Investigation of the Acoustic Vaporization Threshold of Lipid-Coated Perfluorobutane Nanodroplets Using Both High-Speed Optical Imaging and Acoustic Methods

In Vitro and In Vivo Behavioral Evaluation of Condensed Lipid-Coated Perfluorocarbon Nanodroplets

OBJECTIVE

Phase-shift contrast agents consist of a liquid perfluorocarbon core that can be vaporized by ultrasound to generate echogenic contrast with excellent spatiotemporal control. The purpose of the present work was to evaluate the in vitro and in vivo behavior of condensed lipid-shelled nanodroplets (NDs) using different analytical procedures.

METHODS

Perfluorobutane NDs were prepared by condensation of precursor fluorescently labeled lipid-shelled microbubbles (MBs) and were characterized in terms of size distribution, gas core content and in vitro stability in blood, as well as for their acoustic vaporization behavior using a custom-made setup. In particular, the in vivo behavior of the NDs was thoroughly investigated after intravenous bolus injection in rats. To this end, we report, for the first time, the efficient use of three complementary detection procedures to assess the in vivo persistence of NDs: (i) ultrasound contrast imaging of vaporized NDs, (ii) gas chromatography-mass spectrometry to determine the perfluorobutane core content and (iii) fluorescence intensity measurement in the collected blood samples.

RESULTS

The Coulter Counter Multisizer results confirmed the size distribution shift post-condensation. Furthermore, similar PFB concentrations from MB and ND suspensions were obtained, indicating an exceptionally low rate of MB breakage and spontaneous nanodroplet vaporization. As expected, these nanoscale droplets have longer circulation times compared with clinically approved MBs, and only slight variations in half-life were observed between the three monitoring procedures. Finally, echogenic signal observed in focal areas of the liver and spleen after vaporization was confirmed by accumulation of fluorescent nanodroplets in these organs.

CONCLUSION

These results further contribute to our understanding of both the in vitro and in vivo behavior of sono-responsive nanodroplets, which is key to enabling efficient clinical translation.

Ultrasound-responsive microbubbles and nanodroplets: A pathway to targeted drug delivery

Ultrasound-responsive agents have shown great potential as targeted drug delivery agents, effectively augmenting cell permeability and facilitating drug absorption. This review focuses on two specific agents, microbubbles and nanodroplets, and provides a sequential overview of their drug delivery process. Particular emphasis is given to the mechanical response of the agents under ultrasound, and the subsequent physical and biological effects on the cells. Finally, the state-of-the-art in their pre-clinical and clinical implementation are discussed. Throughout the review, major challenges that need to be overcome in order to accelerate their clinical translation are highlighted.

Direct Emulsification of Stable Superheated Perfluorobutane Nanodroplets by Sonication: Addressing the Limitations of the Microbubble Condensation Technique

OBJECTIVE

We have previously determined that direct formulation of a phospholipid-based perfluorobutane (PFB) emulsion using high-pressure homogenization produces monodispersed PFB nanodroplets (NDs) with relatively few non-PFB-filled NDs. In this article, we describe a simpler strategy to reproducibly formulate highly concentrated superheated PFB NDs using a probe sonicator, a more widely available tool.

METHODS

Similar to the homogenization technique, sonicating at low power a solution of phospholipids with condensed PFB at -10°C consistently yields NDs with an encapsulation efficiency close to 100% and very few non-PFB-filled particles.

RESULTS

The PFB emulsion is stable with absence of spontaneous vaporization at 37°C and for more than 14 d when frozen or refrigerated and for 3 d at 25°C. Acoustic droplet vaporization (ADV) occurred at a mechanical index >0.5 and continued to increase thereafter. The ADV threshold was similar for freshly made or frozen emulsion after thawing. In contrast to the microbubble (MB) condensation method, in which the ratio of non-PFB-filled to PFB-filled is 2000:1, particularly if MBs are not washed after formulation, nearly 94% of particles produced by direct sonication are PFB filled.

CONCLUSION

PFB NDs can be manufactured with high yield, stability and reproducibility using a probe sonicator that is available in many laboratories. Their ease of manufacture could spark discoveries into highly impactful ND-based diagnostic and therapeutic applications.

