Highly sensitive, selective, and compact hydrogen (H₂) sensors for safety and process monitoring are needed due to the growing adoption of H₂ as a clean energy carrier. Current resonant frequency-based H₂ sensors face a critical challenge in simultaneously achieving high sensitivity, low operating frequency, and miniaturization while maintaining a high figure of merit (FOM). This study addresses these challenges by introducing a novel piezoelectric micro diagram (PMD) H₂ sensor that achieves an unprecedented FOM exceeding 10⁴. The sensor uniquely integrates a PMD resonator with a palladium (Pd) sensing layer, operating on a stress-based mechanism distinct from traditional mass-loading principles. Despite a low operating frequency of 150 kHz, the sensor demonstrates a remarkable sensitivity of 18.5 kHz/% H₂. Comprehensive characterization also reveals a minimal cross-sensitivity to humidity and common gases and a compact form factor (600 μm lateral length) suitable for IC integration. The sensor's performance was systematically evaluated across various Pd thicknesses (40-125 nm) and piezoelectric stack covering ratios (50% and 70%), revealing a trade-off between sensitivity and response time. This PMD H₂ sensor represents a significant advancement in resonant frequency-based H₂ sensing, offering superior sensitivity, compact size, and robust performance for diverse applications in H₂ detection and monitoring.

Respiration monitoring is vital for human health assessment. Humidity sensing is a promising way to establish a relationship between human respiration and electrical signal. This paper presents a polyimide-based film bulk acoustic resonator (PI-FBAR) humidity sensor operating in resonant frequency and reflection coefficient S₁₁ dual-parameter with high sensitivity and stability, and it is applied in real-time human respiration monitoring for the first time. Both these two parameters can be used to sense different breathing conditions, such as normal breathing and deep breathing, and breathing with different rates such as normal breathing, slow breathing, apnea, and fast breathing. Experimental results also indicate that the proposed humidity sensor has potential applications in predicting the fitness of individual and in the medical field for detecting body fluids loss and daily water intake warning. The respiratory rates measured by our proposed PI-FBAR humidity sensor operating in frequency mode and S₁₁ mode have Pearson correlation of up to 0.975 and 0.982 with that measured by the clinical monitor, respectively. Bland–Altman method analysis results further revealed that both S₁₁ and frequency response are in good agreement with clinical monitor. The proposed sensor combines the advantages of non-invasiveness, high sensitivity and high stability, and it has great potential in human health monitoring.

A high-precision humidity response time measurement has been urgently needed for the fast response humidity sensor in some special situations, for instance, when the quadcopter performs the meteorological observations in the atmospheric boundary layer, the relative humidity changes drastically as it flies into the cloud. So it is essential to gain the accurate response time for these sensors. However, owing to the uncontrollable errors, the conventional methods are difficult to carry out an accurate measurement. Herein, for meeting the requirements above, we designed an accurate measurement method based on a type of switchable chamber and a humidity generator. Among, the chamber was designed through surface molecular dynamics analysis and verified by experiments. It is proved that the switchable chamber with the hydrophobic surface and low water molecule content, combined with the surface to volume ratio S/V =0.2 can achieve the realistic humidity response time of the sensor. For a commercial sensor (SHT20) at 25 ℃, the time corresponding to the 63% and 90% complete responses are 7 s and 49 s measured in this way, which is extraordinarily improved than those traditional ways (51 s and 140 s). As a result, this measurement method could gain the accurate response time for the special situation applications.

This paper presents a polyimide-based film bulk acoustic resonator (PI-FBAR) humidity sensor operating in resonant frequency and reflection coefficient S ₁₁ dual-mode. Models and experiments showed that those two modes are governed by different mechanisms, of which the former is based on the modulation of the humidity on the Young’s modulus of PI, while the latter is based on the modulation of the humidity on dynamic resistance. In the full-scale relative humidity (RH) testing range (20%-80%), the resonant mode achieved a sensitivity of 29.93 kHz/%RH (28.56 ppm/%RH) with ultra-high linearity ( ${R}^{2}=0.9993$ ), and the humidity hysteresis error is only about 1.32% RH. For the S ₁₁ response, an ultra-high average humidity sensitivity of 0.044dB/%RH (2095ppm/%RH) was achieved, and the sensitivity at 20% low relative humidity reached 0.075dB/%RH (3571ppm/%RH), which is more than 2 orders of magnitude higher than that of the resonant frequency response. And the limit of detection (LOD) for the humidity response of S ₁₁ is 0.035% RH, which is half lower than that of the resonant mode. Thus, we can choose a more appropriate sensing mode according to particular application or combine these two mechanisms in one test to expand the sensor competences by taking advantages of each method. In addition, as both of the two modes have high sensitivity to humidity and temperature, we deduced a method to calibrate the temperature for the humidity sensing. Based on this dual-mode mechanism, we can further achieve either temperature-calibrated humidity sensing or dual-variable sensing of humidity and temperature with more accurate sensing results.

Acoustic levitation enables non-contact manipulation of particles, crucial for applications in microassembly, biochemical analysis, and material processing. However, conventional acoustic levitation systems typically rely on bulky transducer arrays or complex architectures, limiting their practical implementation and portability. Here, we demonstrate a miniaturized acoustic particle gripper based on a high transmit (Tx) pressure single Sc_0.3Al_0.7N piezoelectric micromachined ultrasonic transducer (pMUT, 3 × 3 mm²) with an acoustic enclosure specifically designed for mm size single particle trapping. As a result, the proposed system has achieved maximum 2810 Pa at the particle pick-up inlet and successfully demonstrated levitation and holding of a 1 mm foam bead while 360° frame rotation and stable transfer manipulation. The proposed miniaturized system opens up new research for acoustic levitation technology.