Low delay flexible paper-based electrode for capacitive sensor filled through carbon-based materials

Paper-based sensors have good application prospects due to their flexibility and environmental protection. However, paper-based materials are difficult to balance both conductivity and flexibility. This paper mainly reports a high porosity conductive paper based on bamboo wood mixed fiber materials through the wet-manufacture method. The crucial material properties, and long-chain structure of PEDOT:PSS, play an important role in conductivity and flexibility. The relevant experiments show that conductive paper has excellent physical properties and electrical signal transmission capacity. Among them, the conductivity of paper-based electrodes is up to 18.52 S/cm, and there is a low delay in the transmission of the electric signal, when the signal frequency is 300 kHz, the phase frequency angle changes by only 4.9°, not only that, as an electrode, the human pulse signal measured in the capacitive sensor is normal. Under the action of other composite materials, the manufactured electronic paper has a certain mechanical strength, and the thickness is about 90 μm, lower than ordinary paper products. The research shows that the paper-based electrode has the advantages of high conductivity, low hysteresis, light, and thinness, which provides the possibility to be used as a flexible electrode.


Introduction
Owing to the demand for flexible electronic products such as wearable devices (Hou et al. 2022), motion sensors, and flexible supercapacitors, the exploration of electrode materials with high flexibility, low hysteresis, and high conductivity has attracted extensive attention (Dong et al. 2016;Dong et al. 2017;Lin et al. 2016;Lv et al. 2016;Wang et al. 2022;Wu et al. 2016;Yun et al. 2010;Zhu et al. 2019). Electrode materials can be processed into paper-like conductive sheets by using appropriate elastic polymers (such as polydimethylsiloxane (PDMS)) (Wang et al. 2011). The conductive filler is also embedded or chemically bonded to the rubber polymer, making it have excellent electrical conductivity and mechanical tensile properties. (Zhang et al. 2010). But with the rapid development of the electronic industry, the life of electronic products has been much shorter than before. A large amount of e-iste generated every year leads to serious environmental problems, such as sewage discharge, ozone depletion, and climate change. To improve the environmental damage caused by the waste of electronic products, as a flexible, cheap, ubiquitous, and environment-friendly material, the cellulose-rich paper is as low as 10 Ω/sq (Hu et al. 2009). To prepare paper electrodes, carbon nanotubes are also loaded on the paper surface by immersion method and compounded with other conductive material layers, such as polyaniline, manganese dioxide, or nanosilver wire (Choi et al. 2017;Sumboja et al. 2013). In addition, the unique optical properties of cellulose, such as transparency, lead to its emerging applications in flexible displays, which is also an important reason for its use as a carrier of conductive materials (Gao et al. 2013;Zhang et al. 2017). Tin-doped indium oxide, carbon nanotubes, or silver nanowires are deposited on nanocrystalline cellulose (NCC) based films, and transparent conductive nano-paper with surface resistance of 12 Ω/sq and total transmittance of 65% is prepared (Hu et al. 2013). Nonetheless, due to the large fiber and pore size of CNTs, poor dispersion on ordinary solvents and carriers, and poor bonding stability, their retention and stability on paper substrates are usually low, which seriously hinders their further application . Microcrystalline cellulose (MCC) is often used in experiments, due to its large surface area and nano network structure, MCC is often used as the carrier of conductive materials to prepare paper-based electrodes (Hou et al. 2014;Wang et al. 2009).
Bamboo and wood fibers, as two kinds of cellulose fibers that grow naturally and obtain a large amount, have a large number of irregular voids inside, resulting in a large number of irregular pores when the fibers are combined and arranged. These natural pores have super hydrophilic properties and can accommodate more water, simultaneously, the internal natural pore structure can better carry conductive materials dominated by carbon-based materials, moreover, bamboo wood fiber is a kind of natural plant cellulose with both health and environmental protection, it has no pollution in the use process. After use, it can also be destroyed by incineration, which solves the environmental problems caused by the massive abandonment of metal and electronic products. However, due to the problems of high resistance, short service life, and high signal transmission delay, paper-based materials still have great challenges as sensor electrodes or conductive materials (Lin et al. 2016).
