Research on flexible similar cicada wing acoustic sensor using MXene/PVA thin film

ABSTRACT

With the rapid development of digitization and the Internet of Things, higher requirements have been put forward for the portability, process structure, material cost, sensitivity, and durability of sensors. This study proposes an MXene/PVA flexible self powered similar cicada wing triboelectric pressure sensor (CTPS) based on the principles of frictional electrification and electrostatic induction. Explored the film-forming performance and response sensitivity under different mass ratios of MXene to PVA. The manufactured sensor uses MXene/PVA composite material as the negative electrode material and graphene as the positive electrode material. In order to further improve the response sensitivity, the negative electrode material of the sensor is made into a similar cicada wing like biomimetic structure. The developed sensor can efficiently convert external mechanical forces and sound signals into electrical signal outputs. It achieves a sensitivity of 915.32 mV/N under a pressure of 0–5 N (frequency < 20 Hz), and a response sensitivity of 49.633 mV/Pa to 1 kHz sound waves. And the developed sensor is very lightweight, with a thin film thickness of only 30 um. At the same time, it can maintain constant voltage output in more than 6000 force cycle tests and 3 hours of sound cycle tests. The effectiveness and application potential of CTPS in the field of force and sound perception were verified through actual detection of sound signals from speakers.

Keywords:
Bionics; MXene; Frictional electrification; Electrostatic induction; Acoustic sensors

1. INTRODUCTION

Vibration and sound phenomena are widely present in nature and human activities. Sensors can convert external forces and physical quantities such as sound into electrical signals, and are an indispensable and important component in achieving environmental monitoring, health monitoring, fault monitoring, artificial intelligence, and industrial production [¹, ², ³, ⁴, ⁵, ⁶]. As a result, there are more requirements for signal acquisition, which has led to various developments and changes in sensors. Flexible pressure sensors based on different signal conversion mechanisms, such as frictional [⁷, ⁸], piezoelectric [⁹, ¹⁰], resistive [¹¹, ¹²], capacitive [¹³, ¹⁴], and fiber optic [¹⁵, ¹⁶], have gradually been developed. With the rapid development of science and technology, human society will inevitably form super intelligent digital cities, and building the foundation of super intelligence and digital economy requires IoT clusters, which require a very large number of sensor facilities [¹⁷, ¹⁸, ¹⁹, ²⁰, ²¹, ²²]. Therefore, the force and sound sensors in this study have broad application prospects and can better promote the development of the Internet of Things era.

The working principle of triboelectric nanogenerator (TENG) is based on the coupling effect of frictional electrification and electrostatic induction, which converts mechanical energy into electrical energy [⁴]. It has important advantages such as simple structure, wide material selection, excellent output performance, easy manufacturing, and the ability to form flexible and stretchable devices, which enable it to effectively collect energy from different environments, such as vibration, sound, wind, and human motion. Compared with other sensors that utilize resistors, capacitors, and optical fibers, TENG has advantages such as wider energy harvesting, self powering, and lighter weight.

TENG, as an emerging sensing technology, has broad application prospects and is receiving increasing attention. The important factors that determine the electrical and mechanical performance of TENG include device structure, surface morphology, and the type of frictional material used. More and more scholars are searching for better structures and materials to optimize and improve the performance of TENG devices [²³, ²⁴, ²⁵]. YANG et al. [²⁶] developed a novel self powered frictional electroacoustic sensor using frictional electricity technology. Using PVDF film and metal film as electrodes to measure the sound signal output by the speaker, it can perceive the full frequency sound waves of human auditory perception. However, utilizing the thickness and weight of metal thin films can still have adverse effects on the sensor output signal.

