Microfluidics-Enabled Multimaterial Maskless Stereolithographic Bioprinting

Biomimetic gradient scaffolds for the tissue engineering and regeneration of rotator cuff enthesis

Rotator cuff tear is one of the most common musculoskeletal disorders, which often results in recurrent shoulder pain and limited movement. Enthesis is a structurally complex and functionally critical interface connecting tendon and bone that plays an essential role in maintaining integrity of the shoulder joint. Despite the availability of advanced surgical procedures for rotator cuff repair, there is a high rate of failure following surgery due to suboptimal enthesis healing and regeneration. Novel strategies based on tissue engineering are gaining popularity in improving tendon-bone interface (TBI) regeneration. Through incorporating physical and biochemical cues into scaffold design which mimics the structure and composition of native enthesis is advantageous to guide specific differentiation of seeding cells and facilitate the formation of functional tissues. In this review, we summarize the current state of research in enthesis tissue engineering highlighting the development and application of biomimetic scaffolds that replicate the gradient TBI. We also discuss the latest techniques for fabricating potential translatable scaffolds such as 3D bioprinting and microfluidic device. While preclinical studies have demonstrated encouraging results of biomimetic gradient scaffolds, the translation of these findings into clinical applications necessitates a comprehensive understanding of their safety and long-term efficacy.

Harnessing Macromolecular Chemistry to Design Hydrogel Micro- and Macro-Environments

Cell encapsulation within three-dimensional hydrogels is a promising approach to mimic tissues. However, true biomimicry of the intricate microenvironment, biophysical and biochemical gradients, and the macroscale hierarchical spatial organizations of native tissues is an unmet challenge within tissue engineering. This review provides an overview of the macromolecular chemistries that have been applied toward the design of cell-friendly hydrogels, as well as their application toward controlling biophysical and biochemical bulk and gradient properties of the microenvironment. Furthermore, biofabrication technologies provide the opportunity to simultaneously replicate macroscale features of native tissues. Biofabrication strategies are reviewed in detail with a particular focus on the compatibility of these strategies with the current macromolecular toolkit described for hydrogel design and the challenges associated with their clinical translation. This review identifies that the convergence of the ever-expanding macromolecular toolkit and technological advancements within the field of biofabrication, along with an improved biological understanding, represents a promising strategy toward the successful tissue regeneration.

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.

Three dimensional (bio)printing of blood vessels: from vascularized tissues to functional arteries

Tissue engineering has emerged as a strategy for producing functional tissues and organs to treat diseases and injuries. Many chronic conditions directly or indirectly affect normal blood vessel functioning, necessary for material exchange and transport through the body and within tissue-engineered constructs. The interest in vascular tissue engineering is due to two reasons: (1) functional grafts can be used to replace diseased blood vessels, and (2) engineering effective vasculature within other engineered tissues enables connection with the host's circulatory system, supporting their survival. Among various practices, (bio)printing has emerged as a powerful tool to engineer biomimetic constructs. This has been made possible with precise control of cell deposition and matrix environment along with the advancements in biomaterials. (Bio)printing has been used for both engineering stand-alone vascular grafts as well as vasculature within engineered tissues for regenerative applications. In this review article, we discuss various conditions associated with blood vessels, the need for artificial blood vessels, the anatomy and physiology of different blood vessels, available 3D (bio)printing techniques to fabricate tissue-engineered vascular grafts and vasculature in scaffolds, and the comparison among the different techniques. We conclude our review with a brief discussion about future opportunities in the area of blood vessel tissue engineering.

Integrated Sandwich-Paper 3D Cell Sensing Device to In Situ Wirelessly Monitor H2O2 Released from Living Cells

Point-of-care testing (POCT) has attracted great interest because of its prominent advantages of rapidness, precision, portability, and real-time monitoring, thus becoming a powerful biomedical device in early clinical diagnosis and convenient medical treatments. However, its complicated manufacturing process and high expense severely impede mass production and broad applications. Herein, an innovative but inexpensive integrated sandwich-paper three-dimensional (3D) cell sensing device is fabricated to in situ wirelessly detect H2O2 released from living cells. The paper-based electrochemical sensing device was constructed by a sealed sandwiched bottom plastic film/fiber paper/top hole-centered plastic film that was printed with patterned electrodes. A new (Fe, Mn)3(PO4)2/N-doped carbon nanorod was developed and immobilized on the sensing carbon electrode while cell culture solution filled the exposed fiber paper, allowing living cells to grow on the fiber paper surrounding the electrode. Due to the significantly shortening diffusion distance to access the sensing sites by such a unique device and a rationally tuned ratio of Fe2+/Mn2+, the device exhibits a fast response time (0.2 s), a low detection limit (0.4 μM), and a wide detection range (2-3200 μM). This work offers great promise for a low-cost and highly sensitive POCT device for practical clinic diagnosis and broad POCT biomedical applications.

4D-Printable Photocrosslinkable Polyurethane-Based Inks for Tissue Scaffold and Actuator Applications

4D printing recently emerges as an exciting evolution of conventional 3D printing, where a printed construct can quickly transform in response to a specific stimulus to switch between a temporary variable state and an original state. In this work, a photocrosslinkable polyethylene-glycol polyurethane ink is synthesized for light-assisted 4D printing of smart materials. The molecular weight distribution of the ink monomers is tunable by adjusting the copolymerization reaction time. Digital light processing (DLP) technique is used to program a differential swelling response in the printed constructs after humidity variation. Bioactive microparticles are embedded into the ink and the improvement of biocompatibility of the printed constructs is demonstrated for tissue engineering applications. Cell studies reveal above 90% viability in 1 week and ≈50% biodegradability after 4 weeks. Self-folding capillary scaffolds, dynamic grippers, and film actuators are made and activated in a humid environment. The approach offers a versatile platform for the fabrication of complex constructs. The ink can be used in tissue engineering and actuator applications, making the ink a promising avenue for future research.

