A Stretchable and Tough Conductive Elastic Film for Multifunctional Flexible Strain Gauges

As an important branch of modern science and technology, flexible sensor combines the functions of traditional sensors with the characteristics of flexible materials to meet the needs of modern electronic devices for lightweight, flexible and wearable characteristics. However, flexible sensors are faced with problems such as poor stability and weak anti‐interference ability in extreme environments. Here, a strategy of combining thermoplastic polyurethane and conductive carbon black is adopted to obtain conductive elastic film with good conductivity and excellent mechanical properties (TPU@CB film). TPU@CB film exhibits very good mechanical strength (8 MPa). When TPU@CB film is assembled into a sensor, it exhibits wide detection range (600%), low detection limit (0.05%) and short response time (15 ms). Notably, the TPU@CB film demonstrates clear detection capabilities for both ECG and EMG signals of the human body when utilized as an electrode patch. Thanks to lithography, TPU@CB film is machined into strain gauge shape. TPU@CB strain gauge achieves a high signal‐to‐noise ratio and can detect a wide range of frequency vibrations from 0 to 900 Hz, as well as accurately detect the speed of the motor. This provides a feasible way to promote the development of flexible electronics.


Introduction
As an important development direction of modern sensor technology, flexible sensor is playing an important role in various DOI: 10.1002/adsr.202300208application fields.The main feature of flexible sensors is that they can bend, stretch and compress while maintaining function.[3][4][5][6][7][8] However, despite the rapid development of flexible sensors, they still face some important challenges and limitations.We need to realize that relying solely on the intrinsic properties of flexible materials to achieve sensing functions has gradually failed to meet the growing demand. [9,10]19] First of all, the intrinsic properties of the material are the basis of the sensor, and the improvement of the intrinsic properties of the material mainly lies in the choice of flexible substrate and conductive material.The flexible substrate not only provides physical support, but also affects the overall performance, wearability and durability of the sensor.When choosing a flexible substrate, the first and most important thing is that the substrate needs to be flexible and stretchable enough to adapt to a variety of bending and stretching conditions, while maintaining its structural integrity and functionality. [18,20,21]Second, the flexible substrate material should be able to maintain chemical and thermal stability under various environmental conditions, including resistance to moisture, chemical corrosion and temperature changes. [22][25] Many reported flexible sensors use hydrogel as the substrate materials, which is prone to water loss resulting in poor stability. [26]In order to improve the performance of flexible sensors, some stable polymer elastomers have been favored by many researchers. [27]The continuous innovation of flexible substrate materials provides more possibilities for the design and application of flexible sensors.The core function of flexible sensors is to effectively detect and transmit electrical signals, which largely depends on the selection of conductive materials used inside, with the development of science and technology, the selection of conductive materials has been extended from traditional metals and alloys to a variety of advanced conductive polymers and nanomaterials.Conductive materials need to be able to withstand environmental factors such as temperature changes, humidity and chemical corrosion while meeting good electrical conductivity.Carbon-based nanomaterials have attracted much attention for their excellent electrical conductivity, flexibility and stability, and have shown great potential in strengthening composites. [5,28]The development of conductive materials not only promotes the improvement of the performance of flexible sensors, but also provides broad possibilities for the design and application of a new generation of intelligent electronic devices.