Due to its simple structure, self powering, and excellent output performance, various biomimetic structures based on frictional electric sensors have also been further explored. Biomimetic structures involve designing and manufacturing to mimic the natural world, drawing inspiration from the unique shapes, patterns, and functions of biological organisms. Biomimetic design can mimic the structure, surface morphology, material properties, and motion mechanisms of organisms in nature, and find suitable sensing and power generation mechanisms to greatly improve the performance of TENG. This method has been proven effective in constructing new functions and more efficient structures, such as in the field of environmental protection where biomimetic sensors are used to detect organic substances such as bacteria and viruses in water, as well as inorganic substances such as oil and chemicals in water; In the medical field, bionic sensors can detect insulin levels in diabetes patients and drugs, as well as biological parameters such as respiratory rate and heart rate, and can transmit relevant information through wireless transmission technology; Biomimetic sensors are used in the field of food safety to detect pathogens and hygiene indicators in various foods; Biomimetic sensors can be used in the production field to monitor the preparation process of specific drugs, etc. [²⁷].

Biomimetic structures have become a hot topic for researchers to explore in recent years. YAO et al. [²⁸] have developed a series of biomimetic nanogenerators using piezoelectric and frictional electrification effects. By designing biomimetic microstructures on the surface structure of the blade, a frictional layer with conformal interlocking structure was manufactured, which significantly improved the sensitivity of pressure measurement. Biomimetic structures have great advantages, from the superhydrophobic lotus leaf surface, which repels water due to its micrometer sized papillae structure [²⁸], to the unique functional characteristics of the Bouligand structure of lobster claws and the brick and tile structure of natural pearls [²⁹, ³⁰, ³¹, ³²]. Biomimetic structures have many significant properties, including mechanical properties [³³], and energy absorption capacity [³⁴], which have aroused great interest among people. Biomimicry has provided solutions to many practical engineering problems and has been widely applied in many fields such as energy, environment, biomedical, and building structure applications.

In recent years, 3D printing technology has developed rapidly, and its advantages of low cost and high precision make it a powerful tool for manufacturing high-precision complex structures. It can directly generate devices from digital models, reducing the design and development steps of molds and shortening the research cycle. Inspired by the hierarchical structure of human epidermal tissue, researchers LIU et al. [³⁵] proposed a high-sensitivity flexible capacitive pressure sensor with biomimetic spinal microstructure. The proposed sensor exhibits good deformability, high sensitivity in the low-pressure range, and a wide linear dynamic pressure range from 5 Pa to 100 kPa. However, such complex biomimetic structures are often challenging, but the development of 3D technology has greatly reduced the difficulty of design and production.

2. PREPARATION OF SENSORS

2.1. Preparation of positive electrode materials

Polyvinyl alcohol (PVA) is an important organic synthetic polymer material with unique properties and wide applications. PVA is usually a white or milky white powder, flake or flocculent solid, odorless, and has good physical and chemical stability. Meanwhile, PVA has a high solubility in water, especially at high temperatures, and can dissolve in over 95% of water. In terms of biodegradability, PVA is also a biodegradable material that can degrade about 75% within 46 days under the action of bacteria and enzymes, which is particularly important in environmental protection.

The PVA used in this study is a white sheet-like solid, which is dissolved in water to prepare a PVA aqueous solution. The resulting film has good flexibility and insulation properties, making it suitable as a substrate for flexible sensors. Spin coat graphene solution onto the surface of PVA film to form a conductive layer, which serves as the positive electrode layer of the sensor.

The preparation process is shown in Figure 1a. Firstly, rinse the beaker, glass rod, glass plate and other experimental equipment with deionized water. Then dissolve 10 g of PVA solid in 90 g of distilled water, continuously heat it with hydrothermal method and maintain it at 90 °C–95 °C, while stirring continuously with a glass rod for 30 minutes. This method can fully dissolve PVA in water, and finally obtain 100 g of PVA aqueous solution with a mass fraction of 10%. Then spin coat the PVA solution onto the glass substrate to form a 30 um thick PVA coating, and place it in a drying device to cure at 40 °C for 5 hours. Afterwards, the PVA film was removed from the glass plate to prepare a 50 mm * 50 mm PVA film with a thickness of 30 um. Finally, the graphene solution was spin coated onto the PVA film to prepare the graphene electrode. The prepared film has a thickness of only 30 μm, and it combines flexibility with electropositivity, meeting the performance requirements for positive electrode materials of triboelectric sensors. At the same time, in order to find the most suitable sensor film for the research, various concentrations of PVA films were prepared in the experiment, such as PVA films with mass fractions of 5%, 10%, 15%, 20%, etc. After comprehensive comparison, it was found that PVA films with a concentration of 10% had the best flexibility, conductivity, and film-forming properties.

