Biomanufacturing of organ-specific tissues with high cellular density and embedded vascular channels

Multiscale engineering of brain organoids for disease modeling

Brain organoids hold great potential for modeling human brain development and pathogenesis. They recapitulate certain aspects of the transcriptional trajectory, cellular diversity, tissue architecture and functions of the developing brain. In this review, we explore the engineering strategies to control the molecular-, cellular- and tissue-level inputs to achieve high-fidelity brain organoids. We review the application of brain organoids in neural disorder modeling and emerging bioengineering methods to improve data collection and feature extraction at multiscale. The integration of multiscale engineering strategies and analytical methods has significant potential to advance insight into neurological disorders and accelerate drug development.

4D bioprinting of programmed dynamic tissues

Setting time as the fourth dimension, 4D printing allows us to construct dynamic structures that can change their shape, property, or functionality over time under stimuli, leading to a wave of innovations in various fields. Recently, 4D printing of smart biomaterials, biological components, and living cells into dynamic living 3D constructs with 4D effects has led to an exciting field of 4D bioprinting. 4D bioprinting has gained increasing attention and is being applied to create programmed and dynamic cell-laden constructs such as bone, cartilage, and vasculature. This review presents an overview on 4D bioprinting for engineering dynamic tissues and organs, followed by a discussion on the approaches, bioprinting technologies, smart biomaterials and smart design, bioink requirements, and applications. While much progress has been achieved, 4D bioprinting as a complex process is facing challenges that need to be addressed by transdisciplinary strategies to unleash the full potential of this advanced biofabrication technology. Finally, we present future perspectives on the rapidly evolving field of 4D bioprinting, in view of its potential, increasingly important roles in the development of advanced dynamic tissues for basic research, pharmaceutics, and regenerative medicine.

Bioengineering methods for vascularizing organoids

Organoids, self-organizing three-dimensional (3D) structures derived from stem cells, offer unique advantages for studying organ development, modeling diseases, and screening potential therapeutics. However, their translational potential and ability to mimic complex in vivo functions are often hindered by the lack of an integrated vascular network. To address this critical limitation, bioengineering strategies are rapidly advancing to enable efficient vascularization of organoids. These methods encompass co-culturing organoids with various vascular cell types, co-culturing lineage-specific organoids with vascular organoids, co-differentiating stem cells into organ-specific and vascular lineages, using organoid-on-a-chip technology to integrate perfusable vasculature within organoids, and using 3D bioprinting to also create perfusable organoids. This review explores the field of organoid vascularization, examining the biological principles that inform bioengineering approaches. Additionally, this review envisions how the converging disciplines of stem cell biology, biomaterials, and advanced fabrication technologies will propel the creation of increasingly sophisticated organoid models, ultimately accelerating biomedical discoveries and innovations.

3D bioprinting of dense cellular structures within hydrogels with spatially controlled heterogeneity

Embedded bioprinting is an emerging technology for precise deposition of cell-laden or cell-only bioinks to construct tissue like structures. Bioink is extruded or transferred into a yield stress hydrogel or a microgel support bath allowing print needle motion during printing and providing temporal support for the printed construct. Although this technology has enabled creation of complex tissue structures, it remains a challenge to develop a support bath with user-defined extracellular mimetic cues and their spatial and temporal control. This is crucial to mimic the dynamic nature of the native tissue to better regenerate tissues and organs. To address this, we present a bioprinting approach involving printing of a photocurable viscous support layer and bioprinting of a cell-only or cell-laden bioink within this viscous layer followed by brief exposure to light to partially crosslink the support layer. This approach does not require shear thinning behavior and is suitable for a wide range of photocurable hydrogels to be used as a support. It enables multi-material printing to spatially control support hydrogel heterogeneity including temporal delivery of bioactive cues (e.g. growth factors), and precise patterning of dense multi-cellular structures within these hydrogel supports. Here, dense stem cell aggregates are printed within methacrylated hyaluronic acid-based hydrogels with patterned heterogeneity to spatially modulate human mesenchymal stem cell osteogenesis. This study has significant impactions on creating tissue interfaces (e.g. osteochondral tissue) in which spatial control of extracellular matrix properties for patterned stem cell differentiation is crucial.

Hepatic organoids move from adolescence to maturity

Since organoids were developed 15 years ago, they are now in their adolescence as a research tool. The ability to generate 'tissue in a dish' has created enormous opportunities for biomedical research. We examine the contributions that hepatic organoids have made to three areas of liver research: as a source of cells and tissue for basic research, for drug discovery and drug safety testing, and for understanding disease pathobiology. We discuss the features that enable hepatic organoids to provide useful models for human liver diseases and identify four types of advances that will enable them to become a mature (i.e., adult) research tool over the next 5 years. During this period, advances in single-cell RNA sequencing and CRISPR technologies coupled with improved hepatic organoid methodology, which enables them to have a wider range of cell types that are present in liver and to be grown in microwells, will generate discoveries that will dramatically advance our understanding of liver development and the pathogenesis of liver diseases. It will generate also new approaches for treating liver fibrosis, which remains a major public health problem with few treatment options.

A 3D Bioprinted Cortical Organoid Platform for Modeling Human Brain Development

The ability to promote three-dimensional (3D) self-organization of induced pluripotent stem cells into complex tissue structures called organoids presents new opportunities for the field of developmental biology. Brain organoids have been used to investigate principles of neurodevelopment and neuropsychiatric disorders and serve as a drug screening and discovery platform. However, brain organoid cultures are currently limited by a lacking ability to precisely control their extracellular environment. Here, this work employs 3D bioprinting to generate a high-throughput, tunable, and reproducible scaffold for controlling organoid development and patterning. Additionally, this approach supports the coculture of organoids and vascular cells in a custom architecture containing interconnected endothelialized channels. Printing fidelity and mechanical assessments confirm that fabricated scaffolds closely match intended design features and exhibit stiffness values reflective of the developing human brain. Using organoid growth, viability, cytoarchitecture, proliferation, and transcriptomic benchmarks, this work finds that organoids cultured within the bioprinted scaffold long-term are healthy and have expected neuroectodermal differentiation. Lastly, this work confirms that the endothelial cells (ECs) in printed channel structures can migrate toward and infiltrate into the embedded organoids. This work demonstrates a tunable 3D culturing platform that can be used to create more complex and accurate models of human brain development and underlying diseases.

