Direct Ink Writing of Rigid Microparticles

Direct ink writing (DIW) enables 3D printing of macroscopic objects with well-defined structures and compositions that controllably change over length scales of order 100 µm. Unfortunately, only a limited number of materials can be processed through DIW because it imparts stringent rheological requirements on inks. This limitation can be overcome for soft materials, if they are formulated as microparticles that, if jammed, fulfill the rheological requirements to be printed. By contrast, densely packed rigid microparticles with stiffnesses exceeding 2 MPa do not exhibit appropriate rheological properties that enable DIW. Here, an ink composed of up to 60 vol% rigid microparticles with core stiffnesses up to 50 MPa is introduced. To achieve this goal, rigid microparticles possessing soft hydrogel shells are produced. The 3D printed fragile granular structure is transformed into a load-bearing granular material through the formation of a 2nd network within the soft shells and in the interstitial spaces. The potential of these particles is demonstrated to be printed into intricate 3D structures, such as a trophy cup, or cast into flexible macroscopic photonic films.