Viscosity-controlled formation of spherical hydroxyapatite microspheres via spray drying

초록

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) microspheres are widely used in biomedical and separation applications, where particle sphericity and flowability critically influence processing performance and packing behavior. In spray drying, however, maintaining droplet integrity during atomization and rapid solvent evaporation remains a major challenge, often resulting in particle deformation or irregular morphologies. In this study, the role of precursor rheology on the formation of spray-dried HAp microspheres was systematically investigated using poly(vinyl alcohol) (PVA)-modified suspensions with different viscosities. Rheological characterization was correlated with particle morphology, crystal structure, surface properties, and internal microstructure. Particular emphasis was placed on understanding how viscosity-dependent droplet stabilization influences microsphere formation. The results identified precursor viscosity as a key parameter influencing microsphere morphology. Low-viscosity suspensions produced collapsed or irregular particles due to insufficient resistance to droplet deformation during solvent evaporation, whereas excessively high viscosities hindered atomization and resulted in non-uniform particle formation. An optimal viscosity window of approximately 800–900 mPa·s (at 1 s⁻¹) provided the best balance between atomization efficiency and droplet stabilization, producing highly spherical HAp microspheres with an average particle diameter of approximately 40 μm. Structural analyses confirmed that viscosity modification primarily influenced particle morphology without altering the crystalline phase of HAp. To provide a mechanistic interpretation, the morphology evolution was analyzed using the Ohnesorge number, which describes the balance among viscous, inertial, and capillary forces during droplet formation and drying. Increasing Ohnesorge number enhanced droplet stability and improved microsphere sphericity, indicating that microsphere formation is governed not simply by viscosity itself but by the underlying force balance during atomization. These findings establish a quantitative process–structure relationship between precursor rheology and spray-dried microsphere formation and provide practical design guidelines for the scalable production of spherical bioceramic microspheres. © 2026 Institution of Chemical Engineers

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