From Dual-Mode Triboelectric Nanogenerator to Smart Tactile Sensor

Mar 23, 2017 - School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United State. ABSTRACT: Tribo...
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From Dual-Mode Triboelectric Nanogenerator to Smart Tactile Sensor: A Multiplexing Design Tao Li,†,∥ Jingdian Zou,†,∥ Fei Xing,† Meng Zhang,† Xia Cao,*,†,‡ Ning Wang,*,§ and Zhong Lin Wang*,†,⊥ †

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, National Center for Nanoscience and Technology (NCNST), Beijing 100083, China ‡ Research Center for Bioengineering and Sensing Technology, Beijing Key Laboratory for Bioengineering and Sensing Technology, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, China § Center for Green Innovation, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China ⊥ School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United State ABSTRACT: Triboelectric nanogenerators (TENGs) can be applied for the next generation of artificial intelligent products, where skin-like tactile sensing advances the ability of robotics to feel and interpret environment. In this paper, a flexible and thin tactile sensor was developed on the basis of dual-mode TENGs. The effective transduction of touch and pressure stimulus into independent and interpretable electrical signals permits the instantaneous sensing of location and pressure with a plane resolution of 2 mm, a high-pressuresensing sensitivity up to 28 mV·N−1, and a linear pressure detection ranging from 40 to 140 N. Interestingly, this selfpowered dual-mode sensor can even interpret contact and hardness of objects by analyzing the shape of the current peak, which makes this low-cost TENG-based sensor promising for applications in touch screens, electronic skins, healthcare, and environmental survey. KEYWORDS: triboelectric nanogenerator, dual-mode, tactile sensor, artificial intelligence, multiplexing

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Triboelectric sensors (TES) are based on contact electrification and designed to detect tiny pressure or motion in a selfpower mode. Due to advantages such as high sensitivity, light weight, fast response, and low detection limit,6,17,18 great strides have been made in the design and integration of TES during the past several years. For example, Zhu et al. reported a selfpowered sensitive flexible tactile sensor based on singleelectrode triboelectric nanogenerator (TENG).19 Wang et al. proposed a contact-mode TES to measure vibration amplitude quantitatively with high sensitivity.20 Lin et al. fabricated an active sensor array for static and dynamic pressure detection with a modified structure.21 However, simultaneous detection of complex information has not been realized due to the difficulty in decoupling the interfered signals, which makes constructing flexible, low-cost, and sensitive sensors for artificial electronic skin a great challenge.22,23 Herein, we proposed a self-powered, smart, and integrated tactile sensing unit on the basis of the dual-mode TENG for

ntelligent robots have been around long enough for a rich research environment to be created where Sci-Fi movies have become reality.1,2 Unlike traditional industrial robots, which follow scheduled program, the next generation robots are designed to work autonomously in smart homes, outer space exploration, and advanced medical procedures, where they can feel and interpret the environment with the help of all kinds of sensors.3−5 A smart tactile sensor, just like the hand of a person, is critically important because it not only reads physical characteristics such as location, temperature, and shape but also helps to manipulate various objects by feeling hardness, force, and pressure.6 Such bionic functions in the past can only be seen in Sci-Fi movies and now are becoming reality. Traditional tactile sensors can be divided into the following major categories: capacitive,7−9 piezoelectric,10,11 resistive,12,13 and optical.14 However, a main issue in the design of such smart sensors is energy supply because, in some extreme environments, remote locations, and uncharted worlds, recharging or replacing the sensor battery is inconvenient and even impossible while the sensors are required to function long enough to accomplish the application.15,16 © 2017 American Chemical Society

Received: January 18, 2017 Accepted: March 17, 2017 Published: March 23, 2017 3950

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Figure 1. (a) Schematic diagram of the application for an as-designed TES. (b) Illustration of the working principle of the coupled TES. (c) Image of the as-prepared TES. (d) Scanning electron microscopy image of Ag nanowires.

