Paul Prentice

Hybrid gold-iron oxide nanoparticles as a multifunctional platform for biomedical application

BackgroundIron oxide nanoparticles (IONPs) have increasing applications in biomedicine, however fears over long term stability of polymer coated particles have arisen. Gold coating IONPs results in particles of increased stability and robustness. The unique properties of both the iron oxide (magnetic) and gold (surface plasmon resonance) result in a multimodal platform for use as MRI contrast agents and as a nano-heater.ResultsHere we synthesize IONPs of core diameter 30 nm and gold coat using the seeding method with a poly(ethylenimine) intermediate layer. The final particles were coated in poly(ethylene glycol) to ensure biocompatibility and increase retention times in vivo. The particle coating was monitored using FTIR, PCS, UV–vis absorption, TEM, and EDX. The particles appeared to have little cytotoxic effect when incubated with A375M cells. The resultant hybrid nanoparticles (HNPs) possessed a maximal absorbance at 600 nm. After laser irradiation in agar phantom a ΔT of 32°C was achieved after only 90 s exposure (50 μgmL-1). The HNPs appeared to decrease T2 values in line with previously clinically used MRI contrast agent Feridex®.ConclusionsThe data highlights the potential of these HNPs as dual function MRI contrast agents and nano-heaters for therapies such as cellular hyperthermia or thermo-responsive drug delivery.

Laser-nucleated acoustic cavitation in focused ultrasound

Acoustic cavitation can occur in therapeutic applications of high-amplitude focused ultrasound. Studying acoustic cavitation has been challenging, because the onset of nucleation is unpredictable. We hypothesized that acoustic cavitation can be forced to occur at a specific location using a laser to nucleate a microcavity in a pre-established ultrasound field. In this paper we describe a scientific instrument that is dedicated to this outcome, combining a focused ultrasound transducer with a pulsed laser. We present high-speed photographic observations of laser-induced cavitation and laser-nucleated acoustic cavitation, at frame rates of 0.5×10(6) frames per second, from laser pulses of energy above and below the optical breakdown threshold, respectively. Acoustic recordings demonstrated inertial cavitation can be controllably introduced to the ultrasound focus. This technique will contribute to the understanding of cavitation evolution in focused ultrasound including for potential therapeutic applications.

Periodic shock-emission from acoustically driven cavitation clouds: A source of the subharmonic signal

Single clouds of cavitation bubbles, driven by 254kHz focused ultrasound at pressure amplitudes in the range of 0.48-1.22MPa, have been observed via high-speed shadowgraphic imaging at 1×10(6) frames per second. Clouds underwent repetitive growth, oscillation and collapse (GOC) cycles, with shock-waves emitted periodically at the instant of collapse during each cycle. The frequency of cloud collapse, and coincident shock-emission, was primarily dependent on the intensity of the focused ultrasound driving the activity. The lowest peak-to-peak pressure amplitude of 0.48MPa generated shock-waves with an average period of 7.9±0.5μs, corresponding to a frequency of f0/2, half-harmonic to the fundamental driving. Increasing the intensity gave rise to GOC cycles and shock-emission periods of 11.8±0.3, 15.8±0.3, 19.8±0.2μs, at pressure amplitudes of 0.64, 0.92 and 1.22MPa, corresponding to the higher-order subharmonics of f0/3, f0/4 and f0/5, respectively. Parallel passive acoustic detection, filtered for the fundamental driving, revealed features that correlated temporally to the shock-emissions observed via high-speed imaging, p(two-tailed) < 0.01 (r=0.996, taken over all data). Subtracting the isolated acoustic shock profiles from the raw signal collected from the detector, demonstrated the removal of subharmonic spectral peaks, in the frequency domain. The larger cavitation clouds (>200μm diameter, at maximum inflation), that developed under insonations of peak-to-peak pressure amplitudes >1.0MPa, emitted shock-waves with two or more fronts suggesting non-uniform collapse of the cloud. The observations indicate that periodic shock-emissions from acoustically driven cavitation clouds provide a source for the cavitation subharmonic signal, and that shock structure may be used to study intra-cloud dynamics at sub-microsecond timescales.

