High‐Performance and Degradable All‐Paper‐Based Pressure Sensor from Conductive Polymer

Cellulose paper has emerged as an ideal sensing element for wearable pressure sensors owing to its inherent flexibility, high porosity, and light weight. However, traditional paper‐based pressure sensors use metal‐based materials as electrodes, which significantly limits the unique advantages of paper, particularly in terms of degradability. In this study, a degradable pressure sensor is designed by combining the highly conductive poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) Xuan paper electrode with a low‐conductivity PEDOT:PSS tissue paper sensitive layer. Notably, Xuan paper, also called rice paper, has been a prominent substrate owing to its softness and good durability. By introducing a perforated structure in the sensitive layer, a novel sensing mechanism, conductivity conversion under pressure, is realized to improve the sensitivity. The obtained sensor exhibits a high sensitivity (13.9 kPa−1 at < 8.3 kPa, 151 kPa−1 at 8.3–20.8 kPa), ensuring that it can precisely monitor the full‐range human activities. Additionally, as the sensor does not rely on metal materials, it can degrade in water or fire without causing any negative environmental impacts. These findings establish a new approach to producing highly sensitive degradable sensors, which hold significant potential for application in green electronics, paper‐based sensing matrices, new prosthetics, and other fields.


Introduction
The paper has gained considerable attention because of its outstanding flexibility, printability, biodegradability, and cutability. [1][2][3][4] The excellent advantages of this paper, integrated with conducting materials, have presented a considerable potential for wearable electronics such as memory, [5] energy devices, [6,7] transistors, [8,9] and sensors. [10,11] Among these emerging paper electronics, paper-based pressure sensors have demonstrated great importance in real-time physiological monitoring, human-machine interaction, and virtual reality. [12][13][14][15] The key advantage of the paper-based pressure sensor is that it utilizes microstructurelike porous paper as the sensitive layer, eliminating the costly microstructure fabrication using photolithography and etching; thus, the manufacturing process is greatly simplified. [12] Generally, paper-based pressure sensors consist of two main parts: the electrodes and the sensitive layer. [15][16][17][18][19][20][21] For example, Gong et al. utilized metal nanowire paper to fabricate a pressure sensor consisting of a polydimethylsiloxane (PDMS) sheet patterned with an interdigitated Au electrode. [15] Zhan et al. developed a low-power-consumption carbon nanotube (CNT) paper pressure sensor based on a polyimide substrate with interdigitated Ti/Au electrodes. [16] Su et al. combined MXene/bacterial cellulose with copper electrodes to fabricate a paper pressure sensor. [21] However, these paper-based sensors are limited to metal electrodes, which are expensive and fragile. In addition, high-temperature vapor conditions and complex patterned electrode procedures significantly restrict the unique natural advantages of paper, such as its degradability and low cost. Thus, the development of a green, degradable, and cost-effective paper-based pressure sensor is required.
The realization of the high sensitivity of paper-based pressure sensors is another challenge. Typically, high sensitivity implies significant changes in conducting pathways, even at small pressures. Currently, the pressure sensing mechanism of most paperbased pressure sensors relies mainly on the structural variation of the rough surface and internal fibers of the paper. [14][15][16][17][18] Nevertheless, this single sensing mode in the structural variation of Figure 1. a) Schematic illustration showing the fabrication process of the all-paper-based pressure sensor. b) Contact angles of water on Xuan paper and tissue paper. c) FTIR spectra of Xuan paper and tissue paper. d) Current-voltage curves for PEDOT:PSS Xuan paper electrode and PEDOT:PSS tissue paper sensitive layer. e) SEM images of PEDOT:PSS Xuan paper. f) Enlarged SEM images of PEDOT:PSS Xuan paper. g) SEM images of PEDOT:PSS tissue paper. h) Enlarged SEM images of PEDOT:PSS tissue paper.
paper limits the sensitivity of almost all sensors to less than 10 kPa −1 . In addition, owing to the accumulation of deformation in the paper-based sensitive layer, the structural changes in paper under high pressure can be decreased, causing a noticeable decrease in sensitivity in high-pressure regions. Therefore, it is highly desirable to develop a strategy that can integrate high sensitivity into green-degradable paper-based sensors.
