Characteristic Study of Self‐Powered Sensors Based on Native Protein Composite Film

Flexible electronic sensors composed of flexible film and conductive materials play an increasingly important role in wearable and internet information transmission. It has received more and more attention and made some progress over the decades. However, it is still a great challenge to prepare biocompatible and highly transparent conductive films. Egg white is a pure natural protein‐rich material. Hydroxypropylmethyl cellulose has a good compatibility and high transparency, which is an ideal material for flexible sensors. Here, we overcome the problem of poor mechanical flexibility and electrical conductivity of protein, and develop a high transparency and good flexibility hydroxypropylmethyl cellulose/egg white protein composite membrane‐based triboelectric nanogenerator (‘X’‐TENG). The experimental results show that the flexible pressure sensor based on ‘X’‐TENG has a high sensitivity, fast response speed, and low detection limit. It can even be used as a touch/pressure sensing artificial electronic skin. It can also be made into an intelligent waffle keyboard for recording and tracking users of the keyboard. Our strategy may provide a new way to easily build flexible electronic sensors and move toward practical applications.


Introduction
At present, with the rapid development of the internet of things, the demand for various small electronic devices and sensors is gradually increasing.In view of their light weight, portability, and sensitive information transmission, they are widely used in medical monitoring, [1,2] human motion monitoring, [3][4][5][6][7] intelligent keyboard, [8] smart home, and other fields.Since the triboelectric nanogenerator (TENG) was proposed by Academician Zhonglin Wang [9] in 2012, it has caused a major heat wave in the scientific research community.In principle, when two materials, even they are chemically identical, are brought into physical contact, electrostatic charges are created on the surfaces due to the contact electrification effect.Everything can be used as power generation materials for triboelectric nanogenerators because triboelectric nanogenerators are based on the coupling effect of triboelectric electrification and electrostatic induction.It is a new energy harvesting method that converts the small mechanical energy in life into electrical energy. [10,11][27][28][29] These biocompatibility materials have strong electron-loss ability.
However, in the current research field, most of the above mentioned biomaterials are used when adding organic solvents, which leads to poor biocompatibility and even skin allergy in the field of flexible electronic skin, thus limiting their applications.Therefore, it is very important to choose natural materials.For example: egg white (EW), and we all know that eggs are divided into yolk, EW, eggshell membrane, and eggshell.Eggshell membrane is a natural, biologically friendly material.Yan et al. [30] have published articles on triboelectric nanogenerators in 2021 using eggshell membrane.And other researchers, Charoonsuk et al. [31] in the latest research in 2021, added eggshell membrane as a natural filler into chitosan, discussed the performance output of its composite CS/ protein membrane for TENG, and used it as an energy harvester.
Here, we report a green and simple synthetic route to obtain flexible, biocompatible native EW composite membranes.Hydroxypropyl methyl cellulose (HPMC) with good performance and strong electron loss ability was added to dissolve it to obtain HPMC/EW composite membrane (CEWM).We systematically study its triboelectric properties, including functional materials, humidity, area, load, etc. CEWM film exhibits enhanced triboelectric properties and stability compared with pure EW film.Therefore, CEWM triboelectric nanogenerator ('X'-TENG) based on CEWM can be used as a self-powered micro/nano power sources to drive small electronic devices.In addition, it shows a good sensitivity to humidity and pressure, which makes it exhibits Flexible electronic sensors composed of flexible film and conductive materials play an increasingly important role in wearable and internet information transmission.It has received more and more attention and made some progress over the decades.However, it is still a great challenge to prepare biocompatible and highly transparent conductive films.Egg white is a pure natural protein-rich material.Hydroxypropylmethyl cellulose has a good compatibility and high transparency, which is an ideal material for flexible sensors.Here, we overcome the problem of poor mechanical flexibility and electrical conductivity of protein, and develop a high transparency and good flexibility hydroxypropylmethyl cellulose/egg white protein composite membrane-based triboelectric nanogenerator ('X'-TENG).The experimental results show that the flexible pressure sensor based on 'X'-TENG has a high sensitivity, fast response speed, and low detection limit.It can even be used as a touch/pressure sensing artificial electronic skin.It can also be made into an intelligent waffle keyboard for recording and tracking users of the keyboard.Our strategy may provide a new way to easily build flexible electronic sensors and move toward practical applications.potential applications in sensing, and has been used for biomechanical energy harvesting and sensing, applied to smart waffle keyboards.It is expected to be a wearable electronic device for detecting human motion, medical health.

