Cavitation and shock wave effects on biological systems

Abstract

This thesis investigates the effects of cavitation and shock waves on systems of biological or medical relevance. Its main focus lies on high-speed optical observations of cavitation bubble activity after pressure wave excitation and the resulting effects on cells, which comprise cell damage, transient membrane permeabilization, and the detachment of cells adhering to a substrate.In the experimental part of this work different methods have been used to create and excite bubbles. In a first approach preexisting bubbles in the vicinity of rigid boundaries are excited by a laser induced shock wave. The effects of neighboring bubbles and rigid boundaries on bubble dynamics are observed with microscopic high-speed recordings. The bubble dynamics is calculated numerically using the Keller-Miksis model and results are compared with the experimental radius time curves. Also the dynamics of bubbles in the vicinity of cultured fibroblast cells are investigated. These Bubbles are formed from contrast agents for medical ultrasound imaging after spark-induced pressure wave application. Microscopic high-speed images show rapid bubble expansion and collapse. Even at moderate peak negative pressure amplitudes of less than 1 MPa the contrast agent bubbles are observed to expand to more than 30 times their original radius and to rupture cells upon collapse. The bubble dynamics of excited contrast agent bubbles show reasonable agreement with a spherical bubble model during the first oscillation cycle.The destructive behavior of pressure excited bubbles might also be important in ballistic injuries. In this context the effects of high-speed projectiles on tissue substitutes have been investigated. The impact of the bullet generates a shock wave, which travels through the medium. Upon reflection at pressure release boundaries, the shock wave becomes a tensile wave generating cavitation clouds in its wake. The cavitation bubbles might cause secondary damage inside the tissue farther away from the bullet and its fragments.The last experiment deals with the interactions of lithotripter generated shock waves with cancer cells adhering to a substrate. It is shown that shock waves permeabilize adherent cells in vitro through the action of cavitation bubbles. The bubbles are formed in the trailing tensile pulse of a lithotripter generated shock wave. Upon collapse of cavitation bubbles a strong flow field is generated, which accounts for two effects: The detachment of cells from the substrate and the temporary opening of cell membranes followed by molecular uptake.