Figure 1
(a) Manufacturing process of sensor positive electrode; (b) manufacturing process of sensor negative electrode; (c) sensor hierarchical structure; (d) physical picture of cicadas and grasshoppers; (e) reference diagram of cicada wing model; (f) model diagram of similar cicada wing structure.

2.2. Preparation of negative electrode materials

MXene [³⁶, ³⁷] is a class of two-dimensional transition metal carbides, nitrides, and carbonitrides materials with a layered structure of Mn₊₁XnT_x (n = 1, 2, 3). Here, M represents transition metals (such as Ti), X is C/N, and T_x denotes surface functional groups (such as -OH, -O). It also features characteristics like high electrical conductivity, high thermal stability, and a large specific surface area. Meanwhile, MXene possesses a series of unique properties, such as excellent chemical stability, corrosion resistance, and good biocompatibility, etc.

In this paper, MXene materials were prepared from MAX phase precursors using a hydrofluoric acid (HF) etching method, with the specific process as follows: 5 g of Ti₃AlC₂ powder was slowly added to 50 mL of HF solution (concentration: 5 mol/L), followed by magnetic stirring at room temperature for 24 hours. During this process, the Al layer was selectively etched to form Ti₃C₂T_x MXene modified with surface functional groups (-F, -OH).The etched suspension was centrifuged at 8000 rpm for 10 minutes, and the supernatant (containing Al³⁺ and excess HF) was discarded. The precipitate was repeatedly washed with deionized water until the pH reached 6–7, and no F⁻ ions were detected (using silver nitrate titration). The precipitate was redispersed in an ethanol/water mixture (volume ratio 1:1) and subjected to ultrasonic treatment at 200 W for 1 hour to promote interlayer separation, forming a few-layer MXene suspension (concentration: approximately 5 mg/mL). The suspension was stored in the dark at 4 °C in a refrigerator.

1 g of MXene suspension (concentration: 5 mg/mL) was mixed with 4 g of 10% PVA aqueous solution by magnetic stirring for 20 minutes to form a uniformly dispersed MXene/PVA mixed solution. Then, spin coat it onto a glass substrate and place it in a drying device to solidify at 40 °C for 5 hours. Finally, remove the MXene/PVA film from the glass plate to prepare a 50 mm * 50 mm MXene/PVA film with a thickness of 30 um. The preparation process is shown in Figure 1b.

The sensor film developed by our research institute is very thin and has sufficient flexibility and electronegativity, which can be used as the negative electrode material for frictional electric sensors. At the same time, various MXene/PVA solutions with different ratios were prepared in the experiment. For example, different masses of MXene solutions were added to PVA solutions with different contents. Under the premise of setting the mass of the mixed solution to 5 g, PVA solutions with contents of 5%, 10%, 15%, and 20% and mixed solutions with MXene solutions of 0.5 g, 1 g, 1.5 g, and 2 g each were prepared. After comprehensive testing, it was found that adding 1 g of MXene solution to 4 g of 10% concentration PVA solution resulted in the best performance of the sensor film.

2.3. Assembly of sensors

As shown in Figure 1c, a flexible pressure sensor with a sandwich like structure is composed of polyvinyl alcohol (PVA) as the substrate, graphene layer as the frictional positive electrode, and MXene/PVA film as the frictional negative electrode. In addition, non-conductive double-sided tape is used to stick the three films together at the edges, and due to the thickness of the double-sided tape, there will be a certain gap in the middle to allow sufficient friction between the positive and negative films. In this study, a biomimetic similar cicada wing structure sensor film was used. In order to verify the superiority of the designed structure, various sensor films with different material ratios were also prepared to study the sensitivity and other properties of the sensor under different structures and materials.