Result and Discussions
We are not strangers to eggs, it is pure natural protein.In this study, HPMC-modified EW was used to prepare composite membrane.First of all, Figure 1a is the detailed process of making composite membrane.The HPMC powder added in CEWM is shown in Figure 1b.The surface SEM of HPMC powder is shown in Figure 1c.The common single elements of C, Na, O and Cl in the corresponding EDS spectrum analysis are shown in Figure 1d. Figure 1e shows the product photo of CEWM.
The tensile test of CEWM was performed, as shown in Figure 1f.The breaking extension of CEWM was 7% and the breaking strength was 5 MPa.The embedded figure is the real picture of stretching, where the measured thickness, width, and length of CEWM are 0.12, 10.45, and 18.34 mm, respectively.The EW, HPMC powder, and the CEWM were also tested by infrared (IR) spectroscopy.Figure 1g shows the IR spectra of EW, HPMC powder, and CEWM, respectively.It can be clearly seen that the infrared spectrum of EW has obviously different functional groups at the marked 3012, 1639, and 1535 cm À1 , while the infrared spectrum of HPMC can be seen in 2908, 1458, 1045, and 796 cm À1 have obvious characteristic peaks.Finally, compared with the infrared spectrum analysis of EW and HPMC, it can be seen that in the infrared spectrum of HPMC/EW protein composite membrane (CEWM), it has a perfect fusion around 3024 cm À1 to appear EW-3012 cm À1 The characteristic peaks of cm À1 and HPMC-2908 cm À1 , and the characteristic peaks of EW-1639 cm À1 also appeared in the CEWM infrared spectrum analysis, as well as HPMC-1458 cm À1 , 1045 cm À1 .The characteristic peaks of 796 cm À1 is also exhibited.This is enough to indicate that our first proposed HPMC-EW composite natural protein-based friction layer has some significance.As revealed by the cross-sectional scanning electron microscope (SEM), the microstructure of CEWM, at 10 and 100 lm (Figure 1h,i).And the HPMC has high transparency, good solubility, and still has high transparency after mixed with EW. Figure 1j shows the CEWM (HPMC/EW film), when the double layer folding, the English letter "Nanoenergy and Nanosystems" in the cover of "NENS 2016" can be clearly seen through the membrane.
In order to better explain the application of this composite membrane in sensors, we design it as a flexibility HPMC/EW protein composite membrane-based triboelectric nanogenerator ('X'-TENG), which is connected in parallel on both sides.It is not only novel in structure, but also can improve its output performance.Figure 2a is shown below.And we used the cardboard with moderate hardness and easy bending as the substrate, and adhered sponge, Al, and PTFE on the same leaf, and Al and compound EW membrane (CEWM) on the other leaf, respectively.Higher output can be achieved when tapping gently.Figure 2b shows the working principle of 'X'-TENG under the condition of vertical contact separation mode.Figure 2b(i) in the initial state, fully contacted, no charge generated, no potential difference between the two electrodes.In Figure 2b(ii), when the force pressed on 'X'-TENG is unloaded, the surface of PTFE and CEWM is separated due to inertia and the elasticity of the card itself, and the potential difference between the two electrodes is formed in open circuit.The current direction is shown in the diagram.When the distance between the two polymer surfaces  3a, that is, the comparison of the transferred charges of the two fiber/protein composite membrane-based triboelectric nanogenerators is not very obvious, but it can also be seen that the transferred charge of the HPMC/EW protein composite membrane-based triboelectric nanogenerator was slightly higher than that of the CNF-C 2.5/EW protein composite membrane-based tribo-nanogenerator.By comparison, we finally chose HPMC to be added to natural EW to make a protein composite membrane as the natural polymer friction layer in this study.Figure 3b is the output performance of open circuit voltage, short circuit current and transfer charge of 'X'-TENG after HPMC is dissolved in EW at different ratios.It can be clearly seen from the comparison line graphs that the three line graphs all increase first and then decrease, and it can be seen from the graph that when 20% HPMC is dissolved in 25 mL EW, the prepared HPMC/EW protein composite membrane is used as the natural polymer friction layer of 'X'-TENG has the best output performance.Figure 3c-e shows the open-circuit voltage, short-circuit current, and transfer charge of 'X'-TENG at different frequencies.And the open-circuit voltage and transfer charge are relatively stable.The current increases with the increase of frequency.Figure 3f-h using Kapton, PET, FEP, PVC, PTFE several different friction layers for comparative testing, measurement of 'X'-TENG output performance comparison.It can be seen that the open-circuit voltage, short-circuit current, and transfer charge corresponding to FEP are the largest, with the voltage up to 40 V, the short-current up to 2.5 lA, and the transfer charge of 24 nC.Therefore, PTFE-HPMC/EW protein composite membrane was selected as the two friction layer materials in this paper.Here, in order to facilitate the experimental comparison and measurement, a linear motor is used to simulate the contact separation working mode of the 'X'-TENG.