3D-Bioprinted GelMA Scaffold with ASCs and HUVECs for Engineering Vascularized Adipose Tissue

The purpose of tissue engineering is to reconstruct parts of injured tissues and to resolve the shortage of organ donations. However, the main concern is the limited size of engineered tissue due to insufficient oxygen and nutrition distribution in large three-dimensional (3D) tissue constructs. To provide better support for cells inside the scaffolds, the vascularization of blood vessels within the scaffold could be a solution. This study compared the effects of different culturing systems using human adipose tissue-derived stem/stromal cells (ASCs), human umbilical vein endothelial cells (HUVECs), and coculture of ASCs and HUVECs in 3D-bioprinted gelatin methacrylate (GelMA) hydrogel constructs. The in vitro results showed that the number of live cells was highest in the coculture of ASCs and HUVECs in the GelMA hydrogel after culturing for 21 days. Additionally, the tubular structure was the most abundant in the GelMA hydrogel, containing both ASCs and HUVECs. In the in vivo test, blood vessels were present in both the HUVECs and the coculture of ASCs and HUVECs hydrogels implanted in mice. However, the blood vessel density was the highest in the HUVEC and ASC coculture groups. These findings indicate that the 3D-bioprinted GelMA hydrogel coculture system could be a promising biomaterial for large tissue engineering applications.

Cost-Effective Laboratory Matrix Projection Micro-Lithography System

This paper presents a home-built projection lithographer designed to transfer the image from a DLP (digital light processing) projector MEMS matrix onto the microscope objective's field of view, where a photoresist-covered substrate is placed. The photoresist is exposed using blue light with a wavelength of 450 nm. To calibrate the device and adjust focal lengths, we utilize a red light that does not affect the photoresist. The substrate is located on a movable platform, allowing the exposure field to be shifted, enabling the exposure of designs with lateral sizes of 1 × 1 cm2 at a resolution of a few micrometers. Our setup showcases a 2 μm resolution for the single frame 200 × 100 μm2, and a 5 μm resolution for 1 × 1 cm2 with field stitching. The exposure speed, approximately 1 mm2/100 s, proves to be sufficient for a variety of laboratory prototyping needs. This system offers a significant advantage due to its utilization of easily accessible and budget-friendly components, thereby enhancing its accessibility for a broader user base. The exposure speed and resolution meet the requirements for laboratory prototyping in the fields of 2D materials, quantum optics, superconducting microelectronics, microfluidics, and biology.

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.

New Insights and Advanced Strategies for In Vitro Construction of Vascularized Tissue Engineering

Inadequate vascularization is a significant barrier to clinical application of large-volume tissue engineered grafts. In contrast to in vivo vascularization, in vitro prevascularization shortens the time required for host vessels to grow into the graft core and minimizes necrosis in the core region of the graft. However, the challenge of prevascularization is to construct hierarchical perfusable vascular networks, increase graft volume, and form a vascular tip that can anastomose with host vessels. Understanding advances in in vitro prevascularization techniques and new insights into angiogenesis could overcome these obstacles. In the present review, we discuss new perspectives on angiogenesis, the differences between in vivo and in vitro tissue vascularization, the four elements of prevascularized constructs, recent advances in perfusion-based in vitro prevascularized tissue fabrication, and prospects for large-volume prevascularized tissue engineering.

Bioprinted Human Lung Cancer-Mimics for Tissue Diagnostics Applications

Developing a reproducible and secure supply of customizable control tissues that standardizes for the cell type, tissue architecture, and preanalytics of interest for usage in applications including diagnostic, prognostic, and predictive assays, is critical for improving our patient care and welfare. The conventionally adopted control tissues directly obtained from patients are not ideal because they oftentimes have different amounts of normal and neoplastic elements, differing cellularity, differing architecture, and unknown preanalytics, in addition to the limited supply availability and thus associated high costs. In this study, we demonstrated a strategy to stably produce tissue-mimics for diagnostics purposes by taking advantage of the three-dimensional (3D) bioprinting technology. Specifically, we take anaplastic lymphoma kinase-positive (Alk+) lung cancer as an example, where a micropore-forming bioink laden with tumor cells was combined with digital light processing-based bioprinting for developing native-like Alk+ lung cancer tissue-mimics with both structural and functional relevancy. It is anticipated that our proposed methodology will pave new avenues for both fields of tissue diagnostics and 3D bioprinting significantly expanding their capacities, scope, and sustainability.

Development of digital light processing-based multi-material bioprinting for fabrication of heterogeneous tissue constructs

Human tissues and organs have heterogeneous structures with multiple property gradients, which are difficult to restore by single-material bioprinting technology. The advances in multi-material bioprinting technologies have shown great promise in replicating tissue-engineered constructs with one or more functional gradients. In this study, a multi-material 3D printing system based on digital light processing (DLP) was developed, which could efficiently complete multi-material bioprinting tasks. An appropriate concentration of an ultraviolet absorber was selected to improve the printability of channels, and meanwhile, curing parameters were optimized to improve the printing accuracy. The regulation of the mechanical properties of 3D printed constructs was also explored, which offered guidance on the printing of constructs with mechanical anisotropy. In addition, a cell-laden tracheal construct was bioprinted with a biomimetic heterogeneous structure and mechanical gradient, which could support superior cell viability during a 7-day culture. This study showed that the DLP-based process has the capability of building constructs with complex structures and multiple materials, exhibiting the potential to be used in the biofabrication of heterogeneous and functionally-graded tissues and organs for regenerative medicine.

From materials to clinical use: advances in 3D-printed scaffolds for cartilage tissue engineering

Osteoarthritis caused by articular cartilage defects is a particularly common orthopedic disease that can involve the entire joint, causing great pain to its sufferers. A global patient population of approximately 250 million people has an increasing demand for new therapies with excellent results, and tissue engineering scaffolds have been proposed as a potential strategy for the repair and reconstruction of cartilage defects. The precise control and high flexibility of 3D printing provide a platform for subversive innovation. In this perspective, cartilage tissue engineering (CTE) scaffolds manufactured using different biomaterials are summarized from the perspective of 3D printing strategies, the bionic structure strategies and special functional designs are classified and discussed, and the advantages and limitations of these CTE scaffold preparation strategies are analyzed in detail. Finally, the application prospect and challenges of 3D printed CTE scaffolds are discussed, providing enlightening insights for their current research.

