and Schwann cells (SCs) is produced. During characterization with microCT and SEM porosity (55%), pore sizes (675 ± 40 µm), and fiber diameters (382 ± 25 µm) are determined. Electrospun fibers have degree of alignment of 7°, indicating high potential for guidance. On Day 14, PC12 cells migrated from proximal to distal end of nerve guide when SCs are seeded on the guide. After 28 days, over 95% of PC12 are alive and aligned. PC12 cells express early differentiation marker beta-tubulin 10 times more than late marker NeuN. In a 10 mm rat sciatic nerve injury, functional recovery evaluated by using static sciatic index (SSI) is observed in mat-free guides and guides containing mat and SCs. Nerve conduction velocities are also improved in these groups. Histological stainings showed tissue growth around nerve guides with highest new tissue organization being observed with mat and cell-free guides. These suggest 3D-printed PCL nerve guides have significant potential for treatment of peripheral nerve injuries.

Comprehensive Review on Design and Manufacturing of Bio-scaffolds for Bone Reconstruction

Bio-scaffolds are synthetic entities widely employed in bone and soft-tissue regeneration applications. These bio-scaffolds are applied to the defect site to provide support and favor cell attachment and growth, thereby enhancing the regeneration of the defective site. The progressive research in bio-scaffold fabrication has led to identification of biocompatible and mechanically stable materials. The difficulties in obtaining grafts and expenditure incurred in the transplantation procedures have also been overcome by the implantation of bio-scaffolds. Drugs, cells, growth factors, and biomolecules can be embedded with bio-scaffolds to provide localized treatments. The right choice of materials and fabrication approaches can help in developing bio-scaffolds with required properties. This review mostly focuses on the available materials and bio-scaffold techniques for bone and soft-tissue regeneration application. The first part of this review gives insight into the various classes of biomaterials involved in bio-scaffold fabrication followed by design and simulation techniques. The latter discusses the various additive, subtractive, hybrid, and other improved techniques involved in the development of bio-scaffolds for bone regeneration applications. Techniques involving multimaterial printing and multidimensional printing have also been briefly discussed.

3D Cell Culture Systems: Tumor Application, Advantages, and Disadvantages

The traditional two-dimensional (2D) in vitro cell culture system (on a flat support) has long been used in cancer research. However, this system cannot be fully translated into clinical trials to ideally represent physiological conditions. This culture cannot mimic the natural tumor microenvironment due to the lack of cellular communication (cell-cell) and interaction (cell-cell and cell-matrix). To overcome these limitations, three-dimensional (3D) culture systems are increasingly developed in research and have become essential for tumor research, tissue engineering, and basic biology research. 3D culture has received much attention in the field of biomedicine due to its ability to mimic tissue structure and function. The 3D matrix presents a highly dynamic framework where its components are deposited, degraded, or modified to delineate functions and provide a platform where cells attach to perform their specific functions, including adhesion, proliferation, communication, and apoptosis. So far, various types of models belong to this culture: either the culture based on natural or synthetic adherent matrices used to design 3D scaffolds as biomaterials to form a 3D matrix or based on non-adherent and/or matrix-free matrices to form the spheroids. In this review, we first summarize a comparison between 2D and 3D cultures. Then, we focus on the different components of the natural extracellular matrix that can be used as supports in 3D culture. Then we detail different types of natural supports such as matrigel, hydrogels, hard supports, and different synthetic strategies of 3D matrices such as lyophilization, electrospiding, stereolithography, microfluid by citing the advantages and disadvantages of each of them. Finally, we summarize the different methods of generating normal and tumor spheroids, citing their respective advantages and disadvantages in order to obtain an ideal 3D model (matrix) that retains the following characteristics: better biocompatibility, good mechanical properties corresponding to the tumor tissue, degradability, controllable microstructure and chemical components like the tumor tissue, favorable nutrient exchange and easy separation of the cells from the matrix.

