Ionic Hydrogels with Biomimetic 4D-Printed Mechanical Gradients: Models for Soft-Bodied Aquatic Organisms

Direct-ink writing (DIW), a rapidly growing and advancing form of additive manufacturing, provides capacities for on-demand tailoring of materials to meet specific requirements for final designs. The penultimate challenge faced with the increasing demand of customization is to extend beyond modification of shape to create 4D structures, dynamic 3D structures that can respond to stimuli in the local environment. Patterning material gradients is foundational for assembly of 4D structures, however, there remains a general need for useful materials chemistries to generate gray scale gradients via DIW. Here, presented is a simple materials assembly paradigm using DIW to pattern ionotropic gradients in hydrogels. Using structures that architecturally mimic sea-jelly organisms, the capabilities of spatial patterning are highlighted as exemplified by selectively programming the valency of the ion-binding agents. Spatial gradients, when combined with geometry, allow for programming the flexibility and movement of iron oxide nanoparticle–loaded ionotropic hydrogels to generate 4D-printed structures that actuate in the presence of local magnetic fields. This work highlights approaches to 4D design complexity that exploits 3D-printed gray-scale/gradient mechanics.