Synchronous Intravital Imaging and Cavitation Monitoring of Antivascular Focused Ultrasound in Tumor Microvasculature Using Monodisperse Low Boiling Point Nanodroplets

Focused ultrasound-stimulated microbubbles can induce blood flow shutdown and ischemic necrosis at higher pressures in an approach termed antivascular ultrasound. Combined with conventional therapies of chemotherapy, immunotherapy, and radiation therapy, this approach has demonstrated tumor growth inhibition and profound synergistic antitumor effects. However, the lower cavitation threshold of microbubbles can potentially yield off-target damage that the polydispersity of clinical agent may further exacerbate. Here we investigate the use of a monodisperse nanodroplet formulation for achieving antivascular effects in tumors. We first develop stable low boiling point monodisperse lipid nanodroplets and examine them as an alternative agent to mediate antivascular ultrasound. With synchronous intravital imaging and ultrasound monitoring of focused ultrasound-stimulated nanodroplets in tumor microvasculature, we show that nanodroplets can trigger blood flow shutdown and do so with a sharper pressure threshold and with fewer additional events than conventionally used microbubbles. We further leverage the smaller size and prolonged pharmacokinetic profile of nanodroplets to allow for potential passive accumulation in tumor tissue prior to antivascular ultrasound, which may be a means by which to promote selective tumor targeting. We find that vascular shutdown is accompanied by inertial cavitation and complex-order sub- and ultraharmonic acoustic signatures, presenting an opportunity for effective feedback control of antivascular ultrasound.

Effect of Phase-Change Nanodroplets and Ultrasound on Blood-Brain Barrier Permeability In Vitro

Phase-change nanodroplets (PCND;NDs) are emulsions with a perfluorocarbon (PFC) core that undergo acoustic vaporisation as a response to ultrasound (US). Nanodroplets change to microbubbles and cavitate while under the effect of US. This cavitation can apply forces on cell connections in biological barrier membranes, such as the blood-brain barrier (BBB), and trigger a transient and reversible increased permeability to molecules and matter. This study aims to present the preparation of lipid-based NDs and investigate their effects on the brain endothelial cell barrier in vitro. The NDs were prepared using the thin-film hydration method, followed by the PFC addition. They were characterised for size, cavitation (using a high-speed camera), and PFC encapsulation (using FTIR). The bEnd.3 (mouse brain endothelial) cells were seeded onto transwell inserts. Fluorescein with NDs and/or microbubbles were applied on the bEND3 cells and the effect of US on fluorescein permeability was measured. The Live/Dead assay was used to assess the BBB integrity after the treatments. Size and PFC content analysis indicated that the NDs were stable while stored. High-speed camera imaging confirmed that the NDs cavitate after US exposure of 0.12 MPa. The BBB cell model experiments revealed a 4-fold increase in cell membrane permeation after the combined application of US and NDs. The Live/Dead assay results indicated damage to the BBB membrane integrity, but this damage was less when compared to the one caused by microbubbles. This in vitro study shows that nanodroplets have the potential to cause BBB opening in a similar manner to microbubbles. Both cavitation agents caused damage on the endothelial cells. It appears that NDs cause less cell damage compared to microbubbles.

Three-dimensional super resolution ultrasound imaging with a multi-frequency hemispherical phased array

BACKGROUND

High resolution imaging of the microvasculature plays an important role in both diagnostic and therapeutic applications in the brain. However, ultrasound pulse-echo sonography imaging the brain vasculatures has been limited to narrow acoustic windows and low frequencies due to the distortion of the skull bone, which sacrifices axial resolution since it is pulse length dependent.

PURPOSE

To overcome the detect limit, a large aperture 256-module sparse hemispherical transmit/receive array was used to visualize the acoustic emissions of ultrasound-vaporized lipid-coated decafluorobutane nanodroplets flowing through tube phantoms and within rabbit cerebral vasculature in vivo via passive acoustic mapping and super resolution techniques.

METHODS

Nanodroplets were vaporized with 55 kHz burst-mode ultrasound (burst length = 145 μs, burst repetition frequency = 9-45 Hz, peak negative acoustic pressure = 0.10-0.22 MPa), which propagates through overlying tissues well without suffering from severe distortions. The resulting emissions were received at a higher frequency (612 or 1224 kHz subarray) to improve the resulting spatial resolution during passive beamforming. Normal resolution three-dimensional images were formed using a delay, sum, and integrate beamforming algorithm, and super-resolved images were extracted via Gaussian fitting of the estimated point-spread-function to the normal resolution data.

