Directional Resonant MEMS Acoustic Sensor and Associated Acoustic Vector Sensor.

This paper reports on the design, modeling, analysis, and evaluation of a micro-electromechanical systems acoustic sensor and the novel design of an acoustic vector sensor array (AVS) which utilized this acoustic sensor. This research builds upon previous work conducted to develop a small, lightweight, portable system for the detection and location of quiet or distant acoustic sources of interest. This study also reports on the underwater operation of this sensor and AVS. Studies were conducted in the lab and in the field utilizing multiple acoustic sources (e.g., generated tones, gun shots, drones). The sensor operates at resonance, providing for high acoustic sensitivity and a high signal-to-noise ratio (SNR). The sensor demonstrated a maximum SNR of 88 dB with an associated sensitivity of -84.6 dB re 1 V/µPa (59 V/Pa). The sensor design can be adjusted to set a specified resonant frequency to align with a known acoustic signature of interest. The AVS demonstrated an unambiguous, 360-degree, in-plane, azimuthal coverage and was able to provide an acoustic direction of arrival to an average error of within 3.5° during field experiments. The results of this research demonstrate the potential usefulness of this sensor and AVS design for specific applications.

Dual Band MEMS Directional Acoustic Sensor for Near Resonance Operation.

In this paper, we report on the design and characterization of a microelectromechanical systems (MEMS) directional sensor inspired by the tympana configuration of the parasitic fly Ormia ochracea. The sensor is meant to be operated at resonance and act as a natural filter for the undesirable frequency bands. By means of breaking the symmetry of a pair of coupled bridged membranes, two independent bending vibrational modes can be excited. The electronic output, obtained by the transduction of the vibration to differential capacitance and then voltage through charge amplifiers, can be manipulated to tailor the frequency response of the sensor. Four different frequency characteristics were demonstrated. The sensor exhibits, at resonance, mechanical sensitivity around 6 µm/Pa and electrical sensitivity around 13 V/Pa. The noise was thoroughly characterized, and it was found that the sensor die, rather than the fundamental vibration, induces the predominant part of the noise. The computed average signal-to-noise (SNR) ratio in the pass band is about 91 dB. This result, in combination with an accurate dipole-like directional response, indicates that this type of directional sensor can be designed to exhibit high SNR and selectable frequency responses demanded by different applications.

MEMS Underwater Directional Acoustic Sensor in Near Neutral Buoyancy Configuration.

A MEMS directional acoustic sensor housed in an air cavity and operated underwater in a near-neutral buoyancy configuration is demonstrated. The sensor consists of two wings connected by a bridge and attached to a substrate by two centrally mounted torsional legs. The frequency response showed two resonant peaks corresponding to a rocking mode (wings moving in opposite directions) and a bending mode (wings moving in the same direction). Initial tests of the sensor using a shaker table showed that the response is highly dependent on the vibration direction. In air, the sensor showed a maximum sensitivity of about 95 mV/Pa with a cosine directional response. Underwater, the maximum sensitivity was about 37 mV/Pa with a similar cosine directional response. The measured maximum SNR was about 38 dB for a signal generated by a sound stimulus of 1 Pa when the sensor is operated near the bending resonance. The results indicate that this type of MEMS sensor can be operated in a near-neutral buoyant configuration and achieve a good directional response.

Validated model of platelet slip at stenosis and device surfaces.

Platelets are central to thrombosis. However, it is unknown whether platelets slip at vascular or device surfaces. The presence of platelet slip at a surface would interrupt physical contact between the platelet and that surface, and therefore diminish adhesion and thrombosis. Unfortunately, no existing technology can directly measure platelet slip in a biological environment. The objective of this study was to explore whether microspheres-modeling platelets-slip at different vascular and device surfaces in an acrylic scaled-up model coronary artery. The microspheres (3.12 µm diameter) were suspended in a transparent glycerol/water experimental fluid, which flowed continuously at Reynolds numbers typical of coronary flow (200-400) through the model artery. We placed a series of axisymmetric acrylic stenoses (cross-sectional area reduction [CSAr], 20-90%) into the model artery, both without and with a central cylinder present (modeling a percutaneous interventional guide wire, and with a scaled-up Doppler catheter mounted upstream). We used laser Doppler velocimetry (LDV) to measure microsphere velocities within, proximal and distal to each stenosis, and compared to computer simulations of fluid flow with no-slip. For validation, we replaced the acrylic with paraffin stenoses (more biologically relevant from a surface roughness perspective) and then analyzed the signal recorded by the scaled-up Doppler catheter. Using the LDV, we identified progressive microsphere slip proportional to CSAr inside entrances for stenoses ≥60% and ≥40% without and with cylinder present, respectively. Additionally, microsphere slip occurred universally along the cylinder surface. Computer simulations indicated increased fluid shear rates (velocity gradients) at these particular locations, and logistic regression analysis comparing microsphere slip with fluid shear rate resulted in a c-index of 0.989 at a cut-point fluid shear rate of (10.61 [cm-1]×mean velocity [cm×sec-1]). Moreover, the presence of the cylinder caused disordering of microsphere shear rates distal to higher grade stenoses, indicating a disturbance in their flow. Finally, despite lower precision, the signal recorded by the scaled-up Doppler catheter nonetheless indicated slip at the entry into and at most locations distal to the 90% stenosis. Our validated model establishes proof of concept for platelet slip, and platelet slip explains several important basic and clinical observations. If technological advances allow confirmation in a true biologic environment, then our results will likely influence the development of shear-dependent antiplatelet drugs. Also, adding shear rate information, our results provide a direct experimental fluid dynamic foundation for antiplatelet-focused antithrombotic therapy during coronary interventions directed towards higher grade atherosclerotic stenoses.