The humidity in the environment seriously affects the output performance of the TENGs.Here, we conducted an experimental investigation and test, placing the humidifier in a sealed glass cover to affect the HPMC/EW protein composite membrane.By adjusting the humidifier to change the humidity in its closed environment, thereby simulating the effect of environmental humidity on the output of 'X'-TENG in a real-world scenario.Figure 4a-c shows the changes of open circuit voltage, short circuit current and transfer charge when the humidity is increasing.The voltage is reduced from 25 to 7 V, the current is Energy Environ.Mater.2024, 7, e12492 reduced from 0.37 to 0.15 lA, and the charge is reduced from 24 to 2.5 nC. Figure 4d shows the intuitive color distribution and the numerical distribution of open circuit voltage, short circuit current and transfer charge under different humidities.Here we have analyzed that the higher the humidity, the lower the output performance of the CEWM-based triboelectric nanogenerator is because with the increase of humidity, the water molecules in the air will stay on the surface of the CEWM protein composite film, thus affecting its performance.The magnitude of the surface charge, ultimately, has a shielding effect on the work of CEWM-based triboelectric nanogenerators.Therefore, in order to avoid the influence of humidity instability on the output performance of CEWM-based triboelectric nanogenerators in practical applications, we usually seal and assemble CEWM-based triboelectric nanogenerators when using them.The threedimensional figure in Figure 4e shows that the opencircuit voltage, short-circuit current, and transfer charge increase from 15 V, 1.3 lA, 7.4 nC up to 33 V, 3 lA, 27 nC when the contact area increases gradually from 8.75 to 78 cm 2 .This also shows that CEWM-based triboelectric nanogenerators can choose appropriate contact areas for different applications.Figure 4f,g show that as the external resistance value increased from 1.01 to 1111.11MΩ, the peak value of output current decreased and the peak value of output voltage increased gradually, while the output power first increased and then decreased.When the resistance value was adjusted to 71.11 MΩ, the power reached the maximum value of 6.65 lW. Figure 4h shows the stable output of 'X'-TENG within 220 s.It can be seen that the average stable output reaches 74 V, and the embedded figure is the output amplification figure of about 6 s.It can be seen that the triboelectric nanogenerator based on the HPMC/EW protein composite film can perform stable work for a long time.
Because the designed 'X'-TENG is a vertical contact mode of triboelectric nanogenerator, CEWM has a strong triboelectric positive polarity, which can generate electrical output when contacting/separating from the PTFE active layer with the opposite triboelectric tendency.'X'-TENG and the PTFE layer can produce periodic electrical output through a continuous contact/separation process under the external force.We have done the touching and pressing tests.The sensitivity of the 'X'-TENG device is measured by pressing different fingers, as shown in Figure 5a.Interestingly, the shape of the output signal varies with the pressing of different fingers.In Figure 5b, the voltage waveform of the pressure sensor based on 'X'-TENG finger pressing is shown.When a finger is pressed on the device, only one peak is detected, while when two fingers and three fingers increase to five fingers pressed on the device, two, three, and five peaks appear respectively.Figure 5c is the current sensitivities of different finger numbers, respectively.It can  be seen that the number of fingers is proportional to the current signal peaks.Here, we can apply it to touch/pressure sensing artificial electronic skin by signal testing.
As a triboelectric bio-nanogenerator, under force power by a linear motor, the 7 9 5 cm 2 'X'-TENG was used to charge commercial capacitors with different capacitances.As shown in Figure 6a(i,ii), the length and width of the selected friction layer are measured.Figure 6b is a photograph of 'X'-TENG under a linear motor test.And the 'X'-TENG can light 120 LEDs under human pressing, as shown in Fig- ure 6c. Figure 6d shows the voltage diagram for charging capacitors with 3.3, 10, 22 and 47 lF capacities.Among them, after 'X'-TENG is rectified by rectifier bridge, 22 lF capacitor in 2 min can reach 3 V, can directly drive the electronic watch, as shown in Figure 6e.At the same time, the 47 lF capacitor can reach a voltage value of 22 V at 5 min, which can drive a household hygrometer directly, as shown in Figure 6f.Since CEWM has good softness and transparency, we set it as a kind of waffle keyboard through the structure of waffle.When different people 'A' and 'B' use the keyboard respectively, they can produce different sensing performance for tracking and recording the user of the keyboard.Figure 6g is the process simulation diagram, which is people transmitting their signals through the 6514 electrometer system to the computer display screen when using the smart waffle keyboard.We choose different people 'A' and 'B' to test respectively, and record the sensing signal when pressing 'Q-W-E-R-T' different letters in turn.The test results are compared as shown in Figure 6h.There are obvious differences, which are enough to track users of the waffle keyboard.We also selected 'B' users for more smart waffle keyboard tests.Figure 6i shows that the two peaks are almost identical for using the keyboard to tap 'T-E-N-G' sensor signal records.Figure 6j is the sensing signal when pressing out 'B-I-N-N' in turn, each color indicates the output of the same alphabet.Figure 6k also obtained the signal sensing in two knocks 'X-J-H', the voltage values are 2.092, 1.731, 1.626 V, respectively.In addition, the response time of the induction signal after hitting the keyboard is within 1 s, which shows excellent sensitivity.

