Pengfei Niu

Among the diverse ultrasound transducers deployed in wearable continuous imaging applications, piezoelectric micromachined ultrasonic transducer (PMUT), developed via microelectromechanical system (MEMS) technology, exhibit significant potential for wearable use, owing to their device miniaturization and low power consumption. However, PMUT arrays often suffer from poor imaging performance and no efficient method exists to evaluate large-scale arrays' performance other than post-fabrication measurement. This study proposes a mathematical model of mutual radiation impedance to predict the vibration amplitude of each cell in a PMUT array, thereby guiding optimal cell arrangement for simultaneous enhancement of sensitivity, bandwidth, and acoustic uniformity. The elevation aperture of linear PMUT arrays is reduced based on the beam-formed acoustic output, improving the integration and wearability of rigid silicon-based transducers. An ultra-narrow 64-element linear AlN PMUT array with a 7:1 aspect ratio (1 cm × 0.15 cm, aperture area 0.15 cm²) was fabricated for experimental validation. The device demonstrates a pulse-echo fractional bandwidth exceeding 60% at a center frequency of 7 MHz, along with an image contrast of ~50 dB. Its lateral and axial resolutions are measured to be ~0.26 mm and ~0.25 mm, respectively. Continuous and real-time clear phased-array imaging of multiple superficial organs in volunteers, including carotid artery, thyroid gland, and dorsalis pedis artery, demonstrates the qualified ultrasound performance of our designed PMUT array. Our work establishes a systematic framework for developing large-scale PMUT arrays enabling the realization of wearable ultrasound patch with compact form factor, high fidelity imaging quality, superior spatial resolution and low power consumption.

Current ultrasound sensors used for wearable blood pressure monitoring typically require driving voltages above 15 V. This study aims to enhance device sensitivity to enable low-voltage operation, thereby reducing power consumption and improving user safety. We propose an ultrathin curved piezoelectric micromachined ultrasonic transducer (PMUT) featuring a 0.5 µm aluminum nitride (AlN) film, which exhibits a resonance frequency of 7.5 MHz in water. The curved PMUT array exhibits a 1.8× improvement in unit-area transmitting sensitivity and a 2.6× enhancement in unit-area echo sensitivity compared to a planar PMUT array fabricated with the same piezoelectric material. Continuous in vivo monitoring of radial artery diameter was performed using a 2.5 V driving voltage, validating its feasibility for noninvasive blood pressure measurement. The proposed curved PMUT offers an innovative solution for wearable ultrasonic sensing, featuring high sensitivity, low power consumption, and enhanced circuit integration, positioning it as a promising candidate for next-generation physiological monitoring devices.

Objective: Wearable sweat sensors offer a promising route for non-invasive health monitoring; however, their accuracy and reliability remain limited. This limitation arises primarily because wearable sweat sensors typically collect and analyze sweat samples that are composed of mixed secretions from multiple sweat glands, resulting in averaged measurements that obscure the intrinsic concentrations and fluctuations of sweat biomarkers. Accurate sweat detection requires sampling sweat from a single sweat gland, yet no available method currently enables such high-resolution sampling. This study presents a novel single-sweat-gland sampling strategy, which is a class of patches containing dozens of openings to isolate individual sweat glands. By combining Poisson distribution theory with infrared thermography, a sweat patch incorporating 36 microchambers (400 µm in diameter) is designed to isolate sweat from individual glands. Computational modeling and Monte Carlo simulations involving 1000 random patch placements across diverse subjects demonstrate a 100% success rate in capturing sweat from single glands, without the need for vigorous physical exertion. The system functions effectively across different subjects under moderate sweating conditions and is adaptable to real-world applications. This high-resolution sampling strategy enables more accurate biomarker quantification and holds significant potential for improving sweat-blood biomarker correlation, advancing the development of reliable wearable diagnostics.

Skin measurement plays a crucial role in skincare assessment. High-frequency ultrasound showing benefits of non-invasiveness, high-resolution and large penetration, which would significantly improve the accuracy for skincare assessment. However, commercial piezoelectric ultrasound probes of high cost and large size are lack of comfort and ease of use. In contrast, traditional high-frequency piezoelectric micromechanical ultrasound transducers (PMUTs), with smaller-sized but relatively narrow bandwidth, in not suitable for high accuracy ultrasound imaging. In this study, we present a PMUT design with a large bandwidth for skin evaluation. By reducing the array spacing, both the bandwidth and the sensitivity of the PMUT device were simultaneously enhanced successfully. The 10MHz PMUT with a bandwidth of 40.7% was flexibly packaged as an ultrasound patch with a thickness of less than 1 mm. Experiments on simulated skin models and ex vivo porcine ear tissues demonstrated that the developed PMUT achieved an axial resolution of 0.2 mm and measured tissue layer thicknesses consistent with actual values, which confirmed the feasibility to measure the skin thicknesses. This study lays the foundation for developing high-performance high-frequency PMUTs, which would significantly advance the development of wearable ultrasound sensing devices. [2025-0161]

