Wayne Kreider Wayne Kreider's research has contributed important new understanding to the fundamental science of bubbles subjected to high-amplitude acoustic fields. Shock waves—sometimes greater than 1000 atmospheres of pressure—are used in lithotripsy, the most common treatment for kidney stones, and to treat a range of orthopedic injuries. High intensity focused ultrasound (HIFU) is used to treat uterine fibroids and holds promise for a range of conditions. Therapies rely on the mechanical disruption of tissue by the production and excitation of gas bubbles and heating of the tissue by absorption of the acoustic wave. Wayne was one of the first researchers to recognize that these two effects must be understood in concert, not separately. Wayne notes, "When I began this work, the paradigm in the HIFU research community was to consider bubbles containing only non-condensable gases rather than bubbles with both non-condensable gases and vapor. During HIFU treatment tissue is heated and eventually water in the tissue reaches boiling temperatures at which point the bubbles would be predominantly vaporous. My idea was to write a numerical model to better understand how a bubble is affected by both large acoustic pressures as well as elevated temperatures." The most basic effect of vapor trapping is that when a bubble collapses violently, the vapor cushions the collapse so that more energy is conserved and thus available as the bubble rebounds. Wayne's model of gas-vapor bubbles accounts for liquid compressibility, heat transfer, vapor transport, vapor trapping by noncondensable gases, diffusion of noncondensable gases, and heating of the liquid at the bubble wall. Beyond writing the code to simulate the physical processes, he knew that he would have to set up an experiment to quantify the phenomena. Wayne envisioned experiments run with a lithotripter, water tank, high-speed camera, and cavitation detectors – all pieces of equipment available in his advisors' laboratory. It would be critical, however, to control the percentage of dissolved gas in the water tank. Putting mechanical engineering and plumbing skills to use, he designed and built a water filter system that could de-gas regular tap water to precise values, plus heat the water to various temperatures (human body temperature and beyond). The water was also kept in circulation between the experimental tank and the filtration system to maintain dissolved gas and temperature constants during the trials. His efforts were doubly successful. "The observed trends in the bubble rebounds were supported by the model predictions, and using my experimental data in conjunction with the physical understanding supplied by the model, we can now use a more physically realistic model to investigate cavitation behavior in the acoustic and thermal fields characteristic of therapeutic ultrasound," says Wayne. The fundamental physics of Wayne's theoretical and experimental work shows the dominant effect of vapor trapping in bubbles excited acoustically. His co-workers at APL-UW are now using the appearance of boiling in tissue, which is easy to detect, as an exact measure of the acoustic exposure and heating in the body, both of which are hard to measure but important to know. Wayne's discovery has led to collaborative work on cavitation clouds (his model shows that faster shock wave rates in lithotripsy are less effective because too many bubbles are created, reducing pressures exerted on the kidney stone), and to hypotheses concerning sonar effects on deep-diving whales, whose blood and tissues become supersaturated with dissolved gases.