A Flexible, Wearable, Humidity‐Resistant and Self‐Powered Sensor Fabricated by Chitosan‐Critic Acid Film and its Applications in Human Motion Monitoring and Energy Harvesting

Being a resource‐abundant natural material, chitosan shows good frictional electrical properties and has become one of the most used materials for triboelectric nanogenerators (TENGs). To improve water resistance and mechanical properties, herein, a strategy is proposed to modify the chitosan film by citric acid. The results show that both humidity‐resistant and mechanical properties of the modified films can be notably enhanced due to the formation of a network structure between chitosan molecular chains. Excellent output performance of 77.5 V and 2.66 µA is achieved in the TENG based on the modified film with a mass ratio of chitosan/citric acid = 3:1. Under a relative humidity (RH) of 80%RH, it can output 70.16% of the voltage compared to that under 20%RH. Self‐powered tactile sensor based on the TENG exhibits a sensitive response to pressure and bending and humidity resistance, giving the sensor tremendous promise for a wider range of human motion monitoring and energy harvesting.


Introduction
With the booming of technology, various smart electronic devices are widely used in our daily life.Nowadays, the mainstream of research is the pursuit of miniaturization and portability of DOI: 10.1002/adsr.202300129electronic products.Therefore, the ways of providing energy to the devices are also innovating.At present, the main energy supply method is mainly batteries, but they must be charged in a timely manner to avoid depletion.In addition, waste batteries need to be properly disposed of, otherwise, they will cause serious pollution to the environment.For this reason, researchers have started to search for sustainable and pollution-free methods of charging, and converting mechanical energy into electrical energy for power supply has become a research hotspot.[3][4] They are also used to fabricate wearable sensors [5,6] and devices for medical applications, [7,8] human motion monitor, energy harvester, [9][10][11] and so on.
Currently, the main materials used in TENGs are petroleumbased films. [12,13]With the shrinkage and scarcity of resources and expanding environmental pollution, the full conversion of biological resources is crucial for green and sustainable development.Researchers have turned their attention to natural renewable polymer materials, such as cellulose, [14] chitosan, [15] lipid, [16] and protein. [17]As a natural biopolymer material, chitosan is mainly sourced from the shells of crustaceans in the ocean, from which the chitosan can be easily obtained at low cost.Hence, chitosan has emerged as one of the most important materials for TENGs. [18,19]Its good film-forming and flexibility can also be applied to human motion energy collection and power microelectronic devices.Also, chitosan-based self-powered devices have sensitive responsiveness and can be used for real-time touch response sensor applications.[22] Due to abundant hydrophilic functional groups, the chitosan is a natural hydrophilic biomass.The mechanical properties of chitosan film are prone to swell and crack when exposed to water, which means it cannot be used under high humidity conditions. [23,24]herefore, improving the humidity resistance of the chitosan film has been the focus and bottleneck of research.In this work, a chitosan-based film with excellent water resistance, mechanical properties, triboelectricity, and softness was synthesized to solve the above difficulties by cross-linking and adding a plasticizer.The design idea is shown in Figure 1.A series of composite films with different mass ratios of chitosan/citric acid were prepared by the solution casting method.Glycerol is also added as a plasticizer to maintain the softness of the films.Due to the amide reaction between the carboxyl group of citric acid and the amino group of chitosan, a cross-linked mesh structure and the decrease of hydrophilic functional groups are formed to ensure water resistance and humidity-resistant in the proper ratio of chitosan to citric acid.The addition of glycerol and citric acid can improve the amount of electron-donating groups and thus resulting in low electronegativity and higher triboelectricity of chitosan-based film.The TENGs based on the modified films show notably improved output performance as compared to pure chitosan film, giving the films promising applications in a wide range of conditions with longer service time and ideal endurance in high humidity environment.The chitosanbased film with mass ratio of chitosan/citric acid being 3:1 exhibits the best comprehensive performance.The TENG fabricated based on this film shows an excellent output performance of 77.5 V and 2.66 μA under external force of 30 N and 1 Hz.Under 80%RH, it can output 70.16% of the voltage compared to that under 20% relative humidity (RH).The flexible, humidityresistant TENG was also used for human motion energy harvesting and monitoring.

