Design of soft matter for additive processing

Digital assembly via extrusion-based additive manufacturing, or three-dimensional (3D) printing, grants the opportunity to attain exquisite control over material structure and composition at the local (‘voxel’) level. The synthetic incorporation of a diverse array of chemistries into 3D-printed soft materials has expanded its use into many application areas. However, substantial opportunity exists for synthesizing materials in which the functional microstructure (at both filler and molecular levels) interacts with the processing flows of extrusion-based manufacturing to achieve unique and enhanced properties. Here we articulate principles for designing and synthesizing soft materials with the potential to generate printed structures with superlative mechanical and stimuli-responsive properties. Specifically, we consider the rheological requirements of printing via direct ink writing and materials extrusion, and examine materials that show printing-directed alignment or trapping of tailored non-equilibrium structures. Finally, we discuss characterization approaches that connect filament-level microstructure with macroscopic behaviour, thus ‘closing the loop’ of material development. Collectively, these create the potential for additive manufacturing to achieve voxel-level control of composition, microstructure and properties.