Void-free 3D Bioprinting for In-situ Endothelialization and Microfluidic Perfusion

Integrated Cross-Scale Manipulation and Modulable Encapsulation of Cell-Laden Hydrogel for Constructing Tissue-Mimicking Microstructures

Engineered microstructures that mimic in vivo tissues have demonstrated great potential for applications in regenerative medicine, drug screening, and cell behavior exploration. However, current methods for engineering microstructures that mimic the multi-extracellular matrix and multicellular features of natural tissues to realize tissue-mimicking microstructures in vitro remain insufficient. Here, we propose a versatile method for constructing tissue-mimicking heterogeneous microstructures by orderly integration of macroscopic hydrogel exchange, microscopic cell manipulation, and encapsulation modulation. First, various cell-laden hydrogel droplets are manipulated at the millimeter scale using electrowetting on dielectric to achieve efficient hydrogel exchange. Second, the cells are manipulated at the micrometer scale using dielectrophoresis to adjust their density and arrangement within the hydrogel droplets. Third, the photopolymerization of these hydrogel droplets is triggered in designated regions by dynamically modulating the shape and position of the excitation ultraviolet beam. Thus, heterogeneous microstructures with different extracellular matrix geometries and components were constructed, including specific cell densities and patterns. The resulting heterogeneous microstructure supported long-term culture of hepatocytes and fibroblasts with high cell viability (over 90%). Moreover, the density and distribution of the 2 cell types had significant effects on the cell proliferation and urea secretion. We propose that our method can lead to the construction of additional biomimetic heterogeneous microstructures with unprecedented potential for use in future tissue engineering applications.

Advancing Synthetic Hydrogels through Nature-Inspired Materials Chemistry

Synthetic extracellular matrix (ECM) mimics that can recapitulate the complex biochemical and mechanical nature of native tissues are needed for advanced models of development and disease. Biomedical research has heavily relied on the use of animal-derived biomaterials, which is now impeding their translational potential and convoluting the biological insights gleaned from in vitro tissue models. Natural hydrogels have long served as a convenient and effective cell culture tool, but advances in materials chemistry and fabrication techniques now present promising new avenues for creating xenogenic-free ECM substitutes appropriate for organotypic models and microphysiological systems. However, significant challenges remain in creating synthetic matrices that can approximate the structural sophistication, biochemical complexity, and dynamic functionality of native tissues. This review summarizes key properties of the native ECM, and discusses recent approaches used to systematically decouple and tune these properties in synthetic matrices. The importance of dynamic ECM mechanics, such as viscoelasticity and matrix plasticity, is also discussed, particularly within the context of organoid and engineered tissue matrices. Emerging design strategies to mimic these dynamic mechanical properties are reviewed, such as multi-network hydrogels, supramolecular chemistry, and hydrogels assembled from biological monomers.

Bioprinted Multi-Composition Array Mimicking Tumor Microenvironments to Evaluate Drug Efficacy with Multivariable Analysis

Current organ-on-a-chip technologies confront limitations in effectively recapitulating the intricate in vivo microenvironments and accommodating diverse experimental conditions on a single device. Here, a novel approach for constructing a multi-composition tumor array on a single microfluidic device, mimicking complex transport phenomena within tumor microenvironments (TMEs) and allowing for simultaneous evaluation of drug efficacy across 12 distinct conditions is presented. The TME array formed by bioprinting on a microfluidic substrate consists of 36 individual TME models, each characterized by one of three different compositions and tested under four varying drug concentrations. Notably, the TME model exhibits precise compartmentalization, fostering the development of self-organized vascular endothelial barriers surrounding breast cancer spheroids affecting substance transport. Multivariable screening and analysis of diverse conditions, including model complexity, replicates, and drug concentrations, within a single microfluidic platform, highlight the synergistic potential of integrating bioprinting with microfluidics to evaluate drug responses across diverse TME conditions comprehensively.

A review of biomacromolecule-based 3D bioprinting strategies for structure-function integrated repair of skin tissues

When skin is damaged or affected by diseases, it often undergoes irreversible scar formation, leading to aesthetic concerns and psychological distress for patients. In cases of extensive skin defects, the patient's life can be severely compromised. In recent years, 3D printing technology has emerged as a groundbreaking approach to skin tissue engineering, offering promising solutions to various skin-related conditions. 3D bioprinting technology enables the precise fabrication of structures by programming the spatial arrangement of cells within the skin tissue and subsequently printing skin replacements either in a 3D bioprinter or directly at the site of the defect. This study provides a comprehensive overview of various biopolymer-based inks, with a particular emphasis on chitosan (CS), starch, alginate, agarose, cellulose, and fibronectin, all of which are natural polymers belonging to the category of biomacromolecules. Additionally, it summarizes artificially synthesized polymers capable of enhancing the performance of these biomacromolecule-based bioinks, thereby composing hybrid biopolymer inks aimed at better application in skin tissue engineering endeavors. This review paper examines the recent advancements, characteristics, benefits, and limitations of biological 3D bioprinting techniques for skin tissue engineering. By utilizing bioinks containing seed cells, hydrogels with bioactive factors, and biomaterials, complex structures resembling natural skin can be accurately fabricated in a layer-by-layer manner. The importance of biological scaffolds in promoting skin wound healing and the role of 3D bioprinting in skin tissue regeneration processes is discussed. Additionally, this paper addresses the challenges and constraints associated with current 3D bioprinting technologies for skin tissue and presents future perspectives. These include advancements in bioink formulations, full-thickness skin bioprinting, vascularization strategies, and skin appendages bioprinting.

Tissue-Engineered Microvessels: A Review of Current Engineering Strategies and Applications

Microvessels, including arterioles, capillaries, and venules, play an important role in regulating blood flow, enabling nutrient and waste exchange, and facilitating immune surveillance. Due to their important roles in maintaining normal function in human tissues, a substantial effort has been devoted to developing tissue-engineered models to study endothelium-related biology and pathology. Various engineering strategies have been developed to recapitulate the structural, cellular, and molecular hallmarks of native human microvessels in vitro. In this review, recent progress in engineering approaches, key components, and culture platforms for tissue-engineered human microvessel models is summarized. Then, tissue-specific models, and the major applications of tissue-engineered microvessels in development, disease modeling, drug screening and delivery, and vascularization in tissue engineering, are reviewed. Finally, future research directions for the field are discussed.

Three-dimensional printing and decellularized-extracellular-matrix based methods for advances in artificial blood vessel fabrication: A review

Blood vessels are the tubes through which blood flows and are divided into three types: millimeter-scale arteries, veins, and capillaries as well as micrometer-scale capillaries. Arteries and veins are the conduits that carry blood, while capillaries are where blood exchanges substances with tissues. Blood vessels are mainly composed of collagen fibers, elastic fibers, glycosaminoglycans and other macromolecular substances. There are about 19 feet of blood vessels per square inch of skin in the human body, which shows how important blood vessels are to the human body. Because cardiovascular disease and vascular trauma are common in the population, a great number of researches have been carried out in recent years by simulating the structures and functions of the person's own blood vessels to create different levels of tissue-engineered blood vessels that can replace damaged blood vessels in the human body. However, due to the lack of effective oxygen and nutrient delivery mechanisms, these tissue-engineered vessels have not been used clinically. Therefore, in order to achieve better vascularization of engineered vascular tissue, researchers have widely explored the design methods of vascular systems of various sizes. In the near future, these carefully designed and constructed tissue engineered blood vessels are expected to have practical clinical applications. Exploring how to form multi-scale vascular networks and improve their compatibility with the host vascular system will be very beneficial in achieving this goal. Among them, 3D printing has the advantages of high precision and design flexibility, and the decellularized matrix retains active ingredients such as collagen, elastin, and glycosaminoglycan, while removing the immunogenic substance DNA. In this review, technologies and advances in 3D printing and decellularization-based artificial blood vessel manufacturing methods are systematically discussed. Recent examples of vascular systems designed are introduced in details, the main problems and challenges in the clinical application of vascular tissue restriction are discussed and pointed out, and the future development trends in the field of tissue engineered blood vessels are also prospected.