Research progress and application of liver organoids for disease modeling and regenerative therapy

The liver is a major metabolic organ of the human body and has a high incidence of diseases. In recent years, the annual incidence of liver disease has increased, seriously endangering human life and health. The study of the occurrence and development mechanism of liver diseases, discovery of new therapeutic targets, and establishment of new methods of medical treatment are major issues related to the national economy and people's livelihood. The development of stable and effective research models is expected to provide new insights into the pathogenesis of liver diseases and the search for more effective treatment options. Organoid technology is a new in vitro culture system, and organoids constructed by human cells can simulate the morphological structure, gene expression, and glucose and lipid metabolism of organs in vivo, providing a new model for related research on liver diseases. This paper reviews the latest research progress on liver organoids from the establishment of cell sources and application of liver organoids and discusses their application potential in the field of liver disease research.

Droplet bioprinting of acellular and cell-laden structures at high-resolutions

Advances in digital light projection(DLP) based (bio) printers have made printing of intricate structures at high resolution possible using a wide range of photosensitive bioinks. A typical setup of a DLP bioprinter includes a vat or reservoir filled with liquid bioink, which presents challenges in terms of cost associated with bioink synthesis, high waste, and gravity-induced cell settling, contaminations, or variation in bioink viscosity during the printing process. Here, we report a vat-free, low-volume, waste-free droplet bioprinting method capable of rapidly printing 3D soft structures at high resolution using model bioinks and model cells. A multiphase many-body dissipative particle dynamics model was developed to simulate the dynamic process of droplet-based DLP printing and elucidate the roles of surface wettability and bioink viscosity. Process variables such as light intensity, photo-initiator concentration, and bioink formulations were optimized to print 3D soft structures (∼0.4-3 kPa) with a typical layer thickness of 50µm, an XY resolution of 38 ± 1.5μm and Z resolution of 237 ± 5.4µm. To demonstrate its versatility, droplet bioprinting was used to print a range of acellular 3D structures such as a lattice cube, a Mayan pyramid, a heart-shaped structure, and a microfluidic chip with endothelialized channels. Droplet bioprinting, performed using model C3H/10T1/2 cells, exhibited high viability (90%) and cell spreading. Additionally, microfluidic devices with internal channel networks lined with endothelial cells showed robust monolayer formation while osteoblast-laden constructs showed mineral deposition upon osteogenic induction. Overall, droplet bioprinting could be a low-cost, no-waste, easy-to-use, method to make customized bioprinted constructs for a range of biomedical applications.

Recent advances in scaffolding biomaterials for cultivated meat

The emergence of cultivated meat provides a sustainable and ethical alternative to traditional animal agriculture, highlighting its increasing importance in the food industry. Biomaterial scaffolds are critical components in cultivated meat production for enabling cell adhesion, proliferation, differentiation, and orientation. While there's extensive research on scaffolding biomaterials, applying them to cultivated meat production poses distinct challenges, with each material offering its own set of advantages and disadvantages. This review summarizes the most recent scaffolding biomaterials used in the last five years for cell-cultured meat, detailing their respective advantages and disadvantages. We suggest future research directions and provide recommendations for scaffolds that support scalable, cost-effective, and safe high-quality meat production. Additionally, we highlight commercial challenges cultivated meat faces, encompassing bioreactor design, cell culture mediums, and regulatory and food safety issues. In summary, this review provides a comprehensive guide and valuable insights for researchers and companies in the field of cultivated meat production.

Application of 3D, 4D, 5D, and 6D bioprinting in cancer research: what does the future look like?

The application of three- and four-dimensional (3D/4D) printing in cancer research represents a significant advancement in understanding and addressing the complexities of cancer biology. 3D/4D materials provide more physiologically relevant environments compared to traditional two-dimensional models, allowing for a more accurate representation of the tumor microenvironment that enables researchers to study tumor progression, drug responses, and interactions with surrounding tissues under conditions similar to in vivo conditions. The dynamic nature of 4D materials introduces the element of time, allowing for the observation of temporal changes in cancer behavior and response to therapeutic interventions. The use of 3D/4D printing in cancer research holds great promise for advancing our understanding of the disease and improving the translation of preclinical findings to clinical applications. Accordingly, this review aims to briefly discuss 3D and 4D printing and their advantages and limitations in the field of cancer. Moreover, new techniques such as 5D/6D printing and artificial intelligence (AI) are also introduced as methods that could be used to overcome the limitations of 3D/4D printing and opened promising ways for the fast and precise diagnosis and treatment of cancer.

Organoid bioinks: construction and application

Organoids have emerged as crucial platforms in tissue engineering and regenerative medicine but confront challenges in faithfully mimicking native tissue structures and functions. Bioprinting technologies offer a significant advancement, especially when combined with organoid bioinks-engineered formulations designed to encapsulate both the architectural and functional elements of specific tissues. This review provides a rigorous, focused examination of the evolution and impact of organoid bioprinting. It emphasizes the role of organoid bioinks that integrate key cellular components and microenvironmental cues to more accurately replicate native tissue complexity. Furthermore, this review anticipates a transformative landscape invigorated by the integration of artificial intelligence with bioprinting techniques. Such fusion promises to refine organoid bioink formulations and optimize bioprinting parameters, thus catalyzing unprecedented advancements in regenerative medicine. In summary, this review accentuates the pivotal role and transformative potential of organoid bioinks and bioprinting in advancing regenerative therapies, deepening our understanding of organ development, and clarifying disease mechanisms.

