SnO 2 -supported Pd single atoms coupled with small cluster structures, incorporated into sensing platforms, exhibit exceptional performance and selectivity toward CO.

Nanocrystalline pristine and Pd-loaded tin (IV) oxide (SnO2) nanocomposites with different loadings were synthesized via facile impregnation and in-situ reduction, followed by annealing. The crystal structure and morphology of the samples were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. X-ray photoelectron spectroscopy, Raman spectroscopy and ex-situ extended X-ray absorption fine structure (EXAFS) confirm PdO nanoclusters stabilized on SnO2 surface. Results revealed that Pd/ SnO2 with 2.8 wt% loading exhibits the best sensing performance, including high sensitivity to CO with a low detection limit, fast response, and good selectivity to CO against interfering gases. Its enhanced sensing performance is attributed to both fine structure of PdO, and the synergy between PdO and SnO2 as well as dissimilar defect structures and concentrations. In-situ FTIR measurements unraveled CO adsorption kinetics on Pd/SnO2 under reaction conditions, based on which a possible sensing mechanism is put forth. Namely, Pd and PdO on edges, steps, and terraces of (100) and (111) facets provide favorable adsorption and activation sites for CO, from which activated fragments are spilled over onto SnO2 to react with ionosorbed oxygen, locally decreasing depletion layer and sensor resistance; and concurrently, carbon-related species are formed and decomposed into CO2.

Advancements in piezoelectric micromachined ultrasound transducers (PMUTs) has allowed for the development of vascular ultrasound applications in wearable and at- home settings. Pulse wave imaging (PWI) is an ultrasound-elasticity technique that can estimate local pulse wave velocity at end- diastole (PWVED). To move towards wearable and at-home applications of PWI, the objective of this study was to compare the performance of a PMUT array against an L7-4 for PWI in (n=2) healthy subjects. The B-modes produced from the PMUT array had increased presence of noise and artifacts compared to the L7-4. SNRvPWI was found to be higher in the L7-4 acquisitions ( 47.5 +/- 6.98 dB) compared to the PMUT array (35.66 +/- 8.02 dB). However, there was still good agreement for PWVED between the L7-4 and PMUT array, with a mean absolute difference of 0.18 +/- 0.07 m/s when averaging all PWVED estimates per carotid artery. Additionally, by using 10% (13) of lateral positions for the L7-4 and 30% (20) for the PMUT array, it was possible to estimate PWVED within 5% error compared to using all lateral positions (L7-4: 3.44 +/- 1.06%, PMUT array: 3.57 +/- 0.99%). This work showcases the potential for adapting PWI into wearable and at-home monitoring applications with a PMUT array.

This work presents a gas sensor array based on a micromachined quartz crystal resonators integrated with on-resonator thin-film heaters. The change in the functional material’s gas adsorption behavior at different temperatures is utilized as a parameter for volatile organic compound (VOC) gas species identification. The sensitivity of a quartz resonator coated with a single gas sensitive material at four temperatures controlled by the on-chip heater are studied against six kinds of VOC gas mixtures at multiple concentrations. Principal component analysis (PCA) shows the ability of the device to identify the gas species with data from a single functional material. Combining the technique with multiple functional materials results in a hybrid physical-virtual sensor array which further enhances the efficacy of identification.

This article presents a triaxial microelectromechanical system (MEMS) capacitive accelerometer using a high-voltage biasing technique to achieve high resolution with ultralow power. The accelerometer system generates a differential pair of high voltages to bias the MEMS structure, raising the MEMS signal substantially above the noise floor of the analog front-end (AFE) circuits. With the consequent increased signal-to-noise ratio (SNR), the proposed accelerometer system eliminates the need for a power-hungry low-noise amplifier (LNA) and signal chopping which significantly improves the power-noise tradeoff found in conventionally biased MEMS accelerometers. Moreover, by fine-tuning the bias voltages, the proposed method cancels the electrostatic mismatch in the MEMS due to process variation and ensures robust operation. The proposed accelerometer is composed of one integrated MEMS-CMOS chip and one CMOS-only chip. In postfabrication testing, it achieves a 121- $\mu$ g/ $\surd$ Hz input-referred noise floor with $\pm$ 1.5-g dynamic range, ${<}1$ % linearity error, and 184-nW per-axis power (including high-voltage bias generation). Compared to prior art, the design achieves a 10.3 $\times$ FoM improvement in both power and noise specifications.