Immunomodulatory multicellular scaffolds for tendon-to-bone regeneration

Limited motor activity due to the loss of natural structure impedes recovery in patients suffering from tendon-to-bone injury. Conventional biomaterials focus on strengthening the regenerative ability of tendons/bones to restore natural structure. However, owing to ignoring the immune environment and lack of multi-tissue regenerative function, satisfactory outcomes remain elusive. Here, combined manganese silicate (MS) nanoparticles with tendon/bone-related cells, the immunomodulatory multicellular scaffolds were fabricated for integrated regeneration of tendon-to-bone. Notably, by integrating biomimetic cellular distribution and MS nanoparticles, the multicellular scaffolds exhibited diverse bioactivities. Moreover, MS nanoparticles enhanced the specific differentiation of multicellular scaffolds via regulating macrophages, which was mainly attributed to the secretion of PGE2 in macrophages induced by Mn ions. Furthermore, three animal results indicated that the scaffolds achieved immunomodulation, integrated regeneration, and function recovery at tendon-to-bone interfaces. Thus, the multicellular scaffolds based on inorganic biomaterials offer an innovative concept for immunomodulation and integrated regeneration of soft/hard tissue interfaces.

Reasoning on Pore Terminology in 3D Bioprinting

Terminology is pivotal for facilitating clear communication and minimizing ambiguity, especially in specialized fields such as chemistry. In materials science, a subset of chemistry, the term "pore" is traditionally linked to the International Union of Pure and Applied Chemistry (IUPAC) nomenclature, which categorizes pores into "micro", "meso", and "macro" based on size. However, applying this terminology in closely-related areas, such as 3D bioprinting, often leads to confusion owing to the lack of consensus on specific definitions and classifications tailored to each field. This review article critically examines the current use of pore terminology in the context of 3D bioprinting, highlighting the need for reassessment to avoid potential misunderstandings. We propose an alternative classification that aligns more closely with the specific requirements of bioprinting, suggesting a tentative size-based division of interconnected pores into 'parvo'-(d < 25 µm), 'medio'-(25 < d < 100 µm), and 'magno'-(d > 100 µm) pores, relying on the current understanding of the pore size role in tissue formation. The introduction of field-specific terminology for pore sizes in 3D bioprinting is essential to enhance the clarity and precision of research communication. This represents a step toward a more cohesive and specialized lexicon that aligns with the unique aspects of bioprinting and tissue engineering.

Chemical and Structural Engineering of Gelatin-Based Delivery Systems for Therapeutic Applications: A Review

As a biodegradable and biocompatible protein derived from collagen, gelatin has been extensively exploited as a fundamental component of biological scaffolds and drug delivery systems for precise medicine. The easily engineered gelatin holds great promise in formulating various delivery systems to protect and enhance the efficacy of drugs for improving the safety and effectiveness of numerous pharmaceuticals. The remarkable biocompatibility and adjustable mechanical properties of gelatin permit the construction of active 3D scaffolds to accelerate the regeneration of injured tissues and organs. In this Review, we delve into diverse strategies for fabricating and functionalizing gelatin-based structures, which are applicable to gene and drug delivery as well as tissue engineering. We emphasized the advantages of various gelatin derivatives, including methacryloyl gelatin, polyethylene glycol-modified gelatin, thiolated gelatin, and alendronate-modified gelatin. These derivatives exhibit excellent physicochemical and biological properties, allowing the fabrication of tailor-made structures for biomedical applications. Additionally, we explored the latest developments in the modulation of their physicochemical properties by combining additive materials and manufacturing platforms, outlining the design of multifunctional gelatin-based micro-, nano-, and macrostructures. While discussing the current limitations, we also addressed the challenges that need to be overcome for clinical translation, including high manufacturing costs, limited application scenarios, and potential immunogenicity. This Review provides insight into how the structural and chemical engineering of gelatin can be leveraged to pave the way for significant advancements in biomedical applications and the improvement of patient outcomes.

The Art of Building Living Tissues: Exploring the Frontiers of Biofabrication with 3D Bioprinting

The scope of three-dimensional printing is expanding rapidly, with innovative approaches resulting in the evolution of state-of-the-art 3D bioprinting (3DbioP) techniques for solving issues in bioengineering and biopharmaceutical research. The methods and tools in 3DbioP emphasize the extrusion process, bioink formulation, and stability of the bioprinted scaffold. Thus, 3DbioP technology augments 3DP in the biological world by providing technical support to regenerative therapy, drug delivery, bioengineering of prosthetics, and drug kinetics research. Besides the above, drug delivery and dosage control have been achieved using 3D bioprinted microcarriers and capsules. Developing a stable, biocompatible, and versatile bioink is a primary requisite in biofabrication. The 3DbioP research is breaking the technical barriers at a breakneck speed. Numerous techniques and biomaterial advancements have helped to overcome current 3DbioP issues related to printability, stability, and bioink formulation. Therefore, this Review aims to provide an insight into the technical challenges of bioprinting, novel biomaterials for bioink formulation, and recently developed 3D bioprinting methods driving future applications in biofabrication research.

Bioprinted vascular tissue: Assessing functions from cellular, tissue to organ levels

3D bioprinting technology is widely used to fabricate various tissue structures. However, the absence of vessels hampers the ability of bioprinted tissues to receive oxygen and nutrients as well as to remove wastes, leading to a significant reduction in their survival rate. Despite the advancements in bioinks and bioprinting technologies, bioprinted vascular structures continue to be unsuitable for transplantation compared to natural blood vessels. In addition, a complete assessment index system for evaluating the structure and function of bioprinted vessels in vitro has not yet been established. Therefore, in this review, we firstly highlight the significance of selecting suitable bioinks and bioprinting techniques as they two synergize with each other. Subsequently, focusing on both vascular-associated cells and vascular tissues, we provide a relatively thorough assessment of the functions of bioprinted vascular tissue based on the physiological functions that natural blood vessels possess. We end with a review of the applications of vascular models, such as vessel-on-a-chip, in simulating pathological processes and conducting drug screening at the organ level. We believe that the development of fully functional blood vessels will soon make great contributions to tissue engineering and regenerative medicine.

