Adsorption onto nanocrystalline apatitic calcium phosphates. Applications to growth factors and drugs delivery

Interfacial properties of apatitic calcium phosphate play a crucial role in calcified tissues and biomaterials. Generally the adsorption of molecules on apatitic calcium phosphates is considered to obey to electrostatic interactions. The study of the adsorption of various active molecules such as albumin, heparin, bisphosphonate, and growth factors like BMP-2, VEGF and FGF-2 on such surfaces seems to follow a similar pathway. In all cases the adsorption process can be well described using Langmuir isotherms, although the adsorption process appears generally as irreversible. Further studies of the adsorption reaction reveal, in most cases, an ion exchange mechanism involving the replacement of mineral ions of the apatite surface by molecular ions from the solution. Considering biomimetic apatite nanocrystals, considerable variations of the adsorption parameters are observed depending on the maturation time of the nanocrystals. As the maturation time increases, the adsorption affinity constant increases and the maximum amount adsorbed decreases. The understanding of the adsorption process provides fundamental tools for the development of drug delivery system using apatite materials. Applications to the release of active molecules are examined in the case of apatite coatings.