Candidate Bioinks for Extrusion 3D Bioprinting-A Systematic Review of the Literature

Purpose: Bioprinting is becoming an increasingly popular platform technology for engineering a variety of tissue types. Our aim was to identify biomaterials that have been found to be suitable for extrusion 3D bioprinting, outline their biomechanical properties and biocompatibility towards their application for bioprinting specific tissue types. This systematic review provides an in-depth overview of current biomaterials suitable for extrusion to aid bioink selection for specific research purposes and facilitate design of novel tailored bioinks.

Methods: A systematic search was performed on EMBASE, PubMed, Scopus and Web of Science databases according to the PRISMA guidelines. References of relevant articles, between December 2006 to January 2018, on candidate bioinks used in extrusion 3D bioprinting were reviewed by two independent investigators against standardised inclusion and exclusion criteria. Data was extracted on bioprinter brand and model, printing technique and specifications (speed and resolution), bioink material and class of mechanical assessment, cell type, viability, and target tissue. Also noted were authors, study design (in vitro/in vivo), study duration and year of publication.

Results: A total of 9,720 studies were identified, 123 of which met inclusion criteria, consisting of a total of 58 reports using natural biomaterials, 26 using synthetic biomaterials and 39 using a combination of biomaterials as bioinks. Alginate (n = 50) and PCL (n = 33) were the most commonly used bioinks, followed by gelatin (n = 18) and methacrylated gelatin (GelMA) (n = 16). Pneumatic extrusion bioprinting techniques were the most common (n = 78), followed by piston (n = 28). The majority of studies focus on the target tissue, most commonly bone and cartilage, and investigate only one bioink rather than assessing a range to identify those with the most promising printability and biocompatibility characteristics. The Bioscaffolder (GeSiM, Germany), 3D Discovery (regenHU, Switzerland), and Bioplotter (EnvisionTEC, Germany) were the most commonly used commercial bioprinters (n = 35 in total), but groups most often opted to create their own in-house devices (n = 20). Many studies also failed to specify whether the mechanical data reflected pre-, during or post-printing, pre- or post-crosslinking and with or without cells.

Conclusions: Despite the continued increase in the variety of biocompatible synthetic materials available, there has been a shift change towards using natural rather than synthetic bioinks for extrusion bioprinting, dominated by alginate either alone or in combination with other biomaterials. On qualitative analysis, no link was demonstrated between the type of bioink or extrusion technique and the target tissue, indicating that bioprinting research is in its infancy with no established tissue specific bioinks or bioprinting techniques. Further research is needed on side-by-side characterisation of bioinks with standardisation of the type and timing of biomechanical assessment.

Simultaneous Modeling of Young's Modulus, Yield Stress, and Rupture Strain of Gelatin/Cellulose Acetate Microfibrous/Nanofibrous Scaffolds Using RSM

Electrospinning is a promising method to fabricate bioengineered scaffolds, thanks to utilizing various types of biopolymers, flexible structures, and also the diversity of output properties. Mechanical properties are one of the major components of scaffold design to fabricate an efficacious artificial substitute for the natural extracellular matrix. Additionally, fiber orientations, as one of the scaffold structural parameters, could play a crucial role in the application of fabricated fibrous scaffolds. In this study, gelatin was used as a highly biocompatible polymer in blend with cellulose acetate (CA), a polysaccharide, to enhance the achievable range of mechanical characteristics to fabricated fibrous electrospun scaffolds. By altering input variables, such as polymers concentration, weight ratio, and mandrel rotation speed, scaffolds with various mechanical and morphological properties could be achieved. As expected, the electrospun scaffold with a higher mandrel rotation speed shows higher fiber alignment. A wide range of mechanical properties were gained through different values of polymer ratio and total concentration. A general improvement in mechanical strength was observed by increasing the concentration and CA content in the solution, but contradictory effects, such as high viscosity in more concentrated solutions, influenced the mechanical characteristics as well. A response surface method was applied on experimental results in order to describe a continuous variation of Young's modulus, yield stress, and strain at rupture. A full quadratic version of equations with the 95% confidence level was applied for the response modeling. This model would be an aid for engineers to adjust mandrel rotation speed, solution concentration, and gelatin/CA ratio to achieve desired mechanical and structural properties.

