A capacitive micromachined ultrasound transducer (CMUT) was engineered and functionalized with zeolitic imidazolate framework-8 (ZIF-8) dispersed in a photoresist AZ1512HS (AZ) matrix to function as a gravimetric gas sensor. The sensor response was recorded in the presence of nitrogen, argon, carbon dioxide, and methane gases as well as water, acetylene, a propane/butane mixture, n-hexane, gasoline, and diesel vapors. The photoresist matrix alone was found to have a negligible response to all the gases and vapors, except for water vapor. No visible difference in sensor response was detected when switching from nitrogen to methane gas. However, a strong shift in the sensor resonance frequency was observed when exposed to higher hydrocarbons, ranging from 1 kHz for acetylene to 7.5 kHz for gasoline. Even longer-chain hydrocarbons, specifically kerosene and more so diesel, had a significantly reduced sensor frequency shift compared with gasoline. Sensors functionalized with a thin film of AZ+ZIF-8 demonstrated higher sensitivity in their response to a hydrocarbon molecular mass than without functionalization.

The prototype gravimetric microelectromechanical imine functionalized CO2 sensor equipped with custom electronics and significantly increased stability under sub-percent concentrations is investigated. Instabilities caused by the relative air humidity were eliminated by two in-situ calibration methods: two-channel readings correction and single-channel correction by the reference disturbance. Capacitive micromachined ultrasound transducers (CMUT) were designed for 15 MHz nominal resonance frequency, while actual sensor readings were lower (over 12 MHz) due to the functionalization. The resonance frequency of polyethyleneimine (PEI) functionalized CMUT during sub-percent CO2 detection experienced instabilities of several hundred Hz while the relative humidity of the air was changed between 10% and 35%. Readings of graphene oxide (GOx) functionalized CMUT working in parallel with PEI functionalized CMUT were used for two-channel correction, resulting in highly stable sensor output, characterized by 3.2 Hz standard deviation during almost one hour experiment, while relative humidity varied between 10% and 25%. Single channel reading correction method resulted in larger residual instability, characterized by a standard deviation of up to 36 Hz over a two-hour experiment, however, it was found to be more effective for longer humidity-induced drifts, since reference disturbance of known relative humidity of 75% maintained by saturated NaCl solution technique was used.

The combined static displacement of specifically arranged microscale electrostatic elements was analyzed to investigate a fully hermetic, perpendicular-to-plane type of electrostatic microactuator. The concept of asymmetric perimeter-supported electrostatic plates is introduced featuring softened partial clamping of plate perimeter. When engaged by an electrostatic field, asymmetrically clamped square plates tend to develop a rotating momentum, which is transferred to the background plate. Therefore, larger displacement range of the combined membrane actuator is possible by increasing of the number of elementary electrostatic cells, but not the electrostatic gap. It was shown by finite element modeling that a combination of 400 asymmetric electrostatic cells over a square background plate (the membrane actuator), roughly measured at 1 mm2, has a potential to produce a static deflection at the central point of the membrane, that is perpendicular to the plane, reaching 2 & mu;m at 45 V or up to 8 & mu;m at higher voltages. This result was supported by a physical model built at 1000:1 scale using stereolithographic three-dimensional printing. A potential application of the proposed perpendicular to plane membrane actuator is microfluidic systems.

The effect of microchannel height on acoustic streaming velocity and capacitive micromachined ultrasound transducer (CMUT) cell damping was investigated. Microchannels with heights ranging from 0.15 to 1.75 mm were used in experiments, and computational microchannel models with heights varying from 10 to 1800 micrometers were simulated. Both simulated and measured data show local minima and maxima of acoustic streaming efficiency associated with the wavelength of the `bulk acoustic wave excited at 5 MHz frequency. Local minima occur at microchannel heights that are multiples of half the wavelength (150 μm), which are caused by destructive interference between excited and reflected acoustic waves. Therefore, microchannel heights that are not multiples of 150 μm are more favorable for higher acoustic streaming effectiveness since destructive interference decreases the acoustic streaming effectiveness by more than 4 times. On average, the experimental data show slightly higher velocities for smaller microchannels than the simulated data, but the overall observation of higher streaming velocities in larger microchannels is not altered. In additional simulation, at small microchannel heights (10–350 μm), local minima at microchannel heights that are multiples of 150 μm were observed, indicating the interference between excited and reflected waves and causing acoustic damping of comparatively compliant CMUT membranes. Increasing the microchannel height to over 100 μm tends to eliminate the acoustic damping effect as the local minima of the CMUT membrane swing amplitude approach the maximum value of 42 nm, which is the calculated amplitude of the freely swinging membrane under the described conditions. At optimum conditions, an acoustic streaming velocity of over 2 mm/s in a 1.8 mm-high microchannel was achieved.

