Direct Bioprinting of Vessel-Like Tubular Microfluidic Channels

Recent Advances in Hydrogel-Based 3D Bioprinting and Its Potential Application in the Treatment of Congenital Heart Disease

Congenital heart disease (CHD) is the most common birth defect, requiring invasive surgery often before a child's first birthday. Current materials used during CHD surgery lack the ability to grow, remodel, and regenerate. To solve those limitations, 3D bioprinting is an emerging tool with the capability to create tailored constructs based on patients' own imaging data with the ability to grow and remodel once implanted in children with CHD. It has the potential to integrate multiple bioinks with several cell types and biomolecules within 3D-bioprinted constructs that exhibit good structural fidelity, stability, and mechanical integrity. This review gives an overview of CHD and recent advancements in 3D bioprinting technologies with potential use in the treatment of CHD. Moreover, the selection of appropriate biomaterials based on their chemical, physical, and biological properties that are further manipulated to suit their application are also discussed. An introduction to bioink formulations composed of various biomaterials with emphasis on multiple cell types and biomolecules is briefly overviewed. Vasculogenesis and angiogenesis of prefabricated 3D-bioprinted structures and novel 4D printing technology are also summarized. Finally, we discuss several restrictions and our perspective on future directions in 3D bioprinting technologies in the treatment of CHD.

Bridging barriers: advances and challenges in modeling biological barriers and measuring barrier integrity in organ-on-chip systems

Biological barriers such as the blood-brain barrier, skin, and intestinal mucosal barrier play key roles in homeostasis, disease physiology, and drug delivery - as such, it is important to create representative in vitro models to improve understanding of barrier biology and serve as tools for therapeutic development. Microfluidic cell culture and organ-on-a-chip (OOC) systems enable barrier modelling with greater physiological fidelity than conventional platforms by mimicking key environmental aspects such as fluid shear, accurate microscale dimensions, mechanical cues, extracellular matrix, and geometrically defined co-culture. As the prevalence of barrier-on-chip models increases, so does the importance of tools that can accurately assess barrier integrity and function without disturbing the carefully engineered microenvironment. In this review, we first provide a background on biological barriers and the physiological features that are emulated through in vitro barrier models. Then, we outline molecular permeability and electrical sensing barrier integrity assessment methods, and the related challenges specific to barrier-on-chip implementation. Finally, we discuss future directions in the field, as well important priorities to consider such as fabrication costs, standardization, and bridging gaps between disciplines and stakeholders.

Bioprinting of Perfusable, Biocompatible Vessel-like Channels with dECM-Based Bioinks and Living Cells

There is a growing interest in the production of bioinks that on the one hand, are biocompatible and, on the other hand, have mechanical properties that allow for the production of stable constructs that can survive for a long time after transplantation. While the selection of the right material is crucial for bioprinting, there is another equally important issue that is currently being extensively researched-the incorporation of the vascular system into the fabricated scaffolds. Therefore, in the following manuscript, we present the results of research on bioink with unique physico-chemical and biological properties. In this article, two methods of seeding cells were tested using bioink B and seeding after bioprinting the whole model. After 2, 5, 8, or 24 h of incubation, the flow medium was used in the tested systems. At the end of the experimental trial, for each time variant, the canals were stored in formaldehyde, and immunohistochemical staining was performed to examine the presence of cells on the canal walls and roof. Cells adhered to both ways of fiber arrangement; however, a parallel bioprint with the 5 h incubation and the intermediate plating of cells resulted in better adhesion efficiency. For this test variant, the percentage of cells that adhered was at least 20% higher than in the other analyzed variants. In addition, it was for this variant that the lowest percentage of viable cells was found that were washed out of the tested model. Importantly, hematoxylin and eosin staining showed that after 8 days of culture, the cells were evenly distributed throughout the canal roof. Our study clearly shows that neovascularization-promoting cells effectively adhere to ECM-based pancreatic bioink. Summarizing the presented results, it was demonstrated that the proposed bioink compositions can be used for bioprinting bionic organs with a vascular system formed by endothelial cells and fibroblasts.

Biofabrication of engineered blood vessels for biomedical applications

To successfully engineer large-sized tissues, establishing vascular structures is essential for providing oxygen, nutrients, growth factors and cells to prevent necrosis at the core of the tissue. The diameter scale of the biofabricated vasculatures should range from 100 to 1,000 µm to support the mm-size tissue while being controllably aligned and spaced within the diffusion limit of oxygen. In this review, insights regarding biofabrication considerations and techniques for engineered blood vessels will be presented. Initially, polymers of natural and synthetic origins can be selected, modified, and combined with each other to support maturation of vascular tissue while also being biocompatible. After they are shaped into scaffold structures by different fabrication techniques, surface properties such as physical topography, stiffness, and surface chemistry play a major role in the endothelialization process after transplantation. Furthermore, biological cues such as growth factors (GFs) and endothelial cells (ECs) can be incorporated into the fabricated structures. As variously reported, fabrication techniques, especially 3D printing by extrusion and 3D printing by photopolymerization, allow the construction of vessels at a high resolution with diameters in the desired range. Strategies to fabricate of stable tubular structures with defined channels will also be discussed. This paper provides an overview of the many advances in blood vessel engineering and combinations of different fabrication techniques up to the present time.

Recent Developments in Bio-Ink Formulations Using Marine-Derived Biomaterials for Three-Dimensional (3D) Bioprinting

3D bioprinting is a disruptive, computer-aided, and additive manufacturing technology that allows the obtention, layer-by-layer, of 3D complex structures. This technology is believed to offer tremendous opportunities in several fields including biomedical, pharmaceutical, and food industries. Several bioprinting processes and bio-ink materials have emerged recently. However, there is still a pressing need to develop low-cost sustainable bio-ink materials with superior qualities (excellent mechanical, viscoelastic and thermal properties, biocompatibility, and biodegradability). Marine-derived biomaterials, including polysaccharides and proteins, represent a viable and renewable source for bio-ink formulations. Therefore, the focus of this review centers around the use of marine-derived biomaterials in the formulations of bio-ink. It starts with a general overview of 3D bioprinting processes followed by a description of the most commonly used marine-derived biomaterials for 3D bioprinting, with a special attention paid to chitosan, glycosaminoglycans, alginate, carrageenan, collagen, and gelatin. The challenges facing the application of marine-derived biomaterials in 3D bioprinting within the biomedical and pharmaceutical fields along with future directions are also discussed.

