Acoustic Nanodrops for Biomedical Applications

Effect of Ambient Conditions on Acoustic Activation of the Perfluoropropane Droplets Within the Infarct Zone

BACKGROUND

Acoustically activated perfluoropropane droplets (PD) formulated from lipid encapsulated microbubble preparations produce a delayed myocardial contrast enhancement that preferentially highlights the infarct zones (IZ). Since activation of PDs may be temperature sensitive, it is unclear what effect body temperature (BT) has on acoustic activation (AA).

OBJECTIVE

We sought to determine whether the microvascular retention and degree of myocardial contrast intensity (MCI) would be affected by BT at the time of intravenous injection.

METHODS

We administered intravenous (IV) PD in nine rats following 60 min of ischemia followed by reperfusion. Injections in these rats were given at temperatures above and below 36.5°C, with high MI activation in both groups at 3 or 6 min following IV injection (IVI). In six additional rats (three in each group), IV PDs were given only at one temperature (<36.5°C or ≥36.5°C), permitting a total of 12 comparisons of different BT. Differences in background subtracted MCI at 3-6 min post-injection were compared in the infarct zone (IZ) and remote zone (RZ). Post-mortem lung hematoxylin and eosin (H&E) staining was performed to assess the effect potential thermal activation on lung tissue.

RESULTS

Selective MCI within the IZ was observed in 8 of 12 rats who received IVI of PDs at <36.5°C, but none of the 12 rats who had IVI at the higher temperature (p < 0.0001). Absolute MCI following droplet activation was significantly higher in both the IZ and RZ when given at the lower BT. H&E indicated significant red blood extravasation in 5/7 rats who had had IV injections at higher BT, and 0/7 rats who had IV PDs at <36.5°C.

CONCLUSIONS

Selective IZ enhancement with AA of intravenous PDs is possible, but temperature sensitive. Thermal activation appears to occur when PDs are given at higher temperatures, preventing AA, and increasing unwanted bioeffects.

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.

Breast Tomographic Ultrasound: The Spectrum from Current Dense Breast Cancer Screenings to Future Theranostic Treatments

This review provides unique insights to the scientific scope and clinical visions of the inventors and pioneers of the SoftVue breast tomographic ultrasound (BTUS). Their >20-year collaboration produced extensive basic research and technology developments, culminating in SoftVue, which recently received the Food and Drug Administration's approval as an adjunct to breast cancer screening in women with dense breasts. SoftVue's multi-center trial confirmed the diagnostic goals of the tissue characterization and localization of quantitative acoustic tissue differences in 2D and 3D coronal image sequences. SoftVue mass characterizations are also reviewed within the standard cancer risk categories of the Breast Imaging Reporting and Data System. As a quantitative diagnostic modality, SoftVue can also function as a cost-effective platform for artificial intelligence-assisted breast cancer identification. Finally, SoftVue's quantitative acoustic maps facilitate noninvasive temperature monitoring and a unique form of time-reversed, focused US in a single theranostic device that actually focuses acoustic energy better within the highly scattering breast tissues, allowing for localized hyperthermia, drug delivery, and/or ablation. Women also prefer the comfort of SoftVue over mammograms and will continue to seek out less-invasive breast care, from diagnosis to treatment.

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.

Sonosensitive Cavitation Nuclei-A Customisable Platform Technology for Enhanced Therapeutic Delivery

Ultrasound-mediated cavitation shows great promise for improving targeted drug delivery across a range of clinical applications. Cavitation nuclei-sound-sensitive constructs that enhance cavitation activity at lower pressures-have become a powerful adjuvant to ultrasound-based treatments, and more recently emerged as a drug delivery vehicle in their own right. The unique combination of physical, biological, and chemical effects that occur around these structures, as well as their varied compositions and morphologies, make cavitation nuclei an attractive platform for creating delivery systems tuned to particular therapeutics. In this review, we describe the structure and function of cavitation nuclei, approaches to their functionalization and customization, various clinical applications, progress toward real-world translation, and future directions for the field.

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.

Analytic prediction of droplet vaporization events to estimate the precision of ultrasound-based proton range verification

BACKGROUND

The safety and efficacy of proton therapy is currently hampered by range uncertainties. The combination of ultrasound imaging with injectable radiation-sensitive superheated nanodroplets was recently proposed for in vivo range verification. The proton range can be estimated from the distribution of nanodroplet vaporization events, which is stochastically related to the stopping distribution of protons, as nanodroplets are vaporized by protons reaching their maximal LET at the end of their range.

