Coated microbubbles exploit shell buckling to swim

Abstract Cleverly engineered microswimmers show great promise in various biomedical applications. Current realizations are generally either too slow, hardly manoeuvrable, or non biocompatible. To overcome these limitations, we consider lipid coated microbubbles that are already approved for clinical use as diagnostic ultrasound contrast agents. Through a combination of experiments and numerical simulations, we investigate the swimming motion of these microbubbles under external cyclic overpressure. Reproducible and non-destructive cycles of deflation and re-inflation of the microbubble generate a net displacement, through hysteretic buckling events. Our model predicts swimming speeds of the order of m/s, which falls in the range of blood flow velocity in large vessels. Unlike the acoustic radiation force technique, where the displacement is always directed along the axis of ultrasound propagation, here, the direction of propulsion is controlled in the shell reference frame. This provides a solution toward controlled steering in ultrasound molecular imaging and drug delivery