Mn‐Based Superparamagnetic Hollow Hydroxyapatite Microspheres with Integrated Magnetic Resonance Imaging Responsiveness and Sustained Drug Release

Hollow hydroxyapatite (HAp)‐based ceramic microspheres are attractive microcarrier architectures for integrating molecular loading, sustained release, and additional imaging responsiveness. In this study, MnFe ₂ O ₄ ‐incorporated hollow HAp microspheres were constructed through an improved two‐step solvothermal method to integrate MRI responsiveness and sustained model drug release within a single ceramic composite microcarrier. The obtained microspheres exhibited a hollow porous architecture, with a representative diameter of ≈1.5 μm and a shell thickness of ≈200 nm. Structural and elemental analyses confirmed the formation of hollow HAp–MnFe ₂ O ₄ composite microspheres with Mn‐ and Fe‐containing components distributed in the shell. The microspheres showed clear magnetic responsiveness, with a saturation magnetization of 19.45 emu g ⁻¹ , and exhibited concentration‐dependent in vitro T ₂ ‐weighted MRI responsiveness with a transverse relaxivity of 39.12 mM ⁻¹ s ⁻¹ . Using metformin hydrochloride as a model drug, the microspheres achieved a drug loading efficiency of ≈63.2% and a cumulative release of 85.66% over 7 days in PBS. Live/dead staining further indicated preliminary cytocompatibility under the tested extraction/culture conditions. These results demonstrate a ceramic composite microcarrier platform integrating MRI responsiveness and sustained molecular release.