Using Sound to Sterilize Medical Instruments

Originally Published MDDI February 2003 R&D DIGEST

Originally Published MDDI February 2003

R&D DIGEST

Cunefare's test chamber has demonstrated that enhanced transient cavitation can kill bacterial spores. 

Cavitation, an acoustic phenomenon often studied for its effects on submarines, could be the basis for an improved disinfection method. Researchers at Georgia Institute of Technology (GIT, Atlanta) and Georgia State University (Atlanta) believe the patented technique may be able to quickly kill microorganisms on medical instruments without using heat or harsh chemicals. Conventional heat treatments can damage costly devices, such as endoscopes.

Cavitation occurs when acoustic energy applied to a liquid induces the creation of voids, or bubbles, that release energy when they collapse. The phenomenon has been studied for years because it can damage submarines' propellers when they are operating at certain depths. 

Stephen Carter, DDS, an Atlanta-area dentist, proposed using an enhanced form of cavitation to disinfect instruments and obtained a patent for the idea in 1994. He is now working with Kenneth Cunefare, PhD, associate professor at the GIT School of Mechanical Engineering, to develop the technique. 

Carter first proposed that rapid decompression might be able to kill microbes by breaking their cell walls. Even explosive decompression, however, failed to kill all of the bacterial spores. He then suggested enhancing the technique by combining pressure with powerful cycles of ultrasonic energy. The researchers pressurize the test chamber while inducing cavitation, creating a form of transient cavitation that causes violent collapse of the bubbles. The method takes advantage of the “anomalous depth effect,” in which the impact of bubble collapse increases dramatically when subjected to roughly twice normal atmospheric pressure.

When applied to a solution of 66% isopropyl alcohol containing two strains of bacterial spores as markers, the enhanced cavitation reduced the bacterial count by more than 90%, says Cunefare. Both the alcohol and the increase in pressure were found to be necessary to kill the spores with cavitation.

Subsequent studies suggest that acoustic disinfection can be carried out more quickly than existing heat and chemical techniques. The researchers believe this could offer a number of advantages. “We believe that our methods will sterilize in shorter periods of time, which would be a substantial advantage for expensive medical equipment,” says Carter. In addition to reducing the amount of time that expensive equipment is out of service, the method could also minimize the risk of cross-transmission of infection caused by contaminated instruments, he adds. 

The actual mechanism by which the method works will be the focus of further study. Says Donald Ahearn, PhD, professor emeritus of biology at Georgia State University, “We don't know exactly how the cells die, but we know the end phenomenon.” He adds, “Increased pressure and disinfectant molecules are somehow enhanced by the cavitation process, but the physiology of the death has yet to be determined.” Ahearn performed the biological assays during the study. 

Cunfare explains that similar ultrasound methods have been used to make the skin sufficiently permeable to admit drug cmpounds. The reseacher speculates that the cavitation technique may induce a similar effect that makes bacterial cell walls permeable enough to admit the alcohol molecules. In addition, Ahearn believes the method will work against viral organisms.
 
The researchers are seeking support from the National Institutes of Health to optimize the technique and assess the effectiveness of other additives. They also hope to scale up the method to a practical size, ensure that it will adequately kill microorganisms, and assess potential for damaging medical instruments. 

The researchers also plan to improve the techniques being used to couple power into the fluid in order to treat larger liquid volumes. They beleive that because the amount of energy that can be induced into a liquid depends on the surface area, there may be limits to the volume that can be treated by inducing energy from the boundaries.

Copyright ©2003 Medical Device & Diagnostic Industry

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