In this paper, flexible and strong paper-based substrates are prepared from different kinds of cellulose fibers and loaded with CNT and superconducting carbon black (SPC) to prepare paper electrodes. We use paper-based porous structures to uniformly load and stabilize CNT and SPC to improve their conductivity and durability. To further improve the dispersion and stability of CNT on the paper-based surface, a small amount of MCC is added to help disperse carbon nanotubes. Not only that, the conductive paper is modified with polyacrylamide (PAM) to make it have better physical properties. Simultaneously, glycerol (Gl) is added to ensure the smooth surface of the conductive paper to make it meet the application standard. The conductivity and physical properties of conductive paper with different fiber substrates before and after adding PEDOT:PSS are studied and discussed, and the possibility of the paper-based electrode as a flexible capacitance sensor electrode is discussed. were used as experimental materials. All the materials were used without further purification.

Materials
Fabrication and properties of the conductive paper As shown in Fig. 1, soaked wood or bamboo fiber materials in a beaker for 6 h and then put them into a beater for grinding for 20 min. Drained the ground fiber materials and put them into an oven for drying. Put 200 mg of fiber material into a beaker containing 20 ml of deionized water, stirred at 70 °C at 1600 rpm on a magnetic stirrer, added 10 ml of CNT, PEDOT:PSS, and 100 mg of SPC into the beaker, continue to stir for 30 min, added 50 mg of MCC to the beaker for filling, and added 200 µl Gl and a small amount of PAM were used as binders to improve the physical properties of materials.
After mixing, poured the fiber suspension in the beaker into the mold and let it stand for 10 min, next put it into the ultrasonic cleaner to remove the bubbles, then put it into the vacuum box to remove the air repeatedly for three times, and finally placed the mold in the drying box at 80 °C and dried it for 1.5 h to obtain the finished product.
The conductive paper prepared according to the experimental conditions had a length of 80 mm, a width of 50 mm, and a thickness of 90 μm. The mass was 300 mg weighed by an electronic balance. By measuring the average resistance at both ends of the conductive paper to be 9.5 Ω, the square resistance calculated by the formula was 6 Ω/sq, and the calculated conductivity was 18.52 S/cm.

Characterization
Conductive papers after manufacturing molding, the basic appearance parameters of conductive papers measurement, through the vernier caliper and screw micrometer measurement of the length, width, and thickness of conductive papers data, through the digital multimeter (Tektronix, DMM4020 5 − 1/2, American) measurement of conductive papers resistance, through the four-probe test method to measure and calculate the square resistance of conductive papers. The conductivity of the conductive papers was calculated from the measured thickness and square resistance of the conductive papers.
Scanning electron microscopy (SEM, Gemini360, ZEISS, Germany) was used to observe the surface characteristics of different conductive papers at EHT = 20 kV and WD = 12 mm.
Fourier transform infrared (FTIR) spectrometer (Nicolet iS50, Thermo Scientific, USA) equipped with an attenuated total reflectance (ATR) was used to measure the molecular groups inside different conductive papers at 500-4000 cm − 1 with a resolution of 2 cm − 1 , and the specific components were proved by specific wave peak analysis.
The phase diagram and surface characteristics of conductive papers were observed by atomic force microscopy (AFM, Dimension® Icon™, Bruker Dimension, Germany). The probe was a silicon probe with a resonance frequency of 331.179 kHz. The scanning range was 10 × 10 μm, 1 × 10 μm, and the scanning frequency was 0.5 Hz.

Flexibility and service life test
In the flexibility test, the conductive papers were cut to 10 × 50 mm, and the flexibility of the conductive papers was proved through the winding, twisting, and bending experiments of the conductive papers. In the service life test, conductive papers were cut to 30 × 30 mm, reciprocating bending 1000 times, and recorded resistance changes every 100 times. Calculated the resistance change rate of conductive papers: where R and R 0 represented the resistance before and after the experiment, and all tests were repeated five times and averaged.