RESULTS

With super resolution techniques, the mean lateral (axial) full-width-at-half-maximum image intensity was 16 ± 3 (32 ± 6) μm, and 7 ± 1 (15 ± 2) μm corresponding to ∼1/67 of the normal resolution at 612 and 1224 kHz, respectively. The mean positional uncertainties were ∼1/350 (lateral) and ∼1/180 (axial) of the receive wavelength in water. In addition, a temporal correlation between nanodroplet vaporization and the transmit waveform shape was observed, which may provide the opportunity to enhance the signal-to-noise ratio in future studies.

CONCLUSIONS

Here, we demonstrate the feasibility of vaporizing nanodroplets via low frequency ultrasound and simultaneously performing spatial mapping via passive beamforming at higher frequencies to improve the resulting spatial resolution of super resolution imaging techniques. This method may enable complete four-dimensional vascular mapping in organs where a hemispherical array could be positioned to surround the target, such as the brain, breast, or testicles.

Effect of tissue viscoelasticity and adjacent phase-changed microbubbles on vaporization process and direct growth threshold of nanodroplet in an ultrasonic field

Understanding the behavior of nanodroplets converted into microbubbles with applied ultrasound is an important problem in tumor therapeutical and diagnostic applications. In this study, a comprehensive model is proposed to investigate the vaporization process and the direct growth threshold of the nanodroplet by following the vapor bubble growth, especially attention devoted to the effect of tissue viscoelasticity and adjacent phase-changed microbubbles (PCMBs). It is shown that the ultrasonic energy must be sufficiently strong to counterbalance the natural condensation of the vapor bubble and the tissue stiffness-inhibitory effect. The softer tissue with a lower shear modulus favors the vaporization process, and the nanodroplet has a lower direct growth threshold in the softer tissue. Moreover, the adjacent PCMBs show a suppression effect on the vaporization process due to the negative value of the secondary Bjerknes force, implying an attractive force, preventing the nanodroplet from escaping from the constraint of the adjacent PCMBs. However, according to the linear scattering theory, the attractive force signifies that the constraint is weak, causing the direct growth threshold to increase in the range of 0.09-0.24 MPa. The weak increase in threshold demonstrates that the direct growth threshold is relatively unaffected by the adjacent PCMBs. The prediction results of our model are in good agreement with the experiment results obtained by the echo enhancement method, in which the threshold is relatively independent of the intermediate concentration. The findings presented here provide physical insight that will be further helpful in understanding the complex behavior of the nanodroplet responses to ultrasound in practical medical applications.

Factors Influencing the Repeated Transient Optical Droplet Vaporization Threshold and Lifetimes of Phase Change, Perfluorocarbon Nanodroplets

Perfluorocarbon nanodroplets (PFCnDs) are sub-micrometer emulsions composed of a surfactant-encased perfluorocarbon (PFC) liquid and can be formulated to transiently vaporize through optical stimulation. However, the factors governing repeated optical droplet vaporization (ODV) have not been investigated. In this study, we employ high-frame-rate ultrasound (US) to characterize the ODV thresholds of various formulations and imaging parameters and identify those that exhibit low vaporization thresholds and repeatable vaporization. We observe a phenomenon termed "preconditioning", where initial laser pulses generate reduced US contrast that appears linked with an increase in nanodroplet size. Variation in laser pulse repetition frequency is found not to change the vaporization threshold, suggesting that "preconditioning" is not related to residual heat. Surfactants (bovine serum albumin, lipids, and zonyl) impact the vaporization threshold and imaging lifetime, with lipid shells demonstrating the best performance with relatively low thresholds (21.6 ± 3.7 mJ/cm2) and long lifetimes (t1/2 = 104 ± 21.5 pulses at 75 mJ/cm2). Physiological stiffness does not affect the ODV threshold and may enhance nanodroplet stability. Furthermore, PFC critical temperatures are found to correlate with vaporization thresholds. These observations enhance our understanding of ODV behavior and pave the way for improved nanodroplet performance in biomedical applications.