PURPOSE

Here, we aim to estimate the range estimation precision of this technique. As for any stochastic measurement, the precision will increase with the sample size, that is, the number of detected vaporizations. Thus, we first develop and validate a model to predict the number of vaporizations, which is then applied to estimate the range verification precision for a set of conditions (droplet size, droplet concentration, and proton beam parameters).

METHODS

Starting from the thermal spike theory, we derived a model that predicts the expected number of droplet vaporizations in an irradiated sample as a function of the droplet size, concentration, and number of protons. The model was validated by irradiating phantoms consisting of size-sorted perfluorobutane droplets dispersed in an aqueous matrix. The number of protons was counted with an ionization chamber, and the droplet vaporizations were recorded and counted individually using high frame rate ultrasound imaging. After validation, the range estimate precision was determined for different conditions using a Monte Carlo algorithm.

RESULTS

A good agreement between theory and experiments was observed for the number of vaporizations, especially for large (5.8 ± 2.2 µm) and medium (3.5 ± 1.1 µm) sized droplets. The number of events was lower than expected in phantoms with small droplets (2.0 ± 0.7 µm), but still within the same order of magnitude. The inter-phantom variability was considerably larger (up to 30x) than predicted by the model. The validated model was then combined with Monte Carlo simulations, which predicted a theoretical range retrieval precision improving with the square-root of the number of vaporizations, and degrading at high beam energies due to range straggling. For single pencil beams with energies between 70 and 240 MeV, a range verification precision below 1% of the range required perfluorocarbon concentrations in the order of 0.3-2.4 µM.

CONCLUSION

We proposed and experimentally validated a model to provide a quick estimate of the number of vaporizations for a given set of conditions (droplet size, droplet concentration, and proton beam parameters). From this model, promising range verification performances were predicted for realistic perfluorocarbon concentrations. These findings are an incentive to move towards preclinical studies, which are critical to assess the achievable droplet distribution in and around the tumor, and hence the in vivo range verification precision.

Photoacoustic Vaporization of Endoskeletal Droplets Loaded with Zinc Naphthalocyanine

Vaporizable endoskeletal droplets are solid hydrocarbons in liquid fluorocarbon droplets in which melting of the hydrocarbon phase leads to the vaporization of the fluorocarbon phase. In prior work, vaporization of the endoskeletal droplets was achieved thermally by heating the surrounding aqueous medium. In this work, we introduce a near-infrared (NIR) optically absorbing naphthalocyanine dye (zinc 2,11,20,29-tetra-tert-butyl-2,3-naphthalocynanine) into the solid hydrocarbon (eicosane, n-C₂₀H₄₂) core of liquid fluorocarbon (C₅F₁₂) drops suspended in an aqueous medium. Droplets with a uniform diameter of 11.7 ± 0.7 μm were formed using a flow-focusing microfluidic device. The solid hydrocarbon formed a crumpled spherical structure within the liquid fluorocarbon droplet. The photoactivation behavior of these dye-containing endoskeletal droplets was investigated using NIR laser irradiation. When exposed to a pulsed laser of 720 nm wavelength, the dye-containing droplets vaporized at an average laser fluence of 65 mJ/cm², whereas blank droplets without the dye did not vaporize at any fluence up to 100 mJ/cm². Furthermore, dye-loaded droplets with a smaller, polydisperse size distribution were prepared using a simple shaking method and studied in a flow phantom for their photoacoustic signal and ultrasound contrast imaging. These results demonstrate that dye-containing endoskeletal droplets can be made to vaporize by externally applied optical energy. Such droplets may be useful for a variety of photoacoustic applications for sensing, imaging, and therapy.

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.

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.

Polyvinyl Alcohol Cryogels for Acoustic Characterization of Phase-Change Contrast Agents

Phase-change contrast agents (PCCAs) consisting of lipid-encapsulated low-boiling-point perfluorocarbons can be used in conjunction with ultrasound for diagnostic and therapeutic applications. One benefit of PCCAs is site-specific activation, whereby the liquid core is acoustically vaporized into a bubble detectable via ultrasound imaging. For further evaluation of PCCAs in a variety of applications, it is useful to disperse these nanodroplets into an acoustically compatible stationary matrix. However, many traditional phantom preparations require heating, which causes premature thermal activation of low-boiling-point PCCAs. Polyvinyl alcohol (PVA) cryogels do not require heat to set. Here we propose a simple method for the incorporation of the low-boiling-point PCCAs using octafluoropropane (OFP) and decafluorobutane (DFB) into PVA cryogels for a variety of acoustic characterization applications. We determined the utility of the phantoms by activating droplets with a focused transducer, visualizing the lesions with ultrasound imaging. At 1 MHz, droplet activation was consistently observed at 2.0 and 4.0 MPa for OFP and DFB, respectively.