3D printing microporous scaffolds from modular bioinks containing sacrificial, cell-encapsulating microgels

Microgel-based biomaterials have inherent porosity and are often extrudable, making them well-suited for 3D bioprinting applications. Cells are commonly introduced into these granular inks post-printing using cell infiltration. However, due to slow cell migration speeds, this strategy struggles to achieve depth-independent cell distributions within thick 3D printed geometries. To address this, we leverage granular ink modularity by combining two microgels with distinct functions: (1) structural, UV-crosslinkable microgels made from gelatin methacryloyl (GelMA) and (2) sacrificial, cell-laden microgels made from oxidized alginate (AlgOx). We hypothesize that encapsulating cells within sacrificial AlgOx microgels would enable the simultaneous introduction of void space and release of cells at depths unachievable through cell infiltration alone. Blending the microgels in different ratios produces a family of highly printable GelMA : AlgOx microgel inks with void fractions ranging from 0.03 to 0.35. As expected, void fraction influences the morphology of human umbilical vein endothelial cells (HUVEC) within GelMA : AlgOx inks. Crucially, void fraction does not alter the ideal HUVEC distribution seen throughout the depth of 3D printed samples. This work presents a strategy for fabricating constructs with tunable porosity and depth-independent cell distribution, highlighting the promise of microgel-based inks for 3D bioprinting.

Rapid magnetically directed assembly of pre-patterned capillary-scale microvessels

Capillary scale vascularization is critical to the survival of engineered 3D tissues and remains an outstanding challenge for the field of tissue engineering. Current methods to generate micro-scale vasculature such as 3D printing, two photon hydrogel ablation, angiogenesis, and vasculogenic assembly face challenges in rapidly creating organized, highly vascularized tissues at capillary length-scales. Within metabolically demanding tissues, native capillary beds are highly organized and densely packed to achieve adequate delivery of nutrients and oxygen and efficient waste removal. Here, we adopt two existing techniques to fabricate lattices composed of sacrificial microfibers that can be efficiently and uniformly seeded with endothelial cells (ECs) by magnetizing both lattices and ECs. Ferromagnetic microparticles (FMPs) were incorporated into microfibers produced by solution electrowriting (SEW) and fiber electropulling (FEP). By loading ECs with superparamagnetic iron oxide nanoparticles (SPIONs), the cells could be seeded onto magnetized microfiber lattices. Following encapsulation in a hydrogel, the capillary templating lattice was selectively degraded by a bacterial lipase that does not impact mammalian cell viability or function. This work introduces a novel approach to rapidly producing organized capillary networks within metabolically demanding engineered tissue constructs which should have broad utility for the fields of tissue engineering and regenerative medicine.

Biomaterials / bioinks and extrusion bioprinting

Bioinks are formulations of biomaterials and living cells, sometimes with growth factors or other biomolecules, while extrusion bioprinting is an emerging technique to apply or deposit these bioinks or biomaterial solutions to create three-dimensional (3D) constructs with architectures and mechanical/biological properties that mimic those of native human tissue or organs. Printed constructs have found wide applications in tissue engineering for repairing or treating tissue/organ injuries, as well as in vitro tissue modelling for testing or validating newly developed therapeutics and vaccines prior to their use in humans. Successful printing of constructs and their subsequent applications rely on the properties of the formulated bioinks, including the rheological, mechanical, and biological properties, as well as the printing process. This article critically reviews the latest developments in bioinks and biomaterial solutions for extrusion bioprinting, focusing on bioink synthesis and characterization, as well as the influence of bioink properties on the printing process. Key issues and challenges are also discussed along with recommendations for future research.

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.

Multimaterial 3D and 4D Bioprinting of Heterogenous Constructs for Tissue Engineering

Additive manufacturing (AM), which is based on the principle of layer-by-layer shaping and stacking of discrete materials, has shown significant benefits in the fabrication of complicated implants for tissue engineering (TE). However, many native tissues exhibit anisotropic heterogenous constructs with diverse components and functions. Consequently, the replication of complicated biomimetic constructs using conventional AM processes based on a single material is challenging. Multimaterial 3D and 4D bioprinting (with time as the fourth dimension) has emerged as a promising solution for constructing multifunctional implants with heterogenous constructs that can mimic the host microenvironment better than single-material alternatives. Notably, 4D-printed multimaterial implants with biomimetic heterogenous architectures can provide a time-dependent programmable dynamic microenvironment that can promote cell activity and tissue regeneration in response to external stimuli. This paper first presents the typical design strategies of biomimetic heterogenous constructs in TE applications. Subsequently, the latest processes in the multimaterial 3D and 4D bioprinting of heterogenous tissue constructs are discussed, along with their advantages and challenges. In particular, the potential of multimaterial 4D bioprinting of smart multifunctional tissue constructs is highlighted. Furthermore, this review provides insights into how multimaterial 3D and 4D bioprinting can facilitate the realization of next-generation TE applications.

Volumetric Printing Across Melt Electrowritten Scaffolds Fabricates Multi-Material Living Constructs with Tunable Architecture and Mechanics

Major challenges in biofabrication revolve around capturing the complex, hierarchical composition of native tissues. However, individual 3D printing techniques have limited capacity to produce composite biomaterials with multi-scale resolution. Volumetric bioprinting recently emerged as a paradigm-shift in biofabrication. This ultrafast, light-based technique sculpts cell-laden hydrogel bioresins into 3D structures in a layerless fashion, providing enhanced design freedom over conventional bioprinting. However, it yields prints with low mechanical stability, since soft, cell-friendly hydrogels are used. Herein, the possibility to converge volumetric bioprinting with melt electrowriting, which excels at patterning microfibers, is shown for the fabrication of tubular hydrogel-based composites with enhanced mechanical behavior. Despite including non-transparent melt electrowritten scaffolds in the volumetric printing process, high-resolution bioprinted structures are successfully achieved. Tensile, burst, and bending mechanical properties of printed tubes are tuned altering the electrowritten mesh design, resulting in complex, multi-material tubular constructs with customizable, anisotropic geometries that better mimic intricate biological tubular structures. As a proof-of-concept, engineered tubular structures are obtained by building trilayered cell-laden vessels, and features (valves, branches, fenestrations) that can be rapidly printed using this hybrid approach. This multi-technology convergence offers a new toolbox for manufacturing hierarchical and mechanically tunable multi-material living structures.

Expanding Embedded 3D Bioprinting Capability for Engineering Complex Organs with Freeform Vascular Networks

Creating functional tissues and organs in vitro on demand is a major goal in biofabrication, but the ability to replicate the external geometry of specific organs and their internal structures such as blood vessels simultaneously remains one of the greatest impediments. Here, this limitation is addressed by developing a generalizable bioprinting strategy of sequential printing in a reversible ink template (SPIRIT). It is demonstrated that this microgel-based biphasic (MB) bioink can be used as both an excellent bioink and a suspension medium that supports embedded 3D printing due to its shear-thinning and self-healing behavior. When encapsulating human-induced pluripotent stem cells, the MB bioink is 3D printed to generate cardiac tissues and organoids by extensive stem cell proliferation and cardiac differentiation. By incorporating MB bioink, the SPIRIT strategy enables the effective printing of a ventricle model with a perfusable vascular network, which is not possible to fabricate using extant 3D printing strategies. This SPIRIT technique offers an unparalleled bioprinting capability to replicate the complex organ geometry and internal structure in a faster manner, which will accelerate the biofabrication and therapeutic applications of tissue and organ constructs.

