Flexible pressure sensor constructed by polyurethane composite conductive sponge

As the main core component of wearable devices, flexible strain sensors have broad application prospects in health monitoring, motion monitoring, human-machine interface, rehabilitation, entertainment technology and other fields. In this paper, a rectangular sandwich resistive pressure sensor is constructed with porous conductive sponge, and its working mechanism is analyzed. The linearity of the sensor is improved and the stress range is increased by gel modification. Through experimental tests, it can withstand more than 80% compressive strain, and shows a sensitivity of 0.398 kPa−1 in the range of 6 ∼ 11 kPa; the maximum range is close to 40 kPa, and the minimum detection limit is 20 Pa; under constant loading/releasing speed, the response/recovery time is about 133/150 ms; it also shows good linearity and stability. With the help of a single sensor entity, Morse code can be sent, and some human activity signals can be measured, such as speech recognition, weighing measurement, limb movement; and 8 sensors create an interesting smart insole for gait recognition. The results show that piezoresistive sensors with porous composite materials have broad application prospects in motion monitoring and human-computer interaction.


Introduction
With the rapid rise and development of wearable electronic devices, the demand for high-performance flexible sensors is increasing.Flexible strain sensors have attracted extensive research attention in recent years, and have shown broad application prospects in the fields of motion monitoring, medical equipment, consumer electronics, electronic skin and soft robots [1][2][3][4][5][6].For example, in prosthetics and robotics applications, strain sensors can monitor the bending of joints, fingers, etc., in order to achieve health status assessment [7,8].Several attempts have been made to achieve this.Most of the traditional strain sensors are made of rigid conductive materials, and their technology is relatively mature, but their flexibility is limited.They can only detect tiny strains within a range of about 5%, far lower than the strain range of skin [9,10].Flexible strain sensors based on conductive nanomaterials (carbon nanotubes, graphene, silver nanomaterials, metal nanoparticles, etc.) and elastic substrates have been widely reported, among which porous materials have the advantages of low density, high flexibility, good heat dissipation and can withstand a wide range of compressive strains, and have been favored by many researchers in recent years [11][12][13][14][15].
Jung et al [16] made a pressure-sensitive polydimethylsiloxane (PDMS) sponge sensor with high elasticity and high wear resistance based on three-dimensional microporous graphene coating by using sugar template process and simple method of impregnation coating based on graphene ink.Graphene film coating is coated on PDMS sponge skeleton, which is used as sensitive material of piezoresistive strain sensor.Under different strain conditions, the sensor exhibits very stable, repeatable and reversible resistance changes, and shows highly stable mechanical properties in various tensile stress-strain tests.Li et al [17] modified polyurethane (PU) sponge skeleton with chitosan, obtained CS@PU sponge with positive charge, and then dipped Ti3C2Tx MXene sheet with negative charge to manufacture flexible piezoresistive MXene@CS@PU sensor.Because of the high compressive resilience of PU sponge and its polar interaction with MXene sheet, the sensor shows high compressive performance and stable response performance.Yang et al [18] made a flexible microfluidic capacitive pressure sensor by coating two indium tin oxide polyethylene terephthalate (ITO-PET) films on the top and bottom of PU sponge filled with ionic liquid dielectric layer.When the sensor is subjected to force, it will deform, which leads to the increase of contact area between ITO-PET electrode and dielectric layer, and the decrease of relative distance between two ITO-PET electrodes, which makes the capacitance of the sensor increase.Sharma et al [19] developed a stretchable multifunctional sensor composed of stretchable porous membrane with multi-scale arranged pores and silver nanowire electrodes.The sensor has different pore sizes and gradient arrangement in the film, which makes it possible to detect bending degree and pressure at the same time, and shows high linear sensitivity.
Although many efforts have been made, the development of flexible sensors with high sensitivity and wide range is still a hot spot worthy of research at present.The sensor made in this study uses porous materials, and the flexibility of flexible porous materials makes it easier to adapt to various shapes and bends.This helps to improve the stability and reliability of the sensor in different application scenarios.The manufacturing process of the sensor is simple and the cost is low.As shown in figure 1, the conductive metal PU sponge (CMPU) was impregnated in the doped solution composed of polyvinyl alcohol (PVA) and CaCl 2 , and the lightweight composite conductive voltage resistive sponge was prepared as a strain sensor.Its main material is PU foam sponge as the basis, after physical vapor deposition conductive treatment and nickel, copper and other metal electrodeposition, has all-round electrical conductivity and good electrical conductivity.By dipping process, the sensor forms a double sensitive layer composed of metal coating and conductive gel layer, which has high sensing performance.