Characterising the chemical and physical properties of phase-change nanodroplets

Phase-change nanodroplets have attracted increasing interest in recent years as ultrasound theranostic nanoparticles. They are smaller compared to microbubbles and they may distribute better in tissues (e.g. in tumours). They are composed of a stabilising shell and a perfluorocarbon core. Nanodroplets can vaporise into echogenic microbubbles forming cavitation nuclei when exposed to ultrasound. Their perfluorocarbon core phase-change is responsible for the acoustic droplet vaporisation. However, methods to quantify the perfluorocarbon core in nanodroplets are lacking. This is an important feature that can help explain nanodroplet phase change characteristics. In this study, we fabricated nanodroplets using lipids shell and perfluorocarbons. To assess the amount of perfluorocarbon in the core we used two methods, 19F NMR and FTIR. To assess the cavitation after vaporisation we used an ultrasound transducer (1.1 MHz) and a high-speed camera. The 19F NMR based method showed that the fluorine signal correlated accurately with the perfluorocarbon concentration. Using this correlation, we were able to quantify the perfluorocarbon core of nanodroplets. This method was used to assess the content of the perfluorocarbon of the nanodroplets in solutions over time. It was found that perfluoropentane nanodroplets lost their content faster and at higher ratio compared to perfluorohexane nanodroplets. The high-speed imaging indicates that the nanodroplets generate cavitation comparable to that from commercial contrast agent microbubbles. Nanodroplet characterisation should include perfluorocarbon concentration assessment as critical information for their development.

Thermal and Acoustic Stabilization Of Volatile Phase-Change Contrast Agents Via Layer-By-Layer Assembly

OBJECTIVE

Phase-change contrast agents (PCCAs) are perfluorocarbon nanodroplets (NDs) that have been widely studied for ultrasound imaging in vitro, pre-clinical studies, and most recently incorporated a variant of PCCAs, namely a microbubble-conjugated microdroplet emulsion, into the first clinical studies. Their properties also make them attractive candidates for a variety of diagnostic and therapeutic applications including drug-delivery, diagnosis and treatment of cancerous and inflammatory diseases, as well as tumor-growth tracking. However, control over the thermal and acoustic stability of PCCAs both in vivo and in vitro has remained a challenge for expanding the potential utility of these agents in novel clinical applications. As such, our objective was to determine the stabilizing effects of layer-by-layer assemblies and its effect on both thermal and acoustic stability.

METHODS

We utilized layer-by-layer (LBL) assemblies to coat the outer PCCA membrane and characterized layering by measuring zeta potential and particle size. Stability studies were conducted by; 1) incubating the LBL-PCCAs at atmospheric pressure at 37∘C and 45∘C followed by; 2) ultrasound-mediated activation at 7.24 MHz and peak-negative pressures ranging from 0.71 - 5.48 MPa to ascertain nanodroplet activation and resultant microbubble persistence. The thermal and acoustic properties of decafluorobutane gas-condensed nanodroplets (DFB-NDs) layered with 6 and 10 layers of charge-alternating biopolymers, (LBL6NDs and LBL10NDs) respectively, were studied and compared to non-layered DFB-NDs. Half-life determinations were conducted at both 37∘C and 45∘C with acoustic droplet vaporization (ADV) measurements occurring at 23∘C.

DISCUSSION

Successful application of up to 10 layers of alternating positive and negatively charged biopolymers onto the surface membrane of DFB-NDs was demonstrated. Two major claims were substantiated in this study; namely, (1) biopolymeric layering of DFB-NDs imparts a thermal stability up to an extent; and, (2) both LBL6NDs and LBL10NDs did not appear to alter particle acoustic vaporization thresholds, suggesting that the thermal stability of the particle may not necessarily be coupled with particle acoustic vaporization thresholds.

CONCLUSION

Results demonstrate that the layered PCCAs had higher thermal stability, where the half-lifes of the LBLxNDs are significantly increased after incubation at 37∘C and 45∘C. Furthermore, the acoustic vaporization profiles the DFB-NDs, LBL6NDs, and LBL10NDs show that there is no statistically significant difference between the acoustic vaporization energy required to initiate acoustic droplet vaporization.

Barrier-breaking effects of ultrasonic cavitation for drug delivery and biomarker release

Recently, emerging evidence has demonstrated that cavitation actually creates important bidirectional channels on biological barriers for both intratumoral drug delivery and extratumoral biomarker release. To promote the barrier-breaking effects of cavitation for both therapy and diagnosis, we first reviewed recent technical advances of ultrasound and its contrast agents (microbubbles, nanodroplets, and gas-stabilizing nanoparticles) and then reported the newly-revealed cavitation physical details. In particular, we summarized five types of cellular responses of cavitation in breaking the plasma membrane (membrane retraction, sonoporation, endocytosis/exocytosis, blebbing and apoptosis) and compared the vascular cavitation effects of three different types of ultrasound contrast agents in breaking the blood-tumor barrier and tumor microenvironment. Moreover, we highlighted the current achievements of the barrier-breaking effects of cavitation in mediating drug delivery and biomarker release. We emphasized that the precise induction of a specific cavitation effect for barrier-breaking was still challenged by the complex combination of multiple acoustic and non-acoustic cavitation parameters. Therefore, we provided the cutting-edge in-situ cavitation imaging and feedback control methods and suggested the development of an international cavitation quantification standard for the clinical guidance of cavitation-mediated barrier-breaking effects.

