Characterisation of microbubble-membrane interactions in ultrasound mediated drug delivery

Cancer imposes a significant disease burden and constitutes a major societal challenge. Despite being widely used, chemotherapy suffers numerous disadvantages, including off- target drug toxicity, poor tumour penetration, and drug resistance. The use of ultrasound in combination with contrast agents has shown promise in enhancing outcomes in the treatment of cancerous and non-cancerous diseases. However, the underlying biophysical processes that underpin their interactions with tissues remain poorly understood. The aim of the research presented in this thesis is to develop methods to elucidate these processes. Development is hindered by the difficulties involved in isolating cellular parameters, noninvasively quantifying biological features at the molecular scale, and recreating a predictable and repeatable ultrasound field. To overcome these challenges, giant unilamellar vesicles made with 1,2-dioleoyl-sn-glycero-3-phosphocholine and cholesterol were used initially as cell models and exposed to therapeutically relevant conditions in a purpose-built acoustofluidic device. The resulting effects on the hydration and permeability of the vesicle membranes and the dynamics of their intravesicle milieu were characterised in situ using quantitative microscopy. Results show that flow, ultrasound, and microbubbles led to an increase in vesicle membrane hydration, while dehydration was seen only in the presence of microbubbles. An increase in permeability was observed for all exposure conditions, and was accentuated when microbubble shell fragments were incorporated in the vesicle bilayer. These findings thus indicate that ultrasound-mediated delivery is governed by a combination of physical and chemical mechanisms that influence the permeation of molecules through lipid membranes. Further, it was shown that exposure to the same conditions led to the onset of streaming flows within the vesicles’ lumen. An experimental proof-of-concept for extending the study in vitro was also presented. Actuating the motion of cytoplasmic constituents may potentially induce a broad range of underexplored and therapeutically relevant bioeffects.