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Correct measurement of the shell properties of coated microbubbles (MBs) isessential to understanding and optimizing their response to ultrasound (US)exposure parameters in diagnostic and therapeutic ultrasound. MBs aresurrounded by blood; however, the influence of the surrounding medium chargeson the MB properties is poorly understood. This study aims to measure themedium charge interactions with MB shells by measuring the frequency-dependentattenuation of the same size MBs in mediums of varying charge density. In-houselipid-coated MBs with C3F8 gas core were made and were isolated to a mean sizeof 2.35um. MBs were diluted to ~8*10^5 MBs/mL in distilled water (DW), and twodifferent concentrations of phosphate-buffered saline solution (PBS-1x andPBS-10x). The frequency-dependent attenuation of the MBs solutions was measuredusing an aligned pair of PVDF transducers with a center frequency of 10MHz and100% bandwidth. The MB shell properties were estimated by fitting the linearequation to experiments. Using a pendant drop tensiometer, the surface tensionof mm-size drops was measured inside DW, PBS-1x and PBS-10x. The frequency ofthe peak attenuation changes at different salinity levels was 13, 7.5 and6.25MHz in DW, PBS-1x and PBS-10x, respectively. The attenuation peak increasedby ~140% with increasing ion density. MBs' estimated shell elasticity decreasedby 64% between DW and PBS-1x and 36% between PBS-1x and PBS-10x. Reduction inthe shell stiffness is in qualitative agreement with the drop surface tensionmeasurements. The shell viscosity was reduced by ~40% between DW and PBS-1x and42% between PBS-1x and PBS-10x. The reduction in the stiffness and viscosity ispossibly due to the formation of a densely charged layer around the shell,further reducing the effective surface tension on the MBs.

Shell elasticity and viscosity of lipid-coated microbubbles are significantly altered in mediums of different ionic strength