Stretchable hybrid electronic network-based e-skin for proximity and multifunctional tactile sensing

doi:10.21203/rs.3.rs-3450412/v1

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Research Article

Stretchable hybrid electronic network-based e-skin for proximity and multifunctional tactile sensing

https://doi.org/10.21203/rs.3.rs-3450412/v1

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published 24 Sep, 2024

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Multifunctional integrated flexible electronic skin (e-skin) is the essential medium for information exchange between humans and machines. Especially, the proximity/ pressure/ strain sensing has become a technological goal for various emerging wearable electronic devices, such as biomonitoring devices, smart electronics, augmented reality, and prosthetics. Herein, a stretchable hybrid electronic network-based e-skin is presented, fabricated by embedding 3D hollow MXene spheres/Ag NWs hybrid nanocomposite into PDMS, which can effectively avoid the electrode falling off due to stress concentration. This e-skin works in noncontact mode (proximity-negative capacitance) and contact mode (pressure-positive capacitance & strain-resistance) for multiplex detection of random external force stimuli without mutual interference. The macroscopic physical structure of stretchable electrodes and the microscopic hybrid three-dimensional conductive network jointly contribute to the good sensing performance of the device. This workprovides an effective and universalstrategy for the application of wearable intelligent electronic products that demand noncontact interaction and multimodal tactile perception.

Hybrid electronic network

E-skin

Proximity sensing

Multifunctional tactile sensing

No interference

The development of electronic devices tends to be lightweight, flexible and multifunctional, with an integrated component applied to our skin or robot surface to track motion and perceive external signals (Yang et al., 2022). Electronic skins (e-skins) are is rapidly developing to meet the needs of the above applications (Rybak et al., 2023; Gao et al., 2023). In terms of tactile sensing, e-skins are already able to detect signals such as proximity, pressure or tension, respectively (Ge et al., 2023; Li et al., 2022; Zhou et al., 2021). However, due to the mutual interference between detected signals, it is difficult for the device to distinguish between many different signals at the same time. For contact sensing, changes in the e-skins geometry caused by mechanical deformation are usually perceived through changes in the electrical characteristics of the device, such as resistance or capacitance (Nguyen, Estlack, Stockton, & Kim, 2019; Han et al., 2023). Furthermore, non-contact sensing, that is, proximity sensation, has attracted extensive attention in the fields of safety precaution and noninvasive medical diagnostics and therapy (Baghelani, Abbasi, Daneshmand, & Light, 2022; Lv et al., 2020; G.-D. Zhao et al., 2022). Therefore, the design of an e-skin that integrates contact and non-contact sensing and can avoid crosstalk between different signals is expected to further accelerate the development of flexible electronic devices.

The key to achieving multi-parameter flexible sensing is to convert composite stimulus signals into separated or coupled signals through sensing modules (Dai et al., 2023; Fan, Yang, & Sun, 2023). More specifically, integrating a negative capacitance proximity sensor, a positive capacitance pressure sensor and a resistive stretchable strain sensor in a single pixel holds the promise of developing flexible electronic devices with sophisticated touch sensing systems similar to human skin. Several strategies have been reported to demonstrate stretchable devices, typically involving elastomer substrates combined with stretchable conductive materials such as carbon nanotubes (CNTs), liquid metals, or nanowires (M.-Y. Liu et al., 2022; Wang et al., 2022; Z. Cui, Poblete, & Zhu, 2019). Inevitably, since the conductive layer is usually deposited on top of the stretchable matrix, it is prone to falling off or cracks under repeated mechanical loads (Qi, Zhang, Tian, Jiang, & Huang, 2021). In addition, single type of conductive nanomaterials is difficult to have both high conductivity and excellent flexibility (Lee et al., 2014). Our previous research results have proved that it is a feasible approach to embed different composite conductive materials into flexible substrates by vacuum force, for example CNTs/Ag NWs embedded in PDMS and MXene spheres/Ag NWs embedded in PDMS (M.-Y. Liu et al., 2022; X.-F. Zhao, Wen, Zhong, et al., 2021; X.-F. Zhao, Wen, Sun, et al., 2021; M. Y. Liu et al., 2022).

