Multiscale Engineered Heterogeneous Hydrogel Composites for Digital Light Processing 3D Printing.

Hydrogel-based bioinks are widely adopted in digital light processing (DLP) 3D printing. Modulating their mechanical properties is especially beneficial in biomedical applications, such as directing cell activity toward tissue regeneration and healing. However, in both monolithic and granular hydrogels, the tunability of mechanical properties is limited to parameters such as cross-linking or packing density. Herein, we present a bioink platform with multiscale heterogeneity for DLP printing, fabricated by incorporating microgels within a cross-linked polymer matrix to form a mechanically tunable heterogeneous hydrogel composite. The properties of the separate components as well as their interactions can be efficiently tailored from both chemical and physical perspectives, enabling control across both nano and micro scales. Monodisperse, spherical gelatin methacryloyl (GelMA) microgels with a stiffness that can be tuned through polymer concentration or cross-link density are fabricated by a high-throughput microfluidic device. Microgels that have been precross-linked through chemical or physical methods are then embedded in a continuous GelMA matrix, where they influence the biomechanical and biochemical characteristics of composites through particle density and encapsulation of cells. Modulation of microgel volume and selecting different printing parameters enables tailoring of the composite compressive modulus across a range of 29 to 244 kPa. Using this composite hydrogel platform as a DLP ink allows for the fabrication of complex 3D structures with macroscale heterogeneity, providing the potential to mimic tissue- and organ-level complexity. This study presents a unique approach to designing heterogeneous hydrogel composites with tunable properties at the nano-, micro-, and macro-scales, and introduces a highly modular hydrogel platform for DLP 3D printing.