Fabrication of Stand-Alone Cell-Laden Collagen Vascular Network Scaffolds Using Fugitive Pattern-Based Printing-Then-Casting Approach

Engineered Microfibers for Tissue Engineering

Hydrogel microfibers are hydrogel materials engineered into fiber structures. Techniques such as wet spinning, microfluidic spinning, and 3D bioprinting are often used to prepare microfibers due to their ability to precisely control the size, morphology, and structure of the microfibers. Microfibers with different structural morphologies have different functions; they provide a flow-through culture environment for cells to improve viability, and can also be used to induce the differentiation of cells such as skeletal muscle and cardiac muscle cells to eventually form functional organs in vitro through special morphologies. This Review introduces recent advances in microfluidics, 3D bioprinting, and wet spinning in the preparation of microfibers, focusing on the materials and fabrication methods. The applications of microfibers in tissue engineering are highlighted by summarizing their contributions in engineering biomimetic blood vessels, vascularized tissues, bone, heart, pancreas, kidney, liver, and fat. Furthermore, applications of engineered fibers in tissue repair and drug screening are also discussed.

3D printing of recombinant collagen/chitosan methacrylate/nanoclay hydrogels loaded with Kartogenin nanoparticles for cartilage regeneration

Cartilage defects are frequently caused by trauma, illness and degradation of the cartilage. If these defects are not sufficiently treated, the joints will degrade irreversibly, possibly resulting in disability. Articular cartilage lacks blood vessels and nerves and is unable to regenerate itself, so the repair of cartilage defects is extremely challenging in clinical treatment. Tissue engineering technology is an emerging technology in cartilage repair and cartilage regeneration. 3D-printed hydrogels show great potential in cartilage tissue engineering for the fabrication of 3D cell culture scaffolds to mimic extracellular matrix. In this study, we construct a 3D-printed hydrogel loaded with nanoparticles by electrostatic interaction and photo cross-linking for the regeneration of cartilage, which has adaptable and drug-continuous release behavior. A photopolymerizable bioink was prepared using recombinant collagen, chitosan, nanoclay Laponite-XLG and nanoparticles loaded with Kartogenin (KGN). This bioink was added with KGN, a small molecule drug that promotes cartilage differentiation, and as a result, the 3D-printed CF/CM/3%LAP/KGN scaffolds obtained by extrusion printing is expected to be used for cartilage repair. It was shown that the 3D-printed scaffolds had good cytocompatibility for human bone marrow mesenchymal stem cells (hBMSCs) and exhibited excellent antimicrobial properties, the continuous release of KGN in the scaffold induced the hBMSCs differentiation into chondrocytes, which significantly enhanced the expression of collagen II and glycosaminoglycan. In vivo studies have shown that implantation of KGN-loaded scaffolds into cartilage-injured tissues promoted cartilage tissue regeneration. This study demonstrated that 3D-printed CF/CM/3%LAP/KGN scaffolds can be used for cartilage repair, which is expected to lead to new healing opportunities for cartilage injury-based diseases.

Design approaches for 3D cell culture and 3D bioprinting platforms

The natural habitat of most cells consists of complex and disordered 3D microenvironments with spatiotemporally dynamic material properties. However, prevalent methods of in vitro culture study cells under poorly biomimetic 2D confinement or homogeneous conditions that often neglect critical topographical cues and mechanical stimuli. It has also become increasingly apparent that cells in a 3D conformation exhibit dramatically altered morphological and phenotypical states. In response, efforts toward designing biomaterial platforms for 3D cell culture have taken centerstage over the past few decades. Herein, we present a broad overview of biomaterials for 3D cell culture and 3D bioprinting, spanning both monolithic and granular systems. We first critically evaluate conventional monolithic hydrogel networks, with an emphasis on specific experimental requirements. Building on this, we document the recent emergence of microgel-based 3D growth media as a promising biomaterial platform enabling interrogation of cells within porous and granular scaffolds. We also explore how jammed microgel systems have been leveraged to spatially design and manipulate cellular structures using 3D bioprinting. The advent of these techniques heralds an unprecedented ability to experimentally model complex physiological niches, with important implications for tissue bioengineering and biomedical applications.

Sacrificial biomaterials in 3D fabrication of scaffolds for tissue engineering applications

Three-dimensional (3D) printing technology has progressed exceedingly in the area of tissue engineering. Despite the tremendous potential of 3D printing, building scaffolds with complex 3D structure, especially with soft materials, still exist as a challenge due to the low mechanical strength of the materials. Recently, sacrificial materials have emerged as a possible solution to address this issue, as they could serve as temporary support or templates to fabricate scaffolds with intricate geometries, porous structures, and interconnected channels without deformation or collapse. Here, we outline the various types of scaffold biomaterials with sacrificial materials, their pros and cons, and mechanisms behind the sacrificial material removal, compare the manufacturing methods such as salt leaching, electrospinning, injection-molding, bioprinting with advantages and disadvantages, and discuss how sacrificial materials could be applied in tissue-specific applications to achieve desired structures. We finally conclude with future challenges and potential research directions.