Advances in 3D Bioprinted Cardiac Tissue Using Stem Cell-Derived Cardiomyocytes

The ultimate goal of cardiac tissue engineering is to generate new muscle to repair or replace the damaged heart. This requires advances in stem cell technologies to differentiate billions of cardiomyocytes, together with advanced biofabrication approaches such as 3D bioprinting to achieve the requisite structure and contractile function. In this concise review, we cover recent progress in 3D bioprinting of cardiac tissue using pluripotent stem cell-derived cardiomyocytes, key design criteria for engineering aligned cardiac tissues, and ongoing challenges in the field that must be addressed to realize this goal.

The emerging role of heart-on-a-chip systems in delineating mechanisms of SARS-CoV-2-induced cardiac dysfunction

Coronavirus disease 2019 (COVID-19) has been a major global health concern since its emergence in 2019, with over 680 million confirmed cases as of April 2023. While COVID-19 has been strongly associated with the development of cardiovascular complications, the specific mechanisms by which viral infection induces myocardial dysfunction remain largely controversial as studies have shown that the severe acute respiratory syndrome coronavirus-2 can lead to heart failure both directly, by causing damage to the heart cells, and indirectly, by triggering an inflammatory response throughout the body. In this review, we summarize the current understanding of potential mechanisms that drive heart failure based on in vitro studies. We also discuss the significance of three-dimensional heart-on-a-chip technology in the context of the current and future pandemics.

Bioengineering toolkits for potentiating organoid therapeutics

Organoids are three-dimensional, multicellular constructs that recapitulate the structural and functional features of specific organs. Because of these characteristics, organoids have been widely applied in biomedical research in recent decades. Remarkable advancements in organoid technology have positioned them as promising candidates for regenerative medicine. However, current organoids still have limitations, such as the absence of internal vasculature, limited functionality, and a small size that is not commensurate with that of actual organs. These limitations hinder their survival and regenerative effects after transplantation. Another significant concern is the reliance on mouse tumor-derived matrix in organoid culture, which is unsuitable for clinical translation due to its tumor origin and safety issues. Therefore, our aim is to describe engineering strategies and alternative biocompatible materials that can facilitate the practical applications of organoids in regenerative medicine. Furthermore, we highlight meaningful progress in organoid transplantation, with a particular emphasis on the functional restoration of various organs.

Biofabrication's Contribution to the Evolution of Cultured Meat

Cultured Meat (CM) is a growing field in cellular agriculture, driven by the environmental impact of conventional meat production, which contributes to climate change and occupies ≈70% of arable land. As demand for meat alternatives rises, research in this area expands. CM production relies on tissue engineering techniques, where a limited number of animal cells are cultured in vitro and processed to create meat-like tissue comprising muscle and adipose components. Currently, CM is primarily produced on a small scale in pilot facilities. Producing a large cell mass based on suitable cell sources and bioreactors remains challenging. Advanced manufacturing methods and innovative materials are required to subsequently process this cell mass into CM products on a large scale. Consequently, CM is closely linked with biofabrication, a suite of technologies for precisely arranging cellular aggregates and cell-material composites to construct specific structures, often using robotics. This review provides insights into contemporary biomedical biofabrication technologies, focusing on significant advancements in muscle and adipose tissue biofabrication for CM production. Novel materials for biofabricating CM are also discussed, emphasizing their edibility and incorporation of healthful components. Finally, initial studies on biofabricated CM are examined, addressing current limitations and future challenges for large-scale production.

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.

Advances in Cell-Rich Inks for Biofabricating Living Architectures

Advancing biofabrication toward manufacturing living constructs with well-defined architectures and increasingly biologically relevant cell densities is highly desired to mimic the biofunctionality of native human tissues. The formulation of tissue-like, cell-dense inks for biofabrication remains, however, challenging at various levels of the bioprinting process. Promising advances have been made toward this goal, achieving relatively high cell densities that surpass those found in conventional platforms, pushing the current boundaries closer to achieving tissue-like cell densities. On this focus, herein the overarching challenges in the bioprocessing of cell-rich living inks into clinically grade engineered tissues are discussed, as well as the most recent advances in cell-rich living ink formulations and their processing technologies are highlighted. Additionally, an overview of the foreseen developments in the field is provided and critically discussed.

The mechanical effects of chemical stimuli on neurospheres

The formulation of more accurate models to describe tissue mechanics necessitates the availability of tools and instruments that can precisely measure the mechanical response of tissues to physical loads and other stimuli. In this regard, neuroscience has trailed other life sciences owing to the unavailability of representative live tissue models and deficiency of experimentation tools. We previously addressed both challenges by employing a novel instrument called the cantilevered-capillary force apparatus (CCFA) to elucidate the mechanical properties of mouse neurospheres under compressive forces. The neurospheres were derived from murine stem cells, and our study was the first of its kind to investigate the viscoelasticity of living neural tissues in vitro. In the current study, we demonstrate the utility of the CCFA as a broadly applicable tool to evaluate tissue mechanics by quantifying the effect that oxidative stress has on the mechanical properties of neurospheres. We treated mouse neurospheres with non-cytotoxic levels of hydrogen peroxide and subsequently evaluated the storage and loss moduli of the tissues under compression and tension. We observed that the neurospheres exhibit viscoelasticity consistent with neural tissue and show that elastic modulus decreases with increasing size of the neurosphere. Our study yields insights for establishing rheological measurements as biomarkers by laying the groundwork for measurement techniques and showing that the influence of a particular treatment may be misinterpreted if the size dependence is ignored.

Utilizing bioprinting to engineer spatially organized tissues from the bottom-up

In response to the growing demand for organ substitutes, tissue engineering has evolved significantly. However, it is still challenging to create functional tissues and organs. Tissue engineering from the 'bottom-up' is promising on solving this problem due to its ability to construct tissues with physiological complexity. The workflow of this strategy involves two key steps: the creation of building blocks, and the subsequent assembly. There are many techniques developed for the two pivotal steps. Notably, bioprinting is versatile among these techniques and has been widely used in research. With its high level of automation, bioprinting has great capacity in engineering tissues with precision and holds promise to construct multi-material tissues. In this review, we summarize the techniques applied in fabrication and assembly of building blocks. We elaborate mechanisms and applications of bioprinting, particularly in the 'bottom-up' strategy. We state our perspectives on future trends of bottom-up tissue engineering, hoping to provide useful reference for researchers in this field.