Shaping Synthetic Multicellular and Complex Multimaterial Tissues via Embedded Extrusion-Volumetric Printing of Microgels

In living tissues, cells express their functions following complex signals from their surrounding microenvironment. Capturing both hierarchical architectures at the micro- and macroscale, and anisotropic cell patterning remains a major challenge in bioprinting, and a bottleneck toward creating physiologically-relevant models. Addressing this limitation, a novel technique is introduced, termed Embedded Extrusion-Volumetric Printing (EmVP), converging extrusion-bioprinting and layer-less, ultra-fast volumetric bioprinting, allowing spatially pattern multiple inks/cell types. Light-responsive microgels are developed for the first time as bioresins (µResins) for light-based volumetric bioprinting, providing a microporous environment permissive for cell homing and self-organization. Tuning the mechanical and optical properties of gelatin-based microparticles enables their use as support bath for suspended extrusion printing, in which features containing high cell densities can be easily introduced. µResins can be sculpted within seconds with tomographic light projections into centimeter-scale, granular hydrogel-based, convoluted constructs. Interstitial microvoids enhanced differentiation of multiple stem/progenitor cells (vascular, mesenchymal, neural), otherwise not possible with conventional bulk hydrogels. As proof-of-concept, EmVP is applied to create complex synthetic biology-inspired intercellular communication models, where adipocyte differentiation is regulated by optogenetic-engineered pancreatic cells. Overall, EmVP offers new avenues for producing regenerative grafts with biological functionality, and for developing engineered living systems and (metabolic) disease models.

Trends in Photopolymerizable Bioinks for 3D Bioprinting of Tumor Models

Three-dimensional (3D) bioprinting technologies involving photopolymerizable bioinks (PBs) have attracted enormous attention in recent times owing to their ability to recreate complex structures with high resolution, mechanical stability, and favorable printing conditions that are suited for encapsulating cells. 3D bioprinted tissue constructs involving PBs can offer better insights into the tumor microenvironment and offer platforms for drug screening to advance cancer research. These bioinks enable the incorporation of physiologically relevant cell densities, tissue-mimetic stiffness, and vascularized channels and biochemical gradients in the 3D tumor models, unlike conventional two-dimensional (2D) cultures or other 3D scaffold fabrication technologies. In this perspective, we present the emerging techniques of 3D bioprinting using PBs in the context of cancer research, with a specific focus on the efforts to recapitulate the complexity of the tumor microenvironment. We describe printing approaches and various PB formulations compatible with these techniques along with recent attempts to bioprint 3D tumor models for studying migration and metastasis, cell-cell interactions, cell-extracellular matrix interactions, and drug screening relevant to cancer. We discuss the limitations and identify unexplored opportunities in this field for clinical and commercial translation of these emerging technologies.

Application of 3D Bioprinting in Liver Diseases

Liver diseases are the primary reason for morbidity and mortality in the world. Owing to a shortage of organ donors and postoperative immune rejection, patients routinely suffer from liver failure. Unlike 2D cell models, animal models, and organoids, 3D bioprinting can be successfully employed to print living tissues and organs that contain blood vessels, bone, and kidney, heart, and liver tissues and so on. 3D bioprinting is mainly classified into four types: inkjet 3D bioprinting, extrusion-based 3D bioprinting, laser-assisted bioprinting (LAB), and vat photopolymerization. Bioinks for 3D bioprinting are composed of hydrogels and cells. For liver 3D bioprinting, hepatic parenchymal cells (hepatocytes) and liver nonparenchymal cells (hepatic stellate cells, hepatic sinusoidal endothelial cells, and Kupffer cells) are commonly used. Compared to conventional scaffold-based approaches, marked by limited functionality and complexity, 3D bioprinting can achieve accurate cell settlement, a high resolution, and more efficient usage of biomaterials, better mimicking the complex microstructures of native tissues. This method will make contributions to disease modeling, drug discovery, and even regenerative medicine. However, the limitations and challenges of this method cannot be ignored. Limitation include the requirement of diverse fabrication technologies, observation of drug dynamic response under perfusion culture, the resolution to reproduce complex hepatic microenvironment, and so on. Despite this, 3D bioprinting is still a promising and innovative biofabrication strategy for the creation of artificial multi-cellular tissues/organs.

Rapid prototyping of high-resolution large format microfluidic device through maskless image guided in-situ photopolymerization

Microfluidic devices have found extensive applications in mechanical, biomedical, chemical, and materials research. However, the high initial cost, low resolution, inferior feature fidelity, poor repeatability, rough surface finish, and long turn-around time of traditional prototyping methods limit their wider adoption. In this study, a strategic approach to a deterministic fabrication process based on in-situ image analysis and intermittent flow control called image-guided in-situ maskless lithography (IGIs-ML), has been proposed to overcome these challenges. By using dynamic image analysis and integrated flow control, IGIs-ML provides superior repeatability and fidelity of densely packed features across a large area and multiple devices. This general and robust approach enables the fabrication of a wide variety of microfluidic devices and resolves critical proximity effect and size limitations in rapid prototyping. The affordability and reliability of IGIs-ML make it a powerful tool for exploring the design space beyond the capabilities of traditional rapid prototyping.

Engineered biomimetic micro/nano-materials for tissue regeneration

The incidence of tissue and organ damage caused by various diseases is increasing worldwide. Tissue engineering is a promising strategy of tackling this problem because of its potential to regenerate or replace damaged tissues and organs. The biochemical and biophysical cues of biomaterials can stimulate and induce biological activities such as cell adhesion, proliferation and differentiation, and ultimately achieve tissue repair and regeneration. Micro/nano materials are a special type of biomaterial that can mimic the microstructure of tissues on a microscopic scale due to its precise construction, further providing scaffolds with specific three-dimensional structures to guide the activities of cells. The study and application of biomimetic micro/nano-materials have greatly promoted the development of tissue engineering. This review aims to provide an overview of the different types of micro/nanomaterials, their preparation methods and their application in tissue regeneration.

3D bioprinting for organ and organoid models and disease modeling

INTRODUCTION

3D printing, a versatile additive manufacturing technique, has diverse applications ranging from transportation, rapid prototyping, clean energy, and medical devices.

AREAS COVERED

The authors focus on how 3D printing technology can enhance the drug discovery process through automating tissue production that enables high-throughput screening of potential drug candidates. They also discuss how the 3D bioprinting process works and what considerations to address when using this technology to generate cell laden constructs for drug screening as well as the outputs from such assays necessary for determining the efficacy of potential drug candidates. They focus on how bioprinting how has been used to generate cardiac, neural, and testis tissue models, focusing on bio-printed 3D organoids.

EXPERT OPINION

The next generation of 3D bioprinted organ model holds great promises for the field of medicine. In terms of drug discovery, the incorporation of smart cell culture systems and biosensors into 3D bioprinted models could provide highly detailed and functional organ models for drug screening. By addressing current challenges of vascularization, electrophysiological control, and scalability, researchers can obtain more reliable and accurate data for drug development, reducing the risk of drug failures during clinical trials.