3D Bioprinting of Neural Tissues

The human nervous system is a remarkably complex physiological network that is inherently challenging to study because of obstacles to acquiring primary samples. Animal models offer powerful alternatives to study nervous system development, diseases, and regenerative processes, however, they are unable to address some species-specific features of the human nervous system. In vitro models of the human nervous system have expanded in prevalence and sophistication, but still require further advances to better recapitulate microenvironmental and cellular features. The field of neural tissue engineering (TE) is rapidly adopting new technologies that enable scientists to precisely control in vitro culture conditions and to better model nervous system formation, function, and repair. 3D bioprinting is one of the major TE technologies that utilizes biocompatible hydrogels to create precisely patterned scaffolds, designed to enhance cellular responses. This review focuses on the applications of 3D bioprinting in the field of neural TE. Important design parameters are considered when bioprinting neural stem cells are discussed. The emergence of various bioprinted in vitro platforms are also reviewed for developmental and disease modeling and drug screening applications within the central and peripheral nervous systems, as well as their use as implants for in vivo regenerative therapies.

3D printing of functional nerve guide conduits

Nerve guide conduits (NGCs), as alternatives to nerve autografts and allografts, have been widely explored as an advanced tool for the treatment of peripheral nerve injury. However, the repairing efficiency of NGCs still needs significant improvements. Functional NGCs that provide a more favorable microenvironment for promoting axonal elongation and myelination are of great importance. In recent years, 3D printing technologies have been widely applied in the fabrication of customized and complex constructs, exhibiting great potential for tissue engineering applications, especially for the construction of functional NGCs. In this review, we introduce the 3D printing technologies for manufacturing functional NGCs, including inkjet printing, extrusion printing, stereolithography-based printing and indirect printing. Further, we summarize the current methods and strategies for constructing functional NGCs, such as designing special conduit architectures, using appropriate materials and co-printing with different biological cues. Finally, the challenges and prospects for construction of functional NGCs are also presented.

Modulating the foreign body response of implants for diabetes treatment

Diabetes Mellitus is a group of diseases characterized by high blood glucose levels due to patients' inability to produce sufficient insulin. Current interventions often require implants that can detect and correct high blood glucose levels with minimal patient intervention. However, these implantable technologies have not reached their full potential in vivo due to the foreign body response and subsequent development of fibrosis. Therefore, for long-term function of implants, modulating the initial immune response is crucial in preventing the activation and progression of the immune cascade. This review discusses the different molecular mechanisms and cellular interactions involved in the activation and progression of foreign body response (FBR) and fibrosis, specifically for implants used in diabetes. We also highlight the various strategies and techniques that have been used for immunomodulation and prevention of fibrosis. We investigate how these general strategies have been applied to implants used for the treatment of diabetes, offering insights on how these devices can be further modified to circumvent FBR and fibrosis.

Biomimicry in 3D printing design: implications for peripheral nerve regeneration

Nerve guide conduits (NGCs) connect dissected nerve stumps and effectively repair short-range peripheral nerve defects. However, for long-range defects, autografts show better therapeutic effects, despite intrinsic limitations. Recent evidence shows that biomimetic design is essential for high-performance NGCs, and 3D printing is a promising fabricating technique. The current work includes a brief review of the challenges for peripheral nerve regeneration. The authors propose a potential solution using biomimetic 3D-printed NGCs as alternative therapies. The assessment of biomimetic designs includes microarchitecture, mechanical property, electrical conductivity and biologics inclusion. The applications of 3D printing in preparing NGCs and present strategies to improve therapeutic effects are also discussed.

