Photo acoustic drug delivery is a technique for localized drug delivery by laser-induced hydrodynamic pressure following cavitation bubble expansion and collapse. Photoacoustic drug delivery was investigated on gelatin-based thrombus models with planar and cylindrical geometries by use of one microsecond laser pulses. Solutions of a hydrophobic dye in mineral oil permitted monitoring of delivered colored oil into clear gelatin-based thrombus models. Cavitation bubble development and photoacoustic drug delivery were visualized with flash photography. This study demonstrated that cavitation is the governing mechanism for photoacoustic drug delivery, and the deepest penetration of colored oil in gels followed the bubble collapse. Spatial distribution measurements revealed that colored oil could be driven a few millimeters into the gels in both axial and radial directions, and the penetration was less than 500 µm when the gelatin structure was not fractured.
This paper presents studies of microsecond ablation phenomena that take place during during laser thrombolysis. The main goals were to optimize laser parameters for efficient ablation, and to investigate the ablation mechanism. Gelatin containing an absorbing dye was used as the clot model. A parametric study was performed to identify the optimal wavelength, spot size, pulse energies, and repetition rate for maximum material removal. The minimum radiant exposures to achieve ablation at any wavelength were measured. The results suggest that most visible wavelengths were equally efficient at removing material at radiant exposures above threshold. Ablation was initiated at surface temperatures just above 100°C. A vapor bubble was formed during ablation. Less than 5% of the total pulse energy is coupled into the bubble energy. A large part of the delivered energy is unaccounted for and is likely released partly as acoustic transients from the vapor expansion and partly wasted as heat. The current laser and delivery systems may not be able to completely remove large clot burden that is sometimes encountered in heart attacks. However, laser thrombolysis may emerge as a favored treatment for strokes where the occlusion is generally smaller and rapid recanalization is of paramount importance. A final hypothesis is that laser thrombolysis should be done at radiant exposures close to threshold to minimize any damaging effects of the bubble dynamics on the vessel wall.
Photoacoustic drug delivery is a technique for delivering drugs to localized areas in the body. In cardiovascular applications, it uses a laser pulse to generate a cavitation bubble in a blood vessel due to the absorption of laser energy by targets (e.g., blood clots) or surrounding liquids (e.g., blood or injected saline). The hydrodynamic pressure arising from the expansion and collapse of the cavitation bubble can force the drug into the clots and tissue wall tissue. Time-resolved particle image velocimetry was used to investigate the flow of liquids during the expansion and collapse of cavitation bubbles near a soft boundary. A gelatin-based thrombus model was used to simulate the blood clot present during laser thrombolysis. An argon laser chopped by an acousto-optic modulator was used for illumination and photography was achieved using a CCD camera. The implications of this phenomenon on practical photoacoustic drug delivery implementation are discussed.
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