Synthesis of Amorphous Hydrogenated Boron Carbide from Orthocarborane Using Argon Bombardment: A Reaxff Molecular Dynamics Study

In this study, the synthesis process of a-B­xC:Hy using argon bombardment from the orthocarborane precursor was modeled by using reactive molecular dynamics (MD). Utilizing the MD simulations, the formation of free radicals as a result of ion bombardment was identified and quantified. Then, the densification process that is aided by the mixture of free radicals and orthocarborane was analyzed. The densification process by creating the initial structure composed of free radicals and orthocarborane with active sites created by partially removing some of the hydrogen atoms from the icosahedral cage was also modelled. Overall, a better understanding of the mechanism of the densification of hydrogenated boron carbide and the roles of Ar bombardment and radical species toward the deposition process were obtained

Controlling microbubble dynamics in ultrasound therapy

Microbubble-mediated focused ultrasound therapies such as blood-brain barrier opening, sonothrombolysis, and sonoporation can non-invasively deliver drugs to a targeted region. Despite the potential impact this technology could have in the clinic, there are currently concerns regarding its efficacy, uniformity and safety. These challenges ultimately stem from a limited ability to control the microbubble dynamics during ultrasound exposure. In the current thesis, we sought to design ultrasound exposure sequences and develop monitoring techniques that promote the desired acoustic cavitation activity and suppress unwanted stimuli that do not produce a safe therapeutic bioeffect. For example, violent cavitation activity could cause irreversible damage within the treatment area. The behaviour of microbubble populations exposed to low-power therapeutic ultrasound was first qualitatively studied using high-speed microscopy. Microbubbles were found to form large clusters within milliseconds of exposure and collectively coalesce into larger bubbles. Based on these observations and findings in the literature, new therapeutic sequences were designed and tested. Rapid short-pulse sonication consisted of μs-long pulses separated by short off-times. When compared to conventional ultrasound sequences, this pulse sequence enhanced the lifetime and mobility of cavitation nuclei and resulted in more uniform acoustic activity distributions. To better monitor ultrasound treatment as it evolves, we developed a method that passively measures microbubble velocities via the Doppler effect emerging in the microbubble acoustic emissions. Using standard passive cavitation detection techniques in one and two dimensions, we estimated microbubble velocities on the order of m s-1 during ultrasound exposure. Finally, we tested our new therapeutic design in a mouse model in order to improve the safety of blood-brain barrier opening. We achieved drug delivery with a similar magnitude but with a better uniformity compared to conventional sequences, thus demonstrating evidence of favourable microbubble dynamics within the targeted region. Taken together, our contribution in ultrasonic stimulation using new sequences and monitoring using passive acoustic detection techniques improves our control of microbubble dynamics in ultrasound therapy and has the potential to promote treatment efficacy and suppress unwanted damage.Open Acces

Generation of heterogeneous cellular structures by sonication

Many materials require functionally graded cellular microstructures whose porosity (i.e. ratio of the void volume to the total volume of a material) is engineered to meet specific requirements and for an optimal performance in diverse applications. Numerous applications have demonstrated the potential of porous materials in areas ranging from biomaterial science through to structural engineering. Polymeric foams are an example of a cellular material whose microstructure can be considered as a blend of material and nonmaterial zones. While a huge variety of foams can be manufactured with homogenous porosity, for heterogeneous foams there are no generic processes for controlling the distribution of porosity throughout the resulting matrix. Motivated by the desire to create a flexible process for engineering heterogeneous foams, this work has investigated how ultrasound, applied during some of the foaming stages of a polyurethane melt, affects both the cellular structure and distribution of the pore size. After reviewing the literature concerning foam chemistry, ultrasound and sonochemistry, series of experiments were performed that used an ultrasonic field created by a sonotrode irradiating in a water bath containing a strategically placed vessel filled with foaming reactants. Prior to this, the acoustic field in the bath had been accurately mapped so that the acoustic pressure conditions within the foam container were known. During the foam polymerisation reaction, the acoustic pressure in the water bath varied causing the bubbles to pulsate in a state of ‘stable cavitation’ (i.e. rectified diffusion). This pulsation of the bubbles pumped gas from the liquid to the gas phase inducing them to increase in volume. The eventual solidification resulted in a porous material with a cellular structure that reflected the acoustic field imposed upon it. The experimental results revealed how the parameters of ultrasound exposure (i.e. frequency and acoustic pressure) influenced the volume and distribution of pores within the final polyurethane matrix: it was found that porosity varies in direct proportion to both the acoustic pressure and the frequency of the ultrasound signal. The effects of ultrasound on porosity demonstrated by this work offer the prospect of a manufacturing process that can control and adjust the cellular geometry of foam and hence ensure that the resulting characteristics of the heterogeneous material match the functional requirements.Engineering and Physical Sciences Research Council (EPSRC)Neilson Endowment Fund, in the Department of Mechanical Engineerin