Progress and limitations in engineering cellular adhesion for research and therapeutics

Intercellular interactions form the cornerstone of multicellular biology. Despite advances in protein engineering, researchers artificially directing physical cell interactions still rely on endogenous cell adhesion molecules (CAMs) alongside off-target interactions and unintended signaling. Recently, methods for directing cellular interactions have been developed utilizing programmable domains such as coiled coils (CCs), nanobody-antigen, and single-stranded DNA (ssDNA). We first discuss desirable molecular- and systems-level properties in engineered CAMs, using the helixCAM platform as a benchmark. Next, we propose applications for engineered CAMs in immunology, developmental biology, tissue engineering, and neuroscience. Biologists in various fields can readily adapt current engineered CAMs to establish control over cell interactions, and their utilization in basic and translational research will incentivize further expansion in engineered CAM capabilities.

Cardiovascular human organ-on-a-chip platform for disease modeling, drug development, and personalized therapy

Cardiovascular organ-on-a-chip (OoC) devices are composed of engineered or native functional tissues that are cultured under controlled microenvironments inside microchips. These systems employ microfabrication and tissue engineering techniques to recapitulate human physiology. This review focuses on human OoC systems to model cardiovascular diseases, to perform drug screening, and to advance personalized medicine. We also address the challenges in the generation of organ chips that can revolutionize the large-scale application of these systems for drug development and personalized therapy.

Quantitative Biofabrication Platform for Collagen-Based Peripheral Nerve Grafts with Structural and Chemical Guidance

Owing to its crucial role in the human body, collagen has immense potential as a material for the biofabrication of tissues and organs. However, highly refined fabrication using collagen remains difficult, primarily because of its notably soft properties. A quantitative biofabrication platform to construct collagen-based peripheral nerve grafts, incorporating bionic structural and chemical guidance cues, is introduced. A viscoelastic model for collagen, which facilitates simulating material relaxation and fabricating collagen-based neural grafts, achieving a maximum channel density similar to that of the native nerve structure of longitudinal microchannel arrays, is established. For axonal regeneration over considerable distances, a gradient printing control model and quantitative method are developed to realize the high-precision gradient distribution of nerve growth factor required to obtain nerve grafts through one-step bioprinting. Experiments verify that the bioprinted graft effectively guides linear axonal growth in vitro and in vivo. This study should advance biofabrication methods for a variety of human tissue-engineering applications requiring tailored cues.

Injectable MSC Spheroid and Microgel Granular Composites for Engineering Tissue

Many cell types require direct cell-cell interactions for differentiation and function; yet, this can be challenging to incorporate into 3-dimensional (3D) structures for the engineering of tissues. Here, a new approach is introduced that combines aggregates of cells (spheroids) with similarly-sized hydrogel particles (microgels) to form granular composites that are injectable, undergo interparticle crosslinking via light for initial stabilization, permit cell-cell contacts for cell signaling, and allow spheroid fusion and growth. One area where this is important is in cartilage tissue engineering, as cell-cell contacts are crucial to chondrogenesis and are missing in many tissue engineering approaches. To address this, granular composites are developed from adult porcine mesenchymal stromal cell (MSC) spheroids and hyaluronic acid microgels and simulations and experimental analyses are used to establish the importance of initial MSC spheroid to microgel volume ratios to balance mechanical support with tissue growth. Long-term chondrogenic cultures of granular composites produce engineered cartilage tissue with extensive matrix deposition and mechanical properties within the range of cartilage, as well as integration with native tissue. Altogether, a new strategy of injectable granular composites is developed that leverages the benefits of cell-cell interactions through spheroids with the mechanical stabilization afforded with engineered hydrogels.

Micromanipulation of Live Microdissected Tissues with a Low-Cost Integrated Robotic Platform

The small amount of human tissue available for testing is a paramount challenge in cancer drug development, cancer disease models, and personalized oncology. Technologies that combine the microscale manipulation of tissues with fluid handling offer the exciting possibility of miniaturizing and automating drug evaluation workflows. This approach minimizes animal testing and enables inexpensive, more efficient testing of samples with high clinical biomimicry using scarce materials. We have developed an inexpensive platform based on an off-the-shelf robot that can manipulate microdissected tissues (µDTs) into user-programmed positions without using intricate microfluidic designs nor any other accessories such as a microscope or a pneumatic controller. The robot integrates complex functions such as vision and fluid actuation by incorporating simple items including a USB camera and a rotary pump. Through the robot's camera, the platform software optically recognizes randomly-seeded µDTs on the surface of a petri dish and positions a mechanical arm above the µDTs. Then, a custom rotary pump actuated by one of the robot's motors generates enough microfluidic lift to hydrodynamically pick and place µDTs with a pipette at a safe distance from the substrate without requiring a proximity sensor. The platform's simple, integrated construction is cost-effective and compact, allowing placement inside a tissue culture hood for sterile workflows. The platform enables users to select µDTs based on their size, place them in user-programmed arrays, such as multi-well plates, and control various robot motion parameters. As a case application, we use the robotic system to conduct semi-automated drug testing of mouse and human µDTs in 384-well plates. Our user-friendly platform promises to democratize microscale tissue research to clinical and biological laboratories worldwide.

Advances in 3D bioprinting for regenerative medicine applications

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

4D printing for biomedical applications

While three-dimensional (3D) printing excels at fabricating static constructs, it fails to emulate the dynamic behavior of native tissues or the temporal programmability desired for medical devices. Four-dimensional (4D) printing is an advanced additive manufacturing technology capable of fabricating constructs that can undergo pre-programmed changes in shape, property, or functionality when exposed to specific stimuli. In this Perspective, we summarize the advances in materials chemistry, 3D printing strategies, and post-printing methodologies that collectively facilitate the realization of temporal dynamics within 4D-printed soft materials (hydrogels, shape-memory polymers, liquid crystalline elastomers), ceramics, and metals. We also discuss and present insights about the diverse biomedical applications of 4D printing, including tissue engineering and regenerative medicine, drug delivery, in vitro models, and medical devices. Finally, we discuss the current challenges and emphasize the importance of an application-driven design approach to enable the clinical translation and widespread adoption of 4D printing.

