# Ex Vivo Exposure to Soft Biological Tissues by the 2-um All-Fiber Ultrafast Holmium Laser System
## 3 Results and Discussion
Histological sections of tissue before the impact of laser radiation (shown in Figure 4a) were used to determine changes in tissues exposed to laser radiation. The image of the histological section (Figure 4b) after CW laser radiation with a power \(p=0.3\) W for t = 30 s confirms only superficial damage and the absence of irreversible damage to the cell structure (myocytes) of muscle tissue. This damage can be indicated as a heat-affected zone (3) of the tissue, which is reversible by its nature and could be repaired in the healing process. The tissue changes are accompanied by edema at the site of exposure and temporary cellular dysfunction. In contrast to the CW radiation, exposure to ultrashort pulsed radiation with the same average power of 0.3 W leads to completely different results (Figure 4c). After 30 s of exposure, there is an ablation zone (AZ), marked with a yellow line, where an ablation crater (1) with rough and charred edges has formed due to the evaporation of the intracellular fluid and the burning of the residual tissue. The ablation zone is followed by the coagulation zone (CZ), marked by the blue line. In turn, the coagulated tissue represents the burned, loose and compact layers formed along the path of laser beam penetration into the tissue depth. The carbonization of myocyte mineral components leads to the formation of a burnt edge (2A). Necrotized cells with vesicularly altered cytoplasm constitute a loose layer (2B). Finally in the coagulation zone, we distinguished a compact layer (2C), where the loss of water components in myocytes leads to their dystrophy. As in the case of CW laser exposure, a heat-affected zone (3) is observed. In both cases (Figure 4c,d), the HAZ zone is not marked with a line, since this zone is larger than the recorded area of the microphotographs. Figure 4: Section of porcine longissimus muscle tissue (**a**) before exposure; and after 30 s exposure by Ho-doped fiber laser systems operating in (**b**) CW mode with a power of 0.3 W (applied energy of 9 J) [41]; (**c**) USP mode with a power of 0.3 W (applied energy of 9 J); (**d**) CW mode with a power of 0.5 W (applied energy of 15 J) [41]. 1—Ablation zone (AZ), 2—Coagulation zone (CZ) with the elements of the coagulation crust and fragmentary charring, 3—Heat-affected zone (HAZ). We also compared the results described above with those obtained after continuous-wave radiation action with a power of 0.5 W in the applied energy range of 2.5-30 J. Figure 4d shows a histological cross-section after 15 J to 0.5 W of CW radiation. Increasing the CW laser power and, consequently, the applied energy led to more destructive effects in the tissue. Therefore, we also observe the following three thermal damage zones: ablation, coagulation and heat-affected zone, as in the case of USP laser radiation. A comparison of the interaction effect between CW and USP radiation was described in [29]. Thus, for cell surgery, the power of an argon laser at 488 nm and 514 nm wavelengths should be more than 1 W [50], which exceeds the average power of a femtosecond laser at 800 nm wavelength by more than three times [34]. [PERSON] et al. explain this by the fact that interactions of CW radiation and pulsed radiation with pulse duration longer than 10 \(\upmu\)s are based on linear absorption, whereas absorption at exposure with ultrashort pulses is nonlinear. Figure 5 compares the diameters of visible thermal damage (HAZ) for two laser modes of exposure, USP radiation of 0.3 W and CW radiation of 0.3 W and 0.5 W. Increasing the time of exposure leads to an increase in the HAZ diameter in both cases. Exposure of USP and CW radiation with an average power of 0.3 W for 5 s (1.5 J) resulted in HAZ diameters of 1000 \(\upmu\)m and 750 \(\upmu\)m, respectively. In the first case, tissue coagulation and the beginning of crater formation was observed, in contrast to the case of the CW laser.