Fluid Viscosity Affects the Fragmentation and Inertial Cavitation Threshold of Lipid-Encapsulated Microbubbles

Area of Science:

• Acoustic Cavitation
• Biomedical Ultrasound
• Microbubble Dynamics

Background:

• Ultrasound and microbubble research for therapeutic applications typically uses low-viscosity fluids (1 cP).
• Physiological conditions involve microbubbles in blood, a higher viscosity fluid (approximately 4 cP).
• Understanding viscosity effects is crucial for translating in vitro findings to in vivo applications.

Purpose of the Study:

• To investigate the influence of fluid viscosity on microbubble behavior under ultrasound exposure.
• To compare microbubble fragmentation and acoustic emissions in 1 cP versus 4 cP fluids.
• To assess the impact of varying acoustic pressures on microbubble dynamics in different viscosities.

Main Methods:

• Employed ultrahigh-speed microscopy to observe individual microbubble (n=220) dynamics.
• Utilized passive cavitation detection to analyze acoustic emissions.
• Studied microbubbles at 1 MHz frequency under pressures ranging from 0.25 to 2 MPa in 1 cP and 4 cP fluids.

Main Results:

• Microbubble fragmentation significantly decreased in 4 cP fluid compared to 1 cP fluid, despite similar maximum radial expansions.
• Microbubble populations in 4 cP fluid showed reduced wideband emissions (up to 10.2 times) and more distinct harmonic peaks.
• Acoustic emissions, including ultraharmonics, became more pronounced with increasing pressure in the higher viscosity fluid.

Conclusions:

• Fluid viscosity is a critical parameter influencing microbubble behavior and acoustic responses during ultrasound therapy.
• In vitro ultrasound studies should incorporate physiologically relevant viscosities to better predict in vivo outcomes.
• Findings suggest that higher viscosity fluids may enhance the stability and acoustic characteristics of microbubbles for therapeutic use.