Nano- and femtosecond high-repetition-rate multipulse laser irradiation of dehydrated bone tissue: role of accumulated heat and model of cooling

A model is proposed to describe the cooling of a biotissue surface after irradiation by a series of laser pulses. The spatial distribution of heat accumulated by the surface of a hard biotissue irradiated by nano- and femtosecond laser pulses is qualitatively estimated as a function of the pulse repetition rate. The obtained results are compared with the data of experimental irradiation of dry bone with both nanosecond and femtosecond laser pulses. The key contribution of the variation of the biotissue optical characteristics to the dependence of heat, accumulated by its surface, on the repetition rate of nanosecond laser pulses is demonstrated. The mechanism of the pulse repetition rate affecting the residual heat in the biotissue is more complex and requires adequate consideration of the change in its optical and thermophysical parameters. The application of the cooling model allowed substantial confirmation of the key role of the accumulated heat in the process of hard tissue carbonisation around the area of laser impact and the necessity of external cooling in the course of high-repetition-rate multipulse laser processing using both nanosecond and femtosecond pulses.