Las técnicas ablativas basadas en energía por radiofrecuencia (RF) se emplean con el fin de lograr un calentamiento seguro y localizado en el tejido biológico. En los últimos años ha habido un rápido crecimiento en el número de nuevos procedimientos médicos que hacen uso de dichas técnicas, lo cual ha ido acompañado de la aparición de nuevos diseños de electrodos y protocolos de aplicación de energía. Sin embargo, existen todavía muchas incógnitas sobre el verdadero comportamiento electro-térmico de los aplicadores de energía, así como de la interacción energía-tejido en aplicaciones concretas. El principal propósito de esta Tesis Doctoral es adquirir un mejor conocimiento de los fenómenos eléctricos y térmicos involucrados en los procesos de calentamiento de tejidos biológicos mediante corrientes de RF. Esto permitirá, por un lado, mejorar la eficacia y seguridad de las técnicas actualmente empleadas en la clínica en campos tan diferentes como la cirugía cardiaca, oncológica o dermatológica; y por otro, sugerir mejoras tecnológicas para el diseño de nuevos aplicadores. La Tesis Doctoral combina dos metodologías ampliamente utilizadas en el campo de la Ingeniería Biomédica, como son el modelado computacional (matemático) y la experimentación (ex vivo e in vivo). En cuanto al área cardiaca, la investigación se ha centrado, por una parte, en mejorar la ablación intraoperatoria de la fibrilación auricular por aproximación epicárdica, es decir, susceptible de ser realizada de forma mínimamente invasiva. Para ello, se ha estudiado mediante modelos matemáticos un sistema de medida de la impedancia epicárdica como método de valoración de la cantidad de grasa previo a la ablación. Por otra parte, se ha estudiado cómo mejorar la ablación de la pared ventricular por aproximación endocárdica-endocárdica (septo interventricular) y endocárdica-epicárdica (pared libre del ventrículo). Con este objetivo, se han comparado mediante modelado por computador la eficacia de los modos de ablación bipolar y unipolar en términos de la transmuralidad de la lesión en la pared ventricular. En lo que respecta al área de cirugía oncológica, la investigación se ha centrado en la resección hepática asistida por RF. Las técnicas de calentamiento por RF deberían ser capaces de minimizar el sangrado intraoperatorio y sellar vasos y ductos mediante la creación de una necrosis coagulativa por calentamiento. Si este calentamiento se produce en las cercanías de grandes vasos, existe un problema potencial de daño a la pared de dicho vaso. En este sentido, se ha evaluado con modelos matemáticos y experimentación in vivo si el efecto del flujo de sangre dentro de un gran vaso es capaz de proteger térmicamente su pared cuando se realiza una resección asistida por RF en sus cercanías. Además, se ha realizado un estudio computacional y experimental ex vivo e in vivo del comportamiento electro-térmico de aplicadores de RF bipolares internamente refrigerados, puesto que representan una opción más segura frente a los monopolares en la medida en que las corrientes de RF fluyen casi exclusivamente por el tejido biológico situado entre ambos electrodos. Respecto al área dermatológica, la investigación se ha centrado en mejorar el tratamiento de enfermedades o desórdenes del tejido subcutáneo (tales como lipomatosis, lipedema, enfermedad de Madelung y celulitis) mediante el estudio teórico de la dosimetría correcta en cada caso. Para ello, se han evaluado los efectos eléctricos, térmicos y termo-elásticos de dos estructuras diferentes de tejido subcutáneo durante el calentamiento por RF, y se ha cuantificado el daño térmico producido en ambas estructuras tras dicho calentamiento / González Suárez, A. (2014). Computational Models and Experimentation for Radiofrequency-based Ablative Techniques [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/36502