Herein, we propose a multifunctional integrated flexible e-skin, which can achieve multiple detection of random proximity/pressure/strain stimulation without mutual interference. This e-skin is constructed by back-to-back crossing stretchable electrodes, which are fabricated by embedding 3D hollow MXene spheres/Ag NWs hybrid nanocomposite into PDMS. The macroscopic physical structure of stretchable electrodes and the microscopic hybrid three-dimensional conductive network jointly contribute to ensure the great sensing performance of this e-skin. The detection results of different compound stimuli demonstrate that the device can distinguish different stimulus signals and basically achieve simultaneous monitoring. Moreover, the whole device exhibits good stability under external stimulation loading. This simple and universal preparation process and sensing principle provide a reference for the quantitative production of flexible electronic devices and multi-mode smart electronics.

2.1 Preparation process of 3D hollow MXene spheres

First, the negatively charged Ti₃C₂T_x dispersion solution (11 Technology Co., Ltd.) and positively charged PS dispersion solution (Shanghai Aladdin Biochemical Technology Co., Ltd) were mixed at the mass ratio of 2: 8, stirred for 30 min, and self-assemble to obtain PS@Ti₃C₂T_x dispersion. Then, PS@Ti₃C₂T_x composite powder was prepared by freeze-drying process. Next, the composite material was placed in a tube furnace and the PS template is removed by high-temperature heating to obtain the 3D hollow MXene spheres. The heating rate of the tube furnace is 5 ℃/min, heating at 500 ℃ for 2 hours, and the filling gas is Ar. Finally, disperse the 3D hollow Ti₃C₂T_x spheres in water for later use (5 mg/ml).

2.2 Preparation process of the 3D hollow MXene spheres/ Ag NWs based stretchable electrodes

3D hollow Ti₃C₂T_x spheres and Ag nanowires (Ag NWs, JCNANO Technology Co., Ltd.) composite solution were used as conductive ink to prepare stretchable electrodes. Firstly, the composite conductive ink was spray coated onto the patterned template, which was attached to the wafer. After the ink dries, the template is removed to obtain a patterned conductive layer. Then, as-prepared standard polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning Co., Ltd.) was spin coated on the patterned conductive layer. Next, process in high vacuum (< 100 Pa) for 2 h to infiltrate PDMS into 3D Ti₃C₂T_x spheres/Ag NWs nanocomposites. Finally, after PDMS curing, the PDMS based patterned and embedded stretchable electrode film was obtained by stripping.

2.3 Characterizations and measurements

The microstructures and elements distributions were observed by the SEM, EDS and TEM, respectively. Proximity/pressure sensing performance was measured by the Impedance analyzer (E4990A, KEYSIGHT®). Strain sensing performance was measured by the SourceMeter (2450, Keithley®). COMSOL Multiphysics software was used to simulate and analyze the sensor structure by finite element method.