Ordered porous RGO/SnO 2 thin films for ultrasensitive humidity detection at room temperature.

This paper presents a model for the resistance of micromachined semiconducting metal-oxide (SMOx) sensors exposed to ternary gas mixtures. This paper also presents a corresponding method for estimating gas concentrations using the measured resistances of an array of on a chip. To our knowledge, this approach has not been reported for multi-gas estimation.

The continuous monitoring of pulse wave velocity (PWV) - an indicator correlated to arterial stiffness, vascular aging, and blood pressure - is receiving growing interest. In a clinical setting, ultrasound allows an accurate assessment of the PWV from the arterial longitudinal section, acquired at high frame rate (HFR). However, it requires a careful and stable alignment between the probe and the vessel itself, which cannot be guaranteed with wearable sensors. In this work we propose a proof-of-concept system to measure PWV using two linear arrays scanning two arterial cross-sections. In principle, they could be integrated into a patch or small wearable device and could loosen the array-to-vessel alignment requirements and, thus, enable wearable applications. The system was proved by exploiting the ultrasound advanced open platform (ULA-OP 256), which was connected to a 7.5-MHz linear array. Experimental data were acquired when investigating different diameter vessels of a flow phantom, whose flow was controlled by a peristaltic pump, modulated by an electro-valve, and monitored by a pressure sensor. PWV estimates obtained with the proposed method lied within the reference method's uncertainty interval (Reference method PWV values: $12.4\pm 2.65\ \mathrm{m}/\mathrm{s}$ for 2-mm vessel and $0.4\pm 1.81\ \mathrm{m}/\mathrm{s}$ for 4-mm. Proposed method PWV estimates: $13.9\pm 2.09\ \mathrm{m}/\mathrm{s}$ for 2-mm vessel and $9.6\pm 1.45\ \mathrm{m}/\mathrm{s}$ for 4-mm).

In this article, a [Formula: see text] piezoelectric micromachined ultrasonic transducer (PMUT) array is designed and driven with one cycle of a 5-MHz sinusoid at 10 [Formula: see text] for radial artery motion tracking. The transmit and receive performance figure of merit (FOM) of an individual PMUT over operating frequency is modeled and validated using laser Doppler vibrometer (LDV) measurements. Given a fixed cross section, the FOM inversely scales with frequency. The array aperture size is selected to obtain enough pressure and received signal to measure the radial artery wall reflection at a 5-mm depth in tissue. The 2-mm acoustic beamwidth provides enough lateral resolution for radial artery wall motion tracking. Single-line ultrasonic pulse-echo measurements with high time resolution, also called M-mode ultrasound imaging, are demonstrated to reproduce a known target motion profile with a precision of around 0.5 [Formula: see text]. In vivo radial artery dynamics are measured by placing the sensor on the wrist of a volunteer. The measured diameter change waveform of the radial artery is consistent with reports in the literature and captures key arterial pulse waveform features, including systolic upstroke, systolic decline, dicrotic notch, and diastolic runoff. The system has sufficient accuracy and precision to measure both the 50 [Formula: see text] overall diameter change and the 5- [Formula: see text] diameter change due to the dicrotic notch. A heart rate of 70 beats/min is also derived. This demonstrates the great potential of custom PMUT arrays for continuous cardiovascular system monitoring.

