Hydroxyapatite kinetic deposition on solid substrates induced by laser-liquid-solid interaction

Hydroxyapatite (HA) is present in the human body as a mineral constituent of the bones and teeth, as well as a major or minor component of kidney stones. HA deposited on different solid substrates can find applications including biomaterials and biosensors. This work deals with the kinetics of the HA growth by applying a novel method of laser-liquid-solid-interaction (LLSI) process on three types of materials (stainless steel, silicon and silica glass). The method allows interaction between a pulsed laser and a substrate immersed in a solution (simulated body fluid, SBF). By a scanning system, a design of seven squares at a distance of 200 μm was created at the end of each sample. In this way the center of the substrate (about 6 × 6 mm) was not irradiated. Following the LLSI process, the samples were left in the irradiated SBF for various intervals of time. Light microscopy (LM) showed surfaces seeded with randomly distributed transparent and white particles. The surface seeding increased with the immersion time and was dependent on the substrate type. Fourier transform infrared (FTIR) spectroscopy showed that in the first stage of soaking (up to 6 h) the observed white particles were calcium phosphate containing. Energy dispersive X-ray (EDX) spectroscopy revealed that the transparent particles were NaCl. In the next stage (after 12 h) vibrational modes typical for HA were clearly observed. Detailed observation with scanning electron microscopy (SEM) after 12 h showed morphology of sphere-like aggregates, grouped in a porous network. Raman spectroscopy, X-ray diffraction (XRD) and EDX confirmed that after 12 h the grown layer was HA. It was found that in comparison to the traditionally employed prolonged soaking in SBF, the applied LLSI process yielded a synergistic effect due to the simultaneous use of the solid substrate, the aqueous solution and the laser energy.