Thermally reversible hydrogels printing of customizable bio-channels with curvature

Replicating intricate bio-channels, akin to expansive vascular networks, offers numerous advantages including self-repair, replacing damaged bio-channels, testing drugs, and biomedical devices. But, crafting multi-sized, editable bio-channels with specific curvatures, particularly using natural polymer-based bio-inks, poses a significant challenge. To address this, this study introduces a temperature-driven indirect printing method, exemplified by the diploic vein. Here, K-carrageenan (kca)-silk fiber (SF)-hyaluronic acid (HA)/hFOB 1.19 (SV40 transfection of human osteoblasts) and kca-collagen-HA/HUVECs (human umbilical vein endothelial cells) are employed to fabricate vascular-like walls and lumens, utilizing their thermoreversible properties to create multi-stage bifurcated lumens. Precise spatial curvature was generated by heating the vascular network wrapped in poly(N-isopropyl acrylamide) (PNIPAAm)-poly(ethylene glycol) diacrylate (PEGDA). Since temperature is specific to the thermal material carrying the cells, the rheological properties of bioinks, modeling temperature parameters, and their impact on printing size was explored. Additionally, mechanical properties and curvature response were characterized to determine the necessary process parameters for achieving the desired size. Ultimately, in vitro bioprinting experiments involving HUVECs and hFOB 1.19 demonstrate cell viability, adhesion, proliferation, and migration within the intraluminal hydrogel scaffold. This approach allows for customizing bio-channel content and controlling curvature programming, providing new prospects for in vitro biochannel production, with potential benefits for pathology research.

Directly coaxial bioprinting of 3D vascularized tissue using novel bioink based on decellularized human amniotic membrane

Despite several progressions in the biofabrication of large-scale engineered tissues, direct biopri nting of perfusable three-dimensional (3D) vasculature remained unaddressed. Developing a feasible method to generate cell-laden thick tissue with an effective vasculature network to deliver oxygen and nutrient is crucial for preventing the formation of necrotic spots and tissue death. In this study, we developed a novel technique to directly bioprint 3D cell-laden prevascularized construct. We developed a novel bioink by mixing decellularized human amniotic membrane (dHAM) and alginate (Alg) in various ratios. The bioink with encapsulated human vein endothelial cells (HUVECs) and a crosslinker, CaCl2, were extruded via sheath and core nozzle respectively to directly bioprint a perfusable 3D vasculature construct. The various concentration of bioink was assessed from several aspects like biocompatibility, porosity, swelling, degradation, and mechanical characteristics, and accordingly, optimized concentration was selected (Alg 4 %w/v - dHAM 0.6 %w/v). Then, the crosslinked bioink without microchannel and the 3D bioprinted construct with various microchannel distances (0, 1.5 mm, 3 mm) were compared. The 3D bioprinted construct with a 1.5 mm microchannels distance demonstrated superiority owing to its 492 ± 18.8 % cell viability within 14 days, excellent tubulogenesis, remarkable expression of VEGFR-2 which play a crucial role in endothelial cell proliferation, migration, and more importantly angiogenesis, and neovascularization. This perfusable bioprinted construct also possess appropriate mechanical stability (32.35 ± 5 kPa Young's modulus) for soft tissue. Taking these advantages into the account, our new bioprinting method possesses a prominent potential for the fabrication of large-scale prevascularized tissue to serve for regenerative medicine applications like implantation, drug-screening platform, and the study of mutation disease.

Sacrificial-Rotating Rod-Based 3D Bioprinting Technique for the Development of an In Vitro Cardiovascular Model

Several studies have attempted to develop complex cardiovascular models, but the use of multiple cell types and poor cell alignments after fabrication have limited the practical application of these models. Among various bioprinting methods, extrusion-based bioprinting is the most widely used in the bioengineering field. This method not only has the potential to construct complex 3D biological structures but it also enables the alignment of cells in the printing direction owing to the application of shear stress to the cells during the printing process. Therefore, this study developed an in vitro cardiovascular model using an extrusion-based bioprinting method that utilizes a rotating rod as a printing platform. The rotating rod was made of polyvinyl alcohol (PVA) and used as a sacrificial rod. This rotating platform approach enabled the printing of longer tubular-vascular structures of multiple shapes, including disease models, and the water-soluble properties of PVA facilitated the isolation of the printed vascular models. In addition, this method enabled the printing of the endothelial cells in the bloodstream direction and smooth muscle cells in the circumferential direction to better mimic the anatomy of real blood vessels. Consequently, a cardiovascular model was successfully printed using a gelatin methacryloyl bioink with cells. In conclusion, the proposed fabrication method can facilitate the fabrication of various cardiovascular models that mimic the alignment of real blood vessels.

Synergistic coupling between 3D bioprinting and vascularization strategies

Three-dimensional (3D) bioprinting offers promising solutions to the complex challenge of vascularization in biofabrication, thereby enhancing the prospects for clinical translation of engineered tissues and organs. While existing reviews have touched upon 3D bioprinting in vascularized tissue contexts, the current review offers a more holistic perspective, encompassing recent technical advancements and spanning the entire multistage bioprinting process, with a particular emphasis on vascularization. The synergy between 3D bioprinting and vascularization strategies is crucial, as 3D bioprinting can enable the creation of personalized, tissue-specific vascular network while the vascularization enhances tissue viability and function. The review starts by providing a comprehensive overview of the entire bioprinting process, spanning from pre-bioprinting stages to post-printing processing, including perfusion and maturation. Next, recent advancements in vascularization strategies that can be seamlessly integrated with bioprinting are discussed. Further, tissue-specific examples illustrating how these vascularization approaches are customized for diverse anatomical tissues towards enhancing clinical relevance are discussed. Finally, the underexplored intraoperative bioprinting (IOB) was highlighted, which enables the direct reconstruction of tissues within defect sites, stressing on the possible synergy shaped by combining IOB with vascularization strategies for improved regeneration.

