Effects of biocompatible encapsulations on the acoustic characteristics of CMUTs

Advances in modern medicine enable the use of medical implants for the treatment of an increasing number of diseases. If different implanted systems need to communicate with each other, data transmission using ultrasound is a promising solution. In this dissertation, an encapsulation strategy, which allows the use of capacitive micromachined ultrasonic transducers (CMUTs) within conventional implant housings, was developed and evaluated for the first time. The novel encapsulation approach consists of a silicone layer for coupling the CMUT to a layer of polyether ether ketone (PEEK) or titanium. Both materials are widely used for medical implant housings. Finite element simulations, complemented by measurements in air and in immersion as well as ex vivo experiments, have shown that effective data transmission with data rates of minimum 0.8 Mbps is possible over at least 6 cm with this encapsulation strategy.Die Fortschritte in der modernen Medizin ermöglichen immer häufiger den Einsatz von medizinischen Implantaten zur Therapie. In Anwendungsfällen, die eine Kommunikation mehrerer implantierter Systeme untereinander erfordern, stellt die Datenübertragung mit Hilfe akustischer Wellen eine vielversprechende Lösung dar. Hierfür ist eine biokompatible Kapselung nötig, die eine effiziente Datenübertragung nicht verhindert. In dieser Arbeit wird erstmals eine Kapselungsstrategie entwickelt und evaluiert, die den Einsatz von kapazitiven mikromechanischen Ultraschallwandlern (CMUTs) innerhalb konventioneller Implantatgehäuse ermöglicht. Die untersuchte neuartige Kapselung besteht aus einer Silikonschicht zur Ankopplung an eine Schicht aus Polyetheretherketon (PEEK) oder Titan, zwei weitverbreitete Materialien für die Kapselung medizinischer Implantate. Finite Elemente Simulationen, Messungen in Luft und Flüssigkeit sowie ex vivo Experimente haben gezeigt, dass mit dieser Kapselungsstrategie eine effektive Datenübertragung über mindestens 6 cm möglich ist. Die in ex vivo Experimenten ermittelten Frequenzbandbreiten der gekapselten CMUTs ermöglichen Datenraten von mindestens 0.8 Mbps. Ein zusätzlicher experimenteller Vergleich mit herkömmlichen Kapselungen für CMUTs bestätigt das große Potenzial der neuartigen Kapselung aus Silikon und PEEK. Abschließend wurden zukünftige Ansatzpunkte zur Steigerung von Signalamplitude und Datenrate identifiziert und diskutiert

Additive manufacturing (3D print) of air-coupled diaphragm ultrasonic transdrucers