Architected Poly(ionic liquid) Composites with Spatially Programmable Mechanical Properties and Mixed Conductivity

Structural electrolytes present advantages over liquid varieties, which are critical to myriad applications. In particular, structural electrolytes based on polymerized ionic liquids or poly(ionic liquids) (pILs) provide wide electrochemical windows, high thermal stability, nonvolatility, and modular chemistry. However, current methods of fabricating structural electrolytes from pILs and their composites present limitations. Recent advances have been made in 3D printing pIL electrolytes, but current printing techniques limit the complexity of forms that can be achieved, as well as the ability to control mechanical properties or conductivity. We introduce a method for fabricating architected pIL composites as structural electrolytes via embedded 3D (EMB3D) printing. We present a modular design for formulating ionic liquid (IL) monomer composite inks that can be printed into sparse, lightweight, free-standing lattices with different functionalities. In addition to characterizing the rheological and mechanical behaviors of IL monomer inks and pIL lattices, we demonstrate the self-sensing capabilities of our printed structural electrolytes during cyclic compression. Finally, we use our inks and printing method to spatially program self-sensing capabilities in pIL lattices through heterogeneous architectures as well as ink compositions that provide mixed ionic-electronic conductivity. Our free-form approach to fabricating structural electrolytes in complex, 3D forms with programmable, anisotropic properties has broad potential use in next-generation sensors, soft robotics, bioelectronics, energy storage devices, and more.

Unleashing the Power of Undifferentiated Induced Pluripotent Stem Cell Bioprinting: Current Progress and Future Prospects

Induced pluripotent stem cell (iPSC) technology has revolutionized various fields, including stem cell research, disease modeling, and regenerative medicine. The evolution of iPSC-based models has transitioned from conventional two-dimensional systems to more physiologically relevant three-dimensional (3D) models such as spheroids and organoids. Nonetheless, there still remain challenges including limitations in creating complex 3D tissue geometry and structures, the emergence of necrotic core in existing 3D models, and limited scalability and reproducibility. 3D bioprinting has emerged as a revolutionary technology that can facilitate the development of complex 3D tissues and organs with high scalability and reproducibility. This innovative approach has the potential to effectively bridge the gap between conventional iPSC models and complex 3D tissues in vivo. This review focuses on current trends and advancements in the bioprinting of iPSCs. Specifically, it covers the fundamental concepts and techniques of bioprinting and bioink design, reviews recent progress in iPSC bioprinting research with a specific focus on bioprinting undifferentiated iPSCs, and concludes by discussing existing limitations and future prospects.

Multiscale embedded printing of engineered human tissue and organ equivalents

Creating tissue and organ equivalents with intricate architectures and multiscale functional feature sizes is the first step toward the reconstruction of transplantable human tissues and organs. Existing embedded ink writing approaches are limited by achievable feature sizes ranging from hundreds of microns to tens of millimeters, which hinders their ability to accurately duplicate structures found in various human tissues and organs. In this study, a multiscale embedded printing (MSEP) strategy is developed, in which a stimuli-responsive yield-stress fluid is applied to facilitate the printing process. A dynamic layer height control method is developed to print the cornea with a smooth surface on the order of microns, which can effectively overcome the layered morphology in conventional extrusion-based three-dimensional bioprinting methods. Since the support bath is sensitive to temperature change, it can be easily removed after printing by tuning the ambient temperature, which facilitates the fabrication of human eyeballs with optic nerves and aortic heart valves with overhanging leaflets on the order of a few millimeters. The thermosensitivity of the support bath also enables the reconstruction of the full-scale human heart on the order of tens of centimeters by on-demand adding support bath materials during printing. The proposed MSEP demonstrates broader printable functional feature sizes ranging from microns to centimeters, providing a viable and reliable technical solution for tissue and organ printing in the future.

Advancements of 3D bioprinting in regenerative medicine: Exploring cell sources for organ fabrication

3D bioprinting has unlocked new possibilities for generating complex and functional tissues and organs. However, one of the greatest challenges lies in selecting the appropriate seed cells for constructing fully functional 3D artificial organs. Currently, there are no cell sources available that can fulfill all requirements of 3D bioprinting technologies, and each cell source possesses unique characteristics suitable for specific applications. In this review, we explore the impact of different 3D bioprinting technologies and bioink materials on seed cells, providing a comprehensive overview of the current landscape of cell sources that have been used or hold potential in 3D bioprinting. We also summarized key points to guide the selection of seed cells for 3D bioprinting. Moreover, we offer insights into the prospects of seed cell sources in 3D bioprinted organs, highlighting their potential to revolutionize the fields of tissue engineering and regenerative medicine.

Magnetically driven formation of 3D freestanding soft bioscaffolds

3D soft bioscaffolds have great promise in tissue engineering, biohybrid robotics, and organ-on-a-chip engineering applications. Though emerging three-dimensional (3D) printing techniques offer versatility for assembling soft biomaterials, challenges persist in overcoming the deformation or collapse of delicate 3D structures during fabrication, especially for overhanging or thin features. This study introduces a magnet-assisted fabrication strategy that uses a magnetic field to trigger shape morphing and provide remote temporary support, enabling the straightforward creation of soft bioscaffolds with overhangs and thin-walled structures in 3D. We demonstrate the versatility and effectiveness of our strategy through the fabrication of bioscaffolds that replicate the complex 3D topology of branching vascular systems. Furthermore, we engineered hydrogel-based bioscaffolds to support biohybrid soft actuators capable of walking motion triggered by cardiomyocytes. This approach opens new possibilities for shaping hydrogel materials into complex 3D morphologies, which will further empower a broad range of biomedical applications.

