This technology uses stop-flow soft-lithography to make micro-particle colloids. First, a light-polymerizing pre-polymer mixture is flowed through a microfluidic chamber. Next, the solution flowing through the chamber is exposed to a pulse of shaped illumination to polymerize the pre-polymer mixture into the desired shape and size. Finally, the flow carries the micro-particles out of the microfluidic chamber for collection. Importantly, all characteristics of the micro-particles can be controlled. The chemical composition dictates the functional properties and physical stiffness of the final micro-particles, and the size/shape are dictated by the illumination filter used to polymerize the particles. Additionally, the particles can be functionalized with coatings such as antibodies or imaging molecules. As a proof of principle, the inventors demonstrated generation of ring-shaped micro-particles with similar size and stiffness to red blood cells (RBCs), and these micro-particle RBC-mimics can deform to fit through capillary-sized openings with physiologically relevant flow forces.