Design and optimization of capacitive micro-machined transducers applied to ultrasound imaging

Les Transducteurs Ultrasonores Capacitifs Micro-usinés (CMUT en anglais) sont étudiés par plusieurs laboratoires internationaux depuis les années 90. Se présentant comme une alternative aux transducteurs traditionnels piézoélectriques, cette technologie inspirée des MEMS (MicroElectroMechanical Systems) a aujourd'hui atteint la maturité suffisante pour voir émerger sa commercialisation. Dans son application historique, l'imagerie médicale, de nombreux acteurs industriels proposent des sondes CMUTs, comme Hitachi, Kolo Medicals ou Butterfly Network pour n'en citer que quelques uns. La recherche autour de ces dispositifs continue néanmoins à travers des études sur la conception, la modélisation, la fabrication ou pour de nouvelles applications. À travers le projet collaboratif TUMAHI (Transducteurs capacitifs Ultrasonores Multi-Application Hautement Intégrés) entre le laboratoire GREMAN et la société VERMON S.A, deux axes d'optimisation ont été étudiés. Le premier objectif consistait à créer et valider expérimentalement une stratégie de conception d'éléments CMUTs pour optimiser la réponse électroacoustique sur la bande passante ou la sensibilité. La principale contrainte était de définir des degrés de liberté et des paramètres fixes pour fabriquer des barrettes CMUTs pour diverses applications et fréquences de travail sur un même wafer. Par l'intermédiaire d'un dispositif centré à 10 MHz pour de l'imagerie médicale, nous avons fait varier les tailles de membranes pour posséder une configuration Large Bande et une configuration Sensible. Une campagne de mesures de pression a été effectuée avec les sondes complètes et prêtes à l'emploi pour valider les critères de conception établis. L'analyse poussée a néanmoins souligné que la démarche de conception pour une sonde CMUT la plus efficace était d'optimiser la fréquence de résonance du premier mode de rayonnement pour qu'elle coïncide avec la fréquence de travail désirée. La seconde étude était focalisée sur la couche de passivation, couche de protection pour les éléments CMUTs traditionnellement conçue en polymère silicone, et visait à implémenter son impact dans les outils de modélisation avec un nouveau modèle de couplage CMUT / matériau viscoélastique. Une fonction de Green en trois dimensions a été utilisée en définissant des équivalences entre propriétés élastiques et viscoélastiques et validée théoriquement avec des problèmes de Lamb 2D et 3D complémentaires. Une confrontation avec des résultats expérimentaux a été engagée en créant une série d'huiles avec des viscosités variables et en excitant des colonnes CMUTs découplées électriquement pour viser un mode de rayonnement plus sensible aux propriétés de cisaillement du milieu. L'influence de la viscosité a été identifiée à travers la variation de la fréquence centrale et du facteur de qualité sur l'impédance électrique mesurée et simulée, posant la base d'une étude de viabilité de la technologie CMUT pour un capteur sensible viscoélastique. La considération de la couche de passivation a permis de retrouver un comportement équivalent à la réalité mais nécessite une base de données complète sur les propriétés viscoélastiques des matériaux dans le domaine ultrasonore pour être présente dans les étapes de conception d'un transducteur CMUT.Capacitive Micromachined Ultrasonic Transducers (CMUT) have been studied by several international laboratories since the 1990s. Presenting itself as an alternative to traditional piezoelectric transducers, this MEMS (MicroElectroMechanical Systems) inspired technology has now reached the maturity needed for commercialization. In its historic application, medical imaging, many industrial actors propose CMUT probes, such as Hotachi, Kolo Medicals or Butterfly Network to name a few. However, research on these devices continues through studies on design, modeling, manufacturing or for new applications. Through the collaborative project TUMAHI (Highly Integrated Multi-Application capacitive Ultrasonic Transducers) between the GREMAN laboratory and VERMON S.A, two areas of optimization were carried out. The first objective was to create and experimentally validate a CMUT element design strategy to optimize electroacoustic response over bandwidth or sensitivity. The main constraint was to define degrees of freedom and fixed parameters to manufacture CMUTs for various applications and working frequency on a single wafer. Through a 10 MHz centered device for medical imaging, we have varied the size of membranes to have a Broadband and Sensitive configuration. A pressure measurement campaign was carried out with complete and ready-to-use probes to validate the established design criteria. However, the in-depth analysis emphasized that the design approach for the most effective CMUT probe was to optimize the resonance frequency of the first radiation mode to coincide with the desired working frequency. The second study focused on the passivation layer, a protective layer for CMUT elements traditionally designed in silicone polymer, and aimed to implement its impact in modeling tools with a new CMUT / viscoelastic material coupling model. A three-dimensional Green function was used by defining equivalences between elastic and viscoelastic properties and theoretically validated with complementary 2D and 3D Lamb problems. A confrontation with experimental results was initiated by creating a series of oils with variable viscosities and by exciting electrically decoupled CMUT columns to target a more sensitive radiation mode to the shear properties of the medium. The influence of viscosity was identified through the variation of the central frequency and quality factor on the measured and simulated electrical impedance, providing the basis for a viability study of CMUT technology for a viscoelastic sensitive sensor. The consideration of the passivation layer has enabled the retrieval of equivalent behavior to reality but requires a complete database on the viscoelastic properties of materials in the ultrasonic domain to be present in the design steps of a CMUT transducer

