Softening of piezoelectric materials facilitates the development of flexible wearables and energy harvesting devices. To achieve a high piezoelectricity and flexibility simultaneously, a variety of efforts have been utilized to manufacture high-performance and flexible piezoelectric nanodevices. An effective approach toward flexible piezoelectric materials is infiltrating protecting polymers with piezoelectric ceramic nanostructures to form piezoceramic-polymer composites. However, due to the mismatch of strength and permittivity between the two phases inside, most developed piezoelectric composites inevitably exhibit reduced power-generation capability and pressure sensitivity, which limits their applications in energy harvesting and sensing.

In this thesis, a modified template-assisted sol-gel method is developed to fabricate piezoelectric composites with three-dimensional interconnected piezoceramic framework. A systematic theoretical and experimental study has been carried out on the manufacturing, characterization and device fabrication of the piezoelectric composites with designed piezoceramic structures. The unique piezoceramic structures provide the composites with enhanced piezoelectric performance as well as multifunctional applications in biomechanical energy harvesting, ultrasonic energy harvesting and human health monitoring.

First, the thesis starts with an introduction of flexible piezoelectric materials, with a focus on the piezoelectric ceramic-polymer composites and their applications in energy harvesting and sensing. Compared with other approaches (e.g., nanowires and thin films by advanced nanofabrication techniques), flexible piezoelectric devices based on ceramic-polymer composites possess the advantages of simple and scalable fabrication process, low cost and enhanced mechanical durability and flexibility under large mechanical deformation. Among these, piezoelectric composites with the 0-3, 1-3, and 3-3 connectivity patterns have received greatest attention and been widely applied in biomechanical energy harvesting, ultrasonic energy harvesting and self-powered sensing.

Second, a flexible, air-permeable, and high-performance piezoceramic textile composite is proposed with a mechanically reinforced hierarchical porous structure. Based on the template-assisted sol-gel method, a three-order hierarchical ceramic textile is constructed by intertwining submillimeter-scale multi-ply ceramic fibers that are further formed by twisting micrometer-scale one-ply ceramic fibrils. Theoretical analysis indicates that large mechanical stress can be induced in the piezoelectric composite with the multi-order hierarchical structure, which benefits the electrical output. Fabricated samples generate an open-circuit voltage of 128 V, a short-circuit current of 120 μA, and an instantaneous power density of 0.75 mW cm⁻², much higher than the previously reported works. The developed multi-order and 3D-interconnected piezoceramic textile shows satisfactory piezoelectricity (d33 of 190 pm V⁻¹), air permeability (45.1 mm s⁻¹), flexibility (Young’s modulus of 0.35 GPa), and toughness (0.125 MJ m⁻³), collectively. The design strategy of obtaining balanced properties promotes the practicality of smart/functional materials in wearables and flexible electronics.

Third, inspired by the natural wood structures, a transmuscular ultrasonic wireless power transfer system is presented based on a flexible wood-templated piezoelectric ultrasonic energy harvester (W-PUEH) in a unidirectional 3D interconnected ceramic-polymer topology. The developed flexible W-PUEH device demonstrates an output voltage of 21 V, an output current of 2 mA, and an average output power density of 304 μW cm^-2, one order of magnitude higher than the state of the art. The transmitted ultrasonic energy through porcine tissue is sufficient to power a wireless transmitter, demonstrating the potential applications of the W-PUEH in the implantable devices for the improvement of life quality and well-being of the recipients.

Finally, a kirigami-structured highly anisotropic piezoelectric network composite sensor is demonstrated, which is able to monitor multiple information of joint motions, including bending direction, bending radius, and motion modes, and to distinguish them simultaneously within one sensor unit. Based on the modified template-assisted processing method, a functional piezoceramic kirigami with a honeycomb network structure is designed and manufactured, which is stretchable (~100% strain), highly sensitive (15.4 mV kPa⁻¹), and highly anisotropic to bending directions (17.3 times from 90° to 0°). An integrated monitoring system is further established to alarm the prolonged sedentary behaviors, facilitating the prevention of upper extremity MSDs.

In summary, this research illuminates the relationship between the mechanical and piezoelectric properties of the piezoelectric composites and the piezoceramic structure within the composites. Based on this, a series of composite-based flexible piezoelectric devices with designed piezoceramic structures have been developed and applied in biomechanical energy harvesting, ultrasonic energy harvesting and human health monitoring. The concept of designing structural piezoceramic framework and manufacturing multifunctional piezoelectric composites provides a new solution for the emerging area of flexible multifunctional piezoelectric devices. The research facilitates the development of future energy harvesters and self-powered fit-and-forget sensors for applications such as Internet of Things, implantable and wearable devices, and human-machine interactions.