Fabrication of MEMS Directional Acoustic Sensors for Underwater Operation.

In this work, microelectromechanical systems (MEMS)-based directional acoustic sensors operating in an underwater environment are explored. The studied sensors consist of a free-standing single wing or two wings pivoted to a substrate. The sensors operate in a narrow frequency band determined by the resonant frequency of the mechanical structure. The electronic readout of the mechanical response is obtained using interdigitated comb finger capacitors attached to the wings. The characteristics of MEMS sensors immersed in silicone oil are simulated using finite element modeling. The performance of the sensors is evaluated both in air and underwater. For underwater testing and operation, the sensors are packaged in a housing containing silicone oil, which was specially developed to present near unity acoustic transmission. The measurements show that the resonant frequency of the sensors obtained in air shifts to a lower frequency when immersed in silicone oil, which is primarily due to the mass loading of the liquid. The peak sensitivity of the MEMS sensors is approximately 6 mV/Pa or -165 dB re 1 V/mPa, and the directional response shows a dipole pattern. The signal-to-noise ratio was found to be about 200 or 23 dB at 1 Pa incident sound pressure. The results show the potential of MEMS sensors to be used in underwater applications for sound source localization.

Bi-material terahertz sensors using metamaterial structures.

In this paper we report on the design, fabrication and characterization of terahertz (THz) bi-material sensors with metamaterial absorbers. MEMS fabrication-friendly SiOx and Al are used to maximize the bimetallic effect and metamaterial absorption at 3.8 THz, the frequency of a quantum cascade laser illumination source. Sensors with different configurations were fabricated and the measured absorption is near 100% and responsivity is around 1.2 deg/µW, which agree well with finite element simulations. The results indicate the potential of using these detectors to fabricate focal plane arrays for real time THz imaging.

Narrowband terahertz emitters using metamaterial films.

In this article we report on metamaterial-based narrowband thermal terahertz (THz) emitters with a bandwidth of about 1 THz. Single band emitters designed to radiate in the 4 to 8 THz range were found to emit as high as 36 W/m(2) when operated at 400 °C. Emission into two well-separated THz bands was also demonstrated by using metamaterial structures featuring more complex unit cells. Imaging of heated emitters using a microbolometer camera fitted with THz optics clearly showed the expected higher emissivity from the metamaterial structure compared to low-emissivity of the surrounding aluminum.

Microelectromechanical systems bimaterial terahertz sensor with integrated metamaterial absorber.

This Letter describes the fabrication of a microelectromechanical systems (MEMS) bimaterial terahertz (THz) sensor operating at 3.8 THz. The incident THz radiation is absorbed by a metamaterial structure integrated with the bimaterial. The absorber was designed with a resonant frequency matching the quantum cascade laser illumination source while simultaneously providing structural support, desired thermomechanical properties and optical readout access. Measurement showed that the fabricated absorber has nearly 90% absorption at 3.8 THz. A responsivity of 0.1°/µW and a time constant of 14 ms were observed. The use of metamaterial absorbers allows for tuning the sensor response to the desired frequency to achieve high sensitivity for potential THz imaging applications.

Electronic phase shift measurement for the determination of acoustic wave DOA using single MEMS biomimetic sensor.