Preparation of Chitosan-Based Film
The chitosan-based film was prepared by solution casting method.The fabricating procedure of the chitosan film is shown in Figure S1 (Supporting Information).First, 3 g chitosan powder ((C 6 H 11 NO 4 ) n , deacetylation ≥ 95%, Rhawn) was dissolved in 1% acetic acid solution (C 2 H 4 O 2 , 99.5% purity, Sinopharm) and 1 g citric acid (C 6 H 8 O 7 , 99.5% purity, Macklin) was dissolved in deionized water.The citric acid solution was added to the chitosan solution and stirred for 1 h at 50 °C with a magnetic stirring rate of 600 rpm.Then, a certain amount of glycerol (C 3 H 8 O 3 , 99.7% purity, Sinopharm) was added dropwise for 10 min, and the resulting solution was clear and transparent.The solution was placed until bubbles disappeared.The solution was poured into culture dishes and kept in a 50 °C oven for 8 h to remove the moisture.After cooling down to room temperature, these films were carefully removed and cut into rectangle-shaped films.At the same time, we prepared chitosan films with different contents of glycerol, chitosan, and citric acid for comparison.These films were designated as CS-Pure, CS-Gly, CS/CA-1, CS/CA-2, and CS/CA-3.Among them, CS-Pure represents chitosan without any additional ingredients and dissolved only with acetic acid; CS-Gly has additional glycerol; CS/CA-1, 2, and 3 have adjusted the concentration of chitosan and added different proportions of citric acid on the basis of CS-Gly.Their mass ratios of chitosan, citric acid, and glycerol were shown in Table 1.

Characterizations and Test
The structure of the chitosan film was examined by X-ray diffraction (XRD, Rigaku Smartlab Beijing Co, Beijing, China) with Cu K radiation.The functional groups were analyzed by Fourier microscopy (Vertex80+ Hyperon 2000, German).

Swelling and Water Uptake Test
Chitosan-based films were chosen to test their swelling behavior in water.These films were cut into rectangular films (2 cm × 2 cm) and immersed in deionized water for 6 h and their weights and lengths before and after the immersion were measured and recorded by precision electronic autobalance and ruler, respectively.

Fabrication of TENGs
The TENGs were composed of chitosan-based film and fluorinated ethylene propylene (FEP) film.Cu foil was attached to one side of the film used as electrode.Polyethylene terephthalate (PET) was used as a base plate.Cu foil was also attached to PET to keep it grounded to avoid external friction interference.
The type of TENGs is contact-separation.

Electrical Output of the TENGs
The output signals of the TENGs were tested by an electrometer (Model 6517B Programmable Electrometer, Keithley, USA) under different external forces and frequencies.The external force was provided by a linear motor (R-LP3, Dreamer, China).The effective area of the TENGs is 4 × 4 cm 2 .For all the testing processes, the electrode-attached chitosan film was kept grounded.

Humidity Resistant Tests of the CS/CA-3 TENG
The humidity tests were performed in an airtight environment.
The environment humidity was controlled by a fan blowing the water mist produced by the humidifier.The internal environment humidity was measured with a hygrometer.The output voltage of the CS/CA-3 TENG was tested by a humidifier after spraying water mist for 1 min.The effective area of the CS/CA-3 TENGs is 2 × 2 cm 2 .