Coupling Two Ultra-high-Speed Cameras to Elucidate Ultrasound Contrast-Mediated Imaging and Therapy

Ultrasound contrast-mediated medical imaging and therapy both rely on the dynamics of micron- and nanometer-sized ultrasound cavitation nuclei, such as phospholipid-coated microbubbles and phase-change droplets. Ultrasound cavitation nuclei respond non-linearly to ultrasound on a nanosecond time scale that necessitates the use of ultra-high-speed imaging to fully visualize these dynamics in detail. In this study, we developed an ultra-high-speed optical imaging system that can record up to 20 million frames per second (Mfps) by coupling two small-sized, commercially available, 10-Mfps cameras. The timing and reliability of the interleaved cameras needed to achieve 20 Mfps was validated using two synchronized light-emitting diode strobe lights. Once verified, ultrasound-activated microbubble responses were recorded and analyzed. A unique characteristic of this coupled system is its ability to be reconfigured to provide orthogonal observations at 10 Mfps. Acoustic droplet vaporization was imaged from two orthogonal views, by which the 3-D dynamics of the phase transition could be visualized. This optical imaging system provides the temporal resolution and experimental flexibility needed to further elucidate the dynamics of ultrasound cavitation nuclei to potentiate the clinical translation of ultrasound-mediated imaging and therapy developments.

Perfluorocarbon nanodroplet size, acoustic vaporization, and inertial cavitation affected by lipid shell composition in vitro

Perfluorocarbon nanodroplets (PFCnDs) are ultrasound contrast agents that phase-transition from liquid nanodroplets to gas microbubbles when activated by laser irradiation or insonated with an ultrasound pulse. The dynamics of PFCnDs can vary drastically depending on the nanodroplet composition, including the lipid shell properties. In this paper, we investigate the effect of varying the ratio of PEGylated to non-PEGylated phospholipids in the outer shell of PFCnDs on the acoustic nanodroplet vaporization (liquid to gas phase transition) and inertial cavitation (rapid collapse of the vaporized nanodroplets) dynamics in vitro when insonated with focused ultrasound. Nanodroplets with a high concentration of PEGylated lipids had larger diameters and exhibited greater variance in size distribution compared to nanodroplets with lower proportions of PEGylated lipids in the lipid shell. PFCnDs with a lipid shell composed of 50:50 PEGylated to non-PEGylated lipids yielded the highest B-mode image intensity and duration, as well as the greatest pressure difference between acoustic droplet vaporization onset and inertial cavitation onset. We demonstrate that slight changes in lipid shell composition of PFCnDs can significantly impact droplet phase transitioning and inertial cavitation dynamics. These findings can help guide researchers to fabricate PFCnDs with optimized compositions for their specific applications.

Bactericidal and anti-biofilm effects of uncharged and cationic ultrasound-responsive nitric oxide microbubbles on Pseudomonas aeruginosa biofilms

Bacterial biofilms are a major and ongoing concern for public health, featuring both inherited genetic resistance traits and a conferred innate tolerance to traditional antibiotic therapies. Consequently, there is a growing need for novel methods of drug delivery, to increase the efficacy of antimicrobial agents. This research evaluated the anti-biofilm and bactericidal effects of ultrasound responsive gas-microbubbles (MBs) of either air or nitric oxide, using an in vitro Pseudomonas aeruginosa biofilm model grown in artificial wound medium. The four lipid-based MB formulations evaluated were room-air MBs (RAMBs) and nitric oxide MBs (NOMBs) with no electrical charge, as well as cationic (+) RAMBs⁺ and NOMBs⁺. Two principal treatment conditions were used: i) ultrasound stimulated MBs only, and ii) ultrasound stimulated MBs with a sub-inhibitory concentration (4 µg/mL) of the antibiotic gentamicin. The total treatment time was divided into a 60 second passive MB interaction period prior to 40 second ultrasound exposure; each MB formulation was tested in triplicate. Ultrasound stimulated RAMBs and NOMBs without antibiotic achieved reductions in biofilm biomass of 93.3% and 94.0%, respectively. Their bactericidal efficacy however was limited, with a reduction in culturable cells of 26.9% and 65.3%, respectively. NOMBs with sub-inhibitory antibiotic produced the most significant reduction in biofilm biomass, corresponding to a 99.9% (SD ± 5.21%); and a 99.9% (SD ± 0.07%) (3-log) reduction in culturable bacterial cells. Cationic MBs were initially manufactured to promote binding of MBs to negatively charged biofilms, but these formulations also demonstrated intrinsic bactericidal properties. In the absence of antibiotic, the bactericidal efficacy of RAMB⁺ and NOMB⁺ was greater that of uncharged counterparts, reducing culturable cells by 84.7% and 86.1% respectively; increasing to 99.8% when combined with antibiotic. This study thus demonstrates the anti-biofilm and bactericidal utility of ultrasound stimulated MBs, and specifically is the first to demonstrate the efficacy of a NOMB for the dispersal and potentiation of antibiotics against bacterial biofilms in vitro. Importantly the biofilm system and complex growth-medium were selected to recapitulate key morphological features of in vivo biofilms. The results us offer new insight for the development of new clinical treatments, for example, in chronic wounds.

