Photo-activated materials have found widespread use in biological and medical applications and are playing an increasingly important role in the nascent field of three-dimensional (3D) bioprinting. Light can be used as a trigger to drive the formation or the degradation of chemical bonds, leading to unprecedented spatiotemporal control over a material's chemical, physical, and biological properties. With resolution and construct size ranging from nanometres to centimeters, light-mediated biofabrication allows multicellular and multimaterial approaches. It promises to be a powerful tool to mimic the complex multiscale organization of living tissues including skin, bone, cartilage, muscle, vessels, heart, and liver, among others, with increasing organotypic functionality. With this review, we comprehensively discuss photochemical reactions, photo-activated materials, and their use in state-of-the-art deposition-based (extrusion and droplet) and vat polymerization-based (one-and two-photon) bioprinting. By offering an up-to-date view on these techniques, we identify emerging trends, focusing on both the chemistry and instrument aspects, thereby allowing the readers to select the best-suited approach. Starting with photochemical reactions and photo-activated materials, we then discuss principles, applications, and limitations of each technique. With a critical eye to the most recent achievements, the reader is guided through this exciting, emerging field with special emphasis on cell-laden hydrogel constructs.
Hydrogels are excellent mimetics of mammalian extracellular matrices and have found widespread use in tissue engineering. Nanoporosity of monolithic bulk hydrogels, however, limits mass transport of key biomolecules. Microgels used in 3D bioprinting achieve both custom shape and vastly improved permissivity to an array of cell functions, however spherical‐microbead‐based bioinks are challenging to upscale, are inherently isotropic, and require secondary crosslinking. Here, bioinks based on high‐aspect‐ratio hydrogel microstrands are introduced to overcome these limitations. Pre‐crosslinked, bulk hydrogels are deconstructed into microstrands by sizing through a grid with apertures of 40–100 µm. The microstrands are moldable and form a porous, entangled structure, stable in aqueous medium without further crosslinking. Entangled microstrands have rheological properties characteristic of excellent bioinks for extrusion bioprinting. Furthermore, individual microstrands align during extrusion and facilitate the alignment of myotubes. Cells can be placed either inside or outside the hydrogel phase with >90% viability. Chondrocytes co‐printed with the microstrands deposit abundant extracellular matrix, resulting in a modulus increase from 2.7 to 780.2 kPa after 6 weeks of culture. This powerful approach to deconstruct bulk hydrogels into advanced bioinks is both scalable and versatile, representing an important toolbox for 3D bioprinting of architected hydrogels.
3D bioprinting allows the fabrication of 3-dimensional (3D) structures containing living cells whose 3D shape and architecture are matched to a patient. The feature is desirable to achieve personalized treatment of trauma or diseases. However, realization of this promising technique in the clinic is greatly hindered by inferior mechanical properties of most biocompatible bioink materials. Here, we report a novel strategy to achieve printing large constructs with high printing quality and fidelity using an extrusion-based printer. We incorporate cationic 2 nanoparticles in an anionic polymer mixture, which significantly improves mechanical properties, printability and printing fidelity of the polymeric bioink due to electrostatic interactions between the nanoparticles and polymers. Addition of cationic-modified silica nanoparticles to an anionic polymer mixture composed of alginate and gellan gum results in significantly increased zero-shear viscosity (1062 %) as well as storage modulus (486 %). As a result, it is possible to print a large (centimeter-scale) porous structure with high printing quality, whereas the use of the polymeric ink without the nanoparticles leads to collapse of the printed structure during printing. We demonstrate such a mechanical enhancement is achieved by adding nanoparticles within a certain size range (<100 nm), and depends on concentration and surface chemistry of the nanoparticles as well as the length of polymers. Furthermore, shrinkage and swelling of the printed constructs during crosslinking are significantly suppressed by addition of nanoparticles compared to the ink without nanoparticles, which leads to high printing fidelity after crosslinking. The incorporated nanoparticles do not compromise biocompatibility of the polymeric ink, where high cell viability (> 90%) and extracellular matrix secretion are observed for cells printed with nanocomposite inks. The design principle demonstrated can be applied for various anionic polymer-based systems, which could lead to achievement of 3D bioprinting-based personalized treatment.
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