Deconvolution of acoustically detected bubble-collapse shock waves.

HIGHLIGHTSBubble collapse shock waves are detected with a non‐flat needle‐type hydrophone.Shock wave characteristics are retrieved through detector deconvolution over a limited bandwidth.Full waveform deconvolution indicates the apparent negative phase is a detector artefact.Magnitude‐only deconvolution is required for quantitative analysis of cavitation emission spectra. ABSTRACT The shock wave emitted by the collapse of a laser‐induced bubble is detected at propagation distances of 30, 40 and 50 mm, using a PVdF needle hydrophone, with a non‐flat end‐of‐cable frequency response, calibrated for magnitude and phase, from 125 kHz to 20 MHz. High‐speed shadowgraphic imaging at 5 × 106 frames per second, 10 ns temporal resolution and 256 frames per sequence, records the bubble deflation from maximum to minimum radius, the collapse and shock wave generation, and the subsequent rebound in unprecedented detail, for a single sequence of an individual bubble. The Gilmore equation for bubble oscillation is solved according to the resolved bubble collapse, and simulated shock wave profiles deduced from the acoustic emissions, for comparison to the hydrophone recordings. The effects of single‐frequency calibration, magnitude‐only and full waveform deconvolution of the experimental data are presented, in both time and frequency domains. Magnitude‐only deconvolution increases the peak pressure amplitude of the measured shock wave by approximately 9%, from single‐frequency calibration, with full waveform deconvolution increasing it by a further 3%. Full waveform deconvolution generates a shock wave profile that is in agreement with the simulated profile, filtered according to the calibration bandwidth. Implications for the detection and monitoring of acoustic cavitation, where the role of periodic bubble collapse shock waves has recently been realised, are discussed.

Directed jetting from collapsing cavities exposed to focused ultrasound

We demonstrate directed jetting from pulsed laser-induced cavities subjected to a burst of focused ultrasound. Alignment of the ultrasound focus and the pressure amplitudes in the vicinity of the cavity dictate the direction and length of the resulting jet, respectively. We interpret our observations in terms of radiation forces exerted on the cavity, due to the pressure gradient introduced to the ultrasound focus by its presence. We support our hypothesis with a linear analysis of the force distribution across the cavity surface, at the moment of maximum inflation, which shows reasonable predictive agreement with the observed jet characteristics.

The importance of physics to progress in medical treatment

Physics in therapy is as diverse as it is substantial. In this review, we highlight the role of physics--occasionally transitioning into engineering--through discussion of several established and emerging treatments. We specifically address minimal access surgery, ultrasound, photonics, and interventional MRI, identifying areas in which complementarity is being exploited. We also discuss some of the fundamental physical principles involved in the application of each treatment to medical practice.

Bifurcation of ensemble oscillations and acoustic emissions from early stage cavitation clouds in focused ultrasound

The acoustic emissions from single cavitation clouds at an early stage of development in 0.521 MHz focused ultrasound of varying intensity, are detected and directly correlated to high-speed microscopic observations, recorded at 1 × 106 frames per second. At lower intensities, a stable regime of cloud response is identified whereby bubble-ensembles exhibit oscillations at half the driving frequency, which is also detected in the acoustic emission spectra. Higher intensities generate clouds that develop more rapidly, with increased nonlinearity evidenced by a bifurcation in the frequency of ensemble response, and in the acoustic emissions. A single bubble oscillation model is subject to equivalent ultrasound conditions and fitted to features in the hydrophone and high-speed spectral data, allowing an effective quiescent radius to be inferred for the clouds that evolve at each intensity. The approach indicates that the acoustic emissions originate from the ensemble dynamics and that the cloud acts as a single bubble of equivalent radius in terms of the scattered field. Jetting from component cavities on the periphery of clouds is regularly observed at higher intensities. The results may be of relevance for monitoring and controlling cavitation in therapeutic applications of focused ultrasound, where the phenomenon has the potential to mediate drug delivery from vasculature.