Conclusion
In this work, an efficient protein-based complex for biosensor applications was innovatively prepared by adding HPMC as a filler and natural EW as a substrate.The electrical output of different types of cellulose for 'X'-TENG was studied.The results showed that the addition of HPMC had higher output performance, and different HPMC content was tested and analyzed.Under the optimum conditions, CEWM formed a good protein composite film.The tensile properties of CEWM were also discussed.And the influencing factors under different frequencies, different humidity and different areas.Finally, by comparing the materials, the PTFE with strong electron capture ability can be selected to interact with it to obtain higher output signals.In addition, under the pressure, 'X'-TENG can achieve an average open circuit voltage of 120 V, a short circuit current of 12 lA, and a transfer charge of 50 nC, which is sufficient to light 120 LEDs.It can also charge the capacitor and drive small electronic devices such as watches and hygrometers.In the sensing field, CEWM can be used as a touch/pressure sensing artificial electronic skin.And we designed an intelligent waffle keyboard based on biological protein composite film that can be used to record and track users using the keyboard.

Experimental Section
Preparation of HPMC/EW composite film: A native egg was separated into beakers by liquid-liquid separation, and the EW of each egg was measured to be approximately 25 mL.Then, HPMC with different contents (10%, 20%, 40%, 60%, and 100%) was weighed on the weighing paper with an analytical balance.HPMCs with different contents were respectively poured into each beaker containing 25 mL EW, and then magnet was added.The mixed film- forming solution was placed in a constant temperature water bath with a temperature of 40 °C and stirred for 35 min until HPMC was completely dissolved in the composite film-forming solution in EW and then ultrasonic defoamed for 10 min.Then the film-forming solution was poured onto dustfree paper, dried at 45 °C for 5 h on PET substrate, and cooled to form the film.
Fabrication of the TENG devices: The working mode of TENGs is vertical contact mode, using Al HPMC/EW conductive electrode as friction layer, polytetrafluoroethylene (PTFE) as dielectric material.
Measurement and characterizations: The surface morphology of HPMC powder and HPMC/EW (EW) composite film was tested by scanning electron microscope (Hitachi s-4800).The tensile property and EDS elemental analysis of HPMC/EW composite membrane were tested by (Yl-s71) peel strength tester and EDS (Nova), respectively.The infrared spectra of natural EW, HPMC powder and HPMC/EW composite membrane were tested by fourier transform infrared spectrometer.Measurement of influencing factors of separated 'X'-TENG connected to linear motor and keithley 6514 system for measuring open circuit voltage, short circuit current and transfer charge.In order to measure the power output of the 'X'-TENG, the ZX21 rotary DC resistance box system was used to provide external load.

