A self-powered temperature-sensitive electronic-skin based on tribotronic effect of PDMS/PANI nanostructures

1. Introduction

In recent years, the heatstroke disease has induced substantial number of serious health crisis, especially in the low-income tropical countries [1,2]. The heatstroke can rapidly develop into multiple organ dysfunction and even death [3]. The heatstroke patients may have severe neurological symptoms, such as varying degrees of disturbance of consciousness, lethargy, stiffness, and coma [3]. The patents need to be physically or medically cooled in time [4]. There are many studies of identifying people with chronic physical conditions, such as handheld quick diagnostic meters or temperature sensors [[5], [6], [7], [8]]. If portable or wearable temperature sensors can be developed for monitoring body temperature in real time, the mortality rate of heatstroke will be greatly reduced. Furthermore, in agriculture production, chemical engineering field and some special animals’ reproduction, the real-time temperature monitoring is also particularly important for the workers and other organisms [9].

Recently, many body electronic devices have been developed [[10], [11], [12], [13], [14], [15], [16]]. Among the flexible electronics, various e-skins have been integrated with human body to achieve certain sensory information and establish human-computer interactions [17], such as mimicking skin tactility [18], retina vision [19,20] or epithelium olfaction [21]. The multifunctional e-skins can enlighten our experimental design for the wearable temperature sensors. On the other hand, it should be noted that the portable power supply unit embedded in this wearable temperature-sensitive e-skin may become one of the bottlenecks for the future applications on human body. The bulk power unit with enough energy may lower down the comfort level, and the small power unit may miss the temperature information collecting in time due to the high recharge frequency [22]. A self-sustainable wearable temperature-sensitive e-skin driven by the human motion may realize long-term and real-time temperature monitoring on body.

In this paper, a new self-powered temperature-sensitive e-skin for real-time monitoring body temperature without external electricity power has been realized. The material system is patterned PDMS/PANI nanostructures, and the working mechanism is based on tribotronic effect. The e-skin attaching on the human body can be easily driven by the mechanical motion energy, and the triboelectric output is significantly dependent on the local surface temperature, acting as both the power source and temperature sensing signal. A practical application of the e-skin for preventing heatstroke disease (detecting the body temperature range of 36.5-42.0 °C) has been simply demonstrated. The present work can promote the development of self-powered wearable temperature sensors.

2. Experimental

2.1. Preparation of patterned Cu network and the PDMS film

In this test, all the solutions were prepared with deionized water. A piece of clean Cu foil (4 cm × 4 cm ×10 μm) was prepared for forming a patterned Cu network (photolithography and wet-etching method). A hydrosol film was attached to the back of the Cu foil. The positive photoresist (3 μm-thickness; RZJ-304, 25 mPa s, Suzhou Ruihong Co., Ltd.) was spin-coated on the surface of the Cu foil, which was stuck on the SiO₂ wafer. The Cu foil with positive photoresist was dried at 100 °C for 120 s and exposed to UV light (365 nm) for 5 s. Next, the treated Cu foil was immersed into the developer solution (RZX-3038, Suzhou Ruihong Co., Ltd.) for 30 s, washed with deionized water and dring in N₂ flow. The cleaned Cu foil was hardened at 120 °C for 120 s. After that, the patterned Cu network was obtained on the SiO₂ wafer using the wet-etching method by immersing the patterned Cu foil into sodium persulfate aqueous solution at 50 °C for 120 s. The PDMS mixture of elastomer and cross-linker (with a mass ratio of 10:1) was ultrasonically treated for 15 min to eliminate bubbles. The PDMS was then spin-coated on the surface of the patterned Cu network. The PDMS/Cu network film was dried at 80 °C for 2 h. Next, the PDMS/Cu network film was lifted off from the SiO₂ wafer and immersed into deionized water for 24 h to remove hydrosol. The patterned Cu was washed by acetone and deionized water for several times to remove all of the positive photoresist. After that, sodium persulfate aqueous was prepared for the secondary wet-etching process, which could form the interval gap between Cu and PDMS. Finally, surface treatment of PDMS was carried out by inductively coupled plasma (ICP-100A, Potentlube Technology Co., Ltd) to enhance the triboelectric output of the e-skin.