3.1 Materials, structures and sensing concept of the multifunctional e-skin.

The exploded-view schematic illustration of the stretchable hybrid electronic network-based multifunctional e-skin is shown in Fig. 1a. This e-skin is completely prepared by essentially flexible and stretchable electrodes, whose functional layer is to embed 3D hollow MXene spheres/Ag NWs into PDMS. For this multi-material composite hybrid penetration networks, Ag NWs provide backbone collecting electrodes for rapid electron transport due to their excellent conductivity (Fig. 1b). 3D hollow MXene sphere, which has flexibility but relatively poor conductivity, further provides a local electron transfer pathway by filling the nanowire space between the main electrodes of the Ag NWs network. Figure 1c shows the electrical transport path equivalent diagram of MXene Spheres/Ag NWs composite transport network. Furthermore, the top and bottom embedded stretchable electrodes are placed back-to-back. The MXene Spheres/Ag NWs composite functional layer adopts the serpentine structure, aiming to improve its stretchability. Fig. S1 illustrates the schematic diagram of the serpentine structure and its key geometrical parameters. The line width w is 0.3 mm, the arc angle θ is 270°, and the radius R is 1 mm. The normalized width of the serpentine structure w* = w/R = 0.3 has been verified to achieve preferable mechanical properties (Jang et al., 2015). Figure 1d illustrates the actual picture of the embedded stretchable electrodes based multifunctional e-skin. The crossing stretchable electrodes is selected as the resistive strain modules. In addition, the overlapping conductive parts of the upper and lower stretchable electrodes and the middle filling layer PDMS are jointly constructed as capacitive pressure sensing module. Moreover, the stretchable electrodes and a close finger or other conductor form a mutual capacitance non-contact sensor. Figure 1e shows the cross-sectional structure diagram of the multifunctional sensing structure for proximity, pressure, and strain. The limiting electric field distribution in the initial and final state of the proximity sensing module is analyzed through finite element simulation (Fig. 1f) (Dehkhoda, Frounchi, & Veladi, 2010; Haque, Lubej, & Briand, 2022). The results indicate that the electric field strength between the conductive plates decreases with the approach of the external conductor. Therefore, this mutual capacitive proximity sensor can also be called the negative capacitive proximity sensor.

3.2 Strain sensing performance of the embedded stretchable electrodes.

Embedded stretchable electrodes have been proven to provide both conductivity and stretchability [19]. The fabrication process of the embedded stretchable electrodes is shown in Fig. 2a. The synthesis of 3D hollow MXene spheres and more detailed content are shown in Fig. S2. It is worth noting that the MXene spheres are prepared by using positively charged polystyrene (PS) spheres as templates, and negatively charged Ti₃C₂T_x-MXene nanosheets are self-assembled on the surface of PS spheres under electrostatic adsorption force. The SEM images and transmission electron microscope (TEM) images of PS@MXene spheres exhibit the independent and tightly wrapped spherical structure (Fig. S3a, S3c). 3D hollow Ti₃C₂T_x-MXene spheres are produced by thermal annealing in argon. The TEM images after heat treatment indicate that the PS template has been completely removed and the 3D spherical morphology can be well maintained, indicating the completion of the preparation of 3D hollow Ti₃C₂T_x-MXene spheres (Fig. 2b). In addition, no stacking of MXene nanosheets are found in the macroscopic characterization results, proving that this process method can effectively avoid the problem of uneven conductivity caused by stacking MXene nanosheets as electrodes (Fig. S3b). Moreover, Fig. 2c shows the embedded coplanar structure formed by the infiltration of PDMS into hybrid composite conductive layer under vacuum force. The energy dispersive spectrometer (EDS) from SEM shows that the hybrid 3D hollow MXene spheres/Ag NWs conductive functional layer constructs an effective permeable conductor network in PDMS (Fig. 2d).

To clarify the strain sensing characteristics of this serpentine embedded electrodes, Fig. 2e shows the resistance changes ΔR/R₀ corresponding to the normal and shear stretched the 3D hollow MXene spheres/Ag NWs based electrode, respectively. The results indicate that the ΔR/R₀ of shear stretching remains basically unchanged, and the ΔR/R₀ during normal stretching is much greater than that during shear stretching.This is explained by the fact that the line width (micrometer level) of a stretchable electrode is much shorter than its length (centimeter level), and the effect of line width changes on resistance during stretching is much smaller than that of length changes. The gauge factor (GF), an important indicator for evaluating the sensitivity of strain sensors, is usually calculated by GF =(ΔR/R₀×100%)/ΔS×100%, where ΔS represents the relative change in loading strain (Shen, Zhou, Gu, Wang, & Wang, 2023). Figure 2f shows the results of relative resistance changes ΔR/R₀ under different strain states ΔS. The fitting curve shows that the GF value of this strain module is as high as 55. Besides, Fig. 2g shows the uniformity test results of the strain sensing arrays, indicating that their initial resistance values are basically equal. Furthermore, Fig. 2h-j show the finite element simulation results of this stretchable electrodes under fixed stretching of 40% in the x-, y-, and xy- directions, respectively. The results show that the strain of the serpentine conductive layer is less than 40%, which indicates that the preparation process can effectively avoid the defect caused by stress concentration in traditional stretchable electrodes.