Slow-Flow Ultrasound Localization Microscopy Using Recondensation of Perfluoropentane Nanodroplets

Ultrasound localization microscopy (ULM) is an emerging, super-resolution imaging technique for detailed mapping of the microvascular structure and flow velocity via subwavelength localization and tracking of microbubbles. Because microbubbles rely on blood flow for movement throughout the vascular space, acquisition times can be long in the smallest, low-flow microvessels. In addition, detection of microbubbles in low-flow regions can be difficult because of minimal separation of microbubble signal from tissue. Nanoscale, phase-change contrast agents (PCCAs) have emerged as a switchable, intermittent or persisting contrast agent for ULM via acoustic droplet vaporization (ADV). Here, the focus is on characterizing the spatiotemporal contrast properties of less volatile perfluoropentane (PFP) PCCAs. The results indicate that at physiological temperature, nanoscale PFP PCCAs with diameters less than 100 nm disappear within microseconds after ADV with high-frequency ultrasound (16 MHz, 5- to 6-MPa peak negative pressure) and that nanoscale PFP PCCAs have an inherent deactivation mechanism via immediate recondensation after ADV. This "blinking" on-and-off contrast signal allowed separation of flow in an in vitro flow phantom, regardless of flow conditions, although with a need for some replenishment at very low flow conditions to maintain count rate. This blinking behavior allows for rapid spatial mapping in areas of low or no flow with ULM, but limits velocity tracking because there is no stable bubble formation with nanoscale PFP PCCAs.

Effect of Thermal History and Hydrocarbon Core Size on Perfluorocarbon Endoskeletal Droplet Vaporization

Vaporizable hydrocarbon-in-fluorocarbon endoskeletal droplets are a unique category of phase-change emulsions with interesting physical and thermodynamic features. Here, we show microfluidic fabrication of various morphologies, such as solid-in-liquid, liquid-in-solid, and Janus-type, of complex solid n-C₂₀H₄₂ or n-C₂₁H₄₄ and liquid n-C₅F₁₂ droplets. Furthermore, we investigated the vaporization behavior of these endoskeletal droplets, focusing on the effects of heat treatment and core size. Comparison of vaporization and differential scanning calorimetry results indicated that vaporization occurs prior to melting of the bulk hydrocarbon phase for C₂₀H₄₂/C₅F₁₀ droplets and near the rotator phase for C₂₁H₄₄/C₅F₁₀ droplets. We found that heat treatment of the droplets increased the fraction of droplets that vaporized and also increased the vaporization temperature of the droplets, although the effect was temporary. Furthermore, we found that changing the relative size of the solid hydrocarbon core compared to the surrounding liquid shell increased the vaporization temperature and the vaporizing fraction. Taken together, these data support the hypothesis that surface melting behavior exhibited by the linear alkane may trigger the fluorocarbon vaporization event. These results may aid in the understanding of the interfacial thermodynamics and transport and the engineering of novel vaporizable endoskeletal droplets for biomedical imaging and other applications.

Phase-shift nanodroplets as an emerging sonoresponsive nanomaterial for imaging and drug delivery applications

Nanodroplets - emerging phase-changing sonoresponsive materials - have attracted substantial attention in biomedical applications for both tumour imaging and therapeutic purposes due to their unique response to ultrasound. As ultrasound is applied at different frequencies and powers, nanodroplets have been shown to cavitate by the process of acoustic droplet vapourisation (ADV), causing the development of mechanical forces which promote sonoporation through cellular membranes. This allows drugs to be delivered efficiently into deeper tissues where tumours are located. Recent reviews on nanodroplets are mostly focused on the mechanism of cavitation and their applications in biomedical fields. However, the chemistry of the nanodroplet components has not been discussed or reviewed yet. In this review, the commonly used materials and preparation methods of nanodroplets are summarised. More importantly, this review provides examples of variable chemistry components in nanodroplets which link them to their efficiency as ultrasound-multimodal imaging agents to image and monitor drug delivery. Finally, the drawbacks of current research, future development, and future direction of nanodroplets are discussed.

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.

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.