Herein, we design a degradable all-paper-based pressure sensor by combining a highly conductive Xuan-paper-based poly (3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) electrode with a low-conductivity perforated PEDOT: PSS tissue paper sensitive layer. Notably, Xuan paper is a handmade Chinese paper produced for calligraphy and painting in China since the Tang Dynasty (618-907 AD). Owing to its softness, water absorption ability, and durability, Xuan paper can also be used as a substrate for manufacturing paper electrodes. This design eliminates the use of metal electrodes and high-temperature patterning processes. Moreover, the sandwich architecture in the device and perforated structure in the sensitive layer introduces a novel sensing mechanism whereby the applied pressure causes conductivity conversion from the sensitive layer to the electrode. As a result, the all-paper-based sensor presents superior pressure sensitivity (13.9 kPa −1 at < 8.3 kPa, 151 kPa −1 at 8.3-20.8 kPa). Owing to its excellent flexibility and superior performance, the fabricated paper-based sensor can be attached to human skin to detect various physiological signals from low to high-pressure ranges (sound, wrist pulses, respirations, and motions). Using the cutting method, a novel cuttable all-paper-based sensor array was successfully fabricated for pressure mapping. More impressively, our all-paper-based pressure sensor can be degraded in water or fire without waste or by-products. The results provide a feasible approach for fabricating high-performance and green electronic skins, showing unconventional application potential in degradable electronics, intelligent robotics, etc. Figure 1a illustrates the main fabrication procedure of the pressure sensor. Xuan paper and tissue paper were used as substrates for fabricating the electrodes and sensitive layers, respectively. The conductive material, PEDOT:PSS, was coated onto the paper substrates by dipping. For the electrode, Xuan paper was dipped in a high concentration of PEDOT:PSS dispersions for 1 h. For the sensitive layer, tissue paper was dipped in a low concentration of PEDOT: PSS for 10 min. After the dipping process, both types of paper were removed from the solution and dried in a natural environment. The three PEDOT:PSS tissue papers were stacked together, and holes were punched to form perforated sensitive layers. Finally, the sensor was fabricated by stacking the top/bottom PEDOT:PSS paper electrodes, perforated PE-DOT:PSS tissue papers, and encapsulation with paper tape. Figure 1a, the right part shows an actual image of the all-paper-based pressure sensor. Figure 1b shows the contact angles of water on the Xuan paper and tissue paper. Notably, similar to tissue paper (contact angle of 21.2°), Xuan paper also showed good hydrophilicity, with a contact angle of 16.1°. Figure 1c shows the Fourier transform infrared (FTIR) spectra of the Xuan paper and tissue paper. Both types of paper present a distinct peak at ≈3335 cm −1 , which can be attributed to the hydroxyl group of cellulose. [22][23][24] Owing to the excellent hydrophilicity and hydroxyl groups, the PE-DOT:PSS aqueous dispersion could easily bind to the cellulose of both types of papers. [24] The corresponding current-voltage curves for Xuan paper and tissue paper after PEDOT:PSS coating are shown in Figure 1d. Compared to pure paper, the conductivity of both the sensitive layer paper and electrode paper significantly increased after being covered with PEDOT:PSS. Interestingly, the conductivity of papers can be easily tuned by controlling the dipping conditions. Figure 1e-h show SEM images of the PEDOT:PSS Xuan paper and PEDOT:PSS tissue paper. Figure S1, Supporting Information shows SEM images of the original Xuan paper and tissue paper. No significant change in the paper morphology was observed after coating PEDOT:PSS. The PEDOT:PSS Xuan paper maintained a rugged surface, and the PEDOT:PSS tissue paper maintained a porous network structure. Rugged Xuan paper can be used as a stable electrode, whereas porous tissue paper can serve as a sensitive layer with a natural microstructure.

Results and Discussion
To investigate the performance of our paper-based pressure sensor, we constructed a sensor signal testing system comprising an electrical measurement unit, a stepper motor with a gauge force meter, and a computer to collect data (Figure 2a). The sensitivity (S), an important factor for evaluating sensor performance, is defined as follows [25,26] where ΔI and I 0 denote the initial current and current change under pressure, respectively, ΔP denotes the pressure change. Figure 2b,c show that the sensitivities obtained from the paper-based sensor are 13.9 kPa −1 with applied pressure below 8.3 and 151 kPa −1 with an applied pressure of 8.3-20.8 kPa, which is higher than most of the reported paper-based pressure sensors. [14][15][16][17][18][19] It is important to achieve high sensitivity and low detection limits in one sensor simultaneously to significantly widen the application of the sensor. For example, Wei et al. reported NO 2 sensors with high sensitivity (60.02% ppm −1 ) and an ultralow limit of detection (6.8 ppb). Further, Wu et al. reported a transparent stretchable polyacrylamide/carrageenan hydrogel-based temperature sensor with high sensitivity (24.54%°C −1 ) and high resolution (0.8°C), which is superior to existing temperature sensors based on hydrogels. [27,28] Figure 2d shows that the paperbased sensor could respond to subtle pressures of 3 Pa, confirming the good low-pressure resolution of the device. To demonstrate the real-time pressure-sensing performance, the current response of the pressure sensor with an incrementally applied pressure is shown in Figure 2e. The paper-based sensor exhibited stable and accurate responses with increasing pressure. Moreover, the paper-based sensor exhibited outstanding reliability. The sensor presents stable current responses during 500 cycles of pressure loading at 1.04 and 5.02 kPa, as shown in Figure 2f.
