In this study, piezoelectric acoustic absorbers employing two receivers and one transmitter with a feedback controller were evaluated. Based on the target and resonance frequencies of the system, resonance and non-resonance models were designed and fabricated. With a lateral size less than half the wavelength, the model had stacked structures of lossy acoustic windows, polyvinylidene difluoride, and lead zirconate titanate-5A. The structures of both models were identical, except that the resonance model had steel backing material to adjust the center frequency. Both models were analyzed in the frequency and time domains, and the effectiveness of the absorbers was compared at the target and off-target frequencies. Both models were fabricated and acoustically and electrically characterized. Their reflection reduction ratios were evaluated in the quasi-continuous-wave and time-transient modes.Sensors 2020, 20, 47 2 of 17 previous studies. However, it is difficult to compare them due to the different approaches involved. Therefore, we believe that only resonance and non-resonance models with similar structures should be compared. In this study, resonance and non-resonance models suitable for the target frequency were designed via mathematical analysis. Based on the designs, the resonance and non-resonance models were fabricated and evaluated. The absorber was configured in the form of tiles, and the lateral size of the structure was equal to half of the wavelength. Our proposed resonance and non-resonance models were fabricated by stacking commercially available lead zirconate titanate (PZT). Various studies have previously investigated the stacking or direct fabrication of devices [14-19]. Materials and Methods ConceptIn the case of a piezoelectric material, vibration along the thickness direction is observed when an electrode is applied, and the sound spreads around both the front and rear sides of the piezoelectric material. The resonance frequency range of a piezoelectric material depends on its thickness. The thickness of a piezoelectric material and its resonance frequency are inversely proportional, i.e., the thinner the piezoelectric material, the higher is its resonance frequency and vice versa.In this paper, we present a model that resonates in the low frequency region of the target frequency and a non-resonance model that deviates from the target frequency. The proposed resonance and non-resonance models incorporated a function that cancelled a certain portion of the incident sound waves.The incident and reflected waves must be separated to cancel the incident sound wave. Two receiving sensors were required to separate the incident and reflected waves, as the signals were measured based on the overlap between these two waves. The incident (P + ) and reflected (P -) acoustic pressures were the input and output of the system, respectively (Figure 1). We calculated the incident and reflected acoustic sensitivities based on the different receiving sensitivities of the two sensors obtained using the Krimholtz-L...

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Intensive research on photoacoustics (PA) for imaging of the living human body, including the skin, vessels, and tumors, has recently been conducted. We propose a PA measurement system based on a capacitive micromachined ultrasonic transducer (CMUT) with waterless coupling, short measurement time (<1 s), backward light irradiation, and a low-profile ultrasonic receiver unit (<1 cm). We fabricate a 64-element CMUT ring array with 6.2 mm diameter and 10.4 MHz center frequency in air, and 100% yield and uniform element response. To validate the PA tissue characterization, we employ pencil lead and red ink as solid and liquid models, respectively, and a living body to target moles and vessels. The system implements a near-field imaging system consisting of a 6 mm polydimethylsiloxane (PDMS) matching layer between the object and CMUT, which has a 3.7 MHz center frequency in PDMS. Experiments were performed in a waterless contact on the PDMS and the laser was irradiated with a 1 cm diameter. The experimental results show the feasibility of this near-field PA imaging system for position and depth detection of skin, mole, vessel cells, etc. Therefore, a system applicable to a low-profile compact biomedical device is presented.

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With the development of wearable devices, strain sensors have attracted large interest for the detection of human motion, movement, and breathing. Various strain sensors consisting of stretchable conductive materials have been investigated based on resistance and capacitance differences according to the strain. However, this method requires multiple electrodes for multipoint detection. We propose a strain sensor capable of multipoint detection with a single electrode, based on the ultrasound pulse–echo method. It consists of several transmitters of carbon nanotubes (CNTs) and a single polyvinylidene fluoride receiver. The strain sensor was fabricated using CNTs embedded in stretchable polydimethylsiloxane. The received data are characterized by the different times of transmission from the CNTs of each point depending on the strain, i.e., the sensor can detect the positions of the CNTs. This study demonstrates the application of the multipoint strain sensor with a single electrode for measurements up to a strain of 30% (interval of 1%). We considered the optical and acoustic energy losses in the sensor design. In addition, to evaluate the utility of the sensor, finger bending with three-point CNTs and flexible phantom bending with six-point CNTs for the identification of an S-curve having mixed expansion and compression components were carried out.