Spatio-temporal evolution of an encapsulated microbubble dynamics during a low intensity focused acoustic vortex excitation

Although low-intensity focused acoustic vortex combined with thin lipid-encapsulated microbubbles shows great potential for targeted and efficient ultrasound therapies, the therapeutic effects remain limited due to the low drug loading efficiency and the stability of lipid microbubbles during low-intensity focused acoustic vortex insonation. To address these limitations, employing thick-shell microbubbles, such as those made from polymers or albumin, may be a viable alternative. One of the primary mechanisms underlying the efficacy of low-intensity focused acoustic vortex therapies combined with lipid microbubbles is the appropriate cavitation type of microbubbles, which are gathered by acoustic radiation force and lead to a series of beneficial mechanical effects on surrounding tissues, closely related to microbubble dynamics. This study investigates the spatiotemporal dynamics of polymer and albumin microbubbles to illustrate their potential applications in targeted ultrasound therapies. Utilizing Lagrangian formalism and Keller-Miksis equation, along with a modified Church model, a numerical model to examine the dynamics of encapsulated microbubbles under low-intensity focused acoustic vortex excitation is developed. The results indicate that, within the parameters examined, the oscillations of both polymer and albumin microbubbles vary as the number of ultrasound cycles increases, influenced by their translational motions, which significantly differ from those observed in traditional ultrasound. During focused acoustic vortex insonation, microbubbles are more likely to move toward the vortex center when the initial radial distance is smaller or the topological charge is larger. Consequently, the variations in microbubble oscillations are determined by both the initial radial distance and the topological charge. This study illustrates that the microbubble excited by the focused acoustic vortex would aggregate to the focal area and oscillate to generate mechanical effects, which might contribute to improving the efficacy of ultrasound therapies.