Solvent-Free Synthesis of Shape-Programmable Hydrogel Particles Using Polyhedral Liquid Marbles

Nonspherical hydrogel particles offer unique anisotropic properties and functionalities that are inaccessible to conventional spherical hydrogels. However, achieving precise shape control without complex fabrication equipment or organic solvents remains a major challenge. Herein, we present a solvent-free and template-guided strategy for synthesizing nonspherical hydrogel particles using polyhedral liquid marbles (LMs) as reaction vessels. The polyhedral LMs were constructed by assembling millimeter-sized hydrophobic solid polymer plates around droplets of poly(ethylene glycol)-based vinyl monomers containing a photoinitiator. Upon UV irradiation, free radical polymerization proceeded efficiently inside the confined droplets, yielding gel particles whose shapes directly reflected the LM geometries, including cubic, tetrahedral, octahedral, dodecahedral, and icosahedral forms. Furthermore, the degree of swelling in water was finely tunable by varying the cross-linking density of the gel network, while the incorporation and removal of NaCl particles enabled the formation of a multihollow internal structure. As a proof of concept, cubic hydrogel particles exhibited a swelling-induced mechanical function, where water absorption was converted into macroscopic mechanical work capable of lifting an external object, demonstrating the mechanical advantage of shape-defined hydrogels. The LM-based method allows for geometrically defined, solvent-free synthesis of anisotropic hydrogel particles using simple procedures without specialized apparatus or organic solvents. The proposed approach provides a versatile platform for producing shape-programmable soft materials that may find applications in soft robotics, stimuli-responsive systems, and 3D hydrogel assemblies.