Multi-time scale characterization of acoustic droplet vaporization and payload release of phase-shift emulsions using high-speed microscopy

Exploring the Postactivation Behavioral Patterns of Intratumorally Injected Theranostic Nanodroplets: An Ultrasound-Only Extravascular Monitoring Technique

Phase-change nanodroplets (PCNDs) are customizable and controllable theranostic agents of particular interest in extravascular therapies such as drug delivery and histotripsy. High-bulk-boiling-point (HBP) PCNDs are preferred for their enhanced thermal stability under physiological temperature to achieve on-demand therapeutic effects on target sites-mainly in tumor tissue. However, the behavioral patterns of high-concentration, heterogeneously distributed HBP PCNDs in vivo have rarely been explored-the foci of PCND-related therapies mostly fall on the final therapeutic effect rather than the detailed behaviors of PCNDs, which may hamper the development and improvement of in vivo treatments with PCNDs. To fill the gap, we demonstrate an ultrasound-only extravascular monitoring technique to analyze the underlying behavioral patterns of intratumorally injected HBP PCNDs. In our hypothesis, recondensation and coalescence are the two predominant patterns influencing the trend of the postactivation signal of PCNDs. A "blink map" method was, thus, proposed to separate the two parts of the signal by recognizing the unique signal pattern of stochastic recondensation, and four derivative metrics were calculated for further analysis. The results revealed the postactivation patterns of PCNDs at different activation-pulse durations and activation stages throughout the activation-imaging period, and several general trends were observed and explained by existing theories, suggesting the feasibility of our extravascular monitoring technique. Overall, this work enriches the knowledge of the characteristics of HBP PCNDs as extravascular theranostic agents, and the monitoring results have the potential to provide timely feedback on PCND-related treatments underway, which may help adjust the treatment strategy and improve the therapeutic efficacy.

The radial dynamics and acoustic emissions of phase-shift droplets are impacted by mechanical properties of tissue-mimicking hydrogels

Acoustic droplet vaporization (ADV) offers a dynamic approach for generating bubbles on demand, presenting new possibilities in biomedical applications. Although ADV has been investigated in various biomedical applications, its potential in tissue characterization remains unexplored. Here, we investigated the effects of surrounding media on the radial dynamics and acoustic emissions of ADV bubbles using theoretical and experimental methodologies. For theoretical studies, bubble dynamics were combined with the Kelvin-Voigt material constitutive model, accounting for viscoelasticity of the media. The radial dynamics and acoustic emissions of the ADV-bubbles were recorded via ultra-high-speed microscopy and passive cavitation detection, respectively. Perfluoropentane phase-shift droplets were embedded in tissue-mimicking hydrogels of varying fibrin concentrations, representing different elastic moduli. Radial dynamics and the acoustic emissions, both temporal and spectral, of the ADV-bubbles depended significantly on fibrin elastic modulus. For example, an increase in fibrin elastic modulus from ≈0.2 kPa to ≈6 kPa reduced the maximum expansion radius of the ADV-bubbles by 50%. A similar increase in the elastic modulus significantly impacted both linear (e.g., fundamental) and nonlinear (e.g., subharmonic) acoustic responses of the ADV-bubbles, by up to 10 dB. The sensitivity of ADV to the surrounding media was dependent on acoustic parameters such as driving pressure and the droplets concentration. Further analysis of the acoustic emissions revealed distinct ADV signal characteristics, which were significantly influenced by the surrounding media.

Repeatable Acoustic Vaporization of Coated Perfluorocarbon Bubbles for Micro-Actuation Inspired by Polypodium aureum

Polypodium aureum, a fern, possesses a specialized spore-releasing mechanism like a catapult induced by the quick expansion of vaporized bubbles. This study introduces lipid-coated perfluorocarbon droplets to enable repeatable vaporization-condensation cycles, inspired by the repeatable vaporization of Polypodium aureum. Lipid-perfluorocarbon droplets have been considered not to exhibit repeatable oscillations due to bubble collapse of the low surface tension of lipid layers. However, a single lipid-dodecafluoropentane droplet with a diameter of 9.17 µm shows expansion-contraction oscillations over 4000 cycles by changing lipid composition and applying a low-power 1.7 MHz ultrasound to induce the partial vaporization of the droplets. The optimal combinations of shell composition, droplet fabrication, and acoustic conditions can minimize the damage on shell structure and promote a quick recovery of damaged shell layers. The highly expanding oscillatory microbubbles provide a new direction for fuel-free micro- or nanobots, as well as biomedical applications of contrast agents and drug delivery.