On the other hand, the structured design of flexible sensors is a key factor in achieving its high performance and versatility. [29,30]hrough a precise structured approach, complex microstructures can be created on a flexible substrate, giving the sensor unique functionality and enhancing its performance.For example, Yunpeng Yang et al. formulated MWCNTS/PDMS composite inks through screen printing to form flexible strain sensors with patterned designs.The sensor can also be used to monitor unknown strains caused by non-uniform microstructures, machining defects, and fatigue caused by prolonged operation of 3D structures. [31]Oluwaseun A et al. designed conductive microstructures in the shape of curved curves and encapsulated them in pre-stretched elastic films to produce sensors that can withstand harsh load conditions.The sensor exhibits high linearity and insensitivity to bending and torsional deformation, which are important for soft device applications. [32]36][37][38] Here, we have prepared thermoplastic polyurethane (TPU) films with excellent mechanical properties by electrospinning as our flexible substrate, which ensures that our flexible devices can adapt to bending and stretching conditions while maintaining their structural integrity and functionality.Carbon-based nanomaterials have attracted much attention for their excellent electrical conductivity, high strength and flexibility, and have shown great potential in strengthening composites.Conductive carbon black (CB) with large specific surface area, good electrical conductivity and excellent stability was selected as the conductive material.Through vacuum filtration, CB and TPU were well combined to obtain TPU@CB film.TPU@CB film obtained a tensile strength of 8 MPa and a tensile strain of 820%.We assembled the TPU@CB film into a sensor that demonstrated a high sensitivity factor (up to 218), a low detection limit (0.05%), and a short response time (15 ms).When TPU@CB film is used as electrode patch, the ECG signal and EMG signal of human body can be well detected.Thanks to flexible machining and photolithography technology, TPU@CB films are machined into strain gauge shapes.The obtained TPU@CB strain gauge can adapt to complex working environment, and is extremely sensitive to strain in a single direction and insensitive to unrelated deformation (bending, rotation, etc.) in other directions, showing a high signal-to-noise ratio.In addition, TPU@CB strain gauges can detect high frequency dynamic loads from 0 to 900 Hz and show higher accuracy than commercial strain gauges.Thanks to its excellent flexibility and the ability to detect high-frequency dynamic loads, TPU@CB strain gauges shows good promise in the detection of some physical parameters of instruments, such as the detection of motor speed.All the test results were compared with those of commercial instruments, and the results showed good accuracy.

Results and Discussion
In this study, we prepared a conductive film based on polyurethane combined with conductive carbon black.Thanks to lithography, we process the film into a strain gauge shape.The strain gauge we obtained has excellent performance and can be used in a variety of complex operating conditions.The preparation process of TPU@CB film is shown in Figure 1a, TPU films are prepared by electrospinning.TPU is a dense mesh structure formed by interweaving fibers together.Its unique structural characteristics make it have excellent mechanical strength and stretchability as well as outstanding fatigue resistance, which ensures that the sensor can maintain very good stability under external tension and long-term use.In addition, TPU film is easy to be processed, which provides the basis for our sensor patterning design, and provides an important premise for us to design sensors with high sensitivity and high signal-to-noise ratio.CB has the advantages of excellent conductivity, stability and large specific surface area.We combined TPU with CB to form a TPU@CB film with both electrical conductivity and mechanical properties.As shown in Figure 1a, CB is coated on TPU fiber by vacuum filtration.It should be pointed out that TPU is hydrophobic, which is not conducive to its combination with CB.Therefore, we used a plasma cleaning machine to treat the surface of TPU.According to the contact Angle test in Figure S1 (Supporting Information), it can be found that the surface of TPU after treatment changes from hydrophobic to hydrophilic, which provides conditions for the combination of TPU and CB.We use a scanning electron microscope to observe the microscopic morphology of our materials.As shown in Figure 1b and Figure S2 (Supporting Information), the TPU film prepared by electrospinning is fibrous in microstructure, and presents micro networks and porous structures.This unique microstructure provides a guarantee for its excellent mechanical properties.CB is a chain structure composed of many nano-sized small particles, as shown in Figure S3 (Supporting Information).The microstructure of TPU@CB film obtained by vacuum filtration is shown in Figure 1c, CB is well coated on each TPU fiber.It can also be seen from the cross section (Figure 1d) that each fiber is well coated with CB, which makes it build a good conductive path, creating the basis for its excellent sensing capability.Figure 1e shows the X-ray diffraction (XRD) of TPU matrix, CB and TPU@CB film in the range of 10°-80°.The TPU matrix shows a strong wide amorphous peak at ≈2 = 20°of the (110) reflection plane, which is related to the short-range regular ordered structure of both hard and soft domains and the amorphous phase disordered structure of TPU matrix. [33]It can be seen from the XRD pattern of CB that there is a crystallization peak at 2 = 21.7°and2 = 43.6°respectively,2 = 21.7°corresponds to the crystallization peak of (002) and 2 = 43°corresponds to the crystallization peak of (10 l).The (10 l) peaks, made up of (100) and (101) peaks, have never been clearly separated, which means that each layer of carbon atoms in CB is not completely stacked.From the XRD pattern of the composite material, it can be found that there is a crystallization peak of TPU at 2 = 20°, but also retains a crystallization peak of CB at 2 = 43°, which indicates that our material is obtained by the combination of TPU and CB.In addition, the content of TPU@CB film was analyzed by thermogravimetric analysis (TGA), as shown in Figure 1f.It can be seen from the weight change that only 2% of conductive carbon black is contained in our film, which proves that conductive carbon black can make the film obtain good sensing performance under a very low load.In Figure 1g, the Raman spectra of TPU@CB film only show the typical peaks of CB, which mainly include G band (1582 cm −1 ) and D band (1350 cm −1 ), indicating that CB is perfectly coated on TPU fiber and therefore has efficient load transfer, which lays the foundation for TPU@CB film to have good sensing performance and mechanical stability.
