Explore the vast range of titles available.

Cellular models are needed to study human development and disease in vitro, and to screen drugs for toxicity and efficacy. Current approaches are limited in the engineering of functional tissue models with requisite cell densities and heterogeneity to appropriately model cell and tissue behaviors. Here, we develop a bioprinting approach to transfer spheroids into self-healing support hydrogels at high resolution, which enables their patterning and fusion into high-cell density microtissues of prescribed spatial organization. As an example application, we bioprint induced pluripotent stem cell-derived cardiac microtissue models with spatially controlled cardiomyocyte and fibroblast cell ratios to replicate the structural and functional features of scarred cardiac tissue that arise following myocardial infarction, including reduced contractility and irregular electrical activity. The bioprinted in vitro model is combined with functional readouts to probe how various pro-regenerative microRNA treatment regimes influence tissue regeneration and recovery of function as a result of cardiomyocyte proliferation. This method is useful for a range of biomedical applications, including the development of precision models to mimic diseases and the screening of drugs, particularly where high cell densities and heterogeneity are important. Cellular models are needed to study disease in vitro and to screen drugs for toxicity and efficacy. Here the authors develop a bioprinting approach to transfer spheroids into self-healing support hydrogels at high resolution, which enables their patterning and fusion into high-cell density microtissues of prescribed spatial organization.

Hydrogels are water-containing polymer networks that have been applied in various biological settings. Burdick and Murphy review recent advances in the development of dynamic hydrogels whose properties and mechanics change in response to biological signals. Hydrogels are water-swollen polymer networks that have found a range of applications from biological scaffolds to contact lenses. Historically, their design has consisted primarily of static systems and those that exhibit simple degradation. However, advances in polymer synthesis and processing have led to a new generation of dynamic systems that are capable of responding to artificial triggers and biological signals with spatial precision. These systems will open up new possibilities for the use of hydrogels as model biological structures and in tissue regeneration.

Hydrogels, networks of water-swollen polymers, are being exploited for the local delivery of cells and biologically relevant molecules. Loebel et al . describe the preparation of supramolecular hydrogels and their characterization. The design of injectable hydrogel systems addresses the growing demand for minimally invasive approaches for local and sustained delivery of therapeutics. We developed a class of hyaluronic acid (HA) hydrogels that form through noncovalent guest–host interactions, undergo disassembly (shear-thinning) when injected through a syringe and then reassemble within seconds (self-healing) when shear forces are removed. Its unique properties enable the use of this hydrogel system for numerous applications, such as injection in vivo (including with cells and therapeutic molecules) or as a 'bioink' in 3D-printing applications. Here, we describe the functionalization of HA either with adamantanes (guest moieties) via controlled esterification or with β-cyclodextrins (host moieties) through amidation. We also describe how to modify the HA derivatives with methacrylates for secondary covalent cross-linking and for reaction with fluorophores for in vitro and in vivo imaging. HA polymers are rationally designed from relatively low-molecular-weight starting materials, with the degree of modification controlled, and have matched guest-to-host stoichiometry, allowing the preparation of hydrogels with tailored properties. This procedure takes 3–4 weeks to complete. We detail the preparation and characterization of the guest–host hydrogels, including assessment of their rheological properties, erosion and biomolecule release in vitro . We furthermore demonstrate how to encapsulate cells in vitro and provide procedures for quantitative assessment of in vivo hydrogel degradation by imaging of fluorescently derivatized materials.

3D bioprinting is a promising approach for the repair of cartilage tissue after damage due to injury or disease; however, the design of 3D printed scaffolds has been limited by the availability of bioinks with requisite printability, cytocompatibility, and bioactivity. To address this, we developed an approach termed in situ crosslinking that permits the printing of non-viscous, photocrosslinkable bioinks via the direct-curing of the bioink with light through a photopermeable capillary prior to deposition. Using a norbornene-modified hyaluronic acid (NorHA) macromer as a representative bioink and our understanding of thiol-ene curing kinetics with visible light, we varied the printing parameters (e.g., capillary length, flow rate, light intensity) to identify printing conditions that were optimal for the ink. The printing process was cytocompatible, with high cell viability and homogenous distribution of mesenchymal stromal cells (MSCs) observed throughout printed constructs. Over 56 days of culture in chondrogenic media, printed constructs increased in compressive moduli, biochemical content (i.e., sulfated glycosaminoglycans, collagen), and histological staining of matrix associated with cartilage tissue. This generalizable printing approach may be used towards the repair of focal defects in articular cartilage or broadly towards widespread biomedical applications across a range of photocrosslinkable bioinks that can now be printed.

