Pushing the rheological and mechanical boundaries of extrusion-based 3D bioprinting

3D bioprinting has long been subjected to trade-offs between physicochemical and biological outcomes. The resulting material properties of the initial bioinks and final printing products usually lie within a moderate range, which limits the application of bioprinting and its products. Recent progress in bioinks and bioprinting techniques has significantly expanded the window of material properties. In this review, I define two bioprinting windows to clarify the trade-offs between physical chemistry and biology and provide a comprehensive overview of recent advances that have pushed the rheological boundaries of bioinks and mechanical boundaries of bioprints, focusing on unusual material properties. I illustrate this with recent examples, consolidate the existing strategies into well-defined categories, highlight the prominent trends, and provide perspectives on additional boundaries.