Hydrophilicity test
In the hydrophilicity test, the conductive papers were cut to 10 × 10 mm and carried out with a contact angle measuring instrument (KRÜSS, Drop Shape Analyzer-DSA25, Germany) in droplet mode with a single drop of 6 µl. The hydrophilicity of the conductive papers was proved by observing the morphological changes of water drop on the conductive papers with the video recording function of the contact angle measuring instrument, the hydrophilicity test was repeated three times and averaged.

Electrical signal analysis
The experiment of electrical signal analysis was carried out by LCR (Gwinstek, LCR − 6300, China) to measure the relationship between phase angle and frequency of conductive papers (10 Hz, 100 Hz, 1 kHz, 10 kHz, 50 kHz, 100 kHz, 300 kHz), the electrical signal analysis test was repeated four times and averaged.

Real-life application
In this experiment, the conductive paper was used as the electrode to prepare the sensor, and the capacitance sensor was fabricated with a "sandwich" structure. PDMS with a thickness of 100 μm was added between the two conductive paper electrodes (PDMS and curing agent were mixed at a volume ratio of 10:1 and heated to cure the sections), and the copper foil was connected to the surface of the conductive paper to form a conductive path for measurement, finally, the sensor was packaged by PDMS. The capacitance sensor was placed on the wrist of the volunteer with medical scotch tape (The experiments involving human subjects have been performed with the full, informed consent of the volunteers), and the copper foil wire was connected to the LCR (Gwinstek, LCR-6300, China). The capacitance value was measured under the condition of 1 V voltage and 100 kHz frequency, the real-life application test was repeated three times and averaged. Table 1 shows the content of components in the conductive papers manufactured according to different ratios. To verify whether the composition of conductive paper fiber substrate material has an impact on the physical properties of conductive papers, three kinds of fiber materials are selected as the substrate under the condition that the qualities of CNT, SPC, Gl, and MCC are guaranteed. They are the wood fiber of code 1, the bamboo fiber of code 2, and the bamboo wood fiber of code 3. Furthermore, to verify whether PEDOT: PSS has an impact on the physical properties and electrical signal transmission capacity of conductive papers, ensure that the selected fiber substrate and other additive components are certain, and conductive papers with three different substrate materials are prepared. Code 4 is compared with code 1, code 5 is compared with code 2, and code 6 is compared with code 3. The above control group is selected to verify the influence of fiber substrate materials and PEDOT:PSS on the physical properties and electrical signal transmission capacity of conductive papers.

Results and discussion
Characterization of conductive papers Figure 2 shows the effect of different base materials on the characterization of the finished product. (a), (b), and (c) in Fig. 2 are SEM characterization of conductive paper code 1, code 2, and code 3 respectively. Different fiber materials make the conductive paper have different surface structures to a certain extent. The fiber length of wood fiber material is about 50-60 μm, and the fiber diameter is about 3 μm, while CNT and SPC can form conductive paths, there are many agglomerates of different sizes (circle in Fig. 2a), which will lead to uneven surfaces and uneven thickness after the manufacture of conductive papers. In addition, too many agglomerates of different sizes will lead to problems of uneven conductive resistance and unstable mechanical properties in the physical properties of conductive paper. In Fig. 2b, the number of agglomerates has been significantly reduced, which is mainly due to the physical properties of bamboo fiber material. The fiber length of bamboo fiber is about 85-95 μm, and the fiber diameter is about 5 μm. In terms of bendable flexibility, the bamboo fiber material is better than wood fiber material, which also explains why the number of clusters in Fig. 2b is reduced compared with Fig. 2a. In Fig. 2c, the number and size of agglomerates are significantly reduced. The main reason is that the longer, thicker and softer bamboo fiber materials are mixed with CNT to form a larger conductive path, while the shorter, thinner and harder wood fiber materials are mixed with SPC, mixed and filled into the structure of bamboo fiber conductive path under the action of Gl and PAM adhesive, the problems of the large number and large size of agglomerates are solved, the conductive paths are relatively stable, the surfaces of the produced conductive papers sample are flat and smooth, the conductive effect is excellent. Figure 3 shows the effect of different base materials on the characterization of finished products after adding PEDOT:PSS materials. With the material of the substrate changing from wood fiber, bamboo fiber to bamboo wood mixed fiber, the agglomerate changed from ellipse to harden. Figure 3a, b, and c show the SEM characterizations of conductive paper code 4, code5, and code 6, due to the long chain molecular structure of PEDOT:PSS, the number of fiber agglomerates have been greatly reduced. However, also due to the physical and chemical characteristics of PEDOT:PSS, some plant fibers form aggregation and hardening (Fig. 3a). In Fig. 3b, the aggregation and hardening size of medium conductive paper is significantly reduced, this is due to the longer fiber length and larger fiber diameter of bamboo fiber compared with wood fiber, making the connection between fibers more compact when forming conductive paths, it can significantly increase the physical strength of conductive papers. In Fig. 3c, the medium conductive paper is composed of materials based on bamboo and wood fibers, the surface is smooth and flat, combine with the conductive paper in Fig. 2c, it is not difficult to see that compared with a single fiber material, composite fiber materials have excellent effects in reducing the generation of agglomerates, the anisotropy of physical properties of conductive papers is reduced to a certain extent.
