Fingerprint imaging is a powerful tool in biometric identification systems. This paper proposes a fingerprint imaging method that involves applying an impedance signal to a fingerprint via ultrasonic impediography using a capacitive micromachined ultrasonic transducer (CMUT). We fabricated CMUT sparse array which has center frequency of 6.25 MHz and collapse voltage of 50 V by local oxidation of silicon (LOCOS) process. We develop a prototype by using waveguide and ultrasonic impediography method based on the CMUT. By using a waveguide made from a hard material, the prototype can ensure device protection and image lateral resolution improvement. The proposed technique successfully images the acoustic input impedance differences between the ridges and valleys of a fingerprint. The images created using direct touch and waveguide models are compared and analyzed. In addition, we present a phenomenon in the waveguide system in terms of energy loss by using numerical simulation. Therefore, this study facilitates ultrasonic fingerprint impediography measurement based on the basic direct touch and quartz glass waveguide system.

Fingerprint imaging has been widely used in biometric identification systems. This work presents a photoacoustic (PA) fingerprint imaging system that provides acoustic resolution using a pulsed laser and focused ultrasound transducer operating as a receiver. This PA system can measure dry fingers with a wide-range laser field based on the differences in the ultrasound coupling between the fingertip areas contacting and not contacting a solid plate. To demonstrate and validate the image accuracy of the PA system, PA fingerprint images were compared to images captured using a pulse-echo ultrasound system and an ink-pressed fingerprint scan.

Here, we report the characteristics of zero-bias capacitive micromachined ultrasonic transducers (CMUT) in various aspects, considering the transmission and reception sensitivity and evaluation of the long-term stability with AC transmission in immersion. The main idea of the zero-bias CMUT implementation is that the charge is injected by the dielectric charging effects in an insulation layer in the pull-in state. The CMUT was fabricated by a local oxidation of silicon (LOCOS) process, and the insulation layer consists of Si3N4-SiO2 and SiO2, which have been commonly used in previous studies. A study on the charging effects is reported to quantitatively observe the voltage shift by charge transfer with time dependency at different temperatures and collapsing time-dependency. Therefore, we successfully implemented a zero-bias CMUT with a transmission efficiency of 4.62-kPa/V at a center frequency of 7.53 MHz in Si3N4-SiO2 and a transmission efficiency of 6.78 kPa/V at a center frequency of 7.86 MHz in SiO2 immersion.

An investigation was made of the key determinants of the detective quantum efficiency at zero spatial frequency [DQE(0)] of a CsI:Tl scintillator based scanning slot x- ray detector for digital mammography. The slot x-ray detector was made of a prismatic type thallium activated CsI scintillator (150 micrometer thick) optically coupled to CCDs by fiber optical image guides. Monte Carlo calculations were performed to generate the CsI:Tl scintillator Swank factor on the basis of the energy deposition from pencil beam x-ray sources and light transmission within the CsI:Tl scintillator. A theoretical expression for the detector DQE(0) was obtained which was used to investigate the detector imaging performance as a function of x-ray energy, x-ray exposure, CCD electronic noise level, and optical coupling efficiency of the fiber optic image guide. The Swank factor of the CsI:Tl scintillator was close to unity at x-ray energies below Iodine K-edge (33.2 keV), but decreased to approximately 0.8 at higher x-ray energies up to 40 keV. DQE(0) of the slot x-ray detector was approximately 75% at 15 keV but decreased to approximately 40% at 30 keV. Optimum DQE(0) performance of the slot x-ray detector was generally obtained at a detector x-ray exposure level above approximately 5 to 10 mR and an electronic noise level below approximately 50 electrons rms. A drop in the optical coupling efficiency of the image guide from 1.0 to 0.3 reduced the detector DQE(0) from approximately 75% to approximately 55% in the mammography x-ray energy range. The key finding in this study is that the choice of the x-ray energy has a major impact on the DQE(0) of a CsI:Tl scintillator based slot x-ray detector. Since the x-ray photon energy also affects x-ray tube loading, mean glandular dose and subject contrast, the choices of optimal x-ray spectra from current mammography x-ray tubes require further investigation.

In this paper, we present a single-shot dual-mode imaging system that uses optical ultrasound (US) as an ultrasonic pulser; thus without transmission circuit. The ultrasonic pulse-echo system comprises an optical US pulser generated by carbon nanotubes (CNTs), which generate a high-power photoacoustic (PA) signal, and a capacitive micromachined ultrasonic transducer (CMUT) receiver. By fabricating a thin CNT-polydimethylsiloxane (PDMS) composite capable of semi-absorption of the laser, a single-shot imaging system was developed. By transmitting semi-penetration light to the object, US and PA imaging were performed in a single shot. A CNT thickness of <1 μm produced a maximum pressure of 154 kPa, and US was received by the CMUT with a 2-MHz center frequency in the PDMS. Additionally, a low-profile and near-depth imaging system was constructed with an intermediate layer of 6-mm PDMS for the dry contact method. We performed a single-shot dual-mode imaging experiment on point and line phantoms, as well as the particle spread in soft tissue. Thus, we examined the feasibility of the near-depth and single-shot dual-mode (US and PA) imaging system capable of dry contact.

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.