Assembly of Self-Powered Tactile Sensor and Skin-Attachable Motion Sensor
A cross-shaped instant lighting tactile sensor was made of acrylic plate as a base plate, chitosan and FEP films as a tactile sensor, and Cu foil as an electrode.When we touch different parts of the tactile sensor, the corresponding LED light will glow.To fabricate the skin-attachable motion sensor, the chitosan film was overlaid with FEP film, and the edges of the films were bonded by glue as shown in Figure S2 (Supporting Information).Cu foil was attached to the outward facing side of the films as electrodes.The sensor was placed on the wrist and elbow.

Statistical Analysis
All data figures in this paper were analyzed using Origin software.All the voltage and current data were measured using DA-ARM1651 software on the Keithley 6517B machine with a sampling interval of 0.001 s.The data is measured with the existing accuracy of the instrument.

Ethical Approval
The human tissue experiment is not involved in this paper, and this experiment obtained informed written consent from all participants.

Characterization of the Chitosan Films
The XRD patterns of the chitosan films were shown in Figure 2a.
The pattern of CS-Pure shows four obvious peaks at 8.4, 11.5, 18.3, and 23.0°.After the addition of glycerol, only one peak at 21.1°can be found in the pattern of the CS-Gly film.The peaks of the CS/CA films appear at almost the same degrees and their crystallinity is improved obviously compared with CS-Gly.But the peak around 11.4°is significantly weaker than that of CS-Pure.
With the increase of citric acid content, the peak intensities of the CS/CA films decrease.This finding indicates that the original structure of chitosan is destroyed by increasing the content of citric acid.
For more accurate analysis, the chitosan films were analyzed through Fourier transform infrared spectroscopy (FTIR) as shown in Figure 2b.The broadband absorption in the range of 3400-3200 cm −1 is due to the stretching vibrations of the nitrogen-hydrogen bond and oxygen-hydrogen bond. [25]The double peaks at 2932 and 2864 cm −1 are attributed to the asymmetric stretching of the carbon-hydrogen bond.To better analyze and describe the intermolecular interactions between chitosan and citric acid, the spectrum range from 1800 to 1200 cm −1 was enlarged in Figure 2c.With the increasing addition of citric acid, the peak intensities at 1713 and 1215 cm −1 become stronger, representing the increase of carboxyl group content of citric acid. [26]he peaks at 1556 and 1535 cm −1 are attributed to the NH 2 bending vibrations of chitosan. [27]As the citric acid addition increases, the intensities of the peaks at 1556 and 1535 cm −1 decrease, which means that the amount of amino group in CS/CA is decreased.Meanwhile, the intensity increase of the peak occurring at 1626 cm −1 is due to the increase of the amide bond. [26,28]The peaks of CS/CA films in the range of 1500 to 1200 cm −1 become broader and shift to low wavenumber compared with the CS-Gly film, which proves the change of chitosan structure after the addition of citric acid.The FTIR results firmly confirm the amidation reaction between the carboxyl of citric acid and amino of chitosan as illustrated in Figure 3a.
