Sound sterilization gives dentist something to smile about
06/01/2003
By Mark A. DeSorbo
SNELLVILLE, GA.—By the time the late '80s rolled around, AIDS and other infectious diseases had frantically intensified the meaning of sterilization. And it was right around this same time that a dentist in this Atlanta suburb began thinking of other ways to sanitize the tools of his trade without using high temperatures or harsh chemicals.
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Over the next few years, Dr. Stephen Carter's research actually yielded a methodology far greater than his expectations. His patented technique uses a form of cavitation—an ultrasonic boom long used as a critical cleaning method in which acoustic energy applied to a liquid creates bubbles that collapse and release energy.
"For me, as a dental practitioner, it will do very little," says Carter. "Where it really has its primary application is cleaning expensive medical and surgical instruments, like bronchoscopes, which are heat-sensitive."
Carter is on to something, for some bronchoscopes, which are used to examine the lungs, have proven difficult to sanitize. And some healthcare settings have identified the devices as sources of deadly lung infections that have plagued several hospitals. (See "Hospital tracks virus," www.cleanrooms.com and "Botched recall investigation continues," CleanRooms, May 2002, page 1.)
"These instruments are exceedingly expensive, so a hospital can only have so many of them," Carter says. "We believe this technique will be able to sterilize these types of instruments within 10 minutes instead of 30, and it also has the potential to minimize the risk of cross-transmission of infection caused by contaminated instruments."
With the help of Ken Cunefare, a professor of mechanical engineering at the Georgia Institute of Technology, Carter developed a test chamber that harnesses an enhanced form of transient cavitation—a more intense bubble collapse—when subjected to a 24-kilohertz chirp under approximately twice the normal atmospheric pressure.
"It's ultrasonics, and there have been a lot of folks working in this field for a very long time," says Cunefare, a specialist in acoustics. "What we have found is a good combination of pressure and fluid that gives a synergistic effect to sterilization efficiencies."
When applied to a solution of 66 percent isopropyl alcohol containing two bacterial spores—bacillus stearothermophilis and bacillus subtilus—the enhanced cavitation reduced the bacterial count by more than 90 percent. Tests, however, showed little effect on the spores from the transient cavitation in plain water, or from cavitation at standard atmospheric pressure.
"You can kill microorganisms in just water by hitting them with ultrasonic waves, but it might take days. Or, you may not kill them at all," Cunefare adds. "But when you combine isopropyl alcohol, elevated pressure and cavitation, you can kill them quickly. Any one method by itself will not be very effective."
As part of their research, Carter and Cunefare blasted a test vial, which contained bacterial spores in the alcohol solution, with cavitation for 10 to 15 minutes. When the power was applied, the test vial filled with foam, which subsided when the energy was cut. Yet, cavitation remained active for up to 60 seconds.
Carter began batting around the idea of transient cavitation to clean the same instruments he uses to keep his patients smiling. Once he figured out that rapid decompression using cavitation kills microbes by shattering cell walls, he obtained a patent for the method in 1994.
"It killed vegetative forms of bacteria, but it would not kill spores," he says. "I wondered what would happen if you used pressure in conjunction with ultrasonic energy, and that was a threshold those germs just can't take."
 An experimental set-up shows the test chamber and measurement equipment used to study a new acoustic technique for disinfecting medical equipment. |
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Carter obtained a patent in 1997 for the method that combined pressure with ultrasonic energy, yet it still couldn't tackle the most resistant microbes. That's when he hooked up with Cunefare, who says Carter was on the right track but needed to increase the amount of energy and change the amount of pressure to maximize the effect.
Last October, Cunefare and Carter obtained the patent for the "apparatus and associated method for decontaminating contaminated matter with ultrasonic transient cavitation."
Carter says Cunefare "helped me develop a concept tester, and the method worked even better. So, we could now assume that the volatility of the liquid enhances the bubble to more optimal sizes so the collapse of the bubble from the sound is greater, which increases the cavitation."
While the method and apparatus could yield innovations in combating biofilms in cleanrooms, food processing and public wastewater treatment, Cunefare says the mechanism that combines the cavitation, pressure and alcohol remains under development.
"We do need to optimize the chemical and energy delivery," he says. "We also need to test for mechanical impact on typical medical devices. Cavitation is an ablative process; it blasts pits into things, so it can damage the instrument and dull sharp edges. The development entails a lot of testing and upgrading the electronics on the test assembly."
To do that, the project needs funding, and Carter and Cunefare are seeking support from National Institutes of Health to develop a better understanding of transient cavitation parameters, as well as optimize the effects and explore other additives to enhance it.
"We are dealing with an area that combines disciplines—physics, microbiology and the needs of the marketplace," Carter says. "It's been hard to find funding, but there's no reason not to do this on a larger scale. It can be done."