3D Bioprinting as a Powerful Technique for Recreating the Tumor Microenvironment

In vitro three-dimensional models aim to reduce and replace animal testing and establish new tools for oncology research and the development and testing of new anticancer therapies. Among the various techniques to produce more complex and realistic cancer models is bioprinting, which allows the realization of spatially controlled hydrogel-based scaffolds, easily incorporating different types of cells in order to recreate the crosstalk between cancer and stromal components. Bioprinting exhibits other advantages, such as the production of large constructs, the repeatability and high resolution of the process, as well as the possibility of vascularization of the models through different approaches. Moreover, bioprinting allows the incorporation of multiple biomaterials and the creation of gradient structures to mimic the heterogeneity of the tumor microenvironment. The aim of this review is to report the main strategies and biomaterials used in cancer bioprinting. Moreover, the review discusses several bioprinted models of the most diffused and/or malignant tumors, highlighting the importance of this technique in establishing reliable biomimetic tissues aimed at improving disease biology understanding and high-throughput drug screening.

Additive Manufacturing and Physicomechanical Characteristics of PEGDA Hydrogels: Recent Advances and Perspective for Tissue Engineering

In this brief review, we discuss the recent advancements in using poly(ethylene glycol) diacrylate (PEGDA) hydrogels for tissue engineering applications. PEGDA hydrogels are highly attractive in biomedical and biotechnology fields due to their soft and hydrated properties that can replicate living tissues. These hydrogels can be manipulated using light, heat, and cross-linkers to achieve desirable functionalities. Unlike previous reviews that focused solely on material design and fabrication of bioactive hydrogels and their cell viability and interactions with the extracellular matrix (ECM), we compare the traditional bulk photo-crosslinking method with the latest three-dimensional (3D) printing of PEGDA hydrogels. We present detailed evidence combining the physical, chemical, bulk, and localized mechanical characteristics, including their composition, fabrication methods, experimental conditions, and reported mechanical properties of bulk and 3D printed PEGDA hydrogels. Furthermore, we highlight the current state of biomedical applications of 3D PEGDA hydrogels in tissue engineering and organ-on-chip devices over the last 20 years. Finally, we delve into the current obstacles and future possibilities in the field of engineering 3D layer-by-layer (LbL) PEGDA hydrogels for tissue engineering and organ-on-chip devices.

Digital light processing-based multi-material bioprinting: Processes, applications, and perspectives

In the past decade, three-dimensional (3D) printing technology based on digital light processing (DLP) has developed rapidly and shown application prospects in several fields such as pneumatic robotics, flexible electronics, and tissue engineering. In particular, DLP-based multi-material printing has been capable of constructing heterogeneous 3D structures with characteristic gradients. DLP 3D printing technology has a wide range of applications in the field of bioprinting due to its high precision and mild printing conditions, including functionalized artificial tissues, medical models, and bioreactors. This paper focuses on the development of DLP-based multi-material 3D printing technology and its applications in the field of bioprinting, followed by giving an outlook on future efforts on overcoming the challenges and obstacles of this promising technique.

Vascularized organ bioprinting: From strategy to paradigm

Over the past two decades, bioprinting has become a popular research topic worldwide, as it is the most promising approach for manufacturing vascularized organ in vitro. However, transitioning bioprinting from simple tissue models to real biomedical applications is still a challenge due to the lack of interdisciplinary theoretical knowledge and perfect multitechnology integration. This review examines the goals of vasculature manufacturing and proposes the objectives in three stages. We then outline a bidirectional manufacturing strategy consisting of top-down reproduction (bioprinting) and bottom-up regeneration (cellular behaviour). We also provide an in-depth analysis of the views from the four aspects of design, ink, printing, and culture. Furthermore, we present the 'constructing-comprehension cycle' research paradigm in Strategic Priority Research Program and the 'math-model-based batch insights generator' research paradigm for the future, which have the potential to revolutionize the biomedical field.

Lithographic Microneedle-Motors from Multimodal Microfluidics for Cargo Delivery

Micromotors have led to an unprecedented revolution in the field of cargo delivery. Attempts in this area trend toward enriching their structures and improving their functions to promote their further applications. Herein, novel microneedle-motors (MNMs) for active drug delivery through a flexible multimodal microfluidic lithographic approach are presented. The multimodal microfluidics is composed of a co-flow geometry-derived droplet fluid and an active cargo mixed laminar flow in a triangular microchannel. The MNMs with sharp tips and spherical fuel-loading cavities are obtained continuously from microfluidics with the assistance of flow lithography. The structural parameters of the MNMs could be precisely tailored by simply choosing the flow speed or the shape of the photomask. As the actives are mixed into the phase solution during the generation, the resultant MNMs are loaded with cargoes for direct applications without any extra complex operation. Based on these features, it is demonstrated that with sharp tips and autonomous movement, the MNMs can efficiently penetrate the tissue-like substrates, indicating the potential in overcoming physiological barriers for cargo release. These results indicate that the proposed multimodal microfluidic lithographic MNMs are valuable for practical active cargo delivery in biomedical and other relative areas.

Poly(ethylene glycol)-Norbornene as a Photoclick Bioink for Digital Light Processing 3D Bioprinting

Digital light processing (DLP) bioprinting is an emerging technology for three-dimensional bioprinting (3DBP) owing to its high printing fidelity, fast fabrication speed, and higher printing resolution. Low-viscosity bioinks such as poly(ethylene glycol) diacrylate (PEGDA) are commonly used for DLP-based bioprinting. However, the cross-linking of PEGDA proceeds via chain-growth photopolymerization that displays significant heterogeneity in cross-linking density. In contrast, step-growth thiol-norbornene photopolymerization is not oxygen inhibited and produces hydrogels with an ideal network structure. The high cytocompatibility and rapid gelation of thiol-norbornene photopolymerization have lent itself to the cross-linking of cell-laden hydrogels but have not been extensively used for DLP bioprinting. In this study, we explored eight-arm PEG-norbornene (PEG8NB) as a bioink/resin for visible light-initiated DLP-based 3DBP. The PEG8NB-based DLP resin showed high printing fidelity and cytocompatibility even without the use of any bioactive motifs and high initial stiffness. In addition, we demonstrated the versatility of the PEGNB resin by printing solid structures as cell culture devices, hollow channels for endothelialization, and microwells for generating cell spheroids. This work not only expands the selection of bioinks for DLP-based 3DBP but also provides a platform for dynamic modification of the bioprinted constructs.

