The piezoelectric effect refers to a physical phenomenon where piezoelectric materials generate an electric field when subjected to mechanical stress or undergo mechanical deformation when subjected to an external electric field. This principle underlies the operation of piezoelectric sensors. Piezoelectric sensors have garnered significant attention due to their excellent self-powering capability, rapid response speed, and high sensitivity. With the rapid development of sensor techniques achieving high precision, increased mechanical flexibility, and miniaturization, a range of flexible electronic products have emerged. As the core constituents of piezoelectric sensors, flexible piezoelectric composite materials are commonly used due to their unique advantages, including high conformability, sensitivity, and compatibility. They have found applications in diverse domains such as underwater detection, electronic skin sensing, wearable sensors, targeted therapy, and ultrasound diagnostics for deep tissue. The advent of flexible piezoelectric composite materials has revolutionized the design concepts and application scenarios of traditional piezoelectric materials, playing a crucial role in the development of next-generation flexible electronic products. This paper reviews the research progress on flexible piezoelectric composite materials, covering their types and typical fabrication techniques, as well as their applications across various fields. Finally, a summary and outlook on the existing issues and future development of these composite materials are provided.

Surface acoustic wave (SAW) devices are essential devices for communication and sensing, but usually have an operation frequency limit well below 20 GHz due to the constraints of material properties and fabrication capability. By using an AlScN/diamond layered structure with a high electromechanical coupling coefficient K² and our proposed two-step exposure electron beam lithography (EBL) process for ultra-fine patterns, we have fabricated SAW devices with resonant frequency up to 33.7 GHz in the Ka-band, the highest one ever reported for SAW devices electrically excited by interdigital transducers (IDTs). Combined with finite element analysis, we identified that series resonances are fundamental and high order Rayleigh modes, and K² are in the range of 1.21%–2.32%, 200% higher compared to those of traditional AlN/diamond-based SAW devices. The high order modes become stronger and dominant, particularly suitable for the development of ultrahigh frequency SAW devices and applications. In addition, the proposed EBL process showed its superb capability to make ultra-fine IDTs down to the nano-scale with excellent smooth edges and uniform patterns, suitable for ultrahigh frequency SAW development.

Surface acoustic wave (SAW) devices using embedded interdigital transducers (IDTs) on an AlN/diamond/Si layered substrate are fabricated, and their performances are investigated. The Sezawa mode is the dominant resonance with the highest resonant frequency up to 17.7 GHz, a signal amplitude of 20 dB, and an electromechanical coupling coefficient of 0.92%. Comparing these SAW devices with those having the conventional IDTs on the same layered structure, the output SAW power and resonant frequency of devices are improved by 10.7% and 1.1%, respectively, for the embedded IDT devices. This is because the different field distribution leads to the different Bragg reflection and phase velocity for the two types of IDTs. The radiation frequency characteristics indicate that the advantages of the embedded IDTs would be useful for high frequency, high power applications such as monolithic integrated millimeter-wave integrated circuit and high speed communications.

Creatures in nature make extensive use of structural color adaptive camouflage to survive. Cholesteric liquid crystals, with nanostructures similar to those of natural organisms, can be combined with actuators to produce bright structural colors in response to a wide range of stimuli. Structural colors modulated by nano-helical structures can continuously and selectively reflect specific wavelengths of light, breaking the limit of colors recognizable by the human eye. In this review, the current state of research on cholesteric liquid crystal photonic actuators and their technological applications is presented. First, the basic concepts of cholesteric liquid crystals and their nanostructural modulation are outlined. Then, the cholesteric liquid crystal photonic actuators responding to different stimuli (mechanical, thermal, electrical, light, humidity, magnetic, pneumatic) are presented. This review describes the practical applications of cholesteric liquid crystal photonic actuators and summarizes the prospects for the development of these advanced structures as well as the challenges and their promising applications.

Self-suspended polyimide (PI) composite films were fabricated. Their diameter and thickness were 2.5 in. and 260 nm, respectively. A high ratio of diameter to thickness of 2.4 × 10⁵ was obtained. The multiwalled carbon nanotubes (MWCNTs) were filled into PI to improve the optical absorption and the photothermal response. By using the N-methyl pyrrolidone solvent for both MWCNTs and poly(amic acid), a homogeneous dispersion of MWCNTs with up to 15% content was obtained. Under the thermal treatment of 300 °C, the composite films were successfully synthesized, which was proved by the measurements of Fourier transform infrared spectrum and x-ray diffraction. Compared with the pure PI film, the optical absorption of the composite films at 532 nm wavelength increased from 10% to 28%. The steady and transient photothermal properties of the composite films were investigated. In the steady state, a temperature rise of 145 °C was obtained under 0.7 W/cm² illuminating laser power density. The temperature increased by 29 times compared with that of the pure PI film. The corresponding photothermal conversion efficiency improved to 70.8%. In the transient state, the photothermal response time was 7.5 ms, which was only half that of the pure PI film. The excellent photothermal properties of these self-suspended MWCNT/PI composite films highlight their potential applications in micro- and nanoscale devices for photo detection and thermal imaging.