Figure 2. Performance and mechanism of TES for contact sensing. (a) Simplified equivalent circuit model of the as-prepared TES for contact sensing. (b) Output current of the TES in response to different contact materials under a pressure of around 100 Pa. (c) Shape of current peak for different contact materials. (d) Possible mechanism for different shapes of the current peak that belong to different materials.

and pressure and can be used in many fields including mobile technology, computers, and e-skin.

simultaneously sensing contact and location and detecting pressure and motion. Under a tiny pressure (∼100 Pa), current signals (≥0.4 μA) are obtained for six kinds of typical materials. Interestingly, this sensor can not only detect tiny pressure and force but also “feel” the hardness of the contact material by quantifing the shape change of current peak, making it possible to create artificial skin that incorporates the ability to feel and touch objects. Additionally, the embedded silver nanowire electrodes also enable pinning down the location of a small object with a plane resolution of 2 mm. At last, the coupled contact-separation mode unit of TES can linearly detect the pressure in the range from 40 to 140 N. Therefore, the asdesigned smart sensor offers simultaneous detection and quantification of information about contact, hardness, location,

RESULTS AND DISCUSSION The TES unit comprises both a single electrode TENG (top part) and a traditional contact-separation TENG (bottom part), as shown in Figure 1a,b. The top part a polydimethylsiloxane (PDMS) film serves as friction layer, whereas the built-in Ti foam functions as an induced electrode (Figure 1c). Four silver nanowires with a diameter of 200 nm and a length of 25 μm (Figure 1d) are patterned onto the surface of the PDMS film. Once a foreign object reaches the PDMS film, electrical signals that are generated due to contact electrification can be utilized to analyze the contact process. An aluminum electrode was placed under Ti foam and connected with the PDMS film by 3951

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Figure 3. Principle and performance of TES for location sensing. (a) Principle of location sensing. (b) Theoretical analysis of voltage distribution with different contact area. (c) Voltage ratio of electrode a to electrode c with 16 test points. (d) Voltage ratio of electrode b to electrode d with 16 test points. (e) Comparison between the test location and the actual location.

is, the fewer the deformation fields even if the applied force is the same. As a result, a sharp current peak is observed. For a soft material, deformation is created evenly and continuously, leading to slow and steady charge accumulation and neutralization. According to this mechanism, the object hardness can be quantified by the width at half-maximum of the peak current. By simply touching the artificial skin, the robot can thus decide how much force to offer to handle the object, which is important for an industrial robot to interpret and handle in a complex environment. The validity of a decision from a robot is a function of the reliability of the data derived from the object. In advanced robotic applications, using only one kind of feedback is sometimes insufficient to achieve the desired goals perfectly.30,31 Ag nanowire electrodes were located on the edge of the PDMS surface, serving as an induced electrode to realize the two-dimensional location sensing. It should be noted that only four silver electrodes were used, which largely decreased the number of sensor terminals.31 The location detection mechanism can be briefly described as follows (Figure 3a). When an object contacts the surface of TES, negative and positive charges are accumulated on the surface of the PDMS film and object. No induced current is generated because the electric field formed by charge separation is restricted between the object and the surface of PDMS. After the separation between the object and PDMS, the charges on the surface of PDMS will induce opposite charges on the electrodes. The quantity of induced charges is related to the distance between the contact part and electrode, which means different current or voltage output signals. The charges induced in the Ag electrode subsequently become constant as the object goes further, suggesting no current flows. When the object comes back again, the charges carried on the object diminish the influence of negative charges on PDMS to the Ag electrode, leading to positive charges flowing to the ground. Thus, the location can be deduced from these voltage variations by a mathematic process.32 According to the finite element analysis, the voltage

four springs to form a traditional two-electrode TENG. Cu nanowires (100 nm in diameter, 20 μm in length) with good stability were decorated on the surface of an Al electrode to improve the output performance.24,25 It is well-known that contact electrification occurs even between two same materials,26,27 which offers triboelectrification an active role for contact feeling. A simplified operation mechanism was put forward and is shown in Figure 2a. Charge accumulation that stems from instantaneous contact electrification generates a potential difference and induces current flow when the electrode is grounded. It should be noted that only a very small force is needed here, which prevents the object from physical damage. Figure 2b exhibits the output currents obtained from six typical materials, including metal (Cu foil, 99% purity), polymer (nylon, poly(methyl methacrylate) (PMMA)), inorganic material (soda-lime glass), and natural polymer (cellulose). The current signals are all more than 0.4 μA, suggesting the universality of contact electrification. Therefore, this TES could be utilized as an active contact sensor. It is essential for a smart tactile sensor to sense multiple information at the same time for immediate decisions.28,29 Figure 2c presents the shape differences among current peaks corresponding to various objects. The tests were driven by a linear motor which moves under the same motion program with the same surface state in order to avoid the external influence. It can be seen that all current peaks reach nearly 1.2 μA for different materials. Typically, PDMS, terylene, copper, and glass are chosen as the experimental subjects. These samples are of different hardness, whereas their shapes change according to the hardness. For stiff materials such as copper and glass, the current increases suddenly, which is significantly different from the slow and continuous current change for soft materials. This sharp contrast may be explained by the stress− pressure coupling deformation model (Figure 2d). The deformation field in the surface of TES results from the stress field induced by applied forces. Therefore, the harder the object 3952