Experimental materials and instruments
The manufacturing process of the sensor is shown in figure 1.Firstly, CMPU (Shenzhen Aodingchang, China) was cut into 1 × 1 × 1 cm 3 cubes by laser marking machine (HG-LU-5, Wuhan Huagong, China), and then compressed 100 times at 85% by digital push & pull tester (HP-50, Yueqing Handpi, China), washed with absolute ethanol ultrasonic wave for 10 min, and then cooked in an oven (101-1SB, Shaoxing Huyue, China) was baked at 150 °C for 30 min (the main purpose of this operation is to age the conductive sponge, weaken its conductivity and improve its conductive stability).PVA (AR, Shanghai Aladdin,China) and CaCl 2 (96% AR, Shanghai Boer, China) were placed in water according to four different mass ratios in table 1, heated to 85 °C and stored for 30 min to completely dissolve the drug and wait for cooling, soak CMPU in solution, squeeze repeatedly to make it fully absorb the solution, then take it out, soak it in borax(AR, Sinopharm, China) aqueous solution with mass fraction of 1% again for 20 min, finally take it out, put it on dust-free cloth and squeeze it repeatedly until CMPU has no obvious leakage, and dry it naturally at room temperature of 25 °C for 12 h.Stick copper foil tape with the same area on the upper surface and lower surface of sponge, and weld the wire with electric soldering iron to lead out the electrode.

Working principle
The PU sensor with porous structure relies on the contact of conductive metal skeleton and conductive hydrogel rich in CaCl 2 in three-dimensional space for sensing.The conductive coating is attached to the sponge skeleton.During the deformation process, pore shape and equivalent resistivity change.During the deformation process, the pore shape and equivalent resistivity change.As shown in figure 2, according to the existing studies [20][21][22] and the actual analysis in this paper, the sensor resistance is mainly affected by three mechanisms: In the process of stress, the microcracks of the metal coating in the vulnerable part of the sponge skeleton expand, leading to the increase of the resistance value; As the contact degree between gels and gels and between gels and skeleton coatings increases, the total resistance decreases slowly; The contact area between the metal coatings increases, providing more conductive paths, and the total resistance of the coating decreases rapidly.When at slight compression, the sponge skeleton is contactless, and the first mechanism plays a leading role, resulting in positive resistance effect of the sensor; When at medium compression, the contact of sponge skeleton is small, but the contact of gel layer is large, and the second mechanism plays a leading role; When the degree of compression is large, the third mechanism dominates due to the high electrical conductivity of the metal coating of the skeleton.After unloading the pressure, the sponge structure springs back and the sensor resistance returns to the initial state.Figure 4 shows the stress-strain curve of the sensor with different PVA mass fractions corrected under compression load.It can be seen from the figure that the increase of PVA proportion in the sample can improve the stiffness of the sensor to a certain extent, thus slightly increasing the upper stress limit of the sensor.Without further explanation, the mass ratio of sample 4 was selected to make the sensor.Like most porous materials, the stress-strain realization of CMPU can be divided into three stages: elastic zone, yield zone and dense zone.

Results and discussion
According to the schematic diagram shown in figure 5(a), a circuit is built to demonstrate the change of resistance of the sensor under stress.R is the current-limiting resistance, and the sensor is equivalent to a potentiometer Rx.As shown in figure 5(b), when no pressure is applied to the sensor, its resistance is larger, resulting in a larger total circuit resistance and a weak light emitted by the LED.As shown in figure 5(c), when a certain pressure is applied to the sensor, the resistance of the sensor decreases, making the total resistance decrease and the LED emit bright light.