2.2. Synthesis of PANI and fabrication of flexible and wearable temperature sensitive e-skin

The temperature sensitive e-skin is mainly fabricated from PANI and PDMS film. The Cu network contains seven temperature sensing units, flexible electrodes and circular terminals. An electrochemical polymerization method was used to synthesis PANI on the Cu network. The polymerization solution was prepared with 0.5 g CSA (Shanghai Macklin Biochemical Co., Ltd.), 0.5 mL aniline (Shanghai Wokai Biotechnology Co., Ltd.) and 100 mL deionized water. An electrochemical workstation (CHI627D, CH Instruments, Inc.) was used to conduct electrochemical deposition of PANI on the Cu network with Cyclic voltammetry (-1.5 V to 1.5 V) and chronoamperometry method (400 s). The electrodes under the PANI and the copper electrode (2 μm-thickness, formed through electron beam evaporation) at the back of the PDMS were linked with wires for measurement.

2.3. Characterization and measurement

Scanning electron microscopy (SEM, Hitachi S4800) was used to study the morphology and structure of the temperature sensing e-skin. A low-noise preamplifier (Model SR570, Stanford Research Systems) was used to collect the outputting triboelectric current signal of the e-skin device. The bending angle and frequency of the e-skin device were controlled by the programmed stepper motor. The ambient temperature was controlled by a microcomputer heating station. The real-time temperature was probed by a four-probe thermometer. All the experimental measurements were conducted at 19.8% relative humidity.

3. Results and discussion

The device structure and fabrication procedure of self-powered temperature-sensitive e-skin are exhibited in Fig. 1. Fig. 1(a) shows the fabrication procedure of the e-skin. PANI acts as both the sensing and friction material [23]. PDMS is the other friction material. The detailed procedure is discussed in the experimental section. The device structure of the e-skin is shown in Fig. 1(b). Seven units are included in one device for detecting the local surface temperature. Considering the differentiation of the temperature on the local skin surface, the annular design for temperature sensing unit can maximize the detection area and increase the triboelectric outputting current, which can ensure the accuracy of temperature detection. In addition, it can better adapt to the bending of different joints. The bottom Kapton film supports the device, ensuring the uniform deformation on the device. The copper electrode is on the back of PDMS. The distance between PANI and PDMS can be easily controlled by the stepper motor system (applying deformation). The e-skin has good flexibility, as shown in Fig. 1(c). Fig. 1(d) shows that the typical size of the device is 4 cm × 4 cm. The e-skin can be attached on the joints of the human body, and the motion can apply deformation on the device. The e-skin can output the current signal through triboelectric effect. The outputting of the e-skin is dependent on the surface temperature of the device. For real-time monitoring body temperature, the triboelectric outputting of the e-skin can be recorded by the computer. By analyzing the change of the outputting signal, the body temperature can be measured.

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Fig. 1.   (a) Fabrication process of the e-skin. (b) Detailed structure of the e-skin. (c, d) Optical pictures of the e-skin.

The morphology and structure of the e-skin are observed by SEM, as shown in Fig. 2. Fig. 2(a) shows one single unit on the e-skin. This unit consists of a series of concentric circular rings, and the surface is covered by a PANI layer. Fig. 2(b) is an enlarged view of the circular ring. It can be seen that the PANI is uniformly coated on the circular ring. Fig. 2(c) shows the connection between the flexible electrode and the sensing unit (circular ring). Fig. 2(d, e and f) shows the gap between PANI and PDMS. Fig. 2(g) shows the SEM image of flexible electrode. Fig. 2(h) shows the SEM image of terminal electrode. More details of the gap between PANI and PDMS are shown in Fig. S1 in Supplement Materials. The gap is the key of the triboelectric process. Fig. 2(i and j) shows the border between PANI and PDMS before and after bending for 20,000 times. It can be clearly observed that there are no cracks or detachments between PANI and PDMS. This result suggests a good stability and flexibility of the e-skin. Fig. 2(k and l) shows the high magnification SEM of PDMS before and after ICP treatment. The surface of PDMS treated with ICP method exhibits a larger triboelectric area, which is beneficial for enhancing triboelectric outputting current of the e-skin.

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Fig. 2.   SEM images of the e-skin. (a) SEM image of one temperature sensing unit. (b) The PANI layer on the temperature sensing unit. (c) The enlargement of the connection section between unit and the flexible electrode. (d-f) The gap between PANI and PDMS. (g) SEM images of flexible electrode (h) and terminal electrode. (i, j) SEM images of the border between PANI and PDMS before and after bending for 20,000 times under 90°. (k) The high magnification SEM image of PDMS before ICP treatment. (l) The high magnification SEM image of PDMS after ICP treatment.