Multicellular Bioprinting of Biomimetic Inks for Tendon-to-Bone Regeneration

Tendon-to-bone interface has a hierarchical structure and gradient component that are conducive to distributing the stresses to achieve movement. Conventional biomaterials lack the capacity to induce synchronous repair of multiple tissues, resulting in the failure of the interface repair. Biomimetic strategies have attracted enormous attention in the field of complex structure regeneration because they can meet the different physiological requirements of multiple tissues. Herein, a biomimetic ink mimicking tendon/bone tissues is developed by combining tendon/bone-related cells and Mo-containing silicate (MS) bioceramics. Subsequently, biomimetic multicellular scaffolds are fabricated to achieve the simulation of the hierarchical structure and cellular composition of tendon-to-bone interfaces by the spatial distribution of the biomimetic inks via 3D bioprinting, which is of great significance for inducing the regeneration of complex structures in the interface region. In addition, attributed to the desirable ionic microenvironment created by MS bioceramics, the biomimetic scaffolds possess the dual function of inducing tendon/bone-related cells tenogenic and osteogenic differentiation in vitro, and promote the integrated regeneration of tendon-to-bone interfaces in vivo. The study offers a feasible strategy to construct biomimetic multicellular scaffolds with bifunction for inducing multi-lineage tissue regeneration, especially for regenerating soft-to-hard tissue interfaces.

High cell density and high-resolution 3D bioprinting for fabricating vascularized tissues

Three-dimensional (3D) bioprinting techniques have emerged as the most popular methods to fabricate 3D-engineered tissues; however, there are challenges in simultaneously satisfying the requirements of high cell density (HCD), high cell viability, and fine fabrication resolution. In particular, bioprinting resolution of digital light processing-based 3D bioprinting suffers with increasing bioink cell density due to light scattering. We developed a novel approach to mitigate this scattering-induced deterioration of bioprinting resolution. The inclusion of iodixanol in the bioink enables a 10-fold reduction in light scattering and a substantial improvement in fabrication resolution for bioinks with an HCD. Fifty-micrometer fabrication resolution was achieved for a bioink with 0.1 billion per milliliter cell density. To showcase the potential application in tissue/organ 3D bioprinting, HCD thick tissues with fine vascular networks were fabricated. The tissues were viable in a perfusion culture system, with endothelialization and angiogenesis observed after 14 days of culture.

Cell-Laden Scaffolds for Vascular-Innervated Bone Regeneration

For regeneration of highly vascularized and innervated tissues, like bone, simultaneous ingrowth of blood vessels and nerves is essential but largely neglected. To address this issue, a "pre-angiogenic" cell-laden scaffold with durable angiogenic functions is prepared according to the bioactivities of silicate bioceramics and the instructive effects of vascular cells on neurogenesis and bone repair. Compared with traditional cell-free scaffolds, the prepared cell-laden scaffolds printed with active cells and bioactive inks can support long-term cell survival and growth for three weeks. The long-lived scaffolds exhibited durable angiogenic capability both in vitro and in vivo. The pre-angiogenic scaffolds can induce the neurogenetic differentiation of neural cells and the osteogenic differentiation of mesenchymal stem cells by the synergistic effects of released bioactive ions and the ability of vascular cells to attract neurons. The enhanced bone regeneration with both vascularization and innervation is attributed to these physiological functions of the pre-angiogenic cell-laden scaffolds, which is defined as "vascular-innervated" bone regeneration. It is suggested that the concept of "vascular-innervated scaffolds" may represent the future direction of biomaterials for complex tissue/organ regeneration.

Selective Formation of Osteogenic and Vasculogenic Tissues for Cartilage Regeneration

Tissue-engineered periosteum substitutes (TEPSs) incorporating hierarchical architecture with osteoprogenitor and vascular niches are drawing much attention as a promising tool to support functional cells in defined zones and nourish the cortical bone. Current TEPSs usually lack technologies to closely observe cell performance, especially at the cell contact interface between distinct compartments containing defined biological configurations and functions. Here, an electrodeposition strategy is reported, which enables the selective formation of TEPSs with osteoprogenitor and vascular niches in a multiphasic scaffold in combination with different human cell types for cartilage regeneration in an in vivo osteochondral defect model. Human umbilical vein endothelial cells (HUVECs), dermal fibroblasts (HDFs), and bone marrow mesenchymal stem cells (hMSCs) are used to mirror both the vascular and osteogenic niches, respectively. It is observed that the intrinsic viscoelastic nature of the porous solid matrix is essential to successfully induce angiogenesis. Coculture of hMSCs with functional cells (HUVECs/HDFs) in TEPSs also effectively promoted periosteal regeneration, including osteogenic and angiogenic processes. The osteoarthritis cartilage histopathology assessment and histologic/histochemical grading system data indicate that the TEPSs containing hMSCs/HUVECs/HDFs exhibit superior potential for cartilage regeneration.

In Situ Endothelialization of Free-Form 3D Network of Interconnected Tubular Channels via Interfacial Coacervation by Aqueous-in-Aqueous Embedded Bioprinting

The challenge of bioprinting vascularized tissues is structure retention and in situ endothelialization. The issue is addressed by adopting an aqueous-in-aqueous 3D embedded bioprinting strategy, in which the interfacial coacervation of the cyto-mimic aqueous two-phase systems (ATPS) are employed for maintaining the suspending liquid architectures, and serving as filamentous scaffolds for cell attachment and growth. By incorporating endothelial cells in the ink phase of ATPS, tubular lumens enclosed by coacervated complexes of polylysine (PLL) and oxidized bacteria celluloses (oxBC) can be cellularized with a confluent endothelial layer, without any help of adhesive peptides. By applying PLL/oxBC ATPS for embedded bioprinting, free-form 3D vascular networks with in situ endothelialization of interconnected tubular lumens are achieved. This simple approach is a one-step process without any sacrificed templates and post-treatments. The resultant functional vessel networks with arbitrary complexity are suspended in liquid medium and can be conveniently handled, opening new routes for the in vitro production of thick vascularized tissues for pathological research, regeneration therapy and animal-free drug development.

A Porous Gelatin Methacrylate-Based Material for 3D Cell-Laden Constructs

3D constructs are fundamental in tissue engineering and cancer modeling, generating a demand for tailored materials creating a suitable cell culture microenvironment and amenable to be bioprinted. Gelatin methacrylate (GelMA) is a well-known functionalized natural polymer with good printability and binding motifs allowing cell adhesion; however, its tight micropores induce encapsulated cells to retain a non-physiological spherical shape. To overcome this problem, blended GelMa is here blended with Pluronic F-127 (PLU) to modify the hydrogel internal porosity by inducing the formation of larger mesoscale pores. The change in porosity also leads to increased swelling and a slight decrease in Young's modulus. All blends form stable hydrogels both when cast in annular molds and bioprinted in complex structures. Embedded cells maintain high viability, and while Neuroblastoma cancer cells typically aggregate inside the mesoscale pores, Mesenchymal Stem Cells stretch in all three dimensions, forming cell-cell and cell-ECM interactions. The results of this work prove that the combination of tailored porous materials with bioprinting techniques enables to control both the micro and macro architecture of cell-laden constructs, a fundamental aspect for the development of clinically relevant in vitro constructs.

