<p>In this dissertation, we show three approaches towards selective vagus nerve stimulation (VNS). First, we investigated VNS using microelectrode with circle and Vicsek fractal shape. Our rat study shows that fractal microelectrode can activate C-fibers in cervical vagus nerve with higher energy efficiency compared to circle microelectrode. Secondly, we developed stretchable and adhesive cuff device for a compliant neural interface for a long-term stability. We designed Y-shaped kirigami thin-film device for stretchable neural interface and applied a tissue-adhesive hydrogel to enable tough adhesion of the cuff electrode, which can be potentially used to fix the position of microelectrode for a reliable selective stimulation with minimal mechanical mismatch. Lastly, we developed a microchannel electrode array device to potentially measure high-quality of single fiber action potential (SFAP) from the abdominal vagal trunk of rat to explore natural patterns selective organ activities which can be used for a fine-tuned selective VNS. Our results show the potential of measuring C-fiber activities evoked by cervical VNS.</p>

This dissertation aims to better understand the evolution of single hydrogen gas bubbles evolved during the water electrolysis at microelectrodes. In particular, the growth and detachment processes were studied in detail experimentally by means of electrochemical and optical methods in terrestrial, micro-, and hypergravity conditions. The combination of microelectrode and sulfuric acid promoting the bubble coalescence results in a periodical growth and the detachment of single bubbles. This provides a systematic view on the phenomena under study. A shadowgraphy system was used to provide general insight into the bubble behaviour, while Particle Tracking Velocimetry (PTV) was used for the flow velocity measurements around the growing hydrogen bubble. By applying high electric potentials considerably exceeding that in industrial electrolysers, it is possible to analyse the evolution of hydrogen bubbles under extreme conditions and for a wide range of electrolyte concentrations, overall shedding more light on bubble dynamics in general, and especially the underlying balance of forces. The growth of single hydrogen bubbles at micro-electrodes was studied in an acidic electrolyte over a wide range of concentrations and cathodic potentials. New bubble growth regimes were identified which differ in terms of whether the bubble evolution proceeds in the presence of a monotonic or oscillatory variation in the electric current and a carpet of microbubbles underneath the bubble. Key features such as the growth law of the bubble radius, the dynamics of the microbubble carpet, the onset time of the oscillations and the oscillation frequencies were characterised as a function of the concentration and electric potential. Furthermore, the system's response to jumps in the cathodic potential was studied. The electrode, tilted to the horizon, promotes faster growth and, therefore, earlier detachment at the smaller volume of the bubble. During its evolution, the bubble moves laterally from the electrode centre, releasing the electrode area and enabling higher electric current, therefore faster hydrogen generation and bubble-bubble coalescence rates. The duration of the bubble position oscillations found on the horizontal electrode gradually reduces upon tilt angle increase, with an almost complete disappearance at 5°. Based on the analysis of the forces involved and their scaling with the concentration, potential and electric current, a sound hypothesis was formulated regarding the mechanisms underlying the micro-bubble carpet and oscillations. A detailed look was also taken on the dynamics of single hydrogen bubbles in microgravity during parabolic flights. Three bubble evolution scenarios were identified depending on the electric potential applied and the acid concentration. The dominant scenario, characterised by lateral detachment of the grown bubble, was studied in detail. For that purpose, the evolution of the bubble radius, electric current and bubble trajectories as well as the bubble lifetime were comprehensively addressed for different potentials and electrolyte concentrations. The bubble-bubble coalescence events, which are responsible for reversals of the direction of bubble motion, were particularly analysed. Finally, as parabolic flights also permit hypergravity conditions, a detailed comparison of the characteristic bubble phenomena at various levels of gravity was drawn. Finally, the Marangoni convection at the foot of hydrogen gas bubbles mainly induced by the thermocapillary effect is systematically studied during the bubble evolution, the bubble position oscillations, at horizontal and tilted electrodes both in terrestrial and hyper-g environments. The flow structure progressively modifies with the bubble evolution or during the bubble position oscillations, i.e. as per electric current and bubble geometry variation. The velocity increases both with the bubble size and the electric current magnitude. It reaches up to 50 mm/s and 125 mm/s shortly before the bubble detachment at horizontal and tilted electrodes, correspondingly. The bubble position oscillations characterised by the large variation of the electric current govern the velocity of around ~80 mm/s at the highest and ~40 mm/s at the lowest positions. In the case of tilted electrodes, both in terrestrial and hyper-g environments, the lateral movement of the bubble enables higher values of the current and, therefore, stronger convection. The non-homogeneous distribution of the electric current lines at the tilted electrode results in the asymmetrical Marangoni convection around the bubble. There is a certain limitation in terms of the maximal magnitude of the velocity at different tilt angles, governed by the optimal size of the bubble and electric current. At last, the effects of the particles and laser used for PTV measurements were shown to reduce the duration of the oscillations and to retard the bubble evolution. Both effects were considered during the measurements.