Fig. 3 shows the temperature sensing performance of the e-skin (one single sensing unit). The applied deformation is 14°, 1 Hz. Fig. 3(a and b) shows the outputting triboelectric current of the temperature sensing unit against different temperatures. For the temperature-rising process, as the temperature is 33.9, 36.3, 38.6, 41.0 and 43.4 °C, the outputting triboelectric current values are 5.3, 6.27, 8.67, 9.81 and 11.86 nA, respectively. The outputting triboelectric current increases as the temperature increases. For the cooling-down process, as the temperature is 43.4, 40.2, 37.8, 35.9 and 33.4 °C, the outputting triboelectric current values are 11.86, 9.37, 6.9, 5.45 and 4.81 nA, respectively. The outputting triboelectric current decreases as the temperature decreases. This result indicates that the e-skin can detect the temperature at the range of 33.0-44.0 °C, which is close to the body temperature range. Moreover, the outputting triboelectric current is significantly dependent on the temperature. Fig. 3(c, d, e and f) shows the enlarged views of the outputting current signals at 38.6, 43.4, 37.8 and 33.4 °C, respectively. The response R% of the temperature sensing unit can be defined as follows [24,25]:

R%=$\frac{|I_{0}-I_{i}|}{ I_{0}}$×100%

Here, I_i and I₀ represent the outputting triboelectric currents against different temperatures and the initial temperature (33.9 °C), respectively. As the temperature is 36.3, 38.6, 41.0, 43.4, 40.2, 37.8, 35.9 and 33.4 °C, the response (R) is 18.3, 63.6, 85.1, 123.8, 76.8, 30.2, 2.8 and 9.3, respectively, as shown in Fig. 3(g and h). The e-skin has variable response against different temperature. These results indicate that the PANI/PDMS based temperature sensing unit has a great sensitivity in both heating and cooling process.

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Fig. 3.   The temperature sensing performance of self-powered e-skin (one unit). (a) The outputting triboelectric current of e-skin against different temperature. (b) The outputting triboelectric current in the heating and cooling process. (c-f) The enlarged views of the outputting current at 38.6 °C, 43.4 °C, 37.8 °C and 33.4 °C. (g, h) The outputting current and response of outputting triboelectric current in the heating and cooling process.

In practical applications, flexibility is an important parameter of e-skin. Thus the device needs to be designed to have good flexibility for accommodating different deformations. In this test, the working frequency is kept at 1 Hz, and the bending angles are 14°, 17° and 20°. Fig. 4(a and b) shows the temperature sensing performance of the e-skin under different bending angles. The outputting triboelectric current increases with increasing bending angle. Fig. 4(a) shows that the outputting triboelectric current of the unit against 43.0 °C is 11.64, 18.83 and 41.47 nA under 14°, 17° and 20°, respectively. And the response is 186.0, 233.3 and 456.6, respectively, as shown in Fig. 4(b). The outputting current of the e-skin under larger bending angles has been shown in Fig. S2. The device can work under different bending angles. In other tests, the e-skin is fixed at a certain bending angle, and the outputting current is only influenced by the temperature.

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Fig. 4.   (a) The outputting current under different bending angles at 33.1 °C, 35.2 °C, 39.3 °C and 43.0 °C. (b) The response of the e-skin under 14°, 17° and 20°. (c) Response and the recovery of the temperature sensing unit after heating (42.0 °C) and cooling (33.4 °C). (d) Stability of the e-skin. (e) The outputting triboelectric current of the e-skin under different frequencies. (f) The temperature resolution of the temperature-sensitive e-skin.

Repeatability and stability are the important factors of the self-powered temperature-sensitive e-skin. Fig. 4(c) shows the outputting triboelectric current of the temperature sensing unit at 33.5 and 42.0 °C, approximately. The applied deformation is 14°, 1 Hz. For the first cycle, as the temperature is 33.4 °C, the outputting triboelectric current value is 6.53 nA. As the temperature increases to 41.9 °C, the outputting triboelectric current value increases to 7.73 nA. For the other two cycles, the current signals are very close to the first cycle. This result demonstrates that the e-skin has a good repeatability of sensing temperature. Fig. S3 shows the dynamic detecting ability and reliability of the temperature-sensitive e-skin among several heating-cooling processes against different temperature (from 30.7-41.9 °C). This result suggests that the e-skin has the dynamic detecting ability and good reliability. As shown in Fig. 4(d), the outputting current keeps at a certain level with 20,000 cycles against 30.2 and 33.6 °C (under 60° and 1 Hz), respectively. This result suggests that the e-skin has relatively high stability and repeatability. Fig. 4(e) shows that under different frequency of applied deformation (1.00, 1.25, 1.67 Hz), the outputting triboelectric current values are almost the same. And the response keeps unchanged. The triboelectric outputting current is dominatingly dependent on the extent of deformation, and the influence of applied frequency (low frequency) can be ignored (the deformation does not change). As the temperature and bending angle do not change, the outputting current is almost a constant. This demonstrates that the temperature sensing unit has a good stability of sensing temperature.