Bio-inspired microfluidics: A review

Biomicrofluidics, a subdomain of microfluidics, has been inspired by several ideas from nature. However, while the basic inspiration for the same may be drawn from the living world, the translation of all relevant essential functionalities to an artificially engineered framework does not remain trivial. Here, we review the recent progress in bio-inspired microfluidic systems via harnessing the integration of experimental and simulation tools delving into the interface of engineering and biology. Development of "on-chip" technologies as well as their multifarious applications is subsequently discussed, accompanying the relevant advancements in materials and fabrication technology. Pointers toward new directions in research, including an amalgamated fusion of data-driven modeling (such as artificial intelligence and machine learning) and physics-based paradigm, to come up with a human physiological replica on a synthetic bio-chip with due accounting of personalized features, are suggested. These are likely to facilitate physiologically replicating disease modeling on an artificially engineered biochip as well as advance drug development and screening in an expedited route with the minimization of animal and human trials.

Chitosan and its derivatives in 3D/4D (bio) printing for tissue engineering and drug delivery applications

Tissue engineering research has undergone to a revolutionary improvement, thanks to technological advancements, such as the introduction of bioprinting technologies. The ability to develop suitable customized biomaterial inks/bioinks, with excellent printability and ability to promote cell proliferation and function, has a deep impact on such improvements. In this context, printing inks based on chitosan and its derivatives have been instrumental. Thus, the current review aims at providing a comprehensive overview on chitosan-based materials as suitable inks for 3D/4D (bio)printing and their applicability in creating advanced drug delivery platforms and tissue engineered constructs. Furthermore, relevant strategies to improve the mechanical and biological performances of this biomaterial are also highlighted.

Advances in Gelatin Bioinks to Optimize Bioprinted Cell Functions

Gelatin is a widely utilized bioprinting biomaterial due to its cell-adhesive and enzymatically cleavable properties, which improve cell adhesion and growth. Gelatin is often covalently cross-linked to stabilize bioprinted structures, yet the covalently cross-linked matrix is unable to recapitulate the dynamic microenvironment of the natural extracellular matrix (ECM), thereby limiting the functions of bioprinted cells. To some extent, a double network bioink can provide a more ECM-mimetic, bioprinted niche for cell growth. More recently, gelatin matrices are being designed using reversible cross-linking methods that can emulate the dynamic mechanical properties of the ECM. This review analyzes the progress in developing gelatin bioink formulations for 3D cell culture, and critically analyzes the bioprinting and cross-linking techniques, with a focus on strategies to optimize the functions of bioprinted cells. This review discusses new cross-linking chemistries that recapitulate the viscoelastic, stress-relaxing microenvironment of the ECM, and enable advanced cell functions, yet are less explored in engineering the gelatin bioink. Finally, this work presents the perspective on the areas of future research and argues that the next generation of gelatin bioinks should be designed by considering cell-matrix interactions, and bioprinted constructs should be validated against currently established 3D cell culture standards to achieve improved therapeutic outcomes.

Building Blood Vessel Chips with Enhanced Physiological Relevance

Blood vessel chips are bioengineered microdevices, consisting of biomaterials, human cells, and microstructures, which recapitulate essential vascular structure and physiology and allow a well-controlled microenvironment and spatial-temporal readouts. Blood vessel chips afford promising opportunities to understand molecular and cellular mechanisms underlying a range of vascular diseases. The physiological relevance is key to these blood vessel chips that rely on bioinspired strategies and bioengineering approaches to translate vascular physiology into artificial units. Here, we discuss several critical aspects of vascular physiology, including morphology, material composition, mechanical properties, flow dynamics, and mass transport, which provide essential guidelines and a valuable source of bioinspiration for the rational design of blood vessel chips. We also review state-of-art blood vessel chips that exhibit important physiological features of the vessel and reveal crucial insights into the biological processes and disease pathogenesis, including rare diseases, with notable implications for drug screening and clinical trials. We envision that the advances in biomaterials, biofabrication, and stem cells improve the physiological relevance of blood vessel chips, which, along with the close collaborations between clinicians and bioengineers, enable their widespread utility.

3D bioprinting and its innovative approach for biomedical applications

3D bioprinting or additive manufacturing is an emerging innovative technology revolutionizing the field of biomedical applications by combining engineering, manufacturing, art, education, and medicine. This process involved incorporating the cells with biocompatible materials to design the required tissue or organ model in situ for various in vivo applications. Conventional 3D printing is involved in constructing the model without incorporating any living components, thereby limiting its use in several recent biological applications. However, this uses additional biological complexities, including material choice, cell types, and their growth and differentiation factors. This state-of-the-art technology consciously summarizes different methods used in bioprinting and their importance and setbacks. It also elaborates on the concept of bioinks and their utility. Biomedical applications such as cancer therapy, tissue engineering, bone regeneration, and wound healing involving 3D printing have gained much attention in recent years. This article aims to provide a comprehensive review of all the aspects associated with 3D bioprinting, from material selection, technology, and fabrication to applications in the biomedical fields. Attempts have been made to highlight each element in detail, along with the associated available reports from recent literature. This review focuses on providing a single platform for cancer and tissue engineering applications associated with 3D bioprinting in the biomedical field.

Tailor-made natural and synthetic grafts for precise urethral reconstruction

Injuries to the urethra can be caused by malformations, trauma, inflammation, or carcinoma, and reconstruction of the injured urethra is still a significant challenge in clinical urology. Implanting grafts for urethroplasty and end-to-end anastomosis are typical clinical interventions for urethral injury. However, complications and high recurrence rates remain unsatisfactory. To address this, urethral tissue engineering provides a promising modality for urethral repair. Additionally, developing tailor-made biomimetic natural and synthetic grafts is of great significance for urethral reconstruction. In this work, tailor-made biomimetic natural and synthetic grafts are divided into scaffold-free and scaffolded grafts according to their structures, and the influence of different graft structures on urethral reconstruction is discussed. In addition, future development and potential clinical application strategies of future urethral reconstruction grafts are predicted.

Biocompatible fluorescent silk fibroin bioink for digital light processing 3D printing

Chemically modified silk fibroin (SF) bioink has been used for three-dimensional (3D) bioprinting in tissue engineering because of its biocompatibility and printability. Also, fluorescent silk fibroin (FSF) from transgenic silkworms has been recently applied in biomedicine because of its fluorescence property. However, the fabrication of fluorescent hydrogel from FSF has not been elucidated. In this study, we showed the fabrication of a digital light processing (DLP) printable bioink from a chemically modified FSF. This bioink was fabricated by covalent conjugation of FSF and glycidyl methacrylate (GMA) and can be printed into various structures, such as the brain, ear, hand, lung, and internal organs. The physical properties of glycidyl methacrylated fluorescent silk fibroin (FSGMA) hydrogel was like the glycidyl methacrylated non-fluorescent silk fibroin (SGMA) hydrogel. The FSGMA hydrogel significantly retains its fluorescence property and has excellent biocompatibility. All these properties make FSGMA hydrogel a potent tool in encapsulated cell tracking and observing the scaffolds' degradation in vivo. This study suggested that our 3D DLP printable FSF bioink could play a promising role in the biomedical field.