The trade-off between the functionalization shift of the informative parameters and sensitivity of capacitive micromachined ultrasound transducers (CMUT)-based CO2 sensors is addressed, and the CMUT surface modification process by thin inkjet-printed polyethyleneimine (PEI) films is optimized. It was shown that by the proper preparation of the active CMUT surface and properly diluted PEI solution, it is possible to minimize the functionalization shift of the resonance frequency and the quality of the resonance and preserve the sensitivity potential. So, after optimization, we demonstrated 23.2 kHz frequency shift readings of the sensor with 16 MHz nominal frequency while in the gas chamber and switching between pure N2 and CO2. After testing the sensors with different PEI film thickness, it was confirmed that a 200 nm average thickness of a PEI film is an optimum, because this is the practical limit of CO2 absorption depth at given conditions. Additionally, we note that modification of the hydrophilic/hydrophobic properties of the CMUT surface allows changing the nanoscale surface roughness of the printed PEI film and controlling the area resolution of the inkjet functionalization by reducing the diameter of a single dot down to 150 μm by a commercially available printer cartridge.

Proposed fully hermetic actuator design is intended for microfluidic valve application in biosensors. Asymmetric CMUT cells were designed as square perimeter-clamped plates with softened clamping on three sides. While engaged by electrostatic field, these cells tend to develop the rotating momentum, which is transferred to the background plate. It was shown that combination of 400 asymmetric CMUT cells over a square background plate (the membrane actuator), roughly measured at 1 mm 2 , has a potential to produce the perpendicular to plane static deflection of the membrane’s central point reaching 2 µm at 45 V or up to 8 µm at higher voltages, without increase of the electrostatic gap of CMUT cells.

This review paper discusses the advances of the gravimetric detection devices based on capacitive micromachined ultrasound transducers structure. Principles of gravimetric operation and device modeling are reviewed through the presentation of an analytical, one-dimensional model and finite element modeling. Additionally, the most common fabrication techniques, including sacrificial release and wafer bonding, are discussed for advantages for gravimetric sensing. As functional materials are the most important part of the selective gravimetric sensing, the review of different functional material properties and coating and application methods is necessary. Particularly, absorption and desorption mechanisms of functional materials, like methylated polyethyleneimine, with examples of applications for gas sensing and using immune complexes for specific biomolecules detection are reviewed.

A gravimetric gas detection device based on surface functionalized Capacitive Micromachined Ultrasound Transducers (CMUTs) was designed, fabricated and tested for detection of carbon dioxide (CO2) and sulfur dioxide (SO2) mixtures in nitrogen. The created measurement setup of continuous data collection, integrated with an in-situ Fourier Transform Infrared (FT-IR) spectroscopy, allows for better understanding of the mechanisms and molecular interactions with the sensing layer (methylated poly(ethylene)imine) and its need of surface functionalization for multiple gas detection. During experimentation with CO2 gases, weak molecular interactions were observed in spectroscopy data. Linear sensor response to frequency shift was observed with CO2 concentrations ranging from 0.16 vol % to 1 vol %. Moreover, the Raman and FT-IR spectroscopy data showed much stronger SO2 and the polymer interactions, molecules were bound by stronger forces and irreversibly changed the polymer film properties. However, the sensor change in resonance frequency in the tested region of 1 vol % to 5 vol % SO2 showed a linear response. This effect changed not only the device resonance frequency but also affected the magnitude of electroacoustic impedance which was used for differentiating the gas mixture of CO2, SO2, in dry N2.

