Coaxial 3D bioprinting of self-assembled multicellular heterogeneous tumor fibers

Advancements in three-dimensional bioprinting for reproductive medicine: a systematic review

Reproductive failure due to age, genetics and disease necessitates innovative solutions. While reproductive tissue transplantation has advanced, ongoing research seeks superior approaches. Biomaterials, bioengineering and additive manufacturing, such as three-dimensional (3D) bioprinting, are harnessed to restore reproductive function. 3D bioprinting uses materials, cells and growth factors to mimic natural tissues, proving popular for tissue engineering, notably in complex scaffold creation with cell distribution. The versatility which is brought to reproductive medicine by 3D bioprinting allows more accurate and on-site applicability to various problems that are encountered in the field. However, in the literature, there is a lack of studies encompassing the valuable applications of 3D bioprinting in reproductive medicine. This systematic review aims to improve understanding, and focuses on applications in several branches of reproductive medicine. Advancements span the restoration of ovarian function, endometrial regeneration, vaginal reconstruction, and male germ cell bioengineering. 3D bioprinting holds untapped potential in reproductive medicine.

Advanced tumor organoid bioprinting strategy for oncology research

Bioprinting is a groundbreaking technology that enables precise distribution of cell-containing bioinks to construct organoid models that accurately reflect the characteristics of tumors in vivo. By incorporating different types of tumor cells into the bioink, the heterogeneity of tumors can be replicated, enabling studies to simulate real-life situations closely. Precise reproduction of the arrangement and interactions of tumor cells using bioprinting methods provides a more realistic representation of the tumor microenvironment. By mimicking the complexity of the tumor microenvironment, the growth patterns and diffusion of tumors can be demonstrated. This approach can also be used to evaluate the response of tumors to drugs, including drug permeability and cytotoxicity, and other characteristics. Therefore, organoid models can provide a more accurate oncology research and treatment simulation platform. This review summarizes the latest advancements in bioprinting to construct tumor organoid models. First, we describe the bioink used for tumor organoid model construction, followed by an introduction to various bioprinting methods for tumor model formation. Subsequently, we provide an overview of existing bioprinted tumor organoid models.

Innovative 3D bioprinting approaches for advancing brain science and medicine: a literature review

The rapid advancements in 3D printing technology have revolutionized the field of tissue engineering, particularly in the development of neural tissues for the treatment of nervous system diseases. Brain neural tissue, composed of neurons and glial cells, plays a crucial role in the functioning of the brain, spinal cord, and peripheral nervous system by transmitting nerve impulses and processing information. By leveraging 3D bioprinting and bioinks, researchers can create intricate neural scaffolds that facilitate the proliferation and differentiation of nerve cells, thereby promoting the repair and regeneration of damaged neural tissues. This technology allows for the precise spatial arrangement of various cell types and scaffold materials, enabling the construction of complex neural tissue models that closely mimic the natural architecture of the brain. Human-induced pluripotent stem cells (hiPSCs) have emerged as a groundbreaking tool in neuroscience research and the potential treatment of neurological diseases. These cells can differentiate into diverse cell types within the nervous system, including neurons, astrocytes, microglia, oligodendrocytes, and Schwann cells, providing a versatile platform for studying neural networks, neurodevelopment, and neurodegenerative disorders. The use of hiPSCs also opens new avenues for personalized medicine, allowing researchers to model diseases and develop targeted therapies based on individual patient profiles. Despite the promise of direct hiPSC injections for therapeutic purposes, challenges such as poor localization and limited integration have led to the exploration of biomaterial scaffolds as supportive platforms for cell delivery and tissue regeneration. This paper reviews the integration of 3D bioprinting technologies and bioink materials in neuroscience applications, offering a unique platform to create complex brain and tissue architectures that mimic the mechanical, architectural, and biochemical properties of native tissues. These advancements provide robust tools for modelling, repair, and drug screening applications. The review highlights current research, identifies research gaps, and offers recommendations for future studies on 3D bioprinting in neuroscience. The investigation demonstrates the significant potential of 3D bioprinting to fabricate brain-like tissue constructs, which holds great promise for regenerative medicine and drug testing models. This approach offers new avenues for studying brain diseases and potential treatments.

Coaxial bioprinting of a stentable and endothelialized human coronary artery-sized in vitro model

Atherosclerosis accounts for two-thirds of deaths attributed to cardiovascular diseases, which continue to be the leading cause of mortality. Current clinical management strategies for atherosclerosis, such as angioplasty with stenting, face numerous challenges, including restenosis and late thrombosis. Smart stents, integrated with sensors that can monitor cardiovascular health in real-time, are being developed to overcome these limitations. This development necessitates rigorous preclinical trials on either animal models or in vitro models. Despite efforts being made, a suitable human-scale in vitro model compatible with a cardiovascular stent has remained elusive. To address this need, this study utilizes an in-bath bioprinting method to create a human-scale, freestanding in vitro model compatible with cardiovascular stents. Using a coaxial nozzle, a tubular structure of human coronary artery (HCA) size is bioprinted with a collagen-based bioink, ensuring good biocompatibility and suitable rheological properties for printing. We precisely replicated the dimensions of the HCA, including its internal diameter and wall thickness, and simulated the vascular barrier functionality. To simplify post-processing, a pumpless perfusion bioreactor is fabricated to culture a HCA-sized model, eliminating the need for a peristaltic pump and enabling scalability for high-throughput production. This model is expected to accelerate stent development in the future.

Coaxial Bioprinting of Enzymatically Crosslinkable Hyaluronic Acid-Tyramine Bioinks for Tissue Regeneration

Three-dimensional (3D) bioprinting has emerged as an important technique for fabricating tissue constructs with precise structural and compositional control. However, developing suitable bioinks with biocompatible crosslinking mechanisms remains a significant challenge. This study investigates extrusion-based bioprinting (EBB) using uniaxial or coaxial nozzles with enzymatic crosslinking (EC) to produce 3D tissue constructs in vitro. Initially, low-molecular-weight dextran-tyramine and hyaluronic acid-tyramine (LMW Dex-TA/HA-TA) bioink prepolymers were evaluated. Enzymatically pre-crosslinking these prepolymers, achieved by the addition of horseradish peroxidase and hydrogen peroxide, produced viscous polymer solutions. However, this approach resulted in inconsistent bioprinting outcomes (uniaxial) due to inhomogeneous crosslinking, leading to irreproducible properties and suboptimal shear recovery behavior of the hydrogel inks. To address these challenges, we explored a one-step coaxial bioprinting system consisting of enzymatically crosslinkable high-molecular-weight hyaluronic acid-tyramine conjugates (HMW HA-TA) mixed with horseradish peroxidase (HRP) in the inner core and a mixture of Pluronic F127 and hydrogen peroxide in the outer shell. This configuration resulted in nearly instantaneous gelation by diffusion of the hydrogen peroxide into the core. Stable hydrogel fibers with desirable properties, including appropriate swelling ratios and controlled degradation rates, were obtained. The optimized bioink and printing parameters included 1.3% w/v HMW HA-TA and 5.5 U/mL HRP (bioink, inner core), and 27.5% w/v Pluronic F127 and 0.1% H2O2 (sacrificial ink, outer shell). Additionally, optimal pressures for the inner core and outer shell were 45 and 80 kPa, combined with a printing speed of 300 mm/min and a bed temperature of 30 °C. The extruded HMW HA-TA core filaments, containing bovine primary chondrocytes (BPCs) or 3T3 fibroblasts (3T3 Fs), exhibited good cell viabilities and were successfully cultured for up to seven days. This study serves as a proof-of-concept for the one-step generation of core filaments using a rapidly gelling bioink with an enzymatic crosslinking mechanism, and a coaxial bioprinter nozzle system. The results demonstrate significant potential for developing designed, printed, and organized 3D tissue fiber constructs.

