Gas-Stabilizing Gold Nanocones for Acoustically Mediated Drug Delivery

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.

Ultrasound nanotheranostics: Toward precision medicine

Ultrasound (US) is a mechanical wave that can penetrate biological tissues and trigger complex bioeffects. The mechanisms of US in different diagnosis and treatment are different, and the functional application of commercial US is also expanding. In particular, recent developments in nanotechnology have led to a wider use of US in precision medicine. In this review, we focus on US in combination with versatile micro and nanoparticles (NPs)/nanovesicles for tumor theranostics. We first introduce US-assisted drug delivery as a stimulus-responsive approach that spatiotemporally regulates the deposit of nanomedicines in target tissues. Multiple functionalized NPs and their US-regulated drug-release curves are analyzed in detail. Moreover, as a typical representative of US therapy, sonodynamic antitumor strategy is attracting researchers' attention. The collaborative efficiency and mechanisms of US and various nano-sensitizers such as nano-porphyrins and organic/inorganic nanosized sensitizers are outlined in this paper. A series of physicochemical processes during ultrasonic cavitation and NPs activation are also discussed. Finally, the new applications of US and diagnostic NPs in tumor-monitoring and image-guided combined therapy are summarized. Diagnostic NPs contain substances with imaging properties that enhance US contrast and photoacoustic imaging. The development of such high-resolution, low-background US-based imaging methods has contributed to modern precision medicine. It is expected that the integration of non-invasive US and nanotechnology will lead to significant breakthroughs in future clinical applications.

Next-Generation Colloidal Materials for Ultrasound Imaging Applications

Ultrasound has important applications, predominantly in the field of diagnostic imaging. Presently, colloidal systems such as microbubbles, phase-change emulsion droplets and particle systems with acoustic properties and multiresponsiveness are being developed to address typical issues faced when using commercial ultrasound contrast agents, and to extend the utility of such systems to targeted drug delivery and multimodal imaging. Current technologies and increasing research data on the chemistry, physics and materials science of new colloidal systems are also leading to the development of more complex, novel and application-specific colloidal assemblies with ultrasound contrast enhancement and other properties, which could be beneficial for multiple biomedical applications, especially imaging-guided treatments. In this article, we review recent developments in new colloids with applications that use ultrasound contrast enhancement. This work also highlights the emergence of colloidal materials fabricated from or modified with biologically derived and bio-inspired materials, particularly in the form of biopolymers and biomembranes. Challenges, limitations, potential developments and future directions of these next-generation colloidal systems are also presented and discussed.

Triple-combination therapy assisted with ultrasound-active gold nanoparticles and ultrasound therapy against 3D cisplatin-resistant ovarian cancer model

Cancer chemotherapy suffers from drug resistance and side effects of the drugs. Combination therapies have been attracted attention to overcome these limitations of traditional cancer treatments. Recently, increasing in intracellular chemotherapeutic concentration in the presence of ultrasonic waves (US) has been shown in the preclinical stage. In addition, some recent studies have shown that nanoparticles increase the effectiveness of ultrasound therapy. In this study, the US-active property of gold nanocones (AuNCs) was utilized for combinational US and cisplatin (Cis) to overcome drug resistance. The effect of the triple combination therapy US + AuNCs + Cis with low-dose Cis on 2/3D models of cisplatin-resistant ovarian cancer cell line (A2780cis) were investigated. In the 2D cell culture, 60% of the A2780cis cell population was suppressed with triple combination therapy; and the long-term therapeutic efficacy of the US + AuNCs + Cis with the low-dose drug was demonstrated by suppressing 83% of colony formation. According to the results in the 3D cell model, 60% of the spheroid formation was suppressed by the triple combination therapy with low-dose Cis. These results not only demonstrate the success of the US + AuNCs + Cis triple combination therapy for its long-term therapeutic effect on resistant cancer cells but also verified that it might enable effective cancer therapy in vivo and clinical stages based on the 3D tumor models. In addition, enhanced anti-cancer activity was demonstrated at the low-dose Cis on drug-resistant cancer cells indicating the triple-combination therapy successfully overcame drug resistance and this is a promising strategy to reduce the side effects of chemotherapy. This work exhibits a novel US and AuNCs-mediated combination cancer therapy, which demonstrates the role of ultrasound-active AuNCs to combat drug resistance with low-dose chemotherapy.