In addition to SEM, AFM is used to observe the surface characteristics of the prepared conductive paper. Figure 4a, b, c are the surface phase diagrams of code 1, code 2, and code 3 conductive paper, and (d), (e), and (f) are the surface phase diagrams of code 4, code 5, and code 6 conductive paper. When other conditions remain unchanged, PEDOT:PSS is added to increase the color of the conductive paper phase diagram. Meanwhile, in the measurement of the micro surface roughness shown in Fig. 5, from the comparison that the roughness of the other two kinds of conductive paper substrates, except the conductive paper with bamboo fiber as the substrate, has decreased to a certain extent after adding PEDOT:PSS, which is mainly due to the selection of fiber materials, in addition to the characteristics conductive papers (a-c), and dimensional data diagram of fiber agglomerates of code 4-6 conductive papers (d) of the adhesive of the PEDOT:PSS long-chain polymer itself. From the SEM in Figs. 2 and 3, the fiber materials are stacked in three-dimensional space, as a result, some fibers are arranged in the vertical direction. As a result of the large length of bamboo fiber materials, the surface roughness of conductive papers is increased to a certain extent.
In the FT-IR, through observation and analysis of the data in Fig. 6, obvious characteristic peaks can be seen at 700-800 cm − 1 of code 4, code 5, and code 6, which are mainly caused by the stretching of the C-H bond of PSS in PEDOT:PSS. At the same time, the C = C correlation at 1400-1500 cm − 1 can distinguish PEDOT. In the 3000-4000 cm − 1 range, the data measured with bamboo fiber as the base material is different from the other four groups of materials containing wood fiber material base, resulting in a lower light transmittance of conductive papers with bamboo fiber as the base than that with wood fiber as the base.
Flexibility test of the flexible conductive paper In Fig. 7, the resistance changes as the number of windings increases, the maximum resistance change of code 1 wood fiber conductive paper is 23%, this is mainly because the length of wood fiber is shorter than that of bamboo fiber (Fig. 2 a), during the winding process, the conductive path with wood fiber as the frame breaks, which affects the resistance changes. Meanwhile, the resistance change rate of conductive papers based on wood fiber is close to linear with the increase in the number of winding turns. In Fig. 7b, the linearity of the resistance change rate of wood fiber is numerically better than that of bamboo fiber and conductive paper based on bamboo and wood fiber. Moreover, the resistance change of code 3 conductive paper during winding is significantly better than that of single fiber conductive paper and wood fiber conductive paper added with PEDOT:PSS, this is because the bamboo fiber and wood fiber are mixed and wound, enabling CNT and SPC to cover the surface to form a conductive path better, the loss of conductive path is less in the winding process, the physical structure plays an important role in flexibility testing.