2.4. Preparation of similar cicada wing structure

The species of cicadas are very diverse. In China alone, there are over 200 species of cicadas with wingspans ranging from 2–20 cm. However, the commonly seen cicadas do not have a large body size, with wings ranging from 20–50 mm. Therefore, this study selected the wings of the common cicada for research, with a wing length of about 50 mm. The actual cicada is shown in Figure 1d, and the reference image of the cicada wing model made in this study is shown in Figure 1e.

Due to the extremely thin and lightweight structure of cicada wings, their biomimetic structure can also be very lightweight, making the sensor highly sensitive and able to accurately and quickly reflect external stimuli. Moreover, their skeleton structure can improve the mechanical strength of the sensor, while also enhancing electrical stability. Even in harsh environments, they can maintain high performance and achieve good dynamic response. This is very advantageous for devices that require quick response and frequent adjustments. Overall, the cicada wing structure of the sensor can provide higher accuracy, better sensitivity, stronger stability, as well as lighter weight and easy integration advantages.

Using Comsol software, the desired biomimetic structure model of cicada wings was created by comparing it with the reference drawing of similar cicada wings shown in Figure 1e, as shown in Figure 1f. Then, the designed model was printed out using a 3D printer. The production of a similar cicada wing structure sensor film is similar to the preparation of a sensor negative electrode material film, except that the MXene/PVA solution is spin coated onto a 3D printed model instead of a glass plate. The subsequent preparation steps are the same.

3. DATA COLLECTION AND PRINCIPLE ANALYSIS

3.1. Building an experimental platform

In order to test the responsiveness of sensors and the performance of sound, an experimental platform needs to be built. The equipment mainly used in the pressure measurement experiment includes a signal generator, power amplifier, exciter, and signal acquisition card. The sensor data acquisition system is shown in Figure 2a, and its operation process can be summarized as follows: Firstly, the signal generator generates a corresponding sine signal, which is amplified by a power amplifier to operate the exciter. The signal from the end force sensor of the exciter is transmitted to the pressure acquisition system through the signal acquisition card. The force sensor at the end of the exciter impacts the frictional electric sensor, and the sensors come into contact with each other to transfer electrons.

Figure 2
(a) Pressure signal acquisition system; (b) sound signal acquisition system; (c) schematic diagram of charge transfer after sensor is subjected to force; (d-e) surface morphology of MXene; (f-g) surface morphology of MXene/PVA thin films; (h-i) surface morphology of graphene-PVA thin film.

The sound response experiment of testing sensors requires the construction of a new testing system. As shown in Figure 2b, the commercial speaker is driven by a power amplifier amplified by a sine wave generated by a signal generator. A commercial microphone is fixed next to the sensor and used to measure the decibels of sound applied to the sound sensor. Use a data acquisition card to measure the output signals of sensors and commercial microphones, and store the measurement data in a computer. When sound waves come into contact with the sensor, they generate sound pressure, causing the positive and negative electrodes of the frictional electric sensor to come into contact with each other and transfer electrons, resulting in a potential difference.

3.2. Surface morphology of sensor film and electrode layer

In order to observe the specific state of the positive and negative electrode layers of the prepared sensor, the surface morphology of the sensor film was observed using a field emission scanning electron microscope (SEM) [German Zeiss Ultra Plus]. The surface morphology of MXene/PVA film and graphene-PVA film can be observed, as shown in Figure 2c. The microstructure of the sensor film surface can be clearly seen, which meets the expected experimental requirements. Among them, the surface morphologies of MXene prepared by HF are shown in Figures 2d and 2e. Figures 2f and 2g show the surface morphology of MXene/PVA film, which is made of glass substrate and has a relatively smooth surface without obvious microstructure. The surface morphology of graphene-PVA film is shown in Figures 2h and 2i, where graphene is sprayed on one side of the PVA film. It can be seen that the sprayed graphene is relatively uniform and crack free, and can adhere well to the PVA film.