We present a triaxial MEMS accelerometer readout circuit (RoC) with 40× signal gain using a high MEMS bias voltage, reducing power by eliminating the need for a chopped AFE chain. The proposed RoC achieves a 121µg/√Hz input referred noise and 1.5g dynamic range at 184nW per-axis power, while maintaining <1% non-linearity and a mechanical full-scale of >20 g, improving FoM by 15.6×.

In this paper, we present a novel approach that aids in the sizing of micron to mm-sized ultrasonic transducers for enhanced resolution and denser integration. This seeks to optimize penetration-depth which is an important metric of a transducer’s efficacy in different acoustic media. It is generally accepted that in conventional transducers used in clinical imaging, penetration depth is greater for lower driving frequencies which suffer lessened attenuation. However, in acoustic characterization studies, the drops in axial pressure by beam-spreading and attenuation haven’t been individually quantified and their relative effects are poorly understood. We, therefore, undertake a closer investigation of factors that shape the axial pressure profile of a generalized transducer element in a weakly attenuating medium like water and in a highly attenuating medium like tissue. We have found that it is critical to quantify the relative contributions of beam-spreading and attenuation in a medium. We introduce a new parameter called ‘transition distance’ ( ${d}_{\textit {Trans}}$ ) to graphically demarcate the regimes in beam-spreading and attenuation dominate to explain how a low frequency (2 MHz) or a high frequency (5 MHz) gives greater penetration depth in different media.

In this paper, a 5 MHz 1.5 × 1.5 mm 2 piezoelectric micromachined ultrasonic transducer (PMUT) array with 2 mm beam width and 5 mm penetration depth is designed to measure the dynamics of the radial artery. Single line ultrasonic pulse echo measurements with high time resolution, also called M-mode ultrasound imaging, are demonstrated to obtain radial artery dynamics to derive cardiovascular system bio-markers such as heart rate and artery diameter change. The extracted radial artery diameter change waveform is consistent with what is reported in the literature and captures the key arterial pulse waveform components including systolic upstroke, systolic decline, dicrotic notch, and diastolic runoff. This demonstrates the great potential of custom-designed PMUT arrays for continuous cardiovascular system monitoring.

All packaged MEMS sensors experience strain arising from CTE (Coefficient of Thermal Expansion) mismatches and from assembly in the final product when deployed in applications. The effect of these stresses on MEMS based SMOx gas sensor performance has not been studied before and can be a significant source of error in gas sensing applications. In this paper, we have studied the stress effect in MOx by – (a) quantifying strain seen in a packaged MOx gas sensor over typical operating temperature conditions and (b) measuring MOx materials under strain to quantify the effect. We have simulated the effect of thermomechanical stresses on a MEMS gas sensor arising from CTE mismatches in the package and MEMS layers using a well-known finite element software Comsol®. While studying the effect of stress/strain on a typical SMOx material (SnO2) using a four-point flexure test over a temperature range of 200-300 °C, we observed a dependency of the relative change in resistance to the particle size of the MOx with applied strain. From these results we see that 1 °C change in ambient temperature, the equivalent amount of ethanol response due to stress can range from 0.026 ppm to 0.3 ppm for material like SnO2.

In this paper, we demonstrate the ability of identifying ppm level of concentrations of three volatile organic compounds using micromachined quartz resonators. 100 nm films of functional polymers were deposited on the resonators for the identification of ethanol, acetone, and toluene. Adsorption of gas vapors by the functional polymer films results in resonance frequency shifts of the quartz resonators. Principal component analysis (PCA) was performed on the raw frequency shift data collected. PCA analysis is able to determine components that allow for the gas species identification. Further transformation of these components allows for clustering by concentration.

A novel and highly sensitive vacuum gauge based on a micromachined AT-cut quartz bulk acoustic wave (BAW) resonator diaphragm is presented. Pressure is sensed by monitoring frequency shifts of these resonators due to the sensor's diaphragm bending. The sensors show a large dynamic range of 1 mTorr–760 Torr with a resolution ∼1.06 mTorr for 82.4 MHz resonators with a high linearity. Here we report on the parametric study to improve sensitivity of these sensors.