As a flexible electronic device, the mechanical strength, stability and sensing properties of the material are particularly important.Thanks to the excellent mechanical properties of TPU and the good electrical conductivity of CB, our material has shown great prospects in terms of mechanical properties and sensing capabilities, and has received people's attention.Therefore, we designed experiments to study the mechanical properties and sensing properties in detail to show its potential in flexible devices.We cut TPU@CB film into the standard sample size for mechanical testing and then test the mechanical properties, as The response curve of the relative resistancestrain of TPU@CB film.c) The electrical signal response corresponding to the application of 50% strain at different tensile speeds.d) Electrical signal response at 5% strain.e) Electrical signal response at ultra-low strain.f) The electrical signal response with 1% and 5% strain superimposed on the premise of fixed 500% tensile strain.g) Electrical signal response of the film in 10 000 repeated load-to-unload tests at 50% strain.h) Demonstration of response time.i) Comparison of performance parameters of various flexible sensors reported in the literature.
shown in Figure 2a.TPU@CB film shows excellent mechanical properties and achieves a tensile strength of 8 MPa at 820% strain during the tensile test.We carried out continuous loading and unloading experiments on TPU@CB film in the strain range of 0%-500% (Figure S4, Supporting Information).The results show that TPU@CB film can still recover to the initial state under a high tensile strain of 500%, this is attributed to the high strength and toughness of the TPU fiber network obtained by electrospinning.Then we assembled TPU@CB films into sensors to explore its sensing capability.Sensitivity factor (GF) is an important evaluation index of flexible sensors.As shown in Figure 2b, the resistance changes and GF value of TPU@CB sensor within the strain range of 0%-600% are recorded.TPU@CB sensor has a high GF value (up to 218) under both small and large strain.The TPU@CB sensor not only has a wide sensing range, but also has excellent sensitivity.In addition, the TPU@CB sensor is applied with different stretching speeds under the same strain to test its response capability under stretching rates, as shown in Figure 2c.The results show that TPU@CB sensor can respond stably under any stretching rate, and the variation amplitude of the obtained resistance signal is independent of the applied stretching rate.The ability to stably perceive minor strains is an important prerequisite for a qualified sensor.We apply 0.5% tensile strain to TPU@CB sensor and remove the load after holding it for 10 s.As shown in Figure 2d, ΔR/R 0 changes immediately when TPU@CB sensor is subjected to a small deformation of 0.5%.In the process of holding 10s, the value of ΔR/R 0 is in a stable state, and then the value of ΔR/R 0 quickly returns to the initial state after unloading.Subsequently, we continuously apply 0.5% tensile strain to TPU@CB sensor, and the results show that the values of ΔR/R 0 also change rapidly and continuously in a stepped manner (Figure S5, Supporting Information), this indicates that TPU@CB sensor has the ability to sense small strain.Then, we designed further experiments to explore its sensing characteristics under small strain in more detail.We designed several sets of repeated loading and unloading experiments within the strain range of 0.05%-1%, as shown in Figure 2e.Under our repeated loading and unloading tests, the values of ΔR/R 0 also show periodic rapid changes, and the signals are extremely stable, which indicates that TPU@CB sensor has stable sensing ability to external small deformation.Similarly, we also explored the sensing performance of TPU@CB sensor under large strain.Repeated loading and unloading experiments were designed within the strain range of 100%-500%, as shown in Figure S6 (Supporting Information).The results show that the resistance changes periodically and is extremely stable, just as in the case of small strain.Many conventional sensors have difficulties in detecting both large and small strains at the same time.However, the TPU@CB sensor can still detect small deformation at very