Air-coupled ultrasound is a non-contact technology that has become increasingly common in Non Destructive Evaluation (NDE) and material evaluation. Normally, the bandwidth of a conventional transducer can be enhanced, but with a cost to its sensitivity. However, low sensitivity is very disadvantageous in air-coupled devices. This thesis proposes a methodology for improving the bandwidth of an air-coupled micro-machined ultrasonic transducer (MUT) without sensitivity loss by connecting a number of resonating pipes of various length to a cavity in the backplate. This design is inspired by the pipe organ musical instrument, where the resonant frequency (pitch) of each pipe is mainly determined by its length. The −6 dB bandwidth of the "pipe organ" inspired air-coupled transducer is 55.7% and 58.5% in transmitting and receiving modes, respectively, which is ∼5 times wider than a custom-built standard device. After validating the concept via a series of single element low-frequency prototypes, two improved designs: the multiple element and the high-frequency single element pipe organ transducers were simulated in order to tailor the pipe organ design to NDE applications.Although the simulated and experimental performance of the pipe organ inspired transducers are proved to be significantly better than the conventional designs, conventional micro-machined technologies are not able to satisfy their required 3D manufacturing resolution. In recent years, there has been increasing interest in using additive manufacturing (3D printing) technology to fabricate sensors and actuators due to rapid prototyping, low-cost manufacturing processes, customized features and the ability to create complex 3D geometries at micrometre scale. This work combines the ultrasonic diaphragm transducer design with a novel stereolithographic additive manufacturing technique. This includes developing a multi-material fabrication process using a commercial digital light processing printer and optimizing the formula of custom-built functional (conductive and piezoelectric) materials. A set of capacitive acoustic and ultrasonic transducers was fabricated using the additive manufacturing technology. The additive manufactured capacitive transducers have a receiving sensitivity of up to 0.4 mV/Pa at their resonant frequency.Air-coupled ultrasound is a non-contact technology that has become increasingly common in Non Destructive Evaluation (NDE) and material evaluation. Normally, the bandwidth of a conventional transducer can be enhanced, but with a cost to its sensitivity. However, low sensitivity is very disadvantageous in air-coupled devices. This thesis proposes a methodology for improving the bandwidth of an air-coupled micro-machined ultrasonic transducer (MUT) without sensitivity loss by connecting a number of resonating pipes of various length to a cavity in the backplate. This design is inspired by the pipe organ musical instrument, where the resonant frequency (pitch) of each pipe is mainly determined by its length. The −6 dB bandwidth of the "pipe organ" inspired air-coupled transducer is 55.7% and 58.5% in transmitting and receiving modes, respectively, which is ∼5 times wider than a custom-built standard device. After validating the concept via a series of single element low-frequency prototypes, two improved designs: the multiple element and the high-frequency single element pipe organ transducers were simulated in order to tailor the pipe organ design to NDE applications.Although the simulated and experimental performance of the pipe organ inspired transducers are proved to be significantly better than the conventional designs, conventional micro-machined technologies are not able to satisfy their required 3D manufacturing resolution. In recent years, there has been increasing interest in using additive manufacturing (3D printing) technology to fabricate sensors and actuators due to rapid prototyping, low-cost manufacturing processes, customized features and the ability to create complex 3D geometries at micrometre scale. This work combines the ultrasonic diaphragm transducer design with a novel stereolithographic additive manufacturing technique. This includes developing a multi-material fabrication process using a commercial digital light processing printer and optimizing the formula of custom-built functional (conductive and piezoelectric) materials. A set of capacitive acoustic and ultrasonic transducers was fabricated using the additive manufacturing technology. The additive manufactured capacitive transducers have a receiving sensitivity of up to 0.4 mV/Pa at their resonant frequency

Optimization of capacitive micromachined ultrasound transducers (CMUTs) for a high-frequency medical ultrasonic imaging system

Conventional ultrasonic imaging systems use piezoelectric transducers for the generation and reception of the acoustic signal. Since its invention in 1994, the Capacitive Micromachined Ultrasound Transducer (CMUT) has been subjected to research as an alternative technology. Major advantages of the CMUT over traditional piezoelectric ultrasound transducers include higher bandwidth, higher sensitivity, CMOS compatibility, and ease of manufacturing (by the use of standard lithography techniques.) Increasing the dynamic range, decreasing the parasitic capacitance and cross coupling are the major goals in CMUT designing specially for medical imaging applications. The work in this thesis aims the optimization of a high-frequency (20 MHz) CMUT array to be used for high-resolution medical imaging. The figure of merit has been chosen as the signal-to-noise ratio of the electrical return signal, which required the construction of a model for the entire pulse-echo operation. Such a model consists of: (1) a circuit model for the device itself, (2) a model for the radiation impedance, and (3) a model for the propagation medium. The CMUT model has been extensively studied in the literature. An already existing circuit model has been used in the simulations. The radiation impedance of the CMUT array was computed using Finite Element Analysis (FEA) software packages COMSOL Multiphysics® and ANSYS®, and converted to an equivalent circuit to represent the load in the circuit simulator. The pulse-echo model, which is entirely implemented in LTspice circuit simulator, was then used to optimize CMUT parameters that include radius, membrane thickness, and gap height to maximize signal-to-noise ratio

CMUT Crosstalk Reduction Using Crosslinked Silica Aerogel

Inter-element acoustic crosstalk is one of the major concerns which restricts the potential deployment of Capacitive Micromachined Ultrasonic Transducers (CMUTs) in Nondestructive Evaluation (NDE) despite its superior transduction capabilities. This thesis investigates the causes of acoustic crosstalk in CMUTs and develops a novel method of CMUT crosstalk reduction by passivating the CMUT top surface by a thin layer of Di-isocyanate enhanced crosslinked silica aerogel. This powerful technique derives its inspiration from the Scholte waves attenuation techniques as used in boreholes at the permeable formations. Analytical and 3D finite element analysis in MATLAB and COMSOL Multiphysics, respectively, show that the developed technique can minimize the crosstalk due to Scholte waves at the fluid-solid interfaces by at least 5 dB more at the nearest neighbor as compared to other published techniques. An added advantage of the developed technique is that the level of Scholte wave attenuation can be controlled by altering the porosity of the aerogel layer. A simple and cost-effective fabrication process employing sol-gel and ambient pressure drying processes for the aerogel layer deposition has been developed that doesn’t interfere with the basic CMUT operation or fabrication techniques

