**4.1. Cavitation bubble dynamics**

Urinary calculi are crystalline deposits, also known as kidney/ureter/bladder/urethra stones or uroliths, which occur in the urinary system. The presence of urinary tract stones often causes the personal severe discomfort and pain. Even though laser lithotripsy has become the most popular treatment choice for kidney stone disease, the mechanism calculus disin‐ tegration by laser pulse remains unclear. This is due to the multiple physical/chemical pro‐ cesses involved in laser pulse‐caused calculus damage and their sub‐microsecond timescales. Cavitation bubble [9–16] dynamics are the centerpiece of the physical processes that link the whole energy flow chain from laser pulse to calculus damage.

In this study, cavitation bubble dynamics was investigated by a high‐speed camera and a needle hydrophone. We keep the following three questions in mind when performing the investigation:


The results revealed the following:


The transient pressure trace by a hydrophone located ∼10 mm away from the fiber tip reveals that the first shock wave appears immediately after the injection of the laser pulse. However, the second and the highest transient pressure peak corresponds to the first col‐ lapse of the cavitation bubble as shown in **Figure 16**. Since pressure peak can be much larger than the magnitude of the first shock wave, the latter is often overlooked. It should be noted that the first shock wave is more evident (higher peak pressure) at high laser pulse energies or short‐pulse widths as present in the Tm:YAG laser case.

Finally, we observed no discernible difference of cavitation bubble dynamics when switch‐ ing between 273‐ and 365‐μm core fibers. This is most likely because the cavitation bubble dynamics relates more to the pulse energy and pulse width, as opposed to the pulse intensity. A more detailed investigation of the relationship between cavitation bubble dynamics and calculus damage (fragmentation/dusting) will be conducted as a future study.

#### **4.2. Fiber‐tip damage mechanism**

Admittedly, a retrospective study in Ref. [6] revealed superior stone‐free rates results for renal stones <1.5 cm for URS compared with SWL. However, the fibers used in URS as energy delivery devices often suffer distal‐end damage. This is usually referred as fiber‐end burn‐ back [20–23]. Fiber‐tip burn‐back results in a reduced transmission of laser energy, which significantly reduces the efficiency of stone comminution. Though it is known that the higher the energy fluence (which is the ratio of the laser energy over the cross‐section area of the fiber core), the faster the fiber‐tip degradation, the damage mechanism of the fiber tip is still unclear.

In this study, fiber‐tip degradation was investigated by the visualization of shock wave, cavita‐ tion/bubble dynamics, and calculus debris ejection with a high‐speed camera and the Schlieren method. The primary chromophore of 2.01‐μm Holmium laser is water, which is critical for fragmentation of the calculus during laser lithotripsy [19]. The shock wave [9, 10, 12–14] that the laser pulse generated is a disturbance wave that is faster than a sound wave. Because of the transparency of water liquid, pressure wave imaging inside water is not as straightforward. However, the Schlieren method can reveal the acoustic wave inside water [15], just like an X‐ray which can reveal the invisible pressure variation inside a transparent matter.

Laser energy‐induced shock wave, cavitation/bubble dynamics, and stone debris ejection were recorded by a high‐speed camera with a frame rate of 10,000–930,000 fps. The shock wave is successfully detected using the Schlieren imaging technique. The results suggested that using a high‐speed camera and the Schlieren method to visualize the shock wave pro‐ vided valuable information about time‐dependent acoustic energy propagation and its interaction with cavitation and calculus. We successfully observed the shock wave gener‐ ated immediately after the injection of the laser pulse. This "first" shock wave was also detected by a transient pressure sensor (hydrophone) as discussed in Section 3.1.3. By plot‐ ting the shock‐wave displacement curve against time, we revealed that the acoustic wave speed that was more than 1 mm away from the fiber tip was 1.45 mm/μs or 1450 m/s. This is comparable to the sound speed in water (1484 m/s). Furthermore, it is in good agreement with Ref. [12] that indicates shock waves only exist within a millimeter of the fiber tip and travel faster than the sound speed. Therefore, in the current study that utilized a high‐speed camera with a frame rate of 1 million fps, or 1 μs per frame, the acoustic wave will travel 1.48 mm during a camera frame. Apparently, a high‐speed camera with frame rate well at or near 10 million fps is desired to resolve more detail of the shock‐wave dynamics generated by laser pulses.