Meanwhile, the resistance change rate of the conductive papers with PEDOT:PSS is less than that without PEDOT:PSS, this is due to the function of PEDOT:PSS as the binder and conductive agent, water-soluble PEDOT:PSS long molecular ring is added to the suspension, tightly adsorbed between the fibers, it plays the role of both binder and conductive path, for the above reasons make the conductive paper reduce the possibility of damage to the conductive path during winding (Fig. 7c). But also because of the addition of PEDOT:PSS, the wood fiber conductive paper will no longer have the linear ability of resistance change.
In Fig. 8, as the bending angle of conductive paper increases, the change of resistance also increases, the overall trend is that the conductive paper based on a single wood fiber has the worst physical properties in the torsion experiment, the conductive paper based on bamboo wood mixed fiber has better torsional physical properties when other added materials remain unchanged, more than this, the conductive paper with PEDOT:PSS has better torsional physical properties than the conductive paper without PEDOT:PSS. From the data, the resistance of code 1 conductive paper increases to about 27% of the original resistance when it is rotated 1080°, and the resistance of code 4 conductive paper added with PEDOT:PSS increases to 18% of the original resistance. Among them, the resistance change of code 6 conductive paper with the best performance is only 10% when it is twisted by 1080°, it is about 1/3 of the resistance change of code 1 conductive paper.
In the ideal state, with the change of torsion angle, the ratio of resistance change to original resistance is 0, however, the internal conductive principle of conductive papers is due to the conductive path built by filling the fiber structure with conductive materials, this causes the conductive fiber path to be twisted and damaged during the torsion of the conductive paper, reducing the number of conductive paths, the reduction of the conductive path will reduce the electron throughput per unit time and increase the resistance. The resistance changes of code 4, code 5, and code 6 conductive paper with PEDOT:PSS are less than that without PEDOT:PSS, the main reason is that PEDOT:PSS plays the role of binder and conductive materials in the conductive paper while improving the physical properties of conductive papers, the number of conductive paths is increased, reduced the damage to the conductive path in the process of torsion, so that the resistance changes of conductive papers does not exceed 18%.
As the bending angle of the conductive papers increases, the resistance changes of the conductive papers also increase (Fig. 9). With the increase of Fig. 7 Resistance changes data diagram of conductive papers winding on a rod with a radius of 1.8 mm (a), the linearity analysis diagram of conductive papers winding data (b), and schematic diagram of the change of conductive path before and after the conductive papers winding experiment (c) bending angle, the resistance change of medium code 1 wood fiber conductive paper is close to a straight line, which the linearity is good. In contrast, the conductive paper based on bamboo fiber and bamboo wood mixed fiber has poor linearity, because of the short length of the wood fiber, the conductive fiber in the conductive paper will be damaged with the increase of the bending angle during the bending process. The damage degree is related to the bending angle, and the two are approximately proportional to each other. As in the bending experiment (Fig. 9), the resistance change of code 1 conductive paper is the largest when bending, and the resistance change by about 14%, compared with code 1 conductive paper, the resistance change of code 2 conductive paper and code 3 conductive paper is reduced in varying degrees, this is mainly due to the longer fiber length and larger fiber diameter of bamboo fiber so that the conductive paper based on bamboo fiber will not have the damage law similar to that wood fiber conductive paper in the bending process, nonetheless, this leads to unpredictable changes in the bending process of conductive papers based on bamboo fiber, adding wood fiber as a mixture can effectively improve this situation. Therefore, the resistance transformation curve of conductive paper based on code 3 bamboo wood fiber is better than that of conductive paper based on code 2 bamboo fiber.