No description available.

No description available.

Bioimpedance presents a versatile, label-free means of monitoring biological cells and their responses to physical, chemical and biological stimuli. Breast cancer is the second most common type of cancer among women in the United States. Although significant progress has been made in diagnosis and treatment of this disease, there is a need for robust, easy-to-use technologies that can be used for the identification and discrimination of critical subtypes of breast cancer in biopsies obtained from patients. This dissertation makes contributions in three major areas towards addressing the goal. First, we developed miniaturized bioimpedance sensors using MEMS and microfluidics technology that have the requisite traits for clinical use including reliability, ease-of-use, low-cost and disposability. Here, we designed and fabricated two types of bioimpedance sensors. One was based on electric cell-substrate impedance sensing (ECIS) to monitor cell adhesion based events and the other was a microfluidic device with integrated microelectrodes to examine the biophysical properties of single cells. Second, we examined a panel of triple negative breast cancer (TNBC) cell lines and a hormone therapy resistant model of breast cancer in order to improve our understanding of the bioimpedance spectra of breast cancer subtypes. Third, we explored strategies to improve the sensitivity of the microelectrodes to bioimpedance measurements from breast cancer cells. We investigated nano-scale coatings on the surface of the electrode and geometrical variations in a branched electrode design to accomplish this. This work demonstrates the promise of bioimpedance technologies in monitoring diseased cells and their responses to pharmaceutical agents, and motivates further research in customization of this technique for use in personalized medicine. / Ph. D.

Silicon intracortical microelectrodes arrays (MEA) provide some of the best performance for brain computer interfaces but suffer from lifetime limitations that prevent clinical adoption. A primary contributor of lifetime limitations is the chronic foreign body response (FBR) which remains active in response to the brain tissue strain caused by implanted probes during brain micromotion. Designing probes with softer materials can reduce the chronic FBR and improve device lifetime. A high-aspect ratio CNT MEA could provide several advantages as an intracortical MEA but has yet to be mechanically characterized. CNT MEAs are infiltrated with 3 degrees of carbon infiltration and mechanically tested to determine their Young’s modulus. A novel dual deflection (DD) test is designed to measure the cantilever bending of the CNT electrodes and determine their modulus. The CNT diameters of the microelectrodes are measured with SEM as a quantification method of carbon infiltration. A total of 64 CNT microelectrodes (MEs) are tested with the DD test and the modulus is calculated for a subset of 59 CNT MEs. The modulus results are grouped by infiltration level for statistical calculations. The DD test measured a mean modulus of 19.6 ± 14.5 MPa, 67.7 ± 22.7 MPa, and 168 ± 62.3 MPa for arrays fabricated with 0 second, 15 second, and 30 second infiltrations, respectively. A finite element analysis (FEA) model is developed to examine the brain tissue strain under a static, 10 µm, micromotion deflection of penetrating probes with the maximum (1.7 GPa), median (72 MPa), and minimum (3.9 MPa) CNT moduli we measured, as well as the modulus of silicon (165 GPa) for comparison. The max CNT and Si probes induced similar strain in the brain model at the probe tip, while the minimum and median CNT probes showed almost no strain at the tip. The model implies that for the CNT MEA probe geometry, significant reductions in the brain tissue strain can be achieved by CNT MEs with moduli in tens of MPa range, which can be fabricated with 15 seconds of carbon infiltration. Microelectrodes with 15 seconds of infiltration also demonstrated increased resilience compared to the other infiltration time groups, indicating that CNT MEAs with 15 seconds of infiltration might be a viable candidate material for a neural probe array.

