Ultrasound, applied in combination with microbubbles, has potential as a means to enhance the uptake of therapeutic agents, which could include drugs and nucleic acids, into biological cells. This process is commonly referred to as 'sonoporation', and the enhanced uptake can be caused through the incident ultrasonic pressure fi eld causing radial oscillations (cavitation) in the microbubbles, amongst other possibilities. However, the mechanisms responsible for any resultant increase in cell membrane permeability are not yet fully understood. This project focussed on achieving a more fundamental understanding of these salient processes by building on a platform of previous work within the group. One strand of the project involved a complete characterisation of the performance of a rotating mirror high speed camera (Cordin 550-62) that was previously used by our group [and others] to investigate microbubble cavitation phenomena and interactions with proximal cell membranes. Speci cally, I present herein an investigation into the image formation process with this type of camera, the essence of which stymied previous data interpretations. I demonstrate that an inherent asynchrony in the exposure of pixels within individual image frames leads to a temporal anomaly. This was achieved using low cost, flashing LED lights and resulted in the extraction of an algorithm to correct for the temporal anomaly. In a slightly diff erent context, the delivery of suitable ultrasonic fields is necessary to achieve a uniform treatment across a therapeutic target. This thesis also reports on a study on the design of ultrasonic lenses to alter the focal region of a focussed ultrasound transducer with the aim of producing focal regions that can enable sonoporation of tumours of varying sizes. We show that the use of lenses can be an inexpensive alternative to more complex systems such as phased array transducers. Design modelling and experimental testing of lens prototypes are presented along with preliminary results with tissue mimicking polyacrylamide gel phantoms. The target environment in which the process of sonoporation will be clinically useful (i.e. in the physiological circulation) can be simpli ed as a microfluidic system. One strategy for bubble mediated therapy involves the use of a pro-drug approach, that is, when two otherwise benign ingredients are loaded onto separate microbubble populations, but can become mixed at the anatomical target site by the action of focussed ultrasound whereupon a potent drug is produced. The required mixing can be achieved by the violent coalescence of nearby cavitating bubbles, their reaction product then being released and di ffused into the interiour of nearby cells through sonoporation. A study related to this field is presented here where laser induced thermocapillary flows are shown to cause mixing of the content of a drop in a microfluidic channel in a bid to understand the mixing process at a level that may assist future microbubble engineering strategy. To summarise then, the work presented in this thesis has consolidated earlier unpublished data sets achieved by the group, providing new and exacting experimental evidence and an accurate algorithm that will facilitate post-processing of that earlier data (Chapters 2-3). Moreover, group aspirations to translate earlier in-vitro work on sonoporation towards next phase medical-phantom exposures have been boosted through the provision of a new direction involving acoustic lensing, the experimental data from which was used to completely validate existing models for our own design scenarios (Chapter 4). Finally, previous unpublished observations on microbubble coalescence undertaken by the group suggested a means to implement pro-drug delivery with direct in-situ mixing. Such suggestions were explored within microfluidic contexts using lasers to control and visualise the mixing processes that might arise in such situations (Chapter 5). All of these new insights have served to consolidate the group's previous and as yet unpublished data, opening the way for dissemination with confidence in the integrity of that data, and have also extended group capability and expertise in the areas of MHz-rate high speed framing cameras, the fabrication of acoustic lenses, and with microfluidic mixing.
|Date of Award
|Engineering and Physical Sciences Research Council
|Paul Campbell (Supervisor)
- High speed imaging
- Thermocapillary flow pattern