For Immediate Release
Preliminary studies by scientists at the Georgia Institute of Technology
and Georgia State University showed
the technique killed more than 90 percent of bacteria in a test vial that
also contained a mild solution of isopropyl alcohol. Results of the work
were presented December 5 at the First Pan-American/Iberian Meeting on
Acoustics in Cancun, Mexico.
"Complex and extremely expensive endoscopes and related surgical
equipment are very vulnerable to heat, and they are challenging to clean,"
explained Dr. Stephen Carter, an Atlanta-area dentist who is working with
Georgia Tech Professor Kenneth
Cunefare to develop the technique. "We believe that our methods
will sterilize in shorter periods of time, which would be a substantial
advantage for expensive medical equipment."
The patented technique uses a form of cavitation, a phenomenon in which
acoustic energy applied to a liquid induces the creation of voids -- or
bubbles -- that release energy when they collapse. By pressurizing their
test chamber while inducing cavitation, Cunefare and Carter create a form
of transient cavitation that causes violent collapse of the bubbles.
The enhanced cavitation takes advantage of the "anomalous depth
effect," in which the impact of bubble collapse increases dramatically
when subjected to roughly twice normal atmospheric pressure. Scientists
have studied the phenomenon for years because it can damage submarines'
propellers when operating at certain depths.
When applied to a solution of 66 percent isopropyl alcohol containing
two forms of "marker" bacterial spores -- Bacillus stearothermophilus
and Bacillus subtilis -- the enhanced cavitation reduced the bacterial
count by more than 90 percent, Cunefare said. Research indicates that
both the alcohol solution and increased pressure are necessary for killing
the spores with cavitation.
Tests showed little effect on the spores from the transient cavitation
in plain water, or from cavitation of the alcohol solution at standard
As part of their evaluation, researchers subjected a test vial of fluid
containing bacterial spores in the alcohol solution to short bursts of
cavitation over a period of 10-15 minutes. When the power was applied,
the test vial appeared to be filled with foam that subsided when the power
was switched off, Cunefare said. The cavitation was active for up to 60
seconds of the test period.
Because acoustic disinfection could be carried out more quickly than
existing heat and chemical techniques, Carter believes it could offer
significant cost advantages by reducing the amount of time that expensive
equipment is out of service. And it would also have the potential for
minimizing the risk of cross transmission of infection caused by contaminated
instruments, he added.
The idea for using transient cavitation to disinfect instruments originated
with Carter, who had been interested in a new approach for sterilizing
the growing number of instruments that are vulnerable to damage from traditional
heat disinfection. He reasoned that rapid decompression might be able
to kill microbes by breaking their cell walls, and obtained a patent for
the idea in 1994.
Subsequent testing, however, showed that even "explosive decompression"
failed to kill the hardiest of bacterial spores, so Carter sought to enhance
the technique by combining pressure with powerful cycles of ultrasonic
energy. Though he obtained a patent for that approach in 1997, it still
was unable to kill the toughest of microbes. Undaunted, he approached
Georgia Tech for help.
A professor in the School of Mechanical
Engineering and a specialist in acoustics, Cunefare determined that
Carter was on the right track, but needed to increase the amount of ultrasonic
energy and change the pressure to optimize the effects of cavitation.
Working together, Carter and Cunefare selected the right combination of
energy, pressure and alcohol content to dramatically reduce the number
of bacterial spores.
The mechanism by which the cavitation, pressure and alcohol combine to
kill bacteria remains under study.
"We don't know exactly how the cells die, but we know the end phenomenon,"
Ahearn, professor emeritus of biology at Georgia State University.
"Increased pressure and disinfectant molecules are somehow enhanced
by the cavitation process, but the physiology of the death has yet to
Ultrasound has been used elsewhere to make skin permeable enough to admit
drug compounds. Cunefare suspects that the cavitation may induce a similar
effect, making the bacterial cell walls permeable enough to admit the
Though the researchers studied only the technique's effect on bacteria, Ahearn -- who did the biological assays for the study -- expects it would also work against viral organisms that can also be troublesome.
The researchers are now seeking support from the National Institutes
of Health to optimize the technique, scale it up to a practical size,
ensure that it would adequately kill the microorganisms -- and assess
the potential for damaging medical instruments.
"We are seeking funding to further develop our understanding of
the parameters that affect the transient cavitation," Cunefare explained.
"We want to optimize the effects, and explore other additives that
might enhance it."
The researchers will also have to improve techniques for coupling power
into the fluid in order to treat larger volumes of liquid. Since 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. They also need new power electronics and transducers
that can operate continuously.
Beyond sterilization of medical instruments, Cunefare also sees potential
applications in the continuous treatment of water and wastewater, and
potentially in low-temperature pasteurization of food products such as
milk or orange juice.
In October 2002, Carter and Cunefare received a U.S. patent, "Apparatus and Associated Method for Decontaminating Contaminated Matter with Ultrasonic Transient Cavitation." They hope to interest a medical equipment manufacturer in commercializing the technique.
RESEARCH NEWS & PUBLICATIONS OFFICE
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TECHNICAL CONTACT: Kenneth Cunefare (404-894-4726); E-mail: (email@example.com); Fax: (404-894-7790).
WRITER: John Toon