Heart-on-a-chip systems with tissue-specific functionalities for physiological, pathological, and pharmacological studies

Recent advances in heart-on-a-chip systems hold great promise to facilitate cardiac physiological, pathological, and pharmacological studies. This review focuses on the development of heart-on-a-chip systems with tissue-specific functionalities. For one thing, the strategies for developing cardiac microtissues on heart-on-a-chip systems that closely mimic the structures and behaviors of the native heart are analyzed, including the imitation of cardiac structural and functional characteristics. For another, the development of techniques for real-time monitoring of biophysical and biochemical signals from cardiac microtissues on heart-on-a-chip systems is introduced, incorporating cardiac electrophysiological signals, contractile activity, and biomarkers. Furthermore, the applications of heart-on-a-chip systems in intelligent cardiac studies are discussed regarding physiological/pathological research and pharmacological assessment. Finally, the future development of heart-on-a-chip toward a higher level of systematization, integration, and maturation is proposed.

Molecular mechanisms of cellular dysfunction in testes from men with non-obstructive azoospermia

Male factor infertility affects 50% of infertile couples worldwide; the most severe form, non-obstructive azoospermia (NOA), affects 10-15% of infertile males. Treatment for individuals with NOA is limited to microsurgical sperm extraction paired with in vitro fertilization intracytoplasmic sperm injection. Unfortunately, spermatozoa are only retrieved in ~50% of patients, resulting in live birth rates of 21-46%. Regenerative therapies could provide a solution; however, understanding the cell-type-specific mechanisms of cellular dysfunction is a fundamental necessity to develop precision medicine strategies that could overcome these abnormalities and promote regeneration of spermatogenesis. A number of mechanisms of cellular dysfunction have been elucidated in NOA testicular cells. These mechanisms include abnormalities in both somatic cells and germ cells in NOA testes, such as somatic cell immaturity, aberrant growth factor signalling, increased inflammation, increased apoptosis and abnormal extracellular matrix regulation. Future cell-type-specific investigations in identifying modulators of cellular transcription and translation will be key to understanding upstream dysregulation, and these studies will require development of in vitro models to functionally interrogate spermatogenic niche dysfunction in both somatic and germ cells.

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.

Engineering Mesoscopic 3D Tumor Models with a Self-Organizing Vascularized Matrix

Advanced in vitro systems such as multicellular spheroids and lab-on-a-chip devices have been developed, but often fall short in reproducing the tissue scale and self-organization of human diseases. A bioprinted artificial tumor model is introduced with endothelial and stromal cells self-organizing into perfusable and functional vascular structures. This model uses 3D hydrogel matrices to embed multicellular tumor spheroids, allowing them to grow to mesoscopic scales and to interact with endothelial cells. It is shown that angiogenic multicellular tumor spheroids promote the growth of a vascular network, which in turn further enhances the growth of cocultivated tumor spheroids. The self-developed vascular structure infiltrates the tumor spheroids, forms functional connections with the bioprinted endothelium, and can be perfused by erythrocytes and polystyrene microspheres. Moreover, cancer cells migrate spontaneously from the tumor spheroid through the self-assembled vascular network into the fluid flow. Additionally, tumor type specific characteristics of desmoplasia, angiogenesis, and metastatic propensity are preserved between patient-derived samples and tumors derived from this same material growing in the bioreactors. Overall, this modular approach opens up new avenues for studying tumor pathophysiology and cellular interactions in vitro, providing a platform for advanced drug testing while reducing the need for in vivo experimentation.

Small Joint Organoids 3D Bioprinting: Construction Strategy and Application

Osteoarthritis (OA) is a chronic disease that causes pain and disability in adults, affecting ≈300 million people worldwide. It is caused by damage to cartilage, including cellular inflammation and destruction of the extracellular matrix (ECM), leading to limited self-repairing ability due to the lack of blood vessels and nerves in the cartilage tissue. Organoid technology has emerged as a promising approach for cartilage repair, but constructing joint organoids with their complex structures and special mechanisms is still challenging. To overcome these boundaries, 3D bioprinting technology allows for the precise design of physiologically relevant joint organoids, including shape, structure, mechanical properties, cellular arrangement, and biological cues to mimic natural joint tissue. In this review, the authors will introduce the biological structure of joint tissues, summarize key procedures in 3D bioprinting for cartilage repair, and propose strategies for constructing joint organoids using 3D bioprinting. The authors also discuss the challenges of using joint organoids' approaches and perspectives on their future applications, opening opportunities to model joint tissues and response to joint disease treatment.

Embedded 3D bioprinting - An emerging strategy to fabricate biomimetic & large vascularized tissue constructs

Three-dimensional bioprinting is an advanced tissue fabrication technique that allows printing complex structures with precise positioning of multiple cell types layer-by-layer. Compared to other bioprinting methods, extrusion bioprinting has several advantages to print large-sized tissue constructs and complex organ models due to large build volume. Extrusion bioprinting using sacrificial, support and embedded strategies have been successfully employed to facilitate printing of complex and hollow structures. Embedded bioprinting is a gel-in-gel approach developed to overcome the gravitational and overhanging limits of bioprinting to print large-sized constructs with a micron-scale resolution. In embedded bioprinting, deposition of bioinks into the microgel or granular support bath will be facilitated by the sol-gel transition of the support bath through needle movement inside the granular medium. This review outlines various embedded bioprinting strategies and the polymers used in the embedded systems with advantages, limitations, and efficacy in the fabrication of complex vascularized tissues or organ models with micron-scale resolution. Further, the essential requirements of support bath systems like viscoelasticity, stability, transparency and easy extraction to print human scale organs are discussed. Additionally, the organs or complex geometries like vascular constructs, heart, bone, octopus and jellyfish models printed using support bath assisted printing methods with their anatomical features are elaborated. Finally, the challenges in clinical translation and the future scope of these embedded bioprinting models to replace the native organs are envisaged.