Biofabricating the vascular tree in engineered bone tissue

The development of tissue engineering strategies for treatment of large bone defects has become increasingly relevant, given the growing demand for bone substitutes. Native bone is composed of a dense vascular network necessary for the regulation of bone development, regeneration and homeostasis. A major obstacle in fabricating living, clinically relevant-sized bone mimics (1-10 cm³) is the limited supply of nutrients, including oxygen to the core of the construct. Therefore, strategies to support vascularization are pivotal for the development of tissue engineered bone constructs. Creating a functional bone construct integrated with a vascular network, capable of delivering the necessary nutrients for optimal tissue development is imperative for translation into the clinics. The vascular system is composed of a complex network that runs throughout the body in a tree-like hierarchical branching fashion. A significant challenge for tissue engineering approaches lies in mimicking the intricate, multi-scale structures consisting of larger vessels (macro-vessels) which interconnect with multiple sprouting vessels (microvessels) in a closed network. The advent of biofabrication has enabled complex, out of plane channels to be generated and has laid the groundwork for the creation of multi-scale vasculature in recent years. This review highlights the key state-of-the-art achievements for the development of vascular networks of varying scales in the field of biofabrication with a particular focus for its application in developing a functional tissue engineered bone construct. STATEMENT OF SIGNIFICANCE: There is a growing need for bone substitutes to overcome the limited supply of patient-derived bone. Bone tissue engineering aims to overcome this by combining stem cells with scaffolds to restore missing bone. The current bottleneck in upscaling is the lack of an integrated vascular network, required for the delivery of nutrients to cells. 3D bioprinting techniques has enabled the creation of complex hollow structures of varying dimensions that resemble native blood vessels. The convergence of multiple materials, cell types and fabrication approaches, opens the possibility of developing clinically-relevant sized vascularized bone constructs. This review provides an up-to-date insight of the technologies currently available for the generation of complex vascular networks, with a focus on their application in bone tissue engineering.

Bilayer hydrogel dressing with lysozyme-enhanced photothermal therapy for biofilm eradication and accelerated chronic wound repair

Biofilms are closely associated with the tough healing and dysfunctional inflammation of chronic wounds. Photothermal therapy (PTT) emerged as a suitable alternative which could destroy the structure of biofilms with local physical heat. However, the efficacy of PTT is limited because the excessive hyperthermia could damage surrounding tissues. Besides, the difficult reserve and delivery of photothermal agents makes PTT hard to eradicate biofilms as expectation. Herein, we present a GelMA-EGF/Gelatin-MPDA-LZM bilayer hydrogel dressing to perform lysozyme-enhanced PTT for biofilms eradication and a further acceleration to the repair of chronic wounds. Gelatin was used as inner layer hydrogel to reserve lysozyme (LZM) loaded mesoporous polydopamine (MPDA) (MPDA-LZM) nanoparticles, which could rapidly liquefy while temperature rising so as to achieve a bulk release of nanoparticles. MPDA-LZM nanoparticles serve as photothermal agents with antibacterial capability, could deeply penetrate and destroy biofilms. In addition, the outer layer hydrogel consisted of gelatin methacryloyl (GelMA) and epidermal growth factor (EGF) promoted wound healing and tissue regeneration. It displayed remarkable efficacy on alleviating infection and accelerating wound healing in vivo. Overall, the innovative therapeutic strategy we came up with has significant effect on biofilms eradication and shows promising application in promoting the repair of clinical chronic wounds.

3D bioprinting of in situ vascularized tissue engineered bone for repairing large segmental bone defects

Large bone defects remain an unsolved clinical challenge because of the lack of effective vascularization in newly formed bone tissue. 3D bioprinting is a fabrication technology with the potential to create vascularized bone grafts with biological activity for repairing bone defects. In this study, vascular endothelial cells laden with thermosensitive bio-ink were bioprinted in situ on the inner surfaces of interconnected tubular channels of bone mesenchymal stem cell-laden 3D-bioprinted scaffolds. Endothelial cells exhibited a more uniform distribution and greater seeding efficiency throughout the channels. In vitro, the in situ bioprinted endothelial cells can form a vascular network through proliferation and migration. The in situ vascularized tissue-engineered bone also resulted in a coupling effect between angiogenesis and osteogenesis. Moreover, RNA sequencing analysis revealed that the expression of genes related to osteogenesis and angiogenesis is upregulated in biological processes. The in vivo 3D-bioprinted in situ vascularized scaffolds exhibited excellent performance in promoting new bone formation in rat calvarial critical-sized defect models. Consequently, in situ vascularized tissue-engineered bones constructed using 3D bioprinting technology have a potential of being used as bone grafts for repairing large bone defects, with a possible clinical application in the future.

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3D bioprinting was used to fabricate in situ vascularized tissue engineered bone.

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In situ bioprinted endothelial cells exhibited uniform distribution and greater seeding efficiency.

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3D-bioprinted scaffold produced coupling between angiogenesis and osteogenesis.

Tissue Engineering Cartilage with Deep Zone Cytoarchitecture by High-Resolution Acoustic Cell Patterning

The ultimate objective of tissue engineering is to fabricate artificial living constructs with a structural organization and function that faithfully resembles their native tissue counterparts. For example, the deep zone of articular cartilage possesses a distinctive anisotropic architecture with chondrocytes organized in aligned arrays ≈1-2 cells wide, features that are oriented parallel to surrounding extracellular matrix fibers and orthogonal to the underlying subchondral bone. Although there are major advances in fabricating custom tissue architectures, it remains a significant technical challenge to precisely recreate such fine cellular features in vitro. Here, it is shown that ultrasound standing waves can be used to remotely organize living chondrocytes into high-resolution anisotropic arrays, distributed throughout the full volume of agarose hydrogels. It is demonstrated that this cytoarchitecture is maintained throughout a five-week course of in vitro tissue engineering, producing hyaline cartilage with cellular and extracellular matrix organization analogous to the deep zone of native articular cartilage. It is anticipated that this acoustic cell patterning method will provide unprecedented opportunities to interrogate in vitro the contribution of chondrocyte organization to the development of aligned extracellular matrix fibers, and ultimately, the design of new mechanically anisotropic tissue grafts for articular cartilage regeneration.

Bioengineering for vascularization: Trends and directions of photocrosslinkable gelatin methacrylate hydrogels

Gelatin methacrylate (GelMA) hydrogels have been widely used in various biomedical applications, especially in tissue engineering and regenerative medicine, for their excellent biocompatibility and biodegradability. GelMA crosslinks to form a hydrogel when exposed to light irradiation in the presence of photoinitiators. The mechanical characteristics of GelMA hydrogels are highly tunable by changing the crosslinking conditions, including the GelMA polymer concentration, degree of methacrylation, light wavelength and intensity, and light exposure time et al. In this regard, GelMA hydrogels can be adjusted to closely resemble the native extracellular matrix (ECM) properties for the specific functions of target tissues. Therefore, this review focuses on the applications of GelMA hydrogels for bioengineering human vascular networks in vitro and in vivo. Since most tissues require vasculature to provide nutrients and oxygen to individual cells, timely vascularization is critical to the success of tissue- and cell-based therapies. Recent research has demonstrated the robust formation of human vascular networks by embedding human vascular endothelial cells and perivascular mesenchymal cells in GelMA hydrogels. Vascular cell-laden GelMA hydrogels can be microfabricated using different methodologies and integrated with microfluidic devices to generate a vasculature-on-a-chip system for disease modeling or drug screening. Bioengineered vascular networks can also serve as build-in vasculature to ensure the adequate oxygenation of thick tissue-engineered constructs. Meanwhile, several reports used GelMA hydrogels as implantable materials to deliver therapeutic cells aiming to rebuild the vasculature in ischemic wounds for repairing tissue injuries. Here, we intend to reveal present work trends and provide new insights into the development of clinically relevant applications based on vascularized GelMA hydrogels.