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

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

Vat Photopolymerization 3D-Printing of Dynamic Thiol-Acrylate Photopolymers Using Bio-Derived Building Blocks

As an energy-efficient additive manufacturing process, vat photopolymerization 3D-printing has become a convenient technology to fabricate functional devices with high resolution and freedom in design. However, due to their permanently crosslinked network structure, photopolymers are not easily reprocessed or repaired. To improve the environmental footprint of 3D-printed objects, herein, we combine the dynamic nature of hydroxyl ester links, undergoing a catalyzed transesterification at elevated temperature, with an acrylate monomer derived from renewable resources. As a sustainable building block, we synthesized an acrylated linseed oil and mixed it with selected thiol crosslinkers. By careful selection of the transesterification catalyst, we obtained dynamic thiol-acrylate resins with a high cure rate and decent storage stability, which enabled the digital light processing (DLP) 3D-printing of objects with a structure size of 550 µm. Owing to their dynamic covalent bonds, the thiol-acrylate networks were able to relax 63% of their initial stress within 22 min at 180 °C and showed enhanced toughness after thermal annealing. We exploited the thermo-activated reflow of the dynamic networks to heal and re-shape the 3D-printed objects. The dynamic thiol-acrylate photopolymers also demonstrated promising healing, shape memory, and re-shaping properties, thus offering great potential for various industrial fields such as soft robotics and electronics.

Tunable and Compartmentalized Multimaterial Bioprinting for Complex Living Tissue Constructs

Recapitulating inherent heterogeneity and complex microarchitectures within confined print volumes for developing implantable constructs that could maintain their structure in vivo has remained challenging. Here, we present a combinational multimaterial and embedded bioprinting approach to fabricate complex tissue constructs that can be implanted postprinting and retain their three-dimensional (3D) shape in vivo. The microfluidics-based single nozzle printhead with computer-controlled pneumatic pressure valves enables laminar flow-based voxelation of up to seven individual bioinks with rapid switching between various bioinks that can solve alignment issues generated during switching multiple nozzles. To improve the spatial organization of various bioinks, printing fidelity with the z-direction, and printing speed, self-healing and biodegradable colloidal gels as support baths are introduced to build complex geometries. Furthermore, the colloidal gels provide suitable microenvironments like native extracellular matrices (ECMs) for achieving cell growths and fast host cell invasion via interconnected microporous networks in vitro and in vivo. Multicompartment microfibers (i.e., solid, core-shell, or donut shape), composed of two different bioink fractions with various lengths or their intravolume space filled by two, four, and six bioink fractions, are successfully printed in the ECM-like support bath. We also print various acellular complex geometries such as pyramids, spirals, and perfusable branched/linear vessels. Successful fabrication of vascularized liver and skeletal muscle tissue constructs show albumin secretion and bundled muscle mimic fibers, respectively. The interconnected microporous networks of colloidal gels result in maintaining printed complex geometries while enabling rapid cell infiltration, in vivo.

Microfluidic Formulation of Topological Hydrogels for Microtissue Engineering

Microfluidics has recently emerged as a powerful tool in generation of submillimeter-sized cell aggregates capable of performing tissue-specific functions, so-called microtissues, for applications in drug testing, regenerative medicine, and cell therapies. In this work, we review the most recent advances in the field, with particular focus on the formulation of cell-encapsulating microgels of small "dimensionalities": "0D" (particles), "1D" (fibers), "2D" (sheets), etc., and with nontrivial internal topologies, typically consisting of multiple compartments loaded with different types of cells and/or biopolymers. Such structures, which we refer to as topological hydrogels or topological microgels (examples including core-shell or Janus microbeads and microfibers, hollow or porous microstructures, or granular hydrogels) can be precisely tailored with high reproducibility and throughput by using microfluidics and used to provide controlled "initial conditions" for cell proliferation and maturation into functional tissue-like microstructures. Microfluidic methods of formulation of topological biomaterials have enabled significant progress in engineering of miniature tissues and organs, such as pancreas, liver, muscle, bone, heart, neural tissue, or vasculature, as well as in fabrication of tailored microenvironments for stem-cell expansion and differentiation, or in cancer modeling, including generation of vascularized tumors for personalized drug testing. We review the available microfluidic fabrication methods by exploiting various cross-linking mechanisms and various routes toward compartmentalization and critically discuss the available tissue-specific applications. Finally, we list the remaining challenges such as simplification of the microfluidic workflow for its widespread use in biomedical research, bench-to-bedside transition including production upscaling, further in vivo validation, generation of more precise organ-like models, as well as incorporation of induced pluripotent stem cells as a step toward clinical applications.

MSCs-laden injectable self-healing hydrogel for systemic sclerosis treatment

As a novel cellular therapy, the anti-inflammatory and immunomodulatory virtues of mesenchymal stem cells (MSCs) make them promising candidates for systemic sclerosis (SSc) treatment. However, the clinical efficacy of this stratagem is limited because of the short persistence time, poor survival, and engraftment of MSCs after injection in vivo. Herein, we develop a novel MSCs-laden injectable self-healing hydrogel for SSc treatment. The hydrogel is prepared using N, O-carboxymethyl chitosan (CS-CM) and 4-armed benzaldehyde-terminated polyethylene glycol (PEG-BA) as the main components, imparting with self-healing capacity via the reversible Schiff-base connection between the amino and benzaldehyde groups. We demonstrate that the hydrogel laden with MSCs not only promoted the proliferation of MSCs and increased the cellular half-life in vivo, but also improve their immune-modulating functions. The tube formation assay indicates that the MSCs could significantly promote angiopoiesis. Moreover, the MSCs-laden hydrogel could inhibit fibrosis by modulating the synthesis of collagen and ameliorate disease progression in SSc disease model mice after subcutaneous injection of bleomycin. All these results highlight this novel MSCs-laden hydrogel and its distinctive functions in treatment of chronic SSc, indicating the additional potential to be used widely in the clinic.

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We proposed novel MSCs-laden injectable self-healing hydrogel for SSc treatment.

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The hydrogel was constructed by PEG-BA and CS-CM.

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MSCs-laden hydrogel promotes angiopoiesis and inhibit fibrosis.

