Fabrication and application of flexible piezoelectric sensor based on electrospun PVDF-TrFE nanofibers

Flexible piezoelectric sensors have recently demonstrated great promise for use in wearable electronics and electronic skin. In this study, electrospinning was used to generate piezoelectric PVDF-TrFE nanofiber sensors. The relationship between nanofiber alignment orientation and piezoelectric property was examined. The experimental results demonstrated that the piezoelectric property improved with the increasing of nanofiber alignment orientation. The response time of piezoelectric sensor was 2 ms, and the sensitivity was 0.446 V/N and 0.029 V/N at vibration frequency of 5 Hz and 1 Hz, respectively. In addition, the applications of flexible piezoelectric sensor in human joint movements and surface contact have been demonstrated, indicates the potential in the field of motion monitoring.


Introduction
Flexible piezoelectric sensors with high flexibility, high sensitivity, low power consumption, and quick response times have demonstrated significant potential for wearable electronics and electronic skin applications in recent years [1].In comparison to inorganic materials, polyvinylidene fluoride (PVDF) and its copolymer poly(vinyl fluoride-co-tetrafluoroethylene) (PVDF-TrFE) are among the most piezoelectric materials known for their superior piezoelectric, pyroelectric, and ferroelectric properties [2].The unique characteristics of electrospun nanofibers, such as small diameter, high flexibility, high specific surface area, and capacity to form various structures have attracted the attention of researchers in various fields [3].
In this study, PVDF-TrFE nanofibers were prepared by electrospinning.The effects of different drum speeds on the arrangement orientation and piezoelectric properties of the nanofibers were investigated.The application of piezoelectric sensor in partial human joint detection and surface contact detection was also demonstrated.= 500000 g/mol) power was dissolved in N, N-dimethylacetamide (DMAC) with the mass concentration of 22 wt%, and then stirred at room temperature in a magnetic mixer for 12h.The electrospinning processes were carried out on a commercial apparatus (Narai NL-E-ES-2), as the illustrated schematic was shown in figure 1(a).The spinneret was connected to the positive terminal of a high-voltage DC power supply with a voltage of 14 kV, and the collection drum was connected to the negative terminal with a voltage of 0 kV.The inner and outer diameter of spinneret was 0.35 mm and 0.62 mm, respectively.The distance between the spinneret and drum was 15 cm, and the diameter of drum was 12 cm.The supply rate of PVDF-TrFE/DMAC solution was set to 8 μL/min.The electrospinning period of each nanofiber membrane was 2 hours.The morphology of nanofiber membrane was observed by a scanning electron microscopy (SEM, Zeiss Sigma500).

Packaging of Piezoelectric Sensor
The fabricated piezoelectric sensor is shown in figure 1(b) and (c).The electrodes were screen printed directly on the PVDF-TrFE nanofiber membrane to ensure the contact between electrodes and the sensing layer.In addition, the electrode printed nanofiber membrane was casted by PDMS and cured to form a flexible piezoelectric sensor.

Morphology of PVDF-TrFE Nanofiber
The SEM images of PVDF-TrFE nanofiber prepared at different drum rotation speeds are shown in figure 2. The orientation of the nanofibers increases with the increasing of drum rotation speed.The arrangement of nanofibers was not obvious and displayed a disorderly state when the drum speed was lower than 600 rpm, and ordered nanofibers could be obtained at 1500 rpm.

Testing of Piezoelectric Sensors
The PVDF-TrFE nanofiber membranes prepared at different rotational speeds were encapsulated to obtain piezoelectric sensors.The sensors were placed on a shaker table for sinusoidal vibration at 5 Hz, and the force was controlled to be 1 N.The open-circuit voltage was then measured, as shown in figure 3. The open-circuit voltage of the electrospun piezoelectric nanofiber membrane gradually increases as the drum collection speed increases.The average open-circuit voltages were 168 mV, 98 mV, 133 mV, 232 mV, and 310 mV when drum rotation speed were 0 rpm, 200 rpm, 600 rpm, 1000 rpm and 1500 rpm, respectively, as shown in figure 3(a).Figure 3(b) indicates that the sensor generated at a rotation speed of 1500 rpm has a good repeatability.The response time was about 2 ms, as shown in figure 3(c).The results demonstrated that the piezoelectric properties can be improved as the nanofiber arrangement increases with the increasing of drum rotation speed.The open-circuit voltage versus impact stress was measured separately to examine the sensitivity of the sensor, as shown in Figure 4.The output signal of the sensor increased linearly with the impact stress, and the sensitivities were about 0.446 V/N and 0.029 V/N at 5 Hz and 1 Hz, respectively.

Applications of Piezoelectric Sensors
According to the experimental results, the sensor has a quick response time and a sufficient output signal, which can be used to detect human joint movements and have potential uses in the field of contact monitoring.Figure 5 shows the bending test curve of the flexible piezoelectric sensor at the human finger and wrist.When the finger and wrist joints were bent, the sensor was stretched and squeezed, and the open-circuit voltage increased.In addition, the amplitude of the output voltage increased with the speed and range of the movement.To detect the strain distribution on the surface of an object, a "3×3" piezoelectric pressure sensor array was generated.The open-circuit voltage of the sensor can map the spatial distribution of strain on the sensor array for different states of strikes, and the results can be regarded as a strain intensity map as shown in figure 6.When the pressure sensor array was in contact with an object, the spatial distribution of pressure responded to pressures that applied to different areas, and the output voltage responded to the specific value of the pressure.The results show that the sensor array presented significant voltages increment in the contact region, indicated the feasibility to identify the spatial distribution of force, and had a potential application in electronic skin and large area wearable devices.

Conclusions
In this paper, PVDF-TrFE nanofiber were prepared by electrospinning, and the relationship between different drum rotational speeds, fiber alignment orientation, and piezoelectric properties were investigated.The results showed that the piezoelectric properties of the nanofiber membranes increased with the increasing of drum rotation speed, as well as the nanofiber alignment orientation.In addition, the sensitivities of the sensors were 0.446 V/N and 0.029 V/N at vibration frequencies of 5 Hz and 1 Hz, respectively.The sensors had fast response and obvious output signals that could be used to detect human joint activities and contact detection and have potential applications in motion monitoring.

Figure 1 .
Figure 1.(a) Schematic diagram of electrospinning setup.(b) The structure of piezoelectric sensor.(c) The photograph of piezoelectric sensor.

Figure 3 .
Figure 3. (a) The open-circuit peaks voltage of PVDF-TrFE nanofibers prepared at different drum rotation speeds.(b) The overall waveform of the open-circuit voltage with a drum speed of 1500 rpm.(c) A single waveform of open-circuit voltage with a drum speed of 1500 rpm.

Figure 5 .
Figure 5. Joint bending test of the human body part of the sensor.(a) Fingers.(b) Wrist.

Figure 6 .
Figure 6.Strain spatial distribution test of sensor array.(a) The palm was in an open state.(b) The palm was clenched like a fist.(c) Letter "L".(d) Letter "K".(e) Letter "H".