Applications of 3D Bioprinting Technology to Brain Cells and Brain Tumor Models: Special Emphasis to Glioblastoma

Primary brain tumor is one of the most fatal diseases. The most malignant type among them, glioblastoma (GBM), has low survival rates. Standard treatments reduce the life quality of patients due to serious side effects. Tumor aggressiveness and the unique structure of the brain render the removal of tumors and the development of new therapies challenging. To elucidate the characteristics of brain tumors and examine their response to drugs, realistic systems that mimic the tumor environment and cellular crosstalk are desperately needed. In the past decade, 3D GBM models have been presented as excellent platforms as they allowed the investigation of the phenotypes of GBM and testing innovative therapeutic strategies. In that scope, 3D bioprinting technology offers utilities such as fabricating realistic 3D bioprinted structures in a layer-by-layer manner and precisely controlled deposition of materials and cells, and they can be integrated with other technologies like the microfluidics approach. This Review covers studies that investigated 3D bioprinted brain tumor models, especially GBM using 3D bioprinting techniques and essential parameters that affect the result and quality of the study like frequently used cells, the type and physical characteristics of hydrogel, bioprinting conditions, cross-linking methods, and characterization techniques.

Direct Fabrication of Glycoengineered Cells via Photoresponsive Thiol-ene Reaction

Three-dimensional printing of cell constructs with high-cell density, shape fidelity, and heterogeneous cell populations is an important tool for investigating cell sociology in living tissues but remains challenging. Herein, we propose an artificial intercellular adhesion method using a photoresponsive chemical cue between a thiol-bearing polymer and a methacrylate-bearing cell membrane. This process provided cell fabrication containing 108 cells/mL, embedded multiple cell populations in one structure, and enabled millimeter-sized scaleup. Our approach allows for the artificial cell construction of complex structures and is a promising bioprinting strategy for engineering tissues that are structurally and physiologically relevant.

Knockout cancer by nano-delivered immunotherapy using perfusion-aided scaffold-based tumor-on-a-chip

Cancer is a multifactorial disease produced by mutations in the oncogenes and tumor suppressor genes, which result in uncontrolled cell proliferation and resistance to cell death. Cancer progresses due to the escape of altered cells from immune monitoring, which is facilitated by the tumor's mutual interaction with its microenvironment. Understanding the mechanisms involved in immune surveillance evasion and the significance of the tumor microenvironment might thus aid in developing improved therapies. Although in vivo models are commonly utilized, they could be better for time, cost, and ethical concerns. As a result, it is critical to replicate an in vivo model and recreate the cellular and tissue-level functionalities. A 3D cell culture, which gives a 3D architecture similar to that found in vivo, is an appropriate model. Furthermore, numerous cell types can be cocultured, establishing cellular interactions between TME and tumor cells. Moreover, microfluidics perfusion can provide precision flow rates, thus simulating tissue/organ function. Immunotherapy can be used with the perfused 3D cell culture technique to help develop successful therapeutics. Immunotherapy employing nano delivery can target the spot and silence the responsible genes, ensuring treatment effectiveness while minimizing adverse effects. This study focuses on the importance of 3D cell culture in understanding the pathophysiology of 3D tumors and TME, the function of TME in drug resistance, tumor progression, and the development of advanced anticancer therapies for high-throughput drug screening.

Advances in 3D bioprinting for regenerative medicine applications

Biofabrication techniques allow for the construction of biocompatible and biofunctional structures composed from biomaterials, cells and biomolecules. Bioprinting is an emerging 3D printing method which utilizes biomaterial-based mixtures with cells and other biological constituents into printable suspensions known as bioinks. Coupled with automated design protocols and based on different modes for droplet deposition, 3D bioprinters are able to fabricate hydrogel-based objects with specific architecture and geometrical properties, providing the necessary environment that promotes cell growth and directs cell differentiation towards application-related lineages. For the preparation of such bioinks, various water-soluble biomaterials have been employed, including natural and synthetic biopolymers, and inorganic materials. Bioprinted constructs are considered to be one of the most promising avenues in regenerative medicine due to their native organ biomimicry. For a successful application, the bioprinted constructs should meet particular criteria such as optimal biological response, mechanical properties similar to the target tissue, high levels of reproducibility and printing fidelity, but also increased upscaling capability. In this review, we highlight the most recent advances in bioprinting, focusing on the regeneration of various tissues including bone, cartilage, cardiovascular, neural, skin and other organs such as liver, kidney, pancreas and lungs. We discuss the rapidly developing co-culture bioprinting systems used to resemble the complexity of tissues and organs and the crosstalk between various cell populations towards regeneration. Moreover, we report on the basic physical principles governing 3D bioprinting, and the ideal bioink properties based on the biomaterials' regenerative potential. We examine and critically discuss the present status of 3D bioprinting regarding its applicability and current limitations that need to be overcome to establish it at the forefront of artificial organ production and transplantation.

Future trends of additive manufacturing in medical applications: An overview

Additive Manufacturing (AM) has recently demonstrated significant medical progress. Due to advancements in materials and methodologies, various processes have been developed to cater to the medical sector's requirements, including bioprinting and 4D, 5D, and 6D printing. However, only a few studies have captured these emerging trends and their medical applications. Therefore, this overview presents an analysis of the advancements and achievements obtained in AM for the medical industry, focusing on the principal trends identified in the annual report of AM3DP.