Ultrasound-triggered oxygen-loaded nanodroplets enhance and monitor cerebral damage from sonodynamic therapy

In sonodynamic therapy, cellular toxicity from sonosensitizer drugs, such as 5-aminolevulinic acid hydrochloride (5-ALA), may be triggered with focused ultrasound through the production of reactive oxygen species (ROS). Here we show that by increasing local oxygen during treatment, using oxygen-loaded perfluorocarbon nanodroplets (250 +/- 8 nm), we can increase the damage induced by 5-ALA, and monitor the severity by recording acoustic emissions in the brain. To achieve this, we sonicated the right striatum of 16 healthy rats after an intravenous dose of 5-ALA (200 mg/kg), followed by saline, nanodroplets, or oxygen-loaded nanodroplets. We assessed haemorrhage, edema and cell apoptosis immediately following, 24 hr, and 48 hr after focused ultrasound treatment. The localized volume of damaged tissue was significantly enhanced by the presence of oxygen-loaded nanodroplets, compared to ultrasound with unloaded nanodroplets (3-fold increase), and ultrasound alone (40-fold increase). Sonicating 1 hr following 5-ALA injection was found to be more potent than 2 hr following 5-ALA injection (2-fold increase), and the severity of tissue damage corresponded to the acoustic emissions from droplet vaporization. Enhancing the local damage from 5-ALA with monitored cavitation activity and additional oxygen could have significant implications in the treatment of atherosclerosis and non-invasive ablative surgeries.

High-Frequency Ultrasound Activation of Perfluorocarbon Nanodroplets for Treatment of Glaucoma

Elevated intraocular pressure (IOP) is the most prevalent risk factor for initiation and progression of neurodegeneration in glaucoma. Ocular hypertension results from increased resistance to aqueous fluid outflow caused by reduced porosity and increased stiffness of tissues of the outflow pathway. Acoustic activation and resulting bioeffects of the perfluorocarbon (PFC) nanodroplets (NDs) introduced into the anterior chamber (AC) of the eye could potentially represent a treatment for glaucoma by increasing permeability in the aqueous outflow track. To evaluate the potential of NDs to enter the outflow track, 100-nm diameter perfluoropentane (PFP) NDs with a lipid shell were injected into the AC of ex vivo pig eyes and in vivo rat eyes. The NDs were activated and imaged with 18- and 28-MHz linear arrays to assess their location and diffusion. NDs in the AC could also be visualized using optical coherence tomography (OCT). Because of their higher density with respect to aqueous humor, some NDs settled into the iridocorneal angle where they entered the outflow pathway. After acoustic activation of the NDs at the highest acoustic pressure, small gas bubbles were observed in the AC. After two days, no acoustic activation events were visible in the AC of the rats and their eyes showed no evidence of inflammation.

Acoustic Modulation Enables Proton Detection With Nanodroplets at Body Temperature