Recent Advances in Multi-Material 3D Printing of Functional Ceramic Devices

In recent years, functional ceramic devices have become smaller, thinner, more refined, and highly integrated, which makes it difficult to realize their rapid prototyping and low-cost manufacturing using traditional processing. As an emerging technology, multi-material 3D printing offers increased complexity and greater freedom in the design of functional ceramic devices because of its unique ability to directly construct arbitrary 3D parts that incorporate multiple material constituents without an intricate process or expensive tools. Here, the latest advances in multi-material 3D printing methods are reviewed, providing a comprehensive study on 3D-printable functional ceramic materials and processes for various functional ceramic devices, including capacitors, multilayer substrates, and microstrip antennas. Furthermore, the key challenges and prospects of multi-material 3D-printed functional ceramic devices are identified, and future directions are discussed.

Bioprinting of 3D Adipose Tissue Models Using a GelMA-Bioink with Human Mature Adipocytes or Human Adipose-Derived Stem Cells

Adipose tissue is related to the development and manifestation of multiple diseases, demonstrating the importance of suitable in vitro models for research purposes. In this study, adipose tissue lobuli were explanted, cultured, and used as an adipose tissue control to evaluate in vitro generated adipose tissue models. During culture, lobule exhibited a stable weight, lactate dehydrogenase, and glycerol release over 15 days. For building up in vitro adipose tissue models, we adapted the biomaterial gelatin methacryloyl (GelMA) composition and handling to homogeneously mix and bioprint human primary mature adipocytes (MA) and adipose-derived stem cells (ASCs), respectively. Accelerated cooling of the bioink turned out to be essential for the homogeneous distribution of lipid-filled MAs in the hydrogel. Last, we compared manual and bioprinted GelMA hydrogels with MA or ASCs and the explanted lobules to evaluate the impact of the printing process and rate the models concerning the physiological reference. The viability analyses demonstrated no significant difference between the groups due to additive manufacturing. The staining of intracellular lipids and perilipin A suggest that GelMA is well suited for ASCs and MA. Therefore, we successfully constructed physiological in vitro models by bioprinting MA-containing GelMA bioinks.

Three-dimensional in vitro culture models in oncology research

Cancer is a multifactorial disease that is responsible for 10 million deaths per year. The intra- and inter-heterogeneity of malignant tumors make it difficult to develop single targeted approaches. Similarly, their diversity requires various models to investigate the mechanisms involved in cancer initiation, progression, drug resistance and recurrence. Of the in vitro cell-based models, monolayer adherent (also known as 2D culture) cell cultures have been used for the longest time. However, it appears that they are often less appropriate than the three-dimensional (3D) cell culture approach for mimicking the biological behavior of tumor cells, in particular the mechanisms leading to therapeutic escape and drug resistance. Multicellular tumor spheroids are widely used to study cancers in 3D, and can be generated by a multiplicity of techniques, such as liquid-based and scaffold-based 3D cultures, microfluidics and bioprinting. Organoids are more complex 3D models than multicellular tumor spheroids because they are generated from stem cells isolated from patients and are considered as powerful tools to reproduce the disease development in vitro. The present review provides an overview of the various 3D culture models that have been set up to study cancer development and drug response. The advantages of 3D models compared to 2D cell cultures, the limitations, and the fields of application of these models and their techniques of production are also discussed.

Advances in Translational 3D Printing for Cartilage, Bone, and Osteochondral Tissue Engineering

The regeneration of 3D tissue constructs with clinically relevant sizes, structures, and hierarchical organizations for translational tissue engineering remains challenging. 3D printing, an additive manufacturing technique, has revolutionized the field of tissue engineering by fabricating biomimetic tissue constructs with precisely controlled composition, spatial distribution, and architecture that can replicate both biological and functional native tissues. Therefore, 3D printing is gaining increasing attention as a viable option to advance personalized therapy for various diseases by regenerating the desired tissues. This review outlines the recently developed 3D printing techniques for clinical translation and specifically summarizes the applications of these approaches for the regeneration of cartilage, bone, and osteochondral tissues. The current challenges and future perspectives of 3D printing technology are also discussed.

3D Bioprinted Hydrogel Microfluidic Devices for Parallel Drug Screening

Conventional high-throughput screening (HTS) platforms suffer from the need for large cell volumes, high reagent consumption, significant assembly cost, and handling efforts. The assembly of three-dimensional (3D) bioprinted hydrogel-based microfluidic chips within platforms can address these problems. We present a continuous and seamless manufacturing approach to create a bioprinted microfluidic chips with a circular pattern scalable toward HTS platforms. Digital light processing 3D bioprinting is used to tune the local permeability of our chip, made of polyethylene glycol diacrylate and cell-laden gelatin methacryloyl, for creating predefined gradients of biochemical properties. We measured the flow-induced physical characteristics, the mass transport of drug agents, and the biological features of the proposed chip. We measured reactive oxygen species from the encapsulated cells through an integrated process and showed the capacity of the hydrogel-based chip for creating drug/agent gradients. This work introduces a chip design based on a hydrogel that can be changed and could be used for modern HTS platforms such as in vitro organoids.

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.

Nanomaterial integrated 3D printing for biomedical applications

3D printing technology, otherwise known as additive manufacturing, has provided a promising tool for manufacturing customized biomaterials for tissue engineering and regenerative medicine applications. A vast variety of biomaterials including metals, ceramics, polymers, and composites are currently being used as base materials in 3D printing. In recent years, nanomaterials have been incorporated into 3D printing polymers to fabricate innovative, versatile, multifunctional hybrid materials that can be used in many different applications within the biomedical field. This review focuses on recent advances in novel hybrid biomaterials composed of nanomaterials and 3D printing technologies for biomedical applications. Various nanomaterials including metal-based nanomaterials, metal-organic frameworks, upconversion nanoparticles, and lipid-based nanoparticles used for 3D printing are presented, with a summary of the mechanisms, functional properties, advantages, disadvantages, and applications in biomedical 3D printing. To finish, this review offers a perspective and discusses the challenges facing the further development of nanomaterials in biomedical 3D printing.

