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.
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 µm 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 µm 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.
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