UC Riverside

Current therapy to remove vascular lesions involves long laser pulses to coagulate the blood vessels with partial success. Small blood vessels, with thermal relaxation time shorter than the long laser pulses, remain. Skin models incorporating light scattering were irradiated with nanosecond laser pulses. The objective is to destroy blood vessels taking advantage of the short laser pulse high intensity to induce plasma mediated cavitation bubbles. These bubbles may serve as photodisruption mechanism of blood vessels. It was found that permanent or transient bubbles were produced depending on the laser dose, number of pulses and repetition rate. Scattering added to the skin models increased the threshold fluence for plasma formation.

Such fast energy deposition from nanosecond laser pulses implies mechanical effects. Laser energy is coupled to a material through a combination of linear and nonlinear absorption. The first results in heat generation and thermoelastic expansion; while the second results in an expanding plasma formation that creates a shock wave and a cavitation bubble. It was found that the shock wave emitted upon plasma formation is spherical while the pressure wave emitted by pure linear absorption has a plane and cylindrical components. For irradiation of an absorbing solution with no plasma formation, the local pressure was calculated using empirical correlations. The low local pressure explains the bubble formation at low temperature increments.

When nanosecond laser pulses are applied to absorbing microspheres, thermoelastic expansion of the microparticles originates pressure waves. A melanoma detector takes advantage of this principle. Excessive energy creates bubbles around melanosomes damaging the plasma membrane. Optimum laser parameters for this application must be found. Melanoma cells were irradiated at 355 and 532 nm wavelengths to determine cell survival rate, compare the photoacoustic signal, determine the critical laser fluence for melanin leakage and study the intracellular interactions and their effect on the plasma membrane integrity. Cell survival decreased with increasing laser fluence, although the decrement is more pronounced at 355 nm. Melanin leaks from cells equally for both wavelengths. No significant difference in photoacustic signal was found between wavelengths. Damage to plasma membrane due to bubble formation was imaged.