In force measurement applications, a piezoelectric force sensor is one of the most popular sensors due to its advantages of low cost, linear response, and high sensitivity. Piezoelectric sensors effectively convert dynamic forces to electrical signals by the direct piezoelectric effect, but their use has been limited in measuring static forces due to the easily neutralized surface charge. To overcome this shortcoming, several static (either pure static or quasistatic) force sensing techniques using piezoelectric materials have been developed utilizing several unique parameters rather than just the surface charge produced by an applied force. The parameters for static force measurement include the resonance frequency, electrical impedance, decay time constant, and capacitance. In this review, we discuss the detailed mechanism of these piezoelectric‐type, static force sensing methods that use more than the direct piezoelectric effect. We also highlight the challenges and potentials of each method for static force sensing applications.

In this paper, a flexible piezo-composite transducer composed of the active piezoelectric material (PZT-5H) and passive polymer matrix (PDMS) was developed to achieve excellent flexibility, sensitivity and bandwidth for biomedical and industrial applications. The flexible electrodes (silver nanowires (AgNWs) mixed with PDMS) were deposited on the transducers using a spray coating method providing a sufficient conductivity of an electrode through the repeated bending tests. The flexible 1-3 composite transducer exhibited robust mechanical flexibility (curved radius < 5 mm) without failure due to the stretchable mechanical properties of PDMS and AgNWs network, which indicates that AgNW/PDMS electrode is a promising alternative to conventional metal-type electrodes such as Au film. The acoustic characterization suggests that the -6 dB fractional bandwidth (FBD, ~49 %) and transmitting sensitivity (~107 mV/V) of the prototyped transducer are promising for non-destructive testing (NDT), biomedical imaging and therapy.

Due to the rapid developments in materials science and fabrication techniques, wearable devices have recently received increased attention for biomedical applications, particularly in medical ultrasound imaging, sensing, and therapy. Ultrasound is ubiquitous in biomedical applications because of its non-invasive nature, nonionic radiating, high precision, and real-time capabilities. While conventional ultrasound transducers are rigid and bulky, flexible transducers can be conformed to curved body areas for continuous sensing without restricting tissue movement or transducer shifting. This article comprehensively reviews the application of flexible ultrasound transducers in the field of biomedical imaging, sensing, and therapy. First, we review the background of flexible ultrasound transducers. Following that, we discuss advanced materials and fabrication techniques for flexible ultrasound transducers and their enabling technology status. Lastly, we highlight and summarize some promising preliminary data with recent applications of flexible ultrasound transducers in biomedical imaging, sensing, and therapy. We also provide technical barriers, challenges, and future perspectives for further research and development.

The generation of ultrasonic surface waves with a photoacoustic-laser-source has become useful for the noncontact nondestructive testing and evaluation (NDT&E) of materials and structures. In this work, a hybrid ultrasound based NDT&E method is proposed based on the photoacoustic-laser-source as a noncontact Lamb wave generator by incorporating a line-arrayed patterned candle soot nanoparticle-polydimethylsiloxane (CSNPs-PDMS) patch as the signal amplifier and with a narrow bandwidth. The CSNP-PDMS composite has been investigated as the functional patch for its laser energy absorption efficiency, fast thermal diffusion, and large thermoelastic expansion capabilities. The signal amplitude (in mW) from the CSNP-PDMS patch exhibits 2.3 times higher amplitude than the no patch condition and a narrower bandwidth than other conditions. Furthermore, improvement in the sensitivity is also achieved through the selection of the aluminum nitride sensing system. The overall combination of the Lamb wave generation and receiver-sensing system in this study is found to be very promising for a broad range of noncontact NDT&E applications.

This article aims to develop a semi-noncontact stress-sensing system using a laser-generated ultrasound (LGU) wave assisted by candle soot nanoparticle (CSNP) composite. While the acoustoelastic effect is commonly targeted to measure the stress level, efforts to combine it with the LGU wave signal have been lacking due to weak signal intensity. In this study, the CSNP-based transducer is designed to potentiate the photoacoustic energy conversion. To demonstrate the wave propagation with the designed parameters, a numerical simulation was first conducted. The experimental results showed that a laser intensity of 6.5 mJ/cm ² was enough to generate the subsurface longitudinal (SSL) wave from the CSNP composite transducer. The normal beam projection is the most effective wave-generation method, exhibiting the highest signal magnitude compared with inclined projection cases. Finally, the laser-assisted stress-sensing system was assessed by increasing the internal pressure of an air tank. The sensitivity of the developed sensor system was estimated to be 0.296 ns/MPa, showing a correlation of 0.983 with the theoretical prediction. The proposed sensing system can be used to monitor the structural integrity of nuclear power plants.

We present the development of a 6 × 6 piezoelectric array sensor for measuring elasticity and force. The proposed sensor employs an impedance measurement technique, sensing the acoustic load impedance of a target by measuring the electrical impedance shift of face-shear mode PMN–PT (lead magnesium niobate–lead titanate) single crystal elements. Among various modes of PMN–PT single crystals, the face-shear mode was selected due to its especially high sensitivity to acoustic loads. To verify the elasticity sensing performance, gelatin samples with different elastic moduli were prepared and tested. For the force measurement test, different magnitudes of force were loaded to the sensing layer whose acoustic impedance was varied with applied forces. From the experimental results, the fabricated sensor showed an elastic stiffness sensitivity of 23.52 Ohm/MPa with a resolution of 4.25 kPa and contact force sensitivity of 19.27 Ohm/N with a resolution of 5.19 mN. In addition, the mapping experiment of elasticity and force using the sensor array was successfully demonstrated.