An innovative approach to supporting laser lithotripsy in kidney stones

In a procedure called laser lithotripsy, urologists use a small laser under video guidance to break apart painful, potentially harmful kidney stones.

It is better for the patient if urologists can break down the kidney stones as finely as possible, preferably into dust that can be safely aspirated – but the use of more powerful lasers creates additional heat that can damage surrounding tissue and injure the patient.

Of course, you don’t want to over-pump energy into your kidneys, as this is very dangerous. In our work, we show how to better use laser energy that is already being used.”

Assistant Prof. Po-Chun Hsu, University of Chicago Pritzker School of Molecular Engineering (UChicago PME)

Hsu is co-author of a new paper published in , a collaboration between engineers and physicians at UChicago PME and Duke University who have pioneered improving the effectiveness of lasers in treating kidney stones without changing lasers. This work could result in shorter surgeries, faster recoveries and fewer recurrences of a disease that affects 11% of Americans and increased national health care spending by more than $2 billion in 2000 alone.

“This is a classic example of how connecting the dots can create something that is transformative,” said Hsu, whose research focused mainly on building materials and heat-reflecting fabrics.

Co-author Michael Lipkin, a urologist at Duke Health, described the collaboration between engineers and physicians as an opportunity for both parties.

“This is a great opportunity for a clinician to be able to work with world-class scientists to attack a problem that has direct benefits for our patients,” Lipkin said. “These types of partnerships are fertile ground for great ideas that change the world.”

Solution within solution

To improve laser performance without changing the laser itself, the interdisciplinary team needed an innovative solution. An innovative solution.

Doctors use saline – slightly salty water – to expand the empty part of the kidney and maintain visibility during the procedure. Much of the laser energy is typically dissipated in the saline as heat. The researchers found that adding dark nanoparticles, which absorb laser waves, to this salt solution causes the laser to be focused on the stone, rather than reflected or scattered.

This improves the amount of laser energy transferred to and absorbed by kidney stones, a feature that many believe cannot be easily manipulated, said corresponding author, Duke University engineering professor Pei Zhong.

“Each laser has its own wavelength due to the technology with which it was generated. People thought, ‘If the wavelength is fixed, you can’t change the absorption of the laser in the working fluid or in the rock you want to target,'” Zhong said. “Nanofluid brings a new dimension, independent of the stone composition, independent of the laser, which can have an impact on this very complex physical procedure.”

But not every nanofluid is suitable for a medical procedure, said first author Qingsong Fan, a postdoctoral researcher at UChicago PME.

“First of all, the solution should be absorbent at the laser wavelength, which is about 2,000 nanometers, or two micrometers,” Fan said. “The second criterion is that the nanoparticles disperse well in water, because this is how we irrigate the kidney. The third – and most important – is the criterion that it should be safe.”

Tests on lab-grown kidney stones showed the team achieved all three goals. The nanofluid improved stone ablation efficiency by 38–727% in spot treatment and 26–75% in scanning treatment. Immersing living cells in the nanofluid for various periods of time, up to 24 hours, showed that the effective nanoparticle solution was also non-toxic and safe.

In practice, however, this material will never come into contact with cells for nearly that long. Lithotripsy is an outpatient procedure that takes about 30 minutes. Hsu hopes that improving absorption efficiency could reduce that time to 10 minutes.

“If you spend too much time doing this surgery, waste heat from the laser will build up, which will actually be more harmful than the ablation itself,” Hsu said.

Different stones, different lasers

The study focused on holmium-yttrium-aluminum-garnet (Ho-YAG) lasers and lab-grown kidney stones. The gold standard of laser lithotripsy, Ho-YAG, is by far the most popular – but not the only – type of laser used.

“Some lasers perform well for dusting, others do better for fragmentation, but no laser can perform exceptionally well for both dusting and fragmentation,” Zhong said. “Unless you work at a large hospital such as the University of Chicago or Duke, community physicians may not be able to afford multiple lasers. The nanofluid has the potential to increase the effectiveness of each laser in a variety of clinical scenarios.”

Next steps include tests to see how the new technique works using other popular lithotripsy lasers and how it affects real, rather than lab-grown, kidney stones.

Co-authored by Christine Payne and Donald M. Alstadt, chair of the Department of Mechanical Engineering and Materials Science. Thomas Lord at Duke University, called the study “a good example of translating basic research into clinical applications.”

“One of the most exciting aspects of this research is how a team of scientists and clinicians are working together, using their expertise to answer an important question – how to better treat kidney stones,” Payne said.