Direct Ink Writing of 3D Functional Materials

New methods for materials fabrication at the microand nanoscale will drive scientific and technological advances in areas of materials science, chemistry, physics, and biology. The broad diversity of potentially relevant materials, length scales, and architectures underscores the need for flexible patterning approaches. One important example is the fabrication of 3D periodic structures comprised of colloidal, polymeric, or semiconductor materials. These structures may find potential application as sensors, microfluidic networks, photonicbandgap materials, tissue-engineering scaffolds, and drugdelivery devices. Several strategies have recently emerged for precisely assembling 3D periodic arrays, including colloidal-epitaxy, standard-lithographic, and direct-write techniques. Of these, only the latter approach offers the materials flexibility, low cost, and ability to construct arbitrary 3D structures required for advances across multidisciplinary boundaries. The term “direct ink writing” describes fabrication methods that employ a computer-controlled translation stage, which moves a pattern-generating device, that is, an ink-deposition nozzle, to create materials with controlled architecture and composition. Several direct ink writing techniques have been introduced that are capable of patterning materials in three dimensions. They can be divided into filamentary-based approaches, such as robocasting (or robotic deposition), micropen writing, and fused deposition, and droplet-based approaches, such as ink-jet printing, and hot-melt printing (see Fig. 1). Many ink designs have been employed including highly shear thinning colloidal suspensions, colloidal gels, polymer melts, dilute colloidal fluids, waxes, and concentrated polyelectrolyte complexes. These inks solidify either through liquid evaporation, gelation, or a temperatureor solvent-induced phase change. Through careful control of ink composition, rheological behavior, and printing parameters, 3D structures that consist of continuous solids, high aspect ratio (e.g., parallel walls), or spanning features can be constructed. Of these, the latter structures offer the greatest challenge for designing inks, because they contain self-supporting features that must span gaps in the underlying layer(s). This feature article focuses primarily on our recent efforts to design concentrated colloidal, fugitive organic, and polyelectrolyte inks for the direct ink writing of 3D periodic architectures with filamentary features ranging from hundreds of micrometers to sub-micrometer in size, and their respective applications as functional composites, microfluidic networks, and templates for photonic bandgap materials and inorganic–organic hybrid structures. Future opportunities and current challenges for this novel patterning approach are also highlighted.