Piezoelectric Fibers for Conformal Acoustics

The fi rst modern ultrasound transducer was developed after the tragedy of the Titanic in 1912, when the need arose for an echolocating device to detect icebergs and, later on, submarines. The success of sonar technology stimulated research in this fi eld and led to the development of a large number of important applications such as non-destructive testing (NDT) and medical imaging, with an ever higher accuracy both in direction and range. In recent years, new uses of ultrasound transducers have emerged, particularly in - but not limited to - the medical fi eld, such as photoacoustic imaging of the brain, [ 1 ] bone healing, [ 2 ] focused ultrasound surgery, [ 3 ] and ocean observation. [ 4 ] These applications ideally require large-area, fl exible, and addressable transducer arrays, where the amplitude and phase of each individual device can be independently controlled. Several approaches have been pursued to address this challenge. A specially active area of research has been electroactive polymers, which have been widely investigated as an alternative to piezoceramics. [ 5 ] In particular, the large values of piezoelectric constants for poly(vinylidene fl uoride) (PVDF) and copolymers such as poly(vinylidene fltrifl uoroethylene) (P(VDF-TrFE)), coupled with their mechanical compliance and processability make them especially suited for large-area fl exible applications. Furthermore, their large bandwidth and low acoustic impedance are an advantage for many medical uses. Another promising path involves capacitive micromachined ultrasonic transducers (CMUTs). [ 6 ] These approaches, however, leave some key challenges unresolved: device reliability issues due to charge trapping, constraints on the device area due to wafer-based processes, acoustic cross-talk, and complex electronic integration. [ 7 ] Recently developed multimaterial fi bers present a number of attractive properties that can address these issues. The preform-based thermal drawing process offers a scalable means of producing kilometer-long fi ber devices with submillimetric cross-sectional dimensions. [ 8 ] These long and fl exible fi bers can easily be assembled into large-scale conformal constructs, such as fabrics and sparse meshes. [ 9 ] Furthermore, monolithic integration of electrodes into the fi ber enables straightforward electrical connection of the device to an external electrical circuit. [ 8 ] The latest advances in this fi eld have led to the development of P(VDF-TrFE)-based piezoelectric fi bers that are capable of acoustic emission and detection over a broad range of frequencies, from the tens of Hz to the tens of MHz. [ 10 ] While the small cross-sectional area of these fi bers enables both miniaturization and fl exibility, it seemingly involves an equally small active area that potentially limits the fi ber performance. Here we introduce a new large-active-area fi ber design that addresses a fundamental tradeoff between the requirement of maximizing the surface area of the acoustic device, and the inherent energy penalty associated with generating large area interfaces. This is achieved by confi ning the low viscosity crystalline ferroelectric medium between highly viscous boundaries, thus controlling the kinetics of the fi ber thermal drawing process. We perform interference experiments to show that this novel device is coherent in the axial dimension, and we demonstrate how the coherence property combined with the mechanical fl exibility enables acoustic wavefront shaping through precise control over the fi ber curvature. Although the single fi ber itself presents unique opportunities for minimally invasive sensing and imaging, in this work we take it one step further and analyze collective effects in multiple fi bers. Coherent interferences and beam steering capabilities of multi-fi ber phased arrays are demonstrated, thus establishing the possibility of assembling large-scale fi ber constructs such as fabrics and meshes. Piezoelectric transducers can be used both in emission and reception, depending on the application. When the transducer operates as a receiver, internal electrodes with a large surface area are desirable as they lead to a higher number of charges generated, and thus a better device sensitivity. As a transmitter, the fi ber reaches maximal strain when the electric fi eld is maximal, while remaining below the breakdown voltage. In these two regimes, the large-effective-area fi ber design introduced in this paper enhances the transducer performance. Indeed, for a given cross-sectional area, a folded structure enables a severalfold increase in the electrode area and allows high electric fi elds without a high applied voltage since the piezoelectric layer is thinned down. This approach was developed for piezoceramic actuator stacks, and is of particular interest for use with our fabrication process. The fi bers are produced by a thermal drawing technique from a macroscopic preform, which has the desired