La réfrigération magnétique est une alternative verte et théoriquement plus efficace que les systèmes de refroidissement classiques utilisant des cycles de détente/compression de gaz nocifs pour l'environnement comme les CFC et les HCFC. Malheureusement, les meilleurs matériaux utilisés actuellement dans les prototypes de réfrigération magnétique sont très dispendieux (5000$/kg pour le Gd) ce qui limite leur utilisation. La découverte de l'effet magnétocalorique géant en 1997 près de la température ambiante a fait exploser le nombre de publications dans le domaine. La recherche du matériau idéal était lancée. Les principales caractéristiques recherchées sont un grand effet magnétocalorique et une grande capacité réfrigérante. L'effet magnétocalorique correspond au changement d'entropie lors de l'application d'un champ magnétique. Elle est importante près des transitions magnétiques. Parmi les familles de matériaux étudiées pour leur effet magnétocalorique, on retrouve les manganites. Avec des structures cristallines apparentées, le La[indice inférieur]2NiMnO[indice inférieur]6 (LNMO) et le Pr[indice inférieur]2NiMnO[indice inférieur]6 (PNMO), des doubles pérovskites, possèdent des transitions magnétiques légèrement sous la température ambiante, soit 280 K et 212 K. De plus, le caractère isolant, la stabilité et le faible coût de ces matériaux leur procurent un net avantage pour leur intégration dans des systèmes de réfrigération magnétique. Dans ce mémoire, la croissance par ablation laser pulsé de couches minces de doubles pérovskites (La[indice inférieur]2NiMnO[indice inférieur]6, Pr[indice inférieur]2NiMnO[indice inférieur]6) et d'hétérostructures de ces composés a été effectuée. Une caractérisation de la structure des échantillons à l'aide de la diffraction des rayons X a permis d'analyser les variations des paramètres de réseau en plan et hors plan en fonction de la température et de la pression d'oxygène lors de la croissance. La texture des couches a également été mesurée. La structure des échantillons a pu être mise en relation avec les propriétés magnétiques des matériaux. La variation de pression d'oxygène lors de la croissance permet de contrôler la proportion des phases ordonnée et désordonnée magnétiquement dans les échantillons de La[indice inférieur]2NiMnO[indice inférieur]6. L'aimantation à saturation ainsi que les températures de transition des phases ordonnée et désordonnée du LNMO sont obtenues à l'aide de mesures d'aimantation en fonction du champ magnétique et en fonction de la température respectivement. L'effet magnétocalorique a été mesuré sur tous les échantillons pour des gammes de températures allant de 10 K à 320 K. La variation d'entropie maximale de 2,1 J/kgK pour un champ magnétique de 0-7T est obtenue pour l'échantillon à 300 mTorr. Par contre, la présence de la phase désordonnée dans certains échantillons élargit le pic de variation d'entropie en fonction de la température augmentant ainsi la capacité réfrigérante de l'échantillon. La capacité réfrigérante est alors comparable à celle du Gd[indice inférieur]5Ge[indice inférieur]2Si[indice inférieur]2. De plus, une variation d'entropie en forme de plateau sur une très large gamme de température (55 K à 298 K) maximise l'efficacité des cycles thermodynamiques. Un plateau s'étalant sur une aussi grande gamme de température n'avait jamais encore été observé. Une autre technique pour élargir le pic de variation d'entropie est de combiner deux matériaux possédant des transitions magnétiques rapprochées en température. Une bicouche de LNMO/PNMO et une tricouche de LNMO/LPNMO(LaPrNiMnO[indice inférieur]6)/PNMO ont donc été déposées. Un plateau de variation d'entropie a été obtenue sur une gamme de température allant de 152 K à 298 K. Par contre, des problèmes dans la croissance du LPNMO ont nui au magnétisme et réduit grandement l'effet magnétocalorique. La faible aimantation rémanente, le faible champ coercitif et la nature isolante des échantillons leurs procurent également un avantage pour une application dans un système de réfrigération magnétique.