Associative Liquid-In-Liquid 3D Printing Techniques for Freeform Fabrication of Soft Matter

Shaping soft materials into prescribed 3D complex designs has been challenging yet feasible using various 3D printing technologies. For a broader range of soft matters to be printable, liquid-in-liquid 3D printing techniques have emerged in which an ink phase is printed into 3D constructs within a bath. Most of the attention in this field has been focused on using a support bath with favorable rheology (i.e., shear-thinning behavior) which limits the selection of materials, impeding the broad application of such techniques. However, a growing body of work has begun to leverage the interaction or association of the two involved phases (specifically at the liquid-liquid interface) to fabricate complex constructs from a myriad of soft materials with practical structural, mechanical, optical, magnetic, and communicative properties. This review article has provided an overview of the studies on such associative liquid-in-liquid 3D printing techniques along with their fundamentals, underlying mechanisms, various characterization techniques used for ensuring the structural stability, and practical properties of prints. Also, the future paths with the potential applications are discussed.

Realizations of vascularized tissues: From in vitro platforms to in vivo grafts

Vascularization is essential for realizing thick and functional tissue constructs that can be utilized for in vitro study platforms and in vivo grafts. The vasculature enables the transport of nutrients, oxygen, and wastes and is also indispensable to organ functional units such as the nephron filtration unit, the blood-air barrier, and the blood-brain barrier. This review aims to discuss the latest progress of organ-like vascularized constructs with specific functionalities and realizations even though they are not yet ready to be used as organ substitutes. First, the human vascular system is briefly introduced and related design considerations for engineering vascularized tissues are discussed. Second, up-to-date creation technologies for vascularized tissues are summarized and classified into the engineering and cellular self-assembly approaches. Third, recent applications ranging from in vitro tissue models, including generic vessel models, tumor models, and different human organ models such as heart, kidneys, liver, lungs, and brain, to prevascularized in vivo grafts for implantation and anastomosis are discussed in detail. The specific design considerations for the aforementioned applications are summarized and future perspectives regarding future clinical applications and commercialization are provided.

Biohybrid materials: Structure design and biomedical applications

Biohybrid materials are proceeded by integrating living cells and non-living materials to endow materials with biomimetic properties and functionalities by supporting cell proliferation and even enhancing cell functions. Due to the outstanding biocompatibility and programmability, biohybrid materials provide some promising strategies to overcome current problems in the biomedical field. Here, we review the concept and unique features of biohybrid materials by comparing them with conventional materials. We emphasize the structure design of biohybrid materials and discuss the structure-function relationships. We also enumerate the application aspects of biohybrid materials in biomedical frontiers. We believe this review will bring various opportunities to promote the communication between cell biology, material sciences, and medical engineering.

Three-Dimensional Printing in Stimuli-Responsive Yield-Stress Fluid with an Interactive Dual Microstructure

Yield-stress support bath-enabled three-dimensional (3D) printing has been widely used in recent years for diverse applications. However, current yield-stress fluids usually possess single microstructures and still face the challenges of on-demand adding and/or removing support bath materials during printing, constraining their application scope. This study aims to propose a concept of stimuli-responsive yield-stress fluids with an interactive dual microstructure as support bath materials. The microstructure from a yield-stress additive allows the fluids to present switchable states at different stresses, facilitating an embedded 3D printing process. The microstructure from stimuli-responsive polymers enables the fluids to have regulable rheological properties upon external stimuli, making it feasible to perfuse additional yield-stress fluids during printing and easily remove residual fluids after printing. A nanoclay-Pluronic F127 nanocomposite is studied as a thermosensitive yield-stress fluid. The key material properties are characterized to unveil the interactions in the formed dual microstructure and microstructure evolutions at different stresses and temperatures. Core scientific issues, including the filament formation principle, surface roughness control, and thermal effects of the newly added nanocomposite, are comprehensively investigated. Finally, three representative 3D structures, the Hall of Prayer, capsule, and tube with changing diameter, are successfully printed to validate the printing capability of stimuli-responsive yield-stress fluids for fabricating arbitrary architectures.