Performance evaluation
Because the conductivity of the conductive gel produced is much lower than that of the conductive metal coating, the conductive metal coating has a greater effect on the overall resistance during the sensor strain process.As shown in figure 6(a), a digital push & pull tester was used to carry out static compression test on the sensor.At 80% compression, the original CMPU has an initial resistance of 15.3 Ω and a termination resistance of 1.2 Ω.This resistance value is too small, the direct construction of the circuit anti-interference ability is very poor, generally need to cooperate with the clamp resistance to construct Wheatstone bridge, and the operation amplifier circuit to obtain the small voltage signal for amplification and processing, not suitable for large-scale application.In addition, the resistance value of the original CMPU will increase with time in the use process, which is extremely unstable, CMPU must be properly handled.When the CMPU underwent overcompression and hot drying, the cracks of the conductive coating on the skeleton increased and the crack degree increased, accompanied by a small amount of fracture of the skeleton.The overall resistance value increased significantly and tended to be stable.The typical initial resistance and terminal resistance measured were 1.89 kΩ and 1.4 Ω, respectively.Finally, the initial resistance and termination resistance of the typical PVA/CaCl 2 @CMPU modified by the conductive gel are 1.56 kΩ and 1.7 Ω, respectively.
S is used to characterize the stress sensitivity of the sensor.The equations are as follows: Where ΔR is the change in the sensor resistance value, R 0 is the initial resistance value of the sensor, ΔP is the change in pressure.
The CMPU without gel modification was selected to make the sensor and PVA/CaCl 2 @CMPU sensor for comparison, and the performance graphs are shown in figures 6(b) and (c), respectively.It can be seen that after gel modification, the sensor sensitivity slightly decreases and the maximum stress detection limit increases significantly, which is about 40 kPa.In the range of 0 ∼ 6 kPa, the sensor sensitivity is 0.146 kPa −1 , and the correlation coefficient R 2 of linear fitting is 0.985.In the range of 6 ∼ 11 kPa, the sensitivity was 0.398 kPa −1 , and the R 2 is 0.989.In the range of 11 ∼ 40 kPa, the conductive skeleton of the sensor is gradually fully contacted, and its resistance value changes very little, resulting in a small sensitivity of 0.0034 kPa −1 , and the R 2 is 0.736.After gel modification, the linearity of each stage of the sensor increases significantly, and the main causes are analyzed as follows.After the conductive hydrogel is modified with porous sponge, the pressure distribution in the sensor can be uniform due to the softness and elasticity of the hydrogel, the local stress concentration can be reduced, and the pressure exerted on the sensor can be better dispersed and transmitted.The gel has a certain strengthening effect on the structure of porous sponge, enhances the stability of the structure, reduces the  nonlinear deformation caused by pressure, and slightly reduces the sensitivity to pressure.The gel fills part of the pores in the flexible porous material, reduces the influence of the pores on the conductive network, provides more conductive paths during the sensor strain process, and can make the conductive network more uniform.Especially at medium compression, the protruding gel portion provides conductive contact ahead of time before the skeleton enters large area contact.In short, through reasonable gel modification, the random error of the sensor can be reduced, the stability and linearity of the output can be improved, and the range can be improved.
Figure 6(d) shows the volt-ampere characteristic curves of the sensor with different compression degrees in the ±1 V power supply range, and its voltage and current show good linear correlation.It shows that the sensor has good ohm connectivity and excellent static pressure stability.
The initial equivalent conductivity of CMPU is about 5.291 × 10 -2 S m −1 , and it increases sharply in the process of large compression, while the conductivity of gel is relatively low, about 1.2 × 10 -3 S m −1 .Since the conductivity of CMPU and gel is not of the same order of magnitude, and the volume fraction of gel is also very small, the effect of gel modification on the overall resistance of the sensor is not great.With the increase of the stress range, the sensitivity of the gel-modified sensor decreases slightly.
As shown in figure 7, at the compression/release speed of 60 mm s −1 , the response time of the sensor is about 133 ms and the recovery time is about 150 ms when the compression degree is 80%.When the load is withdrawn, the sensor resistance value can quickly return to the initial value.Figure 8 shows the sensor output waveform of 1000 working cycles under 60% compression.It can be seen that the output amplitude remains stable during the working period, and the waveform of multiple cycles is repeated well, indicating that the sensor has good stability and long service life.

Typical application
The CMPU sensors can be used for information encryption.As shown in figure 9(a), when the sensor is fixed on the index finger of the human right hand, the bending motion of the finger joint causes the sensor to deform, resulting in a change in resistance, and eventually produce a different pulse level signal.A continuous high level represents a line in Morse code, and the resulting short spike level represents a point in Morse code.As shown in figure 9(b), this combination of dots and lines can represent 26 different English letters.As shown in figures 9(c), (d), regular finger movements can alternately produce sustained high levels and spike high levels, sending out word signals representing 'IOP', 'MRX', which can be used for information exchange between special populations.
The sensor is also suitable for measuring continuous signals of various actions.As shown in figure 10(a), mount the sensor at the position of the human throat and pronounce 'Shaanxi' twice at intervals, by observing the output of the sensor, it can be found that the two signals have veyr similar characteristics, indicating that the sensor can be used for speech signal recognition to a certain extent.Figure 10(b) shows the response of the sensor to the placement and removal of a grain of rice, and its stress is estimated to be 20 Pa.These weak signals are mainly identified by the crack propagation effect of sensors in linear region.Figure 10(c) shows the response of the sensor mounted on the left mouse button to stand-alone and double-click signals.Several flexible pressure sensors similar to those in this study are listed and their performance is compared, as shown in table 2. Through comparison, it is found that the developed sensor has good sensitivity, wide working range and good response performance.