Finite element modeling and validation of a soft array of spatially coupled dielectric elastomer transducers

Dielectric elastomer (DE) transducers are suitable candidates for the development of compliant mechatronic devices, such as wearable smart skins and soft robots. If many independently-controllable DEs are closely arranged in an array-like configuration, sharing a common elastomer membrane, novel types of cooperative and soft actuator/sensor systems can be obtained. The common elastic substrate, however, introduces strong electro-mechanical coupling effects among neighboring DEs, which highly influence the overall membrane system actuation and sensing characteristics. To effectively design soft cooperative systems based on DEs, these effects need to be systematically understood and modeled first. As a first step towards the development of soft cooperative DE systems, in this paper we present a finite element simulation approach for a 1-by-3 silicone array of DE units. After defining the system constitutive equations and the numerical assumptions, an extensive experimental campaign is conducted to calibrate and validate the model. The simulation results accurately predict the changes in force (actuation behavior) and capacitance (sensing behavior) of the different elements of the array, when their neighbors are subjected to different electro-mechanical loads. Quantitatively, the model reproduces the force and capacitance responses with an average fit higher than 93% and 92%, respectively. Finally, the validated model is used to perform parameter studies, aimed at highlighting how the array performance depends on a relevant set of design parameters, i.e. DE-DE spacing, DE-outer structure spacing, membrane pre-stretch, array scale, and electrode shape. The obtained results will provide important guidelines for the future design of cooperative actuator/sensor systems based on DE transducers

High frequency CMUT for continuous monitoring of red blood cells aggregation

Récemment, de nombreuses recherches ont démontré que le transducteur ultrasonore micro-usiné capacitif CMUT peut être une alternative aux transducteurs piézoélectriques dans différents domaines, y compris l’imagerie par ultrasons médicaux. Des travaux antérieurs ont démontré les avantages de CMUT en termes de production à haute fréquence, de sensibilité, de compatibilité avec la technologie complémentaire métal – oxyde – semi-conducteur et de coût de fabrication peu élevé. Ce travail montrera les travaux préliminaires en vue de la fabrication d'un transducteur à ultrasons utilisant des CMUT pour mesurer en continu l'agrégation des globules rouges. Les cellules CMUT ont été conçues et simulées pour obtenir des fréquences de résonance et des dimensions spécifiques répondant à cet objectif, à l'aide de la modélisation par éléments finis avec COMSOL Multiphysics. Des simulations par ultrasons (logiciel Field II) ont été utilisées pour caractériser les faisceaux ultrasonores émis et reçus afin de concevoir la distribution géométrique des cellules. La fabrication a été réalisée en utilisant une photolithographie multicouche et des dépôts. Huit masques ont été conçus pour chaque couche de dépôt. Les masques ont été conçus pour comporter quatre groupes de CMUT, le premier émettant et recevant à 40 MHz, le second émettant à 30 MHz et recevant à 40 MHz, le troisième émettant à 20 MHz et recevant à 30 MHz, et le dernier émettant à 10 MHz. MHz et réception à 30 MHz. La fréquence change avec le rayon de chaque cellule CMUT, mais les dimensions de l'épaisseur sont les mêmes pour toutes les cellules, les épaisseurs des membranes et des couches isolantes sont de 0,3 µm et l'intervalle de vide est de 0,1 µm. Les matrices CMUT ont été fabriquées à l'aide de la technologie de couche de libération sacrificielle du laboratoire Polytechnique LMF.Research has demonstrated that Capacitive Micro machined Ultrasonic Transducer (CMUT) can be an alternative to piezoelectric transducers in different domains including medical ultrasound imaging. Previous work showed advantages of CMUT in terms of high frequency production, sensitivity, its compatibility with complementary metal–oxide–semiconductor technology and its low cost of fabrication. This work will show preliminary work toward fabricating an ultrasound transducer using CMUTs to continuously measure Red Blood Cells aggregation. CMUTs cells were designed and simulated to obtain specific resonant frequencies and dimension that fulfill that purpose using finite element modeling with COMSOL Multiphysics. Ultrasound simulations (Field II software) were used to characterize the emitted and received US beams to design the cells geometrical distribution. Fabrication was done using multilayered photolithography and depositions. Eight masks were designed for each deposition layer. The masks were designed to have four groups of CMUTs, one emitting and receiving at 40MHz, a second emitting at 30 MHz and receiving at 40 MHz, a third one emitting at 20 MHz and receiving at 30 MHz, and a last one emitting at 10 MHz and receiving at 30 MHz. The frequency changes with the radius of each CMUT cell but the thickness dimensions are the same for all the cells, the membranes and insulation layers thicknesses are 0.3 µm and the vacuum gap is 0.1 µm. The CMUT arrays were fabricated using sacrificial release layer technology in Polytechnic LMF Lab

