Expansion Of Cavitation Bubble Research Articles

Recent developments in SWL physics research.

Two projects in our laboratory highlight some recent developments in shockwave lithotripsy (SWL) physics research. In the first project, we developed a prototype of a piezoelectric annular array (PEAA) shockwave generator that can be retrofitted on a Dornier HM-3 lithotripter for active control of cavitation during SWL. The PEAA generator, operating at 15 kV, produces a peak positive pressure of approximately 8 MPa with a -6-dB beam diameter of 5 mm. The shockwave generated by the PEAA was used to control and force the collapse of cavitation bubbles induced by a laboratory electrohydraulic shockwave lithotripter with a truncated HM-3 reflector. With optimal time delay between the lithotripter pulse and the PEAA-generated shockwave, the collapse of cavitation bubbles near the stone surface could be intensified, and the resultant stone fragmentation in vitro could be significantly improved. In the second project, high-speed shadowgraph imaging was used to visualize the dynamics of lithotripter-induced bubble oscillation in a vascular phantom. Compared with the free bubble oscillation in water, the expansion of cavitation bubble(s) produced in silicone tubes and a 200-microm cellulose hollow fiber by either a Nortech EHL or a Dornier XL-1 lithotripter was found to be significantly constrained. Rupture of the cellulose hollow fiber was observed consistently after about 20 shocks from the XL-1 lithotripter at an output voltage of 20 kV. These results confirm experimentally that SWL-induced cavitation in vivo can be significantly constrained by the surrounding tissue, and large intraluminal bubble expansions could cause rupture of capillaries and small blood vessels.

Bubble dynamics, shock waves and sonoluminescence

Sound and light emission by bubbles is studied experimentally. Single bubbles kept in a bubble trap and single lasergenerated bubbles are investigated using ultrafast and highspeed photography in c...

Influence of pulse duration on mechanical effects after laser-induced breakdown in water

The influence of the pulse duration on the mechanical effects following laser-induced breakdown in water was studied at pulse durations between 100 fs and 100 ns. Breakdown was generated by focusing laser pulses into a cuvette containing distilled water. The pulse energy corresponded to 6-times breakdown threshold energy. Plasma formation and shock wave emission were studied photographically. The plasma photographs show a strong influence of self-focusing on the plasma geometry for femtosecond pulses. Streak photographic recording of the shock propagation in the immediate vicinity of the breakdown region allowed the measurement of the near-field shock pressure. At the plasma rim, shock pressures between 3 and 9 GPa were observed for most pulse durations. The shock pressure rapidly decays proportionally to r−(2⋯3) with increasing distance r from the optical axis. At a 6 mm distance of the shock pressure has dropped to (8.5±0.6) MPa for 76 ns and to <0.1 MPa for femtosecond pulses. The radius of the cavitation bubble is reduced from 2.5 mm (76 ns pulses) to less than 50 μm for femtosecond pulses. Mechanical effects such as shock wave emission and cavitation bubble expansion are greatly reduced for shorter laser pulses, because the energy required to produce breakdown decreases with decreasing pulse duration, and because a larger fraction of energy is required to overcome the heat of vaporization with femtosecond pulses.

Shock wave emission and cavitation bubble generation by picosecond and nanosecond optical breakdown in water

Shock wave emission and cavitation bubble expansion after optical breakdown in water with Nd:YAG laser pulses of 30-ps and 6-ns duration is investigated for energies between 50 μJ and 10 mJ which are often used for intraocular laser surgery. Time-resolved photography is applied to measure the position of the shock front and the bubble wall as a function of time. The photographs are used to determine the shock front and bubble wall velocity as well as the shock wave pressure as a function of time or position. Calculations of the bubble formation and shock wave emission are performed using the Gilmore model of cavitation bubble dynamics and the Kirkwood–Bethe hypothesis. The calculations are based on the laser pulse duration, the size of the plasma, and the maximally expanded cavitation bubble, i.e., on easily measurable parameters. They yield the dynamics of the bubble wall, the pressure evolution inside the bubble, and pressure profiles in the surrounding liquid at fixed times after the start of the laser pulse. The results of the calculations agree well with the experimental data. A large percentage of the laser pulse energy (up to 72%) is transformed into the mechanical energy ES and EB of the shock wave and cavitation bubble, whereby the partitioning between ES and EB is approximately equal. 65%–85% of ES is dissipated during the first 10 mm of shock wave propagation. The pressure at the plasma rim ranges from 1300 MPa (50 μJ, 30 ps) to 7150 MPa (10 mJ, 6 ns). The calculated initial shock wave duration has values between 20 and 58 ns, the duration measured 10 mm away from the plasma is between 43 and 148 ns. A formation phase of the shock front occurs after the ns pulses, but not after the ps pulses where the shock front exists already 100 ps after the start of the laser pulse. After shock front formation, the pressure decays approximately proportional to r−2, and at pressure values below 100 MPa proportional to r−1.06. The maximum bubble wall velocity ranges from 390 to 2450 m/s. The calculations of bubble and shock wave dynamics can cover a large parameter range and may thus serve as a tool for the optimization of laser parameters in medical laser applications.

Drug delivery with microsecond laser pulses into gelatin

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.

Mechanisms of intraocular photodisruption with picosecond and nanosecond laser pulses.

Nd:YAG laser photodisruption with nanosecond (ns) pulses is an established method for intraocular surgery. In order to assess whether an increased precision can be achieved by the use of picosecond (ps) pulses, the plasma size, the shock wave characteristics, and the cavitation bubble expansion after optical breakdown with ps- and ns-laser pulses were investigated by time-resolved photography and acoustic measurements. Nd:YAG laser pulses with a duration of 30 ps and 6 ns, respectively, were focused into a water-filled glass cuvette. Frequency doubled light from the same laser pulses was optically delayed between 2 ns and 136 ns and used as illumination light source for photography. Since the individual events were well reproducible, the shock wave and bubble wall position could be determined as a function of time. From the slope of these r(t) curves, the shock wave and bubble wall velocities were determined, and the shock wave pressure was calculated from the shock velocity. The plasma size at various laser pulse energies was measured from photographs of the plasma radiation. The breakdown thresholds at 30 ps and 6 ns pulse duration were found to be 15 microJ and 200 microJ, respectively. At threshold, ps-plasmas are shorter than ns-plasmas, but at the same pulse energy they are always approximately 2.5 times longer. The initial shock pressures were 17 kbar after ps-pulses with an energy of 50 microJ, and 21 kbar after 1 mJ ns-pulses. The pressure amplitude decayed much faster after the ps-pulses. The maximum expansion velocity of the cavitation bubble was 350 m/s after a 50 microJ ps-pulse, but 1,600 m/s after a 1 mJ ns-pulse. The side effects of intraocular microsurgery associated with shock wave emission and cavitation bubble expansion can be considerably reduced by the use of ps-pulses, and new applications of photodisruption may become possible.