Micro Sponge Balls: Preparation and Characterization of Sponge-like Cryogel Particles of Poly(2-hydroxyethyl methacrylate) via the Inverse Leidenfrost Effect

Here, we report a method to prepare cryogel particles with sponge-like mechanical properties, including high porosity and high elasticity. The preparation process of the cryogel particles of poly(2-hydroxythyl methacrylate) can be summarized in the two following steps: preparation of frozen droplets using the inverse Leidenfrost effect, followed by cryo-gelation by frozen polymerization. First, a polymer precursor was dropwise added into bulk liquid nitrogen (−196 °C). Then, frozen droplets were created by the inverse Leidenfrost effect, which were subsequently polymerized in liquid paraffine (−15 °C). After thawing and drying, the cryogel particles were obtained. The monolithic super-macroporous structure was observed by scanning electron microscopy (SEM). The mechanical properties of the cryogel particles were studied via compression–swelling tests. At maximum compression, the particles achieved 94.3% degree of deformation; remarkably, they returned to their original shape under the swelling state. The strategy proposed herein, which combines the inverse Leidenfrost effect with a cryo-polymerization technique, could be applied to prepare various polymer particles without employing surfactants.