Investigation of acoustic crosstalk effects in CMUT arrays

Capacitive Micromachined Ultrasonic Transducers (CMUTs) have demonstrated significant potential to advance the state of medical ultrasound imaging beyond the capabilities of the currently employed piezoelectric technology. Because they rely on well-established micro-fabrication techniques, they can achieve complex geometries, densely populated arrays, and tight integration with electronics, all of which are required for advanced intravascular ultrasound (IVUS) applications such as high-frequency or forward-looking catheters. Moreover, they also offer higher bandwidth than their piezoelectric counterparts. Before CMUTs can be effectively used, they must be fully characterized and optimized through experimentation and modeling. Unfortunately, immersed transducer arrays are inherently difficult to simulate due to a phenomenon known as acoustic crosstalk, which refers to the fact that every membrane in an array affects the dynamic behavior of every other membrane in an array as their respective pressure fields interact with one another. In essence, it implies that modeling a single CMUT membrane is not sufficient; the entire array must be modeled for complete accuracy. Finite element models (FEMs) are the most accurate technique for simulating CMUT behavior, but they can become extremely large considering that most CMUT arrays contain hundreds of membranes. This thesis focuses on the development and application of a more efficient model for transducer arrays first introduced by Meynier et al. [1], which provides accuracy comparable to FEM, but with greatly decreased computation time. It models the stiffness of each membrane using a finite difference approximation of thin plate equations. This stiffness is incorporated into a force balance which accounts for effects from the electrostatic actuation, pressure forces from the fluid environment, mass and damping from the membrane, etc. For fluid coupling effects, a Boundary Element Matrix (BEM) is employed that is based on the Green's function for a baffled point source in a semi-infinite fluid. The BEM utilizes the nodal mesh created for the finite difference method, and relates the dynamic displacement of each node to the pressure at every node in the array. Use of the thin plate equations and the BEM implies that the entire CMUT array can be reduced to a 2D nodal mesh, allowing for a drastic improvement in computation time compared with FEM. After the model was developed, it was then validated through comparison with FEM. From these tests, it demonstrated a capability to accurately predict collapse voltage, center frequency, bandwidth, and pressure magnitudes to within 5% difference of FEM simulations. Further validation with experimental results revealed a close correlation with predicted impedance/admittance plots, radiation patterns, frequency responses, and noise current spectrums. More specifically, it accurately predicted how acoustic crosstalk would create sharp peaks and notches in the frequency responses, and enhance side lobes and nulls in the angular radiation pattern. Preliminary design studies with the model were also performed. They revealed that membranes with larger lateral dimensions effectively increased the bandwidth of isolated membranes. They also demonstrated potential for various crosstalk reduction techniques in array design such as disrupting array periodicity, optimizing inter-membrane pitch, and adjusting the number of membranes per element. It is expected that the model developed in this thesis will serve as a useful tool for future iterations of CMUT array optimizations.MSCommittee Chair: Dr. F. Levent Degertekin; Committee Member: Dr. Karim Sabra; Committee Member: Dr. Suresh Sitarama

Experiment and simulation validated analytical equivalent circuit model for piezoelectric micromachined ultrasonic transducers