high tensile strain (500%), as shown in Figure 2f.TPU@CB sensor can detect superimposed small strains of 1% and 5% at a tensile strain of 500%.The pursuit of stability and durability of sensing devices is eternal, which can ensure that the performance of our sensing devices will not be degraded during long-term use.As shown in Figure 2g, we have carried out 10 000 loadingunloading cycle experiments on the sensor under 50% strain.During the whole long-term stretching cycle, it can be seen from the amplified images of the electrical signal response at each stage that the electrical signal response is always in a stable state, indicating that TPU@CB sensor has excellent stability.In addition, the response time is crucial for the sensor, and fast response time is conducive to the sensor to achieve high-frequency sensing resolution.We apply a tiny strain to the TPU@CB sensor, and obtain its response time by detecting the time when its electrical signal changes.As shown in Figure 2h, TPU@CB sensor has a very small response time (15 ms).It is worth mentioning that, as shown in Figure 2i and Table S1 (Supporting Information), TPU@CB sensor is superior to sensors reported in many literatures in terms of sensing performance and durability.These results demonstrate that TPU@CB sensors have stable sensing capabilities, rapid response to various strains, and excellent durability, which are expected to help address the existing limitations of flexible sensors.
At present, many reported flexible devices still face great challenges in terms of stable operation under v extreme ambient temperatures.Thanks to the excellent stability of TPU and CB, our TPU@CB films have no signs of failure under extreme ambient temperatures and maintain constant performance.Many commercial sensors are mostly made of metal, due to the high thermal conductivity of metal materials, it is easy to be affected by the ambient temperature when working.Constantan is a coppernickel alloy, which is not easy to change its properties with temperature changes, so it has become the preferred material for many electronic devices.We used a thermal conductivity meter to compare the thermal conductivity of TPU@CB film and Constanta.As shown in Figure 3a, the thermal conductivity of TPU@CB film is 0.1196 W m*k −1 , which is much lower than 19.6 W m*k −1 of Constanta.The small thermal conductivity ensures that TPU@CB film will not be affected by the ambient temperature when working.This is very important.Then, we assembled TPU@CB film into a simple sensor, and conducted cyclic tensile test of 50% strain on the sensor at ambient temperatures of room temperature, −40 and 40 °C respectively, as shown in Figure 3b.The effect of ambient temperature on its performance is observed through its output electrical signal.We can see that the electrical signal output of TPU@CB sensor at −40 and 40 °C is almost the same as that at room temperature, which proves that the sensor assembled by TPU@CB film is little affected by the ambient temperature and can still work normally and stably under extreme ambient temperature.Then, we placed TPU@CB film at −40 and 40 °C respectively for 2 h and took it out to test its mechanical strength immediately, as shown in Figure 3c.From the stress-strain curve, we can see that TPU@CB film can maintain a tensile strength of ≈8 Mpa at −40 and 40 °C, which is similar to the tensile strength at room temperature.It indicates that the mechanical properties of TPU@CB film are not affected by extreme ambient temperature.In addition, with the help of electrochemical workstation and four-eye probe tester, we tested the impedance and conductivity of TPU@CB film at extreme ambient temperatures, as shown in Figure 3d,e.It is found that at extreme low temperature (−40 °C), the resistance of TPU@CB film increases, but the increase is very slight.The conductivity of TPU@CB film at −40 °C is 5.36 S m −1 , which is slightly lower than that at 40 °C (6.25 S m −1 ).This is due to the fact that the charge transfer speed of TPU@CB film slows down at low temperature.These results show that TPU@CB film has excellent electrical conductivity over a wide temperature range, which is not only less affected by ambient temperature, but also can adapt to complex working conditions.