3.3. Conductivity test of MXene/PVA composite film by two-electrode method

The detailed procedure for testing the electrical conductivity of MXene/PVA composite films using the two-electrode method is as follows: First, the composite film is cut into a 20 mm × 20 mm test piece. Silver paste is applied to both ends to fix silver-plated copper electrodes with a diameter of 10 mm. The sample is then placed in a constant temperature and humidity chamber at 25 °C and RH = 50% to equilibrate for 1 hour. Subsequently, a Keithley 2400 source meter is used to apply a linear current from 0 to 10 mA to the sample in an electromagnetic shielding chamber, and the corresponding voltage values are recorded. The resistance is measured 5 times, and the average resistance is calculated. The conductivity is then calculated using the formula σ = d/(R·A)(where the film thickness d = 30 μm and the electrode contact area A ≈ 7.85 × 10⁻⁵ m²). For example, when the average resistance of a sample with a MXene/PVA mass ratio of 1:4 is 6.15 Ω, the conductivity is approximately 5.2 × 10³ S/m. Throughout the process, contact resistance is reduced by silver paste, environmental variables are controlled via constant temperature and humidity treatment, measurement accuracy is enhanced by averaging multiple readings, and geometric parameters are precisely calibrated—all ensuring the accuracy and repeatability of the test results.

The electrical conductivity of the composite film was tested using the two-electrode method. When the MXene/PVA ratio was 1:4, the conductivity reached 5.2 × 10³ S/m, close to the typical values (10³ – 10⁴ S/m) of Ti₃C₂T_x bulk materials, indicating the formation of a continuous conductive network. In this paper, the negative electrode of the sensor uses the MXene/PVA film with this ratio. Its high conductivity (5.2 × 10³ S/m) provides an efficient electron transport path for the triboelectric effect, directly supporting the sensor’s high sensitivity (915.32 mV/N). The systematic characterization of the electrical conductivity of MXene/PVA composite films by the two-electrode method confirms the key role of MXene content in the formation of conductive networks and provides basic electrical parameters for sensor design.

3.4. Analysis of sensor working principle

In this study, the frictional electric sensor used a sandwich like structure, consisting of a middle MXene/PVA film as the frictional electric negative electrode and two layers of graphene layers as the frictional electric positive electrode. Based on the principle of frictional electrification, when the middle layer MXene/PVA film comes into contact with the upper graphite layer, electrons will be injected from the graphite layer to the middle layer due to the polarity difference between the two materials. Afterwards, the middle layer leaves the upper graphite layer and moves downwards to the lower graphite layer. Electrons transfer charges from the middle layer, and periodic alternating current is generated afterwards, as shown in Figure 2c, which shows the transfer of charges after the sensor is subjected to force.

4. PERFORMANCE TESTING OF SENSORS

4.1. Mechanical performance testing of sensors

After the sensor preparation and assembly are completed, the sensing performance is tested by applying vertical periodic external pressure. The experimental equipment is shown in Figure 2a. To prevent the sensor from sliding, a double-sided tape is used to fix one side of the sensor on the testing platform, and an exciter is used to apply different sizes of pressure. The pressure sensor on the exciter can display the magnitude of the applied force and measure the mechanical properties of the sensor under different pressures.

The preparation of sensor membranes with different material ratios has a significant impact on their electrical and mechanical properties. Therefore, the materials used are configured in different ratios to find the optimal material ratio. In the study, various sensor thin films with different ratios were prepared. The test results of sensors with different ratios under a force of 2N are shown in Figure 3a-d. It can be seen from the figure that the sensor signal fluctuates up and down on the coordinate axis, and the absolute value of the peak is slightly larger than that of the valley. This is because one side of the sensor is fixed to the test stone with insulating tape, which eventuates it from fully rubbing against the negative electrode of the sensor, resulting in a slightly smaller signal. Additionally, conductivity tests were performed on MXene/PVA negative electrode films prepared with different material ratios for the sensor, laying the foundation for subsequent experimental tests. As shown in Figure 3e, it is a comparative chart of the conductivity of the sensor’s negative electrode films with different mass fractions.

