Flexible PANI/Lotus Stemmed Fiber Sensor for Bifunctional Sensing of Physical and Chemical Stimuli

In recent times, high‐performance flexible sensors that can be fabricated environmentally friendly and low‐costly have received considerable attention owing to their potential applications in wearable health monitoring. The combination of biocompatibility, degradability, and recyclability between the substrate and sensor layer remains a challenge. Here, a highly sensitive multifunctional sensor is developed using a polyaniline (PANI)‐coated lotus stem fabric (PANI/LSF) for body physical and chemical stimuli in a simple way. The lotus stem fabric has excellent breathability, skin compatibility, and lightweight as a flexible substrate. Its high sensitivity at low pressure is experimentally verified, which can reach 0.3 kPa−1 in the 0–1.5 kPa range. In addition, the sensor has low detection limitation and good linearity of response to water molecules and short response and recovery times for water in N2 or normal atmosphere at the range of 3.7–70.5% relative humidity (RH) and 19.6–66.8% RH, respectively. This can be successfully used in the recognition of various breathing behaviors. Therefore, the study has developed a breathable and skin‐friendly bifunctional sensor based on PANI and natural fabrics, which has great potential for application in human health care.

Furthermore, high-sensitivity multifunctional flexible sensors often require excellent flexible substrates to achieve.[24][25][26] In recent years, the research of flexible wearable sensors based on natural degradable fibers has been attracting considerable attention.Cho et al. synthesized Tunicate Cellulose Nanotube Fibers/Carbon nanotube (TCNF/CNT) fibers exhibited excellent gas (NO 2 ) sensing performance with high selectivity and high sensitivity (parts-per-billion detection). [27]In addition, the TCNF/CNT fibers can tolerate complex and harsh deformations, maintain their inherent sensing properties, and can be perfectly integrated with conventional fabrics through a direct weaving process.Many more flexible sensor such as PANI/metal film, [28] Ti 3 C 2 T x , [23] CNT/Ni 2+ , [24] graphene oxide [29] on nonwoven fabric are wildly investigated.Although these flexible biomaterial based devices have great potential for wearing applications, the synthesis involves expensive and complex manufacturing processes.And in most cases, they are not multifunctional, and the bonding between the materials is not very strong, making their wide application quite challenging.Therefore, it remains a task to fabricate flexible low-cost degradable multifunctional sensors that are well bonded and can simultaneously respond differently to physical and chemical stimuli while maintaining low interference.
Herein, we report a breathable, low-cost, and simply prepared bifunctional sensor based on PANI/stemmed fabric for real-time monitoring of human physiological movements such as respiration, pulse joint flexion, etc.A synthetic PANI solution is used to cover the hydroxyl-rich lotus stem fabric (LSF), [30,31] there by forming hydrogen bonds between the PANI and the lotus stem fabric to facilitate better bonding.The current response curves of this new type of sensor exhibits excellent humidity sensitive properties low as 3.7%, showing that the sensor can recognize different relative humidity well and has good linearity, fast response time, and recovery time.In addition, it also has a high sensitivity of low pressures down to 1.2 Pa with high stability.

Characterization of the Sensor
The preparation process of PANI@LSF pressure sensor is shown in Figure 1a.In this process, hydrogen bonds can be formed be-tween PANI and lotus stem fabric, resulting in a tighter connection, as shown in Figure 1b. [32,33]After the lotus stem fibers are coated by PANI, a mesh fabric sensor with interlocking structure and good permeability can be obtained by the conventional flat needle weaving method.Here, the load-stemmed fibers have good elasticity, and when the device is subjected to pressure, the increased contact area of the conductive material significantly reduces the resistance.When the stress is released, the resistance quickly returns to its original state, as showed in Figure 1c.The regular woven spiral structure of the stalk fibers can provide a large deformation possibility, thus laying the foundation for pressure-sensitive performance.Second, the hydrophilic nature of the hydroxyl-rich lotus fibers and their good permeability offer great possibilities for moisture sensing performance.And the good breathability and degradable safety of the fabric provide good comfort and long-term wearability, which makes it a great potential for human physiological motion monitoring such as respiratory behavior.
The as-obtained lotus stem fabric in Figure 1d is composed of multiple fibers with a helical structure.Multiple fibers are screwed together after soaking in PANI solution, and the scanning electron microscope (SEM) is shown in Figure 1e,f.The PANI-soaked stem fibers are coated with relatively uniform PANI, which provides good electrical conductivity and sensing properties for the later sensor devices.Further, the spiral curl of the form could be observed, indicating that the sensor has excellent expansion and deformation capabilities.

