Exogenous mineralization of hard tissues using photo-absorptive minerals and femto-second lasers; the case of dental enamel.

A radical new methodology for the exogenous mineralization of hard tissues is demonstrated in the context of laser-biomaterials interaction. The proposed approach is based on the use of femtosecond pulsed lasers (fs) and Fe3+-doped calcium phosphate minerals (specifically in this work fluorapatite powder containing Fe2O3 nanoparticles (NP)). A layer of the synthetic powder is applied to the surface of eroded bovine enamel and is irradiated with a fs laser (1040 nm wavelength, 1 GHz repetition rate, 150 fs pulse duration and 0.4 W average power). The Fe2O3 NPs absorb the light and may act as thermal antennae, dissipating energy to the vicinal mineral phase. Such a photothermal process triggers the sintering and densification of the surrounding calcium phosphate crystals thereby forming a new, dense layer of typically ∼20 µm in thickness, which is bonded to the underlying surface of the natural enamel. The dispersed iron oxide NPs, ensure the localization of temperature excursion, minimizing collateral thermal damage to the surrounding natural tissue during laser irradiation. Simulated brushing trials (pH cycle and mechanical force) on the synthetic layer show that the sintered material is more acid resistant than the natural mineral of enamel. Furthermore, nano-indentation confirms that the hardness and Young's modulus of the new layers are significantly more closely matched to enamel than current restorative materials used in clinical dentistry. Although the results presented herein are exemplified in the context of bovine enamel restoration, the methodology may be more widely applicable to human enamel and other hard-tissue regenerative engineering. STATEMENT OF SIGNIFICANCE: In this work we provide a new methodology for the mineralisation of dental hard tissues using femtosecond lasers and iron doped biomaterials. In particular, we demonstrate selective laser sintering of an iron doped fluorapatite on the surface of eroded enamel under low average power and mid-IR wavelength and the formation of a new layer to substitute the removed material. The new layer is evaluated through simulated brushing trials and nano-indentation. From the results we can conclude that is more acid resistant than natural enamel while, its mechanical properties are superior to that of current restorative materials. To the best of our knowledge this is the first time that someone demonstrated, laser sintering and bonding of calcium phosphate biomaterials on hard tissues. Although we here we discuss the case of dental enamel, similar approach can be adopted for other hard tissues, leading to new strategies for the fixation of bone/tooth defects.

The effect of cation doping on the morphology, optical and structural properties of highly oriented wurtzite ZnO-nanorod arrays grown by a hydrothermal method.

Undoped and C-doped (C: Mg2+, Ni2+, Mn2+, Co2+, Cu2+, Cr3+) ZnO nanorods were synthesized by a hydrothermal method at temperatures as low as 60 °C. The effect of doping on the morphology of the ZnO nanorods was visualized by taking their cross section and top SEM images. The results show that the size of nanorods was increased in both height and diameter by cation doping. The crystallinity change of the ZnO nanorods due to each doping element was thoroughly investigated by an x-ray diffraction (XRD). The XRD patterns show that the wurtzite crystal structure of ZnO nanorods was maintained after cation addition. The optical Raman-active modes of undoped and cation-doped nanorods were measured with a micro-Raman setup at room temperature. The surface chemistry of samples was investigated by x-ray photoelectron spectroscopy and energy-dispersive x-ray spectroscopy. Finally, the effect of each cation dopant on band-gap shift of the ZnO nanorods was investigated by a photoluminescence setup at room temperature. Although the amount of dopants (Mg2+, Ni2+, and Co2+) was smaller than the amount of Mn2+, Cu2+, and Cr3+ in the nanorods, their effect on the band structure of the ZnO nanorods was profound. The highest band-gap shift was achieved for a Co-doped sample, and the best crystal orientation was for Mn-doped ZnO nanorods. Our results can be used as a comprehensive reference for engineering of the morphological, structural and optical properties of cation-doped ZnO nanorods by using a low-temperature synthesis as an economical mass-production approach.