Penetration depth of laser pulse in water.

3.2 Temperature Rise Effects During TFL Lithotripsy Under different Conditions

A constant-temperature water environment at 37°C was maintained by means of a bath with recirculating warm water. A 10 mL test tube was immersed in the warm water to simulate the renal pelvis environment during lithotripsy. Irrigation with saline for cooling was administered within the tube using a flow-adjustable micro-peristaltic pump. A temperature probe was positioned approximately 10 mm from the fiber tip to record the water temperature, while the overall temperature field was captured by an infrared camera. At a laser power of 50 W and in the absence of irrigation cooling, the temperature of the water near the fiber tip increased rapidly, reaching 62℃ instantaneously (Fig. 4a). With an irrigation flow rate of 10 mL/min and a laser power of 50 W, the water temperature still kept over 54℃ (Fig. 4b).

Temperature distribution in the surrounding water during laser irradiation. (a) Without irrigation cooling. (b) With an irrigation flow rate of 10 mL/min.

At the laser power of 50 W, without irrigation, temperature near the fiber tip rose rapidly, exceeding 60℃ within 10 seconds (Fig. 5), and with irrigation at 10 mL/min, temperature still exceeded 54℃(Fig. 6). The homeostasis temperature surpassed the 43℃ safety threshold at all power, reaching 45℃ at 20 W and 51℃ at 40 W. The water temperature will decline under 40℃ within 3 seconds if the laser stopping.

Water temperature change in the lithotripsy without irrigation cooling.

Dynamic water temperature in the lithotripsy with irrigation cooling at 10 mL/min.

The experiments shown that the irrigation flow rate was the predominant factor. As shown in Fig. 7b, a laser power of 20W required over 30 mL/min irrigation to stabilize near the safety threshold. It could be seen from Fig. 7c that a flow rate of 40–50 mL/min was necessary to maintain safe temperatures when the laser was 40W. Figure 6 shows the temperature rise with different laser powers and an irrigation flow fixed at 10 mL/min. Due to the inherent flow dynamics, temperature fluctuations on the order of 3°C were observed. At this flow rate, a laser power of 10 W elevated the water temperature to the 43°C safety threshold. If the irrigation flow was 50 mL/min, a laser power of 40 W was also found to be operable within safe thermal limits (Fig. 7d). Higher flows increased temperature fluctuations due to enhanced turbulence.

Temperature profiles under different irrigation flow rates and laser powers. Each subplot corresponds to a fixed irrigation flow rate: (a) 20, (b) 30, (c) 40, (d) 50 mL/min. Different curves within each subplot represent different laser powers (10–40 W).

3.3 Lithotripsy Efficiency of Pulsed TFL with Different Parameters

Human urinary calculi exhibit substantial heterogeneity in morphology and composition, which complicates the assessment of TFL lithotripsy efficacy, so artificial samples were utilized for studding the efficacy rules in the experiment. Stones were positioned in water within a beaker, and the moving worktable was adjusted to maintain suitable distance of 0.5-1 mm between the stone surface and the fiber tip (Fig. 1). when the worktable was moving the laser beam would scan across the stone surface. Laser lithotripsy was performed for a duration of 1 minute, and the mass of ablated material was quantified using an analytical balance with different parameters. Upon laser irradiation, a substantial quantity of white powder generated instantaneously. Then they dispersed into the surrounding aqueous medium. A portion of the powder was drawn back towards the fiber tip due to recoil effects. With the TFL average power set to 10 W, the pulse width and repetition rate were modulated to measure the lithotripsy efficiency, as shown in Fig. 8. Effective lithotripsy was initiated only when the pulse width exceeded 200 µs (pulse energy of 0.16 J), and the lithotripsy efficiency increased precipitously with the pulse width increasing. For the 400 µm fiber, the lithotripsy efficiency reached a maximum at a pulse width of about 500 µs, being a slight decrease when the pulse width was further increased. The 600 µm fiber exhibited an analogous efficiency trend, but the peak efficiency occurred at a larger pulse width, and it was in the range of 1500–2000 µs. The fragmentation efficiency achieved by using the 400 µm fiber was 18% higher than that obtained with the 600 µm fiber. It is consistent with previous study ^[24]. The TFL lithotripsy physical process is complex and intimately linked to parameters such as pulse width and peak power density.

lithotripsy efficiency as a function of pulse width.

The ablation morphology of the stones is presented in Fig. 9. features indicative of thermal melting and ablation craters would be seen with larger pulse width. In the working process, the initial portion of the laser pulse interacts with water, generating shockwaves which cause mechanical fracture and spallation of the stone. Once the water was displaced, subsequent laser energy was deposited directly into the stone matrix, resulting in thermal ablation.

Morphology of ablated calculi.

A comparison of lithotripsy speeds at different repetition rates is shown in Fig. 10. As the average power was increased, the lithotripsy speed increased rapidly, but the high lithotripsy efficiency appeared with suitable laser parameters. As shown in Fig. 10, the lithotripsy efficiency was high under the condition of the repetition rate of 20 Hz and pulse width of 500–1000 µs.

lithotripsy speeds with different laser parameters.