A versatile strategy to construct free-standing multi-furcated vessels and a complicated vascular network in heterogeneous porous scaffolds via combination of 3D printing and stimuli-responsive hydrogels

Mimicking complex structures of natural blood vessels and constructing vascular networks in tissue engineering scaffolds are still challenging now. Herein we demonstrate a new and versatile strategy to fabricate free-standing multi-furcated vessels and complicated vascular networks in heterogeneous porous scaffolds by integrating stimuli-responsive hydrogels and 3D printing technology. Through the sol-gel transition of temperature-responsive gelatin and conversion between two physical crosslinking networks of pH-responsive chitosan (i.e., electrostatic network between protonated chitosan and sulfate ion, crystalline network of neutral chitosan), physiologically-stable gelatin/chitosan hydrogel tubes can be constructed. While stimuli-responsive hydrogels confer the formation mechanism of the hydrogel tube, 3D printing confers the feasibility to create a multi-furcated structure and interconnected network in various heterogeneous porous scaffolds. As a consequence, biomimetic multi-furcated vessels (MFVs) and heterogeneous porous scaffolds containing multi-furcated vessels (HPS-MFVs) can be constructed precisely. Our data further confirm that the artificial blood vessel (gelatin/chitosan hydrogel tube) shows good physiological stability, mechanical strength, semi-permeability, hemocompatibility, cytocompatibility and low in vivo inflammatory response. Co-culture of hepatocyte (L02 cells) and human umbilical vein endothelial cells (HUVECs) in HPS-MFVs indicates the successful construction of a liver model. We believe that our method offers a simple and easy-going way to achieve robust fabrication of free-standing multi-furcated blood vessels and prevascularization of porous scaffolds for tissue engineering and regenerative medicine.

Shining a Light on Cancer - Photonics in Microfluidic Tumor Modelling and Biosensing

Microfluidic platforms represent a powerful approach to miniaturizing important characteristics of cancers, improving in vitro testing by increasing physiological relevance. Different tools can manipulate cells and materials at the microscale, but few offer the efficiency and versatility of light and optical technologies. Moreover, light-driven technologies englobe a broad toolbox for quantifying critical biological phenomena. Herein, we review the role of photonics in microfluidic 3D cancer modeling and biosensing from three major perspectives. First, we look at optical-driven technologies that allow biomaterials and living cells to be manipulated with micro-sized precision and the opportunities to advance 3D microfluidic models by engineering cancer microenvironments' hallmarks, such as their architecture, cellular complexity, and vascularization. Second, we delve into the growing field of optofluidics, exploring how optical tools can directly interface microfluidic chips, enabling the extraction of relevant biological data, from single fluorescent signals to the complete 3D imaging of diseased cells within microchannels. Third, we review advances in optical cancer biosensing, focusing on how light-matter interactions can detect biomarkers, rare circulating tumor cells, and cell-derived structures such as exosomes. We overview photonic technologies' current challenges and caveats in microfluidic 3D cancer models, outlining future research avenues that may catapult the field. This article is protected by copyright. All rights reserved.

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

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

Low temperature hybrid 3D printing of hierarchically porous bone tissue engineering scaffolds with in situ delivery of osteogenic peptide and mesenchymal stem cells

Comparing to other conventional fabrication techniques, 3D printing is advantageous in producing bone tissue engineering scaffolds with customized shape, tailored pore size/porosity, required mechanical properties and even desirable biomolecule delivery capability. However, towards scaffolds with a large volume, it is highly difficult to enable seed cells to migrate to the central region of the scaffolds, resulting in an inhomogeneous cell distribution, and hence lowering the bone forming ability. To address this problem, in this investigation, cell-laden bone tissue engineering scaffolds consisting of osteogenic peptide loaded β-tricalcium phosphate/poly(lactic-co-glycolic acid) (OP/TCP/PLGA) nanocomposite scaffolds and rat bone marrow derived mesenchymal stem cells (rBMSCs)-laden Gelatin/GelMA hydrogel fillers were produced through a "dual-nozzle" cryogenic hybrid 3D printing. The cell-laden scaffolds exhibited a bi-phasic structure and were mechanically similar to human cancellous bone. OP can be released from the hybrid scaffolds in a sustained manner. rBMSCs laden in the hydrogel patterns exhibited a high viability during and after cryogenic hybrid 3D printing process and can be further released from the hydrogel struts and achieve cell anchorage on the surface of adjacent OTP struts. The OP released from OTP struts enhanced rBMSCs proliferation. Comparing to rBMSC-laden hybrid scaffolds without OP incorporation, the rBMSC-laden hybrid scaffolds incorporated with OP significantly up-regulated the osteogenic differentiation of rBMSCs, by showing a higher level of alkaline phosphatase (ALP) expression and calcium deposition. This "proof-ofconcept" study provided a facile method to form cell-laden bone tissue engineering scaffolds with not only required mechanical strength, biomimetic structure and prolonged biomolecule release but also excellent cell delivery capability with uniform cell distribution.

Development and Characterization of Complementary Polymer Network Bioinks for 3D Bioprinting of Soft Tissue Constructs

The development of three-dimensional (3D) bioprinting has been hindered by a narrow "biofabrication window" with a limited variety of feasible bioinks which are compatible with both high printability and well cytocompatibility. Herein, a generalizable strategy using complementary polymer network (CPN) bioinks has been developed in the current study, to address the conflict between the printability and cytocompatibility of bioinks in extrusion 3D bioprinting, especially for the manufacture of soft tissue constructs. In our strategy, CPN bioinks are formed though mixing two interpenetrated polymer networks, one of which is a photocrosslinkable polymer network, and the other is a dynamic polymer network crosslinked by reversible covalent linkage, thereby endowed with well reversible thixotropy. Compatible with well printability, shape fidelity, and cytocompatibility, the utilization of CPN bioinks provides a viable solution for extrusion 3D bioprinting of photocrosslinkable biomaterials at a low concentration, thus suitable for soft tissue construct fabrication. Briefly, this study is testified to be a successful attempt to extend the bioink diversity within the "biofabrication window", and offers a novel insight into designing more feasible bioinks based on their special rheological properties, for further tissue engineering and biomedicine application. This article is protected by copyright. All rights reserved.

