Time-resolved microscopy reveals the driving mechanism of particle formation during ultrashort pulse laser ablation of dentin-like ivory

Abstract

In dental health care, the application of ultrashort laser pulses enables dental tissue ablation free from thermal side effects, such as melting and cracking. However, these laser types create undesired micro- and nanoparticles, which might cause a health risk for the patient or surgeon. The aim of this study was to investigate the driving mechanisms of micro- and nanoparticle formation during ultrashort pulse laser ablation of dental tissue. Time-resolved microscopy was chosen to observe the ablation dynamics of mammoth ivory after irradiation with 660 fs laser pulses. The results suggest that nanoparticles might arise in the excited region. The thermal expansion of the excited material induces high pressure in the surrounding bulk tissue, generating a pressure wave. The rarefaction wave behind this pressure wave causes spallation, leading to ejection of microparticles.

Ultrafast dynamics of hard tissue ablation using femtosecond‐lasers

“…In contrast, the ultimate tensile strength of kidney stones is only in the order a few MPa and gall stones should have similar mechanical properties. Because of their structural similarity , it is expected that dentin shows nearly the same ablation behavior as the mammoth ivory while the other samples may show however different behaviors regarding photomechanical ablation.…”

“…Time‐resolved investigations of the femtosecond‐laser‐induced ablation dynamics of dentin‐like mammoth ivory in the top view perspective revealed that material removal is based on the combination of 2 mechanisms . The first mechanism was found to be similar to the ablation of dielectrics: multiphoton absorption and avalanche ionization cause laser‐induced breakdown in the excited region , and expansion of the generated plasma leads to material removal.…”

“…Because the excitation time is much shorter than the propagation time of a stress wave across the excited volume, a so called “stress confinement” is achieved inside the excited volume . The stress is released by means of pressure waves causing ablation also around the excited volume, where the stress limit of the material is exceeded, which leads to the ablation of particles with a size of a few micrometer .…”

“…In particular, optical properties rather influence the laser absorption process, whereas the tensile strength has an impact on the photomechanical ablation component. Moreover, the propagation of multiple laser‐induced wave fronts was observed in the pump‐probe microscopy images in . Analogous to surface acoustic wave measurement techniques pump‐probe microscopy should provide also access to the elastic constants.…”

“…For this purpose, we applied a time‐resolved pump‐probe microscopy setup to investigate the femtosecond‐laser‐pulse‐induced ablation mechanisms on 4 different types of hard tissues (dentin, chicken bone, gallstone and kidney stones). Images of the ablation process were captured at delay times between 1 picosecond and 10 microseconds, the transient relative reflectivity changes were measured, as well as the velocities of laser‐induced longitudinal pressure waves, Rayleigh waves and shock waves in air were determined.…”