Effects of titanium surface modifications on bonding behavior of hydroxyapatite ceramics and titanium by hydrothermal hot-pressing

The mechanical and chemical stability of the interfaces in bioactive materials: The substrate-bioactive surface layer and hydroxyapatite-bioactive surface layer interfaces

Bioactive materials should maintain their properties during implantation and for long time in contact with physiological fluids and tissues. In the present research, five different bioactive materials (a bioactive glass and four different chemically treated bioactive titanium surfaces) have been studied and compared in terms of mechanical stability of the surface bioactive layer-substrate interface, their long term bioactivity, the type of hydroxyapatite matured and the stability of the hydroxyapatite-surface bioactive layer interface. Numerous physical and chemical analyses (such as Raman spectroscopy, macro and micro scratch tests, soaking in SBF, Field Emission Scanning Electron Microscopy equipped with Energy Dispersive Spectroscopy (SEM-EDS), zeta potential measurements and Fourier Transformed Infra-Red spectroscopy (FTIR) with chemical imaging) were used. Scratch measurements evidenced differences among the metallic surfaces concerning the mechanical stability of the surface bioactive layer-substrate interface. All the surfaces, despite of different kinetics of bioactivity, are covered by a bone like carbonate-hydroxyapatite with B-type substitution after 28 days of soaking in SBF. However, the stability of the apatite layer is not the same for all the materials: dissolution occurs at pH around 4 (close to inflammation condition) in a more pronounced way for the surfaces with faster bioactivity together with detachment of the surface bioactive layer. A protocol of characterization is here suggested to predict the implant-bone interface stability.

Kinetics of hydrothermal crystallization under saturated steam pressure and the self-healing effect by nanocrystallite for hydroxyapatite coatings

Hydroxyapatite coatings (HACs) with a low crystalline state were prepared using the plasma spraying process followed by hermetic autoclaving hydrothermal treatment at 100, 150 and 200 degrees C. Experimental evidence confirmed that the HACs became significantly crystallized and the content of amorphous calcium phosphate decreased by performing the autoclaving hydrothermal treatment in an ambient saturated steam pressure system. The obvious chemisorbed hydroxy groups (OH) peak in the X-ray photoelectron spectra detected from the hydrothermally crystallized HAC specimens means that the hydroxyl-deficient state of plasma-sprayed HACs is significantly improved by the abundant replenished OH groups. The HA nanocrystallite observed from scanning electron microscopy and transmission electron microscopy images within hydrothermally treated HACs is the result of nucleation and grain growth through the replenishment of OH groups into the hydroxyl-deficient HA crystal structure. The microstructural self-healing effect is a result of reduction in defects (pores, microcracks and lamellar boundaries) due to new-growth HA nanocrystallite. According to the systematic derivation of the Arrhenius equation, the HA crystallization is a second-order Arrhenius reaction kinetics. Besides the effects of heating temperature and an atmosphere with abundant water molecules, the saturated steam pressure is a crucial factor which significantly improves the crystallization rate constant and further reduces the activation energy for the hydrothermal HA crystallization.

Mechanochemical Effects on the Synthesis of Copper Orthophosphate and cyclo-Tetraphosphate Bulks by the Hydrothermal Hot Pressing Method

Copper orthophosphate, Cu₃(PO₄)₂, and cyclo-tetraphosphates, Cu₂P₄O₁₂, were synthesized using phosphoric acid and basic copper carbonate, and then treated with a planetary mill for up to 360 minutes. The un-milled and milled samples were characterized by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. SEM images, particle size distribution, specific surface area, UV-Vis reflectance spectra were also used to evaluate the materials. The un-milled and milled materials were used to fabricate copper phosphate bulks by a hydrothermal hot pressing method. The influence of powder condition on the sintering behavior of the copper phosphates was studied.