3D Bioprinting of Collagen-based Microfluidics for Engineering Fully-biologic Tissue Systems

Microfluidic and organ-on-a-chip devices have improved the physiologic and translational relevance of in vitro systems in applications ranging from disease modeling to drug discovery and pharmacology. However, current manufacturing approaches have limitations in terms of materials used, non-native mechanical properties, patterning of extracellular matrix (ECM) and cells in 3D, and remodeling by cells into more complex tissues. We present a method to 3D bioprint ECM and cells into microfluidic collagen-based high-resolution internally perfusable scaffolds (CHIPS) that address these limitations, expand design complexity, and simplify fabrication. Additionally, CHIPS enable size-dependent diffusion of molecules out of perfusable channels into the surrounding device to support cell migration and remodeling, formation of capillary-like networks, and integration of secretory cell types to form a glucose-responsive, insulin-secreting pancreatic-like microphysiological system.

Bioprinting Using Organ Building Blocks: Spheroids, Organoids, and Assembloids

Three-dimensional (3D) bioprinting, a promising advancement in tissue engineering technology, involves the robotic, layer-by-layer additive biofabrication of functional 3D tissue and organ constructs. This process utilizes biomaterials, typically hydrogels and living cells, following digital models. Traditional tissue engineering uses a classic triad of living cells, scaffolds, and physicochemical signals in bioreactors. A scaffold is a temporary, often biodegradable, support structure. Tissue engineering primarily falls into two categories: (i) scaffold based and (ii) scaffold free. The latter, scaffold-free 3D bioprinting, is gaining increasing popularity. Organ building blocks (OBB), capable of self-assembly and self-organization, such as tissue spheroids, organoids, and assembloids, have begun to be utilized in scaffold-free bioprinting. This article discusses the expanding range of OBB, presents the rapidly evolving collection of bioprinting and bioassembly methods using these OBB, and finally, outlines the advantages, challenges, and future perspectives of using OBB in organ printing.

Microinstrumentation for Brain Organoids

Brain organoids are three-dimensional aggregates of self-organized differentiated stem cells that mimic the structure and function of human brain regions. Organoids bridge the gaps between conventional drug screening models such as planar mammalian cell culture, animal studies, and clinical trials. They can revolutionize the fields of developmental biology, neuroscience, toxicology, and computer engineering. Conventional microinstrumentation for conventional cellular engineering, such as planar microfluidic chips; microelectrode arrays (MEAs); and optical, magnetic, and acoustic techniques, has limitations when applied to three-dimensional (3D) organoids, primarily due to their limits with inherently two-dimensional geometry and interfacing. Hence, there is an urgent need to develop new instrumentation compatible with live cell culture techniques and with scalable 3D formats relevant to organoids. This review discusses conventional planar approaches and emerging 3D microinstrumentation necessary for advanced organoid-machine interfaces. Specifically, this article surveys recently developed microinstrumentation, including 3D printed and curved microfluidics, 3D and fast-scan optical techniques, buckling and self-folding MEAs, 3D interfaces for electrochemical measurements, and 3D spatially controllable magnetic and acoustic technologies relevant to two-way information transfer with brain organoids. This article highlights key challenges that must be addressed for robust organoid culture and reliable 3D spatiotemporal information transfer.

Skeletal Muscle Spheroids as Building Blocks for Engineered Muscle Tissue

Spheroids exhibit enhanced cell-cell interactions that facilitate improved survival and mimic the physiological cellular environment in vivo. Cell spheroids have been successfully used as building blocks for engineered tissues, yet the viability of this approach with skeletal muscle spheroids is poorly understood, particularly when incorporated into three-dimensional (3D) constructs. Bioprinting is a promising strategy to recapitulate the hierarchical organization of native tissue that is fundamental to its function. However, the influence of bioprinting on skeletal muscle cell spheroids and their function are yet to be interrogated. Using C2C12 mouse myoblasts and primary bovine muscle stem cells (MuSCs), we characterized spheroid formation as a function of duration and cell seeding density. We then investigated the potential of skeletal muscle spheroids entrapped in alginate bioink as tissue building blocks for bioprinting myogenic tissue. Both C2C12 and primary bovine MuSCs formed spheroids of similar sizes and remained viable after bioprinting. Spheroids of both cell types fused into larger tissue clusters over time within alginate and exhibited tissue formation comparable to monodisperse cells. Compared to monodisperse cells in alginate gels, C2C12 spheroids exhibited greater MyHC expression after 2 weeks, while cells within bovine MuSC spheroids displayed increased cell spreading. Both monodisperse and MuSC spheroids exhibited increased expression of genes denoting mid- and late-stage myogenic differentiation. Together, these data suggest that skeletal muscle spheroids have the potential for generating myogenic tissue via 3D bioprinting and reveal areas of research that could enhance myogenesis and myogenic differentiation in future studies.

Embedded 3D Bioprinting for Engineering Miniaturized In Vitro Tumor Models

Embedded extrusion 3D bioprinting is a rapidly emerging additive manufacturing methodology that provides a precise spatial deposition of synthetic or natural-origin low-viscosity bioinks during the extrusion printing process. Such a strategy has to date unlocked the freeform extrusion biofabrication of complex micro-to-macro-scale living architectures for numerous applications, including tissue engineering and in vitro disease modeling. In this chapter, we describe a suspension bioprinting methodology leveraging a continuous viscoelastic biopolymer supporting bath functionalized with divalent calcium cations to enable a rapid processing of user-defined bioinks toward architecturally complex 3D in vitro tumor models. This highly simple and cost-effective viscoelastic supporting bath enables a full freeform biofabrication of cell-laden 3D tumor-mimetic architectures that exhibit structural stability in culture post-printing. The cytocompatibility of the supporting bath, its ease of removal from biofabricated living constructs, and its adaptability for processing different ECM-mimetic bioinks open avenues for multi-scale fabrication of numerous types of physiomimetic 3D tumor models for preclinical screening of candidate therapeutics.

