Bubble nucleation and dynamics in acoustic droplet vaporization: a review of concepts, applications, and new directions

Abstract

The development of phase-shift droplets has broadened the scope of ultrasound-based biomedical applications. When subjected to sufficient acoustic pressures, the perfluorocarbon phase in phase-shift droplets undergoes a phase-transition to a gaseous state. This phenomenon, termed acoustic droplet vaporization (ADV), has been the subject of substantial research over the last two decades with great progress made in design of phase-shift droplets, fundamental physics of bubble nucleation and dynamics, and applications. Here, we review experimental approaches, carried out via high-speed microscopy, as well as theoretical models that have been proposed to study the fundamental physics of ADV including vapor nucleation and ADV-induced bubble dynamics. In addition, we highlight new developments of ADV in tissue regeneration, which is a relatively recently exploited application. We conclude this review with future opportunities of ADV for advanced applications such as in situ microrheology and pressure estimation.

Keywords: Acoustic droplet vaporization; Bubble dynamics; High-speed microscopy; Microrheology; Phase-shift droplets; Ultrasound.

Figure 1

Capturing the dynamics of acoustic droplet vaporization (ADV) of a phase-shift droplet requires ultra-high-speed microscopy. ADV dynamics (A) have been typically studied via brightfield microscopy in a back illumination configuration using a high intensity flash lamp. The study of how ADV impacts payload release in a droplet with a double emulsion morphology (B), the perfluorocarbon (PFC) phase of the droplet (C), the shell of the droplet (D) as well as ADV-generated cellular bioeffects (E) are performed via fluorescence microscopy in a front illumination configuration using a high intensity pulsed or continuous laser. Brightfield and fluorescent images shown in A-E were taken by an ultra-high-speed camera (SIM802 model, Specialised Imaging Ltd. Pitstone, UK). Scale bar: 15 μm. All schematics in this article were created with BioRender.com.

Figure 2

Ultra-high-speed microscopy captured the dynamics of acoustic droplet vaporization for different phase-shift droplets before, during, and after ultrasound exposure: perfluoropentane (PFP), perfluorohexane (PFH), and perfluorooctane (PFO). Droplets were embedded in 10 mg/mL fibrin hydrogels. Four distinct ADV dynamics were observed: stable bubble formation without recondensation (PFP) (A), stable bubble formation with recondensation (PFH) (B), transient bubble formation with recondensation (PFO) (C), and fragmentation (PFO) (D). Acoustic parameters were as follows: a single burst of 6 µs, 2.5 MHz, and 6.5 MPa peak rarefactional pressure. An exposure time of 100 ns was used. Frames containing ultrasound are denoted with an asterisk. Scale bar: 15 µm. Panels A-D reprinted from Aliabouzar, Kripfgans , with permission from Elsevier.

Figure 3

Timeline displaying models developed to study acoustic droplet vaporization (ADV) of a phase-shift droplet as well as stability and growth dynamics of ADV-induced bubbles. Models are indicated by publication date and first author.

Figure 4

Acoustic droplet vaporization (ADV) enables dynamic modulation of release kinetics from phase-shift droplets with a double emulsion structure. Tuning the acoustic conditions (e.g., frequency) and thermophysical properties of droplets (e.g., bulk boiling point) can result in different bubble dynamics and resulting release kinetics (A). Droplets, loaded with Alexa Flour 488-labeled dextran (green), were embedded in a 5 mg/mL fibrin hydrogel that was labeled with Alexa Flour 647-labeled fibrinogen (red). Confocal microscopy images show that the droplet contained the dextran payload before the arrival of ultrasound (B, I). ADV-induced stable bubble formation completely released the payload for lower bulk boiling point droplets and lower excitation frequencies (B, II). For higher bulk boiling point droplets, repeated vaporization and recondensation partially released the payload (B, III). Other ADV dynamics such as asymmetrical collapse of the bubble, by positioning droplets near a rigid boundary, and fragmentation have been reported which result in complete release of the payload without stable bubble formation (B, IV). Scale bar: 5 µm. Panel (B, III) reprinted from Aliabouzar, Kripfgans , with permission from Elsevier.

Figure 5

Acoustic droplet vaporization (ADV) in a hydrogel matrix can significantly affect its microstructural and micromechanical properties. ADV-induced changes have been studied in fibrin hydrogels using confocal microscopy and atomic force microscopy (A). Comparing confocal images of a phase-shift droplets before (B) and after (C) ADV indicates local compaction at the bubble-fibrin interface, as seen by an increase in fluorescence intensity that longitudinally persisted (E). Both one- (F) and two-dimensional (G) mapping of Young’s moduli adjacent to the bubble were performed, showing ADV-induced stiffening proximal to the bubble-fibrin interface. Scale bar: 300 μm. Panels B, C, D, and G reprinted from Farrell, Aliabouzar , with permission from Elsevier. Panels E and F reprinted from Humphries, Aliabouzar with permission from John Wiley and Sons.

Figure 6

Matrix stiffening in fibrin-based acoustically responsive scaffolds (ARSs), induced by acoustic droplet vaporization (ADV), has been investigated in two separate biomedical applications. In one study, cell signaling was modulated in ARSs containing phase-shift droplets and triple negative breast cancer cells. Activities (i.e., cytoplasmic-to-nuclear ratio (CNR)) of extracellular signal-regulated kinase (ERK), one of the major kinases driving cancer progression, correlated inversely with distance from the ADV-induced bubble (A). In another study, the level of α-smooth muscle actin (α-SMA) expression was significantly elevated in human dermal fibroblasts proximal to bubbles compared to distal cells, irrespective of the addition of exogenous transforming growth factor-β1 (TGF-β1). Proximal and distal regions were defined as regions within 0.1 mm and greater than 0.5 mm from the bubble edge, respectively. Statistically significant differences (p < 0.05) are denoted by an asterisk (B). Panel A reprinted from Humphries, Aliabouzar , with permission John Wiley and Sons. Panel B reprinted from Farrell, Aliabouzar with permission from Elsevier.

Figure 7

Ultra-high-speed images of a perfluoropentane (PFP) phase-shift droplet before, during, and after acoustic droplet vaporization (ADV) in acoustically responsive scaffolds with fibrin concentrations of 5 mg/mL and 20 mg/mL (A). The effect of fibrin concentration on the radial dynamics of the PFP droplet was quantified in microsecond timescales (B) as well as 30 s post-ADV (C) for varying fibrin concentrations. Statistically significant differences (p < 0.05) are denoted as α vs. 5 mg/mL fibrin and β vs. 10 mg/mL fibrin. Acoustic parameters were as follows: a single burst of 6 µs, 2.5 MHz, and 6.5 MPa peak rarefactional pressure. Exposure time of 100 ns was used. Scale bar: 15 µm. Panels A-C reprinted from Aliabouzar, Kripfgans , with permission from Elsevier.

Figure 8

Potential application of acoustic droplet vaporization (ADV) as an in situ microrheometer or pressure sensor is schematically shown (A). Changes in the surrounding rheological properties may impact the oscillation amplitude or resonance frequency of ADV-generated bubbles (B). The reduction in the subharmonic response of ADV-generated bubbles as a function of ambient pressure may be used to monitor interstitial fluid pressure (C).