To understand the working principle of the paper-based pressure sensor, the dynamic process of the sensitive layer change under pressure is shown in Figure 3. Figure 3a shows schematics of the sensor changes with an increase in the applied pressure. The sensor changes under pressure were also observed using a 3D microscope. As shown in Figure 3b, the sensor was fixed vertically to the object and pressure was applied. A 3D microscope was used to record real-time changes in the cross-section of the sensor. As shown in Figure 3a,c, in the initial state, the tissue and Xuan paper electrodes between the layers have a number of air gaps and present few contact points, resulting in a low initial current of the sensor. As a slight pressure was applied to the paper, some air gaps disappeared, and the contact points increased, enhancing the sensor current. As the pressure increased, the air gap between the sensitive layers further reduced, and the top paper electrode gradually contacted the bottom paper electrodes. Thus, a high initial resistance is obtained, and the increased pressure induces a conversion in the conductive path from the sensitive layer to the electrode, leading to a sharp increase in the sensor current. After the pressure was removed, the air gaps returned to their original state, the top and bottom electrodes were separated, and the sensor current dropped to its initial state. A particularly innovative design in our pressure sensors is the use of perforated tissue paper as a sensitive layer, which significantly improves sensitivity. The perforated structure not only reduces the compression modulus of the sensitive layer but also introduces a novel pressure-sensing mechanism, conductivity conversion from the sensitive layer to the electrode. As proof of principle, we compared the performance of the sensors with intact tissue paper and perforated tissue paper as the sensitive layer. The device with the perforated tissue paper-sensitive layer exhibited superior sensitivity in both low-and high-pressure regions ( Figure S2, Supporting Information). Specifically, in the low-pressure region, the perforated structure introduces more air into the sensitive layer, thus causing a reduction in the compression modulus of the sensitive layer. A lower compression modulus causes the sensitive layer to deform more under pressure, resulting in an improvement in sensor sensitivity. A larger pressure facilitates the conversion of the conductivity in the functional layer (from lower conductivity PEDOT:PSS to higher conductivity PEDOT:PSS), thus significantly contributing to the high performance of the sensors. In the previously reported paper-based pressure sensor, deformation accumulation causes the conductive paths of the sensor to gradually saturate, which results in an inevitable reduction in sensitivity under high pressure. [15][16][17][18][19] Unlike the aforementioned sensors, the sensing mechanism of our sensor primarily relies on the conductivity conversion in the bifunctional layer (from the sensitive layer to the electrode), ensuring high sensitivity in the high-pressure region. To further explore the hole size and number, we compared the pressuresensing performance of sensors with different holes in the sensitive layer (including different sizes and numbers). Figure S3, Supporting Information shows the sensing performance of the paper-based pressure sensors with different hole sizes. Figure S4, Supporting Information shows the sensing performance of the paper-based pressure sensor with different numbers of holes. As the hole dimension and the number of sensitive layers increased, the sensitivity gradually increased. The sensor with a larger hole size and number presented more contact paths under pressure, thus showing a higher sensitivity. However, the larger hole size in the sensitive layer also implies that the papers can be broken more easily. Considering that good stability and high sensitivity are necessary for a pressure sensor, we selected an optimized hole dimension of 1.5 mm.
In addition to improving sensitivity, our all-paper material design ensures that each part of the device can be cut freely, which is promising for diverse device designs. As shown in Figure 4a, a 4 × 4 paper-based sensing matrix was constructed using the conventional cutting method. Figure S5, Supporting Information shows the circuit diagram of the sensor array mapping measurement. Figure 4b,c shows the electrical results of the paper-based sensing matrix when square objects were placed at different positions in the matrix. The current change in the sensor provides clear feedback on the shape of the object and the loading area. Moreover, our all-paper-based sensing matrix also allows trajectory tracking. As shown in Figure 4d, the finger first touched the paper-based sensing matrix, which was located in the upper right corner, and then moved to the lower left corner. The corresponding current variation in the paper-based matrix clearly records the trajectory of finger movements. These results confirm the excellent spatial recognition capability of the paper-based sensing matrix.
