All-Aqueous Printing of Viscoelastic Droplets in 3D Space

Analogous to pixels of two-dimensional (2D) pictures, voxels –– in the form of small cubes or spheres –– are the basic building blocks of three-dimensional (3D) objects. In principle, the location, composition, and properties of individual voxels and voxel-voxel interactions can be precisely defined to match the artistry of biological tissues. Realizing voxelated bioprinting would dramatically expand the capability of existing 3D bioprinting technologies, which largely rely on the assembly of one-dimensional (1D) bio-ink filaments. However, it remains a grand challenge to generate, deposit, and assemble individual droplets on-demand in 3D space. This project seeks to establish the foundational knowledge and to provide a new way of precisely manipulating viscoelastic droplets in 3D space. The knowledge and tools will make a positive impact on particle synthesis, encapsulation, miniaturized soft robots, and tissue engineering. By providing opportunities for interdisciplinary research and organizing local scientific activities, this program will train students from diverse backgrounds to be next-generation scientific leaders in soft matter and complex fluids. Through outreach interactions with local high schools, the PI will leverage the vast number of high-speed videos generated from this project to increase the pipeline of underrepresented minority students in STEM fields. The research goal of this project is to elucidate the nonlinear fluid dynamics associated with all-aqueous printing viscoelastic droplets in yield-stress fluids. A printing platform will be developed to study the dynamics of droplet printing in real time. Moreover, a strategy will be developed to independently control the viscosity and relaxation time of inks. Using these tools and materials, three questions will be explored: (1) How to generate a viscoelastic droplet of good roundness without the help of interfacial tension? (2) What are the mechanisms for detaching the print nozzle from a viscoelastic droplet? (3) What are the parameters determining the roundness of a relaxed droplet? Corroborating experiments with theory and modeling, universal scaling laws will be developed to predict the dependence of droplet fidelity on printing conditions and the nonlinear rheological properties of inks. Finally, exploiting controlled polymer swelling, it will be printed 3D structures consisting of interconnected yet distinguishable hydrogel particles of different properties. These studies are expected to help establish the foundational science for voxelated bioprinting. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.