Designing simple biomaterials to replicate the biochemical and mechanical properties of tissues is an ongoing challenge in tissue engineering. For several decades, new biomaterials have been engineered using cytocompatible chemical reactions and spontaneous ligations via click chemistries to generate scaffolds and water swollen polymer networks, known as hydrogels, with tunable properties. However, most of these materials are static in nature, providing only macroscopic tunability of the scaffold mechanics, and do not reflect the dynamic environment of natural extracellular microenvironment. For more complex applications such as organoids or co‐culture systems, there remain opportunities to investigate cells that locally remodel and change the physicochemical properties within the matrices. In this review, advanced biomaterials where dynamic covalent chemistry is used to produce stable 3D cell culture models and high‐resolution constructs for both in vitro and in vivo applications, are discussed. The implications of dynamic covalent chemistry on viscoelastic properties of in vitro models are summarized, case studies in 3D cell culture are critically analyzed, and opportunities to further improve the performance of biomaterials for 3D tissue engineering are discussed.
Gelatin methacryloyl (GelMA) hydrogel scaffolds and GelMA-based bioinks are widely used in tissue engineering and bioprinting due to their ability to support cellular functions and new tissue development. Unfortunately, while terminal sterilization of the GelMA is a critical step for translational tissue engineering applications, it can potentially cause thermal or chemical modifications of GelMA. Thus, understanding the effect of terminal sterilization on GelMA properties is an important, though often overlooked, aspect of material design for translational tissue engineering applications. To this end, we characterized the effects of FDA-approved terminal sterilization methods (autoclaving, ethylene oxide treatment, and gamma (γ)-irradiation) on GelMA prepolymer (bioink) and GelMA hydrogels in terms of the relevant properties for biomedical applications, including mechanical strength, biodegradation rate, cell culture in 2D and 3D, and printability. Autoclaving and ethylene oxide treatment of the GelMA decreased the stiffness of the hydrogel, but the treatments did not modify the biodegradation rate of the hydrogel; meanwhile, γ-irradiation increased the stiffness, reduced the pore size and significantly slowed the biodegradation rate. None of the terminal sterilization methods changed the 2D fibroblast or endothelial cell adhesion and spreading. However, ethylene oxide treatment significantly lowered the fibroblast viability in 3D cell culture. Strikingly, γ-irradiation led to significantly reduced ability of the GelMA prepolymer to undergo sol-gel transition. Furthermore, printability studies showed that the bioinks prepared from γ-irradiated GelMA had significantly reduced printability as compared to the GelMA bioinks prepared from autoclaved or ethylene oxide treated GelMA. These results reveal that the choice of the terminal sterilization method can strongly influence important properties of GelMA bioink and hydrogel. Overall, this study provides further insight into GelMA-based material design with consideration of the effect of terminal sterilization.
Cardiovascular
diseases remain the leading cause of death worldwide.
Patency rates of clinically utilized small diameter synthetic vascular
grafts, such as Dacron and expanded polytetrafluoroethylene (ePTFE),
to treat cardiovascular disease are inadequate because of the lack
of endothelialization. Sodium trimetaphosphate (STMP) cross-linked
poly(vinyl alcohol) (PVA) could be potentially employed as blood-compatible
small diameter vascular graft for the treatment of cardiovascular
disease. However, PVA severely lacks cell adhesion properties, and
the efforts to endothelialize STMP-PVA have been insufficient to produce
a functioning endothelium. To this end, we developed a one-pot method
to conjugate cell-adhesive protein via hydroxyl-to-amine coupling
using carbonyldiimidazole by targeting residual hydroxyl groups on
cross-linked STMP-PVA hydrogel. Primary human umbilical vascular endothelial
cells (HUVECs) demonstrated significantly improved cells adhesion,
viability, and spreading on modified PVA. Cells formed a confluent
endothelial monolayer, and expressed vinculin focal adhesions, cell–cell
junction protein zonula occludens 1 (ZO1), and vascular endothelial
cadherin (VE-Cadherin). Extensive characterization of the blood-compatibility
was performed on modified PVA hydrogel by examining platelet activation,
platelet microparticle formation, platelet CD61 and CD62P expression,
and thrombin generation, which showed that the modified PVA was blood-compatible.
Additionally, grafts were tested under whole, flowing blood without
any anticoagulants in a nonhuman primate, arteriovenous shunt model.
No differences were seen in platelet or fibrin accumulation between
the modified-PVA, unmodified PVA, or clinical, ePTFE controls. This
study presents a significant step in the modification of PVA for the
development of next generation in situ endothelialized synthetic vascular
grafts.
A common indication for corneal transplantation, which is the most transplanted tissue, is a dysfunctional corneal endothelium due to Fuchs' endothelial dystrophy (FED). FED is diagnosed by the presence of in vivo pathological microtopography on the Descemet membrane, which is called corneal guttata. Minimally invasive corneal endothelial cell regenerative procedures such as endothelial cell injection therapy and Rho kinase inhibitor pharmacotherapy have been proposed as alternatives to conventional corneal transplantation for FED patients. However, the effect of guttata on monolayer reformation following such therapies is unknown and there is no equivalent in vitro or animal model to study monolayer reformation. Using a synthetic guttata FED disease model, the formation of the monolayer is investigated to evaluate the efficacy of both therapies. Results obtained suggest that guttata dimensions, density, and spacing greatly affect the fate of corneal endothelial cells in terms of migratory behavior and monolayer reformation. Densely packed synthetic guttata mimicking late-stage FED hinders monolayer reformation, while synthetic guttata of lower height and density show improved monolayer formation. These results suggest that severity of the FED, as determined by height and density of existing guttata, can potentially attenuate corneal endothelial monolayer formation of corneal cell injection therapy and pharmacotherapy.
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