Laser welding of biological tissues is a noncontact suture technology. Compared to the traditional contact needle and thread suture, it has several advantages, such as simpler operation, faster speed, quicker postoperative recovery, and comparatively insignificant scars. Initial success has already been achieved in the welding of the skin, blood vessels, and lung tissue. As an ultrafast pulsed laser, the femtosecond laser has an extremely high peak power and an exceptionally short action time, that helps to avoid the linear absorption, transfer, and diffusion of energy to a large extent. Thus, it has been widely used in the medical field. However, there are only a few reports on the technology of using femtosecond laser for welding biological tissues. The mechanism of interaction between the femtosecond laser and tissue is not yet clear, and the influence of laser process parameters on the incision fusion effect of isolated skin tissue needs to be further studied. Therefore, in this study, we analyze the effects of laser power, defocus amount, and other process parameters on the fusion morphology of in vitro skin tissue, its incision tensile strength, and thermal damage by a mono-factorial experimental method. All the process parameters of the femtosecond laser are also optimized. We believe that our experiment and results will be helpful in determining the effect of femtosecond laser parameters on the fusion effect of biological tissue and promote further research on the laser welding of biological tissue.

This study adopts the mono-factorial experiment method considering that there are many factors affecting tissue fusion, while retaining the other process parameters. The four factors of the femtosecond laser power, defocus amount, scanning speed, and number of scannings are varied and each factor is set to 3-4 levels. Femtosecond laser-welding experiments are performed on in vitro pigskin. Subsequently, using a tensile force meter, a tensile strength test is conducted to obtain the tensile strength of the incision. During the welding process, the temperature of the isolated skin tissue is detected in real time using an infrared thermal imager and the temperature-change curve of the isolated skin tissue is then obtained. The temperature curve is fitted and the data is substituted into the Arrhenius equation to calculate the tissue thermal damage parameters. Then, the changes in the appearance, tensile strength, and thermal damage of the welded tissue with laser power, defocus amount, scanning speed, and number of scannings are obtained. The process parameters are optimized based on the tissue appearance, tensile strength, and thermal damage after welding.

The laser power and scanning speed have a greater impact on the appearance of the in vitro skin tissue (Figs. 2 and 4), whereas the defocus amount has a minimal effect (Fig. 3). The tensile strength of the in vitro skin tissue after welding rapidly increases with an increase in laser power, but only gradually increases when the laser power exceeds a certain value. Further, it gradually decreases with an increase in scanning speed. When the number of scannings is increased, the tensile strength initially increases and subsequently decreases. In contrast, the defocus amount has no apparent effect on the tensile strength of the welded structure (Fig. 6). The thermal damage rapidly increases with the increase in laser power, whereas it increases more gradually with scanning time. With an increase in scanning speed, it initially decreases and then observably increases. Thermal damage to the isolated skin tissue after welding is also less affected (Fig. 7). The experimental results indicate that the femtosecond laser process parameters are successfully optimized; there is satisfactory tissue fusion after welding, and the surface is smooth (Fig. 8). The tensile strength of the in vitro skin tissue is 16.25 N/cm^-2. The thermal damage parameter of tissue obtained by calculation is 0.00538, which is smaller than that of the thermal damage generated by continuous laser at the same tensile strength.

In this study, the influence of laser power, defocus amount, scanning speed, and number of scannings on the appearance and performance of in vitro skin tissue after welding is studied through mono-factorial experiments. The results show that by using femtosecond laser to weld in vitro skin tissue, the tissue can achieve improved fusion. The results show that laser power and scanning speed are two important factors that determine the appearance and tensile strength of in vitro skin tissue after welding, and the laser power has a decisive effect on the thermal damage of tissue. In cases where the tissue does not exhibit irreversible thermal damage and maintains a certain tensile strength, the thermal damage to the tissue is approximately 10^-3 orders of magnitude. The tensile strength of the isolated skin tissue incision after welding is enhanced by decreasing the power and scanning speed of the laser and increasing the number of scannings. Based on this, we optimize the femtosecond laser process parameters. The post-weld incision has higher connection strength, and the thermal damage is less than that generated by continuous laser exposure under the same tensile strength, indicating that the use of a femtosecond laser for in vitro skin tissue welding can reduce tissue thermal damage to a greater extent and maintain tissue activity.