Under high-intensity laser pulses the morphology and size of the crater induced in biotissue depends both on biotissue properties and on irradiation conditions. The objective of this study is to correlate the wound morphology and patterns of microdestruction with the mechanisms of interaction. Along with the regime of superficial layerwise evaporation of tissue induced by C02 medical, or selective photocoagulation induced by copper vapor lasers, the present work describes the regime of bulk selective damage of coloration centers that are irregularly distributed in the deep of the tissue. Photo induced disruption is the mechanism responsible for it.
The results are presented on the study of the damage of melanin granules contained in ex-vivo animal skin specimens under a series of single Nd:glass laser pulses of nanosecond duration, with intensity I ~ 108 W/cm2. The mechanism responsible for tissue damage at the given conditions is shown to be a selective photodisruption occurring due to preferential energy absorption by endogenic skin pigment. For such a complicated multicomponent structure as biotissue comprising substructures inhomogeneously distributed in the bulk of it and differing in physical and chemical properties, such a parameter as volume energy density becomes a decisive one for tissue damage. When its magnitude
reaches the damage threshold value within the absorption loci it breaks whereas the surrounding tissue remains undisturbed.
At laser pulse treatment of skin, the radiation intensity decreases exponentially versus depth; the exponent index includes the biotissue absorption coefficient. The intensity is maximal at the surface leading to preferential heating of superficial layers of skin and if the temperature rises to about 300 C° then both liquid and hard components of the upper layer boil and evaporate. The major part of the incident energy is spent on evaporation of the upper layers rather than deep bulk layers since during a laser pulse the heat front doesn’t spread inside the medium due to a limited thermal conductivity. However, in present work the study was carried out of the case when a nanosecond pulse of near IR optical range strikes the inhomogeneous biotissue like skin comprising substructures with differing optical and physical properties, such a parameter as volume energy density becomes a decisive one for its damage. Its magnitude reaches the damage threshold value only within the substructure whereas the surrounding tissue stays undisturbed.
Under high-intensity laser pulses the morphology and size of a crater created in biotissue depends both on biotissue properties and on irradiation mode. Wounds have been investigated formed in biotargets under single as well as repetitive laser pulses of nanosecond duration of visible and near IR-range. The analysis of the produced damage has demonstrated that its morphology and size depends on ablation mechanism. The latter in its turn depends on irradiation temporal characteristics for a given specimen and wavelength.
In present work an analysis of the existing experimental data has been carried out on the ablation of biological tissue (skin, cornea, bone) under high-irradiance laser beam ~109W/cm2 at different wavelengths λ = 0.266, 0.532, 1,064, 2.7, 2.8, 2.9 μm. Comparison of the dependence of the ablation threshold values versus the intensity and wavelength of the single incident pulse to the data obtained in experiments on hgih power pulses interaction with organic low-density targets of agar-agar allows to reveal some common mechanisms. For the above threshold intensities, the temporal pulse shape becomes decisive and together with tissue transmittacne at a definite wavelength determines the depth and character of tissue disruption and may be changed according to therapeutic destination.