The CS-Pure film is orange transparent and slightly hard.The CS-Gly film shows orange transparent but exhibits great softness, stretchability, and foldability.The CS/CA films are soft and foldable.The CS/CA-1 film is light orange transparent.The CS/CA-2 and CS/CA-3 films are light orange and translucent.Figure 2d shows, representatively, the photo images of the CS-Pure and CS/CA3 films.The pristine CS-Pure film can hardly be bent.It is transparent, hard, and fragile.While the CS/CA-3 film can be freely folded like thin paper, indicating that the addition of glycerin can significantly improve the flexibility.It is found that the flexibility of the CS/CA films strongly depends on the content of the glycerol.Without glycerol, the CS/CA films are easily cracked.When the mass ratio of chitosan to citric acid is beyond 2:1, the films are extremely sticky and easily denatured.The translucency of the CS/CA films is due to the amide reaction between the carboxyl group on the citric acid and the amino group on the chitosan molecule to form a reticulated cross-linked structure when the content of citric acid is low.
Swelling and water uptake test results were shown in Figures 2e,f.The CS-Pure film is prone to swelling when encountering water, with its length being increased from 2 to 4.8 cm after immersing in water for 6 h.A similar result was also found for the CS-Gly and CS/CA-1 films.The three kinds of films can be easily shattered by a small external force after immersion.The CS/CA-2 and CS/CA-3 films show almost no expansion.They maintain good mechanical properties after the immersion.The insets in Figure 2e,f show that the CS/CA-3 film (6 cm × 1.5 cm, thickness 0.3 mm) can bear a weight of 730 g in air and water.The CS/CA-2 film is broken under the same loading in water.The result indicates that the CS/CA-3 film still has good mechanical properties even in water.The swelling is calculated according to the following equation: where, l 0 and l are the length of the film before and after immersion, respectively. [29]The result was shown in Figure 2e.
The swelling value of the CS/CA-3 film is 2.5%, the minimum value among the films.A large value of 140% is found for the CS-Pure film, which is the maximum value among the films.The water uptake value is calculated according to the following equation: where, m 0 and m are the mass of the film before and after immersion, respectively. [29]As shown in Figure 2f, the water uptake value of the CS/CA-3 film is 6.68%, the lowest among these films.
A large value of 1282.4% is achieved for the CS-Pure film.The above results demonstrate that the co-existence of moisture resistance and water resistance in the samples modulated by citric acid doping.
The above results reveal that the excellent properties of water resistance, flexibility, softness, and foldability of the CS/CA-3 film are due to the cross-linked structure and the addition of glycerol.When the mass of citric acid is low, carboxyl groups on one citric acid molecule can react with chitosan molecules to form a cross-linked structure like product I as shown in Figure 3a.When the mass of citric acid is high, only a few carboxyl groups of citric acid can react with chitosan and form a structure like product II.The low swelling value and water uptake value of the CS/CA-3 film are attributed to the decrease of carboxyl group and amino group which are hydrophilic functional groups.As Figure 3b shows, the reticular cross-linked structure results in better water resistance and mechanical properties of the CS/CA-3 film.