3D Electrospun Nanofiber-Based Scaffolds: From Preparations and Properties to Tissue Regeneration Applications

Electrospun nanofibers have been frequently used for tissue engineering due to their morphological similarities with the extracellular matrix (ECM) and tunable chemical and physical properties for regulating cell behaviors and functions. However, most of the existing electrospun nanofibers have a closely packed two-dimensional (2D) membrane with the intrinsic shortcomings of limited cellular infiltration, restricted nutrition diffusion, and unsatisfied thickness. Three-dimensional (3D) electrospun nanofiber-based scaffolds can provide stem cells with 3D microenvironments and biomimetic fibrous structures. Thus, they have been demonstrated to be good candidates for in vivo repair of different tissues. This review summarizes the recent developments in 3D electrospun nanofiber-based scaffolds (ENF-S) for tissue engineering. Three types of 3D ENF-S fabricated using different approaches classified into electrospun nanofiber 3D scaffolds, electrospun nanofiber/hydrogel composite 3D scaffolds, and electrospun nanofiber/porous matrix composite 3D scaffolds are discussed. New functions for these 3D ENF-S and properties, such as facilitated cell infiltration, 3D fibrous architecture, enhanced mechanical properties, and tunable degradability, meeting the requirements of tissue engineering scaffolds were discovered. The applications of 3D ENF-S in cartilage, bone, tendon, ligament, skeletal muscle, nerve, and cardiac tissue regeneration are then presented with a discussion of current challenges and future directions. Finally, we give summaries and future perspectives of 3D ENF-S in tissue engineering and clinical transformation.

Engineering in vitro human neural tissue analogs by 3D bioprinting and electrostimulation

There is a fundamental need for clinically relevant, reproducible, and standardized in vitro human neural tissue models, not least of all to study heterogenic and complex human-specific neurological (such as neuropsychiatric) disorders. Construction of three-dimensional (3D) bioprinted neural tissues from native human-derived stem cells (e.g., neural stem cells) and human pluripotent stem cells (e.g., induced pluripotent) in particular is appreciably impacting research and conceivably clinical translation. Given the ability to artificially and favorably regulate a cell's survival and behavior by manipulating its biophysical environment, careful consideration of the printing technique, supporting biomaterial and specific exogenously delivered stimuli, is both required and advantageous. By doing so, there exists an opportunity, more than ever before, to engineer advanced and precise tissue analogs that closely recapitulate the morphological and functional elements of natural tissues (healthy or diseased). Importantly, the application of electrical stimulation as a method of enhancing printed tissue development in vitro, including neuritogenesis, synaptogenesis, and cellular maturation, has the added advantage of modeling both traditional and new stimulation platforms, toward improved understanding of efficacy and innovative electroceutical development and application.

Mitigation of Sodium Iodate-Induced Cytotoxicity in Retinal Pigment Epithelial Cells in vitro by Transgenic Erythropoietin-Expressing Mesenchymal Stem Cells

Mesenchymal stem cells (MSC) have shown promise in restoring the vision of patients in clinical trials. However, this therapeutic effect is not observed in every treated patient and is possibly due to the inefficacies of cell delivery and high cell death following transplantation. Utilizing erythropoietin can significantly enhance the regenerative properties of MSCs and hence improve retinal neuron survivability in oxidative stress. Hence, this study aimed to investigate the efficacy of conditioned medium (CM) obtained from transgenic human erythropoietin-expressing MSCs (MSC _EPO ) in protecting human retinal pigment epithelial cells from sodium iodate (NaIO₃)-induced cell death. Human MSC and MSC _EPO were first cultured to obtain conditioned media (CM). The IC₅₀ of NaIO₃ in the ARPE-19 culture was then determined by an MTT assay. After that, the efficacy of both MSC-CM and MSC-CM _EPO in ARPE-19 cell survival were compared at 24 and 48 h after NaIO₃ treatment with MTT. The treatment effects on mitochondrial membrane potential was then measured by a JC-1 flow cytometric assay. The MTT results indicated a corresponding increase in cell survivability (5-58%) in the ARPE-19 cell cultures. In comparison to MSC-CM, the use of conditioned medium collected from the MSC-CM _EPO further enhanced the rate of ARPE-19 survivability at 24 h (P < 0.05) and 48 h (P < 0.05) in the presence of NaIO₃. Furthermore, more than 90% were found viable with the JC-1 assay after MSC-CM _EPO treatment, showing a positive implication on the mitochondrial dynamics of ARPE-19. The MSC-CM _EPO provided an enhanced mitigating effect against NaIO₃-induced ARPE-19 cell death over that of MSC-CM alone during the early phase of the treatment, and it may act as a future therapy in treating retinal degenerative diseases.

