Voxelated bioprinting of modular double-network bio-ink droplets

Analogous of pixels to two-dimensional pictures, voxels -- in the form of either small cubes or spheres -- are the basic building blocks of three-dimensional (3D) objects. However, precise manipulation of viscoelastic bio-ink voxels in 3D space represents a grand challenge in both soft matter science and biomanufacturing. Here, we present a voxelated bioprinting technology that enables the digital assembly of interpenetrating alginate and polyacrylamide (PAM) double-network (DN) hydrogel droplets. The hydrogel is crosslinked via additive-free bioorthogonal chemistry involving a pair of stoichiometrically matched polymers. We develop theoretical frameworks to describe the crosslinking kinetics and stiffness of the hydrogels, and construct a diagram-of-state to delineate their mechanical properties. Multi-channel print nozzles are developed to allow on-demand mixing of highly viscoelastic bio-inks without significantly impairing cell viability. Further, we showcase the distinctive capability of voxelated bioprinting by creating highly complex 3D structures such as a hollow sphere composed of interconnected yet distinguishable hydrogel particles. Finally, we validate the cytocompatibility and in vivo stability of the printed DN scaffolds through cell encapsulation and animal transplantation.