Gas-stabilizing nanoparticles for ultrasound imaging and therapy of cancer

The use of ultrasound in the clinic has been long established for cancer detection and image-guided tissue biopsies. In addition, ultrasound-based methods have been widely explored to develop more effective cancer therapies such as localized drug delivery, sonodynamic therapy, and focused ultrasound surgery. Stabilized fluorocarbon microbubbles have been in use as contrast agents for ultrasound imaging in the clinic for several decades. It is also known that microbubble cavitation could generate thermal, mechanical, and chemical effects in the tissue to improve ultrasound-based therapies. However, the large size, poor stability, and short-term cavitation activity of microbubbles limit their applications in cancer imaging and therapy. This review will focus on an alternative type of ultrasound responsive material; gas-stabilizing nanoparticles, which can address the limitations of microbubbles with their nanoscale size, robustness, and high cavitation activity. This review will be of interest to researchers who wish to explore new agents to develop improved methods for molecular ultrasound imaging and therapy of cancer.

Unsupported gold nanocones as sonocatalytic agents with enhanced catalytic properties

Gold catalysts have attracted attention for enabling sustainable chemical processes under ambient conditions. This reactivity is attributed to the small size of the catalysts (<5 nm); however, their size also creates difficulty when removing from product streams and often require rare-metal additives to enhance reaction rate kinetics, thereby limiting the environmental benefits of these catalysts. Comparatively, submicron gold catalysts are easier to separate but are much less reactive under ambient conditions. In this study, we synthesized submicron gas-stabilising gold nanocones (gs-AuNCs) that are acoustically responsive to afford greater reaction rates than other conventional gold catalysts. We explore the catalytic performance of acoustically responsive gs-AuNCs exposed to focussed ultrasound at 5.0 MPa peak negative pressure and 1.1 MHz center frequency. Cavitation nucleated from gs-AuNCs significantly increased the sonocatalytic degradation of water pollutants without the need for co-catalysts. The ability to amplify catalysis with ultrasound by tailoring the morphology of the catalyst to control cavitation opens new paths for future designs of sonocatalysts that may enable a sustainable chemical approach needed for a broad range of industrial processes.

Importance of Nano Medicine and New Drug Therapies for Cancer

Cancer is one of the deadly diseases leading to approximately 7.6 million deaths worldwide, with the mortality rate of 13%, and the number of deaths is expected to increase to 13.1 million within the next 10 years. In controlled drug delivery systems (DDS), the drug is transported to the desired location. Thus, the influence of drugs on vital tissues and undesirable side effects can be minimised. Additionally, DDS protects the drug from rapid degradation or clearance and enhances drug concentration in target tissues, and therefore, minimise the required dose of drug. This modern form of therapy is particularly important when there is a discrepancy between the dose and concentration of a drug. Cell-specific targeting can be achieved by attaching drugs to individually designed carriers. Recent developments in nanotechnology have shown that nanoparticles (particles with diameter < 100 nm in at least one dimension) have great potential as drug carriers. Because of their small size, these nanostructures exhibit unique physicochemical and biological properties that make them a favourable material for biomedical applications. Therefore, in this review, we aimed to describe the importance and types of nanomedicines and efficient ways in which new drug delivery systems for the treatment of cancer can be developed.

Investigating the Acoustic Response and Contrast Enhancement of Drug-Loadable PLGA Microparticles with Various Shapes and Morphologies

Polymer nanoparticles and microparticles have been used primarily for drug delivery. There is now growing interest in further developing polymer-based solid cavitation agents to also enhance ultrasound imaging. We previously reported on a facile method to produce hollow poly(lactic-co-glycolic acid) (PLGA) microparticles with different diameters and degrees of porosity. Here, we investigate the cavitation response from these PLGA microparticles with both therapeutic and diagnostic ultrasound transducers. Interestingly, all formulations exhibited stable cavitation; larger porous and multicavity particles also provided inertial cavitation at elevated acoustic pressure amplitudes. These larger particles also achieved contrast enhancement comparable to that of commercially available ultrasound contrast agents, with a maximum recorded contrast-to-tissue ratio of 28 dB. Therefore, we found that multicavity PLGA microparticles respond to both therapeutic and diagnostic ultrasound and may be applied as a theranostic agent.