In addition, the high flexibility of TPU@CB film allows it to fit perfectly on the dynamic surface of the skin.When TPU@CB film is used as an electrode patch, various physiological information from the skin can be collected, so it has a promising application prospect on flexible electrode patches.For example, we attached TPU@CB film to the arms and legs of volunteers as shown in Figure 3f and connected them to an electrocardiograph.Then an electrocardiogram (ECG) of volunteers was collected by measuring the electrical potential between the TPU@CB film.As shown in Figure 3f, TPU@CB film collects accurate ECG signals and can accurately distinguish P, Q, R, S, and T waves, which can help us detect heart rate and related diseases.Similarly, TPU@CB film was placed on the arm of volunteers in a certain arrangement to detect the electromyography (EMG) signal of volunteers, as shown in Figure 3g.In the two states of fist clenching and extension, we obtain two corresponding EMG signals due to the contraction of different muscle groups.These results show that TPU@CB film can accurately pick up our body ' s physiological signals, which has the potential to help identify a range of diseases.
At present, strain gauges are widely used in many fields such as military, health monitoring and non-destructive testing.It has many advantages such as high accuracy, stable performance and a high signal-to-noise ratio, but most of the strain gauges on the market are made of metal, they are not expandable, there will be impedance mismatch, narrow sensing range, cannot detect complex surfaces and other complex conditions.The soft strain gauge is a promising design example, which brings great hope to solve the above shortcomings of the traditional metal strain gauge.Thanks to flexible machining and photolithography technology, TPU@CB film was processed into strain gauge shape to obtain a soft strain gauge with good mechanical strength, flexibility and high signal-to-noise ratio (TPU@CB strain gauge), as shown in Figure 4a.TPU@CB strain gauges provide us with more accurate test results.We applied 5% tensile strain to TPU@CB strain gauge along the X and Y directions shown in Figure 4b respectively.We found that the strength of electrical signal stretched along the Y direction was much greater than that along the X direction, which was caused by our unique structural design.Therefore, TPU@CB strain gauge can maintain constant resistance under tension along the X-axis direction, showing a strain- ) 5% tensile strain is applied to TPU@CB strain gauge in the X and Y directions, respectively.c) The response curve of the relative resistance-strain of TPU@CB strain gauge.d) The electrical signal response of TPU@CB strain gauge at 1% tensile strain and 90°bending.e) Electrical signal response of TPU@CB strain gauge under puncture, hammer and stretch.f) The electrical signal response of TPU@CB strain gauge is insensitive when pressure is applied to TPU@CB strain gauge under different tensile strains.g) Electrical signal response of TPU@CB film during continuous bending and rotation.h) Electrical signal response of TPU@CB strain gauge during continuous bending and rotation.i) Comparison of SNR between TPU@CB strain gauge and TPU@CB film.insensitive characteristic.However, when the tensile strain is applied along the Y direction, the electrical signal response of the TPU@CB strain gauge is obvious, showing a very sensitive characteristic.This different sensitivity to different directions allows our sensors to clearly distinguish the principal strain direction and obtain more accurate data.Then we measure the sensitivity factor (GF) of TPU@CB strain gauge stretching along the Y direction, as shown in Figure 4c.According to the relative resistance-strain curve of TPU@CB strain gauge, we can calculate that the value of GF of TPU@CB strain gauge is between 57-496.The GF value of TPU@CB strain gauge is 2-3 times that of TPU@CB film on the whole, which is due to the structure of our strain gauge, so that it has a more obvious electrical signal response under the same strain condition when stretched in a certain direction.In practical application scenarios, the sensing device may be affected by external deformation while experiencing tensile strain, and this special sensitivity to only a single direction allows the sensing device to avoid being affected by unrelated deformation in other directions when receiving signals, which is very important.For example, the signal response capability of TPU@CB strain gauge is evaluated under the conditions of 1% tensile strain and 90°bending.It is found that the electrical signal response of TPU@CB strain gauge is obvious under 1% tensile strain, while the signal response of TPU@CB strain gauge under 90°bending is not obvious, as shown in Figure 4d.Similarly, TPU@CB strain gauges show a sensitive response to 1% tensile strain, but are less responsive to puncture and hammer tests (Figure 4e).We applied pressure to TPU@CB strain gauge under a certain tensile condition to collect electrical signals, as shown in Figure 4f.We applied 5%, 10% and 30% tensile strain to TPU@CB strain gauge respectively, and then applied 100kpa pressure in the direction indicated in the Figure 4f.The results show that the electrical signal response of TPU@CB strain gauge is very obvious when we apply tensile strain to TPU@CB strain gauge, but the electrical signal response is very weak during the subsequent pressure experiment.TPU@CB strain gauge shows excellent resistance to pressure interference under the premise of tensile strain.Compared with the traditional sensing elements which use the intrinsic properties of the material, the structured soft strain gauge shows excellent signal-to-noise ratio and high resolution.As shown in Figure 4g,h, we applied the TPU@CB film and TPU@CB strain gauge to the waist of the volunteer, and the electrical signal detected when the volunteer bent over was often accompanied by some irrelevant noise (torsion) interference, so that the low s SNR ratio would lead to the reduction of the accuracy of our acquisition signal, and even the inability to distinguish the main signal.The SNR of motion signal detected by the TPU@CB strain gauge is 55 dB (Figure 4i), which is much higher than TPU@CB film (33 dB).These results show that TPU@CB strain gauge can resist the influence of irrelevant noise, has the advantages of high precision of signal acquisition, a and shows a good application prospect in the field of flexible electronics.
