UCST‐Driven Shape Morphing in Digital Light Processing 4D‐Printed Hydrogels

ABSTRACT

The field of 4D‐printed hydrogels is largely dominated by heat‐induced contracting systems, while thermally expanding materials remain rare and challenging to implement in high‐resolution architectures. UCST‐based hydrogels could fill this gap, yet their use requires simultaneously preserving the interactions responsible for the transition and ensuring sufficient network robustness for 3D processing. As a result, UCST‐driven shape morphing has never been demonstrated in fully DLP‐printed hydrogels. Here, we introduce a minimalist system based on poly(acrylic acid‐ co ‐acrylamide) that satisfies these constraints and enables reversible and predictable UCST‐based shape transformation. Through a rational exploration of monomer concentration, crosslink density, and composition, we identify the structural requirements necessary for maintaining high thermal response. This understanding provides a formulation window for achieving 4D morphing. A first approach relies on printing objects with macroporosity to accelerate swelling in selected regions, generating transient volume differences that drive anisotropic shape changes within a single material. In parallel, multimaterial printing through vat‐switching enables the precise placement of UCST‐responsive and inert domains, making simple geometries exhibit localized deformation. In more complex structures, these selectively responsive regions can generate movements without altering their overall shape. This work opens a clear pathway to implement UCST‐based responses in 4D printed soft materials.