Microscale interactions between ultrasound stimulated microbubbles and the fibrin networks of clots

Abstract

While the concept of microbubble mediated sonothrombolysis is now well established, a detailed mechanistic understanding of this process remains both elusive as well as necessary in order to facilitate the development of improved exposure methods. As lytic effects may ultimately arise from microscale bubble-clot interactions, we have employed high-speed microscopy and two-photon microscopy to examine these interactions first in (transparent, fluorescently tagged) fibrin clots, and then in blood clots. Bubble “population” studies in fibrin clots show the prominent role of primary and secondary radiation forces: bubbles are first directed toward the clot boundary, where their concentrations increase and interactions such as clustering and coalescence occur frequently. A subset of bubbles penetrate into the clots, disrupting the fibrin network structure along their paths. Once initiated, the resulting tunnels act in subsequent exposures as conduits for bubbles to enter and access deeper points within the clot. Using optical tweezers, individual bubble experiments reveal that bubble entry into the clots, along with the accompanying network damage and fluid uptake, are a function of the network pore size, bubble size, and the exposure scheme. With blood clots, the erosion surface evolves in a complex manner, involving the ejection of erythrocytes and the development and progression of a cell depleted fibrin network zone. The characteristics of the erosion front are highly dependent on exposure conditions.