Capacitive Micromachined Ultrasonic Transducers (CMUTs) demonstrate highly competitive advantages in fields such as underwater near-field communication, short-to-medium range precision imaging, and target detection. However, compared to traditional piezoelectric transducers, their lower sensitivity limits the application of CMUTs in these areas. This paper designs, simulates, fabricates, and experimentally validates CMUTs with membranes having a large radius-to- thickness ratio. It systematically studies the relationship between CMUT membrane sizes and both resonant frequency and transmission sensitivity, and discusses the effects of the stress stiffening effect under large deflections on the resonant frequency and the dynamic velocity of the membrane. A larger radius-to-thickness ratio helps to improve sensitivity, but the accompanying stress stiffening effect leads to an increase in frequency. It is necessary to ensure that the ratio of membrane deflection to thickness does not exceed 40%, in which case the frequency deviation due to stress stiffening effects does not exceed 10%. These design concepts are well validated through finite element method (FEM) simulation and experimental results. The acoustic characterization experiments in water show that the CMUT with a membrane thickness of 5 μm and a radius-to-thickness ratio of 44:1, has a center frequency of 112.4 kHz, a transmitting voltage response (TVR) of 148.15 dB (μPa·m/V), and a -6 dB fractional bandwidth of 138.4%. The outstanding transmitting voltage response and large fractional bandwidth make the CMUT designed in this paper highly competitive in areas such as underwater target detection, imaging, and near-field communication.

Wearable sweat-sensing platforms represent a transformative advancement in noninvasive, real-time health monitoring, enabling personalized healthcare. For in vivo applications, sensor substrate materials require biocompatibility, secure adhesion, and, preferably, environmental sustainability. However, existing substrate materials fail to meet some of those requirements. This study introduces bacterial cellulose (BC) as a novel sensor substrate, leveraging its printability, biocompatibility, self-adhesion, and eco-friendliness. A wearable sweat sensor was fabricated by screen-printing conductive inks onto BC films. A key challenge addressed was the hydrophilicity of BC, which can cause liquid penetration and disrupt signal stability. To solve this, an approach was developed where the electrical tracks are sandwiched between two hydrophobic layers to fully avoid liquid interference and ensure stable electrochemical performance. The sensor was further functionalized with the alcohol oxidase enzyme to enable reliable alcohol detection in sweat at the relevant concentration range. This work demonstrates the feasibility of BC-based sensors for their application in wearable health monitoring, meanwhile promoting sustainable technological innovations in personalized healthcare and well-being technologies.

This article uses the Piezoelectric Micromachined Ultrasonic Transducer (PMUT) to verify the feasibility of ultra-thin structures in wearable urine volume detection devices. The properties of PMUT allow it to operate without a backing layer or impedance matching, offering a unique advantage in miniaturization. It is fixed to the reinforced flexible printed circuit (FPC) through wire-bonding, then encapsulated in solid silicone, keeping the overall thickness within 1mm, making it more comfortable to wear without significantly affecting performance. Arranging four chips in a T-shaped configuration and simultaneously activating them to acquire positional information are obtained in practical verification. Ellipses are fitted in the XY and XZ planes using coordinates from 3 chips (middle, left, right) and 2 chips (middle, top), respectively, combined with the distance between the bladder's walls. These data were used to construct an ellipsoid to estimate volume, followed by linearization of the output data. Incremental water injection into a pig bladder resulted in a nonlinear error of 3.78%, demonstrating the system's effectiveness for long-term wearable monitoring of bladder dysfunction.

Piezoelectric micromachined ultrasonic transducers (PMUTs) play a crucial role in the advancement of portable and wearable ultrasound imaging devices. Matching strategy of conventional thickness mode transducer for acoustic propagation is quite well developed yet. However, PMUTs which employ thin film’s flexural mode vibration lack systematic acoustic matching guidelines critical for optimizing performance. This study introduces the first comprehensive design framework for PMUT matching layers, integrating theoretical modeling, finite element method analysis and ultrasound characterization as well as phased-array ultrasound imaging validation. Being different from conventional thickness mode devices, the results indicate that a matching layer cannot simultaneously improve PMUT’s sensitivity and bandwidth (BW). Employing the matching layer thickness to be a quarter ultrasound wavelength, materials having an acoustic impedance above 1.5 MRayl enhance sensitivity, whereas those with an impedance below 1.5 MRayl improve BW. For our developed PMUT having center frequency of 6.3 MHz, a PDMS with an acoustic impedance of 1 MRayl was selected as matching layer, and it increases the PMUT’s BW from 20% to 39% while resulting in a 41% reduction in sensitivity. The results align closely with the theoretical predictions. To enhance PMUT’s sensitivity, matching materials like polybutadiene and thermoplastic polyurethane are more effective. These guidelines provide a foundation for rapid material selection and dimensional optimization, facilitating the broader application of miniaturized PMUTs in medical imaging and industrial sensing.

As a key component in motion state sensing, the accuracy of accelerometers directly affect the performance and safety of their systems. However, their sensitivity can influenced due to various factors during operation, necessitating both pre-deployment and periodic calibration. Traditional primary calibration methods rely on laser interferometers, yet their trueness and stability are compromised by environmental disturbances affecting the laser wavelength, making on-site traceability challenging. Addressing these issues, this study combines a Cr self-traceable grating with a homodyne littrow grating interferometer and primary vibration calibration technique. Leveraging the high line density, stability, and traceability of the self-traceable grating, it achieves high accuracy and on-site traceable calibration. In experiments, the effectiveness of this calibration system was validated using commercial accelerometers, achieving a measurement uncertainty of 0.25%. This calibration technique not only enhances the accuracy and environmental adaptability of accelerometer calibration but also addresses the critical issue of on-site traceability, representing the future direction of in-field accelerometer calibration.