Temporal Enzymatic Treatment to Enhance the Remodeling of Multiple Cartilage Microtissues into a Structurally Organized Tissue

Scaffold-free tissue engineering aims to recapitulate key aspects of normal developmental processes to generate biomimetic grafts. Although functional cartilaginous tissues are engineered using such approaches, considerable challenges remain. Herein, the benefits of engineering cartilage via the fusion of multiple cartilage microtissues compared to using (millions of) individual cells to generate a cartilaginous graft are demonstrated. Key advantages include the generation of a richer extracellular matrix, more hyaline-like cartilage phenotype, and superior shape fidelity. A major drawback of aggregate engineering is that individual microtissues do not completely (re)model and remnants of their initial architectures remain throughout the macrotissue. To address this, a temporal enzymatic (chondroitinase-ABC) treatment is implemented to accelerate structural (re)modeling and shown to support robust fusion between adjacent microtissues, enhance microtissue (re)modeling, and enable the development of a more biomimetic tissue with a zonally organized collagen network. Additionally, enzymatic treatment is shown to modulate matrix composition, tissue phenotype, and to a lesser extent, tissue mechanics. This work demonstrates that microtissue self-organization is an effective method for engineering scaled-up cartilage grafts with a predefined geometry and near-native levels of matrix accumulation. Importantly, key limitations associated with using biological building blocks can be alleviated by temporal enzymatic treatment during graft development.

Biofabrication methods for reconstructing extracellular matrix mimetics

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

Nested Biofabrication: Matryoshka-Inspired Intra-Embedded Bioprinting

Engineering functional tissues and organs remains a fundamental pursuit in bio-fabrication. However, the accurate constitution of complex shapes and internal anatomical features of specific organs, including their intricate blood vessels and nerves, remains a significant challenge. Inspired by the Matryoshka doll, here a new method called "Intra-Embedded Bioprinting (IEB)" is introduced building upon existing embedded bioprinting methods. a xanthan gum-based material is used which served a dual role as both a bioprintable ink and a support bath, due to its unique shear-thinning and self-healing properties. IEB's capabilities in organ modeling, creating a miniaturized replica of a pancreas using a photocrosslinkable silicone composite is demonstrated. Further, a head phantom and a Matryoshka doll are 3D printed, exemplifying IEB's capability to manufacture intricate, nested structures. Toward the use case of IEB and employing an innovative coupling strategy between extrusion-based and aspiration-assisted bioprinting, a breast tumor model that included a central channel mimicking a blood vessel, with tumor spheroids bioprinted in proximity is developed. Validation using a clinically-available chemotherapeutic drug illustrated its efficacy in reducing the tumor volume via perfusion over time. This method opens a new way of bioprinting enabling the creation of complex-shaped organs with internal anatomical features.

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.

Engineering Biomimetic Microenvironment for Organoid

Organoid is an emerging frontier technology in the field of life science, in which pluripotent stem cells or tissue-derived differentiated/progenitor cells form 3D structures according to their multi-directional differentiation potential and self-assembly ability. Nowadays, although various types of organoids are widely investigated, their construction is still complicated in operation, uncertain in yield, and poor in reproducibility for the structure and function of native organs. Constructing a biomimetic microenvironment for stem cell proliferation and differentiation in vitro is recognized as a key to driving this field. This review reviews the recent development of engineered biomimetic microenvironments for organoids. First, the composition of the matrix for organoid culture is summarized. Then, strategies for engineering the microenvironment from biophysical, biochemical, and cellular perspectives are discussed in detail. Subsequently, the newly developed monitoring technologies are also reviewed. Finally, a brief conclusion and outlook are presented for the inspiration of future 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.

Facile Photopatterning of Perfusable Microchannels in Synthetic Hydrogels to Recreate Microphysiological Environments

The fabrication of perfusable hydrogels is crucial for recreating in vitro microphysiological environments. Existing strategies to fabricate complex microchannels in hydrogels involve sophisticated equipment/techniques. A cost-effective, facile, versatile, and ultra-fast methodology is reported to fabricate perfusable microchannels of complex shapes in photopolymerizable hydrogels without the need of specialized equipment or sophisticated protocols. The methodology utilizes one-step ultraviolet (UV) light-triggered cross-linking and a photomask printed on inexpensive transparent films to photopattern PEG-norbornene hydrogels. Complex and intricate patterns with high resolution, including perfusable microchannels, can be fabricated in <1 s. The perfusable hydrogel is integrated into a custom-made microfluidic device that permits connection to external pump systems, allowing continuous fluid perfusion into the microchannels. Under dynamic culture, human endothelial cells form a functional and confluent endothelial monolayer that remains viable for at least 7 days and respond to inflammatory stimuli. Finally, approach to photopattern norbornene hyaluronic acid hydrogels is adapted, highlighting the versatility of the technique. This study presents an innovative strategy to simplify and reduce the cost of biofabrication techniques for developing functional in vitro models using perfusable three-dimensional (3D) hydrogels. The approach offers a novel solution to overcome the complexities associated with existing methods, allowing engineering advanced in vitro microphysiological environments.

Direct-Ink-Writing 3D-Printed Bioelectronics

The development of wearable and implantable bioelectronics has garnered significant momentum in recent years, driven by the ever-increasing demand for personalized health monitoring, remote patient management, and real-time physiological data collection. The elevated sophistication and advancement of these devices have thus led to the use of many new and unconventional materials which cannot be fulfilled through traditional manufacturing techniques. Three-dimension (3D) printing, also known as additive manufacturing, is an emerging technology that opens new opportunities to fabricate next-generation bioelectronic devices. Some significant advantages include its capacity for material versatility and design freedom, rapid prototyping, and manufacturing efficiency with enhanced capabilities. This review provides an overview of the recent advances in 3D printing of bioelectronics, particularly direct ink writing (DIW), encompassing the methodologies, materials, and applications that have emerged in this rapidly evolving field. This review showcases the broad range of bioelectronic devices fabricated through 3D printing including wearable biophysical sensors, biochemical sensors, electrophysiological sensors, energy devices, multimodal systems, implantable devices, and soft robots. This review will also discuss the advantages, existing challenges, and outlook of applying DIW 3D printing for the development of bioelectronic devices toward healthcare applications.