Infiltration from Suspension Systems Enables Effective Modulation of 3D Scaffold Properties in Suspension Bioprinting

Bioprinting is a biofabrication technology which allows efficient and large-scale manufacture of 3D cell culture systems. However, the available biomaterials for bioinks used in bioprinting are limited by their printability and biological functionality. Fabricated constructs are often homogeneous and have limited complexity in terms of current 3D cell culture systems comprising multiple cell types. Inspired by the phenomenon that hydrogels can exchange liquids under the infiltration action, infiltration-induced suspension bioprinting (IISBP), a novel printing technique based on a hyaluronic acid (HA) suspension system to modulate the properties of the printed scaffolds by infiltration action, was described in this study. HA served as a suspension system due to its shear-thinning and self-healing rheological properties, simplicity of preparation, reusability, and ease of adjustment to osmotic pressure. Changes in osmotic pressure were able to direct the swelling or shrinkage of 3D printed gelatin methacryloyl (GelMA)-based bioinks, enabling the regulation of physical properties such as fiber diameter, micromorphology, mechanical strength, and water absorption of 3D printed scaffolds. Human umbilical vein endothelial cells (HUVEC) were applied as a cell culture model and printed within cell-laden scaffolds at high resolution and cell viability with the IISBP technique. Herein, the IISBP technique had been realized as a reliable hydrogel-based bioprinting technique, which enabled facile modulation of 3D printed hydrogel scaffolds properties, being expected to meet the scaffolds requirements of a wide range of cell culture conditions to be utilized in bioprinting applications.

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.

Microfluidic Tissue Engineering and Bio-Actuation

Bio-hybrid technologies aim to replicate the unique capabilities of biological systems that could surpass advanced artificial technologies. Soft bio-hybrid robots consist of synthetic and living materials and have the potential to self-assemble, regenerate, work autonomously, and interact safely with other species and the environment. Cells require a sufficient exchange of nutrients and gases, which is guaranteed by convection and diffusive transport through liquid media. The functional development and long-term survival of biological tissues in vitro can be improved by dynamic flow culture, but only microfluidic flow control can develop tissue with fine structuring and regulation at the microscale. Full control of tissue growth at the microscale will eventually lead to functional macroscale constructs, which are needed as the biological component of soft bio-hybrid technologies. This review summarizes recent progress in microfluidic techniques to engineer biological tissues, focusing on the use of muscle cells for robotic bio-actuation. Moreover, the instances in which bio-actuation technologies greatly benefit from fusion with microfluidics are highlighted, which include: the microfabrication of matrices, biomimicry of cell microenvironments, tissue maturation, perfusion, and vascularization.

Application Status of Sacrificial Biomaterials in 3D Bioprinting

Additive manufacturing, also known as three-dimensional (3D) printing, relates to several rapid prototyping (RP) technologies, and has shown great potential in the manufacture of organoids and even complex bioartificial organs. A major challenge for 3D bioprinting complex org unit ans is the competitive requirements with respect to structural biomimeticability, material integrability, and functional manufacturability. Over the past several years, 3D bioprinting based on sacrificial templates has shown its unique advantages in building hierarchical vascular networks in complex organs. Sacrificial biomaterials as supporting structures have been used widely in the construction of tubular tissues. The advent of suspension printing has enabled the precise printing of some soft biomaterials (e.g., collagen and fibrinogen), which were previously considered unprintable singly with cells. In addition, the introduction of sacrificial biomaterials can improve the porosity of biomaterials, making the printed structures more favorable for cell proliferation, migration and connection. In this review, we mainly consider the latest developments and applications of 3D bioprinting based on the strategy of sacrificial biomaterials, discuss the basic principles of sacrificial templates, and look forward to the broad prospects of this approach for complex organ engineering or manufacturing.

3D Bioprinted GelMA-Nanoclay Hydrogels Induce Colorectal Cancer Stem Cells Through Activating Wnt/β-Catenin Signaling

Cancer stem cells (CSCs) are a rare cell population in tumors that are responsible for tumor recurrence and metastasis. They are a priority as therapeutic targets, however, assays targeting CSCs have been limited by expanding and maintaining CSCs in vitro. Here, the authors find that gelatin methacryloyl (GelMA)-nanoclay hybrid hydrogels can induce and enrich colorectal CSCs assisted by three-dimensional (3D) bioprinting. The presence of the nanoclay increases the printability, Young's modulus, pore size, and cytocompatibility of the hydrogels. Bioprinted GelMA-nanoclay hydrogels promote the formation of spheroids expressing elevated levels of the stemness markers LGR5, CD133, CD26, and SOX2. Cancer cells grown in GelMA-nanoclay hydrogel possess higher self-renewal and differentiation capacity in vitro and higher tumorigenic capacity in vivo. GelMA-nanoclay hydrogels induce CSCs by stimulating the activation of the Wnt/β-catenin signaling pathway. Further studies demonstrate that spheroids from GelMA-nanoclay hydrogels possess increased stemness, higher consistency, yield, and sensitivity to the anti-CSC compounds compared to the classic CSC-enrichment model. Collectively, this study may provide a valuable biomaterial and method for inducing and enriching CSCs, to facilitate the effective CSC-targeting drug screening.

Computational Modeling and Experimental Characterization of Extrusion Printing into Suspension Baths

The extrusion printing of inks into suspension baths is an exciting tool, as it allows the printing of diverse and soft hydrogel inks into 3D space without the need for layer-by-layer fabrication. However, this printing process is complex and there have been limited studies to experimentally and computationally characterize the process. In this work, hydrogel inks (i.e., gelatin methacrylamide (GelMA)), suspension baths (i.e., agarose, Carbopol), and the printing process are examined via rheological, computational, and experimental analyses. Rheological data on various hydrogel inks and suspension baths is utilized to develop computational printing simulations based on Carreau constitutive viscosity models of the printing of inks within suspension baths. These results are then compared to experimental outcomes using custom print designs where features such as needle translation speed, defined in this work as print speed, are varied and printed filament resolution is quantified. Results are then used to identify print parameters for the printing of a GelMA ink into a unique guest-host hyaluronic acid suspension bath. This work emphasizes the importance of key rheological properties and print parameters for suspension bath printing and provides a computational model and experimental tools that can be used to inform the selection of print settings.

Tunable Microgel-Templated Porogel (MTP) Bioink for 3D Bioprinting Applications

Micropores are essential for tissue engineering to ensure adequate mass transportation for embedded cells. Despite the considerable progress made by advanced 3D bioprinting technologies, it remains challenging to engineer micropores of 100 µm or smaller in cell-laden constructs. Here, a microgel-templated porogel (MTP) bioink platform is reported to introduce controlled microporosity in 3D bioprinted hydrogels in the presence of living cells. Templated gelatin microgels are fabricated with varied sizes (≈10, ≈45, and ≈100 µm) and mixed with photo-crosslinkable formulations to make composite MTP bioinks. The addition of microgels significantly enhances the shear-thinning and self-healing viscoelastic properties and thus the printability of bioinks with cell densities up to 1 × 108 mL-1 in matrix. Consistent printability is achieved for a series of MTP bioinks based on different component ratios and matrix materials. After photo-crosslinking the matrix phase, the templated microgels dissociated and diffused under physiological conditions, resulting in corresponding micropores in situ. When embedding osteoblast-like cells in the matrix phase, the MTP bioinks support higher metabolic activity and more uniform mineral formation than bulk gel controls. The approach provides a facile strategy to engineer precise micropores in 3D printed structures to compensate for the limited resolution of current bioprinting approaches.

