Cavitation in Biological and Bioengineering Contexts

There are an increasing number of biological and bioengineering contexts in which cavitation is either utilized to create some desired effect or occurs as a byproduct of some other process. In this review an attempt will be made to describe a cross-section of these cavitation phenomena. In the byproduct category we describe some of the cavitation generated by head injuries and in artifical heart valves. In the utilization category we review the cavitation produced during lithotripsy and phacoemulsification. As an additional example we describe the nucleation suppression phenomena encountered in supersaturated oxygen solution injection. Virtually all of these cavitation and nucleation phenomena are critically dependent on the existence of nucleation sites. In most conventional engineering contexts, the prediction and control of nucleation sites is very uncertain even when dealing with a simple liquid like water. In complex biological fluids, there is a much greater dearth of information. Moreover, all these biological contexts seem to involve transient, unsteady cavitation. Consequently they involve the difficult issue of the statistical coincidence of nucleation sites and transient low pressures. The unsteady, transient nature of the phenomena means that one must be aware of the role of system dynamics in vivo and in vitro. For example, the artificial heart valve problem clearly demonstrates the importance of structural flexibility in determining cavitation occurrence and cavitation damage. Other system issues are very important in the design of in vitro system for the study of cavitation consequences. Another common feature of these phenomena is that often the cavitation occurs in the form of a cloud of bubbles and thus involves bubble interactions and bubble cloud phenomena. In this review we summarize these issues and some of the other characteristics of biological cavitation phenomena. INTRODUCTION One of the first people to recognize the occurrence of cavitation in living organisms was E.N.Harvey whose work eventually encompassed many aspects of bubble formation in plants and in animals. His considerations of the state of tension in the sap of trees led him to important deliberations on the ability of a liquid to sustain tension and, consequently, on a model (''the Harvey nucleus'') that he used to explain the existence of stable cavitation nuclei in a liquid [1-5]. This model was an important precursor to our current understanding of the existence of stabilized nuclei in the cracks and interstices of solid surfaces in contact with a liquid. Harvey went on to an erudite and broad ranging research career, that included investigations of the existence of cavitation nuclei in blood and in animal tissue [1,2], studies of the ''bends'' or bubble formation during decompression [6], and investigations of cavitation during wounding by high velocity missiles [7] as well as a host of studies of bioluminescence. He has been called the “Dean of Bioluminescence” but could also be identified as the father of cavitation studies in the biological environment. Today, we recognize that the processes that involve cavitation in a biological or bioengineering context are so wide-ranging that it would be impossible (and excessively tedious) to attempt a comprehensive review. Rather we will focus on the different types of cavitation that occur or are generated, attempt to find common themes and thereby suggest avenues of basic research that might help advance these applications. The cavitation that occurs in biological, bioengineering and biomedical contexts can be divided into that which is deliberately induced in order to generate some beneficial effect and that which occurs as an undesirable byproduct of some other procedure or device. We begin by reviewing the former. ULTRASOUND By far the commonest deliberate generation of cavitation in medicine is though the use of ultrasound. For a comprehensive recent review of the therapeutic effects of ultrasound the reader is referred to the excellent review by Bailey et al. [8]. Though the normal use of ultrasound is to emulsify unwanted tissue or to pulverize unwanted solid material, it is also beginning to be used for hemostasis (to stop bleeding in internal organs [9] such as the liver [10] and spleen [11]), for tumor necrosis [8] and for immunotherapy [8]. Two different tissue destruction techniques are used and are described in the following paragraphs. In some applications an ultrasonically vibrating probe is placed in close proximity to the tissue or solid material. The cavitation induced at the tip of this probe creates the desired effect when it is placed close to the