Low-frequency ultrasound triggers drug release from perfluorocarbon droplets at low intensities

Perfluorocarbon droplets represent a promising platform for ultrasound-triggered drug delivery. Their liquid core can vaporize upon the application of a pressure wave such as ultrasound, resulting in the controlled and targeted release of their drug cargo. These carriers are highly tunable, exhibit a sharp on-off behavior and have longer lifetimes compared to ultrasound-responsive microbubbles. However, despite increasing efforts in the field, achieving an acceptable balance between stability and a safe activation threshold remains challenging. Indeed, stable droplet formulations are usually activated at very high ultrasound pressures, exceeding 1 MPa, raising safety concerns. In this study, we investigated the response of stable surfactant-shelled perfluoropentane droplets to low-intensity pulsed ultrasound. We demonstrated that a significant drug release can be triggered at a pressure as low as 100 kPa using a very low frequency (i.e., 38 kHz). Therefore, we focused on characterizing the behavior of this droplet formulation under 38 kHz ultrasound stimulation, optimizing a protocol able to achieve efficient drug release while adhering to safety guidelines, thus facilitating a future in vivo translation. We systematically examined how drug delivery efficiency varies with different stimulation parameters, including pressure, pulse repetition frequency and duty cycle. Additionally, high-speed camera imaging and ultrasound imaging were performed to further elucidate the response mechanisms of perfluorocarbon droplets to low-frequency ultrasound.