Multiparametric Material Functionality of Microtissue-Based In Vitro Models as Alternatives to Animal Testing

With the definition of the 3R principle by Russel and Burch in 1959, the search for an adequate substitute for animal testing has become one of the most important tasks and challenges of this time, not only from an ethical, but also from a scientific, economic, and legal point of view. Microtissue-based in vitro model systems offer a valuable approach to address this issue by accounting for the complexity of natural tissues in a simplified manner. To increase the functionality of these model systems and thus make their use as a substitute for animal testing more likely in the future, the fundamentals need to be continuously improved. Corresponding requirements exist in the development of multifunctional, hydrogel-based materials, whose properties are considered in this review under the aspects of processability, adaptivity, biocompatibility, and stability/degradability.

Bioink with cartilage-derived extracellular matrix microfibers enables spatial control of vascular capillary formation in bioprinted constructs

Microvasculature is essential for the exchange of gas and nutrient for most tissues in our body. Some tissue structures such as the meniscus presents spatially confined blood vessels adjacent to non-vascularized regions. In biofabrication, mimicking the spatial distribution of such vascular components is paramount, as capillary ingrowth into non-vascularized tissues can lead to tissue matrix alterations and subsequent pathology. Multi-material 3D bioprinting can potentially resolve anisotropic tissue features, although building complex constructs comprising stable vascularized and non-vascularized regions remains a major challenge. Here, we developed endothelial cell(EC)-laden pro- and anti-angiogenic bioinks, supplemented with bioactive matrix-derived microfibers (MFs) that were created from type I collagen sponges (col-1) and cartilage decellularized extracellular matrix (CdECM). EC-driven capillary network formation started two days after bioprinting. Supplementing cartilage-derived MFs to endothelial-cell laden bioinks reduced the total length of neo-microvessels by 29% after 14 days, compared to col-1 MFs-laden bioinks. As a proof of concept, the bioinks were bioprinted into an anatomical meniscus shape with a biomimetic vascularized outer and non-vascularized inner region, using a microgel suspension bath. The constructs were cultured up to 14 days, with in the outer zone the HUVEC-, mural cell-, and col-1 MF-laden pro-angiogenic bioink, and in the inner zone a meniscus progenitor cell (MPC)- and CdECM MF-laden anti-angiogenic bioink, revealing successful spatial confinement of the nascent vascular network only in the outer zone. Further, to co-facilitate both microvessel formation and MPC-derived matrix formation, we formulated cell culture medium conditions with a temporal switch. Overall, this study provides a new strategy that could be applied to develop zonal biomimetic meniscal constructs. Moreover, the use of ECM-derived MFs to promote or inhibit capillary networks opens new possibilities for the biofabrication of tissues with anisotropic microvascular distribution. These have potential for many applications including in vitro models, cancer progression, and testing anti-angiogenic therapies.

3D In Vitro Platform for Cell and Explant Culture in Liquid-like Solids

Existing 3D cell models and technologies have offered tools to elevate cell culture to a more physiologically relevant dimension. One mechanism to maintain cells cultured in 3D is by means of perfusion. However, existing perfusion technologies for cell culture require complex electronic components, intricate tubing networks, or specific laboratory protocols for each application. We have developed a cell culture platform that simply employs a pump-free suction device to enable controlled perfusion of cell culture media through a bed of granular microgels and removal of cell-secreted metabolic waste. We demonstrated the versatile application of the platform by culturing single cells and keeping tissue microexplants viable for an extended period. The human cardiomyocyte AC16 cell line cultured in our platform revealed rapid cellular spheroid formation after 48 h and ~90% viability by day 7. Notably, we were able to culture gut microexplants for more than 2 weeks as demonstrated by immunofluorescent viability assay and prolonged contractility.

Advances in hydrogel-based vascularized tissues for tissue repair and drug screening

The construction of biomimetic vasculatures within the artificial tissue models or organs is highly required for conveying nutrients, oxygen, and waste products, for improving the survival of engineered tissues in vitro. In recent times, the remarkable progress in utilizing hydrogels and understanding vascular biology have enabled the creation of three-dimensional (3D) tissues and organs composed of highly complex vascular systems. In this review, we give an emphasis on the utilization of hydrogels and their advantages in the vascularization of tissues. Initially, the significance of vascular elements and the regeneration mechanisms of vascularization, including angiogenesis and vasculogenesis, are briefly introduced. Further, we highlight the importance and advantages of hydrogels as artificial microenvironments in fabricating vascularized tissues or organs, in terms of tunable physical properties, high similarity in physiological environments, and alternative shaping mechanisms, among others. Furthermore, we discuss the utilization of such hydrogels-based vascularized tissues in various applications, including tissue regeneration, drug screening, and organ-on-chips. Finally, we put forward the key challenges, including multifunctionalities of hydrogels, selection of suitable cell phenotype, sophisticated engineering techniques, and clinical translation behind the development of the tissues with complex vasculatures towards their future development.

Development and Characterization of Gelatin-Norbornene Bioink to Understand the Interplay between Physical Architecture and Micro-Capillary Formation in Biofabricated Vascularized Constructs

The principle challenge for engineering viable, cell-laden hydrogel constructs of clinically-relevant size, is rapid vascularization, in order to moderate the finite capacity of passive nutrient diffusion. A multiscale vascular approach, with large open channels and bulk microcapillaries may be an admissible approach to accelerate this process, promoting overall pre-vascularization for long-term viability of constructs. However, the limited availability of bioinks that possess suitable characteristics that support both fabrication of complex architectures and formation of microcapillaries, remains a barrier to advancement in this space. In this study, gelatin-norbornene (Gel-NOR) is investigated as a vascular bioink with tailorable physico-mechanical properties, which promoted the self-assembly of human stromal and endothelial cells into microcapillaries, as well as being compatible with extrusion and lithography-based biofabrication modalities. Gel-NOR constructs containing self-assembled microcapillaries are successfully biofabricated with varying physical architecture (fiber diameter, spacing, and orientation). Both channel sizes and cell types affect the overall structural changes of the printed constructs, where cross-signaling between both human stromal and endothelial cells may be responsible for the reduction in open channel lumen observed over time. Overall, this work highlights an exciting three-way interplay between bioink formulation, construct design, and cell-mediated response that can be exploited towards engineering vascular tissues.

Engineering the multiscale complexity of vascular networks

The survival of vertebrate organisms depends on highly regulated delivery of oxygen and nutrients through vascular networks that pervade nearly all tissues in the body. Dysregulation of these vascular networks is implicated in many common human diseases such as hypertension, coronary artery disease, diabetes and cancer. Therefore, engineers have sought to create vascular networks within engineered tissues for applications such as regenerative therapies, human disease modelling and pharmacological testing. Yet engineering vascular networks has historically remained difficult, owing to both incomplete understanding of vascular structure and technical limitations for vascular fabrication. This Review highlights the materials advances that have enabled transformative progress in vascular engineering by ushering in new tools for both visualizing and building vasculature. New methods such as bioprinting, organoids and microfluidic systems are discussed, which have enabled the fabrication of 3D vascular topologies at a cellular scale with lumen perfusion. These approaches to vascular engineering are categorized into technology-driven and nature-driven approaches. Finally, the remaining knowledge gaps, emerging frontiers and opportunities for this field are highlighted, including the steps required to replicate the multiscale complexity of vascular networks found in nature.