Pressure Sensing Performance and Sensory of Physical Behavior
The response curve is segmented into three intervals as shown in Figure 2a.The sensitivity of the sensor was up to 0.3 kPa −1 in the range of 0-1.5 kPa, 0.1 kPa −1 in the range of 1.5-11 kPa, and 0.06 kPa −1 in the range of 11-21 kPa, indicating that it is more sensitive to low pressure.[36][37][38] The response curves at different pressures are shown in Figure 2b, which demonstrates that the device is capable of good recognition of different pressure.In Figure 2c, the I-V curves at different pressure are shown, displaying a significant linearity in the voltage range of −1-1 V.As can be seen, the conductivity of the sensor is notably stable at different pressure and increases significantly with the increasing pressure.As shown in Figure 2d, a small piece of paper is placed on the sensor, generating a pressure of about 1.2 Pa.The sensor outputs a very clear response signal, demonstrating that the device has a low detection limit and a high recognition capability.
Moreover, response and recovery time is one of the key parameters to evaluate the performance of piezoresistive sensor devices.As evident from Figure 2e, the fast response time (640 ms) and recovery time (620 ms) of the PANI/LSF sensor can illustrate its ability to monitor human motion in real time.In addition, the repeatability of the sensor at different pressures and the durability test under long-term cyclic load are shown in Figure 2f,g, respectively.It shows excellent stability, after more than 4000 cycles at 4 kPa (Figure 2g).From the SEM, it is observed that after cycling tests, PANI still remains uniformly coated on the stem fibers, which may be a result of the hydrogen bonds formed between PANI and the stem fibers.The good stability of this material makes it promising for the monitoring of daily physiological movements of human body.
Benefiting from the extraordinary sensing performance of this breathable fabric sensor, such as excellent sensitivity, good stability, fast response/recovery time, PANI/LSF piezoresistive sensors are suitable for real-time detection and differentiation of various human movements.Therefore, by sticking the device on the human skin and joints, a real-time motion of the human pulse, fingers, wrist, and elbow is detected, as shown in Figure 3.In Figure 3a, the typical P, T, and D waves of the pulse can be clearly observed, suggesting that it has outstanding recognition capability.In addition, some joint flexion activities such as wrist (Figure 3b), fingers (Figure 3c-e), and elbow (Figure 3f) are also identified.The above results illustrate that this fabric sensor has excellent stability for the testing of physiological movements.Consequently, the fabric sensor has potential applications in physiological monitoring.