3.3 Proximity sensing performance of the multifunctional e-skin.

To determine the proximity sensing capabilities of the embedded stretchable electrodes based multifunctional e-skin, two crossed embedded stretchable electrodes are fixed back-to-back (Fig. 3a). The plates of the capacitive proximity sensing module are composed of overlapping parts of upper and lower stretchable electrodes. Human hands or metal conductors act as the external plate that approach the e-skin in a vertical direction. Previous studies have shown that the working principle of capacitive proximity sensing module is related to the edge field effect of capacitors, as shown in Fig. 3b (Sarwar et al., 2017; P. Zhao et al., 2020). There is a potential difference between the upper and lower plates, and the generated electric field is mostly distributed between the electrodes (C_p). A small portion of the electric field starts to diverge from the high potential electrode and eventually returns to the low potential electrode. This portion of the electric field passing through the external space is called the edge field (C_f). When a finger approaches the upper surface of the sensor, some edge fields will be intercepted by the finger, pointing towards the finger instead of returning to the lower electrode, resulting in a decrease in capacitance value. The key to the operation of capacitive proximity sensors is the coupling between the edge field of the sensor and the capacitance of the approaching object. With the decrease of the distance between the approaching object and the device or the increase of the interaction area, the edge field near the object increases, the capacitive coupling becomes stronger, and the capacitive response of the proximity sensor becomes more significant. Figure 3c show the finite element simulation results of the proximity sensing module at different states. The results indicate that the electric field intensity between the upper and lower plates decreases as the outer conductor approaches, resulting in a decrease in capacitance value.

Figure 3d shows the relative capacitance changes ΔC/C₀ as a function of the proximity distance. The results indicate that the ΔC/C₀ of the device tends to zero in the initial state. And the absolute value of the relative capacitance change gradually increases as the hand approaches the surface. Figure 3e shows the real-time response results by acting fixed distance for four cycles. The e-skin exhibits stable and consistent response results in terms of proximity sensing, demonstrating the reproducibility of the proximity sensing module. Moreover, the response-recovery time of the proximity sensing module is ~ 0.6 s, which almost realizes the real-time noncontact detection. Meanwhile, both the output capacitance and the distance of the external conductor keep good synchronization, which further proves the real-time monitoring effect of the proximity sensing module (Fig. 3f). In addition, to explore the spatial recognition performance of proximity sensing arrays, static and dynamic environmental tests are conducted on the e-skin. The initial capacitance value of each capacitor in the e-skin is basically equal, as shown in Fig. S4. As shown in Fig. 3g, h, different numbers of fingers are kept at a fixed distance from the e-skin. The detection results indicate that the relative capacitance variation trend of the sensing array corresponds to the position of the fingers. The distribution of proximity monitoring results corresponds to the positions of the fingers. Furthermore, when the finger keeps a fixed distance over the e-skin (Fig. 3i), the negative capacitance change results can basically distinguish the sliding trajectory of the finger (Fig. 3j). The above research results indicate that the e-skin can achieve proximity sensing, revealing its potential applications in the field of non-contact sensing.

