(161v) Templating Hydrogels Using Fractal Flow Processing

Polymeric hydrogels, water-laden 3D cross-linked networks, find broad application as advanced biomaterials and functional materials due to their biocompatibility, stimuli-responsiveness, and affordability. In these materials, the cross-linking density reports critical material properties such as elasticity, permeability, transparency, and swelling propensity, but can be challenging to alter across the sample volume polymerized. To improve spatial control over gel mesostructure, we developed a novel processing scheme that uses laminar flows to direct the organization of polymeric cross-linking density across a single sample. Custom-designed static mixers force disparate streams through serpentine splitting, rotation, and recombination elements. These elements multiply the incoming 2D concentration field across the cross-sectional area while preserving its relative spacing and orientation. Since the replication of the pattern is fractal in nature, the heterogeneous distribution is efficiently multiplied and shrunk before it is dissipated by diffusive mixing. These so-called “fractal” mixers are well-suited to assemble fluid streams parallel and perpendicular to one another, ultimately enabling the fabrication of laminated and dendritic structures with micron-scale feature sizes. To demonstrate the utility of this method for hydrogel structuring, we template poly(ethylene glycol) diacrylate gels with long-range, highly ordered variations in cross-linking density. Gel precursors of different polymer compositions are first blended with a poly(acrylic acid) microgel dispersion, which serves as a yield-stress carrier fluid that promotes plug flow and preserves pattern fidelity through rotational flow elements. These precursors are processed through the mixer and photopolymerized upon extrusion, thus securing the ordered concentration distribution. Structure replication is confirmed by tracking the distribution of fluorescent dyes in operando and post-polymerization. Improvements in the bulk mechanical strength and actuation upon swelling are emblematic of the benefits afforded by the control over the orientation and connectivity of the self-similar cross-linking density distribution. We anticipate that this continuous processing strategy could be readily scaled-up to high-throughputs or incorporated in-line into 3D printing instruments to impart hierarchal structure at the filament level of hydrogels. Moreover, the chemistry-agnostic operating principles of fractal processing make it a promising strategy to design the architecture of other classes of soft material composites.