After adding PEDOT:PSS, the flexibility of conductive papers with different fiber materials is significantly improved. After the conductive paper of code 4, code 5, and code 6 materials are bent for 180°, the change of resistance reduce to 4%, 5%, and 4.9% respectively, compared with the proportion without PEDOT:PSS, it is reduced by about 3-4% points, it is proved that PEDOT:PSS plays a role in it. As the binder, it reduces the damage of the conductive path to a certain extent, so that the resistance changes of conductive papers with the increase of bending angle are less than that of conductive papers without PEDOT:PSS. Figure 10 shows the resistance stability of conductive papers. With the increase in the number of test cycles, the change of resistance gradually increases. In Fig. 10, the differences are particularly obvious after 500 cycles of conductive papers without PEDOT:PSS, which does not form a linear distribution and has low linearity, this is because part of the conductive path has been nearly destroyed after multiple cycles inside the fiber, which makes the service life of the conductive paper low, to increase the service life of conductive papers, other substances need to be added for improvement. The conductive paper added with PEDOT:PSS has better performance in the service life test. Among them, the resistance of code 6 conductive paper increases by only about 12% after 1000 times of reciprocating 90° bending, which is far lower than the bamboo wood mixed material samples without PEDOT:PSS, as well as other wood fiber base conductive paper and bamboo fiber base conductive paper added with PEDOT:PSS. Nonetheless, a single wood fiber or bamboo fiber has its limitations as the base of conductive papers, in which the longer bamboo fiber is mixed with the shorter wood fiber to form a fiber frame, the physical structure formed after filling PEDOT:PSS used as the binder and conductive material improves the mechanical properties and service life of conductive papers and ensures that they can well reduce the bending strain of multiple bending and reduce the fracture and damage of fiber during bending. Concurrently, the chemical effect of PEDOT:PSS molecular long-chain structure adsorbed with CNT and SPC makes the conductive paper maintain lasting electrical continuity. Figure 11 shows the relationship between different conductive paper materials and the water absorption rate of conductive papers, the contact angles between the water droplet and the conductive paper when the water droplet of code 1, 2 and 3 conductive papers just drip are 31.5°, 46.4°, and 35.9°, it shows that the surface of conductive papers based on bamboo and wood fiber is hydrophilic, but the strength of hydrophilic water absorption varies due to different fibers. In addition, the contact angles of code 4, code 5, and code 6 conductive paper with PEDOT:PSS added are 39.6°, 34.9°, and 47.7° respectively, although watersoluble PEDOT:PSS is added, the droplet contact angle does not decrease when the droplet is just in contact, this is because the manufacturing process is prepared by suspension deposition, which makes the surfaces on both sides of the conductive paper form a phenomenon similar to smooth hardening, while there are pits in varying degrees on the surface of the ordinary conductive paper without PEDOT:PSS to better absorb water droplet. Figure 12 shows the changes trend of the contact angle between water droplet and paper in the hydrophilicity test of conductive papers, and Fig. 12a shows the overall test histogram of conductive papers. In the conductive paper without PEDOT:PSS, the water drop contact angle of code 1 and code 3 conductive paper based on wood fiber and bamboo wood fiber is lower than that of code 2 bamboo fiber conductive paper, the surface bonding degree of paperbased structure built by wood fiber and bamboo wood fiber is not as close as bamboo fiber, so the contact angle of water drop contact is small. Figure 12b is the broken line trend diagram of the water drop contact angle. In Fig. 12b, the surface water absorption effect of wood fiber conductive paper is better than the other two kinds of conductive papers when water drops into contact, after adding PEDOT:PSS, the surface water absorption of bamboo fiber conductive papers is better than the other two. This is because PEDOT:PSS will bond and modify the surface of shorter wood fiber, resulting in the hardening of wood fiber and bamboo wood fiber and weakening of surface water absorption capacity. Figure 12c and d are broken line trend charts of water droplet dripping on the surface of conductive papers for 10 and 20 s. From the broken line, the water absorption capacity of bamboo fiber conductive paper after adding PEDOT:PSS is better than before adding, from the overall trend, the longer length of bamboo fiber makes the internal Fig. 11 Schematic diagram of water drop contact angle in hydrophilicity experiment of conductive papers pores of paper-based structure less, and the anisotropy of bamboo fiber makes the fiber itself absorb water faster than wood fiber. It can be seen from the broken line trend in Fig. 12d that the water droplet of the three kinds of conductive papers added with PEDOT:PSS is fully absorbed 20 s after the experiment, which is due to the long-chain molecular structure of PEDOT:PSS. After the fiber and pore absorb water, the molecular