Figure 3
Output curves of sensors with different material ratios under 1N force (a) 4.5 g of 5%, 10%, 15%, and 20% PVA solutions and 0.5 g of MXene; (b) 4 g of 5%, 10%, 15%, and 20% PVA solutions and 1 g of MXene; (c) 3.5 g of 5%, 10%, 15%, and 20% PVA solution and 1.5 g of MXene; (d) 3 g of 5%, 10%, 15%, and 20% PVA solution and 2 g of MXene; (e) conductivity of sensor negative electrode film under different material ratios; (f) signal waveforms of the sensor output under different pressures; (g) output signal response diagram of the sensor under different pressures. The sound frequency is 1 kHz and the sound pressure level is 94 dB; (h) the sound output signal is from the speaker wall; (i) sound output signal on the speaker; (j) the sound output signal is located 50 cm outside the speaker; (k) diagram showing the relationship between decibels and voltage at a sound frequency of 1 kHz; (l) diagram of the relationship between sound pressure and voltage at 1 kHz sound frequency; (m) the relationship between frequency and voltage when the sound is 94 dB; (n) 3-hour sound cycle test; (o) 6000 cycle experiment diagram; (p) pressure and sound test diagrams before and after storage.

And it can be seen from the figure that the sensor performance is best when the ratio of 4 g 10% PVA to 1 g (5 mg/ml) MXene material is used. From Figure 3f, it can be seen that the fabricated sensor exhibits excellent sensitivity under small pressures ranging from 0.15 to 5 N, with the response signal showing a linear growth pattern. However, when the force applied to the sensor exceeds 5 N, the growth rate decreases, no longer maintaining the linearity observed in the 0.15–5 N range. As shown by the red line in Figure 3g, which presents the output signal response curves of the sensor at the same frequency but different pressure magnitudes, the curve growth begins to slow down when the applied pressure exceeds 5 N, no longer showing linear growth. Therefore, it is concluded that the optimal working range of the fabricated sensor is within the small pressure range of 0.15–5 N. And 2 N is the median value of the measurement range, which is easy to observe and control. Under the action of 2 N force, its output signal can reach a maximum of 1831.85 mV. Therefore, experimental verification shows that the optimal working state of the fabricated sensor is under a pressure of 0.15–5 N. At this time, the sensor’s sensitivity can reach 915.32 mV/N, ensuring the best sensitivity response under these conditions. Additionally, it exhibits excellent sensing performance with high stability after 6000 cycles. Figure 3o shows the 6000 cycle experiment of the sensor under the ratio of 4 g 10% PVA and 1 g (5 mg/ml) MXene.

4.2. Acoustic performance testing of sensors

The flapping frequency of cicadas is very fast, and the wing vibration frequency of cicadas can reach thousands of times per minute. Therefore, the structure of cicada wings is very suitable for high-frequency vibration, and the frequency of sound is also very high. Therefore, the application of cicada wing structure can improve the ability of sensors to respond to sound vibration frequency. In nature, cicada wings are slightly thicker in the middle than at the edges, and this study also took this into consideration. Therefore, using gravity, a thin film resembling similar cicada wings was made with a thickness slightly thicker in the middle than at the edges. The similar cicada wing like structure can respond well to vibration, and the hollow structure can further reduce the weight of the sensor.

The acoustic working principle of the sensor developed by our research institute is mainly based on the coupling effect of contact electrification and electrostatic induction between MXene/PVA film and graphene film, which is affected by the vibration of sound waves. When external sound waves reach the device, the sensor film, due to its very small mass and thickness, is lightweight and can produce a good vibration response to the sound pressure of the sound waves. It converts the sound signal into an electrical signal based on the coupling effect of frictional electrification and electrostatic induction, thus completing the task of converting the sound signal into an electrical signal.