Based on its excellent sensitivity and wide detection range, our paper-based sensor showed great utility in detecting diverse human activities, as illustrated in Figure 5a. Owing to the excellent flexibility and biocompatibility, the sensor can be directly attached to the human body using paper tape. The pressure sensor was successfully applied to the detection of human signals from small pressures (e.g., throat movement) to large pressures (e.g., gait). To demonstrate good sensing ability, we fabricated five independent sensors and attached them to different parts of the human body using paper tape. The corresponding electrical signals of each sensor were recorded by a source meter. Figure 5b shows the sensor attached to the throat of a human. The muscle movements in the throat caused by opening the mouth can be easily recorded by changes in current signals. This feature is useful in human-machine interactions and vocal cord recovery. Owing to their sensitivity and quick sensing ability, paper-based pressure sensors can also be used for Morse code preparation. As shown in Figure 5c, the real-time current changes are decrypted by controlling different press times and can be used for outputting the command, such as "SENSOR." Heart rate is considered an essential sign, which can be used to evaluate the health status of a person. [29,30] Figure 5d shows the pressure sensor attached to the human wrist to monitor the heart rate. The measured heart rate was ≈90, which corresponded well with the heart rate of an  adult in good health. The inset of Figure 5d shows a typical pulse with three distinct peaks: the percussion wave (P 1 ), tidal wave (P 2 ), and diastolic wave (P 3 ). Furthermore, paper-based pressure sensors have an outstanding ability to monitor human motion. Our sensor can be fixed on a finger to detect bending states, as shown in Figure 5e. The sensor signals exhibited different intensity responses at different joint flexion angles. Figure 5f illustrates that our sensor was fixed on the elbow to record the current response under bending-relaxing conditions. This feature is useful in avoiding joint injuries during exercise. Furthermore, the pressure sensor can be attached to the insole to further detect different movement states, acting as a stable gait monitor, as shown in Figure 5g. The frequency of the current signal was 39 times per min. In the running state, the frequency increased to 73 per min, and sharper peaks were observed. Owing to the increasing demand for intelligent consumer products, there is a tendency to design waterproof and sweat-proof sensors that can withstand sweat and moisture environments. For example, Wu et al. developed a hydrophobic strain sensor with high strainsensing performance. [31] It is suitable for various applications, such as the real-time detection of various human activities in an underwater environment. In our experiments, we used degraded paper as the encapsulation layer to highlight the degradability of the all-paper sensors. In practical applications involving sweat and moisture, waterproof paper can be used as an encapsulation layer to ensure the proper functionality of the sensor.
Most previously reported flexible pressure sensors have been fabricated using non-degradable materials, which inevitably cause acute environmental pollution. Therefore, it is highly desirable to combine green and degradable materials with wearable sensors. [32][33][34][35][36][37] For example, Ding et al. prepared a high-performance sensor based on a polyvinyl alcoholcellulose nanofibril double-network organohydrogel, which not only shows high sensing performance, such as high sensitivity and low limit of detection but also exhibits good degradability and eco-friendliness. [34,35] Among these degradable materials, paper, mainly derived from natural plants, has emerged as an excellent green candidate owing to its sustainability, biocompatibility, and degradability. [38][39][40][41] However, the presence of non-paper components in previous paper-based pressure sensors, such as metal electrodes on polymer films, limits the advantages of paper for green degradable electronics. In our work, as shown in Figure 6a, the electrodes and sensitive layers originate from paper, which is combined with water-soluble organic PEDOT:PSS, ensuring the degradability of the whole device. Further, the degradation products are harmless and non-toxic and can be recycled back into the natural biosphere. As a proof of concept, Figure 6b,c shows our all-paper-based sensors that are degraded in water and soil.
As shown in Figure 6b, the sensor was immersed in water and then softened and degraded through simple physical agitation. We also buried the sensor in the soil and placed it in a natural environment. On the one hand, rainwater washout can cause the dissolution of soluble PEDOT:PSS and the breakage of paper. However, the cellulose of paper can be decomposed in the presence of bacteria, fungi, and other microorganisms. [ 41 ] As shown in Figure 6c, as the degradation time increased, the sensor fractured and gradually decomposed in the soil. Further increase in the degradation time or selecting soil with more microorganisms could allow the sensor to degrade completely. Figure S6, Supporting Information shows that the all-paper-based sensor can also be incinerated by fire and almost without harmful products, the main products of which are carbon dioxide. These results confirm the excellent degradability and disposability of all-paperbased sensors, showing their significant importance in alleviating environmental pollution.

