Application of Electrospun Piezoelectric Fibers

Piezoelectricity is the ability of a material to transform mechanical energy into electrical energy. Numerous applications have been developed to make use of this property ranging from sensors to power generation. Using electrospinning, piezoelectric material can be made into the form of nanofibers which have the potential for application in microdevices or significantly improve the performance of piezoelectric devices. For material such as polyvinylidene fluoride (PVDF), electrospinning has been shown to increase crystal phase that exhibits piezoelectric properties.

Piezoelectric effect may be used for strain sensing applications. To improve the piezoresponse of electrospun PVDF fibers to dynamic strain, Dhakras et al (2012) added hydrated salt, nickel chloride hexahydrate (NiCl₂.6H₂O) to PVDF solution for electrospinning. The presence of NiCl₂.6H₂O was found to increase the polar β phase by about 30% which leads to a 3 times increase of peak to peak piezo-voltage generation compared to electrospun pure PVDF. Such improvement was attributed to ionic interaction between the salt and polymer molecules. Presence of small quantity of water due to the hygroscopic nature of the salt may also form hydrogen bond between water of crystallization and PVDF chains. At NiCl₂.6H₂O) concentration greater than 0.5wt%, the piezoelectric response starts to drop. This is probably due to excessive water content in the fiber.

Electrospun piezoelectric polyvinylidene fluoride (PVDF) nanofiber has the potential to be used as sensing element in a nanoelectromechanical sensors (NEMS). Sengupta et al (2017) showed that a single suspended nanofiber has a piezoelectric coefficient (d₃₃) of -58.77 pm/V. They were able to incorporate the single nanofiber onto a micro-electro-mechanical systems (MEMS), and demonstrate its ability to sense dynamic flow generated by oscillatory dipole stimulus.

A potential use of piezoelectric effect is in power generation. A study by Chang (2009) found that the average energy conversion efficiency for a single PVDF nanofiber was 12.5%, going as high as 21.8%, which is much higher than the energy conversion efficiency of PVDF thin film which average about 1.3%. This gives electrospun fibers the potential for use as power generator. Lu et al (2016) stacked multiple layers of electrets films made of electrospun PVDF/PTFE nanofibers. The device comprised of the electrets films sandwiched between flexible copper electrodes films. In their setup, a three layers configuration gave the best output with an instantaneous output power of 45.6 µW.

Schematic (a), photograph (b), and working principle (c) of the paper-based low-frequency eKEH [Lu et al. J. Phys.: Conf. Ser. 2016; 773: 012032. doi: 10.1088/1742-6596/773/1/012032. This work is licensed under a Creative Commons Attribution 3.0 Unported License].

Piezoelectric material is able to move with electrical stimulation and this makes it suitable for use as transducer. Ren et al (2014) explored the possibility of using electrospun piezoelectric poly(γ-benzyl-α,L-glutamate) (PBLG) nanofibers in velocity and pressure microphone. The microphone is constructed from aligned PBLG hot pressed into film form, cut along the shear direction (45° to fiber direction) and coated on both side with evaporated Al before attaching to Mylar film. Although the highest piezoelectric coefficient is in the direction of its fiber (d₃₃) mode, this is not suitable for microphone application because capacitance in this direction is extremely small. After compression into thin film, its capacitance across the film is relatively large and its shear piezoelectric coefficient d₁₄ at approximately -1 pC/N is feasible. The constructed diaphragm has a sensitivity around -60 dBV/Pa, which is about 30dB below that of a typical electret microphone in its responses as velocity and pressure sensors in the audio frequency range. Electrospun membrane as transducer may also be used as sound detectors to capture the mechanical vibration of human throat cords. This was demonstrated by Lee et al (2018) who fabricated aligned PVDF fibers by using near-field electrospinning (NFES) on printing paper. The constructed membrane was cut into strips and a single strip was pasted on the throat. By having the aligned dipoles y of the aligned fibers in a top-down position on the throat, it was able to detect unique characteristic waveform generated by different actions such as coughs, hums, shaking head, swallowing and nod action. This would be useful for further applications in pattern recognition and machine learning. The sound detector was able to capture the audios of Canon-in-D emitted from a loudspeaker with relative voltage output corresponds to nearly synchronous response to meter-measured profiles of the original audios. Wang et al (2020) developed a non-resonating acoustic sensor based on the ability of sparsely distributed freely suspended electrospun piezoelectric fibers to pick up acoustic signals. A nanofiber mesh made of poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) with an average fiber diameter of 307 nm was produced by a dynamic near field electrospinning method. This nanofiber mesh was found to be sensitive to acoustic waves from 200 Hz to 500 Hz which covers the most common sound frequencies encountered daily hence the acoustic sensors made from the electrospun (P(VDF-TrFE)) fibers were able to detect changes in the source frequencies. As the sound wave travels perpendicular to the suspended nanofibers, it causes them to vibrate and generate a voltage output. Further tests showed that the nanofiber mats were able to differentiate between different sound frequencies.