Have You Heard About Sonochemistry?

The honk of a car horn, the tone of an AMBER alert, and the beep of a heart rate monitor are all auditory sensations that can spell the difference between life and death. Yet sound does not always need to be heard to save lives. Ultrasound describes sounds with frequencies higher than that of the human hearing range, of 20 Hz to 20 kHz. While acoustic technologies have existed in the medical field since the early 19th century, ultrasonic applications like ultrasound imaging emerged in the 1950s. In the 1980s however, a field called sonochemistry, which explores the application of ultrasound to cause chemical reactions, was invented. While instruments like stethoscopes will stay relevant for many more decades, sonochemistry and many other applications of ultrasound are becoming the first choice for many doctors. Because these minimally invasive instruments allow for immediate and precise results, they may replace many of the technologies and practices we use today. Whether by creating nanotechnologies that can transport and release virtually anything or causing chemical reactions directly within the body, sonochemistry is a field that will continue to evolve.

In a recent study, researchers at RMIT University in Australia utilized sonochemistry to reach a breakthrough in nanotechnology. They devised a way to produce metal-organic frameworks (MOFs)—highly permeable polymers composed of a metal ion center and additionally linked organic molecules—with great efficiency. MOFs are highly versatile as they can be used in any application that requires the transport of substances on a microscopic scale, making them highly valuable in medical and biotechnical fields. MFM-300(Al), an MOF that traps toxic gas molecules, and Fe-BTC, an MOF that removes metals like lead and mercury from water, are just two of the 80,000+ varieties of MOFs.

MOFs are manufactured through a resource-intensive process. First, the organic and inorganic molecules must be assembled in a high surface tension solvent which allows them to stay in place. Then, in a process called activation, the MOF-solvent complex is heated for hours in the solvent with a low boiling temperature until the solvents inside and outside the MOF are exchanged. Finally, the product is put in a vacuum, leaving the inside empty and ready for use. In contrast, the new technique developed by RMIT scientists uses high-frequency sound waves delivered from a special microchip that precisely alters the frequency of the sound, to meticulously order and move the molecules forming the MOFs. This method allows scientists to relocate the molecules while also removing the solvent without the use of activation. Because the frequency of the ultrasound is so high, the sound wave is over 100,000 times larger than the molecules themselves. Currently, this technique is only verified to work with iron and copper-based materials; nonetheless, it serves as a proof that MOFs can be manufactured in mere minutes.

The same group of researchers also proved that sonochemistry can also be used for drug delivery and vaccinations. Using ultrasound’s ability to move molecules once again, the researchers manipulated molecules found in vaccines using high-frequency sound waves, thereby restructuring them. While MOFs can create nanoparticles that house vaccines, the team also guided the contained vaccine to its destination in the body. Lead researcher Leslie Yeo at RMIT explains that his team has established a range of frequencies that create unique interactions between a variety of materials: ultrasound-driven chemistry involving “low” frequencies between 10 kHz to 3 MHz to not only help raise the temperature of a solution but also help guide molecules along a wave-like formation. This discovery opens the doors to a host of new, non-invasive, and painless methods of drug delivery, as large molecules like vaccine antibodies can be inserted in MOFs to be delivered. Additionally, existing nebulizers—machines that administer liquid medicine as gases—can be made more efficient and accurate with the ability to create a finer suspension. Despite being an emerging technology, these techniques are relatively cheap to administer, making it possible to scale up production and create large trials before they are officially released.

RMIT University is not the only institution advancing the applications of sonochemistry. The French National Institute for Health and Medical Research also uses sound to treat cancer without the need of surgery. The treatment involves administering ultrasound applicators outside the body to cause small movements in the body that bring the cancerous tissue closer for removal by heat. This alternative is both safe and effective, and serves as a new hope for patients, especially those whose bodies are still recovering from treatments like surgery. Likewise, a team from the University of Michigan used an ultrasound method, called histotripsy, to remove tumors in the prostate. This method works by creating micro-sized bubbles within the tumor, which disrupts cellular functions and breaks down tumor cells. Additionally, scientists at Duke University are experimenting with a technique called virtual palpation, which uses the ultrasound’s ability to permeate through molecules to scan tissues and analyze their stiffness. This technique has primarily been used to detect irregularities in heart, liver, and breast tissues.

Though it has existed for over 30 years, the field of sonochemistry is still in its early stages of development. Despite being overshadowed by recent advancements in artificial intelligence and space travel, the lesser known field of sonochemistry can still lend itself to great scientific progress in the years to come. MOFs are powerful tools, but they are only one example of how sonochemistry propels multiple areas of research. Perhaps the future could see devices that use ultrasound to manipulate the molecules in our atmosphere to reduce the effects of global warming. Additionally, patients can expect to see vaccinations as a pill guided by ultrasound rather than an injection. Though sonochemistry is not the first scientific field that people ordinarily think of, it still contributes to many of the scientific advancements of the 21st century—albeit silently.