Polyelectrolyte Inks for Direct Writing of 3-D Microperiodic Structures
Direct-write assembly of 3D microwebs comprised of charged polyelectrolyte filaments has been demonstrated using fluid inks that readily flow through fine deposition nozzles (~ 1 μm diameter or less), and then rapidly solidify in a coagulation reservoir. A chemically tailored ink/reservoir combination was created along with carefully engineered deposition parameters. The ink design is based on concepts developed through the understanding of spider silk spinning. Concentrated, non-stoichiometric mixtures of polyanions and polycations are mixed to create the desired initial ink rheology. By regulating the ratio of anionic to cationic groups and combining these species under specific solution conditions that promote polyelectrolyte exchange reactions, we can produce homogeneous fluids (40-50 wt% polyelectrolyte in aqueous solution) with the requisite viscosity needed for deposition through micro-capillary nozzles. These concentrated polyelectrolyte inks rapidly coagulate to yield self-supporting filaments (or rods) upon deposition into an alcohol/water coagulation reservoir. The exact coagulation mechanism was studied in detail. The resulting rise in ink elasticity, driven by electrostatics in water-rich or solvent quality effects in alcohol-rich reservoirs, depends strongly on reservoir composition. By carefully matching the coagulation kinetics to the deposition rate, the deposited ink filament is elastic enough to promote shape retention, yet maintains sufficient flexibility for continuous flow and adherence to the substrate and underlying patterned layers. These inks allow the direct writing of complex, 3-D structures with feature sizes (< 1 μm) that are 100 times smaller than those achieved by other multilayer printing techniques. Another inherent feature of our 3-D microwebs is that they possess a net surface charge, and can therefore be used as scaffolds to further direct materials self-assembly. We have demonstrated that oppositely charged gold nanoparticles can be adsorbed onto the polyelectrolyte filament surfaces via electrostatic-driven attractions. By carefully controlling the net filament charge, the nanoparticle charge and concentration, as well as the adsorption time, we have created 3-D patterned gold nanoparticle arrays of varying density. By combining directed- and self-assembly approaches, we have introduced a new route for 3D patterning of materials that is underpinned by our fundamental understanding of the phase behavior, rheology, and assembly of complex fluids comprised of nanoscale building blocks.
The ability to pattern fine-scale structures in three dimensions is critical for several emerging technologies, including tissue engineering scaffolds, microfluidic devices, and photonic band gap materials. Our new direct writing technique offers the ability to construct materials in arbitrary patterns, with characteristic feature sizes nearly two orders of magnitude below those achieved by other 3-D printing techniques and that approach those produced by state-of-the-art, 2-D nanolithographic techniques (such as dip-pen nanolithography). Direct-write assembly offers a powerful route for patterning 3-D structures of arbitrary design and functionality. By rational extension, other polyelectrolyte mixtures, e.g., those based on biologically, electrically, or optically active polyelectrolytes, could be developed as novel inks. Furthermore, they can be coated with other functional moieties, such as nanoparticles. These complex 3-D structures may serve as bio-scaffolds that direct cell-scaffold interactions, microfluidic networks that manipulate fluid flow, templates for photonic materials that control light propagation, or sensor arrays that respond to environmental stimuli.
Senior FS-MRL PI: J. Lewis