Humidity Sensing Performance
It is well known that the conductivity of PANI is changed due to the proton exchange reaction between PANI molecular chains and water molecules.[41] Consequently, the material has been developed as an attractive moisture-sensitive material.A sensing device is constructed to investigate the humidity sensing performance of the PANI/LSF sensor (Figure 4a).To exam the basic humidity sensing properties, the current variation curves of the fabric sensor in the humidity range of 22.8-70.5% relative humidity (RH) with N 2 as a background gas are shown in Figure 4b.The fabric sensor responds to water due to the presence of hydrophilic hydroxyl groups and permeable voids on its surface, which provide the necessary pore channels for water molecules.The sensor therefore exhibits excellent linear variation as well as resolution.Even in the low humidity range of 3.7-15.5%RH, it still has a clearly differentiated current response signal and significant linearity, as shown in the insets in Figure 4b,c.And its response time and recovery time were 53 and 54 s at 56.5% RH (Figure 4d), which is much faster than many other PANI-based humidity sensors, as shown in Table 1.In Figure 4e, the current variation curve of the sensor at 10% RH for repeated tests is presented.And its maximum current is always stable in a range without any significant drop, demonstrating its exceptional stability.As a result, the PANI/LSF sensor can provide a stable response to different humidity levels under N 2 background conditions that are not disturbed by other redox gases.
Subsequently, in order to improve the practical application, a study of humidity performance is investigated in a dry air background.In the experiment, the humidity could only drop to 18% RH after a long period of dry air is passed in, so here the current response curve is tested in the humidity range of 19.6 to 66.8% RH.The dynamic current variation of PANI/LSF sensor under different relative humidity conditions is shown in Figure 4f, from which it can be found that the sensor can still identify different humidity levels well in the air background.As can be noted in the inset of Figure 4f, it still has a fast response time (56 s) and a recovery time (48 s).It is evident from Figure 4g that the current of PANI/LSF sensor shows a linear trend of increasing as the relative humidity changes from 19.6% RH to 66.8% RH, indicating that the PANI/LSF sensor has a great linearity in response to water molecules.Figure 4h shows the cyclic response of this sensor at 43.7% RH, which shows its excellent repeatability.The sensors have excellent humidity sensing performance even when interfered with by other oxidizing gases such as oxygen, which can provide a strong basis for practical application.
The consistent response of PANI/LSF sensors to humidity inspires us to detect human respiration.Disease or organ failure  can lead to changes in respiratory function. [47,48]Real-time respiratory monitoring is an effective method for respiratory diagnosis and assessment of physical function.The devices were used to test normal breathing, rapid breathing, and deep breathing states.From Figure 4i, it can be observed that there is a clear difference in the frequency of the current change and its intensity between the three types of breathing.The magnification reveals that the respiration is stable in the same state, while the current variation between different respiration is quite different, and the deep respiration has the most significant current variation because more water molecules enter the fabric surface (Figure 4j-l).Therefore the sensor has the potential to be used for portable wearable respiratory monitoring applications.

Conclusion
In conclusion, a simple weaving method is used to design a flexible and breathable bifunctional sensor for physical and chemical stimulation.As a flexible piezoresistive sensor, its sensitivity can reach 0.3 kPa −1 in the range of 0-1.5 kPa, while its response does not show any significant degradation after a long cycle test.It shows fast response and recovery times of 640 and 620 ms based on the excellent elasticity of the load-stemmed fibers.In addition, PANI/LSF shows excellent humidity sensing properties with fast response time and recovery time.This can be attributed to the enhanced hydrophilicity of PANI by the combination of PANI with the load-stemmed fibers.The study confirms that the effective combination of natural fibers and PANI material greatly improves the performance of the sensor.We believe that this simple design concept can provide meaningful guidance for future wearable electronics.