Les biofilms sont des communautés de microorganismes incorporés dans une matrice exo-polymérique complexe. Ils sont reconnus pour jouer un rôle important comme barrière de diffusion dans les systèmes environnementaux et la santé humaine, donnant lieu à une résistance accrue aux antibiotiques et aux désinfectants. Comme le transfert de masse dans un biofilm est principalement dû à la diffusion moléculaire, il est primordial de comprendre les principaux paramètres influençant les flux de diffusion. Dans ce travail, nous avons étudié un biofilm de Pseudomonas fluorescens et deux hydrogels modèles (agarose et alginate) pour lesquels l’autodiffusion (mouvement Brownien) et les coefficients de diffusion mutuels ont été quantifiés. La spectroscopie par corrélation de fluorescence a été utilisée pour mesurer les coefficients d'autodiffusion dans une volume confocal de ca. 1 m3 dans les gels ou les biofilms, tandis que les mesures de diffusion mutuelle ont été faites par cellule de diffusion. En outre, la voltamétrie sur microélectrode a été utilisée pour évaluer le potentiel de Donnan des gels afin de déterminer son impact sur la diffusion. Pour l'hydrogel d'agarose, les observations combinées d'une diminution du coefficient d’autodiffusion et de l’augmentation de la diffusion mutuelle pour une force ionique décroissante ont été attribuées au potentiel de Donnan du gel. Des mesures de l'effet Donnan (différence de -30 mV entre des forces ioniques de 10-4 et 10-1 M) et l'accumulation correspondante d’ions dans l'hydrogel (augmentation d’un facteur de 13 par rapport à la solution) ont indiqué que les interactions électrostatiques peuvent fortement influencer le flux de diffusion de cations, même dans un hydrogel faiblement chargé tel que l'agarose. Curieusement, pour un gel plus chargé comme l'alginate de calcium, la variation de la force ionique et du pH n'a donné lieu qu'à de légères variations de la diffusion de sondes chargées dans l'hydrogel. Ces résultats suggèrent qu’en influençant la diffusion du soluté, l'effet direct des cations sur la structure du gel (compression et/ou gonflement induits) était beaucoup plus efficace que l'effet Donnan. De même, pour un biofilm bactérien, les coefficients d'autodiffusion étaient pratiquement constants sur toute une gamme de force ionique (10-4-10-1 M), aussi bien pour des petits solutés chargés négativement ou positivement (le rapport du coefficient d’autodiffusion dans biofilm sur celui dans la solution, Db/Dw ≈ 85 %) que pour des nanoparticules (Db/Dw≈ 50 %), suggérant que l'effet d'obstruction des biofilms l’emporte sur l'effet de charge. Les résultats de cette étude ont montré que parmi les divers facteurs majeurs qui affectent la diffusion dans un biofilm environnemental oligotrophe (exclusion stérique, interactions électrostatiques et hydrophobes), les effets d'obstruction semblent être les plus importants lorsque l'on tente de comprendre la diffusion du soluté. Alors que les effets de charge ne semblaient pas être importants pour l'autodiffusion de substrats chargés dans l'hydrogel d'alginate ou dans le biofilm bactérien, ils ont joué un rôle clé dans la compréhension de la diffusion à travers l’agarose. L’ensemble de ces résultats devraient être très utiles pour l'évaluation de la biodisponibilité des contaminants traces et des nanoparticules dans l'environnement. / Biofilms are primarily communities of microorganisms embedded in a complex exopolymer matrix. They are thought to play an important role as diffusive barriers in environmental systems and human health, resulting in increased resistance to disinfectants and antibiotics. Since mass transport in a biofilm is primarily due to molecular diffusion, it is critical to understand the main parameters influencing diffusive fluxes in a biofilm. In this thesis, a Pseudomonas fluorescens biofilm and two model hydrogels, (agarose and calcium alginate), were investigated. Both self-diffusion (Brownian motion) and mutual diffusion coefficients were quantified. Fluorescence correlation spectroscopy was used to measure the self-diffusion coefficients in a ca. 1 m3 confocal volume in the gels or biofilms, whereas a diffusion cell setup was employed for mutual diffusion measurements. In addition, microelectrode voltammetry was used to evaluate Donnan potential of the gels in order to determine its impact on diffusion. For the agarose hydrogel, the combined observations of a decreasing self-diffusion coefficient coupled with increasing mutual diffusion as a function of a decreasing ionic strength have been attributed to the gel’s Donnan potential. Measurements of the Donnan effect (difference of -30 mV between ionic strengths of 10-4 and 10-1 M) and the corresponding accumulation of ions in the hydrogel (13x enhancement with respect to the bulk solution) indicated that electrostatic interactions can strongly influence the diffusive flux of cations, even in a weakly charged hydrogel, such as agarose. Somewhat surprisingly, for a more highly charged gel such as calcium alginate, varying ionic strength and pH resulted in only small changes to the diffusion of charged probes in the hydrogel. These results suggested that the direct effect of the cations on gel structure (due to an induced swelling or compression) was much more effective than the Donnan effect when influencing solute diffusion. Similarly, for a bacterial biofilm, self-diffusion coefficients were virtually constant across a range of examined ionic strengths (10-4-10-1 M) for both negatively and positively charged small solutes (Db/Dw≈85%) and nanoparticles (Db/Dw≈50%), suggesting that the obstruction effect of the biofilms again overwhelmed the charge effect. The results of this work indicated that among the various major factors affecting diffusion in an oligotrophic environmental biofilm (steric exclusion, hydrophobic and electrostatic interactions), obstruction effects appeared to be the most important when attempting to understand the solute diffusion. While charge effects did not appear to be important to the self-diffusion of charged substrates in the alginate hydrogel or bacterial biofilm, they were key to understanding diffusion through another gel, with numerous biomedical and environmental applications, i.e. agarose. These results should be extremely useful when evaluating the bioavailability of the trace contaminants and nanoparticles in the environment.