Human stones frequently exhibit porous structures. Moreover, during clinical lithotripsy, the smaller fragmented particles can be expelled spontaneously through the ureter. Consequently, laser lithotripsy does not invariably produce fine dust. Porous stone samples measuring 10 mm³ were placed in a mesh bag with 3 mm squares suspended in water (Fig. 11). Lithotripsy was performed by guiding the fiber manually. The procedure was deemed complete when all stone fragments had passed through the mesh. The efficiency under this simulation condition was analyzed.

Schematic of the net bag setup for fragment collection.

Under the condition of the laser power of 20 W and single pulse energies ranging from 0.15 J to 2 J, the lithotripsy efficiency is plotted in Fig. 12. When the single pulse width exceeded 250 µs, further increase in pulse width did not significantly alter the lithotripsy efficiency, which stabilized around 156 mg/min. The fragmentation morphology was predominantly characterized by coarse fragments, with a maximum fragment size of 2.8 mm. If only larger stone fragments were crushed, the crushing efficiency of 1 ms pulse width exceeded 40% than it of 250 µs pulse width. Microscopic examination under different conditions confirmed that no thermal ablation features were produced in porous stones.

lithotripsy efficiency with error bars indicating variability.

The TFL lithotripsy validation experiment was conducted using a porous calcium oxalate human kidney stone sample (Fig. 13), and the experimental results followed similar patterns. The pulsed laser frequency was set at 10 Hz in the experiment. The laser pulse width of 500 µs already showed a certain ability to fragment stones, while the laser with 1 ms pulse width had a stronger ability to break larger stones.

lithotripsy experiment with human calcium oxalate stone

3.4 Retropulsion and Traction Dynamics in TFL Lithotripsy

The generation and collapse of vapor bubbles during TFL lithotripsy can induce retropulsion and traction forces on the stone. The traction effect can limit the dispersion of larger fragments, facilitating the procedure and mitigating the need for additional anti-retropulsion devices ^[2]. Stone samples (3×3×3 mm³) were prepared, and the traction characteristics with different laser parameters were recorded in the experiment. Notable traction was observed only when the pulse energy exceeded 0.3J and the repetition rate was higher than 5 Hz. The most effective traction was achieved within the parameter range of 0.5 J and 40 Hz. However, the stone rotated at the fiber tip when the repetition rate over 30 Hz in the experiment. it makes the laser difficult to align the stone and is unable to effectively break stones in surgery, so during lithotripsy, only selecting appropriate parameters can improve the crushing stones really.

4. Discussion

Ureteral mucosal tissue can suffer irreversible damage if exposed to an environment of 45°C for more than 8 minutes, and higher temperatures can cause damage more quickly, leading to complications such as strictures. During stone surgery, the cooling water needs to flow through the gaps of the inserted ureteral sheath and flexible ureteroscope, which imposes significant flow limitations and cannot ensure a consistently unobstructed state. It results in high risk of burns. Based on the experimental results, it can be seen that when perfusion is not smooth, such as the flow rate is less than 10 ml/min, the water temperature rises to above 53°C within 10 seconds with high laser power of 50 W, which can cause significant damage to soft tissues. Synthesis of the data presented in Figs. 5 and Figs. 6 indicates that TFL lithotripsy with an average output power below 20 W is associated with a relatively high safety margin. If higher power is clinically necessary, the irrigation flow rate must be meticulously monitored during the procedure to ensure that the heat is carried away in time.

The unit energy lithotripsy efficiency is important for surgery and high efficiency can reduce heat deposition effectively and lower the risk of burns. Experimental results show that when using a 400 µm optical fiber, the efficiency of stone pulverization no longer changes significantly with increasing pulse width once the laser pulse width exceeds 300 µs (energy of 0.24 J). The similar patterns are observed in experiments on the particulate fragmentation of porous stone models, but for large stones, the ability of long-pulse-width lasers to break them is significantly enhanced. Additionally, experiments on the dynamics of stone fragmentation show that when the pulse frequency exceeds 30 Hz, small stones, such as size of 3 mm³, tend to rotate rapidly around the fiber tip, it making the laser aim at the stone more difficultly and reducing effective fragmentation ability. To ensure the necessary fragmentation speed, the average power cannot be too low, so in actual stone fragmentation procedures, doctors usually prefer to use a relatively long pulse width, typically reaching or exceeding 1 ms. According to the experimental results in Figs. 3, the penetration depth of the laser pulse in water exceeds 5 millimeters under this condition. Meanwhile, it is difficult to determine whether it has irradiated soft tissue because of the 1940 nm wavelength laser being invisible in the stone-fragment environment. It is easy to cause radiation damage to the surrounding soft tissues.

In summary, the lithotripsy effect and safety mainly depend on laser parameters, such as pulse width and average power ^[25]. In TFL lithotripsy surgery, special attention should be paid to laser radiation and high-temperature damage to surrounding tissues when using high pulse energy and high-power. In terms of parameter optimization, when the stone is large, using high pulse energy can improve lithotripsy efficiency, as the large stone may block the laser and have low laser leakage. When the stone is small, the pulse width should be reduced promptly to ensure safe lithotripsy.