A group of researchers recently published a paper in the journal Macromolecular Materials and Engineering that reviewed the latest developments toward improving the piezoelectricity of electrospun polymer nanofibers for wearable electronics.
Study: A Review on Wearable Electrospun Polymeric Piezoelectric Sensors and Energy Harvesters. Image Credit: Kateryna Kon/Shutterstock.com
In recent years, wearable energy harvesters and sensors have gained significant attention for several applications, including human-machine interfaces, robotics, and personalized healthcare.
Piezoelectric materials are used most extensively in wearable electronics owing to their unique ability to harvest energy from surrounding sources. In wearable sensors, these materials can also be used as sensing elements.
Several studies have investigated the feasibility of using electrospun piezoelectric polymer nanofibers for wearable electronics owing to their biocompatibility, ease of processing, high flexibility, and higher piezoelectric property compared to their corresponding cast films.
However, these nanofibers display lower piezoelectric coefficients compared to piezoceramic materials. Recently, significant efforts have been made to improve the piezoelectricity of electrospun polymer nanofibers.
In this paper, the authors reviewed the new strategies, structural designs, and materials adopted to improve the piezoelectricity of electrospun polymer nanofibers to realize high-performance electrospun polymer nanofiber-based piezoelectric energy harvesters and sensors for wearable technologies.
Polyvinylidene fluoride (PVDF) and its copolymers such as PVDF homopolymers, poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) polymers, and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymers, polyacrylonitrile (PAN), cellulose, and polylactic acid (PLA) are typically used to fabricate electrospun nanofiber-based piezoelectric wearable devices.
Among these piezoelectric polymers, PVDF and its copolymers possess the highest piezoelectricity. Although PVDF-TrFE has greater piezoelectricity than PVDF, it is more expensive compared to PVDF. However, PVDF lacks certain unique properties present in other polymers. For instance, PAN can be used at high temperatures due to its higher thermal stability compared to PVDF, while PLA and cellulose are more eco-friendly.
Strategies to Enhance the Piezoelectric Performance of Electrospun Polymer Nanofibers
Polymer Blend/Polymer Nanocomposite Design
The use of additives, such as polymers, non-piezoelectric fillers, and piezoelectric fillers in the piezoelectric polymers can modify their orientations, molecular dipoles, and crystalline structure and affect their piezoelectric properties by influencing their electrical and mechanical properties.
Wearable piezoelectric nanofiber mats possessing high piezoelectricity were fabricated by integrating piezoelectric ceramics with various piezoelectric polymers. Boron nitride (BN), zinc oxide (ZnO), and barium titanate (BaTiO3) in different forms, such as nanowires, nanosheets, nanofibers, nanoflakes, nanorods (NRs), and nanoparticles, can be combined with piezoelectric polymers to synthesize flexible piezoelectric devices.
For instance, the piezoelectricity of PAN nanofibers was improved using BaTiO3 nanoparticles by promoting 31-helix transformation into the planar zigzag. Similarly, monolayer BN improved the piezoelectricity of PVDF owing to its robust in-plane piezoelectric properties.
ZnO nanospheres increased the PVDF nanofiber piezoelectricity by increasing the crystallinity degree and β-phase content of PVDF. PVDF samples with five wt% ZnO displayed the highest open circuit voltage of 11 V.
Surface functionalization of nanofillers also effectively improved the piezoelectricity by promoting strong interaction between PVDF and nanofillers and better dispersion. For instance, the use of five wt% carbon-coated ZnO in PVDF increased the output voltage by five times to almost 37 V compared to pure PVDF.
Electrically conductive nanomaterials, such as magnetic materials, MXenes, metal nanomaterials such as gold nanoparticles, and carbon-based nanomaterials such as multi-walled carbon nanotubes (MWCNTs) can be used to improve the piezoelectricity of piezoelectric polymers.
These conductive nanofillers improve the induced charge transfer and increase the β-phase content of piezoelectric polymers to enhance the piezoelectric output. For instance, the use of MWCNTs increased the polar β-phase content and crystallinity of PVDF-TrFE nanofibers, resulting in a higher piezoelectric output. The maximum open circuit voltage of 18.23 V was observed in PVDF-TrFE electrospun nanofibers at three wt% MWCNTs.
The addition of another polymer can also improve the mechanical and piezoelectric properties of piezoelectric polymer nanofibers. For instance, the addition of elastic thermoplastic polyurethane (TPU) and polydimethylsiloxane (PDMS) polymers into the electrospinning PVDF solution increased the stretchability and elasticity of PVDF. The maximum failure strain of PVDF/TPU at a 1:3 ratio was 85% compared to 12.5% in pure PVDF.
Structural engineering of fibrous-based sensors and nanogenerators can improve their piezoelectric properties.
Nanoparticle-coated polymer nanofiber mats typically demonstrate a higher piezoelectric output. For instance, a significantly higher output of 245.63 nW cm2 was realized when the PVDF nanofiber mat was electrosprayed with ZnO NRs in place of dispersing ZnO NRs into the PVDF solution before electrospinning the fibers.
Coaxial electrospinning can fabricate an extensive range of nanofibers with better piezoelectric and mechanical properties in a core-sheath configuration by combining different complementary materials.
Core-shell nanofibrous configuration is suitable for polymers, such as PDMS and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), that cannot be electrospun easily.
Polymers with a coaxial core-shell structure demonstrate a higher polymer density and β-phase content. For instance, a core-shell nanofiber composed of PEDOT:PSS as core and PVDF-TrFE as shell in a 4:1 ratio displayed an 8.76 V output voltage, which was almost 10 times higher than pure PVDF-TrFE nanofibers.
Other Hierarchical Designs
Recently, a unique design was developed in which the electrospun nanofiber mat was twinned into a yarn structure, which can be integrated with textiles for wearable electronics.
Similarly, a novel structured yarn composed of a conductive core with wrapped PVDF nanofibers that was successively coated with a PVDF and a silver layer displayed 0.52 V average peak voltage and 5.54 μW cm−3 power density under 0.02 MPa cyclic compression. The peak voltage and power density values were considerably higher than the yarn manufactured from only PVDF nanofibers.
Parameters Influencing the Electrospun Nanofiber Properties
Ambient parameters, solution parameters, and electrospinning parameters primarily influence the electrospun polymer nanofiber properties, including piezoelectricity, β-phase formation, and morphology.
The ambient parameters include temperature and relative humidity, while the solution parameters include the molecular weight of the polymer, solution concentration, and solvent. The electrospinning parameters include collector type, voltage, the distance between tip to the collector, and feed rate.
Significant progress has been achieved in the development of high-performance and flexible piezoelectric polymer nanofibers with enhanced piezoelectricity. However, more research is required to further improve the piezoelectric output of piezoelectric polymers using novel methods.
New ways must be identified to incorporate fillers in piezoelectric polymers to maximize the improvement in the piezoelectric output and mechanical properties. Additionally, the challenges associated with the development of stretchable piezoelectric devices and the use of piezoelectric sensors in wearable garments and accessories must also be addressed to realize the large-scale adoption of electrospun polymer nanofibers for wearable electronics.
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Peng, S., Huang, S., Wu, S. et al. A Review on Wearable Electrospun Polymeric Piezoelectric Sensors and Energy Harvesters. Macromolecular Materials and Engineering 2022. https://onlinelibrary.wiley.com/doi/10.1002/mame.202200442