Fabrication of Biomedical Scaffolds Using Biodegradable Polymers

Degradable polymers are used widely in tissue engineering and regenerative medicine. Maturing capabilities in additive manufacturing coupled with advances in orthogonal chemical functionalization methodologies have enabled a rapid evolution of defect-specific form factors and strategies for designing and creating bioactive scaffolds. However, these defect-specific scaffolds, especially when utilizing degradable polymers as the base material, present processing challenges that are distinct and unique from other classes of materials. The goal of this review is to provide a guide for the fabrication of biodegradable polymer-based scaffolds that includes the complete pathway starting from selecting materials, choosing the correct fabrication method, and considering the requirements for tissue specific applications of the scaffold.

Modeling the Mechanobiology of Cancer Cell Migration Using 3D Biomimetic Hydrogels

Understanding how cancer cells migrate, and how this migration is affected by the mechanical and chemical composition of the extracellular matrix (ECM) is critical to investigate and possibly interfere with the metastatic process, which is responsible for most cancer-related deaths. In this article we review the state of the art about the use of hydrogel-based three-dimensional (3D) scaffolds as artificial platforms to model the mechanobiology of cancer cell migration. We start by briefly reviewing the concept and composition of the extracellular matrix (ECM) and the materials commonly used to recreate the cancerous ECM. Then we summarize the most relevant knowledge about the mechanobiology of cancer cell migration that has been obtained using 3D hydrogel scaffolds, and relate those discoveries to what has been observed in the clinical management of solid tumors. Finally, we review some recent methodological developments, specifically the use of novel bioprinting techniques and microfluidics to create realistic hydrogel-based models of the cancer ECM, and some of their applications in the context of the study of cancer cell migration.

Neural tissue engineering: the influence of scaffold surface topography and extracellular matrix microenvironment

Strategies using surface topography, contact guidance and biomechanical cues in the design of scaffolds as an ECM support for neural tissue engineering.

3D printing of tissue engineering scaffolds: a focus on vascular regeneration

Tissue engineering is an emerging means for resolving the problems of tissue repair and organ replacement in regenerative medicine. Insufficient supply of nutrients and oxygen to cells in large-scale tissues has led to the demand to prepare blood vessels. Scaffold-based tissue engineering approaches are effective methods to form new blood vessel tissues. The demand for blood vessels prompts systematic research on fabrication strategies of vascular scaffolds for tissue engineering. Recent advances in 3D printing have facilitated fabrication of vascular scaffolds, contributing to broad prospects for tissue vascularization. This review presents state of the art on modeling methods, print materials and preparation processes for fabrication of vascular scaffolds, and discusses the advantages and application fields of each method. Specially, significance and importance of scaffold-based tissue engineering for vascular regeneration are emphasized. Print materials and preparation processes are discussed in detail. And a focus is placed on preparation processes based on 3D printing technologies and traditional manufacturing technologies including casting, electrospinning, and Lego-like construction. And related studies are exemplified. Transformation of vascular scaffolds to clinical application is discussed. Also, four trends of 3D printing of tissue engineering vascular scaffolds are presented, including machine learning, near-infrared photopolymerization, 4D printing, and combination of self-assembly and 3D printing-based methods.