The primary purpose of this work was to evaluate the capability of nanoparticles to transform electromagnetic energy at microwave frequencies into therapeutic heating. Targeted nanoparticles, in conjunction with microwave irradiation, can increase the temperatures of the targeted area over the peripheral region. Therefore, to become clinically viable, microwave absorbing nanoparticles must first be identified, and a system to monitor the treatment must be developed. In this study, ultrasound temperature imaging was used to monitor the temperature of deep lying structures. First, a material-dependent quantity to correlate the temperature induced changes in ultrasound images (i.e. apparent time shifts) to differential temperatures was gathered for a tissue-mimicking phantom, porcine longissimus dorsi muscle, and porcine fat. Then microwave nanoabsorbers were identified using an infrared radiometer. The determined nanoabsorbers were then injected into ex-vivo porcine longissimus dorsi muscle tissue. Ultrasound imaging frames were gathered during microwave treatment of the inoculated tissue. Finally, the ultrasound frames were analyzed using the correlation between temperature and apparent shifts in ultrasound for porcine muscle tissue. The outcome was depth-resolved temperature profiles of the ex-vivo porcine muscle during treatment. The results of this study show that magnetite is a microwave nanoabsorber that increases the targeted temperature of microwave hyperthermia treatments. Overall, there is clinical potential to use microwave nanoabsorbers to increase the efficiency of microwave hyperthermia treatments. / text

Ag thin and thick films have been experimentally deposited using a technique called Laser Ablation of a Microparticle Aerosol (LAMA). This technique is based on a supersonic jet accelerating NPs of a few nm diameter up to 1000 m/s and operating at room temperature. The deposited films have experimentally demonstrated interesting properties such as dense growth with good adherence on the substrate. Aerosol feed rates have been fixed to 10 mg/h which corresponds to rate depositions of 10¹⁰ to 10¹¹ NPs/s/cm². In order to model this deposition technique and possibly be able to predict the morphology and structure of deposited films using computational methods, we have designed MD programs simulating the depositions of several Ag nanoclusters onto a substrate at a fixed temperature (300 K). The variation of parameters such as cluster size, cluster impact energy, and deposition rate has influenced the morphology and structure of the deposited films. Cluster diameters have been set to 3 nm or 5 nm, cluster velocities set to 200 m/s (0.022 eV/atom), 400 m/s (0.069 eV/ atom), or 800 m/s (0.358 eV/atom), and the deposition rate adjusted to ensure relaxation times between impactions of 5 ps to 20 ps. The evolution of deposited film density, adherence, and crystal arrangement has been analyzed with the variation of the aforementioned parameters. The highest cluster velocities have enabled the deposition of smoother, denser, and more adherent films. NCs with an initial velocity of 200 m/s have shown ratios of flattening equal to 50 % as opposed to 85% flattening for NCs deposited at 800 m/s. These observations have enabled us to draw qualitative conclusions on the film density The deposited films are less porous when the cluster impaction velocity increases. Atomic mixing between substrate and impacted NC atoms increased with increasing deposition velocity, which can perhaps be correlated to an increase of adherence, assuming that more mixing will create stronger molecular binding in the cluster-substrate interaction. Finally, complete epitaxial growth was observed for the highest impaction velocities only, which indicates that recrystalization can occur for this range of impact energies (0.3 eV/atom - 0.5 eV/atom). Although experimental results have given more quantitative data on film density and sticking ratios, they agree with our modeling, and this comparison allows us to validate our MD simulations. However, some limitations have been faced, mainly because of long computing time requirements that a single laptop computer has not been able to support. / text

A common analytical problem in applying LA sampling concerns dealing with large planar samples, e.g. gel plates, Si wafers, tissue sections or geological samples. As the current state of the art stands, there are two solutions to this problem: either sub-sample the substrate or build a custom cell. Both have their inherent drawbacks. With sub-sampling, the main issue is to ensure that a representative is sample taken to correctly determine the analytes of interest. Constructing custom cells can be time consuming, even for research groups that are experienced or skilled, as they have to be validated before data can be published. There are various published designs and ideas that attempt to deal with the issue of large samples, all of which ultimately enclose the sample in a box. The work presented in this thesis shows a viable alternative to enclosed sampling chambers. The non-contact cell is an open cell that uses novel gas dynamics to remove the necessity for an enclosed box and, therefore, enables samples of any arbitrary size to be sampled. The upper size limit of a sample is set by the travel of the XY stages on the laser ablation system, not the dimensions of the ablation cell.