Figure 1 .
Figure 1.Fabrication of CEWM and other property characterization.a) The fabrication process of the CEWM.b) Physical photograph of HPMC powder.c) SEM image of HPMC powder.d) EDS analysis of HPMC powder.e) HPMC/EW film display prepared.f) The tensile test of as-prepared HPMC/EW composite film.g) The IR spectra of EW solution, HPMC powder and CEWM, respectively.h, i) SEM image of HPMC/EW composite film.j) Transparency display of CEWM under double folding.

Figure 2 .
Figure 2. The structure, principle, potential distribution and output performance of 'X'-TENG.a) The basic structure of 'X'-TENG.b) The working principle diagram of 'X'-TENG.c) The electrical potential distribution of 'X'-TENG under different working state.d-f) Electrical output of the 'X'-TENG.

Figure 3 .
Figure 3.Comparison of output performance of the different 'X'-TENG at different cellulose materials, ratios, frequency, friction layers.a) Comparison of voltage, current and charge of different synthetic films at different frequencies.b) Output performance of composite membranes with different HPMC ratios.c-e) The voltage, current and charge output of the CEWM at different frequencies.f-h) The 'X'-TENG output under different friction layers.

Figure 4 .
Figure 4. Output performance and stability of 'X'-TENG under different humidity, area and load.a-c) The voltage, current, and charge output of the CEWM at different humidities.d) Intuitive color distribution of voltage, current, and charge output at different humidity.e) Three-dimensional cylindrical distribution of voltage, current, and charge in different areas.f) Output voltage and output current of 'X'-TENG at different load resistances.g) Output power of 'X'-TENG at different load resistances.h) Stable output voltage of 'X'-TENG.

Figure 5 .
Figure 5. Sensitivity of 'X'-TENG device pressed by different fingers and different number of fingers.a) Current signal of different fingers pressing 'X'-TENG.b, c) Respectively represent the sensitivity of the 'X'-TENG device to the voltage and current measured by pressing different fingers.