Three-dimensional (3D) bioprinting strategies use computer-aided processes to enable automated simultaneous spatial patterning of cells and/or biomaterials. These technologies are suitable for a broad range of biomedical applications owing to their capability to produce structurally sophisticated and functionally relevant tissue constructs. Extrusion-based 3D bioprinting strategies were among the first modalities developed and are now arguably the most widely used for producing 3D tissue constructs. These technologies have rapidly evolved over the past two decades, providing a powerful tool set for the biofabrication of tissues that can facilitate translational efforts in the field. In this Primer, we describe the methodology of 3D extrusion bioprinting, focusing on the selection of hardware, software and bioinks. We expand upon recent advances in 3D extrusion bioprinting by illustrating the key variations that promote its biofabrication abilities. Finally, we provide an outlook on possible future refinements of the technology. 3D extrusion bioprinting methods can be used to produce tissue constructs in vitro and in situ and are arguably the most commonly used bioprinting strategies. In this Primer, Zhang and colleagues describe the variants of 3D extrusion bioprinting methods and their specific applications, considerations for the formulation of bioinks and strategies for assessing print quality. The authors conclude by looking to recent and upcoming developments in 4D printing and artificial intelligence-assisted dynamic printing strategies.

3D extrusion bioprinting

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

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Engineering the multiscale complexity of vascular networks

Decellularized extracellular matrix: New promising and challenging biomaterials for regenerative medicine.

Three-dimensional (3D) bioprinting of vascular tissues that are mechanically and functionally comparable to their native counterparts is an unmet challenge. Here, we developed a tough double-network hydrogel (bio)ink for microfluidic (bio)printing of mono- and dual-layered hollow conduits to recreate vein- and artery-like tissues, respectively. The tough hydrogel consisted of energy-dissipative ionically cross-linked alginate and elastic enzyme–cross-linked gelatin. The 3D bioprinted venous and arterial conduits exhibited key functionalities of respective vessels including relevant mechanical properties, perfusability, barrier performance, expressions of specific markers, and susceptibility to severe acute respiratory syndrome coronavirus 2 pseudo-viral infection. Notably, the arterial conduits revealed physiological vasoconstriction and vasodilatation responses. We further explored the feasibility of these conduits for vascular anastomosis. Together, our study presents biofabrication of mechanically and functionally relevant vascular conduits, showcasing their potentials as vascular models for disease studies in vitro and as grafts for vascular surgeries in vivo, possibly serving broad biomedical applications in the future.

https://www.science.org/doi/pdf/10.1126/sciadv.abq6900?download=true

Microfluidic bioprinting of tough hydrogel-based vascular conduits for functional blood vessels

Functional segmental trachea reconstruction remains a remarkable challenge in the clinic. To date, functional trachea regeneration with alternant cartilage‐fibrous tissue‐mimetic structure similar to that of the native trachea relying on the three‐dimensional (3D) bioprinting technology has seen very limited breakthrough. This fact is mostly due to the lack of tissue‐specific bioinks suitable for both cartilage and vascularized fibrous tissue regeneration, as well as the need for firm interfacial integration between stiff and soft tissues. Here, a novel strategy is developed for 3D bioprinting of cartilage‐vascularized fibrous tissue‐integrated trachea (CVFIT), utilizing photocrosslinkable tissue‐specific bioinks. Both cartilage‐ and fibrous tissue‐specific bioinks created by this study provide suitable printability, favorable biocompatibility, and biomimetic microenvironments for chondrogenesis and vascularized fibrogenesis based on the multicomponent synergistic effect through the hybrid photoinitiated polymerization reaction. As such, the tubular analogs are successfully bioprinted and the ring‐to‐ring alternant structure is tightly integrated by the enhancement of interfacial bonding through the amidation reaction. The results from both the trachea regeneration and the in situ trachea reconstruction demonstrate the satisfactory tissue‐specific regeneration along with realization of mechanical and physiological functions. This study thus illustrates the 3D‐bioprinted native tissue‐like trachea as a promising alternative for clinical trachea reconstruction.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/advs.202202181

Functional Trachea Reconstruction Using 3D‐Bioprinted Native‐Like Tissue Architecture Based on Designable Tissue‐Specific Bioinks

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adfm.202011252

Light-Activated Decellularized Extracellular Matrix-Based Bioinks for Volumetric Tissue Analogs at the Centimeter Scale

Tremendous advances have been made toward accurate recapitulation of the human intestinal system in vitro to understand its developmental process, and disease progression. However, current in vitro models are often confined to 2D or 2.5D microarchitectures, which is difficult to mimic the systemic level of complexity of the native tissue. To overcome this problem, we develop physiologically relevant intestinal models with a 3D hollow tubular structure using 3D bioprinting strategy. A tissue-specific biomaterial, colon-derived decellularized extracellular matrix (Colon dECM) has been developed and it provides significant maturation-guiding potential to human intestinal cells. To fabricate a perfusable tubular model, we develop a simultaneous printing process of multiple materials through concentrically assembled nozzles and employ a light-activated Colon dECM bioink by supplementing with ruthenium/sodium persulfate as a photoinitiator. The bioprinted intestinal tissue models show spontaneous 3D morphogenesis of the human intestinal epithelium without any external stimuli. In consequence, the printed cells form multicellular aggregates and cysts and then differentiate into several types of enterocytes, building junctional networks. This system can serve as a platform to evaluate the effects of potential drug-induced toxicity on the human intestinal tissue and create a co-culture model with commensal microbes and immune cells for future therapeutics. This article is protected by copyright. All rights reserved.

A Bioprinted Tubular Intestine Model Using a Colon-Specific Extracellular Matrix Bioink

Facile Bioprinting Process for Fabricating Size-Controllable Functional Microtissues Using Light-Activated Decellularized Extracellular Matrix-Based Bioinks

Recent advances in biofabrication techniques, including 3D bioprinting, have allowed for the fabrication of cardiac models that are similar to the human heart in terms of their structure (e.g., volumetric scale and anatomy) and function (e.g., contractile and electrical properties). The importance of developing techniques for assessing the characteristics of 3D cardiac substitutes in real time without damaging their structures has also been emphasized. In particular, the heart has two primary mechanisms for transporting blood through the body: contractility and an electrical system based on intra and extracellular calcium ion exchange. This review introduces recent trends in 3D bioprinted cardiac tissues and the measurement of their structural, contractile, and electrical properties in real time. Cardiac models have also been regarded as alternatives to animal models as drug-testing platforms. Thus, perspectives on the convergence of 3D bioprinted cardiac tissues and their assessment for use in drug development are also presented.

Byeongmin Kang

Papers

Light-Activated Decellularized Extracellular Matrix-Based Bioinks for Volumetric Tissue Analogs at the Centimeter Scale

A Bioprinted Tubular Intestine Model Using a Colon-Specific Extracellular Matrix Bioink

Facile Bioprinting Process for Fabricating Size-Controllable Functional Microtissues Using Light-Activated Decellularized Extracellular Matrix-Based Bioinks

3D bioprinted and integrated platforms for cardiac tissue modeling and drug testing.