Real-time spatiotemporal characterization of mechanics and sonoporation of acoustic droplet vaporization in acoustically responsive scaffolds

Abstract

Phase-shift droplets provide a flexible and dynamic platform for therapeutic and diagnostic applications of ultrasound. The spatiotemporal response of phase-shift droplets to focused ultrasound, via the mechanism termed acoustic droplet vaporization (ADV), can generate a range of bioeffects. Although ADV has been used widely in theranostic applications, ADV-induced bioeffects are understudied. Here, we integrated ultra-high-speed microscopy, confocal microscopy, and focused ultrasound for real-time visualization of ADV-induced mechanics and sonoporation in fibrin-based, tissue-mimicking hydrogels. Three monodispersed phase-shift droplets—containing perfluoropentane (PFP), perfluorohexane (PFH), or perfluorooctane (PFO)—with an average radius of ∼6 μm were studied. Fibroblasts and tracer particles, co-encapsulated within the hydrogel, were used to quantify sonoporation and mechanics resulting from ADV, respectively. The maximum radial expansion, expansion velocity, induced strain, and displacement of tracer particles were significantly higher in fibrin gels containing PFP droplets compared to PFH or PFO. Additionally, cell membrane permeabilization significantly depended on the distance between the droplet and cell (d), decreasing rapidly with increasing d. Significant membrane permeabilization occurred when d was smaller than the maximum radius of expansion. Both ultra-high-speed and confocal images indicate a hyper-local region of influence by an ADV bubble, which correlated inversely with the bulk boiling point of the phase-shift droplets. The findings provide insight into developing optimal approaches for therapeutic applications of ADV.

Ultra-high-speed dynamics of acoustic droplet vaporization in soft biomaterials: Effects of viscoelasticity, frequency, and bulk boiling point

“…A radial strain of ∼ 2 was measured for a PFP bubble with a Λ max of ∼ 3.5 in 1 % fibrin hydrogels. The ADV-induced strains were hyperlocal and decreased significantly with distance from the bubbles [54] . Radial strain fields (logarithmic Hencky strain) were shown to decay as ∼ , where is the distance from the center of the laser-induced bubble [55] .…”

“…Larger Λ max in ADV-induced bubbles similarly leads to larger strains to the surrounding. Ultra-fast tracking of particle displacements induced by ADV indicated that radial strains (engineering strains) were largest at maximum bubble radius and correlated directly with Λ max [54] . A radial strain of ∼ 2 was measured for a PFP bubble with a Λ max of ∼ 3.5 in 1 % fibrin hydrogels.…”

“…The ratio of R max /R rest was 2.4, 2.2, 2.3, and 1.9 for ADV-generated bubbles at 2.5 MHz in fibrin hydrogels of 0.2 % (w/v), 1 % (w/v), 4 % (w/v), and 8 % (w/v) with PFP phase-shift droplets, respectively. Knowing R rest and R max can help in applications such as sonoporation where the distance of cells with respect to phase-shift droplets (d) can define cellular bioeffects such as irreversible sonoporation (d < R rest ), reversible sonoporation (R rest < d < R max ), or no sonoporation (d > R max ) [54] , [63] , [64] .…”