The role of debris in fiber‐tip damage is also studied. Fiber‐tip damage/degradation/burn‐ back mechanism is an intricate subject due to its numerous physical phenomena, for example, sonic shock waves, self‐focusing of the laser beam, and transient thermal surge, and so on. It covers three areas of degradation mechanisms:


The transient pressure trace by a hydrophone located ∼10 mm away from the fiber tip reveals that the first shock wave appears immediately after the injection of the laser pulse. However, the second and the highest transient pressure peak corresponds to the first col‐ lapse of the cavitation bubble as shown in **Figure 16**. Since pressure peak can be much larger than the magnitude of the first shock wave, the latter is often overlooked. It should be noted that the first shock wave is more evident (higher peak pressure) at high laser pulse energies

Finally, we observed no discernible difference of cavitation bubble dynamics when switch‐ ing between 273‐ and 365‐μm core fibers. This is most likely because the cavitation bubble dynamics relates more to the pulse energy and pulse width, as opposed to the pulse intensity. A more detailed investigation of the relationship between cavitation bubble dynamics and

Admittedly, a retrospective study in Ref. [6] revealed superior stone‐free rates results for renal stones <1.5 cm for URS compared with SWL. However, the fibers used in URS as energy delivery devices often suffer distal‐end damage. This is usually referred as fiber‐end burn‐ back [20–23]. Fiber‐tip burn‐back results in a reduced transmission of laser energy, which significantly reduces the efficiency of stone comminution. Though it is known that the higher the energy fluence (which is the ratio of the laser energy over the cross‐section area of the fiber core), the faster the fiber‐tip degradation, the damage mechanism of the fiber tip is still

In this study, fiber‐tip degradation was investigated by the visualization of shock wave, cavita‐ tion/bubble dynamics, and calculus debris ejection with a high‐speed camera and the Schlieren method. The primary chromophore of 2.01‐μm Holmium laser is water, which is critical for fragmentation of the calculus during laser lithotripsy [19]. The shock wave [9, 10, 12–14] that the laser pulse generated is a disturbance wave that is faster than a sound wave. Because of the transparency of water liquid, pressure wave imaging inside water is not as straightforward. However, the Schlieren method can reveal the acoustic wave inside water [15], just like an X‐ray

Laser energy‐induced shock wave, cavitation/bubble dynamics, and stone debris ejection were recorded by a high‐speed camera with a frame rate of 10,000–930,000 fps. The shock wave is successfully detected using the Schlieren imaging technique. The results suggested that using a high‐speed camera and the Schlieren method to visualize the shock wave pro‐ vided valuable information about time‐dependent acoustic energy propagation and its interaction with cavitation and calculus. We successfully observed the shock wave gener‐ ated immediately after the injection of the laser pulse. This "first" shock wave was also detected by a transient pressure sensor (hydrophone) as discussed in Section 3.1.3. By plot‐ ting the shock‐wave displacement curve against time, we revealed that the acoustic wave speed that was more than 1 mm away from the fiber tip was 1.45 mm/μs or 1450 m/s. This is comparable to the sound speed in water (1484 m/s). Furthermore, it is in good agreement with Ref. [12] that indicates shock waves only exist within a millimeter of the fiber tip and

which can reveal the invisible pressure variation inside a transparent matter.

calculus damage (fragmentation/dusting) will be conducted as a future study.

or short‐pulse widths as present in the Tm:YAG laser case.

102 Updates and Advances in Nephrolithiasis - Pathophysiology, Genetics, and Treatment Modalities

**4.2. Fiber‐tip damage mechanism**

unclear.

The function of the stone particle in fiber‐tip degradation mechanism has two folds: kinetic impulsion and thermal heating/melting. **Figure 25** depicts the stone particle clusters hover‐ ing around the fiber‐end area and the fiber‐end degradation/deformation after 1.5‐kJ laser power dose and laser‐stone interaction in our test. Our investigation reveals that the fiber‐tip degradation/deformation is more significant when the spacing between the fiber end and stone surface is less. And with similar spacing, the 45° incidence angle leads to less tip surface degradation/deformation as compared to 0° incidence ones.

More investigation should be performed to find out the predominant degradation mechanism by the stone particle (thermal or kinetic), cavitation bubble dynamics, and balance between degradation/burn‐back control and the stone dusting efficiency.

**Figure 25.** The stone particle clusters hovering around the fiber‐end area (a) and the fiber‐end degradation/deformation after 1.5‐kJ laser power dose and laser‐stone interaction (b).