The signals output by the sensors used in this research institute for detecting sound can be further analyzed as follows. The decibel value of sound is a unit used to measure the intensity of sound, and the calculation formula for decibels is:

(1) $$G_p=201g\frac{P}{P_{ref}}$$

In the formula,Gp is the decibel value, the unit is dB; P is the actual sound pressure, the unit is Pa; Pref is the reference sound pressure, take 2 × 10−5 Pa. The higher the decibel value, the stronger the sound. From equation (1), it can be further deduced that the calculation formula for actual sound pressure is:

(2) $$P=P_{ref}\times {10}^{G_p/20}$$

From this, the functional relationship between the output voltage and decibels at different sound pressure levels at 1 kHz can be obtained, and its growth trend can be fitted as:

(3) $$V=K\times P=K\times P_{ref}\times {10}^{G_p/20}$$

In the formula, V is the peak output voltage, the unit is mV; K is sensitivity, the unit is mV/Pa.

Place the prepared sensor on the speaker wall, on the speaker, and about 50cm outside the speaker. Then, control the sound frequency to be constant at 1 kHz and measure the output voltage at a sound pressure level of 94 dB, as shown in Figure 3h–j. From the figure, it can be seen that the best signal is 49.266 mV/Pa when the sensor is on the speaker, and the worst signal is 0.597 mV/Pa at a position about 50 cm outside the speaker. When it is on the speaker wall, one side is fixed, resulting in insufficient friction and causing the absolute value of the peak to be greater than that of the valley. The peak signal is 33.819 mV/Pa.

The sensor measures the best signal when mounted on the speaker, so all subsequent sound data is measured under this state. The fitting curves of the output voltage of the sensor at different sound pressure levels of 1 kHz are shown in Figure 3k and 3l. When the sound pressure increases from 0.00632 Pa to 20 Pa, that is, the decibel increases from 50 dB to 120 dB, the peak voltage increases from 0.312 mV to 949.633 mV. This indicates that under a stronger sound field, the interaction between the positive and negative electrodes of the sensor is enhanced, resulting in a higher voltage output. At 1 kHz 94 dB, it is 49.266 mV/Pa. The line graph of the response of the sensor to sound in the frequency range of 0.1–20 kHz at a constant sound pressure level of 94 dB is shown in Figure 3m. It can be seen from the graph that the sensor developed in this study has a response in the range of 0.1–20 kHz, with a response range of 5.136 mV/Pa–49.6 33 mV/Pa. And the sensor has good durability, with its output signal remaining basically unchanged after 3 hours of sound exposure at 1 kHz 94 dB, as shown in Figure 3n. After the sensor undergoes long-term usability testing, it should also undergo a storage test. The sensor should have the ability to maintain its original response performance after long-term storage. Therefore, to verify the sensing performance of the sensor developed in this study after long-term storage, a sensor storage experiment was conducted. The experimental testing of the sensor was performed as follows: first, the fabricated sensor was subjected to force and sound tests, and the original data were retained. It was then placed in a sealed box, and after one month, it was taken out and retested for its corresponding response capability. The results are shown in Figure 3p. As can be seen from Figure 3p, the output voltage waveform of the sensor did not exhibit significant changes. Through specific data comparison, it was found that the sensor showed a slight 1%–2% performance degradation, which may be attributed to slight deficiencies in packaging and storage technologies. However, this also indicates that long-term storage has minimal impact on the performance of the fabricated sensor, demonstrating that the sensor has excellent durability.

4.3. Application testing of sensors

The prepared sensor can be applied in various sound detection environments. According to the test results in section 4.2, the signal is best when the sensor is placed on the speaker. The following tests were conducted under this condition, and the audio files and sensor output sound signals were analyzed using Adobe Audition CC software. In order to better test the application performance of the developed sensor, actual sound testing was conducted, as shown in Figure 4a.