An analytical Mason equivalent circuit is derived for a circular, clamped plate piezoelectric micromachined ultrasonic transducer (pMUT) design in 31 mode, considering an arbitrary electrode configuration at any axisymmetric vibration mode. The explicit definition of lumped parameters based entirely on geometry, material properties, and defined constants enables straightforward and wide-ranging model implementation for future pMUT design and optimization. Beyond pMUTs, the acoustic impedance model is developed for universal application to any clamped, circular plate system, and operating regimes including relevant simplifications are identified via the wave number-radius product ka. For the single-electrode fundamental vibration mode case, sol-gel Pb(Zr[subscript 0.52])Ti[subscript 0.48]O[subscript 3] (PZT) pMUT cells are microfabricated with varying electrode size to confirm the derived circuit model with electrical impedance measurements. For the first time, experimental and finite element simulation results are successfully applied to validate extensive electrical, mechanical, and acoustic analytical modeling of a pMUT cell for wide-ranging applications including medical ultrasound, nondestructive testing, and range finding.Masdar Institute of Science and Technology (Massachusetts Institute of Technology Cooperative Agreement Grant 6923443)National Science Foundation (U.S.). Graduate Research Fellowshi

A Feasibility Study of Micromachined Ultrasonic Transducers Functionalized for Ethanol Dectection

The chemical sensing system plays an important role in medical and environmental monitoring. Gases exhaled by humans include nitrogen, oxygen, water vapor, carbon dioxide and volatile organic compounds (VOCs). The VOCs are important and provide valuable information for non- invasive diagnosis. For instance, ethanol detection is beneficial for checking blood alcohol. In time blood alcohol level checking before checking can prevent a person from unsafe driving. Due to the extremely low concentration of the target gases, a gas sensor with high sensitivity, selectivity and low detection limit is required. There is a high demand for low cost, fast, accurate and easy-to-use self-check diagnosis devices. With low cost and high portability, micro-electromechanical systems (MEMS) sensors have been extensively studied for chemical sensing, which provide a cheap self-diagnosis solution. Capacitive Micromachined Ultrasonic Transducers (CMUTs) and Piezoelectric Micromachined Ultrasonic Transducer (PMUTs), which both work based on the mass-loading effect, are considered as the promising types of MEMS sensors for gas sensing. Since they are fabricated in a batch manner with the similar process of silicon-based integrated circuits, CMUTs and PMUTs are able to provide massive parallelism, easy integration with microelectronic circuits, and a higher quality factor. In this research, studied the feasibility of using PMUTs and CMUTs fabricated by our lab for ethanol detection through simulation and experiments. Models for are built via COMSOL for PMUT and CMUT respectively. The simulation results of a single sensing element demonstrated that both CMUTs and PMUTs show great potential for gas sensors. The chemical experiments through frequency response measurement exhibit that both the PMUTs and CMUTs are effective for ethanol detection based on the mass-loading effect. When the gas analyte is attached to the sensing layer, a higher resonance frequency of the transducer induces a higher frequency shift, which means the higher resonance frequency of transducer, the higher sensitivity of a gas sensor is and the lower concentration of ethanol can be detected. Additionally, a CMUT array is also applied to ethanol detection. It provides a good preliminary study of the CMUTs functionalized with more sensing materials for chemical detection in future

Using ANN and combined capacitive sensors to predict the void fraction for a two-phase homogeneous fluid independent of the liquid phase type

Measuring the void fraction of different multiphase flows in various fields such as gas, oil, chemical, and petrochemical industries is very important. Various methods exist for this purpose. Among these methods, the capacitive sensor has been widely used. The thing that affects the performance of capacitance sensors is fluid properties. For instance, density, pressure, and temperature can cause vast errors in the measurement of the void fraction. A routine calibration, which is very grueling, is one approach to tackling this issue. In the present investigation, an artificial neural network (ANN) was modeled to measure the gas percentage of a two-phase flow regardless of the liquid phase type and changes, without having to recalibrate. For this goal, a new combined capacitance-based sensor was designed. This combined sensor was simulated with COMSOL Multiphysics software. Five different liquids were simulated: oil, gasoil, gasoline, crude oil, and water. To estimate the gas percentage of a homogeneous two-phase fluid with a distinct type of liquid, data obtained from COMSOL Multiphysics were used as input to train a multilayer perceptron network (MLP). The proposed neural network was modeled in MATLAB software. Using the new and accurate metering system, the proposed MLP model could predict the void fraction with a mean absolute error (MAE) of 4.919