Sensing technology based on soft materials has brought great hope for the development of sensing equipment.Thanks to the high precision and good flexibility of TPU@CB strain gauge, TPU@CB strain gauge can detect some small and high frequency dynamic loads of planar or curved structures, which greatly broadens the application range in practice.As shown in Figure 5a, a TPU@CB strain gauge is attached to the upper surface of the cantilever beam, and then a shaker is used to excite the cantilever beam with a small vibration at a specific frequency.The variation of resistance signal of TPU@CB strain gauge is collected, and then the corresponding spectrum diagram is obtained through FFT processing.The frequency extracted from the spectrum diagram is used to observe the monitoring capability of TPU@CB strain gauge on continuous high frequency dynamic load.At the same time, the commercial strain gauge is attached to the lower surface of the cantilever beam at the same position for the same signal collection.The signals collected by the TPU@CB strain gauge and the commercial strain gauge are compared to evaluate the performance of TPU@CB strain gauge.We applied vibration excitation of 1, 30, 100 and 900 Hz to the cantilever beam through the shaker, and detected the resistance changes of TPU@CB strain gauge, as shown in Figure 5b.The electrical signal response showed similar sinusoidal waveform at both low frequency and high frequency.After obtaining the frequency component through FFT processing, we can obtain the corresponding spectrum diagram as shown in Figure 5c.In the spectrum diagram of frequency excitation of 1, 30, 100 and 900 Hz, the extracted frequencies are 1.013, 29.979, 99.927 and 900.001Hz respectively.This is almost the same as the excitation frequency of the shaker to the cantilever beam.Similarly, the signals collected by commercial strain gauges also obtained similar results (Figure S7, Supporting Information).After FFT processing, the frequencies extracted from the spectrogram were 0.994, 29.959, 99.985 and 899.75 Hz, respectively.In order to further evaluate the capability of TPU@CB strain gauge to monitor high frequency dynamic loads, we used TPU@CB strain gauge and commercial strain gauge respectively to collect multiple sets of low frequency and high frequency excited signals and processed them by FFT.Then, the frequencies that can be extracted from the spectra obtained by TPU@CB strain gauge and commercial strain gauge are compared with the output frequency of the exciter.The accuracy of their signal acquisition is compared by mathematical statistics.As shown in Figure S8 (Supporting Information), the accuracy of signal collected by TPU@CB strain gauge is higher than commercial strain gauge no matter under the excitation of low frequency or high frequency vibration, which is attributed to the fact that the GF value of TPU@CB strain gauge is much higher than commercial strain gauge.Based on the excellent ability of TPU@CB strain gauge to monitor high frequency dynamic load, we attach TPU@CB strain gauge to the surface of a singlephase motor to detect the vibration frequency of the motor under different voltages and thus determine the speed of the motor (Figure 5d).It should be noted that the correlation coefficient between the obtained characteristic frequency and the motor speed is 8, which is due to the fact that the motor has 8 blades and produces 8 cycles of disturbance for every turn.We adjust the input voltage of the motor to 100, 105 and 110 V, and then detect the resistance change of TPU@CB strain gauge under different voltages, as shown in Figure S9 (Supporting Information).After FFT processing, the frequencies extracted from the spectrum diagram are 422.901Hz, 604.178Hz and 762.584Hz respectively (Figure 5e).After calculation, the speed of the motor under 100, 105 and 110 V voltage is 3165, 4530 and 5715 RPM respectively as shown in the Figure 5f.To verify the accuracy of the results, we calibrate the speed of the motor at 100, 105 and 110 V with commercial speed measuring instruments.As shown in Figure S10 (Supporting Information), a reflective coating is affixed to the motor, and the speed is determined by the reflection of the reflective coating on infrared light during rotation.It is obtained that the motor speeds at 100, 105 and 110 V are 3243, 4375 and 5815 RPM respectively, which are almost consistent with the re-sults measured by TPU@CB strain gauge.These results show that TPU@CB strain gauges are very reliable in detecting high frequency dynamic loads and have good commercial prospects.