Bioink Formulation and Machine Learning-Empowered Bioprinting Optimization

Bioprinting enables the fabrication of complex, heterogeneous tissues through robotically-controlled placement of cells and biomaterials. It has been rapidly developing into a powerful and versatile tool for tissue engineering. Recent advances in bioprinting modalities and biofabrication strategies as well as new materials and chemistries have led to improved mimicry and development of physiologically relevant tissue architectures constituted with multiple cell types and heterogeneous spatial material properties. Machine learning (ML) has been applied to accelerate these processes. It is a new paradigm for bioprinting. In this review, we explore current trends in bioink formulation and how ML has been used to accelerate optimization and enable real-time error detection as well as to reduce the iterative steps necessary for bioink formulation. We examined how rheometric properties, including shear storage, loss moduli, viscosity, shear-thinning property of biomaterials affect the printability of a bioink. Furthermore, we scrutinized the interplays between yield shear stress and the printability of a bioink. Moreover, we systematically surveyed the application of ML in precision in situ surgical site bioprinting, closed-loop AI printing, and post-printing optimization.

Role of nanostructured materials in hard tissue engineering

The rise in the use of biomaterials in bone regeneration in the last decade has exponentially multiplied the number of publications, methods, and approaches to improve and optimize their functionalities and applications. In particular, biomimetic strategies based on the self-assembly of molecules to design, create and characterize nanostructured materials have played a very relevant role. We address this idea on four different but related points: self-setting bone cements based on calcium phosphate, as stable tissue support and regeneration induction; metallic prosthesis coatings for cell adhesion optimization and prevention of inflammatory response exacerbation; bio-adhesive hybrid materials as multiple drug delivery localized platforms and finally bio-inks. The effect of the physical, chemical, and biological properties of the newest biomedical devices on their bone tissue regenerative capacity are summarized, described, and analyzed in detail. The roles of experimental conditions, characterization methods and synthesis routes are emphasized. Finally, the future opportunities and challenges of nanostructured biomaterials with their advantages and shortcomings are proposed in order to forecast the future directions of this field of research.

Dual-core coaxial bioprinting of double-channel constructs with a potential for perfusion and interaction of cells

Coaxial bioprinting of hydrogel tubes has tremendous potential in the fabrication of highly complex large-scale vascularized structures, however, constructs with bioinks of simultaneous weak printability and perfusable networks have not been reported. Here, we report a coaxial printing method in which double-channel filaments are three-dimensional (3D) extrusion-bioprinted using a customized dual-core coaxial nozzle. The filament in one channel can perform core/shell role and the other channel can play a role in perfusion. These parallel channels within filaments are separated by an interval wall of alginate, whose thickness (∼50μm) is beneficial to supplement nutrients via perfusion. Different cell-laden hydrogels of weak mechanics were used to test the adaptability and perfusability of our method, and the results showed that dynamic perfusion maintained higher viability and functions than static culture. By combining with a bioprinter, 8-layer perfusable double-channel constructs were fabricated, and the cell viabilities gradually decreased with the reduction in nutrients and oxygen in the downstream medium. Furthermore, the double-channel filaments were tested as a platform to mimic dynamic functions between cells through sequential perfusion by using Mouse insulinoma 6 (Min6) and Hepatocellular carcinoma (HepG2) as the model cells. These results demonstrated the insulin secreted by Min6 upstream simulated and increased the uptake of glucose by the downstream HepG2 cells. In conclusion, our study provided evidence for the probability of all-in-one fabrication of 3D double-channel perfusable constructs with high simplicity, expansibility, and versability. Our strategy has significant potential for building large-scale tissue constructs for applications in tissue engineering, possibly even in drug screening and regenerative medicine.

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.

Microfluidic Coaxial Bioprinting of Hollow, Standalone, and Perfusable Vascular Conduits

Three-dimensional bioprinting represents promising approach for fabricating standalone and perfusable vascular conduits using biocompatible materials. Here we describe a step-by-step method by using a multichannel coaxial extrusion system (MCCES) and a blend bioink constituting gelatin methacryloyl, sodium alginate, and eight-arm poly(ethylene glycol)-acrylate with a tripentaerythritol core for the fabrication of standalone circumferentially multilayered hollow tubes. This microfluidic bioprinting method allows the fabrication of perfusable vascular conduits with a core lumen, an inner endothelial layer resembling the tunica intima, and an outer smooth muscle cell layer resembling the tunica media of the blood vessel. Biocompatible and perfusable blood vessels with a widely tunable size range in terms of luminal diameters and wall thicknesses can be successfully fabricated using the MCCES.

Advanced Strategies for 3D Bioprinting of Tissue and Organ Analogs Using Alginate Hydrogel Bioinks

Alginate is a natural polysaccharide that typically originates from various species of algae. Due to its low cost, good biocompatibility, and rapid ionic gelation, the alginate hydrogel has become a good option of bioink source for 3D bioprinting. However, the lack of cell adhesive moieties, erratic biodegradability, and poor printability are the critical limitations of alginate hydrogel bioink. This review discusses the pivotal properties of alginate hydrogel as a bioink for 3D bioprinting technologies. Afterward, a variety of advanced material formulations and biofabrication strategies that have recently been developed to overcome the drawbacks of alginate hydrogel bioink will be focused on. In addition, the applications of these advanced solutions for 3D bioprinting of tissue/organ mimicries such as regenerative implants and in vitro tissue models using alginate-based bioink will be systematically summarized.

Microfluidic models of the human circulatory system: versatile platforms for exploring mechanobiology and disease modeling

The human circulatory system is a marvelous fluidic system, which is very sensitive to biophysical and biochemical cues. The current animal and cell culture models do not recapitulate the functional properties of the human circulatory system, limiting our ability to fully understand the complex biological processes underlying the dysfunction of this multifaceted system. In this review, we discuss the unique ability of microfluidic systems to recapitulate the biophysical, biochemical, and functional properties of the human circulatory system. We also describe the remarkable capacity of microfluidic technologies for exploring the complex mechanobiology of the cardiovascular system, mechanistic studying of cardiovascular diseases, and screening cardiovascular drugs with the additional benefit of reducing the need for animal models. We also discuss opportunities for further advancement in this exciting field.