MEMS acoustic sensors have been developed to mimic the highly-accurate sound-locating system of the Ormia ochracea fly, which detects sound wavelengths much larger than its hearing organ. A typical ormia-based MEMS directional sound sensor possesses two coupled wings that vibrate in response to sound according to a superposition of its two main resonant modes, rocking and bending. Vibrations are transduced into electronic signals by interdigitated comb finger capacitors at each wing's end along with a capacitance measuring circuitry. A sensor designed to exhibit resonant modes closely placed in frequency, enhancing their coupling, was operated with a closed cavity behind the wings. Simultaneous and independent measurements of electronic signals generated at each of the single sensor wings were used to determine incident sound direction of arrival (DOA). DOA was found proportional to the phase shift between them and to the difference over the sum of their amplitudes as well. Single sensor phase shift DOA measurement presented a resolution better than 3° for sound pressure levels of 25 mPa or greater. These results indicate that a single sensor operating in closed-cavity configuration can provide hemispherical unambiguous direction of arrival of sound waves which wavelength is much larger than the sensor size.

A novel noninvasive all optical technique to monitor physiology of an exercising muscle.

An all optical technique based on near-infrared spectroscopy and mid-infrared imaging (MIRI) is applied as a noninvasive, in vivo tool to monitor the vascular status of skeletal muscle and the physiological changes that occur during exercise. A near-infrared spectroscopy (NIRS) technique, namely, steady state diffuse optical spectroscopy (SSDOS) along with MIRI is applied for monitoring the changes in the values of tissue oxygenation and thermometry of an exercising muscle. The NIRS measurements are performed at five discrete wavelengths in a spectral window of 650-850 nm and MIRI is performed in a spectral window of 8-12 microm. The understanding of tissue oxygenation status and the behavior of the physiological parameters derived from thermometry may provide a useful insight into muscle physiology, therapeutic response and treatment.

MEMS terahertz-to-infrared band converter using frequency selective planar metamaterial.

A MEMS terahertz-to-infrared converter has been developed based on the unique properties of metamaterials that allow for selective control of the absorptivity and emissivity of the sensors. The converter consists of a sensing element structurally made of planar metamaterial membranes, connected to a substrate frame by four symmetrically-located thermal insulators. Upon THz absorption, the temperature of the sensing element increases and the outward infrared flux from the backside of the element is read by a commercial long-wave infrared camera. Two configurations were designed and fabricated with metamaterial absorptivity optimized for 3.8 THz and 4.75 THz quantum cascade lasers. The first sensor, fabricated with an oxidized aluminum backside, exhibits higher responsivity, but lower conversion efficiency than the second sensor, fabricated with a metamaterial backside. The spectral characteristics of the metamaterial on the two sides can be optimized to improve both responsivity and sensitivity, while keeping the sensors' thermal time constant sufficiently small for real time imaging. No dedicated electronics or optics are required for readout making metamaterial-based MEMS THz-to-IR converters very attractive for THz imaging as means of a simple attachment to commercial IR cameras.

Bio-Inspired Miniature Direction Finding Acoustic Sensor.

A narrowband MEMS direction finding sensor has been developed based on the mechanically coupled ears of the Ormia Ochracea fly. The sensor consists of two wings coupled at the middle and attached to a substrate using two legs. The sensor operates at its bending resonance frequency and has cosine directional characteristics similar to that of a pressure gradient microphone. Thus, the directional response of the sensor is symmetric about the normal axis making the determination of the direction ambiguous. To overcome this shortcoming two sensors were assembled with a canted angle similar to that employed in radar bearing locators. The outputs of two sensors were processed together allowing direction finding with no requirement of knowing the incident sound pressure level. At the bending resonant frequency of the sensors (1.69 kHz) an output voltage of about 25 V/Pa was measured. The angle uncertainty of the bearing of sound ranged from less than 0.3° close to the normal axis (0°) to 3.4° at the limits of coverage (±60°) based on the 30° canted angle used. These findings indicate the great potential to use dual MEMS direction finding sensor assemblies to locate sound sources with high accuracy.

Real-time imaging using a 2.8 THz quantum cascade laser and uncooled infrared microbolometer camera.

Real-time imaging in the terahertz (THz) spectral range was achieved using a milliwatt-scale, 2.8 THz quantum cascade laser and an uncooled, 160 x 120 pixel microbolometer camera modified with Picarin optics. Noise equivalent temperature difference of the camera in the 1-5 THz frequency range was estimated to be at least 3 K, confirming the need for external THz illumination when imaging in this frequency regime. Despite the appearance of fringe patterns produced by multiple diffraction effects, single-frame and extended video imaging of obscured objects show high-contrast differentiation between metallic and plastic materials, supporting the viability of this imaging approach for use in future security screening applications.