We report the fabrication of highly piezoelectric biocompatible soft fibers containing barium titanate ferroelectric ceramic particles dispersed in electrospun poly lactic acid (PLA). These fibers form mats that have two orders of magnitude larger piezoelectric constant per weight than single crystal barium titanate films. We propose that the observed apparent piezoelectricity results from the electrospinning induced polar alignment of the ferroelectric particles that pole the fibers similar to ferroelectret polymer foams that are poled by corona discharge. Due to the biocompatibility of PLA that encases the ferroelectric particles, these mats can be used in biological applications such as bio-sensors, artificial muscles, and energy harvesting devices.Piezoelectricity (a linear coupling between electric and mechanical signals) was discovered in non centro-symmetric crystals by the Curie brothers in 1880. 1 Since then piezoelectricity has been observed in ceramics, 2 certain synthetic 3 and biological polymers, 4,5 and ferroelectric liquid crystals. 6,7 Today piezoelectric materials are used in ultrasonic and hydroacoustic devices, 8 for frequency standards, 9 and in electromechanical actuators and sensors. 10 Recently certain cellular polymers internally charged by corona discharge (ferroelectrets) have been found to behave like soft sensitive piezoelectrics. 11 In addition to the corona charges in the voids in these foams, they usually also have to be inflated further with high pressure gas. 12 As a result, these ferroelectrets have long-term stability issues. 13 Here, we demonstrate high converse piezoelectric response of fiber mats composed of ferroelectric barium titanate (BT) nanoparticles dispersed in polylactic acid (PLA) and show that the effective piezoelectric constant per weight is two orders of magnitude larger than that of single crystal BT and of pure piezoelectric ceramic fibers. We propose that this unexpected super-piezoelectricity is due to the electrospinning-induced polar alignment of the particles, and the large surface area of the fibers, similar to ferroelectrets, however without the need of corona discharge and inflation processes.Electrospun fibers are formed by drawing a solution through a high electric field gradient generated between a nozzle and a collection plate 14-17 by electric fields of about 1-2 kV/cm, which is smaller than the breakdown field of 3 kV/cm of dry air. This produces very thin fibers with diameters ranging from hundreds of nanometers to several microns. Previously fibers of ferroelectric polymers, such as polyvinylidene fluoride (PVDF), have been electrospun and studied for electro-mechanical energy conversion applications. [18][19][20][21] Several examples of ferroelectric ceramic nanofibers have been fabricated using a sol-gel in the electrospinning process. [22][23][24] While the direct piezoelectric responses of ceramic fibers were demonstrated, 25,26 they are brittle. This problem was recently addressed by electrospinning of multi-material piezoelectric polymer...
Recently soft fiber mats electrospun from solutions of Barium Titanate (BT) ferroelectric ceramics particles and polylactic acid (PLA) were found to have large (d 33 $ 1 nm/V) converse piezoelectric signals offering a myriad of applications ranging from active implants to smart textiles. Here, we report direct piezoelectric measurements (electric signals due to mechanical stress) of the BT/PLA composite fiber mats at several BT concentrations. A homemade testing apparatus provided AC stresses in the 50 Hz-1.5 kHz-frequency range. The piezoelectric constant d 33 $ 0.5 nC/N and the compression modulus Y $ 10 4-10 5 Pa found are in agreement with the prior converse piezoelectric and compressibility measurements. Importantly, the direct piezoelectric signal is large enough to power a small LCD by simple finger tapping of a 0.15 mm thick 2-cm 2 area mat. We propose using these mats in active Braille cells and in liquid crystal writing tablets. V
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