Bioprinting Au Natural: The Biologics of Bioinks

The development of appropriate bioinks is a complex task, dependent on the mechanical and biochemical requirements of the final construct and the type of printer used for fabrication. The two most common tissue printers are micro-extrusion and digital light projection printers. Here we briefly discuss the required characteristics of a bioink for each of these printing processes. However, physical printing is only a short window in the lifespan of a printed construct-the system must support and facilitate cellular development after it is printed. To that end, we provide a broad overview of some of the biological molecules currently used as bioinks. Each molecule has advantages for specific tissues/cells, and potential disadvantages are discussed, along with examples of their current use in the field. Notably, it is stressed that active researchers are trending towards the use of composite bioinks. Utilizing the strengths from multiple materials is highlighted as a key component of bioink development.

Colonization versus encapsulation in cell-laden materials design: porosity and process biocompatibility determine cellularization pathways

Seeding materials with living cells has been-and still is-one of the most promising approaches to reproduce the complexity and the functionality of living matter. The strategies to associate living cells with materials are limited to cell encapsulation and colonization, however, the requirements for these two approaches have been seldom discussed systematically. Here we propose a simple two-dimensional map based on materials' pore size and the cytocompatibility of their fabrication process to draw, for the first time, a guide to building cellularized materials. We believe this approach may serve as a straightforward guideline to design new, more relevant materials, able to seize the complexity and the function of biological materials. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Progress of 3D Bioprinting in Organ Manufacturing

Three-dimensional (3D) bioprinting is a family of rapid prototyping technologies, which assemble biomaterials, including cells and bioactive agents, under the control of a computer-aided design model in a layer-by-layer fashion. It has great potential in organ manufacturing areas with the combination of biology, polymers, chemistry, engineering, medicine, and mechanics. At present, 3D bioprinting technologies can be used to successfully print living tissues and organs, including blood vessels, skin, bones, cartilage, kidney, heart, and liver. The unique advantages of 3D bioprinting technologies for organ manufacturing have improved the traditional medical level significantly. In this article, we summarize the latest research progress of polymers in bioartificial organ 3D printing areas. The important characteristics of the printable polymers and the typical 3D bioprinting technologies for several complex bioartificial organs, such as the heart, liver, nerve, and skin, are introduced.

Composite Inks for Extrusion Printing of Biological and Biomedical Constructs

Extrusion-based three-dimensional (3D) printing is an emerging technology for the fabrication of complex structures with various biological and biomedical applications. The method is based on the layer-by-layer construction of the product using a printable ink. The material used as the ink should possess proper rheological properties and desirable performances. Composite materials, which are extensively used in 3D printing applications, can improve the printability and offer superior performances for the printed constructs. Herein, we review composite inks with a focus on composite hydrogels. The properties of different additives including fibers and nanoparticles are discussed. The performances of various composite inks in biological and biomedical systems are delineated through analyzing the synergistic effects between the composite ink components. Different applications, including tissue engineering, tissue model engineering, soft robotics, and four-dimensional printing, are selected to demonstrate how 3D-printable composite inks are exploited to achieve various desired functionality. This review finally presents an outlook of future perspectives on the design of composite inks.

The potential of microfluidics-enhanced extrusion bioprinting

Microfluidics-enhanced bioprinting holds great promise in the field of biofabrication as it enables the fabrication of complex constructs with high shape fidelity and utilization of a broad range of bioinks with varying viscosities. Microfluidic systems contain channels on the micrometer-scale, causing a change in fluid behaviors, enabling unconventional bioprinting applications such as facilitating the precise spatial positioning and switching between bioinks with higher accuracy compared to traditional approaches. These systems can roughly be divided into three groups: microfluidic chips, co- and triaxial printheads, and printheads combining both. Although several aspects and parameters remain to be improved, this technology is promising as it is a step toward recapitulating the complex native histoarchitecture of human tissues more precisely. In this Perspective, key research on these different systems will be discussed before moving onto the limitations and outlook of microfluidics-enhanced bioprinting as a whole.

Recent advancements in the bioprinting of vascular grafts

Recent advancements in the bioinks and three-dimensional (3D) bioprinting methods used to fabricate vascular constructs are summarized herein. Critical biomechanical properties required to fabricate an ideal vascular graft are highlighted, as well as various testing methods have been outlined to evaluate the bio-fabricated grafts as per the Food and Drug Administration (FDA) and International Organization for Standardization (ISO) guidelines. Occlusive artery disease and cardiovascular disease are the major causes of death globally. These diseases are caused by the blockage in the arteries, which results in a decreased blood flow to the tissues of major organs in the body, such as the heart. Bypass surgery is often performed using a vascular graft to re-route the blood flow. Autologous grafts represent a gold standard for such bypass surgeries; however, these grafts may be unavailable due to the previous harvesting or possess a poor quality. Synthetic grafts serve well for medium to large-sized vessels, but they fail when used to replace small-diameter vessels, generally smaller than 6 mm. Various tissue engineering approaches have been used to address the urgent need for vascular graft that can withstand hemodynamic blood pressure and has the ability to grow and remodel. Among these approaches, 3D bioprinting offers an attractive solution to construct patient-specific vessel grafts with layered biomimetic structures.

Biofabrication of tissue engineering vascular systems

Cardiovascular disease (CVD) is the leading cause of death among persons aged 65 and older in the United States and many other developed countries. Tissue engineered vascular systems (TEVS) can serve as grafts for CVD treatment and be used as in vitro model systems to examine the role of various genetic factors during the CVD progressions. Current focus in the field is to fabricate TEVS that more closely resembles the mechanical properties and extracellular matrix environment of native vessels, which depends heavily on the advance in biofabrication techniques and discovery of novel biomaterials. In this review, we outline the mechanical and biological design requirements of TEVS and explore the history and recent advances in biofabrication methods and biomaterials for tissue engineered blood vessels and microvascular systems with special focus on in vitro applications. In vitro applications of TEVS for disease modeling are discussed.

