A hierarchical assembly strategy for heterogeneous hydrogel constructs from magnetized liquid droplets

Conventional magnetic assembly strategies for hydrogel constructs face fundamental limitations in achieving multiscale structure construction due to transient magnetic response and lack of magnetic domain programming. We present a hierarchical assembly strategy utilizing magnetic memory liquid droplets as guidable building blocks for heterogeneous hydrogel structures. Through interfacial jamming of alginate-surfactant membranes and subsequent photo-crosslinking, we engineer ferromagnetic liquid droplets containing polyethylene glycol (PEG)-modified iron oxide nanoparticles that exhibit magnetic memory. These units enable hierarchical assembly via sequential magnetization and re-magnetization cycles. The molded building block can be reconfigured using induction magnetic dipoles, which determine the building block's magnetic assembly and actuation behavior. Meanwhile, by adjusting the intensity and direction of the external magnetic field, the assembly can perform directional delivery in narrow and tortuous spaces. These biocompatible, complex, and multifunctional hydrogel assemblies can be readily applied to tissue engineering, regenerative medicine, and biomimetics. STATEMENT OF SIGNIFICANCE: Fabricating complex, biocompatible hydrogels with spatial control remains a challenge in tissue engineering. Conventional magnetic assembly methods are limited by transient magnetic responses and lack programmable domain control, hindering multiscale heterogeneous structure formation. This work presents a hierarchical magnetic assembly strategy using magnetized liquid droplets with magnetic memory, enabling heterogeneous hydrogel construction through magnetization-remagnetization cycles, actuation, and reconfiguration under external fields. The strategy's effectiveness is demonstrated by fabricating a blood vessel tissue mimic.