Conclusion
In conclusion, a degradable all-paper-based pressure sensor was designed using highly conductive PEDOT:PSS Xuan paper as the electrode and low-conductive PEDOT:PSS issue paper as the sensitive layer. The conductivities of the electrode and sensitive layer were adjusted by soaking the papers in PEDOT:PSS solutions with different concentrations. By combining the vertical configuration of the device with the perforated structure of the sensitive layer, the sensor presents excellent sensing performances www.advancedsciencenews.com www.advsensorres.com (13.9 kPa −1 at < 8.3 kPa, 151 kPa −1 at 8.3-20.8 kPa). The highpressure sensitivity results from the wrinkled structure of the PEDOT:PSS tissue paper and conductivity conversion from the sensitive layer to the electrode. Benefiting from this design, an all-paper-based sensor array was successfully fabricated by cutting and gluing for pressure mapping. Another attractive attribute of paper-based sensors is the degradability of the entire device, which ensures that the sensor can be degraded in water or fire without forming any waste or by-products. Our design fully demonstrates the promising advantages of paper-based electronics in terms of flexibility and degradability, showing its capability for full-range human activity monitoring, such as breath, wrist pulse, and motion detection.

Experimental Section
Materials and Characterization: PEDOT:PSS (Clevios PH1000) aqueous solution was purchased from Heraeus. Ethylene glycol (EG) was purchased from Sigma-Aldrich. All the reagents were used without further purification. Fourier-transform infrared spectra of the papers were obtained using Nicolet IS10 (Fisher Scientific). SEM images of the samples were collected using Sigma 500. The electrical curves of the paper-based sensor, including the current-time and current-voltage curves, were obtained from Keithley 2614 B. Pressure and bending tests were performed using a force gauge (Mark-10, M5-05, M5-2) installed on a motorized vertical test stand (ESM301). Informed consent was obtained from the volunteers who participated in the experiments.
Fabrication of PEDOT:PSS Paper Electrodes and Sensitive Layers: A highconcentration PEDOT:PSS solution was prepared by mixing a PEDOT:PSS aqueous solution with 6 vol. % EG. A low-concentration PEDOT:PSS solution was prepared by mixing the PEDOT:PSS aqueous solution with deionized water (volume ratio 1:3). Both the high-and low-concentration solutions were stirred for 12 h. The Xuan paper was dipped into the PEDOT:PSS solution (high concentration) for 1 h to fabricate the paper electrode. To prepare the paper-sensitive layer, the tissue paper was dipped into a PE-DOT:PSS solution (low concentration) for 10 min. After the dipping process, the paper was removed and dried in a natural environment.
Fabrication of Pressure Sensors: First, the PEDOT:PSS tissue paper was cut into squares (1.3 × 1.3 cm). Second, three layers of PEDOT:PSS paper were stacked and perforated using a hole punch. Subsequently, the top and bottom electrodes were obtained by cutting the PEDOT:PSS Xuan paper (1.2 × 3 cm). Finally, PEDOT:PSS paper electrodes with PEDOT:PSS papersensitive layers were stacked together and encapsulated using paper tape.
Fabrication of Paper-Based Pressure Sensing Arrays: First, the PE-DOT:PSS Xuan paper electrode was cut into 1 × 9 cm 2 . Next, eight PE-DOT:PSS electrodes were four-four-crossed with a unit space of 1 cm. Subsequently, multilayered PEDOT:PSS tissue papers (1.3 × 1.3 cm) were placed between the intersections. Finally, paper tapes were attached to the sensing units to simply package the arrays. A microcontroller was used to process and pass the electrical signals to the computer.
Measurement of Human Activities: Five of the fabricated sensors were fixed on paper substrates. All the prepared sensors were fixed to paper substrates and secured to the corresponding measurement area of the human body with paper tape. To detect the wrist pulse, Morse code, and throat/finger/elbow movements, the top and bottom PEDOT:PSS electrodes were connected to source meter wires to realize the current measurement. For the sensor to detect the gait states, the PPy electrodes were connected to conductive wires, and the entire sensor was fixed on the insole to achieve electrical measurements. Informed written consent was obtained from all participants.
Statistical Analysis: The data used to extract performance parameters, such as sensitivity, was not pre-processed before the analysis. All experiments were performed at least three times (n ≥ 3) for each sample, and statistical analysis was treated by Origin 2018.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.