Covalent Grafting of Functionalized MEW Fibers to Silk Fibroin Hydrogels to Obtain Reinforced Tissue Engineered Constructs

Hydrogels are ideal materials to encapsulate cells, making them suitable for applications in tissue engineering and regenerative medicine. However, they generally do not possess adequate mechanical strength to functionally replace human tissues, and therefore they often need to be combined with reinforcing structures. While the interaction at the interface between the hydrogel and reinforcing structure is imperative for mechanical function and subsequent biological performance, this interaction is often overlooked. Melt electrowriting enables the production of reinforcing microscale fibers that can be effectively integrated with hydrogels. Yet, studies on the interaction between these micrometer scale fibers and hydrogels are limited. Here, we explored the influence of covalent interfacial interactions between reinforcing structures and silk fibroin methacryloyl hydrogels (silkMA) on the mechanical properties of the construct and cartilage-specific matrix production in vitro. For this, melt electrowritten fibers of a thermoplastic polymer blend (poly(hydroxymethylglycolide-co-ε-caprolactone):poly(ε-caprolactone) (pHMGCL:PCL)) were compared to those of the respective methacrylated polymer blend pMHMGCL:PCL as reinforcing structures. Photopolymerization of the methacrylate groups, present in both silkMA and pMHMGCL, was used to generate hybrid materials. Covalent bonding between the pMHMGCL:PCL blend and silkMA hydrogels resulted in an elastic response to the application of torque. In addition, an improved resistance was observed to compression (∼3-fold) and traction (∼40-55%) by the scaffolds with covalent links at the interface compared to those without these interactions. Biologically, both types of scaffolds (pHMGCL:PCL and pMHMGCL:PCL) showed similar levels of viability and metabolic activity, also compared to frequently used PCL. Moreover, articular cartilage progenitor cells embedded within the reinforced silkMA hydrogel were able to form a cartilage-like matrix after 28 days of in vitro culture. This study shows that hybrid cartilage constructs can be engineered with tunable mechanical properties by grafting silkMA hydrogels covalently to pMHMGCL:PCL blend microfibers at the interface.

Mending a broken heart by biomimetic 3D printed natural biomaterial-based cardiac patches: a review

Myocardial infarction is one of the major causes of mortality as well as morbidity around the world. Currently available treatment options face a number of drawbacks, hence cardiac tissue engineering, which aims to bioengineer functional cardiac tissue, for application in tissue repair, patient specific drug screening and disease modeling, is being explored as a viable alternative. To achieve this, an appropriate combination of cells, biomimetic scaffolds mimicking the structure and function of the native tissue, and signals, is necessary. Among scaffold fabrication techniques, three-dimensional printing, which is an additive manufacturing technique that enables to translate computer-aided designs into 3D objects, has emerged as a promising technique to develop cardiac patches with a highly defined architecture. As a further step toward the replication of complex tissues, such as cardiac tissue, more recently 3D bioprinting has emerged as a cutting-edge technology to print not only biomaterials, but also multiple cell types simultaneously. In terms of bioinks, biomaterials isolated from natural sources are advantageous, as they can provide exceptional biocompatibility and bioactivity, thus promoting desired cell responses. An ideal biomimetic cardiac patch should incorporate additional functional properties, which can be achieved by means of appropriate functionalization strategies. These are essential to replicate the native tissue, such as the release of biochemical signals, immunomodulatory properties, conductivity, enhanced vascularization and shape memory effects. The aim of the review is to present an overview of the current state of the art regarding the development of biomimetic 3D printed natural biomaterial-based cardiac patches, describing the 3D printing fabrication methods, the natural-biomaterial based bioinks, the functionalization strategies, as well as the in vitro and in vivo applications.

Application of three-dimensional (3D) bioprinting in anti-cancer therapy

Three-dimensional (3D) bioprinting is a novel technology that enables the creation of 3D structures with bioinks, the biomaterials containing living cells. 3D bioprinted structures can mimic human tissue at different levels of complexity from cells to organs. Currently, 3D bioprinting is a promising method in regenerative medicine and tissue engineering applications, as well as in anti-cancer therapy research. Cancer, a type of complex and multifaceted disease, presents significant challenges regarding diagnosis, treatment, and drug development. 3D bioprinted models of cancer have been used to investigate the molecular mechanisms of oncogenesis, the development of cancers, and the responses to treatment. Conventional 2D cancer models have limitations in predicting human clinical outcomes and drug responses, while 3D bioprinting offers an innovative technique for creating 3D tissue structures that closely mimic the natural characteristics of cancers in terms of morphology, composition, structure, and function. By precise manipulation of the spatial arrangement of different cell types, extracellular matrix components, and vascular networks, 3D bioprinting facilitates the development of cancer models that are more accurate and representative, emulating intricate interactions between cancer cells and their surrounding microenvironment. Moreover, the technology of 3D bioprinting enables the creation of personalized cancer models using patient-derived cells and biomarkers, thereby advancing the fields of precision medicine and immunotherapy. The integration of 3D cell models with 3D bioprinting technology holds the potential to revolutionize cancer research, offering extensive flexibility, precision, and adaptability in crafting customized 3D structures with desired attributes and functionalities. In conclusion, 3D bioprinting exhibits significant potential in cancer research, providing opportunities for identifying therapeutic targets, reducing reliance on animal experiments, and potentially lowering the overall cost of cancer treatment. Further investigation and development are necessary to address challenges such as cell viability, printing resolution, material characteristics, and cost-effectiveness. With ongoing progress, 3D bioprinting can significantly impact the field of cancer research and improve patient outcomes.

3D Bioprinting of Hyaline Articular Cartilage: Biopolymers, Hydrogels, and Bioinks

The musculoskeletal system, consisting of bones and cartilage of various types, muscles, ligaments, and tendons, is the basis of the human body. However, many pathological conditions caused by aging, lifestyle, disease, or trauma can damage its elements and lead to severe disfunction and significant worsening in the quality of life. Due to its structure and function, articular (hyaline) cartilage is the most susceptible to damage. Articular cartilage is a non-vascular tissue with constrained self-regeneration capabilities. Additionally, treatment methods, which have proven efficacy in stopping its degradation and promoting regeneration, still do not exist. Conservative treatment and physical therapy only relieve the symptoms associated with cartilage destruction, and traditional surgical interventions to repair defects or endoprosthetics are not without serious drawbacks. Thus, articular cartilage damage remains an urgent and actual problem requiring the development of new treatment approaches. The emergence of biofabrication technologies, including three-dimensional (3D) bioprinting, at the end of the 20th century, allowed reconstructive interventions to get a second wind. Three-dimensional bioprinting creates volume constraints that mimic the structure and function of natural tissue due to the combinations of biomaterials, living cells, and signal molecules to create. In our case-hyaline cartilage. Several approaches to articular cartilage biofabrication have been developed to date, including the promising technology of 3D bioprinting. This review represents the main achievements of such research direction and describes the technological processes and the necessary biomaterials, cell cultures, and signal molecules. Special attention is given to the basic materials for 3D bioprinting-hydrogels and bioinks, as well as the biopolymers underlying the indicated products.