Conclusion
In summary, by combining TPU and CB, we obtained TPU@CB film with excellent mechanical and conductive properties, which overcomes the limitations of low mechanical strength and poor stability of flexible sensors, and also has excellent sensing performance.TPU@CB films can also be used in electrode patches to detect physiological signals related to the human body.TPU@CB films are machined into strain gauge shapes that exhibit a high signal-to-noise ratio and can detect a wide range of frequency vibrations from 0 to 900 Hz.It can also monitor some complex curved surface structures such as detecting the speed of curved structure motor.All the features show that this study shows a good application prospect in flexible electronics, and provides a feasible idea for the development of flexible electronics.

Figure 1 .
Figure 1.Preparation and characterization of TPU@CB film.a) Schematic diagram of the preparation of TPU@CB film.b) SEM image of TPU film.C)SEM image of TPU@CB film.d) SEM image of a TPU@CB film cross-section.e) The XRD pattern of TPU@CB film.f) TGA curves of TPU fiber and TPU@CB film.g) FT-IR patterns of TPU fiber and TPU@CB film.

Figure 2 .
Figure 2. Mechanical and sensing properties of TPU@CB film.a) Stress-strain curves of TPU@CB film.b)The response curve of the relative resistancestrain of TPU@CB film.c) The electrical signal response corresponding to the application of 50% strain at different tensile speeds.d) Electrical signal response at 5% strain.e) Electrical signal response at ultra-low strain.f) The electrical signal response with 1% and 5% strain superimposed on the premise of fixed 500% tensile strain.g) Electrical signal response of the film in 10 000 repeated load-to-unload tests at 50% strain.h) Demonstration of response time.i) Comparison of performance parameters of various flexible sensors reported in the literature.

Figure 3 .
Figure 3.The properties of TPU@CB film are almost unaffected by the ambient temperature, and the electromechanical properties of TPU@CB film are shown.a) Comparison of thermal conductivity of TPU@CB film and Constantan.b) Electrical response of the TPU@CB film at extreme temperatures of −40 and 40 °C.c) Stress-strain curves of TPU@CB film at different extreme temperatures d) Nyquist plots of TPU@CB film at different extreme temperatures.e) Conductivity of TPU@CB film at different extreme temperatures.f) The ECG signals detected by the TPU@CB film.g) The EMG signals of human body in different postures were detected by TPU@CB film.

Figure 4 .
Figure 4. Evaluation of sensing performance of TPU@CB strain gauge.a) Physical diagram of flexible strain gauge.b) 5% tensile strain is applied to TPU@CB strain gauge in the X and Y directions, respectively.c) The response curve of the relative resistance-strain of TPU@CB strain gauge.d) The electrical signal response of TPU@CB strain gauge at 1% tensile strain and 90°bending.e) Electrical signal response of TPU@CB strain gauge under puncture, hammer and stretch.f) The electrical signal response of TPU@CB strain gauge is insensitive when pressure is applied to TPU@CB strain gauge under different tensile strains.g) Electrical signal response of TPU@CB film during continuous bending and rotation.h) Electrical signal response of TPU@CB strain gauge during continuous bending and rotation.i) Comparison of SNR between TPU@CB strain gauge and TPU@CB film.

Figure 5 .
Figure 5.Some practical applications of TPU@CB strain gauge.a) Schematic diagram of the vibration system used to collect the vibration signal.b) The resistance changes of TPU@CB strain gauge at different vibration frequencies.c) Spectrum diagram processed by FFT.d) Physical diagram used to measure the motor speed.e) The spectrum diagram of the motor obtained by FFT processing.f) The corresponding motor speed under different voltages.