In Silico Modelling and Optimisation of Synthesised Apatite Coatings for Dental Implants

Abstract

In order to better understand the formation of ordered apatite coatings we have examined the surface energy and chemical effects on the morphology and optimal Ca/P values of nano-crystals synthesised through the precipitation method. This study was aimed at understanding and controlling mineral formation for use in the regenerative dentistry field. Titanium and hydroxyapatite substrate were submerged in a Ca2+ and PO3 4 supersaturated solution and incubated separately at six different temperatures below 100 ˚C. The experiment was repeated using a solution that was buffered with bis-tris in order to maintain pH at 6 throughout the incubation. A range of morphologies were formed and catalogued. Compositional and structural analysis was carried out using standard characterisation techniques. We find that in general, increasing the surface roughness reduces the number of competing calcium phosphate morphologies found. However, there is an upper limit on how much the surface roughness can be increased. A apatite surface roughness between 0.212-0.360 μm can be used to form aligned rod-like nano crystals when incubated at 70 ˚C. Similar results were found on titanium surfaces that were pre-coated with apatite before incubation, indicating that these results could further be applied to implant coating techniques. Buffered samples encourage the formation of dicalcium phosphate over that of apatite at temperatures higher than solutions without bistris buffer. Where buffered samples showed evidence from 5-37 ˚C, but unbuffered samples only show indication at 5 ˚C only. Buffering also prevented the formation of any aligned rod-like apatite coatings. Thus buffered solutions were deemed unsuitable for further study. Supersaturated solution incubated at temperatures 21 ˚C, and 70 ˚C, for 7 days were identified as yielding aligned apatite. Coatings which were then characterised using SEM, EDX, FTIR and XRD. Images were then analysed via ImageJ which was used to quantify surface coverage achieved by coatings, and the size of morphological features. SEM shows interconnected crystals at day 6 which could indicate the formation of an amorphous epitaxial layer. Aligned crystals were formed by a successive layering of rod-like apatite nano-crystals. Pair Distribution Function analysis and Atomistimodelling studies of synthesised apatite powders show that amorphous calcium phosphate plays a key role in the synthesis route. Amorphous calcium phosphate containing bond lengths consistent with apatite and DCPD were found in samples that were between 2-4 mins, after which only crystallised peaks could be found. The following 7 days showed a continual remodelling of bond lengths and bond formation. Furthermore, during the 7 days the bonds would transition between apatite-like and DCPD-like bond lengths in key regions. Bond length and bond angle analysis revealed that with an increase in time, there was an increase in the number of bond lengths and angle present in the data. This meant that there was an increase in the level of disorder even though there was clearly a presence of a crystalline phase throughout the 7 days incubation. This could suggest the presence of an amorphous phase throughout the 7 days.