Ultrasonic Implantation and Imaging of Sound-Sensitive Theranostic Agents for the Treatment of Arterial Inflammation

For site-specific diseases such as atherosclerosis, it is desirable to noninvasively and locally deliver therapeutics for extended periods of time. High-intensity focused ultrasound (HIFU) provides targeted drug delivery, yet remains unable to sustain delivery beyond the HIFU treatment time. Furthermore, methods to validate HIFU-enhanced drug delivery remain limited. In this study, we report on HIFU-targeted implantation of degradable drug-loaded sound-sensitive multicavity PLGA microparticles (mcPLGA MPs) as a theranostic agent for the treatment of arterial lesions. Once implanted into the targeted tissue, mcPLGA MPs eluted dexamethasone for several days, thereby reducing inflammatory markers linked to oxidized lipid uptake in a foam cell spheroid model. Furthermore, implanted mcPLGA MPs created hyperechoic regions on diagnostic ultrasound images, and thus noninvasively verified that the target region was treated with the theranostic agents. This novel and innovative multifunctional theranostic platform may serve as a promising candidate for noninvasive imaging and treatment for site-specific diseases such as atherosclerosis.

Nanostructured TiO2 cavitation agents for dual-modal sonophotocatalysis with pulsed ultrasound

Current sonochemical methods rely on spatially uncontrolled cavitation for radical species generation to promote chemical reactions. To improve radical generation, sonosensitizers have been demonstrated to be activated by cavitation-based light emission (sonoluminescence). Unfortunately, this process remains relatively inefficient compared to direct photocatalysis, due to the physical separation between cavitation event and sonosensitizing agent. In this study, we have synthesized nanostructured titanium dioxide particles to couple the source for cavitation within a photocatalytic site to create a sonophotocatalyst. In doing so, we demonstrate that site-controlled cavitation from the nanoparticles using pulsed ultrasound at reduced acoustic powers resulted in the sonochemical degradation methylene blue at rates nearly three orders of magnitude faster than other titanium dioxide-based nanoparticles by conventional methods. Sonochemical degradation was directly proportional to the measured cavitation produced by these sonophotocatalysts. Our work suggests that simple nanostructuring of current sonosensitizers to enable on-site cavitation greatly enhances sonochemical reaction rates.

Gas-Stabilizing Sub-100 nm Mesoporous Silica Nanoparticles for Ultrasound Theranostics

Recent studies have demonstrated that gas-stabilizing particles can generate cavitating micron-sized bubbles when exposed to ultrasound, offering excellent application potential, including ultrasound imaging, drug delivery, and tumor ablation. However, the majority of the reported gas-stabilizing particles are relatively large (>200 nm), and smaller particles require high acoustic pressures to promote cavitation. Here, this paper reports the preparation of sub-100 nm gas-stabilizing nanoparticles (GSNs) that can initiate cavitation at low acoustic intensities, which can be delivered using a conventional medical ultrasound imaging system. The highly echogenic GSNs (F127-hMSN) were prepared by carefully engineering the surfaces of ∼50 nm mesoporous silica nanoparticles. It was demonstrated that the F127-hMSNs could be continuously imaged with ultrasound in buffer or biological solutions or agarose phantoms for up to 20 min. Also, the F127-hMSN can be stored in phosphate-buffered saline for at least a month with no loss in ultrasound responsiveness. The particles significantly degraded when diluted in simulated body fluids, indicating possible biodegradation of the F127-hMSNs in vivo. Furthermore, at ultrasound imaging conditions, F127-hMSNs did not cause detectable cell death, supporting the potential safety of these particles. Finally, strong cavitation activity generation by the F127-hMSNs under high-intensity focused ultrasound insonation was demonstrated and applied to effectively ablate cancer cells.

