Applications are emerging that require the creation of fine-scale structures in three dimensions — examples include scaffolds for tissue engineering1, micro-fluidic devices2 and photonic materials that control light propagation over a range of frequencies3. But writing methods such as dip-pen nanolithography4 and ink-jet printing5 are either confined to two dimensions or beset by wetting and spreading problems. Here we use concentrated polyelectrolyte inks to write three-dimensional microperiodic structures directly without using masks. Our technique enables us to write arbitrary three-dimensional patterns whose features are nearly two orders of magnitude smaller than those attained with other multilayer printing techniques6.
For this direct-write process, we have developed fluid inks that flow readily through fine deposition nozzles and solidify rapidly in a coagulation reservoir (Fig.1). The inks are made up of concentrated polyelectrolyte complexes that consist of non-stoichiometric mixtures7 of polyanions (polyacrylic acid, PAA) and polycations (polyethylenimine, PEI, or polyallylamine hydrochloride, PAH). By regulating the ratio of anionic (COO−Na+) to cationic (NHx+) groups and combining these species under solution conditions that promote polyelectrolyte exchange reactions8, we produce homogeneous fluids (40–50 wt % polyelectrolyte in aqueous solution; for methods, see supplementary information) with the requisite viscosity (about 5–150 pascal seconds) for deposition through microcapillary nozzles of varying diameter (0.5–5.0 µm).
When deposited in an alcohol/water coagulation reservoir, these concentrated polyelectrolyte inks coagulate to form self-supporting filaments or rods (Fig.1). The reservoir composition strongly affects both the ink's elasticity and the coagulation mechanism, which is driven by electrostatic effects in water-rich reservoirs and by solvent effects in alcohol-rich reservoirs. For example, the elasticity of PAA/PEI ink (ratio of anionic/cationic groups: [COO−Na+]/[NHx+]=5.7) rises dramatically from 1 Pa (fluid phase) to nearly 105 Pa (coagulated phase) upon deposition in a reservoir containing 83–88% isopropyl alcohol. Under these conditions, the deposited ink filament is elastic enough to retain its shape while being able to flow and adhere to the substrate and underlying patterned layers.
Images of three-dimensional microperiodic lattices and radial arrays assembled from the PAA/PEI ink are shown in Fig.1c,d. These structures can have solid or porous walls and rod-like filaments that span the web, as well as tight and broad-angled features. A three-axis (x,y,z) micropositioning device deposits the ink, sequentially building layered, patterned structures (see supplementary information). After producing a two-dimensional patterned layer, the nozzle is raised in the z-direction to generate the next layer. The process is repeated until the full three-dimensional structure is fabricated (building time is about 5 min).
The versatility of these inks is demonstrated by microperiodic structures made from PAA/PAH ink ([COO−Na+]/[NH3+]=0.5), which consist of a patterned array of polyelectrolyte filaments with a net positive surface charge (see supplementary information). These three-dimensional polyelectrolyte scaffolds open up new opportunities for the electrostatic layer-by-layer assembly9 of materials, which has previously been limited to thin-film9 or discrete colloidal structures10.
Direct-write assembly is a powerful way to create three-dimensional microscale structures of arbitrary design and functionality. A variety of inks could be developed using other polyelectrolyte mixtures, based for example on biologically, electrically or optically active polyelectrolytes. The structures created could direct cell-scaffold interactions11, manipulate fluid flow2, control light propagation3, or respond to environmental stimuli12.
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The authors declare no competing financial interests.
Three panels showing the electrostatic assembly of negatively charged, gold nanoparticles onto polyelectrolyte scaffolds: (a) 3-D periodic scaffold (filament diameter = 2 µm, 4 layers); (b), (c) Nanoparticle coverage on rod surface after 5 min of immersion in a dilute gold nanoparticle solution. (JPG 60 kb)
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Gratson, G., Xu, M. & Lewis, J. Direct writing of three-dimensional webs. Nature 428, 386 (2004). https://doi.org/10.1038/428386a
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