Extrusion bioprinting of cellular aggregates improves mesenchymal stem cell proliferation and differentiation

3D extrusion bioprinting brings the prospect of stem cell-based therapies in regenerative medicine. These bioprinted stem cells are expected to proliferate and differentiate to form the desired organoids into 3D structures, which is critical for complex tissue construction. However, this strategy is hampered by low reproducible cell number and viability, and organoid immaturity due to incomplete differentiation of stem cells. Hence, we apply a novel extrusion-based bioprinting process with cellular aggregates (CA) bioink, in which the encapsulated cells are precultured in hydrogels to undergo aggregation. In this study, alginate-gelatin-collagen (Alg-Gel-Col) hydrogel containing mesenchymal stem cells (MSCs) were precultured for 48 h to form CA bioink and resulted in high cell viability and printing fidelity. Meanwhile, MSCs in CA bioink showed high proliferation, stemness and lipogenic differentiative potential in contrast to that in single cell (SC) bioink and hanging drop cell spheroid (HDCS) bioink, which indicated the considerable potential for complex tissue construction. In addition, the printability and efficacy of human umbilical cord MSCs (hUC-MSCs) were further confirmed the translational potential of this novel bioprinting method.

Functional Skeletal Muscle Regeneration Using Muscle Mimetic Tissue Fabricated by Microvalve-Assisted Coaxial 3D Bioprinting

3D-printed artificial skeletal muscle, which mimics the structural and functional characteristics of native skeletal muscle, is a promising treatment method for muscle reconstruction. Although various fabrication techniques for skeletal muscle using 3D bio-printers are studied, it is still challenging to build a functional muscle structure. A strategy using microvalve-assisted coaxial 3D bioprinting in consideration of functional skeletal muscle fabrication is reported. The unit (artificial muscle fascicle: AMF) of muscle mimetic tissue is composed of a core filled with medium-based C2C12 myoblast aggregates as a role of muscle fibers and a photo cross-linkable hydrogel-based shell as a role of connective tissue in muscles that enhances printability and cell adhesion and proliferation. Especially, a microvalve system is applied for the core part with even cell distribution and strong cell-cell interaction. This system enhances myotube formation and consequently shows spontaneous contraction. A multi-printed AMF (artificial muscle tissue: AMT) as a piece of muscle is implanted into the anterior tibia (TA) muscle defect site of immunocompromised rats. As a result, the TA-implanted AMT responds to electrical stimulation and represents histologically regenerated muscle tissue. This microvalve-assisted coaxial 3D bioprinting shows a significant step forward to mimicking native skeletal muscle tissue.

Recent Developments in 3D Bio-Printing and Its Biomedical Applications

The fast-developing field of 3D bio-printing has been extensively used to improve the usability and performance of scaffolds filled with cells. Over the last few decades, a variety of tissues and organs including skin, blood vessels, and hearts, etc., have all been produced in large quantities via 3D bio-printing. These tissues and organs are not only able to serve as building blocks for the ultimate goal of repair and regeneration, but they can also be utilized as in vitro models for pharmacokinetics, drug screening, and other purposes. To further 3D-printing uses in tissue engineering, research on novel, suitable biomaterials with quick cross-linking capabilities is a prerequisite. A wider variety of acceptable 3D-printed materials are still needed, as well as better printing resolution (particularly at the nanoscale range), speed, and biomaterial compatibility. The aim of this study is to provide expertise in the most prevalent and new biomaterials used in 3D bio-printing as well as an introduction to the associated approaches that are frequently considered by researchers. Furthermore, an effort has been made to convey the most pertinent implementations of 3D bio-printing processes, such as tissue regeneration, etc., by providing the most significant research together with a comprehensive list of material selection guidelines, constraints, and future prospects.

Recent Developments of Silk-Based Scaffolds for Tissue Engineering and Regenerative Medicine Applications: A Special Focus on the Advancement of 3D Printing

Regenerative medicine has received potential attention around the globe, with improving cell performances, one of the necessary ideas for the advancements of regenerative medicine. It is crucial to enhance cell performances in the physiological system for drug release studies because the variation in cell environments between in vitro and in vivo develops a loop in drug estimation. On the other hand, tissue engineering is a potential path to integrate cells with scaffold biomaterials and produce growth factors to regenerate organs. Scaffold biomaterials are a prototype for tissue production and perform vital functions in tissue engineering. Silk fibroin is a natural fibrous polymer with significant usage in regenerative medicine because of the growing interest in leftovers for silk biomaterials in tissue engineering. Among various natural biopolymer-based biomaterials, silk fibroin-based biomaterials have attracted significant attention due to their outstanding mechanical properties, biocompatibility, hemocompatibility, and biodegradability for regenerative medicine and scaffold applications. This review article focused on highlighting the recent advancements of 3D printing in silk fibroin scaffold technologies for regenerative medicine and tissue engineering.

Engineering Heart Valve Interfaces Using Melt Electrowriting: Biomimetic Design Strategies from Multi-Modal Imaging

Interfaces within biological tissues not only connect different regions but also contribute to the overall functionality of the tissue. This is especially true in the case of the aortic heart valve. Here, melt electrowriting (MEW) is used to engineer complex, user-defined, interfaces for heart valve scaffolds. First, a multi-modal imaging investigation into the interfacial regions of the valve reveals differences in collagen orientation, density, and recruitment in previously unexplored regions including the commissure and inter-leaflet triangle. Overlapping, suturing, and continuous printing methods for interfacing MEW scaffolds are then investigated for their morphological, tensile, and flexural properties, demonstrating the superior performance of continuous interfaces. G-codes for MEW scaffolds with complex interfaces are designed and generated using a novel software and graphical user interface. Finally, a singular MEW scaffold for the interfacial region of the aortic heart valve is presented incorporating continuous interfaces, gradient porosities, variable layer numbers across regions, and tailored fiber orientations inspired by the collagen distribution and orientation from the multi-modal imaging study. The scaffold exhibits similar yield strain, hysteresis, and relaxation behavior to porcine heart valves. This work demonstrates the ability of a bioinspired approach for MEW scaffold design to address the functional complexity of biological tissues.

Fusion between Glioma Stem Cells and Mesenchymal Stem Cells Promotes Malignant Progression in 3D-Bioprinted Models

The interaction between glioma stem cells (GSCs) and mesenchymal stem cells (MSCs) in the glioma microenvironment is considered to be an important factor in promoting tumor progression, but the mechanism is still not fully elucidated. To further elucidate the interaction between GSCs and MSCs, two 3D-bioprinted tumor models (low-temperature molding and coaxial bioprinting) were used to simulate the tumor growth microenvironment. Cell fusion between GSCs and MSCs was found by the method of Cre-LoxP switch gene and RFP/GFP dual-color fluorescence tracing. The fused cells coexpressed biomarkers of GSCs and MSCs, showing stronger proliferation, cloning, and invasion abilities than GSCs and MSCs. In addition, the fused cells have stronger tumorigenic properties in nude mice, showing the pathological features of malignant tumors. In conclusion, GSCs and MSCs undergo cell fusion in 3D-bioprinted models, and the fused cells have a higher degree of malignancy than parental cells, which promotes the progression of glioma.

