The biomedical materials research community frequently accepts that amorphous calcium phosphate (ACP) can be adsorbed and assimilated more readily by living organisms to produce new bone tissue than crystallized calcium phosphates such as hydroxyapatite (HAP). Previous studies also confirm that ACP has improved bioactivity compared to HAP since more adhesion and proliferation of osteogenic cells are observed on the ACP substrates. However, we note that the different size -effects of calcium phosphates are not taken into account in these studies and the used ACP are always smaller than the HAP. Our recent study reveals that the dimensions of nanoparticles are directly related to the bioactivities of calcium phosphates, e.g. the smaller nanocrystallites have a greater promotion effect on the proliferation of bone marrow mesenchymal stem cells (BMSCs). In order to understand the influence of crystallinity of calcium phosphate on the osteogenic cells correctly, it is critical to use ACP and HAP nanoparticles which have the same size distribution in such comparisons. In the present work, ∼20 nm ACP and HAP particles are synthesized and the effects of crystallinity of calcium phosphates are studied. The adhesion, proliferation, and differentiation of BMSCs are measured on ACP and HAP films, which are compared at the same size scale. It is surprising that more cells adsorb and proliferate on the film of well crystallized HAP than those on the ACP film. Alkaline phosphatase (ALP) activity assay and reverse transcription-polymerase chain reaction (RT-PCR) assay are also used to evaluate the differentiation of BMSCs. The results show that the differentiation of BMSCs to osteoblasts is promoted significantly by NanoHAP. The current experimental phenomena clearly demonstrate that the crystallized phase of calcium phosphate, HAP, provides a better substrate for BMSCs than the amorphous one, ACP, when the factor of size effect is removed. A new view on the relationship between the crystallinity of calcium phosphate and the responses of BMSCs indicates the importance of size and phase controls in the application of biomedical materials.
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