Superheated nanodroplet (ND) vaporization by proton radiation was recently demonstrated, opening the door to ultrasound-based in vivo proton range verification. However, at body temperature and physiological pressures, perfluorobutane nanodroplets (PFB-NDs), which offer a good compromise between stability and radiation sensitivity, are not directly sensitive to primary protons. Instead, they are vaporized by infrequent secondary particles, which limits the precision for range verification. The radiation-induced vaporization threshold (i.e., sensitization threshold) can be reduced by lowering the pressure in the droplet such that ND vaporization by primary protons can occur. Here, we propose to use an acoustic field to modulate the pressure, intermittently lowering the proton sensitization threshold of PFB-NDs during the rarefactional phase of the ultrasound wave. Simultaneous proton irradiation and sonication with a 1.1 MHz focused transducer, using increasing peak negative pressures (PNPs), were applied on a dilution of PFB-NDs flowing in a tube, while vaporization was acoustically monitored with a linear array. Sensitization to primary protons was achieved at temperatures between [Formula: see text] and 40 °C using acoustic PNPs of relatively low amplitude (from 800 to 200 kPa, respectively), while sonication alone did not lead to ND vaporization at those PNPs. Sensitization was also measured at the clinically relevant body temperature (i.e., 37 °C) using a PNP of 400 kPa. These findings confirm that acoustic modulation lowers the sensitization threshold of superheated NDs, enabling a direct proton response at body temperature.

Sonoporation based on repeated vaporization of gold nanodroplets

Background

Gold nanodroplets (AuNDs) have been proposed as agents for photothermal therapy and photoacoustic imaging. Previously, we demonstrated that the sonoporation can be more effectively achieved with synchronized optical and acoustic droplet vaporization. By applying a laser pulse at the rarefactional phase of the ultrasound (US) pulse, the vaporization threshold can be reached at a considerably lower laser average power. However, a large loading quantity of the AuNDs may increase the risk of air embolism. The destruction of phase‐shifted AuNDs at the inertial cavitation stage leads to a reduced drug delivery performance. And it also causes instability of echogenicity during therapeutic monitoring.

Purpose

In this study, we propose to further improve the sonoporation effectiveness with repeated vaporization. In other words, the AuNDs repeatedly undergo vaporization and recondensation so that sonoporation effects are accumulated over time at lower energy requirements. Previously, repeated vaporization has been demonstrated as an imaging contrast agent. In this study, we aim to adopt this repeated vaporization scheme for sonoporation.

Methods

Perfluoropentane NDs with a shell made of human serum albumin were used as the US contrast agents. Laser pulses at 808 nm and US pulses of 1 MHz were delivered for triggering vaporization and inertial cavitation of NDs. We detected the vaporization and cavitation effects under different activation firings, US peak negative pressures (PNPs), and laser fluences using 5‐ and 10‐MHz focused US receivers. Numbers of calcein‐AM and propidium iodide signals uptake by BNL hepatocarcinoma cancer cells were used to evaluate the sonoporation and cell death rate of the cells.

Results

We demonstrate that sonoporation can be realized based on repeatable vaporization instead of the commonly adopted inertial cavitation effects. In addition, it is found that the laser fluence and the acoustic pressure can be reduced. As an example, we demonstrate that the acoustic and optical energy for achieving a similar level of sonoporation rate can be as low as 0.44 MPa for the US PNP and 4.01 mJ/cm² for the laser fluence, which are lower than those with our previous approach (0.53 MPa and 4.95 mJ/cm², respectively).

Conclusion

We demonstrated the feasibility of vaporization‐based sonoporation at a lower optical and acoustic energy. It is an advantageous method that can enhance drug delivery efficiency, therapeutic safety and potentially deliver an upgraded gene therapy strategy for improved theragnosis.

Sub-millimetre precision of drug delivery in the brain from ultrasound-triggered nanodroplets

Drug-loaded nanoscale cavitation agents, called nanodroplets, are an attractive solution to enhance and localize drug delivery, offering increased stability and prolonged half-life in circulation compared to microbubbles. However, the spatial precision with which drug can be released and delivered into brain tissue from such agents has not been directly mapped. Decafluorobutane lipid-shell droplets (206 +/- 6 nm) were loaded with a fluorescent blood-brain barrier (BBB)-penetrating dye (Nile Blue) and vaporized with ultrasound (1.66 MHz, 10 ms pulse length, 1 Hz pulse repetition frequency), generating transient echogenic microbubbles and delivering the encapsulated dye. The distribution and intensity of released fluorophore was mapped in a tissue-mimicking phantom, and in the brain of rats (Sprague Dawley, N = 4, n = 16). The release and distribution of dye was found to be pressure-dependent (0.2-3.5 MPa) and to occur only above the vaporization threshold of the nanodroplets (1.5 +/- 0.25 MPa in vitro, 2.4 +/- 0.05 MPa in vivo). Dye delivery was achieved with sub-millimetre spatial precision, covering an area of 0.4 to 1.5 mm in diameter, determined by the sonication pressure. The distribution and intensity of dye released at depth in the brain followed the axial pressure profile of the ultrasound beam. Nile Blue (354 Da, LogP 2.7) was compared to Nile Red (318 Da, LogP 3.8) and Quantum Dots (CdSe/ZnS, 5 nm diameter) to visualize the role of molecule size and lipophilicity in crossing the intact BBB following triggered release. Acoustic emissions were shown to predict the successful delivery of the BBB-penetrating dye and the extent of the distribution, demonstrating the theranostic capabilities of nanoscale droplets to precisely localize drug delivery in the brain.

