Piezoelectric Fibers Endow Textiles with Acoustic Functionalities

As hierarchical structures that contain multiple interfaces that serve as scattering points, textiles are traditionally thought of as structures that damp vibrations and sound. Harnessing the sensitivity of a piezoelectric composite in a fiber form factor, however, we introduce new ways for textiles to interact with sound by incorporating the “acoustic fiber” into fabrics. First, we weave an acoustic fiber into traditional fabrics to develop fabric microphones to capture sound. The acoustic fibers are created using a thermal drawing technique in which viscoelastic materials flow in a laminar regime, maintaining the cross-sectional geometry of the preform. In the fiber device, the piezoelectric domain of P(VDF-TrFE) with 20 wt% barium titanate nanoparticles is sandwiched between two carbon-loaded polycarbonate electrodes, each containing two copper wires that are fed in during the draw. The active layer and electrodes are encapsulated by a soft SEBS elastomer cladding. The measured d₃₁ piezoelectric coefficient of the active domain of the fiber is 46 pC/N, which is more than double the values reported previously in literature (~20 pC/N). Coupling the acoustic characterization with laser vibrometry measurements, we deduce that the modulus of the fabric influences the curvature of the fiber, which is key to its electrical output. Through an axial bending mechanism, the fiber couples to the fabric to detect nanometer-scale vibrations in the fabric that result from the incoming acoustic pressure. In addition to fabric microphones, the acoustic fiber lends itself to many diverse applications. An array of fibers can be used to determine the direction of incident sound, and the fiber woven into a shirt can detect heartbeat and breathing rate. Additionally, rather than damping sound, textiles can be used as speakers, creating sound by applying a voltage to an acoustic fiber on fabric. Recent advances in these particular applications will be discussed.