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

Phase-shift droplets are a highly adaptable platform for biomedical applications of ultrasound. The spatiotemporal response of phase-shift droplets to focused ultrasound above a certain pressure threshold, termed acoustic droplet vaporization (ADV), is influenced by intrinsic features (e.g., bulk boiling point) and extrinsic factors (e.g., driving frequency and surrounding media). A deep understanding of ADV dynamics is critical to ensure the robustness and repeatability of an ADV-assisted application. Here, we integrated ultra-high-speed imaging, at 10 million frames per second, and confocal microscopy for a full-scale (i.e., from nanoseconds to seconds) characterization of ADV. Experiments were conducted in fibrin-based hydrogels to mimic soft tissue environments. Effects of fibrin concentration (0.2 to 8 % (w/v)), excitation frequency (1, 2.5, and 9.4 MHz), and perfluorocarbon core (perfluoropentane, perfluorohexane, and perfluorooctane) on ADV dynamics were studied. Several fundamental parameters related to ADV dynamics, such as expansion ratio, expansion velocity, collapse radius, collapse time, radius of secondary rebound, resting radius, and equilibrium radius of the generated bubbles were extracted from the radius vs time curves. Diffusion-driven ADV-bubble growth was fit to a modified Epstein-Plesset equation, adding a material stress term, to estimate the growth rate. Our results indicated that ADV dynamics were significantly impacted by fibrin concentration, frequency, and perfluorocarbon liquid core. This is the first study to combine ultra-high-speed and confocal microscopy techniques to provide insights into ADV bubble dynamics in tissue-mimicking hydrogels.

A new sonoablation using acoustic droplet vaporization and focused ultrasound: A feasibility study

BACKGROUND

Histotripsy and boiling histotripsy are two methods of mechanical ablation that use high-pressure focused ultrasound (FUS).

PURPOSE

Here, a new bubble sonoablation technique was investigated using low-pressure FUS in combination with local injection of perfluoropentane (PFP) in rabbit liver.

METHODS

Fifteen healthy New Zealand white rabbits were treated with FUS alone, FUS + PFP or PFP alone. FUS was performed using a single-element focused transducer (frequency 596 kHz, 0.27 ms pulses, 0.54% duty cycle, and peak negative pressure 2.0 MPa). Ten minutes before FUS treatment, the PFP droplet was locally injected into the rabbit liver, where the ultrasound was focused. Contrast-enhanced ultrasound (CEUS) of the liver was performed, and the temperature at the liver surface in the targeted liver region was recorded during treatment. The livers were collected for pathological examination. Statistical significance was set at p < 0.05. Paired t-tests were used to compare the pre- and post-treatment values. One-way analysis of variance was performed to compare multiple groups, and the least significant difference method was used for further comparisons between the two groups.

RESULTS

Analysis of CEUS data showed that the values of area under the curve (AUC) were significantly different in the PFP + FUS group pre- (10453.644 ± 1182.93) and post-treatment (4058.098 ± 2720.41), and the AUC values of PFP + FUS post-treatment (4058.098 ± 2720.41) were also significantly lower than those of the FUS (9946.694 ± 1071.54) and the PFP (10364.794 ± 2181.53) groups. The peak intensity values also showed the same results, the value of peak intensity of PFP+FUS post-treatment was 82.958 ± 13.99, whereas there was no difference between FUS (106.61 ± 7.61) and PFP (104.136 ± 10.55). Hematoxylin and eosin (H&E) staining revealed that the pathological damage ratings of the PFP + FUS, PFP, and FUS groups were grade 3, grade 1, and grade 0, respectively. Specifically, the area of liver necrosis in the PFP + FUS group (0.99 ± 0.29 cm2 ) was 198 times higher than that in the PFP group (0.005 ± 0.008 cm2 ), whereas no necrosis was observed in the livers treated with FUS alone. Simultaneously, the number of vacuoles in the liver of the PFP + FUS group (35.50 ± 23.31) was approximately five times that of the PFP group (7.00 ± 12.88), whereas no vacuoles were found in the liver treated with FUS alone.

