Acoustic streaming in soft tissue-mimicking materials

Abstract

Acoustic streaming - the displacement of fluid by sound - has been proposed as the mechanism for therapeutic effects, such as drug distribution enhancement and neurostimulation, however, there have been no direct observation or characterisation of this effect in soft tissue microenvironments, making it difficult to optimise and control. Post-mortem and indirect analyses of ultrasound-exposed tissue have revealed changes in drug or fluid distribution, and neuronal excitation; but it has remained uncertain whether these biological outcomes were due to acoustic streaming or other responses, such as acoustic cavitation. An aim of this thesis was to be the first to directly observe ultrasound-induced streaming during sonication in a tissue-mimicking material. Initially, conventional tissue-mimicking materials were used for ultrasound imaging and it was concluded that these tissue substitutes were not suitable to demonstrate acoustic streaming due to their microstructure. A material was then developed (macroporous polyacrylamide) that mimicked the porous structure of tissue (interconnected pores) and a dye and a video camera were used to track fluid movement. When applied above an acoustic intensity threshold, a continuous focused ultrasound beam was shown to push the dye axially i.e. in the direction of wave propagation and in the radial direction. Dye displacement or clearance increased with ultrasound intensity and was modelled using an adapted version of Eckart’s acoustic streaming velocity equation. It was shown that the resulting dye clearance was due to the ultrasound and not due to thermal diffusion. No mechanical damage or microstructural changes were observed in the sonicated region when assessed using scanning electron microscopy. No thermal damage was observed when assessed using a needle thermocouple. This study showed that acoustic streaming can occur in soft porous materials and provides a mechanistic basis for future technologies using streaming for therapeutic or diagnostic purposes.