ring of PEDOT:PSS locks H 2 O molecules to a certain extent, increases water absorption, and improves the hydrophilicity of conductive papers. Figure 13a shows the relationship between different conductive paper materials and the change of water drop contact angle. At the moment of contact, the conductive paper absorbs a small amount of water, which is contacted and absorbed by the surface of the paper-based. Concurrently, the greater the angle change, the better the internal water absorption of the conductive paper. The wood fiber paper without PEDOT:PSS has the least water absorption, and the bamboo fiber starts to absorb water slowly, but the overall absorption is more, which is not only the pore structure inside the conductive paper but also because the bamboo fiber itself is porous, the pores of the fiber itself absorb some water, resulting in a large change in the contact angle. Fig. 12 Data diagram of 0-20 s contact angle variation in the hydrophilic experiment of conductive papers (a), contact angle data diagram of water droplet contacting the surface of con-ductive papers at 0 s (b), contact angle data diagram of conductive papers surface after 10 s (c), and contact angle data diagram of conductive papers surface after 20 s (d) Figure 13b reflects the relationship between different conductive paper materials and the resistance ratio before and after the hydrophilicity test, the grid histogram is the resistance value before the hydrophilicity test of conductive papers, and the gray histogram is the resistance of water inside the conductive paper after the hydrophilicity test. After adding deionized water with extremely poor conductivity, the resistance change rate of conductive papers from code 1 conductive paper to code 6 conductive paper, resistance change rate is getting lower and lower, proving that the effect of deionized water on conductive paper is low. The electronic change rate of the conductive paper added with PEDOT:PSS has little change after contacting water droplet, the minimum is only about 2.8%, and there is no significant change in the conductivity of the conductive paper. To sum up, the structure of conductive papers and their internal substances are conducive to full contact with the hydrophilic electrolytes and provide the possibility of using human paper-based electrodes. Figure 14 shows the relationship between different conductive paper materials and electrical signal hysteresis. The overall trend is shown in Fig. 14c. The phase frequency angle characteristic diagram shown in Fig. 14c is the unique phase-frequency characteristic of the circuit in Fig. 14c-i, which shows that there are capacitors and inductors in the conductive paper. The larger the amplitude of the phase frequency angle, the more capacitors and inductors exist, and the more hysteresis in the process of electrical signal transmission. Among them, the phase frequency angle of code 1 conductive paper changes the most on the whole. At 300 kHz, the phase frequency angle changes by about 6.5°, mainly due to the short length of wood fiber and the ease to become fiber agglomerates. From the SEM (Fig. 2a), fiber cluster winding and fiber aggregation and hardening are generated due to the random arrangement of fibers in the manufacturing process of code 1 conductive paper based on wood fibers, which makes similar fiber clusters form more capacitance, in addition, the fiber clusters have winding conductive fibers that form inductive elements (Fig. 14b).

Electrical signal analysis of conductive papers
From the numerical point of view, the change of phase frequency angle of code 6 conductive paper is the smallest with the change of frequency, The main reason is that the bamboo wood fiber is used as the base, and PEDOT:PSS as the binder is added, which makes the conductive paper of bamboo wood fiber reduce the fiber agglomeration and fiber hardening, and greatly reduce the additional capacitance and inductance of fiber agglomeration and aggregation hardening. The phase frequency angle hysteresis of code 6 conductive paper is about 0.014° at the frequency of 1 kHz and only about 4.9° at the frequency of 300 kHz. In the same period, the hysteresis change of phase frequency angle is about 5.86%. The above data show the characteristics of low delay and low hysteresis of conductive papers, which shows that code 6 conductive paper is suitable to be used as the electrode for transmitting signals, which proves that the conductive paper in this study can become the electrode of the capacitive sensor.

Application of paper-based electrode in the sensor
Pulse beat information is an important physiological index in the clinical examination of cardiovascular diseases, and it is often concerned by people in daily life, which has important clinical application value. The paper-based electrode in this study can transmit electrical signals well and has a good performance in the service life test. It is possible to be used as the electrode of the capacitive sensor. Manufacturing from the "sandwich" flexible capacitive pressure sensor is prepared by the self-packaging method. The PDMS is used as the intermediate dielectric layer, the conductive paper is installed on both sides of the PDMS as the sensor electrode, and the conductive paper electrode is connected with the copper foil wire. Finally, the overall sensor is packaged with PDMS (Fig. 15a).