3D Bioprinting: An Enabling Technology to Understand Melanoma

Melanoma is a potentially fatal cancer with rising incidence over the last 50 years, associated with enhanced sun exposure and ultraviolet radiation. Its incidence is highest in people of European descent and the ageing population. There are multiple clinical and epidemiological variables affecting melanoma incidence and mortality, such as sex, ethnicity, UV exposure, anatomic site, and age. Although survival has improved in recent years due to advances in targeted and immunotherapies, new understanding of melanoma biology and disease progression is vital to improving clinical outcomes. Efforts to develop three-dimensional human skin equivalent models using biofabrication techniques, such as bioprinting, promise to deliver a better understanding of the complexity of melanoma and associated risk factors. These 3D skin models can be used as a platform for patient specific models and testing therapeutics.

3D Coaxial Bioprinting: Process Mechanisms, Bioinks and Applications

In the last decade, bioprinting has emerged as a facile technique for fabricating tissues constructs mimicking the architectural complexity and compositional heterogeneity of native tissues. Amongst different bioprinting modalities, extrusion-based bioprinting (EBB) is the most widely used technique. Coaxial bioprinting, a type of EBB, enables fabrication of concentric cell-material layers and enlarges the scope of EBB to mimic several key aspects of native tissues. Over the period of development of bioprinting, tissue constructs integrated with vascular networks, have been one of the major achievements made possible largely by coaxial bioprinting. In this review, current advancements in biofabrication of constructs with coaxial bioprinting are discussed with a focus on different bioinks that are particularly suitable for this modality. This review also expounds the properties of different bioinks suitable for coaxial bioprinting and then analyses the key achievements made by the application of coaxial bioprinting in tissue engineering, drug delivery and in-vitro disease modelling. The major limitations and future perspectives on the critical factors that will determine the ultimate clinical translation of the versatile technique are also presented to the reader.

Three-dimensional-engineered bioprinted in vitro human neural stem cell self-assembling culture model constructs of Alzheimer's disease

The pathogenic cascade of Alzheimer's disease (AD) characterized by amyloid-β protein accumulation is still poorly understood, partially owing to the limitations of relevant models without in vivo neural tissue microenvironment to recapitulate cell-cell interactions. To better mimic neural tissue microenvironment, three-dimensional (3D) core-shell AD model constructs containing human neural progenitor cells (NSCs) with 2% matrigel as core bioink and 2% alginate as shell bioink have been bioprinted by a co-axial bioprinter, with a suitable shell thickness for nutrient exchange and barrier-free cell interaction cores. These constructs exhibit cell self-clustering and -assembling properties and engineered reproducibility with long-term cell viability and self-renewal, and a higher differentiation level compared to 2D and 3D MIX models. The different effects of 3D bioprinted, 2D, and MIX microenvironments on the growth of NSCs are mainly related to biosynthesis of amino acids and glyoxylate and dicarboxylate metabolism on day 2 and ribosome, biosynthesis of amino acids and proteasome on day 14. Particularly, the model constructs demonstrated Aβ aggregation and higher expression of Aβ and tau isoform genes compared to 2D and MIX controls. AD model constructs will provide a promising strategy to facilitate the development of a 3D in vitro AD model for neurodegeneration research.

A scalable coaxial bioprinting technology for mesenchymal stem cell microfiber fabrication and high extracellular vesicle yield

Mesenchymal stem cell (MSC)-derived extracellular vesicles (EVs) are promising candidates for regenerative medicine; however, the lack of scalable methods for high quantity EV production limits their application. In addition, signature EV-derived proteins shared in 3D environments and 2D surfaces, remain mostly unknown. Herein, we present a platform combining MSC microfiber culture with ultracentrifugation purification for high EV yield. Within this platform, a high quantity MSC solution (∼3 × 10⁸total cells) is encapsulated in a meter-long hollow hydrogel-microfiber via coaxial bioprinting technology. In this 3D core-shell microfiber environment, MSCs express higher levels of stemness markers (Oct4, Nanog, Sox2) than in 2D culture, and maintain their differentiation capacity. Moreover, this platform enriches particles by ∼1009-fold compared to conventional 2D culture, while preserving their pro-angiogenic properties. Liquid chromatography-mass spectrometry characterization results demonstrate that EVs derived from our platform and conventional 2D culturing have unique protein profiles with 3D-EVs having a greater variety of proteins (1023 vs 605), however, they also share certain proteins (536) and signature MSC-EV proteins (10). This platform, therefore, provides a new tool for EV production using microfibers in one culture dish, thereby reducing space, labor, time, and cost.

3D bioprinting: current status and trends—a guide to the literature and industrial practice

The multidisciplinary research field of bioprinting combines additive manufacturing, biology and material sciences to create bioconstructs with three-dimensional architectures mimicking natural living tissues. The high interest in the possibility of reproducing biological tissues and organs is further boosted by the ever-increasing need for personalized medicine, thus allowing bioprinting to establish itself in the field of biomedical research, and attracting extensive research efforts from companies, universities, and research institutes alike. In this context, this paper proposes a scientometric analysis and critical review of the current literature and the industrial landscape of bioprinting to provide a clear overview of its fast-changing and complex position. The scientific literature and patenting results for 2000–2020 are reviewed and critically analyzed by retrieving 9314 scientific papers and 309 international patents in order to draw a picture of the scientific and industrial landscape in terms of top research countries, institutions, journals, authors and topics, and identifying the technology hubs worldwide. This review paper thus offers a guide to researchers interested in this field or to those who simply want to understand the emerging trends in additive manufacturing and 3D bioprinting.

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The promising rise of bioprinting in revolutionalizing medical science: Advances and possibilities

Bioprinting is a relatively new yet evolving technique predominantly used in regenerative medicine and tissue engineering. 3D bioprinting techniques combine the advantages of creating Extracellular Matrix (ECM)like environments for cells and computer-aided tailoring of predetermined tissue shapes and structures. The essential application of bioprinting is for the regeneration or restoration of damaged and injured tissues by producing implantable tissues and organs. The capability of bioprinting is yet to be fully scrutinized in sectors like the patient-specific spatial distribution of cells, bio-robotics, etc. In this review, currently developed experimental systems and strategies for the bioprinting of different types of tissues as well as for drug delivery and cancer research are explored for potential applications. This review also digs into the most recent opportunities and future possibilities for the efficient implementation of bioprinting to restructure medical and technological practices.

