Insight into the Internal Structure of Biogenic, Synthetic and Geological Apatite by Electron Microscopy and X‐Ray Scattering

Calcium phosphate is the inorganic component (biomineral) of hard tissue, i.e., bone and teeth, of many higher organisms, including humans. Calcium phosphate is also synthetically prepared for biomedical application, usually as calcium phosphate ceramics for bone substitution and as calcium phosphate nanoparticles for drug delivery and imaging. Finally, hydroxyapatite occurs as a mineral in geology, sometimes in cm‐sized single crystals. Two types of nanocrystalline biological apatite (human tooth enamel and shark tooth enameloid), one single‐crystalline geological apatite, one sintered hydroxyapatite, and four different types of calcium phosphate nanoparticles are analyzed in‐depth for their external and internal structure. Particle size, crystallinity, and crystallite size determine the materials properties, like the solubility under biological conditions, e.g., during resorption by osteoclasts in bone defects or inside cells after uptake by endocytosis. The structure‐sensitive methods electron microscopy (scanning electron microscopy; SEM; and transmission electron microscopy; TEM), X‐ray powder diffraction (XRD; including Rietveld refinement) and total scattering analysis (pair‐distribution function, PDF) are applied. In addition, the sample composition is assessed by elemental analysis, infrared spectroscopy, and thermogravimetry. XRD and PDF showed that all samples consisted of crystallites that are smaller than their overall particle size as determined by electron microscopy.