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Flexible wearable piezoelectric sensors, with their remarkable ability to convert small deformations into electrical energy, have emerged as highly promising technological solutions with numerous potential applications in healthcare monitoring, human-machine interfaces, biomechanics and smart motion science, as well as security and surveillance. Nanofiber mats consist of interconnected nanoscale fibers, typically ranging from hundreds of nanometers to submicrometers in diameter, which have extraordinary responsiveness to mechanical stimuli such as pressure, strain, or vibration, and are perfectly flexible and inherently mechanically strong, making them advantageous components for building piezoelectric sensors. These pads can be fabricated by a variety of techniques, including electrospinning, solution blow-spinning, and others.
The main point of this paper
Electrospinning technology with PVDF nanofiber mats
Electrospinning Technology:
Stretching of polymer solutions or melts using electrostatic forces to form very fine fibers.
PVDF (polyvinylidene fluoride) is an ideal material for electrospinning due to its chemical stability, piezoelectric properties and high dielectric constant.
Types of PVDF nanofiber mats:
Non-aligned structure: randomly aligned nanofibers that are isotropic and suitable for applications that do not require a specific orientation alignment, such as biomedical and piezoelectric sensors.
Aligned structure: nanofibers are aligned in a single direction, achieved by adjusting the collector or external electric field.
Collector type:
Static collectors: metal plates, double plates, round electrodes.
Rotating collectors: e.g. drum collectors, provide additional tensile forces that contribute to fiber alignment and PVDF polarization orientation.
Aligned PVDF nanofiber mats applications:
Excellent electromechanical properties and ultra-high measurement sensitivity.
In energy harvesting devices (PENGs), the electrical output of PENGs based on aligned fiber mesh exceeds that of PENGs based on randomly aligned fiber mesh.
Research Objective:
To investigate the basic principles of PVDF nanofiber formation during electrospinning.
To investigate the effect of process parameters (voltage, humidity, collector speed) on the morphology, alignment and piezoelectric properties of nanofibers.
Research Methods:
Fiber morphology was observed using scanning electron microscopy (SEM).
The beta crystalline phase content was analyzed using X-ray diffraction and Fourier transform infrared (FTIR) spectroscopy.
Piezoelectric properties were observed using shaker and oscilloscope.
Importance of PVDF Nanofiber Felt:
PVDF nanofiber mats are in great demand due to their high sensitivity and long detection range for multi-sensory functionality in wearable electronic devices
Development of Electrospinning Process:
Researchers have successfully developed freely stacked and aligned PVDF nanofiber mats ranging from micron to nanometer diameters to provide stable performance for wearable electronic devices
Process Parameter-Performance Relationships:
An in-depth study of the material preparation, electrospinning, and fiber collection processes revealed the relationship between nanofiber morphology, beta phase fraction, and piezoelectric output and process parameters (voltage, humidity, collector speed)
Fabrication system establishment:
A mature and reliable nanofiber mat manufacturing system has been established, which is capable of mass production of PVDF nanofibers with the required diameter and stable properties
Specific effects of process parameters:
The uniformity of fiber diameter and β-phase content was maintained in good range at 18 kV and 60% relative humidity conditions
When the drum speed was increased to 2000 r/s, both fiber orientation and β-phase content increased
Application of piezoelectric sensors:
Neatly aligned PVDF nanofiber mats were assembled with conductive fabrics to form flexible piezoelectric sensors that successfully monitored different human motions and produced an output voltage of 0.1 V
This work details the electrospinning process under several sets of important processing parameters, including voltage, humidity, and drum speed, and compares the electrical and mechanical properties of these prepared non-aligned and aligned PVDF nanofiber mats. The following conclusions were drawn.
(1) When the voltage was 18 kV and the humidity was 60% RH, the uniformity of fiber diameters and the content of β-phase could be maintained in a favorable range. On this basis, when the drum speed reaches 2000 r/s, the fibers show significant orientation and the β-phase content increases significantly.
(2) Higher drum rotational speeds result in aligned nanopiezoelectric fibers, which have higher electrical and mechanical output properties compared to unaligned piezoelectric fibers.
(3) The wearable flexible sensor maintains good electrical output at a deformation angle of 0-90° and has good torsional resistance.
(4) The prepared wearable flexible sensor has high shape adaptability without affecting its sensing capability. At the same time, its self-powered function eliminates the need for a rigid battery, enabling it to fit well on curved surfaces such as joints for sensing.