Fabrication of Tβ4-Exosome-releasing artificial stem cells for myocardial infarction therapy by improving coronary collateralization

Currently, stem cell transplantations in cardiac repair are limited owing to disadvantages, such as immunological rejection and poor cell viability. Although direct injection of exosomes can have a curative effect similar to that of stem cell transplantation, high clearance hinders its application in clinical practice. Previous reports suggested that induction of coronary collateralization can be a desired method of adjunctive therapy for someone who had missed the optimal operation time to attenuate myocardial ischemia. In this study, to mimic the paracrine and biological activity of stem cells, we developed artificial stem cells that can continuously release Tβ4-exosomes (Tβ4-ASCs) by encapsulating specific exosomes within microspheres using microfluidics technology. The results show that Tβ4-ASCs can greatly promote coronary collateralization in the periphery of the myocardial infarcted area, and its therapeutic effect is superior to that of directly injecting the exosomes. In addition, to better understand how it works, we demonstrated that the Tβ4-ASC-derived exosomes can enhance the angiogenic capacity of coronary endothelial cells (CAECs) via the miR-17-5p/PHD3/Hif-1α pathway. In brief, as artificial stem cells, Tβ4-ASCs can constantly release functional exosomes and stimulate the formation of collateral circulation after myocardial infarction, providing a feasible and alternative method for clinical revascularization.

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Inspired by the paracrine of stem cells, we fabricated artificial stem cells (Tβ4-ASCs) by loading engineered Tβ4-exosomes with microspheres using microfluidics technology.

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Tβ4-ASCs stimulate the formation of coronary collateralization in myocardial infarcted area through a slowly sustained release of engineered Tβ4-exosomes.

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Tβ4-ASCs improve coronary collateralization via the miR-17-5p/PHD3/Hif-1α signaling pathway.

Dynamically Responsive Scaffolds from Microfluidic 3D Printing for Skin Flap Regeneration

Biological scaffolds hold promising perspectives for random skin flap regeneration, while the practical application is greatly limited by their insufficient vascularization ability and the lack of responsiveness during the dynamical healing process. Herein, a novel MXene-incorporated hollow fibrous (MX-HF) scaffold with dynamically responsive channels is presented for promoting vascularization and skin flap regeneration by using a microfluidic-assisted 3D printing strategy. Benefiting from the photothermal conversion capacity of the MXene nanosheets and temperature-responsive ability of poly(NIPAM) hydrogels in the MX-HF scaffolds, they display a near-infrared (NIR)-responsive shrinkage/swelling behavior, which facilitates the cell penetration into the scaffold channels from the surrounding environment. Moreover, by incorporating vascular endothelial growth factor (VEGF) into the hydrogel matrix for controllable delivery, the MX-HF scaffolds can achieve promoted proliferation, migration, and proangiogenic effects of endothelial cells under NIR irradiation. It is further demonstrated in vivo that the NIR-responsive VEGF@MX-HF scaffolds can effectively improve skin flap survival by promoting angiogenesis, decreasing inflammation, and attenuating apoptosis in skin flaps. Thus, it is believed that such responsive MX-HF scaffolds are promising candidates for clinical random skin flap regeneration as well as other diverse tissue engineering applications.

Smart acoustic 3D cell construct assembly with high-resolution

Precise and flexible three-dimensional (3D) cell construct assembly using external forces or fields can produce micro-scale cellular architectures with intercellular connections, which is an important prerequisite to reproducing the structures and functions of biological systems. Currently, it is also a substantial challenge in the bioengineering field. Here, we propose a smart acoustic 3D cell assembly strategy that utilizes a 3D printed module and hydrogel sheets. Digitally controlled six wave beams offer a high degree of freedom (including wave vector combination, frequency, phase, and amplitude) that enables versatile biomimetic micro cellular patterns in hydrogel sheets. Further, replaceable frames can be used to fix the acoustic-built micro-scale cellular structures in these sheets, enabling user-defined hierarchical or heterogeneous constructs through layer-by-layer assembly. This strategy can be employed to construct vasculature with different diameters and lengths, composed of human umbilical vein endothelial cells and smooth muscle cells. These constructs can also induce controllable vascular network formation. Overall, the findings of this work extend the capabilities of acoustic cell assembly into 3D space, offering advantages including innovative, flexible, and precise patterning, and displaying great potential for the manufacture of various artificial tissue structures that duplicate in vivo functions.

[Research advances of three-dimensional bioprinting technology in urinary system tissue engineering]

For the damage and loss of tissues and organs caused by urinary system diseases, the current clinical treatment methods have limitations. Tissue engineering provides a therapeutic method that can replace or regenerate damaged tissues and organs through the research of cells, biological scaffolds and biologically related molecules. As an emerging manufacturing technology, three-dimensional (3D) bioprinting technology can accurately control the biological materials carrying cells, which further promotes the development of tissue engineering. This article reviews the research progress and application of 3D bioprinting technology in tissue engineering of kidney, ureter, bladder, and urethra. Finally, the main current challenges and future prospects are discussed.

A Review of Multi-Material 3D Printing of Functional Materials via Vat Photopolymerization

Additive manufacturing or 3D printing of materials is a prominent process technology which involves the fabrication of materials layer-by-layer or point-by-point in a subsequent manner. With recent advancements in additive manufacturing, the technology has excited a great potential for extension of simple designs to complex multi-material geometries. Vat photopolymerization is a subdivision of additive manufacturing which possesses many attractive features, including excellent printing resolution, high dimensional accuracy, low-cost manufacturing, and the ability to spatially control the material properties. However, the technology is currently limited by design strategies, material chemistries, and equipment limitations. This review aims to provide readers with a comprehensive comparison of different additive manufacturing technologies along with detailed knowledge on advances in multi-material vat photopolymerization technologies. Furthermore, we describe popular material chemistries both from the past and more recently, along with future prospects to address the material-related limitations of vat photopolymerization. Examples of the impressive multi-material capabilities inspired by nature which are applicable today in multiple areas of life are briefly presented in the applications section. Finally, we describe our point of view on the future prospects of 3D printed multi-material structures as well as on the way forward towards promising further advancements in vat photopolymerization.