Applications of 3D bioprinting in tissue engineering: advantages, deficiencies, improvements, and future perspectives

Over the past decade, 3D bioprinting technology has progressed tremendously in the field of tissue engineering in its ability to fabricate individualized biological constructs with precise geometric designability, which offers us the capability to bridge the divergence between engineered tissue constructs and natural tissues. In this work, we first review the current widely used 3D bioprinting approaches, cells, and materials. Next, the updated applications of this technique in tissue engineering, including bone tissue, cartilage tissue, vascular grafts, skin, neural tissue, heart tissue, liver tissue and lung tissue, are briefly introduced. Then, the prominent advantages of 3D bioprinting in tissue engineering are summarized in detail: rapidly prototyping the customized structure, delivering cell-laden materials with high precision in space, and engineering with a highly controllable microenvironment. The current technical deficiencies of 3D bioprinted constructs in terms of mechanical properties and cell behaviors are afterward illustrated, as well as corresponding improvements. Finally, we conclude with future perspectives about 3D bioprinting in tissue engineering.

Fundamentals and Current Strategies for Peripheral Nerve Repair and Regeneration

A body of evidence indicates that peripheral nerves have an extraordinary yet limited capacity to regenerate after an injury. Peripheral nerve injuries have confounded professionals in this field, from neuroscientists to neurologists, plastic surgeons, and the scientific community. Despite all the efforts, full functional recovery is still seldom. The inadequate results attained with the "gold standard" autograft procedure still encourage a dynamic and energetic research around the world for establishing good performing tissue-engineered alternative grafts. Resourcing to nerve guidance conduits, a variety of methods have been experimentally used to bridge peripheral nerve gaps of limited size, up to 30-40 mm in length, in humans. Herein, we aim to summarize the fundamentals related to peripheral nerve anatomy and overview the challenges and scientific evidences related to peripheral nerve injury and repair mechanisms. The most relevant reports dealing with the use of both synthetic and natural-based biomaterials used in tissue engineering strategies when treatment of nerve injuries is envisioned are also discussed in depth, along with the state-of-the-art approaches in this field.

Novel method to produce a layered 3D scaffold for human pluripotent stem cell-derived neuronal cells

BACKGROUND

Three-dimensional (3D) in vitro models have been developed into more in vivo resembling structures. In particular, there is a need for human-based models for neuronal tissue engineering (TE). To produce such a model with organized microenvironment for cells in central nervous system (CNS), a 3D layered scaffold composed of hydrogel and cell guiding fibers has been proposed.

NEW METHOD

Here, we describe a novel method for producing a layered 3D scaffold consisting of electrospun poly (L,D-lactide) fibers embedded into collagen 1 hydrogel to achieve better resemblance of cells' natural microenvironment for human pluripotent stem cell (hPSC)-derived neurons. The scaffold was constructed via a single layer-by-layer process using an electrospinning technique with a unique collector design.

RESULTS

The method enabled the production of layered 3D cell-containing scaffold in a single process. HPSC-derived neurons were found in all layers of the scaffold and exhibited a typical neuronal phenotype. The guiding fiber layers supported the directed cell growth and extension of the neurites inside the scaffold without additional functionalization.

COMPARISON WITH EXISTING METHODS

Previous methods have required several process steps to construct 3D layer-by-layer scaffolds.

CONCLUSIONS

We introduced a method to produce layered 3D scaffolds to mimic the cell guiding cues in CNS by alternating the soft hydrogel matrix and fibrous guidance cues. The produced scaffold successfully enabled the long-term culture of hPSC-derived neuronal cells. This layered 3D scaffold is a useful model for in vitro and in vivo neuronal TE applications.

