Birds are outstanding flyers with high aerodynamic efficiency and agility, especially under dynamic flight conditions. Flight feathers play a key role in achieving these remarkable performances based on their flexible and hierarchical structures. To develop bio-inspired micro air vehicles (MAVs), researchers have adopted rigid feather-shaped panels, membrane-type artificial feathers and natural feathers as part of the morphing wing platform. In this paper, bio-inspired, 3D printed feathers with hierarchical structures resembling natural flight feathers are presented. Moreover, piezoresistive and piezoelectric sensing components are embedded in the 3D printed feather rachis, which can provide sensory information on the aerodynamic forces and feather vibrations. The 3D printed feather transducers are characterized through vibration testing and wind tunnel testing, and are finally integrated into dried, spread wings for aerodynamic force and vibration sensing of the entire wing. Therefore, the 3D printed feather transducers can potentially be used on future MAVs to improve aerodynamic efficiency and allow fly-by-feel sensing.
The aramid nanofibers (ANFs) are isolated from commercial para-aramid fiber, a class of strong and heat-resistant synthetic fibers. They comprise dense inter-molecular hydrogen bonds among highly aligned molecular chains, resulting in outstanding modulus and strength-to-weight properties comparable to carbon fiber and cellulose nanofiber. The aramid nanofibers show a high aspect ratio with a nano-sized diameter and micro-scaled length. These have been spotlighted as promising nano-building blocks with their excellent mechanical properties and thermal stability. In addition, it is an eco-friendly and cost-effective system as one way to recycle waste aramid fibers generated from the rapidly growing aramid textile market. Herein, ANFs-based multidimensional structures with high specific modulus, strength and toughness, such as 1D ANFs assembled filament, 2D ANF nanocomposite film, and 3D ANF reinforced polymer structures, are introduced with various designed fabrication technologies.
Flexible and stretchable stress and strain sensing materials have gained a lot of research interest recently as the development of wearable sensors for health monitoring, motion capturing and soft robotics. In these applications where dynamic stress and strain are common, piezoelectricity becomes a suitable sensing mechanism due to its fast response and high sensitivity. Existing research on flexible piezoelectric materials includes nanocomposites and sandwich composites made of piezoelectric fillers and elastomers. However, the giant modulus mismatch between the two distinct phases makes nanocomposites or sandwich materials prone to inaccurate sensing under large strains due to the weak stress transfer efficiency. In this research, polyvinylidene fluoride (PVDF) and unvulcanized nitrile rubber (NBR) are both dissolved in N,N-dimethylformamide (DMF) and then precipitation printed into a water bath to produce PVDF/NBR polymer blends. The blends are further vulcanized via hot pressing. The resulting blends exhibit polar phases of PVDF, highly uniform blend morphology, as well as excellent stretchability. As a stretchable sensor, the PVDF/NBR (2:8) shows consistent open circuit voltage-strain and open circuit voltage-stress relationships, as well as a high operating strain range up to 70%. Therefore, the PVDF/NBR blend can be used as a promising dynamic stress/strain sensing material for wearable sensors or soft robotic sensors.
Additive manufacturing of poly(vinylidene fluoride) (PVDF) as a piezoelectric material gained wide research interests over the past decade. Promoting the most polar β phase of PVDF during or after the printing process is the key focus to enhance its piezoelectricity. In this paper, a novel additive manufacturing technique termed precipitation printing is developed to produce high-β phase PVDF with the advantages of achieving geometry complexity and fabrication scalability, which is based on the different solubility of PVDF in two mutually miscible solvents. Through dispensing the PVDF/N,N-dimethylformamide (DMF) solution into a water bath, PVDF is continuously precipitated to form a solid structure while DMF diffuses into water. The β phase fraction of printed PVDF is improved to 64.2%. By further hot pressing of precipitation printed PVDF to reduce internal porosity, the piezoelectric d31 and d31 coefficients are measured to be 1.95 pC/N and -6.42 pC/N, respectively. Precipitation printing is also demonstrated to fabricate piezoelectric PVDF energy harvesters, such as a stretching mode strip energy harvester and a heel insole energy harvester. Therefore, precipitation printing provides a new additive manufacturing technique for producing high-β phase PVDF with strong piezoelectric effect, which can be potentially used to produce sensors, energy harvesters and actuators.
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