Role of gold nanoparticles in advanced biomedical applications

Gold nanoparticles (GNPs) have generated keen interest among researchers in recent years due to their excellent physicochemical properties. In general, GNPs are biocompatible, amenable to desired functionalization, non-corroding, and exhibit size and shape dependent optical and electronic properties. These excellent properties of GNPs exhibit their tremendous potential for use in diverse biomedical applications. Herein, we have evaluated the recent advancements of GNPs to highlight their exceptional potential in the biomedical field. Special focus has been given to emerging biomedical applications including bio-imaging, site specific drug/gene delivery, nano-sensing, diagnostics, photon induced therapeutics, and theranostics. We have also elaborated on the basics, presented a historical preview, and discussed the synthesis strategies, functionalization methods, stabilization techniques, and key properties of GNPs. Lastly, we have concluded this article with key findings and unaddressed challenges. Overall, this review is a complete package to understand the importance and achievements of GNPs in the biomedical field.

Ultrasound-Responsive Carriers for Therapeutic Applications

Ultrasound (US)-responsive carriers have emerged as promising theranostic candidates because of their ability to enhance US-contrast, promote image-guided drug delivery, cause on-demand pulsatile release of drugs in response to ultrasound stimuli, as well as to enhance the permeability of physiological barriers such as the stratum corneum, the vascular endothelium, and the blood-brain barrier (BBB). US-responsive carriers include microbubbles MBs, liposomes, droplets, hydrogels, and nanobubble-nanoparticle complexes and have been explored for cavitation-mediated US-responsive drug delivery. Recently, a transient increase in the permeability of the BBB by microbubble (MB)-assisted low-frequency US has shown promise in enhancing the delivery of therapeutic agents in the case of neurological disorders. Further, the periodic mechanical stimulus generated by US-responsive MBs have also been explored in tissue engineering and has directly influenced the differentiation of mesenchymal stem cells into cartilage. This Review discusses the various types of US-responsive carriers and explores their emerging roles in therapeutics ranging from drug delivery to tissue engineering.

Microbubble Agents: New Directions

Microbubble ultrasound contrast agents have now been in use for several decades and their safety and efficacy in a wide range of diagnostic applications have been well established. Recent progress in imaging technology is facilitating exciting developments in techniques such as molecular, 3-D and super resolution imaging and new agents are now being developed to meet their specific requirements. In parallel, there have been significant advances in the therapeutic applications of microbubbles, with recent clinical trials demonstrating drug delivery across the blood-brain barrier and into solid tumours. New agents are similarly being tailored toward these applications, including nanoscale microbubble precursors offering superior circulation times and tissue penetration. The development of novel agents does, however, present several challenges, particularly regarding the regulatory framework. This article reviews the developments in agents for diagnostic, therapeutic and "theranostic" applications; novel manufacturing techniques; and the opportunities and challenges for their commercial and clinical translation.

Precise tailoring of evaporated gold nanocones using electron beam lithography and lift-off

The ability to fabricate nanocones with precise dimensions is essential for several emerging applications. We demonstrate here a method which can be used to fabricate arrays of gold nanocones with high dimensional precision using lithographic and lift-off means. electron beam (ebeam) writing of a spin-coated PMMA-based bilayer resist deposited onto silicon wafers is used to form a shadow mask. This mask gradually closes as the deposition of gold (using ebeam evaporation) proceeds-the result is arrays of gold nanocones on the silicon wafer surface after lift-off of the resist. Observations using scanning electron microscopy enable a statistical study of the dimensions of 360 gold nanocones-the results demonstrate the high precision of the nanocones dimensions. The fabrication process enables the creation of arrays of nanocones with a base diameter varying from 53.6 ± 2.1 nm to 94.1 ± 2.4 nm, a vertical height ranging from 71.3 ± 4.1 nm to 153.4 ± 3.4 nm, and an apex radius of curvature ranging from 8.4 ± 1.2 nm to 11.6 ± 1.5 nm. The results are compared with the predictions of a deposition model which considers the evolving shadow masking during the gold deposition to compute the nanocone profile.

