Oscillations, Waves, and Interactions - GWDG

Dynamics of pulsed laser tissue ablation 247
Figure 19. Recoil-induced material expulsion for liver ablation by (a) 200-µs Er:YAG laser
pulses at 100 J/cm 2 radiant exposure and 1.1 mm spot size, (b) 70-ns Er:YAG laser pulses at
5.4 J/cm 2 radiant exposure and 0.5 mm spot size. The primary material ejection produced
by the phase changes in the target is also visible in all images of (a) and in the first image
of (b).
To understand this discrepancy, we must first consider that the recoil-induced tensile
and shear stresses that contribute to tissue fracture may be considerably smaller
than the measured compressive recoil stress. Moreover, as discussed in Sect. 2, the
dynamic tensile strength of tissue at the extreme strain rates produced in pulsed
laser ablation is much higher than the quasi-static values for the UTS found in the
literature. Finally, tissue fracture will only occur at sufficiently large strain that may
not be achieved by stress transients of very short duration [6,22].
For mechanically weak tissues, the recoil-stress-induced material expulsion produces
craters with a depth much larger then the optical penetration depth and a
diameter much larger than the irradiated spot size, as shown in Fig. 20(a,b). For
mechanically strong tissues, the recoil stress does not lead to material expulsion.
However, it can produce tissue tearing at the sides of the ablation crater as seen in
Fig. 20(c). The cracks and tearing patterns arise preferentially along morphological
structures with reduced mechanical strength such as the transitions between corneal
lamellae, sinusoid spaces holding blood between plates of cells in liver tissue, and
their orientation is also influenced by the weakness of the longitudinal strength of
blood vessels compared to their circumferential strength [106]. Tissue tearing at the
rim of ablation craters was not observed for skin due to its three-dimensional collagen
network that results in an approximately isotropic UTS.