Radio-Frequency (RF) lesioning, or RF ablation, is a method that uses high frequency currents for thermal coagulation of pathological tissue or signal pathways. The current is delivered from an electrode, which also contains a temperature sensor permitting control of the current at a desired target temperature. In the brain, RF lesioning can e.g. be used for treatment of severe chronic pain and movement disorders such as Parkinson’s disease. This thesis focuses on modelling and simulation with the aim of gaining better understanding and predictability of the lesioning process in the central brain.   The finite element method (FEM), together with experimental comparisons, was used to study the effects of electric and thermal conductivity, blood perfusion (Paper I), and cerebrospinal fluid (CSF) filled cysts (Paper II) on resulting lesion volume and shape in brain tissue. The influence of blood perfusion was modelled as an increase in thermal conductivity in non-coagulated tissue. This model gave smaller simulated lesions with increasing blood perfusion as heat was more efficiently conducted from the rim of the lesion. If the coagulation was not taken into consideration, the lesion became larger with increasing thermal conductivity instead, as the increase in conducted heat was compensated for through an increased power output in order to maintain the target temperature. Simulated lesions corresponded well to experimental in-vivo lesions. The electric conductivity in a homogeneous surrounding had little impact but this was not true for a heterogeneous surrounding. CSF has a much higher electric conductivity than brain tissue, which focused the current to the cyst if the electrode tip was in contact with both a cyst and brain tissue. Heating of CSF could also cause considerable convective flow and as a result a very efficient heat transfer. This affected both simulated and experimental lesion sizes and shapes. As a result both very large and very small lesions could be obtained depending on whether sufficient power was supplied or if the heating was mitigated over a large volume.   Clinical (Paper IV) and experimental (Paper III) measurements were used for investigation of changes in reflected light intensity from undamaged and coagulating brain tissue respectively. Monte Carlo (MC) simulations for light transport were made for comparison (Paper V). For the optical measurements, an RF electrode with adjacent optical fibres was used and this electrode was also modelled for the optical simulations. According to the MC simulations, coagulation should make grey matter lighter and white matter darker, while thalamic light grey should remain approximately the same. Experiments in ex-vivo porcine tissue gave an increase in reflected light intensity from grey matter at approximately 50 °C but the signal was very variable and the isotherm 60 °C gave better agreement between simulated and experimental lesions. No consistent decrease in reflected light intensity could be seen during coagulation of white matter. Clinical measurements were performed during the creation of 21 trajectories for deep brain stimulation electrodes. In agreement with the simulations, reflected light intensity was found to differentiate well between undamaged grey, light grey and white matter.   In conclusion, blood perfusion and CSF in particular may greatly affect the lesioning process and can be important to consider when planning surgery. Reflected light intensity seems unreliable for the detection of coagulation in light grey brain matter such as the thalamus. However, it seems very promising for navigation in the brain and for detection of coagulation in other tissue types such as muscle.

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.

Le contrôle de qualité de produits pharmaceutiques implique souvent la détermination de la teneur en minéraux. Que ce soit pour détecter la présence de métaux lourds ou déterminer la concentration des additifs minéraux, la spectrométrie d’émission atomique à plasma à couplage inductif (ICP-AES) est fré- quemment appliquée comme méthode de contrôle de la qualité. Il s’agit d’une méthode reconnue et fiable, mais qui requiert une préparation laborieuse des échantillons, ce qui occasionne des coûts élevés et des délais considérables. L’objectif principal du projet est de proposer une alternative à l’analyse ICP-AES qui permettrait de ra- pidement détecter, et si possible quantifier, la teneur en minéraux traces sans préparation des échantillons. Ceci permettrait d’économiser sur des consommables dispendieux et dangereux, d’économiser du temps de laboratoire et du temps de quarantaine pour les produits finis. Afin d’effectuer ceci, deux technologies d’ana- lyse de procédé sont proposées : la spectrométrie sur plasma induit par laser (LIBS) et la spectrométrie par fluorescence de rayon-X (XRF). Ce projet fait suite à une étude visant à quantifier la composition de comprimés pharmaceutiques multi- vitaminés. Ayant conclu que ces 2 technologies permettent de déterminer la teneur en minéraux dans une gamme de concentrations relativement élevée (1000 – 10 000 ppm), l’objectif présent est de déterminer si ces technologies sont aussi capables de déterminer la teneur d’éléments traces (moins de 50 ppm). Les éléments traces retenus pour cette preuve de concept sont le plomb, le nickel et le cadmium. Le cadre de ce projet s’étend de la fabrication des échantillons jusqu’à l’optimisation des paramètres d’échan- tillonnage ainsi que les paramètres d’acquisition des données spectroscopiques déterminés, l’objectif ultime du projet étant de produire des courbes de calibration pour les trois (3) éléments traces qui sont utilisés pour faire la preuve de concept.