Spatiotemporal Assessment of the Cellular Safety of Cavitation-Based Therapies by Passive Acoustic Mapping

Many useful therapeutic bio-effects can be generated using ultrasound-induced cavitation. However, cavitation is also capable of causing unwanted cellular and vascular damage, which should be monitored to ensure treatment safety. In this work, the unique opportunity provided by passive acoustic mapping (PAM) to quantify cavitation dose across an entire volume of interest during therapy is utilised to provide setup-independent measures of spatially localised cavitation dose. This spatiotemporally quantifiable cavitation dose is then related to the level of cellular damage generated. The cavitation-mediated destruction of equine red blood cells mixed with one of two types of cavitation nuclei at a variety of concentrations is investigated. The blood is placed within a 0.5-MHz ultrasound field and exposed to a range of peak rarefactional pressures up to 2 MPa, with 50 to 50,000 cycle pulses maintaining a 5% duty cycle. Two co-planar linear arrays at 90° to each other are used to generate 400-µm-resolution frequency domain robust capon beamforming PAM maps, which are then used to generate estimates of cavitation dose. A relationship between this cavitation dose and the levels of haemolysis generated was found which was comparable regardless of the applied acoustic pressure, pulse length, cavitation agent type or concentration used. PAM was then used to monitor cellular damage in multiple locations within a tissue phantom simultaneously, with the damage-cavitation dose relationship being similar for the two experimental models tested. These results lay the groundwork for this method to be applied to other measures of safety, allowing for improved ultrasound monitoring of cavitation-based therapies.

Biomedical applications of acoustically responsive phase shift nanodroplets: Current status and future directions

The evolution of ultrasonic contrast agents to enhance the reflectivity of structures in the human body has consolidated ultrasound's stance as a reliable diagnostic imaging modality. A significant development within this field includes the advent of liquid nanodroplets that are capable of vaporising into gaseous microbubbles upon ultrasonic irradiation. This literature review will therefore appraise and summarise the available literature on the generation of phase-shift nanodroplets, their formulations, applications, safety issues, future developments and any implications that may inhibit their clinical implementation. The main findings of this review affirm that phase change nanodroplets do indeed demonstrate functionality in drug delivery and targeting and characterisation of tumours. Its bioeffects however, have not yet been extensively researched, prompting further exploration into how bubble size can be controlled once it has vaporised into microbubbles and the resulting complications. As such, future research should be directed towards determining the safety, longevity and suitability of phase-shift nanodroplets over contrast agents in current clinical use.

The Role of PEG-40-stearate in the Production, Morphology, and Stability of Microbubbles

Phospholipid coated microbubbles are currently in widespread clinical use as ultrasound contrast agents and under investigation for therapeutic applications. Previous studies have demonstrated the importance of the coating nanostructure in determining microbubble stability and its dependence upon both composition and processing method. While the influence of different phospholipids has been widely investigated, the role of other constituents such as emulsifiers has received comparatively little attention. Herein, we present an examination of the impact of polyethylene glycol (PEG) derivatives upon microbubble structure and properties. We present data using both pegylated phospholipids and a fluorescent PEG-40-stearate analogue synthesized in-house to directly observe its distribution in the microbubble coating. We examined microbubbles of clinically relevant sizes, investigating both their surface properties and population size distribution and stability. Domain formation was observed only on the surface of larger microbubbles, which were found to contain a higher concentration of PEG-40-stearate. Lipid analogue dyes were also found to influence domain formation compared with PEG-40-stearate alone. "Squeezing out" of PEG-40-stearate was not observed from any of the microbubble sizes investigated. At ambient temperature, microbubbles formulated with DSPE-PEG(2000) were found to be more stable than those containing PEG-40-stearate. At 37 °C, however, the stability in serum was found to be the same for both formulations, and no difference in acoustic backscatter was detected. This could potentially reduce the cost of PEGylated microbubbles and facilitate simpler attachment of targeting or therapeutic species. However, whether PEG-40-stearate sufficiently shields microbubbles to inhibit physiological clearance mechanisms still requires investigation.