Embedded bioprinting for designer 3D tissue constructs with complex structural organization

3D bioprinting has been developed as an effective and powerful technique for the fabrication of living tissue constructs in a well-controlled manner. However, most existing 3D bioprinting strategies face substantial challenges in replicating delicate and intricate tissue-specific structural organizations using mechanically weak biomaterials such as hydrogels. Embedded bioprinting is an emerging bioprinting strategy that can directly fabricate complex structures derived from soft biomaterials within a supporting matrix, which shows great promise in printing large vascularized tissues and organs. Here, we provide a state-of-the-art review on the development of embedded bioprinting including extrusion-based and light-based processes to manufacture complex tissue constructs with biomimetic architectures. The working principles, bioinks, and supporting matrices of embedded printing processes are introduced. The effect of key processing parameters on the printing resolution, shape fidelity, and biological functions of the printed tissue constructs are discussed. Recent innovations in the processes and applications of embedded bioprinting are highlighted, such as light-based volumetric bioprinting and printing of functional vascularized organ constructs. Challenges and future perspectives with regard to translating embedded bioprinting into an effective strategy for the fabrication of functional biological constructs with biomimetic structural organizations are finally discussed. STATEMENT OF SIGNIFICANCE: It is still challenging to replicate delicate and intricate tissue-specific structural organizations using mechanically-weak hydrogels for the fabrication of functional living tissue constructs. Embedded bioprinting is an emerging 3D printing strategy that enables to produce complex tissue structures directly inside a reservoir filled with supporting matrix, which largely widens the choice of bioprinting inks to ECM-like hydrogels. Here we aim to provide a comprehensive review on various embedded bioprinting techniques mainly including extrusion-based and light-based processes. Various bioinks, supporting matrices, key processing parameters as well as their effects on the structures and biological functions of resultant living tissue constructs are discussed. We expect that it can provide an important reference and generate new insights for the bioprinting of large vascularized tissues and organs with biological functions.

Freeform Liquid 3D Printing of Soft Functional Components for Soft Robotics

Freeform liquid three-dimensional printing (FL-3DP) is a promising new additive manufacturing process that uses a yield stress gel as a temporary support, enabling the processing of a broader class of inks into complex geometries, including those with low viscosities or long solidification kinetics that were previously not processable. However, the full exploitation of these advantages for the fabrication of complex multilateral structures has been hindered by difficulties in controlling the interfaces between inks and supports. In this work, an in-depth study of the rheological properties and interfacial stabilities between a nanoclay-modified support and silicone-based inks enabled a better understanding of the impact printing parameters have on the extruded filament morphology, and thus on printing resolutions. With these improvements, the fabrication of functional multimaterial pneumatic components applied to soft robotics could be demonstrated, exhibiting superior capabilities compared to casting or traditional extrusion-based additive manufacturing in terms of geometric freedom (overhanging and multimaterial structures), tunability of the component's functionality, and robustness between different phases. Overall, the full exploitation of FL-3DP advantages enables a broader design space for features and functionalities in soft robotic components that require complex and robust combinations of materials.

The rheology of direct and suspended extrusion bioprinting

Bioprinting is a tool increasingly used in tissue engineering laboratories around the world. As an extension to classic tissue engineering, it enables high levels of control over the spatial deposition of cells, materials, and other factors. It is a field with huge promise for the production of implantable tissues and even organs, but the availability of functional bioinks is a barrier to success. Extrusion bioprinting is the most commonly used technique, where high-viscosity solutions of materials and cells are required to ensure good shape fidelity of the printed tissue construct. This is contradictory to hydrogels used in tissue engineering, which are generally of low viscosity prior to cross-linking to ensure cell viability, making them not directly translatable to bioprinting. This review provides an overview of the important rheological parameters for bioinks and methods to assess printability, as well as the effect of bioink rheology on cell viability. Developments over the last five years in bioink formulations and the use of suspended printing to overcome rheological limitations are then discussed.

Freeform Three-Dimensionally Printed Microchannels via Surface-Initiated Photopolymerization Combined with Sacrificial Molding

Precise freeform microchannels within an aqueous environment have several biomedical applications but remain a challenge to fabricate. Carbohydrate glass materials have shown potential for three-dimensionally (3D) printing precise, microscale structures and are suitable as a sacrificial material to reconstruct complex channel architectures, but due to the rapid dissolution kinetics in hydrogels and the aqueous environment, protective coatings are required. Here, conformal coatings were applied to carbohydrate structures via surface-initiated photopolymerization (SIP) by incorporating a photoinitiator (PI) into freeform 3D printed isomalt structures using a custom 3D printer. Structures were then immersed into a photocurable prepolymer bath and exposed to light for reaction initiation. To achieve uniform distribution of photoinitiator molecules in 3D printed constructs, miscibility between commercial photoinitiators and isomalt was modeled using the group contribution method. A dye-based, type-two photoinitiator, Eosin Y disodium salt (EY), was selected for its miscibility with isomalt and stability under high temperature. A previously described Eosin Y (EY)/triethanolamine (TEA) radical polymerization system was used to polymerize poly(ethylene glycol) diacrylate (PEGDA). Attenuated total reflectance-Fourier transform infrared (ATR-FTIR), surface morphology, and swelling ratio characterizations via SIP were performed. Coatings around freeform structures and solid surfaces were presented to demonstrate the capability of coating complex architectures. This coating method should facilitate the application of 3D sacrificial molding in a variety of hydrogels toward building biomimetic vascular constructs.