Experimental Section
Materials: Aniline is used by reduced-pressure distillation, and the rest of the reagents are analytically pure.Aniline and N-methylpyrrolidone (NMP) were purchased from Shanghai Maclean Biochemical Technology Co. Ammonium persulfate was purchased from Tianjin Fuchen Chemical Reagent Factory.Anhydrous ethanol, toluene, hydrochloric acid, and ammonia were purchased from Chengdu Kolon Chemical Co.Copper wire, copper foil conductive silver paste, and lotus stalks were purchased from Taobao.com.
Preparation of the PANI Solution: The synthesis of the PANI solution is based on the previous work and details of the synthesis process are shown in Figure 5a. [49]First, the protonic acid-doped PANI powder was synthesized under acidic conditions using chemical oxidation polymerization.Then it was dedoped by stirring in concentrated ammonia.Next, soxhlet extraction was performed with a mixture of toluene, ethanol, and water (v/v = 9:1) under N 2 protection to remove oligomers and salts.The powder of purified PANI was dried in vacuum at 40 °C.The crystal structure was measured by X-ray diffraction (Figure 5b) and the morphology of the PANI powder was confirmed by scan electron microscopy (Figure 5c).The results confirmed that the powder of PANI was highly pure and without byproducts.Subsequently, 0.2 g of PANI powder was dissolved in 8 mL of NMP and stirred magnetically for 48 h to obtain PANI solution.Finally, the PANI solution was centrifuged and the supernatant was taken, resulting in a homogeneous PANI solution.
Preparation of the Breathable Fabric Sensor: The extracted lotus fibers were dispersed and soaked in a PANI solution for one minute, and then the fibers were hung and dried naturally.After drying, two 40 cm long PANI/lotus fibers were woven into a 1.5 × 1.5 cm 2 fabric using the traditional weaving process.Finally, the fabric devices were doped with concentrated HCl for 2 min and then stabilized under natural environment for one day before being fabricated with silver paste and copper foil to obtain sensor devices.
Material Characterization and Performance Testing: The morphology and microstructure of PANI/stemmed fabric were measured by a SEM (Gemini, SEM-300).The composition and crystal structure were analyzed by X-ray diffractometer (XRD, TD-XRD-3500).
The universal testing machine (ZQ-990) was used for pressure loading and the output signal of pressure sensor was collected by the electrochemical workstation (CHI-760).The voltage applied to the sensor during all tests was 0.2 V.The response was defined as (I − I 0 )/I 0 , where I 0 is the initial current and I represents the current after the application of pressure.The sensitivity was being in definition as S = (ΔI/I 0 )/ΔP, where ΔI is the real-time current value I minus the initial current value I 0 , ΔI/I 0 is the relative current change, and ΔP is the amount of pressure change.The response and recovery time were calculated as the time to reach 90% of the total current change.Humidity control and its testing were performed using a self-assembled humidity test platform with real-time current signals from a connected electrochemical workstation.

Figure 1 .
Figure 1.a) Schematic diagram of the preparation process of a PANI/LSF sensor, b) molecular formula for the hydrogen bonding between the load stem fiber and PANI, c) diagram of sensing mechanism, d) the SEM image of lotus stem fabric, and e,f) the SEM image of PANI/LSF.

Figure 2 .
Figure 2. a) Sensitivity of the sensor in the pressure range of 0-20 kPa, b) relative current response of the sensor under step-applied pressure, c) the I-V curves of the sensor under various pressure, d) response curve at 1.2 Pa pressure, e) the response time and recovery time, f) cyclic test curves under different pressures, and g) reproducibility and durability test with an applied loading and unloading pressure of 4 kPa and the SEM image of PANI/LSF after cyclic testing.

Figure 3 .
Figure 3. Application tests of PANI/LSF sensor for real-time human activity monitoring a) pulse record under normal conditions, b) wrist bending, c-e) fingers bending movement, and f) elbow bending movement.

Figure 4 .
Figure 4. a) Schematic of the real-time humidity sensing setup, b) dynamic current variation of PANI/LSF sensors with different relative humidity in N 2 background, c) current versus RH% response curve of PANI/LSF sensor, d) the response time and recovery time of PANI/LSF sensor at 56.5% RH, e) repeatability performance test of PANI/LSF sensor exposed to 10% RH in N 2 background, f) dynamic current variation of PANI/LSF sensor with different relative humidity in a dry air background and the inset shows the response time and recovery time at 43.3% RH, g) current versus RH% response curve of PANI/LSF sensor, h) repeatability performance test of PANI/LSF sensor exposed to 43.7% RH in a dry air background, i) breath testing curves, j) magnified normal breath test curve, k) magnified fast breath test curve, and l) magnified deep breath test curve.

Figure 5 .
Figure 5. a) Flow chart for the preparation of PANI solution, b) XRD pattern of PANI powder, and c) SEM images of PANI powder.

Table 1 .
Comparison of the humidity sensing performance in this work with the reported literature.