Theoretical and experimental investigation of the nonlinear dynamics of nanobubbles excited at clinically relevant ultrasound frequencies and pressures: the role of lipid shell buckling

The usage of microbubbles (MBs) is limited to the blood pool due to their large size yet the detection of biomarkers on tumor cells and effective drug deliveiy, require MBs to reach the tumor tissue outside of the vasculature. To tackle these problems, nanobubbles (NBs) are proposed as a potential alternative. NBs can pass through submicron blood vessels and extravasate to tissue. Due to their higher number density, higher doses of NBs can be delivered to the target. However, despite their potential, the use of NBs has been limited because of the limited information of their complex dynamics In this work, we manufactured lipid and surfactant stabilized C3F8 NBs (mean diameter similar to 200 nin). NB scattering response was investigated by single bubble scattering experiments with narrow band pulses with 16-55 MHz and acoustic pressure of 0.250-1.5 MPa (Vevo-770 Machine, Fujifilm visualsonics), and in-vivo imaging at 18 MHz and 4% power (Vevo 3100, Fujifilmvisualsonics). The nonlinear response of the NBs was numerically studied by solving the Marmottant model for the US pulses used in the experiments. The results were visualized using the resonance curves and bifurcation diagrams of the oscillations of the NBs versus frequency and pressure. Experimental results demonstrate strong echogenicity of NBs at a frequency range of 10-25 MHz, Single NB experiments suggest that NBs generate strong subharmonic and super harmonic responses even at lower acoustic pressures similar to 250 kPa. This contradicts the linear theoretical predictions, as the resonance frequency (fr) of the NBs is calculated to be similar to 130 MHz. Results of numerical simulations show that when the initial surface tension of the NBs is similar to<0.01 N/m, the fr of the NBs rapidly decreases as the acoustic pressure increases. Thus. NBs become active at frequencies below 50 MHz due to the nonlinear behavior of the lipid shell. Bifurcation diagrams confirmed the generation of subharmonics and super harmonics only for NBs which are initially close to the buckling state.