Tomasz Troczynski

Effect of hydrolysis on the phase evolution of water-based sol-gel hydroxyapatite and its application to bioactive coatings

In a previous report, we demonstrated a successful synthesis of crystalline hydroxyapatite (HA) through the use of a water-based sol–gel process. It was shown that the apatite can be obtained at temperatures generally below 400 °C, providing a great advantage for practical bioactive coating purposes. The influence of hydrolysis of phosphorus sol solution on the phase evolution of the resulting HA is the focus of this investigation. Experimental results show that, in the absence of acid catalyst, a long-term hydrolysis, i.e. >4 h, is required for better evolution of apatitic phase. Such a phase evolution is mainly attributed to an increased concentration of apatitic phase, rather than improved crystallinity in the calcined gels. With the aid of acid catalyst, we found that a well-crystalline HA can be synthesized over a time period shorter by 2–3 orders of magnitude than those without catalyst, i.e. a few minutes. In almost all cases, a small amount of tricalcium phosphate (TCP) was detected, which may be explainable by the formation of oligomeric derivatives of the phosphorus sol during synthesis, where calcium phosphate derivatives with lower Ca/P ratio than stoichiometry can be developed. By selecting an optimal sol as a dipping source, highly-porous dental root specimens were coated and a thin, dense, adhesive (upon finger-nail scratching test) coating was achieved after calcinations at 375 °C. An in vitro test also shows a bioactive character of the coating.

In-vitro forming of calcium phosphate layer on sol–gel hydroxyapatite-coated metal substrates

In-vitro deposition of calcium phosphate layer (CPL) on metallic substrate requires special surface preparation in order to provide an interfacial bond. In this work 316 stainless steel surface is modified through deposition of a thin film (∼0.5 μm) of sol–gel hydroxyapatite (SG-HA). This well-bonded film acts as an intermediary and nucleation surface of the CPL film. The SG-HA films were annealed at 375 °C (samples coded 375-ACS) and 400 °C (400-ACS) to achieve different crystallinity of the films, and thus to affect and study the CPL nucleation process. The CPL growth was investigated in terms of deposition kinetics and microstructural development. A deposition rate of dense CPL of about 0.43 μm/day was achieved on the crystallized film of 400-ACS, and 0.22 μm/day of porous CPL on amorphous 375-ACS. A compositional variation of Ca/P ratio across the CPL film thickness (400-ACS) was observed. Lower Ca/P ratio of 1.2 was detected near the substrate-CPL interface and about 1.5 near the solution-CPL interface. Infrared analysis showed the CPL to be of apatitic calcium-deficient structure. Kinetic model explaining the advancement of the CPL upon the in-vitro immersion is proposed.