Rheology and direct write printing of chitosan - graphene oxide nanocomposite hydrogels for differentiation of neuroblastoma cells

The direct write printing method has gained popularity in synthesizing scaffolds for tissue engineering. To achieve an excellent printability of scaffolds, a thorough evaluation of rheological properties is required. We report the synthesis, characterization, rheology, and direct-write printing of chitosan - graphene oxide (CH - GO) nanocomposite hydrogels at a varying concentration of GO in 3 and 4 wt% CH polymeric gels. Rheological characterization of CH - GO hydrogels shows that an addition of only 0.5 wt% of GO leads to a substantial increase in storage modulus (G'), viscosity, and yield stress of 3 and 4 wt% of CH hydrogels. A three-interval thixotropy test (3ITT) shows that 3 wt% CH with 0.5 wt% GO hydrogel has 94% recovery of G' after 7 sequential stress cycles and is the best candidate for direct-write printing. Neuronal cell culture on 3 wt% CH with 0.5 wt% hydrogels reveals that GO promotes the differentiation of SH-SY5Y cells.

3D Bioprinting of Vascularized Tissues for in vitro and in vivo Applications

With a limited supply of organ donors and available organs for transplantation, the aim of tissue engineering with three-dimensional (3D) bioprinting technology is to construct fully functional and viable tissue and organ replacements for various clinical applications. 3D bioprinting allows for the customization of complex tissue architecture with numerous combinations of materials and printing methods to build different tissue types, and eventually fully functional replacement organs. The main challenge of maintaining 3D printed tissue viability is the inclusion of complex vascular networks for nutrient transport and waste disposal. Rapid development and discoveries in recent years have taken huge strides toward perfecting the incorporation of vascular networks in 3D printed tissue and organs. In this review, we will discuss the latest advancements in fabricating vascularized tissue and organs including novel strategies and materials, and their applications. Our discussion will begin with the exploration of printing vasculature, progress through the current statuses of bioprinting tissue/organoids from bone to muscles to organs, and conclude with relevant applications for in vitro models and drug testing. We will also explore and discuss the current limitations of vascularized tissue engineering and some of the promising future directions this technology may bring.

One-Step Formation of Protein-Based Tubular Structures for Functional Devices and Tissues

Tubular biological structures consisting of extracellular matrix (ECM) proteins and cells are basic functional units of all organs in animals and humans. ECM protein solutions at low concentrations (5-10 milligrams per milliliter) are abundantly used in 3D cell culture. However, their poor "printability" and minute-long gelation time have made the direct extrusion of tubular structures in bioprinting applications challenging. Here, this limitation is overcome and the continuous, template-free conversion of low-concentration collagen, elastin, and fibrinogen solutions into tubular structures of tailored size and radial, circumferential and axial organization is demonstrated. The approach is enabled by a microfabricated printhead for the consistent circumferential distribution of ECM protein solutions and lends itself to scalable manufacture. The attached confinement accommodates minute-long residence times for pH, temperature, light, ionic and enzymatic gelation. Chip hosted ECM tubular structures are amenable to perfusion with aqueous solutions and air, and cyclic stretching. Predictive collapse and reopening in a crossed-tube configuration promote all-ECM valves and pumps. Tissue level function is demonstrated by factors secreted from cells embedded within the tube wall, as well as endothelial or epithelial barriers lining the lumen. The described approaches are anticipated to find applications in ECM-based organ-on-chip and biohybrid structures, hydraulic actuators, and soft machines.

Complex 3D bioprinting methods

3D bioprinting technology is evolving in complexity to enable human-scale, high-resolution, and multi-cellular constructs to better mimic the native tissue microenvironment. The ultimate goal is to achieve necessary complexity in the bioprinting process to biomanufacture fully-functional tissues and organs to address organ shortage and lack of patient-specific disease models. In this Review, we presented an in-depth overview of complex 3D bioprinting approaches including evolution of complex bioprinting, from simple gel-casting approach to multi-material bioprinting to omnidirectional bioprinting approaches, and emerging bioprinting approaches, including 4D bioprinting and in situ bioprinting technologies.

On-chip high-definition bioprinting of microvascular structures

'Organ-on-chip' devices which integrate three-dimensional (3D) cell culture techniques with microfluidic approaches have the capacity to overcome the limitations of classical 2D platforms. Although several different strategies have been developed to improve the angiogenesis within hydrogels, one of the main challenges in tissue engineering remains the lack of vascularization in the fabricated 3D models. The present work focuses on the high-definition (HD) bioprinting of microvascular structures directly on-chip using two-photon polymerization (2PP). 2PP is a nonlinear process, where the near-infrared laser irradiation will only lead to the polymerization of a very small volume pixel (voxel), allowing the fabrication of channels in the microvascular range (10-30 µm in diameter). Additionally, 2PP not only enables the fabrication of sub-micrometer resolution scaffolds but also allows the direct embedding of cells within the produced structure. The accuracy of the 2PP printing parameters were optimized in order to achieve high-throughput and HD production of microfluidic vessel-on-chip platforms. The spherical aberrations stemming from the refractive index mismatch and the focusing depth inside the sample were simulated and the effect of the voxel compensation as well as different printing modes were demonstrated. Different layer spacings and their dependency on the applied laser power were compared both in terms of accuracy and required printing time resulting in a 10-fold decrease in structuring time while yielding well-defined channels of small diameters. Finally, the capacity of 2PP to create vascular structures within a microfluidic chip was tested with two different settings, by direct embedding of a co-culture of endothelial- and supporting cells during the printing process and by creating a supporting, cell-containing vascular scaffold barrier where the endothelial cell spheroids can be seeded afterwards. The functionality of the formed vessels was demonstrated with immunostaining of vascular endothelial cadherin (VE-Cadherin) endothelial adhesion molecules in both static and perfused culture.

Chitosan hydrogels in 3D printing for biomedical applications

Tissue engineering and regenerative medicine have entered a new stage of development by the recent progress in biology, material sciences, and particularly an emerging additive manufacturing technique, three-dimensional (3D) printing. 3D printing is an advanced biofabrication technique which can generate patient-specific scaffolds with highly complex geometries while hosting cells and bioactive agents to accelerate tissue regeneration. Chitosan hydrogels themselves have been widely used for various biomedical applications due to its abundant availability, structural features and favorable biological properties; however, the 3D printing of chitosan-based hydrogels is still under early exploration. Therefore, 3D printing technologies represent a new avenue to explore the potential application of chitosan as an ink for 3D printing, or as a coating on other 3D printed scaffolds. The combination of chitosan-based hydrogels and 3D printing holds much promise in the development of next generation biomedical implants.