CONCLUSION

PFP droplets combined with FUS can destroy liver tissue and cause tissue necrosis in the droplet injection area, without affecting the structure of surrounding tissue.

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

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.

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

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.

Acoustic droplet vaporization for on-demand modulation of microporosity in smart hydrogels

Microporosity in hydrogels is critical for directing tissue formation and function. We have developed a fibrin-based smart hydrogel, termed an acoustically responsive scaffold (ARS), which responds to focused ultrasound in a spatiotemporally controlled, user-defined manner. ARSs are highly flexible platforms due to the inclusion of phase-shift droplets and their tunable response to ultrasound through a mechanism termed acoustic droplet vaporization (ADV). Here, we demonstrated that ADV enabled consistent generation of micropores in ARSs, throughout the entire thickness (∼5.5 mm), utilizing perfluorooctane phase-shift droplets. Size characteristics of the generated micropores were quantified in response to critical parameters including acoustic properties, droplet size, and shear elastic modulus of fibrin using confocal microscopy. The findings showed that the length of the generated micropores correlated directly with excitation frequency, peak rarefactional pressure, pulse duration, droplet size, and indirectly with the shear elastic modulus of the fibrin matrix. The ADV-generated micropores in ARSs were further compared with cavitation-mediated micropores in fibrin gels without droplets. Additionally, the Keller-Miksis equation was used to predict an upper bound for micropore formation in ARSs at varying driving frequencies and droplet sizes. Finally, our in vivo studies showed that host cell migration following ADV-induced micropore formation was frequency-dependent, with up to 2.6 times higher cell migration at lower frequencies. Overall, these findings demonstrate a new potential application of ADV in hydrogels. STATEMENT OF SIGNIFICANCE: Interconnected micropores within a hydrogel can facilitate many cell-mediated processes. Most techniques for generating micropores are typically not biocompatible or do not enable controlled, in situ micropore formation. We used an ultrasound-based technique, termed acoustic droplet vaporization, to generate microporosity in smart hydrogels termed acoustically responsive scaffolds (ARSs). ARSs contain a fibrin matrix doped with a phase-shift droplet. We demonstrate that unique acoustic properties of phase-shift droplets can be tailored to yield spatiotemporally controlled, on-demand micropore formation. Additionally, the size characteristics of the ultrasound-generated micropores can be modulated by tuning ultrasound parameters, droplet properties, and bulk elastic properties of fibrin. Finally, we demonstrate significant, frequency-dependent host cell migration in subcutaneously implanted ARSs in mice following ultrasound-induced micropore formation in situ.

Tumor-penetrating nanoplatform with ultrasound "unlocking" for cascade synergistic therapy and visual feedback under hypoxia

BACKGROUND

Combined therapy based on the effects of cascade reactions of nanoplatforms to combat specific solid tumor microenvironments is considered a cancer treatment strategy with transformative clinical value. Unfortunately, an insufficient O₂ supply and the lack of a visual indication hinder further applications of most nanoplatforms for solid tumor therapy.

RESULTS

A visualizable nanoplatform of liposome nanoparticles loaded with GOD, H(Gd), and PFP and grafted with the peptide tLyP-1, named _tLyP-1H(Gd)-GOD@PFP, was constructed. The double-domain peptide tLyP-1 was used to specifically target and penetrate the tumor cells; then, US imaging, starvation therapy and sonodynamic therapy (SDT) were then achieved by the ultrasound (US)-activated cavitation effect under the guidance of MR/PA imaging. GOD not only deprived the glucose for starvation therapy but also produced H₂O₂, which in coordination with ¹O₂ produced by H(Gd), enable the effects of SDT to achieve a synergistic therapeutic effect. Moreover, the synergistic therapy was enhanced by O₂ from PFP and low-intensity focused ultrasound (LIFU)-accelerated redox effects of the GOD. The present study demonstrated that the nanoplatform could generate a 3.3-fold increase in ROS, produce a 1.5-fold increase in the maximum rate of redox reactions and a 2.3-fold increase in the O₂ supply in vitro, and achieve significant tumor inhibition in vivo.

CONCLUSION

We present a visualizable nanoplatform with tumor-penetrating ability that can be unlocked by US to overcome the current treatment problems by improving the controllability of the O₂ supply, which ultimately synergistically enhanced cascade therapy.