Emerging Technologies in Multi-Material Bioprinting

Bioprinting, within the emerging field of biofabrication, aims at the fabrication of functional biomimetic constructs. Different 3D bioprinting techniques have been adapted to bioprint cell-laden bioinks. However, single-material bioprinting techniques oftentimes fail to reproduce the complex compositions and diversity of native tissues. Multi-material bioprinting as an emerging approach enables the fabrication of heterogeneous multi-cellular constructs that replicate their host microenvironments better than single-material approaches. Here, bioprinting modalities are reviewed, their being adapted to multi-material bioprinting is discussed, and their advantages and challenges, encompassing both custom-designed and commercially available technologies are analyzed. A perspective of how multi-material bioprinting opens up new opportunities for tissue engineering, tissue model engineering, therapeutics development, and personalized medicine is offered.

The Importance of Interfaces in Multi-Material Biofabricated Tissue Structures

Biofabrication exploits additive manufacturing techniques for creating 3D structures with a precise geometry that aim to mimic a physiological cellular environment and to develop the growth of native tissues. The most recent approaches of 3D biofabrication integrate multiple technologies into a single biofabrication platform combining different materials within different length scales to achieve improved construct functionality. However, the importance of interfaces between the different material phases, has not been adequately explored. This is known to determine material's interaction and ultimately mechanical and biological performance of biofabricated parts. In this review, this gap is bridged by critically examining the interface between different material phases in (bio)fabricated structures, with a particular focus on how interfacial interactions can compromise or define the mechanical (and biological) properties of the engineered structures. It is believed that the importance of interfacial properties between the different constituents of a composite material, deserves particular attention in its role in modulating the final characteristics of 3D tissue-like structures.

Using Spheroids as Building Blocks Towards 3D Bioprinting of Tumor Microenvironment

Cancer still ranks as a leading cause of mortality worldwide. Although considerable efforts have been dedicated to anticancer therapeutics, progress is still slow, partially due to the absence of robust prediction models. Multicellular tumor spheroids, as a major three-dimensional (3D) culture model exhibiting features of avascular tumors, gained great popularity in pathophysiological studies and high throughput drug screening. However, limited control over cellular and structural organization is still the key challenge in achieving in vivo like tissue microenvironment. 3D bioprinting has made great strides toward tissue/organ mimicry, due to its outstanding spatial control through combining both cells and materials, scalability, and reproducibility. Prospectively, harnessing the power from both 3D bioprinting and multicellular spheroids would likely generate more faithful tumor models and advance our understanding on the mechanism of tumor progression. In this review, the emerging concept on using spheroids as a building block in 3D bioprinting for tumor modeling is illustrated. We begin by describing the context of the tumor microenvironment, followed by an introduction of various methodologies for tumor spheroid formation, with their specific merits and drawbacks. Thereafter, we present an overview of existing 3D printed tumor models using spheroids as a focus. We provide a compilation of the contemporary literature sources and summarize the overall advancements in technology and possibilities of using spheroids as building blocks in 3D printed tissue modeling, with a particular emphasis on tumor models. Future outlooks about the wonderous advancements of integrated 3D spheroidal printing conclude this review.

Coaxially Bioprinted Cell-Laden Tubular-Like Structure for Studying Glioma Angiogenesis

Glioblastomas are the most frequently diagnosed and one of the most lethal primary brain tumors, and one of their key features is a dysplastic vascular network. However, because the origin of the tumor blood vessels remains controversial, an optimal preclinical tumor model must be established to elucidate the tumor angiogenesis mechanism, especially the role of tumor cells themselves in angiogenesis. Therefore, shell-glioma cell (U118)-red fluorescent protein (RFP)/core-human umbilical vein endothelial cell (HUVEC)-green fluorescent protein (GFP) hydrogel microfibers were coaxially bioprinted. U118–RFP and HUVEC–GFP cells both exhibited good proliferation in a three-dimensional (3D) microenvironment. The secretability of both vascular endothelial growth factor A and basic fibroblast growth factor was remarkably enhanced when both types of cells were cocultured in 3D models. Moreover, U118 cells promoted the vascularization of the surrounding HUVECs by secreting vascular growth factors. More importantly, U118–HUVEC-fused cells were found in U118–RFP/HUVEC–GFP hydrogel microfibers. Most importantly, our results indicated that U118 cells can not only recruit the blood vessels of the surrounding host but also directly transdifferentiate into or fuse with endothelial cells to participate in tumor angiogenesis in vivo. The coaxially bioprinted U118–RFP/HUVEC–GFP hydrogel microfiber is a model suitable for mimicking the glioma microenvironment and for investigating tumor angiogenesis.

Bioprinting of a Zonal-Specific Cell Density Scaffold: A Biomimetic Approach for Cartilage Tissue Engineering

The treatment of articular cartilage defects remains a significant clinical challenge. This is partially due to current tissue engineering strategies failing to recapitulate native organization. Articular cartilage is a graded tissue with three layers exhibiting different cell densities: the superficial zone having the highest density and the deep zone having the lowest density. However, the introduction of cell gradients for cartilage tissue engineering, which could promote a more biomimetic environment, has not been widely explored. Here, we aimed to bioprint a scaffold with different zonal cell densities to mimic the organization of articular cartilage. The scaffold was bioprinted using an alginate-based bioink containing human articular chondrocytes. The scaffold design included three cell densities, one per zone: 20 × 106 (superficial), 10 × 106 (middle), and 5 × 106 (deep) cells/mL. The scaffold was cultured in a chondrogenic medium for 25 days and analyzed by live/dead assay and histology. The live/dead analysis showed the ability to generate a zonal cell density with high viability. Histological analysis revealed a smooth transition between the zones in terms of cell distribution and a higher sulphated glycosaminoglycan deposition in the highest cell density zone. These findings pave the way toward bioprinting complex zonal cartilage scaffolds as single units, thereby advancing the translation of cartilage tissue engineering into clinical practice.

Multimaterial bioprinting and combination of processing techniques towards the fabrication of biomimetic tissues and organs

Tissue reconstruction requires the utilization of multiple biomaterials and cell types to replicate the delicate and complex structure of native tissues. Various three-dimensional (3D) bioprinting techniques have been developed to fabricate customized tissue structures; however, there are still significant challenges, such as vascularization, mechanical stability of printed constructs, and fabrication of gradient structures to be addressed for the creation of biomimetic and complex tissue constructs. One approach to address these challenges is to develop multimaterial 3D bioprinting techniques that can integrate various types of biomaterials and bioprinting capabilities towards the fabrication of more complex structures. Notable examples include multi-nozzle, coaxial, and microfluidics-assisted multimaterial 3D bioprinting techniques. More advanced multimaterial 3D printing techniques are emerging, and new areas in this niche technology are rapidly evolving. In this review, we briefly introduce the basics of individual 3D bioprinting techniques and then discuss the multimaterial 3D printing techniques that can be developed based on combination of these techniques for the engineering of complex and biomimetic tissue constructs. We also discuss the perspectives and future directions to develop state-of-the-art multimaterial 3D bioprinting techniques for engineering tissues and organs.

