Acoustic characterization and modeling of poly-lactic acid-encapsulated contrast microbubbles

Abstract

Biodegradable polymers like polylactic acid hold potential for better stability and control over encapsulation properties of ultrasound contrast microbubbles. We report here several interesting acoustic properties of air-filled PLA shelled microbubbles through both in vitro experiments and mathematical modeling. Attenuation measurements with PLA microbubbles (average diameter 1.9 micrometer), indicated a resonance frequency of 2.5–3 MHz, which, in contrary to other encapsulated microbubbles, is lower than the resonance frequency of a free bubble of similar size. Pressure dependent scattering measurements at two different excitation frequencies (2.25 and 3 MHz) show strongly non-linear behavior with distinct second and subharmonic responses. Subharmonic responses are registered above a relatively low generation threshold of 100–150 kPa. To investigate the underlying mechanisms, we utilized several preexisting interfacial models describing encapsulated bubble dynamics. The attenuation data were utilized to determine the interfacial rheological properties of the encapsulation for each of these models. The model predictions are then compared with scattered nonlinear—sub- and second harmonic—responses. Our studies indicate that the extremely low surface elasticity (around 0.01 N/m) and reduced surface tension (0.01–0.03 N/m) along with the polydispersity of the bubble suspension play a critical role in determining the acoustic properties of PLA microbubbles.