The temperature resolution of the temperature-sensitive e-skin has been tested, as shown in Fig. 4(f). The ambient temperature is 41.0 °C and the outputting triboelectric current is 10.17 nA. As the temperature increases to 41.4 °C, the outputting triboelectric current increases to 10.81 nA. It can be seen that the current value against 41.0 °C is different from that against 41.4 °C, while the current value against 41.4 °C is basically similar to that against 41.7 °C. The minimum temperature change of 0.4 °C can be detected by the e-skin. The details of the current change are shown in Fig. S4. This result further demonstrates that the temperature sensing e-skin is very sensitive.

Fig. 5(a and b) shows the control experimental results (without PANI, only PDMS/Cu) for confirming the working mechanism. For the first cycle, as the temperature is 30.4 °C, the outputting triboelectric current is 48.44 nA. As the temperature increase to 40.5 °C, the outputting triboelectric current is 38.91 nA. For the other two cycles, the current signals are very close to the first cycle. The PDMS can be softened at high temperature, which lowers down the triboelectric output. This phenomenon is totally opposite to the PANI/PDMS structure, confirming that the temperature effect of PANI plays a key role in the temperature sensing process [[26], [27], [28]].

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Fig. 5.   Control experimental results and the working mechanism. (a, b) The outputting triboelectric current of the device (Cu/PDMS). (c) The triboelectric process between PANI and PDMS. (d) The temperature-sensitive mechanism of the e-skin.

The working mechanism can be ascribed to the coupling effect of triboelectric and semiconductor temperature characteristics (tribotronic effect), as shown in Fig. 5(c and d). The electricity-generation process is shown in Fig. 5c. In the initial state, as the electron accepting ability of PDMS is much stronger than PANI, negative charges are accumulated on the surface of PDMS and an equal number of positive charges are accumulated on the surface of PANI [[29], [30], [31], [32]]. When PANI and PDMS of the temperature sensing unit are separated from each other by the applied deformation, negative charges can be maintained on the surface of the PDMS layer for a long time due to the good insulating property of PDMS, and the positive charges are driven from the top electrode to the bottom electrode via external circuit to shield the electrostatic field. An induced current can be detected. As PDMS and PANI come into contact again by the applied deformation, positive charges transfer from the bottom electrode to the surface of PANI. And a reverse current is induced by the electrostatic induction process. Fig. 5(d) shows the temperature-sensitive mechanism of the temperature sensing unit. It has been reported that the CSA-doped PANI is a temperature-sensitive semiconductor [33,34]. The carriers density increases with increasing temperature. More charges migrate from PANI to PDMS to shield the electrostatic field through the external circuit, which results in the higher output.

The practical application of the temperature sensing e-skin for detecting the body temperature without external power is simply demonstrated, as shown in Fig. 6. The e-skin is attached on the human arm joint. In future application, considering the random arbitrary arm movement, a simple mechanical part for providing constant bending deformation will be integrated into the system to realize accurate and reliable temperature detection. Under the driven of arm bending, the outputting triboelectric current of the e-skin at normal body temperature (36.5 °C) and high temperature ($\widetilde{4}$2.0 °C) is approximately 10.25 nA and 13.04 nA, respectively. For future applications, the material system and device structure need to be further developed for the comfort and accuracy requirements.

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Fig. 6.   Practical application of the e-skin for detecting body temperature.

4. Conclusion

In summary, a new flexible temperature sensing e-skin for real-time monitoring body temperature has been realized. The e-skin can be feasibly attached on the human body and convert the human-motion energy into outputting triboelectric current without an external electricity power. The outputting triboelectric impulse of the PDMS/PANI units is significantly dependent on the local surface temperature of the e-skin, serving as both the power source and temperature sensing signal. The outputting current of the e-skin increases with increasing surface temperature of the device. The working mechanism can be ascribed to the coupling effect of triboelectric and semiconductor properties (tribotronic effect). And the minimum change of temperature that the electronic skin can detect is $\widetilde{0}$.4 °C. Moreover, it also has good repeatability. A practical application of the e-skin attaching on the human body for detecting the body temperature range of 36.5-42.0 °C has been simply demonstrated. The present results provide a viable method for real-time monitoring body temperature, and can promote the development of wearable temperature sensors and self-powered multifunctional nanosystems.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (No. 11674048).