3.4 Pressure sensing performance of the multifunctional e-skin.

The capacitor composed of the 3D hollow MXene spheres/Ag NWs/PDMS based plates and the intermediate dielectric layer (PDMS) can not only realize proximity sensing, but also realize pressure sensing by reducing the distance of the plates under pressure. The ΔC/C₀ of the pressure module corresponding to different pressures are shown in Fig. 4a. The result indicates that the capacitance curve varies linearly in two regions with the change of pressure. The sensitivity of the capacitive pressure sensor is usually defined as S = (ΔC/C₀)/ΔP (X. Cui et al., 2023; Tan et al., 2022). The sensitivity of the pressure sensing module is 0.11 kPa_− 1 in the pressure region 0-7.5 kPa and in the pressure regions 7.5–20 kPa is 0.03 kPa^− 1. Figure 4b shows the stability of the pressure sensing module. When a quantitative external pressure (0 kPa, 2.4 kPa, 4.8 kPa, 7.2 kPa) is applied to the sensor in a stepped manner, the response of the capacitive pressure sensing module is basically stable. On the contrary, when the external pressure is unloaded, the ΔC/C₀ of the sensing module gradually returns to its initial state, unaffected by previous stimulus. The response-recovery time of the pressure sensing module is ~ 0.82 s and ~ 0.81 s, respectively (Fig. 4c). The above results verify that this sensing module is expected to realize timely pressure sensing in practical applications. Due to the stretchability of the overall e-skin, the signal crosstalk between stretching and pressure is crucial for the accuracy of device sensing performance. Figure 4d shows the response results of the multifunctional e-skin under different stretching states at fixed pressure 10 kPa. The results indicate that as the strain of the device gradually increases, the pressure response decreases accordingly. When the strain range is 0–4%, the influence of fixed pressure response results can be basically ignored. Furthermore, the corresponding pressure response results under different tensile states are shown in Fig. 4e. The results show that the strain has no effect on the corresponding device within 4 kPa. Moreover, Fig. 4f shows the durability test results of the pressure sensing module. After 1,000 cycles of compression-release testing, the response of the sensing module shows no significant difference, indicating that the performance of the sensing module is not decreased, indicating that the pressure sensing module has great durability.

The multifunctional e-skin is able to detect different morphologies such as letters. “A”, “B” and “C” pressure graphics physical maps and letters recognition results are show in Fig. S5. The results of the projected fields show that the e-skin is very sensitive to slight pressure contact. In addition, different gestures have also been proven to be monitored, such as a swipe, where fingers touch slightly and move on the surface of the e-skin (Fig. 4g). The results show that the e-skin is sensitive and accurate for dynamic gesture recognition. If needed, further signal processing can be used to generate gesture animations from the resulting feedback. In addition, the multi-touch feature is another common gesture in commercial touch screen devices, which can detect the presence of one, two, or even more fingers (Fig. S6). The results show that the pressure sensing module can effectively distinguish between multi-point touch positions and pressure levels. It is worth noting that the finger touch process activates neighboring sensing pixel. This can be interpreted as a finger non-contact response perceived by the device. In practical applications, further signal processing can be used to enhance spatial resolution, thereby achieving simultaneous monitoring of proximity and pressure.

3.5 Multiplex detection demonstration of the e-skin.

In order to verify whether the e-skin can achieve comprehensive tactile perception similar to human skin, the multiplex detection experiments are designed to simultaneously monitor noncontact (proximity) and contact (pressure and strain) parameters. The schematic diagram of the multiplexing detection process is shown in Fig. 5a. As is well known, a random external force acting on the e-skin can be broken down into shear force (orange arrow), normal force (green arrow), and force point (black arrow). The shear force and normal force can be monitored by the strain sensing module and pressure sensing module, so that the direction and magnitude of the external force can be obtained. Besides, the distance (blue arrows) between the noncontact conductor and the device can also be obtained. Figure 5b shows the flow chart of the device response to the stimulus signal. When the relative capacitance change ΔC<0, it indicates that the external signal does not touch the surface of the e-skin, and the relative capacitance change reflects the distance between the noncontact signal and the e-skin. Conversely, when ΔC > 0, it indicates that the stimulus signal is in contact with the surface of the e-skin, and the ΔC reflects the pressure of the stimulus signal. Then, the direction of the stimulus signal acting on the device surface is obtained by monitoring the change of the relative resistance ΔR. Figure 5c-f show four groups of comparative experiments under different stimulus signals, respectively. When external stimuli approach the e-skin (Fig. 5c), the ΔC/C₀ is less than 0, and the ΔR/R₀ is equal to 0. When the external force is loaded perpendicular to the e-skin (Fig. 5d), the capacitance response is symmetrical in the Z direction. The maximum ΔC/C₀ represents the magnitude of the external force. The resistance responses ΔR/R₀ in X direction and Y direction are symmetric and their maximum values are basically consistent. When the direction of the external force changes (Fig. 5e), the ΔR/R₀ results of the upper and lower stretchable electrodes are inconsistent. The overall trend in the direction of the external force can be obtained by comparing the strain/pressure response results. Finally, when different types (proximity, pressure and strain) of stimuli work together on the device, comprehensive tactile information can be obtained by monitoring ΔC/C₀ and ΔR/R₀, which provides a reference for the application of smart electronics in the field of contactless interaction and multi-mode touch.

