Effect of Temperature and Substrate on Ordered Apatite Crystals for Coating Dental Implants.

Objectives: Coating dental implants with apatite is well-known to improve the osseointegration and subsequently the healing time. Recently, wet chemical routes to synthesise apatite crystals has gained a lot of interest due to their ability to produce highly-ordered apatite crystals and cost-effectiveness. On the other hand, controlling organised crystal growth is dependent on many parameters such as temperature, substrate, pH and ionic concentration. Therefore, better understanding is needed to investigate the effect of temperature and substrate on the chemistry and organisation of the synthesized apatite crystals.
Methods: Mineralisation experiments were undertaken at different temperatures (37, 54, 70 and 90 ˚C) on different substrates including; titanium, silicon, sapphire, and hydroxyapatite disks. The grown crystals were investigated chemically using FTIR, ¹⁹F MAS-NMR, Synchrotron XRD and morphologically using SEM and TEM.
Results: Self-assembled nanocrystals (dandelion-like structures) were observed to develop from the less-ordered spherical structures. Interestingly, apatite growth on hydroxyapatite substrates shows highly-ordered bundles of crystals with a close likeness to those found in biological hard tissues. Synchrotron XRD data show that at all temperatures, diffraction peaks arose from fluorapatite, however the relative intensity between (012) and (120) reflections varied at different temperatures. This is likely caused by the presence of fluorite (CaF₂), which has Bragg peaks which overlap the (012) and (222) reflections of the apatite phase. Similarly MAS-NMR measurements support the presence of the two phases and confirm that the relative percentage of fluorapatite increases with increasing temperature. FTIR spectra show typical apatite peaks at all temperatures.
Conclusions: Fluorapatite ordered crystals were successfully synthesised at all temperatures. However, there is an optimum temperature to promote fluorapatite formation between 37 and 54 ˚C. Hydroxyapatite surfaces were observed to guide the growth of crystals into highly-ordered crystals that could be investigated further in order to mimic biological hard tissues.