A capacitive micromachined ultrasonic transducer (CMUT)-based sensor modified with methylated poly(ethylenimine) (mPEI) was designed and tested for the detection of two acidic gases: carbon dioxide (CO2) and sulfur dioxide (SO2). Combined gas sensing and Fourier transform infrared spectroscopy of the adsorbed products allowed to simultaneously and in situ determine the types and strength of the molecular interactions responsible for sensing. For CO2, the limit of detection was calculated to be 0.011 CO2 vol % and the limit of quantification was calculated to be 0.033%. For SO2, the limit of detection was calculated as 0.232 SO2 vol % and the limit of quantification was 0.704%. The sensing system exhibited a linear response at lower concentrations for CO2 and linear response for all the tested concentrations of SO2. In situ IR and ex situ Raman showed that CO2 was observed to undergo weak molecular coordination with mPEI while SO2 bound strongly and irreversibly degraded the thin mPEI films.

The improvement of the micromachined ultrasound transducer based (CMUT) biosensor fabrication technology and signal processing, which led to higher signal to noise ratio is reported. The biosensor contains interdigitally arranged CMUT structure with gold-coated analytical area. It is assembled with the plexiglass microchannels. CMUTs were fabricated with the wafer bonding technology for 5 MHz operation in immersion. For signal processing the convolutional neural network (CNN) was developed and trained to classify the sensor data to different propagation delay values. For training of the network 750 thousand signals representing different properties of the bioanalyte and different noise conditions was simulated by the finite time difference domain (FDTD) model. The capability of the CNN algorithm to classify the propagation delay data was compared with the adaptive passband filter signal processing algorithm used in our previous version of the senor. Both sensing channels were run simultaneously with the reference liquids in the microchannel: deionized water switching to 0.9% saline. It was found that CNN channel is capable to improve the signal to noise ratio for this experiment to 75 dB, when the same property for the passband filter channel was only 60 dB. This led to the generalization about the advantage of CNN channel to provide 15 dB less of instrumental noise. Finally, the real-time detection ability of the bovine serum albumin (BSA) deposition on the analytical area of improved sensor was demonstrated.

Elastic modulus of the furniture coatings is an important parameter, which indicates the curing state of the composite resin curable by the ultraviolet radiation. Non-contact way of elastic modulus measurement by the transverse acoustical waves, propagating perpendicular to the surface of a 10-mu m thick polymer, is shown as a suitable principle of the curing state sensing during furniture production. Transverse waves were excited by the cone-shaped ultrasound transducer having its frequency band centered at 1 MHz and captured with a receive transducer within 10- to 30-mm distance away from the excitation point. This setup allows to measure the differences of the time of flight of the pulse of the transverse waves in respect of the curing state in a non-contact regime. Numerical simulation and experimental results have led to the function, which relates the time of flight with the elastic modulus of the polymer coating, which was identified to be less than 1.95 GPa for an insufficiently cured polymer up to a 2.25 GPa in the case when cured as specified by the manufacturer. These values were confirmed by the AFM force-separation curve measurement technique, used here as the reference measurement channel.

Acoustical fluid mixing and streaming in microfluidic chips enhance the detection and fluid routeing capabilities in the lab-on-chip devices. Capacitive micromachined ultrasound transducers (CMUT) are easy to integrate into a closely packed environment, and they can be simultaneously used as integrated sensors and micropumps/mixers. In this paper, particular focus is given to examininge the impact of the acoustical fluid mixing to the kinetics of biochemical interaction. CMUT interdigital transducers for 10-MHz operation in water were designed and fabricated using the surface micromachining technique. Devices use Scholte type waves for biochemical detection and acoustical streaming. They also have the ability to control the directionality of acoustical streaming by +/-90 degrees phase shift. The impact of acoustical streaming to the liquid diffusion kinetics in the microchannel and to the kinetics of adsorption of the bovine serum albumin (BSA) to the gold surface was investigated experimentally. For microfluidic experiments, CMUTs were assembled with 100 mu m deep microchannels. It was determined that acoustical streaming can improve the diffusion rate through the microchannel. Also, it was shown that BSA adsorption rate can be controlled by changing the phase shift during excitation of the Sholte type waves. [2016-0224]