Analysis of the Inhibitory Effect of Oestradiol on Functional GABA/5-HT Relationship in the Rat Suprachiasmatic Area

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A Boundary Element Model for CMUT-Arrays Loaded by a Viscoelastic Medium

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Viscoelasticity Assessment Using Quartz Crystal Microbalance for Accurate Loaded CMUT Modeling

International audienceThe main challenge with Capacitive Micromachined Ultrasonic Transducer (CMUT) probes packaging is to preserve their intrinsic performances. Indeed, careful attention should be paid to the encapsulation of CMUTs and the impact of a viscoelastic acoustic loading (e.g., polymer) on the array performance. Our group developed a Boundary Element Matrix (BEM) dedicated to the acoustic coupling computation between a viscoelastic medium and a CMUT. As a complementary work, the present study aims to set up an experimental protocol to extract viscoelastic properties of different media and use them as model data inputs. This task is performed using Quartz Crystal Microbalance (QCM) with electrical admittance measurements performed in air and in each viscoelastic medium. A complex viscosity value was obtained for vegetable oils and the tested polymer. Then, using a CMUT array, electrical impedance measurements were performed in each medium. Using the QCM extracted media properties, a very good agreement was obtained between CMUT modeling and measurements where viscoelasticity is proven to reduce inter-cell coupling effects. Full electrode loading of a highly viscous polymer overdamp the QCM resonance and highlight its first order equation limits

Serotonin synthesis from tryptophan by hypothalamic cells in serum‐free medium culture

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Retrospective multicentric study on Campylobacter spp. bacteremia in France: the Campylobacteremia study

International audienceAbstract Background Campylobacter spp. bacteremia is a severe infection. A nationwide 5-year retrospective study was conducted to characterize its clinical features and prognostic factors. Methods Patients diagnosed with Campylobacter spp. bacteremia in 37 French hospitals participating in the surveillance network of the National Reference Center for Campylobacter and Helicobacter were included from January 1, 2015, to December 31, 2019. The goal was to analyze the effects of a delay of appropriate antibiotic therapy and other risk factors on 30-day mortality, antibiotic resistance, patient characteristics and prognosis according to the Campylobacter species. Findings Among the 592 patients, Campylobacter jejuni and Campylobacter fetus were the most commonly identified species (42.9 and 42.6%, respectively). The patients were elderly (median age 68 years), and most had underlying conditions, mainly immunodepression (43.4%), hematologic malignancies (25.9%), solid neoplasms (23%) and diabetes (22.3%). C. jejuni and Campylobacter coli were associated with gastrointestinal signs, and C. fetus was associated with secondary localizations. Among the 80 patients (13.5%) with secondary localizations, 12 had endocarditis, 38 vascular, 24 osteo-articular and 9 ascitic fluid infections. The thirty-day mortality rate was 11.7%, and an appropriate antibiotic treatment was independently associated with 30-day survival (odds ratio [OR]=0.47, 95% CI [0.24–0.93], p=0.03). The median efficient therapy initiation delay was quite short (2 days, IQR [0–4]) but it had no significant impact on 30-day mortality (p=0.78). Interpretation Campylobacter spp. bacteremia mainly occurred in elderly immunocompromised individuals with variable clinical presentations according to the species involved. Appropriate antimicrobial therapy was associated with improved 30-day survival

Multicenter Retrospective Study of Vascular Infections and Endocarditis Caused by Campylobacter spp., France

The incidence of campylobacteriosis has substantially increased over the past decade, notably in France. Secondary localizations complicating invasive infections are poorly described. We aimed to describe vascular infection or endocarditis caused by Campylobacter spp. We included 57 patients from a nationwide 5-year retrospective study on Campylobacter spp. bacteremia conducted in France; 44 patients had vascular infections, 12 had endocarditis, and 1 had both conditions. Campylobacter fetus was the most frequently involved species (83%). Antibiotic treatment involved a β-lactam monotherapy (54%) or was combined with a fluoroquinolone or an aminoglycoside (44%). The mortality rate was 25%. Relapse occurred in 8% of cases and was associated with delayed initiation of an efficient antimicrobial therapy after the first symptoms, diabetes, and coexistence of an osteoarticular location. Cardiovascular Campylobacter spp. infections are associated with a high mortality rate. Systematically searching for those localizations in cases of C. fetus bacteremia may be warranted