Recent Advances in the Regenerative Approaches for Traumatic Spinal Cord Injury: Materials Perspective

Spinal cord injury (SCI) is a devastating health condition that may lead to permanent disabilities and death. Understanding the pathophysiological perspectives of traumatic SCI is essential to define mechanisms that can help in designing recovery strategies. Since central nervous system tissues are notorious for their deficient ability to heal, efforts have been made to identify solutions to aid in restoration of the spinal cord tissues and thus its function. The two main approaches proposed to address this issue are neuroprotection and neuro-regeneration. Neuroprotection involves administering drugs to restore the injured microenvironment to normal after SCI. As for the neuro-regeneration approach, it focuses on axonal sprouting for functional recovery of the injured neural tissues and damaged axons. Despite the progress made in the field, neural regeneration treatment after SCI is still unsatisfactory owing to the disorganized way of axonal growth and extension. Nanomedicine and tissue engineering are considered promising therapeutic approaches that enhance axonal growth and directionality through implanting or injecting of the biomaterial scaffolds. One of these recent approaches is nanofibrous scaffolds that are used to provide physical support to maintain directional axonal growth in the lesion site. Furthermore, these preferable tissue-engineered substrates can afford axonal regeneration by mimicking the extracellular matrix of the neural tissues in terms of biological, chemical, and architectural characteristics. In this review, we discuss the regenerative approach using nanofibrous scaffolds with a focus on their fabrication methods and their properties that define their functionality performed to heal the neural tissue efficiently.

Bioprinting Neural Systems to Model Central Nervous System Diseases

To date, pharmaceutical progresses in central nervous system (CNS) diseases are clearly hampered by the lack of suitable disease models. Indeed, animal models do not faithfully represent human neurodegenerative processes and human in vitro 2D cell culture systems cannot recapitulate the in vivo complexity of neural systems. The search for valuable models of neurodegenerative diseases has recently been revived by the addition of 3D culture that allows to re-create the in vivo microenvironment including the interactions among different neural cell types and the surrounding extracellular matrix (ECM) components. In this review, the new challenges in the field of CNS diseases in vitro 3D modeling are discussed, focusing on the implementation of bioprinting approaches enabling positional control on the generation of the 3D microenvironments. The focus is specifically on the choice of the optimal materials to simulate the ECM brain compartment and the biofabrication technologies needed to shape the cellular components within a microenvironment that significantly represents brain biochemical and biophysical parameters.

Guidelines for establishing a 3-D printing biofabrication laboratory

Advanced manufacturing and 3D printing are transformative technologies currently undergoing rapid adoption in healthcare, a traditionally non-manufacturing sector. Recent development in this field, largely enabled by merging different disciplines, has led to important clinical applications from anatomical models to regenerative bioscaffolding and devices. Although much research to-date has focussed on materials, designs, processes, and products, little attention has been given to the design and requirements of facilities for enabling clinically relevant biofabrication solutions. These facilities are critical to overcoming the major hurdles to clinical translation, including solving important issues such as reproducibility, quality control, regulations, and commercialization. To improve process uniformity and ensure consistent development and production, large-scale manufacturing of engineered tissues and organs will require standardized facilities, equipment, qualification processes, automation, and information systems. This review presents current and forward-thinking guidelines to help design biofabrication laboratories engaged in engineering model and tissue constructs for therapeutic and non-therapeutic applications.

Process hybridization schemes for multiscale engineered tissue biofabrication

Recapitulation of multiscale structure-function properties of cells, cell-secreted extracellular matrix, and 3D architecture of natural tissues is central to engineering biomimetic tissue substitutes. Toward achieving biomimicry, a variety of biofabrication processes have been developed, which can be broadly classified into five categories-fiber and fabric formation, additive manufacturing, surface modification, remote fields, and other notable processes-each with specific advantages and limitations. The majority of biofabrication literature has focused on using a single process at a time, which often limits the range of tissues that could be created with relevant features that span nano to macro scales. With multiscale biomimicry as the goal, development of hybrid biofabrication strategies that synergistically unite two or more processes to complement each other's strengths and limitations has been steadily increasing. This work discusses recent literature in this domain and attempts to equip the reader with the understanding of selecting appropriate processes that can harmonize toward creating engineered tissues with appropriate multiscale structure-function properties. Opportunities related to various hybridization schemes and a future outlook on scale-up biofabrication have also been discussed. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Implantable Materials and Surgical Technologies > Nanotechnology in Tissue Repair and Replacement.