In the biomedical field, traditional caries treatment often relies on tooth-grinding machines. However, the significant thermal effects produced by the high-speed friction of drill irreversibly damage the tooth surface. Therefore, in this study, femtosecond laser direct-writing processing technology is used to replace traditional grinding machines for caries treatment. The influence of laser scanning power, speed, and frequency on tooth surface cavity characteristics is studied using a femtosecond laser with high controllability and flexible programmability. The results demonstrate that the scanning power and frequency of the femtosecond laser are positively correlated with the hole depth and bottom roughness, whereas the scanning speed is negatively correlated. The experimental results confirm the feasibility of using a femtosecond laser to remove necrotic dental tissues. The present study facilitates the use of lasers in dental analysis and treatment and contributes to the understanding of the mechanism of laser-tooth interaction.

Carious teeth are first immersed in an acetone solution for 6 h to remove grease and dirt from the carious surface. The caries are then rinsed with anhydrous ethanol to wash away residual tissue and acetone solution. After pretreatment, a circular pattern with a diameter of 1 mm is drawn, and the laser scanning mode is set to “horizontal scanning + vertical scanning”. The 3D moving table is adjusted so that the laser focus is located on the tooth surface. The laser scanning distance, laser scanning speed, laser scanning power, pulse repetition frequency, energy of a single pulse, and laser scanning number are set to 0.01 mm, 5 mm/s, 500 mW, 1 kHz, 500 J, and five, respectively. In this experiment, only the scanning speed, laser power, and number of iterations are varied, and eight control groups are set for each variable. The teeth are processed after setting the parameter settings, and subsequently, the morphology of the processed holes is observed and characterized using a laser confocal microscope.

The morphology of the femtosecond laser-processed holes is observed using a laser confocal microscope, and the holes are constructed and characterized by a three-dimensional model using a analysis software. The relationship between the different processing parameters and hole depth is analyzed and plotted (Figs. 3, 4 and 5). The laser scanning number is set to five and the scanning power is 500 mW [Fig. 3(a)]. After adjusting the scanning speed, femtosecond laser treatment is applied to form holes with certain depths on the tooth surface, and their depth tends to increase as the scanning speed decreases. The laser scanning speed is set to 5 mm/s and the scanning power is 500 mW [Fig. 4(a)]. Upon changing the laser scanning number, femtosecond laser treatment produces holes on the tooth surface, whose depth gradually increases with the laser scanning number. The laser scanning speed is set to 5 mm/s and the laser scanning number is five [Fig. 5(a)]. After adjusting the scanning power, the femtosecond laser treatment also forms holes on the tooth surface, and the depth gradually increases with the scanning power. Data analysis indicates that the hole depth gradually becomes smaller as the scanning speed increases, indicating a negative correlation between them. The hole depth gradually increases with the scanning number and power, that is, positive correlations exist between the scanning number and power with the depth of the hole, and the changes in the scanning number and power have greater influence on the hole depth. Subsequently, the relationship between the different parameters and roughness of the bottom of the hole is analyzed and plotted (Fig. 6). The scanning speed is negatively correlated with the bottom roughness of the hole, whereas the laser scanning number and power is positively correlated with it.

In summary, we use human teeth as the target to investigate the effect of a femtosecond laser on tooth structure under different processing parameters. By adjusting the laser scanning speed, laser scanning number, and power of the femtosecond laser, we process holes with different depths and roughnesses on the tooth surface. Subsequently, we characterize these holes to determine the relationship between the femtosecond laser processing parameters and changes in the tooth structure. The experimental results indicate that femtosecond lasers have broad application prospects in dentistry, and the optimization of their processing parameters can significantly improve the safety and efficacy of dental treatment, thereby providing new ideas and methods for developing future dental treatment technology.