Multifunctional Hierarchically-Assembled Hydrogel Particles with Pollen Grains via Pickering Suspension Polymerization

Hierarchical assembly of heterogeneous particles is of great importance to interface and colloid science. In this work, a facile but powerful approach for the large-scale production of multifunctional hydrogel particles armored with biological colloidal species is developed by combining Pickering stabilization and photopolymerization. Biocompatible hollow pollen grains extracted from naturally occurring pollen species with an average diameter of ∼32 μm serve as universal solid emulsifiers dispersed in an oil phase and are self-assembled at the interface between an oil phase and a photo-cross-linkable hydrogel to make water-in-oil (W/O) emulsion droplets. While droplets are solidified into hydrogel particles by UV-induced free-radical polymerization, self-assembled hollow pollen grains are transformed to a robust shell on hydrogel particles with supracolloidal structures. The physically adsorbed hollow pollen grains on the hydrogel core can be released by a hydration-induced swelling of hollow pollen grains, leading to a transient floating behavior of core–shell particles. The size of the resultant core–shell particles is easily controlled by tailoring the process parameters such as a liquid volume or a loading mass of hollow pollen grains. The incorporation of magnetic or upconverting luminescent nanoparticles into a hydrogel core successfully expands the functionality of core–shell particles that can provide new design opportunities for floating drug delivery or ecofriendly proppants.