A influência da incorporação de água em ablação de tecidos duros dentais por lasers de femtossegundos

Abstract

There is a growing interest in laser micro-machinning of biomaterials and biological tissues since the availability of ultrashort pulsed laser technology. The possibility of ablating tissues with minimal thermal or mechanical is attractive in Dentistry for the definite replacement of conventional rotary drills. Besides that, the utilization of human teeth in scientific researches has been diminished, with the substitution by bovine origin samples, of easier obtaining, having its results extrapolated to human nature. Most existing studies until present days about femtosecond time-scale pulsed ablation are in general based on the variation of parameters exclusively from the laser radiation source, but never related to the nature of the target tissue and its constituents. In this study we aim to evaluate how the hydration state of irradiated tissue influences the ablation process itself, as well as search for differences between human and bovine substrates behavior. Eighty (80) teeth samples, half human, half bovine, subdivided into enamel and dentin were used in this work. The samples were worn so as to have a plane surface of the target tissue and separated into four hydration groups. Quantifications of dehydration suffered during samples preparation in each group were made. Five Power levels, different to enamel and dentin, ranging from 200 mW to 1W of a 1 kHz repetition rate and 70fs output pulsed laser, with three exposition times, were applied at the substrate surface with perpendicular incidence. An analysis of the ablated microcavity edge diameter made possible the verification of dental hard tissues ablation threshold behavior with respect to different structural water levels. The ablation threshold was calculated based on the laser spatial intensity distribution. Also, the diameter of the thermal damages extension behavior in dentin was related to the dehydration of the substrates, where we observed a strong hydration-dependent behavior. Scanning Electron Microscopy was also used, objecting the perception of morphological variations at microcavity edges. A raise on the number of microcracks to a higher dehydration was verified, as well as a decrease on the thermal damage threshold. The results led us to propose the role played by water on enamel and dentin structures and also made it possible to find differences between human and bovine substrates. A higher understanding of the ablation process was achieved, as well as the possibility to establish it as a viable tool to comprehend the microscopic properties of materials.