Biomedical Applications for Gas-Stabilizing Solid Cavitation Agents

For over a decade, advancements in ultrasound-enhanced drug delivery strategies have demonstrated remarkable success in providing targeted drug delivery for a broad range of diseases. In order to achieve enhanced drug delivery, these strategies harness the mechanical effects from bubble oscillations (i.e., cavitation) of a variety of exogenous cavitation agents. Recently, solid cavitation agents have emerged due to their capacity for drug-loading and sustained cavitation duration. Unlike other cavitation agents, solid cavitation agents stabilize gaseous bubbles on hydrophobic surface cavities. Thus, the design of these particles is crucial. In this Review, we provide an overview of the different designs for solid cavitation agents such as nanocups, nanocones, and porous structures, as well as the current status of their development. Considering the numerous advantages of solid cavitation agents, we anticipate further innovations for this new type of cavitation agent across a broad range of biomedical applications.

Ultrasound-mediated cavitation enhances the delivery of an EGFR-targeting liposomal formulation designed for chemo-radionuclide therapy

Nanomedicines allow active targeting of cancer for diagnostic and therapeutic applications through incorporation of multiple functional components. Frequently, however, clinical translation is hindered by poor intratumoural delivery and distribution. The application of physical stimuli to promote tumour uptake is a viable route to overcome this limitation. In this study, ultrasound-mediated cavitation of microbubbles was investigated as a mean of enhancing the delivery of a liposome designed for chemo-radionuclide therapy targeted to EGFR overexpressing cancer. Method: Liposomes (¹¹¹In-EGF-LP-Dox) were prepared by encapsulation of doxorubicin (Dox) and surface functionalisation with Indium-111 tagged epidermal growth factor. Human breast cancer cell lines with high and low EGFR expression (MDA-MB-468 and MCF7 respectively) were used to study selectivity of liposomal uptake, subcellular localisation of drug payload, cytotoxicity and DNA damage. Liposome extravasation following ultrasound-induced cavitation of microbubbles (SonoVue®) was studied using a tissue-mimicking phantom. In vivo stability, pharmacokinetic profile and biodistribution were evaluated following intravenous administration of ¹¹¹In-labelled, EGF-functionalised liposomes to mice bearing subcutaneous MDA-MB-468 xenografts. Finally, the influence of ultrasound-mediated cavitation on the delivery of liposomes into tumours was studied. Results: Liposomes were loaded efficiently with Dox, surface decorated with ¹¹¹In-EGF and showed selective uptake in MDA-MB-468 cells compared to MCF7. Following binding to EGFR, Dox was released into the intracellular space and ¹¹¹In-EGF shuttled to the cell nucleus. DNA damage and cell kill were higher in MDA-MB-468 than MCF7 cells. Moreover, Dox and ¹¹¹In were shown to have an additive cytotoxic effect in MDA-MB-468 cells. US-mediated cavitation increased the extravasation of liposomes in an in vitro gel phantom model. In vivo, the application of ultrasound with microbubbles increased tumour uptake by 66% (p<0.05) despite poor vascularisation of MDA-MB-468 xenografts (as shown by DCE-MRI). Conclusion: ¹¹¹In-EGF-LP-Dox designed for concurrent chemo-radionuclide therapy showed specificity for and cytotoxicity towards EGFR-overexpressing cancer cells. Delivery to tumours was enhanced by the use of ultrasound-mediated cavitation indicating that this approach has the potential to deliver cytotoxic levels of therapeutic radionuclide to solid tumours.