Atrial fibrillation (AF) is the most common cardiac arrhythmia in the United States today. However, treatment options remain limited despite the enormous magnitude of both AF prevalence and the associated economic cost. Of those treatment options that are available, ablation-based interventional methods have demonstrated the highest rates of long-term cure. Unfortunately, these methods have substantially lower efficacy in patients with heavier burdens of disease, thus leaving the most affected individuals with the least hope for successful treatment. The focus of this research is to develop a mechanistically guided approach towards the treatment of multi-wavelet reentry (MWR), one of the primary drivers of AF. For this purpose, we use a computational model of electrical propagation in cardiac tissue to simulate both episodes of fibrillatory activity and the ablative treatment thereof. We demonstrate that the probability of forming the reentrant circuits necessary for continuous electrical activity is a function of the shape and size of a tissue as well as its underlying cellular properties. Ablation at tissue sites with high probability of circuit formation more efficiently reduces the overall duration of fibrillatory episodes than ablation at sites with low probability. We then propose and validate in silico a parameter-based metric for predicting the propensity of an individual tissue to support fibrillation, which we term the fibrillogenicity index. Using this metric, we develop an algorithm for prospectively determining optimized, tissue-specific ablation patterns. Finally, we examine the relationship between multi-wavelet reentry and focal drivers, and demonstrate that MWR and fibrillatory conduction exist along a continuum. We examine the complex interplay between functional and structural substrates within fibrillating tissue and define the mechanisms by which they promote the perpetuation of AF. These findings present a novel theoretical framework for understanding treatment of multi-wavelet reentry driven AF and provide a set of testable predictions that can serve to guide the design of future experimental studies aimed at advancing the rational design of patient-specific ablation sets for treating AF.

The pyrolysis mechanics of Phenolic Impregnated Carbon Ablators (PICA) makes it a valued material for use in thermal protection systems for spacecraft atmospheric re-entry. The present study of the interaction of pyrolysis gases and char with plasma gases in the boundary layer over PICA and its substrate, FiberForm, extends previous work on this topic that has been done in the UVM 30 kW Inductively Coupled Plasma (ICP) Torch Facility. Exposure of these material samples separately to argon, nitrogen, oxygen, air, and carbon dioxide plasmas, and combinations of said test gases provides insight into the evolution of the pyrolysis gases as they react with the different environments. Measurements done to date include time-resolved absolute emission spectroscopy, location-based temperature response, flow characterization of temperature, enthalpy, and enthalpy flux, and more recently, spatially resolved and high-resolution emission spectroscopy, all of which provide measure of the characteristics of the pyrolysis chemistry and material response. Flow characterization tests construct an general knowledge of the test condition temperature, composition, and enthalpy. Tests with relatively inert argon plasmas established a baseline for the pyrolysis gases that leave the material. Key pyrolysis species such as CN Violet bands, NH, OH and Hydrogen Alpha (Hα) lines were seen with relative repeatability in temporal, spectral, and intensity values. Tests with incremental addition, and static mixtures, of reactive plasmas provided a preliminary image of how the gases interacted with atmospheric flows and other pyrolysis gases. Evidence of a temporal relationship between NH and Hα relating to nitrogen addition is seen, as well as a similar relationship between OH and Hα in oxygen based environments. Temperature analysis highlighted the reaction of the material to various flow conditions and displayed the in depth material response to argon and air/argon plasmas. The development of spatial emission analysis has been started with the hope of better resolving the previously seen pyrolysis behavior in time and space.