Printing mesoscale architectures
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Introduction New methods for patterning materials at the mesoscale, which lies between the molecular and macroscopic lengths scales, are driving scientific and technological advances in multiple areas, including lightweight structures, antennas, batteries, displays, and photonics. The broad diversity of potentially relevant materials, length scales, and architectures underscores the need for flexible patterning approaches. The term “threedimensional (3D) printing” describes additive manufacturing methods that employ a computer-controlled translation stage, which moves a pattern-generating device in the form of ink deposition nozzle(s) or laser-writing optics, to fabricate materials layer by layer.1–14 Since the 1980s, several ink- and lightbased techniques have been introduced to pattern materials in three dimensions (Figure 1). Ink-based printing approaches consist of filamentary and droplet-based methods. Printable inks are typically formulated from particulate and polymeric species that are suspended or dissolved in a liquid or heated to achieve the desired rheological (or flow) behavior. Specific parameters of interest include the ink viscosity, surface tension, shear yield stress, and viscoelastic properties (i.e., the shear elastic and loss moduli) which must be tailored for each printing method. In droplet-based methods, materials are deposited using printheads similar to those employed in desktop document printing. Several 3D printing methods rely on this basic approach, including inkjet printing on a powder bed,15 direct inkjet printing,16–18 and hot-melt printing.19
The “inks” are composed of low-viscosity fluids that must be removed by evaporation, ultraviolet- (UV-) curable resins that are polymerized upon printing, or wax-based inks that are heated during droplet formation and then solidified upon impact. The fluid dynamics involved in droplet formation, wetting, and spreading play an important, yet also limiting, role in defining the surface roughness and minimum feature size of materials patterned by inkjet printing methods. Typical values for the ink viscosity, droplet size, and velocity are 2–20 mPa•s, 10–30 μm, and 1–10 m/s, respectively, making it inherently difficult to jet concentrated suspensions or polymer solutions without clogging. Unlike droplet-based methods, filamentary printing methods2,20,21 allow for a broader range of ink designs, feature sizes, and geometries. In this approach, a viscoelastic ink is deposited as a continuous filament in a layer-wise build sequence. In the earliest embodiment, known as fused-deposition modeling,22,23 thermoplastic filaments are fed through a hot extrusion head, printed, and solidified as they cool below their glasstransition temperature. Recently, direct-writing of viscoelastic inks under ambient conditions has emerged as a viable alternative. Several concentrated colloidal,24 nanoparticle,25 fugitive organic (used to sacrificially pattern empty channels in a matrix material),26 and polyelectrolyte3 inks have been developed for printing complex 3D architectures. C
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