Selecting Hydroxyapatite requires matching the purity, particle size, and crystallinity to the end-use. For biomedical implants, prioritize medical-grade HAp with high crystallinity to ensure bioactivity and bone bonding. For dentifrice (toothpaste), choose nano-hydroxyapatite (n-HAp) with a particle size typically between 20-100nm to ensure effective enamel remineralization and dentin tubule occlusion. Always verify the Ca/P molar ratio, which should be approximately 1.67 for stoichiometric purity.