Chitosan based-nanoparticles and nanocapsules: Overview, physicochemical features, applications of a nanofibrous scaffold, and bioprinting

Recent advancements in the synthesis, properties, and applications of chitosan as the second after cellulose available biopolymer in nature were discussed in this review. A general overview of processing and production procedures from A to Z was highlighted. Chitosan exists in three polymorphic forms which differ in degree of crystallinity (α, β, and γ). Thus, the degree of deacetylation, crystallinity, surface area, and molecular mass significantly affect most applications. Otherwise, the synthesis of chitosan nanofibers is suffering from many drawbacks that were recently treated by co-electrospun with other polymers such as polyvinyl alcohol (PVA), polyethylene oxide (PEO), and polycaprolactone (PCL). Ultimately, this review focuses on the area of new trend utilization of chitosan nanoparticles as nanospheres and nanocapsules, in cartilage and bone regenerative medicine. Owing to its biocompatibility, bioavailability, biodegradability, and costless synthesis, chitosan is a promising biopolymeric structure for water remediation, drug delivery, antimicrobials, and tissue engineering.

Recent progress in extrusion 3D bioprinting of hydrogel biomaterials for tissue regeneration: a comprehensive review with focus on advanced fabrication techniques

Over the last decade, 3D bioprinting has received immense attention from research communities to bridge the divergence between artificially engineered tissue constructs and native tissues.

Acoustic Cell Patterning in Hydrogel for Three-Dimensional Cell Network Formation

In the field of engineered organ and drug development, three-dimensional network-structured tissue has been a long-sought goal. This paper presents a direct hydrogel extrusion process exposed to an ultrasound standing wave that aligns fibroblast cells to form a network structure. The frequency-shifted (2 MHz to 4 MHz) ultrasound actuation of a 400-micrometer square-shaped glass capillary that was continuously perfused by fibroblast cells suspended in sodium alginate generated a hydrogel string, with the fibroblasts aligned in single or quadruple streams. In the transition from the one-cell stream to the four-cell streams, the aligned fibroblast cells were continuously interconnected in the form of a branch and a junction. The ultrasound-exposed fibroblast cells displayed over 95% viability up to day 10 in culture medium without any significant difference from the unexposed fibroblast cells. This acoustofluidic method will be further applied to create a vascularized network by replacing fibroblast cells with human umbilical vein endothelial cells.

Solid Organ Bioprinting: Strategies to Achieve Organ Function

The field of tissue engineering has advanced over the past decade, but the largest impact on human health should be achieved with the transition of engineered solid organs to the clinic. The number of patients suffering from solid organ disease continues to increase, with over 100 000 patients on the U.S. national waitlist and approximately 730 000 deaths in the United States resulting from end-stage organ disease annually. While flat, tubular, and hollow nontubular engineered organs have already been implanted in patients, in vitro formation of a fully functional solid organ at a translatable scale has not yet been achieved. Thus, one major goal is to bioengineer complex, solid organs for transplantation, composed of patient-specific cells. Among the myriad of approaches attempted to engineer solid organs, 3D bioprinting offers unmatched potential. This review highlights the structural complexity which must be engineered at nano-, micro-, and mesostructural scales to enable organ function. We showcase key advances in bioprinting solid organs with complex vascular networks and functioning microstructures, advances in biomaterials science that have enabled this progress, the regulatory hurdles the field has yet to overcome, and cutting edge technologies that bring us closer to the promise of engineered solid organs.

Electroconductive Hydrogels for Tissue Engineering: Current Status and Future Perspectives

Over the past decade, electroconductive hydrogels, integrating both the biomimetic attributes of hydrogels and the electrochemical properties of conductive materials, have gained significant attention. Hydrogels, three-dimensional and swollen hydrophilic polymer networks, are an important class of tissue engineering (TE) scaffolds owing to their microstructural and mechanical properties, ability to mimic the native extracellular matrix, and promote tissue repair. However, hydrogels are intrinsically insulating and therefore unable to emulate the complex electrophysiological microenvironment of cardiac and neural tissues. To overcome this challenge, electroconductive materials, including carbon-based materials, nanoparticles, and polymers, have been incorporated within nonconductive hydrogels to replicate the electrical and biological characteristics of biological tissues. This review gives a brief introduction on the rational design of electroconductive hydrogels and their current applications in TE, especially for neural and cardiac regeneration. The recent progress and development trends of electroconductive hydrogels, their challenges, and clinical translatability, as well as their future perspectives, with a focus on advanced manufacturing technologies, are also discussed.

Synthetic Polymers for Organ 3D Printing

Three-dimensional (3D) printing, known as the most promising approach for bioartificial organ manufacturing, has provided unprecedented versatility in delivering multi-functional cells along with other biomaterials with precise control of their locations in space. The constantly emerging 3D printing technologies are the integration results of biomaterials with other related techniques in biology, chemistry, physics, mechanics and medicine. Synthetic polymers have played a key role in supporting cellular and biomolecular (or bioactive agent) activities before, during and after the 3D printing processes. In particular, biodegradable synthetic polymers are preferable candidates for bioartificial organ manufacturing with excellent mechanical properties, tunable chemical structures, non-toxic degradation products and controllable degradation rates. In this review, we aim to cover the recent progress of synthetic polymers in organ 3D printing fields. It is structured as introducing the main approaches of 3D printing technologies, the important properties of 3D printable synthetic polymers, the successful models of bioartificial organ printing and the perspectives of synthetic polymers in vascularized and innervated organ 3D printing areas.

Extrusion and Microfluidic-based Bioprinting to Fabricate Biomimetic Tissues and Organs

Next generation engineered tissue constructs with complex and ordered architectures aim to better mimic the native tissue structures, largely due to advances in three-dimensional (3D) bioprinting techniques. Extrusion bioprinting has drawn tremendous attention due to its widespread availability, cost-effectiveness, simplicity, and its facile and rapid processing. However, poor printing resolution and low speed have limited its fidelity and clinical implementation. To circumvent the downsides associated with extrusion printing, microfluidic technologies are increasingly being implemented in 3D bioprinting for engineering living constructs. These technologies enable biofabrication of heterogeneous biomimetic structures made of different types of cells, biomaterials, and biomolecules. Microfluiding bioprinting technology enables highly controlled fabrication of 3D constructs in high resolutions and it has been shown to be useful for building tubular structures and vascularized constructs, which may promote the survival and integration of implanted engineered tissues. Although this field is currently in its early development and the number of bioprinted implants is limited, it is envisioned that it will have a major impact on the production of customized clinical-grade tissue constructs. Further studies are, however, needed to fully demonstrate the effectiveness of the technology in the lab and its translation to the clinic.