Fix the sensor on the wrist and connect both ends of the sensor to the LCR tester, as shown in Fig. 15b (The experiments involving human subjects have been performed with the full, informed consent of the volunteers). Because the paper-based capacitance sensor is flexible, it can not only sense the direct impact of the surface but also feel the bending force. The principle is that the distance between the two electrodes of the capacitance sensor decreases with the increase of external force and the capacitance value increases. To verify that the sensor is a promising wearable device, place the sensor at the wrist, connect the LCR tester at both ends of the wire, observe the value change on the LCR screen, collect data after the value is stable, and obtain the pulse beat test data, The data curve in Fig. 15c can be obtained by sorting the data.
In Fig. 15c, the sensor has obvious signal characteristics, and the peak value of the red curve is the pulse beat signal. From the peak shape and capacitance, the pulse beating has a frequency and can be observed. With the weakening of the peak value, the pulse beating stops, the sensor itself recovers, the distance between electrodes increases gradually, and the capacitance value returns to the original situation. The stable part in the middle of the peak value is the interval time between pulse beating cycles. At this time, the pulse beating is weak, afterwards, the sensor has no signal and the skin is relaxed. Due to external irresistible factors, the sensor has interference, but on the whole, the trend of the signal is correct. In Fig. 15c, the subject's pulse beat 6 times within 5 s, and the average pulse beat peak interval is 0.811 s. Through calculation, the subject's pulse beat is 74 times in 1 min, while the pulse beats of normal men are 60-100 times/min. according to the experimental data, it is in line with the normal expectation, which proves that the sensor can work normally and the possibility of conductive papers as the electrode of the capacitive sensor. Table 2 shows the comparison of conductivity, hysteresis, and thickness of conductive papers in the previous literature with that in this study. It can be seen that the conductivity of the conductive paper in this study increases and the thickness of the paper decreases compared with the study of (Jabbour et al.2012), the conductivity of the conductive paper in this study is almost ten times that of the conductive paper studied by (Fugetsu et al.2008). Meanwhile, the average thickness of the conductive paper in this study is about half that of the conductive paper studied by (Imai et al. 2010). The hysteresis rate of the conductive paper studied by (Huang et al. 2020) on the transmission of electrical signals is 8.57%, and the hysteresis rate of the conductive paper in this study is about 3% lower than that of the conductive paper studied by (Huang et al. 2020).

Comparison of experimental data
The conductivity increases with the increase of conductive materials in the conductive paper. The conductivity of this paper has been greatly improved to a certain extent. For the thickness of conductive paper, it depends on the method of manufacturing the conductive paper. In this paper, wet deposition is used, which makes the thickness of conductive papers increase compared with that prepared by paper machine. The hysteresis rate is mainly related to the thickness of conductive papers. It is easy to see that the thicker the paper, the higher the hysteresis rate. Changing the proportion and manufacturing method of conductive paper can directly affect the properties of conductive papers.

Conclusion
In this paper, CNT, SPC, PEDOT:PSS, PAM, and other materials are used to investigate the flexibility and conductivity of paper-based materials. While providing conductive paths, the physical properties of conductive papers are improved through covalent bonds and other adsorption relationships. Among them, the conductivity of the conductive paper is 18.52 S/cm. After winding the rod with a radius of 1.8 mm 3 times, the resistance change is less than 10% of the total resistance. Under the same conditions, the resistance change rate is only about 3% after bending 180°, 12% after bending 90° 1000 times, and 12% after twisting 1080°. In terms of physical properties, the hydrophilicity of the paper-based material makes the resistance of the conductive paper almost unchanged before and after water absorption, and the response phase angle is only delayed by 4.9° at 300 kHz. The relevant performance test results of the application on the sensor show that the sensor has good performance, again proving the possibility of using the conductive paper as the electrode of the capacitive sensor in this study. Therefore, the paperbased electrodes in this study have broad application prospects in the long-term use of wearable sensor devices in the future.
Funding This article is not supported by any funding.

Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.