Photoplethysmography (PPG) is central to wrist-worn physiological monitoring, but its signal acquisition is strongly influenced by mechanical and optical coupling at the sensor-skin interface. This study presents an integrated physical characterization of contact force, illumination wavelength, source-detector separation, skin-surface properties, and motion-artifact patterns using a controlled wristwatch platform. In the two tested participants, contact force produced nonmonotonic and metric-dependent changes in signal-to-noise ratio (SNR), perfusion index (PI), and waveform morphology, and the most favorable tested force condition differed between the two cases. Across three illumination wavelengths, increasing the source-detector separation from 4 to 8mm produced distinct wavelength-dependent attenuation of the alternating-current (AC) area, revealing the coupled influence of optical wavelength and sensor geometry on detected pulsatile signals. In the tested participant, hair removal was associated with improved PPG signal characteristics under the tested condition. In ten participants, localized tanning caused pronounced wavelength-dependent reductions in the AC and direct-current (DC) components and harmonic amplitudes, whereas SNR, PI, and relative spectral morphology remained comparatively stable. These results indicate that changes associated with localized tanning should be evaluated jointly using detected signal levels, ratio-based quality metrics, and waveform characteristics rather than any single indicator. Motion artifacts were further examined under two controlled wearing conditions intended to emphasize ambient-light intrusion or coupling-related deformation, and corresponding differential-processing and detection-excision-reconstruction strategies improved signal recovery under the tested conditions. Overall, this work provides an integrated physical framework for mechanical fitting, optical configuration, evaluation of tanning-associated signal changes, and mechanism-matched artifact handling in wrist-worn PPG systems.
Measurement of water cut in two-phase flows (especially oil-water, gas-water, and wet steam) is crucial for in dustrial process control in fields such as petroleum, chemical engineering, and energy. Aiming at the problems of insufficient accuracy and poor robustness of traditional measurement methods under complex conditions, a high-precision measurement method using a microwave resonant cavity sensor is proposed. Combining electro magnetic simulation and multiple experimental setups, we systematically analyze the resonant frequency response characteristics of the sensor under three typical two-phase flow conditions (oil-water, gas-water, and wet steam). Experimental results show that in wet steam measurement, changes in the pressure inside the pipeline cause sig nificant drift in the resonant frequency. This pressure effect is considered a key factor limiting high-precision measurement. To address this issue, we conduct an in-depth study on the relationship between the dielectric constant of the mixture and pressure, and innovatively propose a compensation model that uses pressure as a calibration parameter. Accurate conversion from resonant frequency to water cut is achieved through a pressure-dependent function. The introduction of pressure compensation greatly improves the measurement accuracy of wet steam and gas-water two-phase flows. The calibrated model exhibits excellent accuracy and environmental robustness over the entire range of operating conditions, with measurement errors effectively controlled within the range required for industrial applications.
Skin-conformable temperature sensors with high stability and sensitivity are essential for electronic skin and real-time health monitoring. However, achieving high stability while preserving sensitivity and a broad linear working range remains a challenge. Herein, we report a novel, highly stable resistive temperature sensor based on an all-solution-processed SnO2@Ag core-shell nanowire composite, embedded in an ultrathin polyvinylidene fluoride (PVDF) substrate (similar to 5.1 mu m) via laser direct writing. The conformal SnO2 shell (similar to 2 nm) protects Ag NWs from degradation without compromising conductivity and transparency. The resulting sensor exhibits high optical transparency (89.5% at 550 nm), a linear temperature sensing range of 30-130 degrees C, a temperature coefficient of resistance (TCR) of 0.31% degrees C-& sup1; , and a temperature resolution of 0.5 degrees C. Notably, the sensor demonstrates stable resistance after 300 thermal cycles (30-80 degrees C) and 3000 bending cycles, with a maximum hysteresis error of 1.73% over the full range, highlighting its mechanical and thermal robustness. Furthermore, the core-shell structure imparts robust resistance to moisture, oxidants, and salt solutions, ensuring reliable operation in harsh environments. As a proof-of-concept, the sensor enables precise, skin-conformal measurement of human body temperature, and offers a rapid response/recovery time of 1.5 s/1.2 s for respiratory pattern monitoring, showing its significant potential for electronic skin applications.
Inspired by the dynamic network structure of spider silk that enables efficient dewdrop capture, we report a novel multi-network polydimethylsiloxane (PDMS) composite elastomer designed for flexible sensing applications. The hierarchical architecture was constructed through a sequential fabrication strategy, integrating a hydrogen-bonded polyvinyl alcohol/cellulose nanofiber/glycerol network, a conductive framework of multilayer graphene and multi-walled carbon nanotubes, and a hydrophobic nano-silica reinforced PDMS skeleton. This biomimetic design decouples the traditional trade-off between strength and toughness, achieving an enhanced mechanical toughness of 4200 and 57,000 kPa under tension and compression, respectively. The resulting flexible sensor delivers a wide linear strain sensing range (0-200%), excellent cyclic stability over 10,000 cycles, with response and recovery times of 185 ms and 363 ms. These superior attributes enable the sensor to precisely monitor a wide spectrum of human motions, from subtle muscle movements to large-scale joint flexion, underscoring its strong potential for applications in wearable health monitoring and intelligent human-machine interaction systems.
Conventional hot-film sensors suffer from the substrate heat conduction effect, which limits their static and dynamic characteristics. Dual-layer hot-film sensors employ a guard-heating strategy to reduce the substrate conduction, and experimental and numerical studies have recently demonstrated their effectiveness in enhancing static sensitivity and mitigating the edge diffusion effect. However, the frequency response characteristics of dual-layer hot-film sensors have not yet been experimentally reported. In this paper, the frequency responses of rectangular and trapezoidal sandwiched dual-layer hot-film sensors were investigated and compared for the first time. And a novel pulsatile laminar channel flow device was developed to achieve the in-situ and direct calibration of the sensors. The dual-layer hot-film sensors were dynamically calibrated up to 200Hz over a wall shear stress fluctuation range of 5-35 mPa. Experimental results show that the amplitude-frequency response is significantly improved by the dual-layer sensors, with the -3 dB bandwidth increased by nearly an order of magnitude compared with their single-layer counterparts. However, the improvement in phase lag diminishes with increasing frequency. The dual-layer sensors exhibit a larger phase lag than the single-layer sensors at high frequencies. Time-resolved conjugate heat transfer numerical simulations suggest that this arises from the increased thermal inertia of the thermal boundary layer surrounding the dual-layer sensor at high frequencies, which is an inherent limitation of the dual-layer sensor design. This work deepens the understanding of the dynamic characteristics of dual-layer hot-film sensors and provides support for their future application and development in turbulence measurements.