Preparation and characterization of nanoclay-hydrogel composite support-bath for bioprinting of complex structures

Three-dimensional bioprinting of cell-laden hydrogels in a sacrificial support-bath has recently emerged as a potential solution for fabricating complex biological structures. Physical properties of the support-bath strongly influence the bioprinting process and the outcome of the fabricated constructs. In this study, we reported the application of a composite Pluronic-nanoclay support-bath including calcium ions as the crosslinking agent for bioprinting of cell-laden alginate-based hydrogels. By tuning the rheological properties, a shear-thinning composite support-bath with fast self-recovery behavior was yielded, which allowed continuous printing of complex and large-scale structures. The printed structures were easily and efficiently harvested from the support-bath without disturbing their shape fidelity. Moreover, the results showed that support-bath assisted bioprinting process did not influence the viability of cells encapsulated within hydrogel. This study demonstrates that Pluronic-nanoclay support-bath can be utilized for bioprinting of complex, cell-laden constructs for vascular and other tissue engineering applications.

Cross-Linkable Microgel Composite Matrix Bath for Embedded Bioprinting of Perfusable Tissue Constructs and Sculpting of Solid Objects

Tissue engineering is a rapidly growing field, which requires advanced fabrication technologies to generate cell-laden tissue analogues with a wide range of internal and external physical features including perfusable channels, cavities, custom shapes, and spatially varying material and/or cell compositions. A versatile embedded printing methodology is proposed in this work for creating custom biomedical acellular and cell-laden hydrogel constructs by utilizing a biocompatible microgel composite matrix bath. A sacrificial material is patterned within a biocompatible hydrogel precursor matrix bath using extrusion printing to create three-dimensional features; after printing, the matrix bath is cross-linked, and the sacrificial material is flushed away to create perfusable channels within the bulk composite hydrogel matrix. The composite matrix bath material consists of jammed cross-linked hydrogel microparticles (microgels) to control rheology during fabrication along with a fluid hydrogel precursor, which is cross-linked after fabrication to form the continuous phase of the composite hydrogel. For demonstration, gellan or enzymatically cross-linked gelatin microgels are utilized with a continuous gelatin hydrogel precursor solution to make the composite matrix bath herein; the composite hydrogel matrix is formed by cross-linking the continuous gelatin phase enzymatically after printing. A variety of features including discrete channels, junctions, networks, and external contours are fabricated in the proposed composite matrix bath using embedded printing. Cell-laden constructs with printed features are also evaluated; the microgel composite hydrogel matrices support cell activity, and printed channels enhance proliferation compared to solid constructs even in static culture. The proposed method can be expanded as a solid object sculpting method to sculpt external contours by printing a shell of sacrificial ink and further discarding excess composite hydrogel matrix after printing and cross-linking. While aqueous alginate solution is used as a sacrificial ink, more advanced sacrificial materials can be utilized for better printing resolution.

A mini-review of embedded 3D printing: supporting media and strategies

Embedded 3D printing is an additive manufacturing method based on a material extrusion strategy.

Printing of Hydrophobic Materials in Fumed Silica Nanoparticle Suspension

Freeform three-dimensional (3D) printing of functional structures from liquid hydrophobic build materials is of great significance and widely used in various fields such as soft robotics and microfluidics. In particular, a yield-stress support bath-enabled 3D-printing methodology has been emerging to fabricate complex 3D structures. Unfortunately, the reported support bath materials are either hydrophobic or not versatile enough for the printing of a wide range of hydrophobic materials. The objective of this study is to propose a fumed silica nanoparticle-based yield-stress suspension as a hydrophobic support bath to enable 3D extrusion printing of various hydrophobic ink materials in a printing-then-solidification fashion. Hydrophobic ink is freeform-deposited in a hydrophobic fumed silica-mineral oil suspension and maintains its shape during printing; it is not cured until the whole structure is complete. Various hydrophobic inks including poly(dimethylsiloxane) (PDMS), SU-8 resin, and epoxy-based conductive ink are printed into complex 3D structures in the fumed silica-mineral oil bath and then cured using relevant cross-linking mechanisms, even at a temperature as high as 90 °C, to prove the feasibility and versatility of the proposed printing approach. In addition, the deposited feature can easily reach a much better resolution such as 30 μm for PDMS filaments due to the negligible interfacial tension effect.