In conclusion, we have developed a stretchable hybrid electronic network-based e-skin for proximity, pressure and strain sensing, and the signals do not interfere with each other. The e-skin is constructed by crossing stretchable electrodes, which is fabricated by embedding 3D hollow MXene spheres/Ag NWs hybrid nanocomposite into PDMS. The sensitivity of pressure and strain sensor are S = 0.11 kPa^− 1 and GF = 55, respectively. Different non-contact and contact experiments have verified the excellent sensing characteristics of the e-skin, thus verifying the effectiveness of the design of the micro hybrid three-dimensional conductive network. Moreover, multiplex detection results of the e-skin under different stimulus signals indicate its ability of crosstalk free recognition of different tactile signals and proximity signals. The results provide a facile and universal strategy for the application of intelligent electrons in the field of non-contact interaction and multi-mode touch, such as wearable devices, augmented reality and prosthetic limbs.

Funding

This work is supported by the National Natural Science Foundation of China (No. 62305222).

The authors declare no conflict of interest.

Data Availability statement

The data supporting the findings of this study are available within the paper and its Supplementary Information files. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request.

Author's contribution

X.W. designed the experiments and wrote draft of manuscript. Z. Z. carried out the preparation of materials and contributed to the interpretation of the results. Y. C. and X. S. revised the manuscript and discussed the results. X. Z. and X. G. carried out laboratory research and commented on the manuscript. S. Z. discussed the results and commented on the manuscript.

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No competing interests reported.

SupportingInformation.docx
Supporting Information Fig. S1. Schematic diagram of the patterned template for the embedded stretchable electrodes and its key geometrical parameters. Fig. S2. Synthesis process of the 3D hollow MXene spheres/Ag NWs composite. Fig. S3. SEM images of different preparation stages of embedded stretchable electrode: aPS @MXene spheres, b 3D hollow MXene spheres. c TEM images of PS @MXene spheres. Fig. S4.The initial capacitance value of each capacitor in the e-skin. Fig. S5. “A”, “B” and “C” pressure graphics physical maps and letters recognition results. Fig. S6. Simultaneously detecting charts and results with different numbers of fingers, demonstrating multi-touch capabilities.

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Stretchable hybrid electronic network-based e-skin for proximity and multifunctional tactile sensing

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Abstract

Figures

1 Introduction

2 Experimental section

2.1 Preparation process of 3D hollow MXene spheres

2.2 Preparation process of the 3D hollow MXene spheres/ Ag NWs based stretchable electrodes

2.3 Characterizations and measurements

3 Results and discussion

3.1 Materials, structures and sensing concept of the multifunctional e-skin.

3.2 Strain sensing performance of the embedded stretchable electrodes.

3.3 Proximity sensing performance of the multifunctional e-skin.

3.4 Pressure sensing performance of the multifunctional e-skin.

3.5 Multiplex detection demonstration of the e-skin.

4 Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1