ABSTRACT
Investigation of the cavitation activity during ultrasonic treatment of magnesium particles during nanostructuring has been performed. Cavitation activity is recorded in the continuous mode after switching the ultrasound on with the use of ICA-5DM cavitometer. It has been demonstrated that this characteristic of the cavitation zone may be varied in a wide range of constant output parameters of the generator. The speed and nature of the cavitation activity alteration depended on the concentration of Mg particles in the suspension and the properties of the medium in which the sonochemical treatment has been performed. Three stages of the cavitation area evolution can be distinguished: 1 - the initial increase in cavitation activity, 2 - reaching a maximum with a subsequent decrease, and 3 - reaching the plateau (or the repeated cycles with feedback loops of enlargement/reduction of the cavitation activity). The ultrasonically treated magnesium particles have been characterized by scanning electron microscopy, X-ray diffraction analysis and thermal analysis. Depending on the nature of the dispersed medium the particles can be characterized by the presence of magnesium hydroxide (brucite) and magnesium hydride. It is possible to reach the incorporation of magnesium hydride in the magnesium hydroxide/magnesium matrix by varying the conditions of ultrasonic treatment (duration of treatment, amplitude, dispersed medium etc.). The influence of the magnesium reactivity is also confirmed by the measurements of cavitation activity in organic dispersed media (ethanol, ethylene glycol) and their aqueous mixtures.
ABSTRACT
The intense conditions generated in the core of a collapsing bubble have been the subject of intense scrutiny from fields as diverse as marine biology and nuclear fusion. In particular, the phenomenon of sonoluminescence, whereby a collapsing bubble emits light, has received significant attention. Sonoluminescence has been associated predominantly with millimeter-sized bubbles excited at low frequencies and under conditions far removed from those associated with the use of ultrasound in medicine. In this study, however, we demonstrate that sonoluminescence is produced under medically relevant exposure conditions by microbubbles commonly used as contrast agents for ultrasound imaging. This provides a mechanistic explanation for the somewhat controversial reports of "sonodynamic" therapy, in which light-sensitive drugs have been shown to be activated by ultrasound-induced cavitation. To illustrate this, we demonstrate the activation of a photodynamic therapy agent using microbubbles and ultrasound. Since ultrasound can be accurately focused at large tissue depths, this opens up the potential for generating light at locations that cannot be reached by external sources. This could be exploited both for diagnostic and therapeutic applications, significantly increasing the range of applications that are currently restricted by the limited penetration of light in the tissue.
Subject(s)
Microbubbles , Ultrasonics , Contrast Media/chemistry , Reactive Oxygen Species/metabolismABSTRACT
Ultrasound is used to trigger the cytotoxicity of chemical compounds, known as sonosensitisers, in an approach called sonodynamic therapy (SDT), which is under investigation herein. The generation of reactive oxygen species (ROS) has been proposed as the main biological occurrence that leads to the cytotoxic effects, which are achieved via the synergistic action of two components: the energy-absorbing sonosensitiser and ultrasound (US), which are both harmless per se. Despite some promising results, a lack of investigation into the mechanisms behind US sonosensitiser-mediated ROS generation has prevented SDT from reaching its full potential. The aim of this work is to investigate the US-responsiveness of a variety of metal-porphyrin complexes, free-base porphyrin and Fe(III), Zn(II) and Pd(II) porphyrin, by analyzing their ROS generation under US exposure and related bio-effects. All experiments were also carried out under light exposure and the results were used as references. Our results show that porphyrin ultrasound-responsiveness depends on the metal ion present, with Zn(II) and Pd(II) porphyrin being the most efficient in generating singlet oxygen and hydroxyl radicals. ROS production efficiency is lower after ultrasound exposure than after light exposure, because of the various physico-chemical mechanisms involved in sensitiser activation. US and porphyrin-mediated ROS generation is oxygen-dependent and the activation of porphyrin by US appears to be more compatible with sonoluminescence-based photo-activation rather than a radical path process that occurs via the homolytic bond rupture of water. Notably, the cytotoxicity results reported herein, which are mirrored by ex-cellulo data, confirm that the type of ROS generation achieved by the US activation of intracellular porphyrins is pivotal to the effectiveness of cancer cell killing.
