In an effort to fabricate a wearable piezoelectric energy harvester based on core-shell piezoelectric yarns with external electrodes, flexible piezoelectric nanofibers of BNT-ST (0.78Bi0.5Na0.5TiO3-0.22SrTiO3) and polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) were initially electrospun. Subsequently, core-shell piezoelectric nanofiber yarns were prepared by twining the yarns around a conductive thread. To create the outer electrode layers, the core-shell piezoelectric nanofiber yarns were braided with conductive thread. Core-shell piezoelectric nanofiber yarns with external electrodes were then directly stitched onto the fabric. In bending tests, the output voltages were investigated according to the total length, effective area, and stitching interval of the piezoelectric yarns. Stitching patterns of the piezoelectric yarns on the fabric were optimized based on these results. The output voltages of the stitched piezoelectric yarns on the fabric were improved with an increase in the pressure, and the output voltage characteristics were investigated according to various body movements of bending and pressing conditions.

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An all-in-one energy harvester module comprising a top piezoelectric layer, a bottom piezoelectric layer, and a middle triboelectric layer was fabricated based on flexible piezoceramic nanofibers to serve as a power source for wearable devices. The top and bottom piezoelectric layers were manufactured by modularizing electrospun piezoceramic nanofibers with an interdigitated electrode, and the energy harvesting characteristics were maximized by laminating the single modules in z-axis array arrangements. The triboelectric layer was manufactured by attaching polydimethylsiloxane on both sides of an electrode layer, and the energy harvesting characteristics were controlled according to the surface roughness of the triboelectric modules. The output voltages of the individual energy harvester modules of the all-in-one module were individually or integrally measured by hand pressing the lower and upper parts of the module. The all-in-one energy harvester module generated a maximum voltage (power) of 253 V (3.8 mW), and the time required to charge a 0.1 μF capacitor to 25 V was 40 s. The results of a simulated energy harvesting experiment conducted on the all-in-one energy harvester module showed that 42 LED bulbs arranged in the shape of the “KICET” logo could be turned on in real time without charging, and a mini fan consuming a power of 3.5 W was operated after charging a 10 μF capacitor for 250 s. This work shows the potential of the all-in-one module as an ecofriendly flexible energy harvester for operating wearable devices.

A 3YSZ (3 mol% yttria-stabilized zirconia) ceramic green body with 50 vol% of ceramic content was 3D-printed by supportless stereolithography under optimal drying, debinding, and sintering conditions in order to achieve high strength and density. The viscosity and flowability of the ceramic nanocomposite resins were optimized by adjusting the amounts of non-reactive diluents. The ceramic 3D-printed objects have a high polymer content compared to ceramics samples manufactured by conventional manufacturing processes, and the attraction between layers is weak because of the layer-by-layer additive method. This causes problems such as layer separation and cracking due to internal stress generated when materials such as solvents and polymers are separated from the objects during the drying and debinding processes; therefore, the drying and debinding conditions of 3YSZ ceramic 3D-printed objects were optimized based on thermogravimetry–differential thermal analysis. The sintering conditions at various temperatures and times were analyzed using X-ray diffraction, SEM, and flexural strength analysis, and the body of the 3YSZ ceramic 3D-printed object that sintered at 1450 °C for 150 min had a relative density of 99.95% and flexural strength of 1008.5 MPa. This study widens the possibility of manufacturing ceramic 3D-printed objects with complex shapes, remarkable strength, and unique functionality, enabling their application in various industrial fields.