Beyond total release: Fragment-resolved metrics for ultrasound microbubble-mediated drug delivery

Microbubble-assisted ultrasound has emerged as a pivotal technology for breaching biological barriers and achieving localized drug delivery. However, current evaluative strategies are largely confined to total release efficiency as a single metric, overlooking the physicochemical heterogeneity of products generated by microbubble cavitation. This oversimplification contributes to a translational gap in which high in vitro release does not reliably predict in vivo therapeutic outcomes. To address this limitation, this Review introduces the fragment species spectrum as a conceptual and practical framework. We classify three core fragment species produced under acoustic excitation: free drug (rapid diffusion and instantaneous peak concentrations), vesicular fragments (endocytosis-enabled retention and sustained exposure), and shell debris (contact-mediated interfacial effects that can modulate the local microenvironment). We then synthesize how shell formulation (lipid vs polymer), payload topology (surface-adsorbed vs embedded), and acoustic parameters (e.g., peak negative pressure, pulse duration) shape fragment distributions by modulating shell micromechanics and cavitation dynamics. By integrating evidence from loading-density comparisons (low vs high doxorubicin) and C6 glioma cell studies, we highlight a form-over-amount principle: in specific contexts, vesicular fragments produced under stable cavitation can outperform free drug produced under inertial cavitation despite lower total release. Finally, we propose a task-fragment matching strategy for inverse design of acoustic exposures and formulations to achieve application-specific fragment spectra (e.g., transient blood-brain barrier opening or deep tumor penetration), providing a mechanism-based route toward fragment-guided precision engineering.