Ultraviolet laser interference patterning of hydroxyapatite surfaces

Direct laser interference patterning (DLIP) was used to produce periodic patterns on hydroxyapatite. A Nd:YAG laser operating at 266 and 355 nm wavelengths and a pulse duration of 10 ns was used in these experiments. Line- and cross-like patterns with periodical distances of 10 and 20 μm were fabricated with energy densities between 0.6 and 2.4 J/cm2, and pulse numbers from 1 to 100. In the low/middle laser intensity range it was observed that the structure depth increased with the pulse number. However, for higher energies the patterns smudge due to thermal effects. For single pulse laser experiments, increasing of the laser fluence did not produce deeper structures. In addition, the best results were obtained when using low-medium laser intensities (∼0.6–1.2 J/cm2) and moderate number of laser pulses (20–50), depending on laser wavelength. In addition, at a 355 nm wavelength only patterns with 20 μm periods presented a good quality structure. In contrast, 266 nm wavelengths permitted to improve resolution up to periods of 10 μm due to a higher photochemical contribution to the ablation process. X-ray Photoelectron Spectroscopy (XPS) analysis showed that there are no significant changes in the chemical composition of laser-treated hydroxyapatite.