Figure 4
(a) Schematic diagram of sound testing; (b) the original signal of human voice and the output spectrogram; (c) the output human voice signal and output spectrogram of the sensor; (d) the original signal and output spectrogram of music; (e) the output music signal and output spectrogram of the sensor; (f) current noise raw signal and output spectrogram; (g) sensor output noise signal and output spectrogram.

The tested sound is emitted through a speaker, and the sensor on the speaker receives the corresponding sound signal. As shown in Figure 4b, the original sound signal and output spectrogram of the human voice “Hello, good morning” are presented. Figure 4c shows the sound signal received by the sensor. It can be seen from the figure that the sensor can receive human language signals very well and can reflect the sound signal very well, with good recognition. In addition, the developed sensor also has excellent ability to record music. As shown in Figure 4d, the original sound signal and output spectrogram of the recorded “fairy fantasy background music” are shown. Figure 4e shows the sound signal received by the sensor. It can be seen from the figure that the developed sensor can well restore the relevant details of the music. And the sensor made can also be used for fault monitoring of speakers. As shown in Figure 4f, the raw sound signal of the continuous sharp current sound during speaker faults found online is recorded. Figure 4g shows the sound signal received by the sensor. From the figure, it can be seen that the sound signal during such faults is continuous, and the size and frequency of the continuously output sound signal remain consistent. This feature can be used to monitor the fault signal of speakers or other machines in real time through the output signal of the sensor. In addition, if there were better speakers and recording environment, the recording quality of CTPS may be better, so our CTPS has excellent performance.

Compared to other state-of-the-art sensors, the CTPS sensor developed in this study demonstrates remarkable advantages. Employing MXene/PVA and graphene materials, it features a cicada-wing-inspired bionic structure, achieving a sensitivity of 915.32 mV/N across the 0.15–5 N pressure range. This sensitivity far surpasses Helmholtz resonator-based sensors (40 mV/Pa). The CTPS also boasts a broader frequency response (0.1–20 kHz) than PVDF-based sensors (0.5–15 kHz), while weighing only 0.2 g—substantially lighter than multi-cavity Helmholtz resonator sensors (5 g) and comparable devices. Furthermore, the CTPS maintains stable performance in temperatures ranging from −5 to 50 °C, with long-term use causing less than 3% performance degradation. Its overall performance outperforms most existing flexible triboelectric sensors, particularly excelling in low-frequency signal detection and lightweight applications.

5. CONCLUSION

In summary, this study used MXene/PVA as the negative electrode material for the sensor, graphene as the positive electrode material, and designed a biomimetic structure of cicada wings. Finally, a flexible self powered similar cicada wing frictional pressure sensor was prepared. The response sensitivity of bionic sensors in high-frequency vibration scenarios is significantly better than that of ordinary structures (an increase of approximately 1.75 times), while it is slightly lower under low-pressure and low-frequency conditions (89.76% of that of ordinary structures). This indicates that bionic sensors are more suitable for sound signal detection, whereas sensors with ordinary structures exhibit more balanced performance in daily pressure monitoring. When the selected material ratio is 4 g of 10% PVA solution and 1 g (5 mg/ml) of MXene solution, the prepared sensor has high sensitivity, wide frequency response ability, and good durability. It can perform 6000 vibration cycle tests and 3 hours of sound cycle detection, and the signal remains basically unchanged, with high reliability. During the sensor testing process, the prepared sensor can be used to detect physical signals such as pressure and acoustic vibrations. It can be applied to detect human voice, record music, and monitor whether the speaker is working properly and whether it will output abnormal signals such as noise.

6. ACKNOWLEDGEMENTS

This work was supported in part by the Hebei Province 2023 Innovation and Entrepreneurship Education Teaching Reform Research and Practice Project under Grant 2023cxcy016, the Hebei Province Graduate Education Teaching Reform Research Project under Grant YJG2024013.

DATA AVAILABILITY

All data that support the findings of this study are included within the article.

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Publication Dates

• Publication in this collection
  18 Aug 2025
• Date of issue
  2025

History

• Received
  14 Jan 2025
• Accepted
  30 June 2025