M.Sc Thesis

M.Sc StudentZeltser Dekel Carmel
SubjectDecompression and Ultrasound Effects on Cell Membrane
and the Intramembrane Cavitation Model
DepartmentDepartment of Biomedical Engineering
Supervisor PROFESSOR EMERITUS Eitan Kimmel
Full Thesis textFull thesis text - English Version


Ultrasound is a highly versatile and powerful tool in both therapeutic and diagnostic capacities. It is the phenomenon of acoustic pressure waves advancing in matter at a frequency higher than that of the human audible sound frequencies. In this work, a possible mechanism of gas bubbles interactions with the acoustic field (cavitation), is explored. A better understanding of the cavitation mechanism could help in developing controlled ultrasound-assisted applications in addition to better avoid possible adverse effects.

The Bilayer Sonophore model (BLS model) suggests intramembrane cavitation as a unifying mechanism for US induced cavitation related bioeffects. It hypothesizes that the hydrophobic intramembrane space between the two bilayer leaflets of membranes is capable of inflating and deflating under an ultrasonic field thus creating cavitation nuclei of gas.

Decompression sickness (DCS) in divers was explored as a possible physiological example associated with BLS mechanism.  During a dive the pressure around the body increases with sea depth and in accordance with Henry's law the tissues becomes saturated with gas. Upon rapid ascent (decompression) and in accordance with Boyle's law it can form bubbles in tissue and blood stream and DCS occurs. In this work, it was hypothesized that the DCS mechanism is similar to that suggested in the BLS model as both manifest in the form of bubble formation and require relatively low pressure decrease, and that in fact, bubbles in DCS form in the intramembrane space.

The specific and combined effects of both DC and US on bovine aortic endothelial cells (BAEC) were examined in three experimental set ups. In all experiments a custom made pressure chamber was used. To examine DC effect, the BAEC were labeled with Octadecyl Rhodamine, a self-quenching fluorescent membrane tracer. It was hypothesized that if indeed the membrane inflates during decompression, the fluorescent signal will increase as a result of the self-quenching function. To examine US effect, the backscatter from an US pulse applied to the sample using a focused US transducer was measured over time. The combined effect of DC and US was examined by applying pressure to the sample while still measuring the backscatter. The governing hypothesis in the use of backscattering was that as more bubbles are formed, a higher backscatter signal will be received and that upon pressure application, backscatter will decrease.

The results for the fluorescent DC experiment were inconclusive, with a slight indication of signal increase after DC. In the US backscatter an increase of the signal over time for BAEC, with a decrease of the signal for control experiments using medium was observed. This was found to be statistically significant and is believed to be the result of sonophore formation. The combined case showed increase of the signal during compression, contrary to the hypothesis. It is believed that increased gas diffusion during the compression turned out to be the dominant mechanism, providing more gas for sonophore formation. To conclude, it is believed that the methods used show much promise upon further refinement.