Calcium Hydroxyapatite Biostimulators: A Comparative Study of Biological Response and Particle Morphology

Background/Objectives: Calcium hydroxyapatite (CaHA)-based injectable materials are widely used as dermal biostimulators. In vitro models allow for controlled comparison of cellular responses and particle characteristics across formulations. This study aimed to compare two commercially available CaHA-based materials in terms of fibroblast metabolic activity, extracellular matrix-related gene expression, and microsphere morphology. Methods: Primary human dermal fibroblasts were exposed to two CaHA-based materials (Sample R and Sample S) at 10 mg/mL. Metabolic activity was assessed using the MTT assay at 24, 36, 48, and 72 h. Type I collagen and elastin gene expression were evaluated by RT-qPCR at 72 h. Microsphere morphology was analyzed by scanning electron microscopy (SEM). Results: Both materials increased fibroblast metabolic activity compared with the control at all time points. Early responses were similar, whereas Sample S showed higher activity at 48 and 72 h. At 72 h, both materials increased collagen and elastin gene expression versus the control, with greater responses observed for Sample S. SEM analysis showed predominantly spherical microspheres in both materials, with qualitative differences in surface microtopography. Conclusions: Under controlled in vitro conditions, both CaHA-based materials were biocompatible and modulated fibroblast metabolic activity and extracellular matrix-related gene expression. Differences in particle surface characteristics may contribute to the observed biological profiles. These findings support further studies incorporating extended incubation periods and protein-level analyses.