Biofabrication approaches and regulatory framework of metastatic tumor-on-a-chip models for precision oncology

The complexity of the tumor microenvironment (TME) together with the development of the metastatic process are the main reasons for the failure of conventional anticancer treatment. In recent years, there is an increasing need to advance toward advanced in vitro models of cancer mimicking TME and simulating metastasis to understand the associated mechanisms that are still unknown, and to be able to develop personalized therapy. In this review, the commonly used alternatives and latest advances in biofabrication of tumor‐on‐chips, which allow the generation of the most sophisticated and optimized models for recapitulating the tumor process, are presented. In addition, the advances that have allowed these new models in the area of metastasis, cancer stem cells, and angiogenesis are summarized, as well as the recent integration of multiorgan‐on‐a‐chip systems to recapitulate natural metastasis and pharmacological screening against it. We also analyze, for the first time in the literature, the normative and regulatory framework in which these models could potentially be found, as well as the requirements and processes that must be fulfilled to be commercially implemented as in vitro study model. Moreover, we are focused on the possible regulatory pathways for their clinical application in precision medicine and decision making through the generation of personalized models with patient samples. In conclusion, this review highlights the synergistic combination of three‐dimensional bioprinting systems with the novel tumor/metastasis/multiorgan‐on‐a‐chip systems to generate models for both basic research and clinical applications to have devices useful for personalized oncology.

Molecularly cleavable bioinks facilitate high-performance digital light processing-based bioprinting of functional volumetric soft tissues

Digital light processing bioprinting favors biofabrication of tissues with improved structural complexity. However, soft-tissue fabrication with this method remains a challenge to balance the physical performances of the bioinks for high-fidelity bioprinting and suitable microenvironments for the encapsulated cells to thrive. Here, we propose a molecular cleavage approach, where hyaluronic acid methacrylate (HAMA) is mixed with gelatin methacryloyl to achieve high-performance bioprinting, followed by selectively enzymatic digestion of HAMA, resulting in tissue-matching mechanical properties without losing the structural complexity and fidelity. Our method allows cellular morphological and functional improvements across multiple bioprinted tissue types featuring a wide range of mechanical stiffness, from the muscles to the brain, the softest organ of the human body. This platform endows us to biofabricate mechanically precisely tunable constructs to meet the biological function requirements of target tissues, potentially paving the way for broad applications in tissue and tissue model engineering.

Micropore-Forming Gelatin Methacryloyl (GelMA) Bioink Toolbox 2.0: Designable Tunability and Adaptability for 3D Bioprinting Applications

It is well-known that tissue engineering scaffolds that feature highly interconnected and size-adjustable micropores are oftentimes desired to promote cellular viability, motility, and functions. Unfortunately, the ability of precise control over the microporous structures within bioinks in a cytocompatible manner for applications in 3D bioprinting is generally lacking, until a method of micropore-forming bioink based on gelatin methacryloyl (GelMA) was reported recently. This bioink took advantage of the unique aqueous two-phase emulsion (ATPE) system, where poly(ethylene oxide) (PEO) droplets are utilized as the porogen. Considering the limitations associated with this very initial demonstration, this article has furthered the understanding of the micropore-forming GelMA bioinks by conducting a systematic investigation into the additional GelMA types (porcine and fish, different methacryloyl-modification degrees) and porogen types (PEO, poly(vinyl alcohol), and dextran), as well as the effects of the porogen concentrations and molecular weights on the properties of the GelMA-based ATPE bioink system. This article exemplifies not only the significantly wider range of micropore sizes achievable and better emulsion stability, but also the improved suitability for both extrusion and digital light processing bioprinting with favorable cellular responses.

The tendon microenvironment: Engineered in vitro models to study cellular crosstalk

Tendinopathy is a multi-faceted pathology characterized by alterations in tendon microstructure, cellularity and collagen composition. Challenged by the possibility of regenerating pathological or ruptured tendons, the healing mechanisms of this tissue have been widely researched over the past decades. However, so far, most of the cellular players and processes influencing tendon repair remain unknown, which emphasizes the need for developing relevant in vitro models enabling to study the complex multicellular crosstalk occurring in tendon microenvironments. In this review, we critically discuss the insights on the interaction between tenocytes and the other tendon resident cells that have been devised through different types of existing in vitro models. Building on the generated knowledge, we stress the need for advanced models able to mimic the hierarchical architecture, cellularity and physiological signaling of tendon niche under dynamic culture conditions, along with the recreation of the integrated gradients of its tissue interfaces. In a forward-looking vision of the field, we discuss how the convergence of multiple bioengineering technologies can be leveraged as potential platforms to develop the next generation of relevant in vitro models that can contribute for a deeper fundamental knowledge to develop more effective treatments.

Selection of natural biomaterials for micro-tissue and organ-on-chip models

The desired organ in micro-tissue models of organ-on-a-chip (OoC) devices dictates the optimum biomaterials, divided into natural and synthetic biomaterials. They can resemble biological tissues' biological functions and architectures by constructing bioactivity of macromolecules, cells, nanoparticles, and other biological agents. The inclusion of such components in OoCs allows them having biological processes, such as basic biorecognition, enzymatic cleavage, and regulated drug release. In this report, we review natural-based biomaterials that are used in OoCs and their main characteristics. We address the preparation, modification, and characterization methods of natural-based biomaterials and summarize recent reports on their applications in the design and fabrication of micro-tissue models. This article will help bioengineers select the proper biomaterials based on developing new technologies to meet clinical expectations and improve patient outcomes fusing disease modeling.

Advances in Digital Light Processing of Hydrogels

Hydrogels, three-dimensional (3D) networks of hydrophilic polymers formed in water, are a significant type of soft matter used in fundamental and applied sciences. Hydrogels are of particular interest for biomedical applications, owing to their soft elasticity and good biocompatibility. However, the high water content and soft nature of hydrogels often make it difficult to process them into desirable solid forms. The development of 3D printing (3DP) technologies has provided opportunities for the manufacturing of hydrogels, by adopting a freeform fabrication method. Owing to its high printing speed and resolution, vat photopolymerization 3DP has recently attracted considerable interest for hydrogel fabrication, with digital light processing (DLP) becoming a widespread representative technique. Whilst acknowledging that other types of vat photopolymerization 3DP have also been applied for this purpose, we here only focus on DLP and its derivatives. In this review, we first comprehensively outline the most recent advances in both materials and fabrication, including the adaptation of novel hydrogel systems and advances in processing (e.g., volumetric printing and multimaterial integration). Secondly, we summarize the applications of hydrogel DLP, including regenerative medicine, functional microdevices, and soft robotics. To the best of our knowledge, this is the first time that either of these specific review focuses has been adopted in the literature. More importantly, we discuss the major challenges associated with hydrogel DLP and provide our perspectives on future trends. To summarize, this review aims to aid and inspire other researchers investigatng DLP, photocurable hydrogels, and the research fields related to them.