3D printing involves the development of inks that exhibit the requisite properties for both printing and the intended application. In bioprinting, these inks are often hydrogels with controlled rheological properties that can be stabilized after deposition. Here, an alternate approach is developed where the ink is composed exclusively of jammed microgels, which are designed to incorporate a range of properties through microgel design (e.g., composition, size) and through the mixing of microgels. The jammed microgel inks are shear‐thinning to permit flow and rapidly recover upon deposition, including on surfaces or when deposited in 3D within hydrogel supports, and can be further stabilized with secondary cross‐linking. This platform allows the use of microgels engineered from various materials (e.g., thiol‐ene cross‐linked hyaluronic acid (HA), photo‐cross‐linked poly(ethylene glycol), thermo‐sensitive agarose) and that incorporate cells, where the jamming process and printing do not decrease cell viability. The versatility of this particle‐based approach opens up numerous potential biomedical applications through the printing of a more diverse set of inks. Microgels are jammed to formulate inks useful for 3D printing. Microgels are fabricated on microfluidic devices and jammed to form shear‐thinning solids where individual microgels can be designed with various chemical compositions or to contain cells. The microgel inks can be printed onto surfaces or into 3D hydrogels to produce diverse structures.

Organs are complex systems composed of different cells, proteins and signalling molecules that are arranged in a highly ordered structure to orchestrate a myriad of functions in our body. Biofabrication strategies can be applied to engineer 3D tissue models in vitro by mimicking the structure and function of native tissue through the precise deposition and assembly of materials and cells. This approach allows the spatiotemporal control over cell–cell and cell–extracellular matrix communication and thus the recreation of tissue-like structures. In this Review, we examine biofabrication strategies for the construction of functional tissue replacements and organ models, focusing on the development of biomaterials, such as supramolecular and photosensitive materials, that can be processed using biofabrication techniques. We highlight bioprinted and bioassembled tissue models and survey biofabrication techniques for their potential to recreate complex tissue properties, such as shape, vasculature and specific functionalities. Finally, we discuss challenges, such as scalability and the foreign body response, and opportunities in the field and provide an outlook to the future of biofabrication in regenerative medicine. Biofabrication can be applied to replicate tissues and organs for regenerative medicine and for the creation of 3D in vitro tissue models. In this Review, the recent advances in biomaterials and biofabrication technologies are discussed, and challenges and opportunities are highlighted.

Biofabrication holds the potential to generate constructs that more closely recapitulate the complexity and heterogeneity of tissues and organs than do currently available regenerative medicine therapies. Such constructs can be applied for tissue regeneration or as in vitro 3D models. Biofabrication is maturing and growing, and scientists with different backgrounds are joining this field, underscoring the need for unity regarding the use of terminology. We therefore believe that there is a compelling need to clarify the relationship between the different concepts, technologies, and descriptions of biofabrication that are often used interchangeably or inconsistently in the current literature. Our objective is to provide a guide to the terminology for different technologies in the field which may serve as a reference for the biofabrication community. Biofabrication holds great potential in the fields of regenerative medicine and physiological 3D in vitro models by allowing the manufacture of complex tissue constructs with a higher degree of biomimicry to native tissues than do current biomedical solutions. As the number of biofabrication technologies being developed continues to expand, it is of paramount importance to adopt a concerted terminology framework and avoid generalizations. The ratio between the spatial resolution and the timescale of manufacture could be considered as a reliable measure to aid in the selection of an appropriate biofabrication technology for a desired application.

