Understanding Nonlinear Fluid Dynamics for All-aqueous Printing of a Viscoelastic Droplet in Yield-Stress Fluids

Analogous of pixels to two-dimensional (2D) pictures, voxels – in the form of either small cubes or spherical particles – are the basic building blocks of three-dimensional (3D) object. Voxelated bioprinting may provide a standardized approach for tissue engineering, transforming basic and translational biomedicine. Recently, we proved the concept of a voxelated bioprinting technology that relies on all-aqueous printing of viscoelastic droplets in yield-stress fluids. Despite the practical success, the fundamental science of such a printing process is largely unknown. Here, we study the process of printing an aqueous viscoelastic droplet in an aqueous yield-stress fluid without the help of large interfacial tension. We develop a printing platform that allows for real-time quantification of the printing process. Further, we design hybrid biomaterials to independently control viscosity and shear-thinning properties and identify the parameter space for printing droplets of good fidelity in three-dimensional (3D) space. We discover that the fidelity of droplet printing, defined as droplet roundness and roughness, is determined by the viscosity and shear-thinning properties of bio-inks. Finally, we demonstrate printing 3D structures made of interconnected yet distinguishable droplets made of multi-materials. Our results provide the knowledge and tools for 3D printing of highly viscoelastic droplets, paving the way for engineering highly functional tissues.