<p>The gastrointestinal mucus gel layer and blood flow are two important mechanisms for protection at the pre-epithelial and sub-epithelial levels, respectively. <i>Helicobacter pylori</i> might circumvent these mechanisms and elicit a chronic inflammatory response with consequent ulcers in the stomach and duodenum. In this thesis, the physical state and properties of the adherent mucus gel layer was studied from the stomach to colon. Furthermore, the acute and chronic effects of <i>H. pylori</i> on the integrity of the mucus gel layer and mucosal blood flow were studied in the anesthetized rat.</p><p>A translucent mucus gel covers all studied segments of the gastrointestinal tract during fasting conditions, with the thickest layers in the colon and ileum. Carefully applied suction revealed that the mucus gel was a multi-layered structure comprising a firmly adherent layer covering the mucosa, impossible to remove, and a loosely adherent upper layer. The firmly adherent layer was thick and continuous in the corpus (80μm), antrum (154μm) and colon (116μm), but thin (<20μm) and discontinuous in the small intestine.</p><p>Following mucus removal, a rapid renewal of the loosely adherent layer ensued. The highest rate was observed in the colon with intermediate values in the small intestine. Mucus renewal in the stomach was attenuated on acute luminal application of water extracts from <i>H. pylori</i> (HPE). In animals with a chronic <i>H. pylori</i> infection the mucus renewal rate was unaffected, but the total gastric mucus gel thickness was reduced and the mucus secretory response to luminal acid (pH1) attenuated in the antrum. </p><p>HPE from type I strains acutely reduced corporal mucosal blood flow, measured with laser-Doppler flowmetry, by approximately 15%. The reduction in blood flow was mediated by a heat stable factor other than VacA and CagA. Inhibition of endogenous nitric oxide production with Nω-nitro-l-arginine augmented the decrease. However, ketotifen, a mast cell stabilizer, completely attenuated the effect of the extract as did the platelet activating factor (PAF) receptor-antagonist, WEB2086, thus depicting a detrimental role for the microvascular actions of PAF.</p>