Although cell–matrix adhesive interactions are known to regulate stem cell differentiation, the underlying mechanisms, in particular for direct three-dimensional encapsulation within hydrogels, are poorly understood. Here, we demonstrate that in covalently crosslinked hyaluronic acid (HA) hydrogels, the differentiation of human mesenchymal stem cells (hMSCs) is directed by the generation of degradation-mediated cellular traction, independently of cell morphology or matrix mechanics. hMSCs within HA hydrogels of equivalent elastic moduli that permit (restrict) cell-mediated degradation exhibited high (low) degrees of cell spreading and high (low) tractions, and favoured osteogenesis (adipogenesis). Moreover, switching the permissive hydrogel to a restrictive state through delayed secondary crosslinking reduced further hydrogel degradation, suppressed traction, and caused a switch from osteogenesis to adipogenesis in the absence of changes to the extended cellular morphology. Furthermore, inhibiting tension-mediated signalling in the permissive environment mirrored the effects of delayed secondary crosslinking, whereas upregulating tension induced osteogenesis even in the restrictive environment. Adhesive interactions between stem cells and the extracellular matrix are known to regulate stem cell differentiation, yet the underlying mechanisms are not well understood. It is now shown that fate decisions of stem cells encapsulated in covalently crosslinked hydrogels are regulated, independently of matrix mechanics and cell morphology, by the cellular tension generated from cell-induced degradation of the hydrogels.

Mesenchymal stem cells (MSCs) are multipotent progenitor cells whose plasticity and self-renewal capacity have generated significant interest for applications in tissue engineering. The objective of this study was to investigate MSC chondrogenesis in photo-cross-linked hyaluronic acid (HA) hydrogels. Because HA is a native component of cartilage, and MSCs may interact with HA via cell surface receptors, these hydrogels could influence stem cell differentiation. In vitro and in vivo cultures of MSC-laden HA hydrogels permitted chondrogenesis, measured by the early gene expression and production of cartilage-specific matrix proteins. For in vivo culture, MSCs were encapsulated with and without transforming growth factor beta-3 (TGF-β3) or pre-cultured for 2 weeks in chondrogenic medium before implantation. Up-regulation of type II collagen, aggrecan, and sox 9 was observed for all groups over MSCs at the time of encapsulation, and the addition of TGF-β3 further enhanced the expression of these genes. To assess the influence of scaffold chemistry on chondrogenesis, HA hydrogels were compared with relatively inert poly(ethylene glycol) (PEG) hydrogels and showed enhanced expression of cartilage-specific markers. Differences between HA and PEG hydrogels in vivo were most noticeable for MSCs and polymer alone, indicating that hydrogel chemistry influences the commitment of MSCs to undergo chondrogenesis (e.g., ∼43-fold up-regulation of type II collagen of MSCs in HA over PEG hydrogels). Although this study investigated only early markers of tissue regeneration, these results emphasize the importance of material cues in MSC differentiation microenvironments, potentially through interactions between scaffold materials and cell surface receptors.

A major challenge in tissue engineering is the development of materials that can support angiogenesis, wherein endothelial cells from existing vasculature invade the surrounding matrix to form new vascular structures. To identify material properties that impact angiogenesis, here we have developed an in vitro model whereby molded tubular channels inside a synthetic hydrogel are seeded with endothelial cells and subjected to chemokine gradients within a microfluidic device. To accomplish precision molding of hydrogels and successful integration with microfluidics, we developed a class of hydrogels that could be macromolded and micromolded with high shape and size fidelity by eliminating swelling after polymerization. Using this material, we demonstrate that matrix degradability switches three-dimensional endothelial cell invasion between two distinct modes: single-cell migration and the multicellular, strand-like invasion required for angiogenesis. The ability to incorporate these tunable hydrogels into geometrically constrained settings will enable a wide range of previously inaccessible biomedical applications. The fabrication of vascularized 3D tissues requires an understanding of how material properties govern endothelial cell invasion into the surrounding matrix. Here the authors integrate a non-swelling synthetic hydrogel with a microfluidic device to study chemokine gradient-driven angiogenic sprouting and find that matrix degradability modulates the collectivity of cell migration.