The effect of neural cell integrated into 3D co-axial bioprinted BMMSC structures during osteogenesis

A three-dimensional (3D) bioprinting is a new strategy for fabricating 3D cell-laden constructs that mimic the structural and functional characteristics of various tissues and provides a similar architecture and microenvironment of the native tissue. However, there are few reported studies on the neural function properties of bioengineered bone autografts. Thus, this study was aimed at investigating the effects of neural cell integration into 3D bioprinted bone constructs. The bioprinted hydrogel constructs could maintain long-term cell survival, support cell growth for human bone marrow-derived mesenchymal stem cells (BMMSCs), reduce cell surface biomarkers of stemness, and enhance orthopedic differentiation with higher expression of osteogenesis-related genes, including osteopontin (OPN) and bone morphogenetic protein-2. More importantly, the bioprinted constructs with neural cell integration indicated higher OPN gene and secretory alkaline phosphatase levels. These results suggested that the innervation in bioprinted bone constructs can accelerate the differentiation and maturation of bone development and provide patients with an option for accelerated bone function restoration.

Meet me halfway: Are in vitro 3D cancer models on the way to replace in vivo models for nanomedicine development?

The complexity and diversity of the biochemical processes that occur during tumorigenesis and metastasis are frequently over-simplified in the traditional in vitro cell cultures. Two-dimensional cultures limit researchers' experimental observations and frequently give rise to misleading and contradictory results. Therefore, in order to overcome the limitations of in vitro studies and bridge the translational gap to in vivo applications, 3D models of cancer were developed in the last decades. The three dimensions of the tumor, including its cellular and extracellular microenvironment, are recreated by combining co-cultures of cancer and stromal cells in 3D hydrogel-based growth factors-inclusive scaffolds. More complex 3D cultures, containing functional blood vasculature, can integrate in the system external stimuli (e.g. oxygen and nutrient deprivation, cytokines, growth factors) along with drugs, or other therapeutic compounds. In this scenario, cell signaling pathways, metastatic cascade steps, cell differentiation and self-renewal, tumor-microenvironment interactions, and precision and personalized medicine, are among the wide range of biological applications that can be studied. Here, we discuss a broad variety of strategies exploited by scientists to create in vitro 3D cancer models that resemble as much as possible the biology and patho-physiology of in vivo tumors and predict faithfully the treatment outcome.

Cancer Cell Direct Bioprinting: A Focused Review

Three-dimensional printing technologies allow for the fabrication of complex parts with accurate geometry and less production time. When applied to biomedical applications, two different approaches, known as direct or indirect bioprinting, may be performed. The classical way is to print a support structure, the scaffold, and then culture the cells. Due to the low efficiency of this method, direct bioprinting has been proposed, with or without the use of scaffolds. Scaffolds are the most common technology to culture cells, but bioassembly of cells may be an interesting methodology to mimic the native microenvironment, the extracellular matrix, where the cells interact between themselves. The purpose of this review is to give an updated report about the materials, the bioprinting technologies, and the cells used in cancer research for breast, brain, lung, liver, reproductive, gastric, skin, and bladder associated cancers, to help the development of possible treatments to lower the mortality rates, increasing the effectiveness of guided therapies. This work introduces direct bioprinting to be considered as a key factor above the main tissue engineering technologies.

Recapitulating Tumorigenesis in vitro: Opportunities and Challenges of 3D Bioprinting

Cancer is considered one of the most predominant diseases in the world and one of the principal causes of mortality per year. The cellular and molecular mechanisms involved in the development and establishment of solid tumors can be defined as tumorigenesis. Recent technological advances in the 3D cell culture field have enabled the recapitulation of tumorigenesis in vitro, including the complexity of stromal microenvironment. The establishment of these 3D solid tumor models has a crucial role in personalized medicine and drug discovery. Recently, spheroids and organoids are being largely explored as 3D solid tumor models for recreating tumorigenesis in vitro. In spheroids, the solid tumor can be recreated from cancer cells, cancer stem cells, stromal and immune cell lineages. Organoids must be derived from tumor biopsies, including cancer and cancer stem cells. Both models are considered as a suitable model for drug assessment and high-throughput screening. The main advantages of 3D bioprinting are its ability to engineer complex and controllable 3D tissue models in a higher resolution. Although 3D bioprinting represents a promising technology, main challenges need to be addressed to improve the results in cancer research. The aim of this review is to explore (1) the principal cell components and extracellular matrix composition of solid tumor microenvironment; (2) the recapitulation of tumorigenesis in vitro using spheroids and organoids as 3D culture models; and (3) the opportunities, challenges, and applications of 3D bioprinting in this area.

Advances in 3D Printing for Tissue Engineering

Tissue engineering (TE) scaffolds have enormous significance for the possibility of regeneration of complex tissue structures or even whole organs. Three-dimensional (3D) printing techniques allow fabricating TE scaffolds, having an extremely complex structure, in a repeatable and precise manner. Moreover, they enable the easy application of computer-assisted methods to TE scaffold design. The latest additive manufacturing techniques open up opportunities not otherwise available. This study aimed to summarize the state-of-art field of 3D printing techniques in applications for tissue engineering with a focus on the latest advancements. The following topics are discussed: systematics of the available 3D printing techniques applied for TE scaffold fabrication; overview of 3D printable biomaterials and advancements in 3D-printing-assisted tissue engineering.

Engineering Hydrogel-Based Biomedical Photonics: Design, Fabrication, and Applications

Light guiding and manipulation in photonics have become ubiquitous in events ranging from everyday communications to complex robotics and nanomedicine. The speed and sensitivity of light-matter interactions offer unprecedented advantages in biomedical optics, data transmission, photomedicine, and detection of multi-scale phenomena. Recently, hydrogels have emerged as a promising candidate for interfacing photonics and bioengineering by combining their light-guiding properties with live tissue compatibility in optical, chemical, physiological, and mechanical dimensions. Herein, the latest progress over hydrogel photonics and its applications in guidance and manipulation of light is reviewed. Physics of guiding light through hydrogels and living tissues, and existing technical challenges in translating these tools into biomedical settings are discussed. A comprehensive and thorough overview of materials, fabrication protocols, and design architectures used in hydrogel photonics is provided. Finally, recent examples of applying structures such as hydrogel optical fibers, living photonic constructs, and their use as light-driven hydrogel robots, photomedicine tools, and organ-on-a-chip models are described. By providing a critical and selective evaluation of the field's status, this work sets a foundation for the next generation of hydrogel photonic research.