Conclusions
This study provides a low-cost and simple method to fabricate flexible pressure strain sensors.This study provides a low-cost and simple method for preparing flexible pressure strain sensors.The range of the sensor is 0 ∼ 40 kPa, and a sensitivity of 0.398 kPa −1 in the range of 6 ∼ 11 kPa.At a compression level of 80% and a speed of 60 mm s −1 , the response/recovery time of the sensor is approximately 130/150 ms.In the subsequent compression and release experiments, the sensor shows excellent cyclic stability, and the prepared sensor has certain application value.The results show that when the sponge structure is deformed greatly, the structure of the conductive gel attached to the foam web of the conductive sponge will also undergo secondary deformation at the microscopic level.This change is conducive to improving the linearity of the stress sensor and enlarging the measurement range of stress.

3. 1 .
Sensor characterizationThe surface morphology of CMPU was observed with an optical microscope (DM2700, Leica, Germany), as shown in figure3(a).It can be observed that the skeleton surface of the original CMPU sample is smooth and clean with metallic luster.As shown in figure 3(b), there are a few microcracks in the enlarged skeleton diagram.As shown in figure 3(c), more cracks and larger cracks appeared in the CMPU skeleton after aging treatment, which would lead to the increase of the overall resistance value of the sample.As shown in figure 3(d), the resulting sample conductive skeleton is coated with conductive gel, namely PVA/CaCl 2 @CMPU.

Figure 3 .
Figure 3. (a) Original CMPU optical image; (b) Microcracks in the conductive skeleton; (c) The degree and number of cracks increased after aging treatment; (d) Gel coated skeleton.

Figure 4 .
Figure 4. Stress-strain curves of PVA modified sensors with different mass fractions.

Figure 5 .
Figure 5. (a) Circuit schematic diagram; (b) The sensor does not apply pressure to make the LED glow weakly; (c) The sensor applies pressure to make the LED glow brightly.
Figure 10(d) shows the response of the sensor attached to the elbow of the human body to the extension-contraction stroke of the arm.The researchers cut polyvinyl chloride (PVC) foam into a thick insole.With reference to the typical distribution of human plantar pressure in reference [23-26], hollow out the part shown in figure 11(a).Eight sensors with similar sensing properties were selected and embedded into eight holes, and the measuring circuit was connected to complete the sealing.The schematic diagram of the measurement system is shown in figure 11(b).The circuit is powered by a mobile constant voltage source with a nominal voltage of 5 V.A microcontroller unit (MCU) with built-in analog-to-digital conversion (ADC) is responsible for collecting the voltage signal of each sensor and transmitting it wirelessly through Bluetooth module.The upper computer calculates and processes the collected signals through the Bluetooth receiver to recognize the motion state of the foot.Figure 11(c) is the installation schematic diagram, and figure 11(d) is the voltage time domain signal output generated by the 8-channel sensor when the human body is walking.Figure 11(e) shows the heel and toes of the

Figure 7 .
Figure 7. Response/recovery time of the sensor.

Figure 8 .
Figure 8. Stability test of the sensor at 60% compression for 1000 cycles.

Figure 10 .
Figure 10.(a) Pronunciation 'Shaanxi'; (b) Response to the placement of a rice; (c) Response to mouse clicks; (d) Monitoring of arm extension and contraction.

Figure 11 .
Figure 11.(a) The distribution of sensors; (b) Schematic diagram of signal measurement system; (c) Sensor installation diagram; (d) Time domain signal of voltage output by 8 sensors; (e) Characterization of sensor output signals when the heel and toe of the right foot are raised, and the right foot is tilted inward and outward, respectively; (f) Motion state recognition based on flexible pressure sensor and machine learning.

Table 1 .
Quality distribution of ingredients in dipping solution.

Table 2 .
Compared with the performance of related devices.