The Working Principle of TENGs
To test and assess the electrical output of the CS/CA film, we fabricated kinds of TENGs based on the CS-Pure, CS-Gly, and CS/CA films.The working principle of the TENGs is shown in Figure 3c and it works by the contact and separation between the chitosan and FEP films.When the chitosan film contacts the FEP film, the contacted surfaces of the two films generate opposite polarity and carry the same amount of charges because of the different electronegativity between the two films.When they are separated, electrons on Cu foil flow to keep electrostatic balance through the external load.When they are contacted again, the electrons on the Cu foil flow once again to keep the balance. [30]he alternating current is formed due to the continuous flow of electrons during the constant contact-separation process.

Electrical Output of the TENGs
The output properties of the TENGs were measured at a pressure of 30 N and a frequency of 1 Hz to evaluate the effect of citric acid on the triboelectricity of the chitosan film, and the results were presented in Figures 4a,b.The CS/CA-1 TENG shows the highest voltage and current output among these TENGs.Compared with the CS-Pure TENG, the output performance of other TENGs is improved obviously.When the mass ratio of chitosan to citric acid is changed from 3:1 to 1:1, the output voltage is improved from 77.5 to 82.4 V and the output current is improved from 2.66 to 3.96 μA.With the increase of citric acid addition, the output voltage and current are improved slightly.Compared to the CS-Pure film, the electronegativity of the CS-Gly film is decreased due to the hydroxyl group of glycerol, which is a strong electron-donating group.Carboxyl groups are weak electron withdrawing groups and amide groups are electron-donating groups.Under the combined influence of the above groups, the electronegativity of the film decreases and TENGs fabricated by these films output higher voltage and current with the increase of citric acid.Although the output performance of the CS/CA-1 TENG was the best, the CS/CA-3 TENG was used for the following tests because of the good water resistance and mechanical properties of the CS/CA-3 film.Which can be the most suitable film used to fabricate skin-attachable motion sensor and self-powered tactile sensor.
To test the output performance of the CS/CA-3 TENG, the pressure and impact frequency of external force were changed.The output voltage increases from 43.2 to 84.1 V and the output current increases from 1.32 to 3.56 μA as the external pressure is increased from 10 to 40 N at a fixed frequency of 1 Hz.As Figures 4c,d show, the output value is positively related to the external force.As the external force is increased, the effective friction area between the CS/CA-3 and FEP films increases, resulting in improved output performance of the CS/CA-3 TENG 12 .When the impact frequency is increased from 0.5 to 2.5 Hz at a constant pressure of 20 N, the output voltage is ≈61 V, and the output current increases from 1.08 to 3.18 μA.As Figures 4e,f show, the output current is positively related to the impact frequency but the output voltage keeps a stable value.To further assess the stability and working durability of the CS/CA-3 TENG, it was tested under 20 N pressure and 1 Hz for 1000 s.The output voltage maintains a stable value ≈62 V as shown in Figure 4i, confirming good practicability and long-term-run performance of the TENG.Besides, the output power density of the CS/CA-3 TENG was measured on different external load resistances, and the result was shown in Figure 4g.The highest output power density reaches 10 μWcm −2 when the internal impedance of the TENG matches the external loadings (≈5 MΩ).To show the useful application of the TENG, up to 25 white LED lights can be lit by the TENG as shown in the lower right inset of Figure 4g and Video S1 (Supporting Information), indicating that the ability of the TENG as a direct power source.Besides, three kinds of capacitors were charged by the TENG.They were linked with the TENG and charged via a bridge rectifier.The CS/CA-3 TENG can charge a 10 μF capacitor up to 2.5 V in 100 s, a 33 μF capacitor to 1.83 V in 132 s, and a 47 μF capacitor to 1.62 V in 140 s.These observations imply that the energy transformed by the TENG can be stored in the capacitors as a voltage source.

Humidity Tests of the CS/CA-3 TENG
To further explore the effect of environment humidity on the output of the CS/CA-3 TENG, it was tested under 20 N pressure and 1 Hz in an airtight environment as shown in Figure S3 (Supporting Information).The result was presented in Figure 4j.The voltage output decreases slowly with increasing the relative humidity range from 20 to 60%RH.The output voltage decreases more obviously with the increasing humidity from 70 to 99%RH.The CS/CA-3 TENG can output 70.16% of the voltage under 80%RH and 46.2% of the voltage under 90%RH compared to that under 20%RH.The CS/CA-3 TENG maintains good output performance under high humidity due to the addition of citric acid.Amino groups in chitosan are hydrophilic functional groups, which make chitosan film absorb abundant water molecules in high humidity level.Therefore, the output voltage of TENG based on unmodified chitosan film drops rapidly 20 .Through an amidation reaction between the carboxyl of citric acid and amino of chitosan, the decrease of hydrophilic functional groups reduces the absorption of water molecules and improves the humidity resistance of the CS/CA-3 TENG.To further investigate its use for human motion monitoring as the motions are usually influenced by sweat, the CS/CA-3 TENG was tested after 1 min of spraying water mist on the CS/CA-3 film by a humidifier as shown in Figure 4k.The result was displayed in Figure 4l, in which the recovery time of the CS/CA-3 TENG is ≈20 s.This result indicates that the CS/CA-3 TENG has good output performance in high humidity environments and has good recoverability from high humidity to low humidity.It can be used in a wide range of environment humidity.