Peptide Chitosan/Dextran Core/Shell Vascularized 3D Constructs for Wound Healing

Three-dimensional (3D) bioprinting has emerged to create novel cell-based therapies for regenerative medicine applications. Vascularized networks within engineered constructs are required, and toward this end, we report a promising strategy using core-shell (c/s) extrusion 3D-bioprinting technology that employs biomimetic biomaterials to construct regenerative, prevascularized scaffolds for wound care. A custom-designed cell-responsive bioink consisting of a 13% (w/v) cell-laden gelatin methacryloyl (GelMA) shell surrounding a peptide-functionalized, succinylated chitosan (C)/dextran aldehyde (D) cell-laden core was successfully bioprinted resulting in organized microdesigns exhibiting excellent cell viability and subsequent vessel formation. Our templating strategy takes advantage of GelMA's intrinsic thermoreversible properties of low degree of acryloyl functionalization used in combination with a lightly, chemically cross-linked peptide-CD core to serve as temporal structural supports that stabilize during extrusion onto a cooled platform. Mechanical integrity was further strengthened layer-by-layer via GelMA UV photo-cross-linking. We report the first example of GelMA used in combination with a peptide-CD bioink to c/s 3D-bioprint regenerative, prevascularized constructs for wound care. Particular cell adhesion and proteolytic peptide-CD functionalized pair combinations, P15/MMP-2 and P15/cRGD, were found to significantly increase growth of human bone-marrow-derived mesenchymal stems cells (hBMSCs) and human umbilical vein endothelial cells (HUVECs). The constructs delivered two cell types: hBMSCs in the shell bioink and HUVECs within the core bioink. Cord-like, natural microvascularization was shown with endothelial cell marker expression as confirmed by immunofluorescence (IF) staining exhibiting tubelike structures. In addition, in vitro skin wound healing activity of the construct showed a ∼twofold rate of wound closure. Overall, c/s 3D-bioprinted, peptide-CD/GelMA constructs provided the appropriate microenvironment for in vitro stem and endothelial cell viability, delivery, and differentiation. We foresee these custom constructs as representing a fundamental step toward engineering larger scale regenerative, prevascularized tissues.

3D biofabrication for soft tissue and cartilage engineering

Soft tissue injuries (STIs) affect patients of all age groups and represent a common worldwide clinical problem, resulting from conditions including trauma, infection, cancer and burns. Within the spectrum of STIs a mixture of tissues can be injured, ranging from skin to underlying nerves, blood vessels, tendons and cartilaginous tissues. However, significant limitations affect current treatment options and clinical demand for soft tissue and cartilage regenerative therapies continues to rise. Improving the regeneration of soft tissues has therefore become a key area of focus within tissue engineering. As an emerging technology, 3D bioprinting can be used to build complex soft tissue constructs "from the bottom up," by depositing cells, growth factors, extracellular matrices and other biomaterials in a layer-by-layer fashion. In this way, regeneration of cartilage, skin, vasculature, nerves, tendons and other bodily tissues can be performed in a patient specific manner. This review will focus on recent use of 3D bioprinting and other biofabrication strategies in soft tissue repair and regeneration. Biofabrication of a variety of soft tissue types will be reviewed following an overview of available cell sources, bioinks and bioprinting techniques.

Three-Dimensional Printing Constructs Based on the Chitosan for Tissue Regeneration: State of the Art, Developing Directions and Prospect Trends

Chitosan (CS) has gained particular attention in biomedical applications due to its biocompatibility, antibacterial feature, and biodegradability. Hence, many studies have focused on the manufacturing of CS films, scaffolds, particulate, and inks via different production methods. Nowadays, with the possibility of the precise adjustment of porosity size and shape, fiber size, suitable interconnectivity of pores, and creation of patient-specific constructs, 3D printing has overcome the limitations of many traditional manufacturing methods. Therefore, the fabrication of 3D printed CS scaffolds can lead to promising advances in tissue engineering and regenerative medicine. A review of additive manufacturing types, CS-based printed constructs, their usages as biomaterials, advantages, and drawbacks can open doors to optimize CS-based constructions for biomedical applications. The latest technological issues and upcoming capabilities of 3D printing with CS-based biopolymers for different applications are also discussed. This review article will act as a roadmap aiming to investigate chitosan as a new feedstock concerning various 3D printing approaches which may be employed in biomedical fields. In fact, the combination of 3D printing and CS-based biopolymers is extremely appealing particularly with regard to certain clinical purposes. Complications of 3D printing coupled with the challenges associated with materials should be recognized to help make this method feasible for wider clinical requirements. This strategy is currently gaining substantial attention in terms of several industrial biomedical products. In this review, the key 3D printing approaches along with revealing historical background are initially presented, and ultimately, the applications of different 3D printing techniques for fabricating chitosan constructs will be discussed. The recognition of essential complications and technical problems related to numerous 3D printing techniques and CS-based biopolymer choices according to clinical requirements is crucial. A comprehensive investigation will be required to encounter those challenges and to completely understand the possibilities of 3D printing in the foreseeable future.

3D Bioprinting for Vascularized Tissue-Engineered Bone Fabrication

Vascularization in bone tissues is essential for the distribution of nutrients and oxygen, as well as the removal of waste products. Fabrication of tissue-engineered bone constructs with functional vascular networks has great potential for biomimicking nature bone tissue in vitro and enhancing bone regeneration in vivo. Over the past decades, many approaches have been applied to fabricate biomimetic vascularized tissue-engineered bone constructs. However, traditional tissue-engineered methods based on seeding cells into scaffolds are unable to control the spatial architecture and the encapsulated cell distribution precisely, which posed a significant challenge in constructing complex vascularized bone tissues with precise biomimetic properties. In recent years, as a pioneering technology, three-dimensional (3D) bioprinting technology has been applied to fabricate multiscale, biomimetic, multi-cellular tissues with a highly complex tissue microenvironment through layer-by-layer printing. This review discussed the application of 3D bioprinting technology in the vascularized tissue-engineered bone fabrication, where the current status and unique challenges were critically reviewed. Furthermore, the mechanisms of vascular formation, the process of 3D bioprinting, and the current development of bioink properties were also discussed.