Remote targeted implantation of sound-sensitive biodegradable multi-cavity microparticles with focused ultrasound

Ultrasound-enhanced drug delivery has shown great promise in providing targeted burst release of drug at the site of the disease. Yet current solid ultrasound-responsive particles are non-degradable with limited potential for drug-loading. Here, we report on an ultrasound-responsive multi-cavity poly(lactic-co-glycolic acid) microparticle (mcPLGA MP) loaded with rhodamine B (RhB) with or without 4',6-diamidino-2-phenylindole (DAPI) to represent small molecule therapeutics. After exposure to high intensity focused ultrasound (HIFU), these delivery vehicles were remotely implanted into gel and porcine tissue models, where the particles rapidly released their payload within the first day and sustained release for at least seven days. RhB-mcPLGA MPs were implanted with HIFU into and beyond the sub-endothelial space of porcine arteries without observable damage to the artery. HIFU also guided the location of implantation; RhB-mcPLGA MPs were only observed at the focus of the HIFU away from the direction of ultrasound. Once implanted, DAPI co-loaded RhB-mcPLGA MPs released DAPI into the arterial wall, staining the nucleus of the cells. Our work shows the potential for HIFU-guided implantation of drug-loaded particles as a strategy to improve the local and sustained delivery of a therapeutic for up to two weeks.

Nanoparticle Activation Methods in Cancer Treatment

In this review, we intend to highlight the progress which has been made in recent years around different types of smart activation nanosystems for cancer treatment. Conventional treatment methods, such as chemotherapy or radiotherapy, suffer from a lack of specific targeting and consequent off-target effects. This has led to the development of smart nanosystems which can effect specific regional and temporal activation. In this review, we will discuss the different methodologies which have been designed to permit activation at the tumour site. These can be divided into mechanisms which take advantage of the differences between healthy cells and cancer cells to trigger activation, and those which activate by a mechanism extrinsic to the cell or tumour environment.

Colloids, nanoparticles, and materials for imaging, delivery, ablation, and theranostics by focused ultrasound (FUS)

This review focuses on different materials and contrast agents that sensitize imaging and therapy with Focused Ultrasound (FUS). At high intensities, FUS is capable of selectively ablating tissue with focus on the millimeter scale, presenting an alternative to surgical intervention or management of malignant growth. At low intensities, FUS can be also used for other medical applications such as local delivery of drugs and blood brain barrier opening (BBBO). Contrast agents offer an opportunity to increase selective acoustic absorption or facilitate destructive cavitation processes by converting incident acoustic energy into thermal and mechanical energy. First, we review the history of FUS and its effects on living tissue. Next, we present different colloidal or nanoparticulate approaches to sensitizing FUS, for example using microbubbles, phase-shift emulsions, hollow-shelled nanoparticles, or hydrophobic silica surfaces. Exploring the science behind these interactions, we also discuss ways to make stimulus-responsive, or "turn-on" contrast agents for improved selectivity. Finally, we discuss acoustically-active hydrogels and membranes. This review will be of interest to those working in materials who wish to explore new applications in acoustics and those in acoustics who are seeking new agents to improve the efficacy of their approaches.

Microbubbles, Nanodroplets and Gas-Stabilizing Solid Particles for Ultrasound-Mediated Extravasation of Unencapsulated Drugs: An Exposure Parameter Optimization Study

Ultrasound-induced cavitation has been proposed as a strategy to tackle the challenge of inadequate extravasation, penetration and distribution of therapeutics into tumours. Here, the ability of microbubbles, droplets and solid gas-trapping particles to facilitate mass transport and extravasation of a model therapeutic agent following ultrasound-induced cavitation is investigated. Significant extravasation and penetration depths on the order of millimetres are achieved with all three agents, including the range of pressures and frequencies achievable with existing clinical ultrasound systems. Deeper but highly directional extravasation was achieved with frequencies of 1.6 and 3.3 MHz compared with 0.5 MHz. Increased extravasation was observed with increasing pulse length and exposure time, while an inverse relationship is observed with pulse repetition frequency. No significant cell death or any haemolytic activity in human blood was observed at clinically relevant concentrations for any of the agents. Overall, solid gas-trapping nanoparticles were found to enable the most extensive extravasation for the lowest input acoustic energy, followed by microbubbles and then droplets. The ability of these agents to produce sustained inertial cavitation activity whilst being small enough to follow the drug out of the circulation and into diseased tissue, combined with a good safety profile and the possibility of real-time monitoring, offers considerable potential for enhanced drug delivery of unmodified drugs in oncological and other biomedical applications.