Application of the CS/CA-3 TENG
The CS/CA-3 film is extremely sensitive to external forces, so we fabricated a self-powered tactile sensor as illustrated in Figure 5a.According to the preparation method mentioned above, a crossshaped real-time illumination tactile sensor was fabricated using an acrylic board as the substrate, chitosan, FEP films as a tactile sensor, and Cu foil as an electrode.Each of these substrates utilizes TENG's typical contact-separation structure and is connected to a bulb via wires.When a finger touches the sensor, mechanical energy is transferred to electrical energy by the TENG and the corresponding indicator lights up immediately as shown in Video S2 (Supporting Information).Consequently, a skin-attachable sensor was proposed to collect human activity energy and monitor human motion.The sensor was attached to the elbow and wrist.Figures 5b,c demonstrate the high output voltage of the sensor.When the elbow bends from 0 to 150°, the sensor outputs a voltage of 33 V as shown in Video S3 (Supporting Information).When the wrist bends from 0 to 70°, the sensor outputs a voltage of 40 V as shown in Video S4 (Supporting Information).By outputting pulses, we can obtain information on movement time and amplitude, while also collecting movement energy.These results successfully demonstrate that the self-powered tactile sensor and the skin-attachable sensor based on the CS/CA-3 TENG have broad application prospects in the future.

Conclusion
In summary, we synthesized a flexible, foldable, and hydrophobic chitosan-based TENG by cross-linking with citric acid and plasticizing with glycerol.Due to the cross-link structure of the CS/CA-3 film, it exhibits excellent water resistance, load-bearing property, and triboelectricity as compared with the unmodified chitosan film.The CS/CA-3 TENG outputs a voltage of 77.5 V and a current of 2.66 μA under 30 N pressure at 1 Hz and reaches the highest output power density of 10 μWcm −2 .Under 80%RH, the CS/CA-3 TENG can output 70.16% of the voltage compared to that under 20%RH.Besides, it can maintain good output stability after continuous operation for 1000 cycles.The brilliant water resistance ensures its application in high humidity environments.The TENG is used for self-powered tactile sensors showing the ability to accurately monitor human movements and will have promising applications in flexible, wearable, humidity-resistant, and self-powered electronics and devices.

Figure 1 .
Figure1.The cross-link reaction between chitosan and citric acid enhances water resistance, the addition of glycerol improves the softness, and the addition of glycerol and citric acid results in low electronegativity and higher triboelectricity of the chitosan-based film.

Figure 2 .
Figure 2. a) XRD patterns of the chitosan films with various composite ratios.b) The FTIR spectra of the chitosan films in full range, and c) in range from 1800 to 600 cm −1 .d) The flexibility and foldability performance of the chitosan films.e) Swelling value of the chitosan films soaked in water for 6 h and f) Water uptake value of the chitosan films soaked in water for 6 h.The insets show the additional tensile force that the CS/CA-3 film can carry in air and water.

Figure 3 .
Figure 3. a) Illustration of the amidation reaction between carboxyl of citric acid and amino of chitosan, and two kinds of structures of the products (I and II).b) The cross-linked mesh structure of product I. c) Working principle of the CS/CA TENGs.

Figure 4 .
Figure 4. a) The output voltage and b) current of the TENGs fabricated by different chitosan films.c) The output voltages and d) currents of the CS/CA-3 TENG under various pressures.e) The output voltage and f) current of the CS/CA-3 TENG under different impact frequencies.g) The output voltage and instantaneous power density on different external load resistances.The upper left inset shows the circuit diagram.The lower right inset shows LED lights lighted by the TENG.h) Response of different capacitors charged by the CS/CA-3 TENG.i) Stability test on voltage output of the CS/CA-3 TENG.j) The output voltage of the CS/CA-3 TENG under various humidities between 20 and 99%RH.k) Diagram of high humidity recovery test.l) A recovery period for the CS/CA-3 TENG from high humidity to low humidity.

Figure 5 .
Figure 5. a) Schematic of the self-powered tactile sensor, the corresponding small light bulbs light up when different parts of the sensor are touched (indicated by green blocks).The photograph of b) elbow bending activity c) wrist bending activity and the output voltage pulses using the CS/CA-3 TENG as a motion sensor.

Table 1 .
The ratios of chitosan, citric acid, and glycerol of the prepared films.