We present here our results of designing, testing and researching of capacitive micromachined ultrasound transducers (CMUT) as detectors in biosensing. In our work we used two types of CMUT structure, which correspond to two types of detection: resonance and Scholte waves. We found that biosensing is more efficient when the interdigital (IDT) CMUT structure is used for Scholte waves excitation and receive, since IDT structure enables the real-time biosensing in fluid environment CMUT IDTs for 10 MHz operation in water, with 146 m spaced double fingers were designed and fabricated using the surface micromachining technique. Fabricated CMUTs were tested for their resonance in air and for Scholte-type wave transmission in alcohol/deionized water solutions. The amplitude and phase velocity of the excited and received Scholte waves were measured in a 200 mu m height microchannel, capped with a thick layer of soft polymer, which suppressed the production of non-informative guided waves. Experimental data were also used to verify the adequacy of the finite element model. It was determined that for the analyzed conditions the sensitivity of the measurement channel is expected to be no worse than 2 kHz GPa(-1) in terms of the Scholte wave and CMUT IDT resonance frequency. This leads to a positive conclusion on the feasibility of the new sensor type. Additionally we explored the advantage of the CMUT for liquid mixing and pumping. (C) 2016 E. Sapeliauskas, D. Barauskas, D. Pelenis, G. Vanagas, M. Mikolajunas, D. Virzonis.

Interdigital CMUTs were designed and fabricated in transmit/receive pairs. Each transmitter or receiver has 20 double-phase finger pairs with 146 micrometer pitch and 3 millimeter aperture. The wave transmission distance between the receiver and transmitter is 9 millimeters. Devices were fabricated on the highly doped silicon wafers by the surface micromachining technology using silicon nitride as the structural material and chromium as the sacrificial material. Interface (Scholte type) waves were transmitted and received at 100 V bias and 10 V pp 10 MHz single period harmonic burst excitation. The wave propagation velocity in water and isopropanol were measured by the spectral analysis methods. It gave us 1380 m/s and 1125 m/s phase velocity of the Scholte waves for water and isopropanol, correspondingly. We explain these reference values as specific to the measurement conditions, which are specific to our measurement conditions and particular CMUT assembly with the microchannel.

In this study we present theoretical proof of the principle of using interdigital capacitive micromachined ultrasound transducers (CMUT IDTs) for the detection of phospholipid membrane elasticity. Proof of principle was needed to find out whether the new type of microelectromechanical sensors of the toxins incorporated with the lipid membranes was feasible. CMUT IDTs for 10 MHz operation in water, with 146 mu m spaced double fingers were designed and fabricated using the surface micromachining technique. Fabricated CMUTs were tested for their resonance in air and for Scholte-type wave transmission in deionized water and isopropanol solutions containing 0%, 10% and 20% water. The amplitude and phase velocity of the excited and received Scholte waves were measured in a 200 mu m height microchannel, capped with a thick layer of soft polymer, which suppressed the production of non-informative guided waves. It was determined that the average sensitivity of Scholte wave phase velocity within the given range of solution concentrations is 2.9 m s(-1) per one percent. Experimental data were also used to verify the adequacy of the finite element model, which was found to be suitable for reliable prediction of the phospholipid membrane elasticity impact on the Scholte wave phase velocity or the resonance frequency in the present IDT structure. It was determined that for the analyzed conditions (the elasticity of simulated phospholipid membrane changed from 1 to 5 GPa) the sensitivity of the measurement channel is expected to be no worse than 2 kHz GPa(-1) in terms of the Scholte wave and CMUT IDT resonance frequency. This leads to a positive conclusion on the feasibility of the new sensor type.