3D Bioprinted cancer models: Revolutionizing personalized cancer therapy

After cardiovascular disease, cancer is the leading cause of death worldwide with devastating health and economic consequences, particularly in developing countries. Inter-patient variations in anti-cancer drug responses further limit the success of therapeutic interventions. Therefore, personalized medicines approach is key for this patient group involving molecular and genetic screening and appropriate stratification of patients to treatment regimen that they will respond to. However, the knowledge related to adequate risk stratification methods identifying patients who will respond to specific anti-cancer agents is still lacking in many cancer types. Recent advancements in three-dimensional (3D) bioprinting technology, have been extensively used to generate representative bioengineered tumor in vitro models, which recapitulate the human tumor tissues and microenvironment for high-throughput drug screening. Bioprinting process involves the precise deposition of multiple layers of different cell types in combination with biomaterials capable of generating 3D bioengineered tissues based on a computer-aided design. Bioprinted cancer models containing patient-derived cancer and stromal cells together with genetic material, extracellular matrix proteins and growth factors, represent a promising approach for personalized cancer therapy screening. Both natural and synthetic biopolymers have been utilized to support the proliferation of cells and biological material within the personalized tumor models/implants. These models can provide a physiologically pertinent cell-cell and cell-matrix interactions by mimicking the 3D heterogeneity of real tumors. Here, we reviewed the potential applications of 3D bioprinted tumor constructs as personalized in vitro models in anticancer drug screening and in the establishment of precision treatment regimens.

3D bioprinting of hepatocytes: core-shell structured co-cultures with fibroblasts for enhanced functionality

With the aim of understanding and recapitulating cellular interactions of hepatocytes in their physiological microenvironment and to generate an artificial 3D in vitro model, a co-culture system using 3D extrusion bioprinting was developed. A bioink based on alginate and methylcellulose (algMC) was first shown to be suitable for bioprinting of hepatocytes; the addition of Matrigel to algMC enhanced proliferation and morphology of them in monophasic scaffolds. Towards a more complex system that allows studying cellular interactions, we applied core-shell bioprinting to establish tailored 3D co-culture models for hepatocytes. The bioinks were specifically functionalized with natural matrix components (based on human plasma, fibrin or Matrigel) and used to co-print fibroblasts and hepatocytes in a spatially defined, coaxial manner. Fibroblasts acted as supportive cells for co-cultured hepatocytes, stimulating the expression of certain biomarkers of hepatocytes like albumin. Furthermore, matrix functionalization positively influenced both cell types in their respective compartments by enhancing their adhesion, viability, proliferation and function. In conclusion, we established a functional co-culture model with independently tunable compartments for different cell types via core-shell bioprinting. This provides the basis for more complex in vitro models allowing co-cultivation of hepatocytes with other liver-specific cell types to closely resemble the liver microenvironment.

Natural Biomaterials and Their Use as Bioinks for Printing Tissues

The most prevalent form of bioprinting-extrusion bioprinting-can generate structures from a diverse range of materials and viscosities. It can create personalized tissues that aid in drug testing and cancer research when used in combination with natural bioinks. This paper reviews natural bioinks and their properties and functions in hard and soft tissue engineering applications. It discusses agarose, alginate, cellulose, chitosan, collagen, decellularized extracellular matrix, dextran, fibrin, gelatin, gellan gum, hyaluronic acid, Matrigel, and silk. Multi-component bioinks are considered as a way to address the shortfalls of individual biomaterials. The mechanical, rheological, and cross-linking properties along with the cytocompatibility, cell viability, and printability of the bioinks are detailed as well. Future avenues for research into natural bioinks are then presented.

Biomaterials and 3D Bioprinting Strategies to Model Glioblastoma and the Blood-Brain Barrier

Glioblastoma (GBM) is the most prevalent and lethal adult primary central nervous system cancer. An immunosuppresive and highly heterogeneous tumor microenvironment, restricted delivery of chemotherapy or immunotherapy through the blood-brain barrier (BBB), together with the brain's unique biochemical and anatomical features result in its universal recurrence and poor prognosis. As conventional models fail to predict therapeutic efficacy in GBM, in vitro 3D models of GBM and BBB leveraging patient- or healthy-individual-derived cells and biomaterials through 3D bioprinting technologies potentially mimic essential physiological and pathological features of GBM and BBB. 3D-bioprinted constructs enable investigation of cellular and cell-extracellular matrix interactions in a species-matched, high-throughput, and reproducible manner, serving as screening or drug delivery platforms. Here, an overview of current 3D-bioprinted GBM and BBB models is provided, elaborating on the microenvironmental compositions of GBM and BBB, relevant biomaterials to mimic the native tissues, and bioprinting strategies to implement the model fabrication. Collectively, 3D-bioprinted GBM and BBB models are promising systems and biomimetic alternatives to traditional models for more reliable mechanistic studies and preclinical drug screenings that may eventually accelerate the drug development process for GBM.

Engineering of tissue constructs using coaxial bioprinting

Bioprinting is a rapidly developing technology for the precise design and manufacture of tissues in various biological systems or organs. Coaxial extrusion bioprinting, an emergent branch, has demonstrated a strong potential to enhance bioprinting's engineering versatility. Coaxial bioprinting assists in the fabrication of complex tissue constructs, by enabling concentric deposition of biomaterials. The fabricated tissue constructs started with simple, tubular vasculature but have been substantially developed to integrate complex cell composition and self-assembly, ECM patterning, controlled release, and multi-material gradient profiles. This review article begins with a brief overview of coaxial printing history, followed by an introduction of crucial engineering components. Afterward, we review the recent progress and untapped potential in each specific organ or biological system, and demonstrate how coaxial bioprinting facilitates the creation of tissue constructs. Ultimately, we conclude that this growing technology will contribute significantly to capabilities in the fields of in vitro modeling, pharmaceutical development, and clinical regenerative medicine.

Human mini-brain models

Engineered human mini-brains, made possible by knowledge from the convergence of precision microengineering and cell biology, permit systematic studies of complex neurological processes and of pathogenesis beyond what can be done with animal models. By culturing human brain cells with physiological microenvironmental cues, human mini-brain models reconstitute the arrangement of structural tissues and some of the complex biological functions of the human brain. In this Review, we highlight the most significant developments that have led to microphysiological human mini-brain models. We introduce the history of mini-brain development, review methods for creating mini-brain models in static conditions, and discuss relevant state-of-the-art dynamic cell-culture systems. We also review human mini-brain models that reconstruct aspects of major neurological disorders under static or dynamic conditions. Engineered human mini-brains will contribute to advancing the study of the physiology and aetiology of neurological disorders, and to the development of personalized medicines for them.