<p>The stomach is frequently exposed to hazardous agents and to resist this harsh environment, several protective mechanisms exist. Of special interest is the gastric pathogen <i>Helicobacter pylori </i>which causes gastritis, ulcers and cancer but the mechanism leading to these diseases are still unclear. However it is very likely that <i>H. pylori </i>negatively influence the protection mechanisms that exist in the stomach. </p><p>The aims of the present investigation were first to develop an in vivo mouse model in which different protection mechanisms could be studied, and second to investigate the influence of <i>H. pylori</i> on these mechanisms. </p><p>An in vivo preparation of the gastric mucosa in mice was developed. This preparation allows studies of different gastric mucosal variables and can also be applied for studies in other gastro-intestinal organs. </p><p>Mice chronically infected with <i>H. pylori</i>, were shown to have a reduced ability of the mucosa to maintain a neutral pH at the epithelial cell surface. This could be due to the thinner inner, firmly adherent mucus gel layer, and/or to defective bicarbonate transport across the epithelium. The Cl^-/HCO₃^- exchanger SLC26A9 was inhibited by NH₄⁺, which also is produced by <i>H. pylori</i>. The mRNA levels of SLC26A9 were upregulated in infected mice, suggesting a way to overcome the inhibition of the transporter. Furthermore, the hyperemic response to acid pH 2 and 1.5 was abolished in these mice. The mechanisms by which the bacteria could alter the blood flow response might involve inhibition of the epithelial iNOS.</p><p>Water extracts of <i>H. pylori </i>(HPE) reduces the blood flow acutely through an iNOS and nerve-mediated pathway, possibly through the endogenous iNOS inhibitor ADMA. Furthermore, HPE alters the blood flow response to acid as the hyperemic response to acid pH 0.8 is accentuated in mice treated with HPE. </p>

The gastrointestinal mucus gel layer and blood flow are two important mechanisms for protection at the pre-epithelial and sub-epithelial levels, respectively. Helicobacter pylori might circumvent these mechanisms and elicit a chronic inflammatory response with consequent ulcers in the stomach and duodenum. In this thesis, the physical state and properties of the adherent mucus gel layer was studied from the stomach to colon. Furthermore, the acute and chronic effects of H. pylori on the integrity of the mucus gel layer and mucosal blood flow were studied in the anesthetized rat. A translucent mucus gel covers all studied segments of the gastrointestinal tract during fasting conditions, with the thickest layers in the colon and ileum. Carefully applied suction revealed that the mucus gel was a multi-layered structure comprising a firmly adherent layer covering the mucosa, impossible to remove, and a loosely adherent upper layer. The firmly adherent layer was thick and continuous in the corpus (80μm), antrum (154μm) and colon (116μm), but thin (&lt;20μm) and discontinuous in the small intestine. Following mucus removal, a rapid renewal of the loosely adherent layer ensued. The highest rate was observed in the colon with intermediate values in the small intestine. Mucus renewal in the stomach was attenuated on acute luminal application of water extracts from H. pylori (HPE). In animals with a chronic H. pylori infection the mucus renewal rate was unaffected, but the total gastric mucus gel thickness was reduced and the mucus secretory response to luminal acid (pH1) attenuated in the antrum. HPE from type I strains acutely reduced corporal mucosal blood flow, measured with laser-Doppler flowmetry, by approximately 15%. The reduction in blood flow was mediated by a heat stable factor other than VacA and CagA. Inhibition of endogenous nitric oxide production with Nω-nitro-l-arginine augmented the decrease. However, ketotifen, a mast cell stabilizer, completely attenuated the effect of the extract as did the platelet activating factor (PAF) receptor-antagonist, WEB2086, thus depicting a detrimental role for the microvascular actions of PAF.