An analysis of the acoustic cavitation noise spectrum: The role of periodic shock waves

Research on applications of acoustic cavitation is often reported in terms of the features within the spectrum of the emissions gathered during cavitation occurrence. There is, however, limited understanding as to the contribution of specific bubble activity to spectral features, beyond a binary interpretation of stable versus inertial cavitation. In this work, laser-nucleation is used to initiate cavitation within a few millimeters of the tip of a needle hydrophone, calibrated for magnitude and phase from 125 kHz to 20 MHz. The bubble activity, acoustically driven at f0 = 692 kHz, is resolved with high-speed shadowgraphic imaging at 5 × 106 frames per second. A synthetic spectrum is constructed from component signals based on the hydrophone data, deconvolved within the calibration bandwidth, in the time domain. Cross correlation coefficients between the experimental and synthetic spectra of 0.97 for the f0/2 and f0/3 regimes indicate that periodic shock waves and scattered driving field predominantly account for all spectral features, including the sub-harmonics and their over-harmonics, and harmonics of f0.

Ultrasound Activated Nano-Encapsulated Targeted Drug Delivery and Tumour Cell Poration

INTRODUCTION Recently, ultrasonic drug release has been a focus of many research groups for stimuli responsive drug release. It has been demonstrated that a focused ultrasound (FUS) beam rapidly increases the temperature at the focused tissue area. One potential mechanism of drug targeting is to utilize the induced heat to release or increase penetration of chemotherapy to cancer cells. The efficiency of targeted drug delivery may increase by using FUS beam in conjugation with nano--encapsulated drug carriers.The aim of this study is to investigate the effect of heat and ultrasound on the cellular uptake and therapeutic efficacy of an anticancer drug using Magnetic Resonance Imaging guided Focused Ultrasound (MRgFUS). MATERIALS AND METHODS Human KB cells (CCL-17 cells) were seeded into 96-well plates and heat treated at 37-55°C for 2-10 min. Cell viability was determined using the colorimetric MTT assay. The cells were also subjected to MRgFUS and the degree of cell viability was determined. These experiments were conducted using an ExAblate 2000 system (InSightec, Haifa, Israel) and a GE 1.5 T MRI system, software release 15. RESULTS We have observed a significant decrease in human KB cell viability due to heat (>41°C) in the presence of Doxorubicin (DOX), in comparison with DOX at normal culture temperature (37°C). The synergistic effect of heat with DOX may be explained by several mechanisms. One potential mechanism may be increased penetration of DOX to the cells during heating. In addition, we have shown that ultrasound induced cavitation causes cell necrosis. DISCUSSION AND FUTURE WORK: Further investigation is required to optimize the potential of MRgFUS to enhance cellular uptake of therapeutic agents. A novel delivery nano-vehicle developed by CapsuTech will be investigated with MRgFUS for its potential as a stimuli responsive delivery system.

Spatially controlled sonoporation of prostate cancer cells via ultrasound activated microbubble cavitation

Cells that are exposed to ultrasonic (US) energy, in the presence of ultrasound contrast agent microbubbles, may experience enhanced membrane permeability. If the effective dose of US exceeds some threshold, then cell lysis can result (lethal sonoporation), however for lower doses a transient enhancement of membrane permeability occurs (reversible or non lethal sonoporation). The merits of each mode are clear: lethal sonoporation constitutes a significant tumour therapy weapon, whilst its less intrusive counterpart, reversible sonoporation, represents an effective non-invasive targeted drug delivery technique. Until now, the mechanism of the dynamic interaction between microbubbles and cells has remained unknown. Moreover pores, which are the presumed mode of permeabilization have not been observed in a convincing fashion. We will demonstrate, for the first time, how an innovative hybridization of holographic optical trapping technology, together with the application of MHz pulsed US energy and subsequent high resolution observation using atomic force microscopy has been used to elucidate the fundamental mode for membrane permeabilization during sonoporation.