Influence of shell-thickness-to-radius ratio on buckling and rupture of polymer-shelled ultrasound contrast agents

This study investigated the behavior of polymer-shelled ultrasound contrast agents (UCAs) of different shell-thickness-to-radius ratios (STRRs) when subjected to static overpressure. Three varieties of polylactide shelled UCAs with STRRs of 7.5, 40 and 100 nm/μm were studied. A custom-designed pressure chamber was employed to observe polymer-shelled UCAs through a video microscope. A normalized cross-sectional area of individual UCAs was obtained from the binarized UCA images. Integration of the normalized cross-sectional area over time, defined as Buckling factor (Bf), provided a dimensionless parameter to compare the differences between the buckling behaviors of UCAs with different STRRs. The UCAs with an STRR of 7.5 nm/μm exhibited a distinct buckling behavior prior to shell rupture (Bf<;1), whereas the UCAs with higher STRRs (40 and 100 nm/μm) did not undergo significant pre-rupture buckling (Bf≈1). The observed behavior was in agreement with a recently reported mathematical model and qualitatively consistent with the experimentally observed behavior of UCAs subjected to ultrasound.