In our work we address creation of reliable and cross-selective platform of greenhouse gas sensing for environmental monitoring and safety purposes based on capacitive micromachined ultrasound transducers (CMUT) technology. We emphasize the need of cross-selectivity between CO 2 , CH 4 and H 2 S since mixture of these gases is common in anthropogenic and natural gas emissions, and their impact to the greenhouse effect remains underexplored. CMUT sensors containing five elements with resonance frequencies from 15 to 35 MHz were designed and fabricated by fully CMOS compatible surface micromachining technology. Two parallel measurement channels, one reference and one sensing were arranged to measure the mass loading of the CMUT structure and eliminate the non-informative changes of the working environment as temperature, pressure and humidity. CO 2 sensing experiments show that present sensing setup, when CMUT structure is functionalized by thin polyethyleneimine film, has the 4 Hz/ppm sensitivity.

The array of the CMUT sensors was fabricated by the direct bonding of the silicon membranes provided as a device layer of the SOI wafer with the oxidized and pre-patterned highly doped silicon wafer. The active surface of the sensors was coated with thin gold film. We demonstrate the detection of the formed immune complex over the CMUT surface modified by the bovine leukemia virus antigen BLV gp51. The modified CMUT surface was allowed to interact with the specific antibody anti-gp51 labeled by the horseradish peroxidase. Antibody labels were actvated after the interaction by wetting the sensor surface with tetramethylbenzidine to provide the reference quantification of the formation of the immune complex. CMUT sensor readings (resonance frequency and the resonance value of the impedance real part, “resistance”) were obtained before and after modification by the BLV gp51 and after the interaction with antig-p51. The readings were interpreted and fitted by the finite element analysis. It was determined that increase of the elasticity modulus and stress of the sensor structure caused increase of the resonance frequency and some decrease of the resonance quality (resistance value). After the immune complex is established, we observe decrease of the resonance frequency by 4% on average from the initial value. Finite element analysis model fitting to the experimental results revealed the mass loading function of the CMUT structure in the presence of increased elasticity of the proteins.

Interdigitally arranged and integrated with the polymer microchannels capacitive micromachined ultrasound transducers (CMUT) were designed, fabricated and tested for transverse waves, preferably Scholte type waves, excitation and receive. CMUTs were used for verification of the finite element model dedicated to predict the ability of sensing the elasticity of tethered phospholipid bilayers in liquid environment. Also, the sensitivity of the transducer and microchannel assembly to the liquid properties was tested while recording in the real time the amplitude of the received Scholte type transverse wave. Mixtures of isopropyl alcohol (IPA) and water in fractions of 10:1, 20:1 and 50:1 was used for this test. Finite element analysis predicted that considerable sensitivity of the received Scholte wave amplitude to the tethered phospholipid bilayer elasticity change from 2 to 5 GPa can be expected. The real-time sensing of the liquid mixture concentration changes revealed that even 2% water content increase (50:1 mixture case) can be sensed as 100 mV transition of the Scholte wave amplitude. Greater concentration changes caused up to 700 mV transition of the received wave amplitude.

High-frequency (40 MHz) and low-frequency (7 MHz) capacitive micromachined ultrasound transducers (CMUT) were fabricated and tested for use in gravimetric detection of biomolecules. The low-frequency CMUT sensors have a gold-coated surface, while the high-frequency sensors have a silicon nitride surface. Both surfaces were functionalized with bovine leukemia virus antigen gp51 acting as the antigen. On addition of an a specific antibody labeled with horseradish peroxidase (HRP), the antigen/antibody complex is formed on the surface and quantified by HRP-catalyzed oxidation of tetramethylbenzidine. It has been found that a considerably smaller quantity of immuno complex is formed on the high frequency sensor surface. In parallel, the loading of the surface of the CMUT was determined via resonance frequency and electromechanical resistance readings. Following the formation of the immuno complexes, the resonance frequencies of the low-frequency and high-frequency sensors decrease by up to 420 and 440 kHz, respectively. Finite element analysis reveals that the loading of the (gold-coated) low frequency sensors is several times larger than that on high frequency sensors. The formation of the protein film with pronounced elasticity and stress on the gold surface case is discussed. We also discuss the adoption of this method for the detection of DNA using a hybridization assay following polymerase chain reaction.