Ultrasound and Photoacoustic Imaging of Laser-Activated Phase-Change Perfluorocarbon Nanodroplets

Laser-activated perfluorocarbon nanodroplets (PFCnDs) are emerging phase-change contrast agents that showed promising potential in ultrasound and photoacoustic (US/PA) imaging. Unlike monophase gaseous microbubbles, PFCnDs shift their state from liquid to gas via optical activation and can provide high US/PA contrast on demand. Depending on the choice of perfluorocarbon core, the vaporization and condensation dynamics of the PFCnDs are controllable. Therefore, these configurable properties of activation and deactivation of PFCnDs are employed to enable various imaging approaches, including contrast-enhanced imaging and super-resolution imaging. In addition, synchronous application of both acoustic and optical pulses showed a promising outcome vaporizing PFCnDs with lower activation thresholds. Furthermore, due to their sub-micrometer size, PFCnDs can be used for molecular imaging of extravascular tissue. PFCnDs can also be an effective therapeutic tool. As PFCnDs can carry therapeutic drugs or other particles, they can be used for drug delivery, as well as photothermal and photodynamic therapies. Blood barrier opening for neurological applications was recently demonstrated with optically-triggered PFCnDs. This paper specifically focuses on the activation and deactivation properties of laser-activated PFCnDs and associated US/PA imaging approaches, and briefly discusses their theranostic potential and future directions.

Ultrasonic technologies in imaging and drug delivery

Ultrasonic technologies show great promise for diagnostic imaging and drug delivery in theranostic applications. The development of functional and molecular ultrasound imaging is based on the technical breakthrough of high frame-rate ultrasound. The evolution of shear wave elastography, high-frequency ultrasound imaging, ultrasound contrast imaging, and super-resolution blood flow imaging are described in this review. Recently, the therapeutic potential of the interaction of ultrasound with microbubble cavitation or droplet vaporization has become recognized. Microbubbles and phase-change droplets not only provide effective contrast media, but also show great therapeutic potential. Interaction with ultrasound induces unique and distinguishable biophysical features in microbubbles and droplets that promote drug loading and delivery. In particular, this approach demonstrates potential for central nervous system applications. Here, we systemically review the technological developments of theranostic ultrasound including novel ultrasound imaging techniques, the synergetic use of ultrasound with microbubbles and droplets, and microbubble/droplet drug-loading strategies for anticancer applications and disease modulation. These advancements have transformed ultrasound from a purely diagnostic utility into a promising theranostic tool.

Predicting initial nucleation events occurred in a metastable nanodroplet during acoustic droplet vaporization

Acoustic droplet vaporization (ADV) capable of converting liquid perfluorocarbon (PFC) micro/nanodroplets into gaseous microbubbles has gained much attention due to its medical potentials. However, its physical mechanisms for nanodroplets have not been well understood due to the disappeared superharmonic focusing effect and the prominent Laplace pressure compared to microdroplets, especially for the initial ADV nucleation occurring in a metastable PFC nanodroplet. The classical nucleation theory (CNT) was modified to describe the ADV nucleation via combining the phase-change thermodynamics of perfluoropentane (PFP) and the Laplace pressure effect on PFP nanodroplets. The thermodynamics was exactly predicted by the Redlich-Kwong equation of state (EoS) rather than the van der Waals EoS, based on which the surface tension of the vapor nucleus as a crucial parameter in the CNT was successfully obtained to modify the CNT. Compared to the CNT, the modified CNT eliminated the intrinsic limitations of the CNT, and it predicted a larger nucleation rate and a lower ADV nucleation threshold, which agree much better with experimental results. Furthermore, it indicated that the nanodroplet properties exert very strong influences on the nucleation threshold instead of the acoustic parameters, providing a potential strategy with an appropriate droplet design to reduce the ADV nucleation threshold. This study may contribute to further understanding the ADV mechanism for PFC nanodroplets and promoting its potential theranostic applications in clinical practice.