Structuring Hydrogel Cross-Link Density Using Hierarchical Filament 3D Printing

Polymer hydrogels, water-laden 3D cross-linked networks, find broad application as advanced biomaterials and functional materials because of their biocompatibility, stimuli responsiveness, and affordability. The cross-linking density reports material properties such as elasticity, permeability, and swelling propensity. However, this critical design parameter can be challenging to template locally. Here, we report a continuous processing scheme that uses laminar flow to direct the organization of cross-linking density across a single sample. Dilute and concentrated poly(ethylene glycol) diacrylate solutions are fed into custom serpentine millifluidic devices. These feature a modular sequence of splitting, rotation, and recombination elements, which create patterned streamlines that serve as a template for hierarchical concentration distributions. Poly(acrylic acid) microgels impart viscoplasticity, which stabilizes layered flow during multiplication and ensures reliable advection. The devices produce structured, seamless filaments, which are then arranged into objects using 3D printing, and photopolymerized to secure the heterogeneous distribution. The flow-encoded, multiscale architecture provides mechanical contrast, which is demonstratively exploited to program robust and reversible shape transformations, potentially useful in soft actuator and sensor applications. The unique structures achieved, and the geometrically dictated, chemistry-agnostic operating principles used to achieve them, provides a new means to engineer hydrogels to suit a variety of applications.