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Figure 4. Output performance of the TES and relationship between pressure and open-circuit voltage. (a) Short-circuit current of the TES. (b) Open-circuit voltage of as-prepared TES. (c) Numerical calculations of the potential difference and the gap distance of the TES. (d) Relationship between output voltage and applied pressure.

of the four electrodes changes when the location of the contact moves, as illustrated in Figure 3b. Generally, the closer the electrode is to the contact location, the lower the potential is. The theoretical analysis clearly suggests the feasibility of the location test protocol. In the experiment, the whole electrode area is in nine parts and 16 test points are marked. The peak voltage is used to calculate the opposite voltage ratio because it is more stable and easier to measure. To obtain a twodimensional location for the contact, two voltage ratios of opposite electrodes (denoted as Vi/Viii and Vii/Viv) were measured with a 4 × 4 test point (Figure 3c,d). Vi/Viii and Vii/ Viv monotonously increase when the test point approaches electrodes i and ii. The voltage ratio changes from 0.57 to 1.67 and shows good resolution. Aiming to verify the location ability, we carried out the location test at point (2, 3) (Ei = 0.34 V, Eii = 0.43 V, Eiii = 0.41 V, and Eiv = 0.34 V), and the output voltage was collected and calculated using an intersection method. The calculated location is (1.93, 3.20), which represents good accuracy with a distance deviation of 2.0 mm (Figure 3e). As discussed above, the TES can detect the tiny contact and location, and there is still a demand for pressure sensing in view of the robots’ movement. However, in some robotic motions such as grasping or pushing, it is necessary to know the robots’ exact pressure to avoid possible damage to the objects. Here, the vertical contact-separation mode TENG was incorporated in the as-designed TES owing to its good linear response for external force. As shown in Figure 4a,b, the short-circuit current is around 9 μA, and the open-circuit voltage reaches 90 V. The TES mechanism (two-electrode mode) is based on the coupling of contact electrification and electrostatic induction, whereas the detection of pressure was carried out in a noncontact mode in order to avoid potential damage caused by continuous impact. Initially, the linear motor brings the plate into contact with the other plate, causing contact electrification. The positive and negative charges accumulate on each surface when the external force is withdrawn. With a periodical external

force, AC signals are generated. These triboelectrification charges exist for a long time, and this property supports the pressure detection under a noncontact mode, which means the two friction layers no longer frequently contact each other after the initial triboelectrification. Typically, the potential of the Al electrode is defined as zero; σ represents the triboelectric charge density of PDMS, ε0 is the vacuum permittivity, and d is the gap distance between two plates, according to eq 1:

Voc =

σd ε0

(1)

The open-circuit voltage (Voc) increases linearly along with the vertical gap distance. The Voc−d relationship can be mathematically analyzed by assuming the TES is a parallel-plate capacitor. In this model, the Al electrode and PDMS film are stacked parallel with a gap distance ranging from 0 to 5 mm. The triboelectric charge density on the inner surface of the PDMS film was set by calculating the transfer charge in Figure 4a,b. The calculated potential distribution based on the parallel-plate model is shown with color scaling. As shown in Figure 4c, the potential difference is zero when the two plates are in full contact (d = 0), which reaches a maximum when the gap distance extends to 5 mm. Furthermore, the spring that connects the electrode plates of as-prepared TES follows Hooke’s law. As a result, the open-circuit voltage of TES changes linearly with the external force, and the linearity could be used for dynamic force sensing. A triboelectric test was carried out with a linear motor to verify the relationship between actual pressure and the open-circuit voltage. During the test, the gap distance between two plates was carefully controlled to avoid new contact after the initial triboelectrification. As illustrated in Figure 4d, there exists an output voltage of 7.8 V when a pressure of 40 N is applied. When the pressure increases to 140 N, the output voltage reaches 10.6 V. The fitting curve (red line) suggests that the pressure and output voltage have a linear relationship with a correlation 3953

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Figure 5. Real response behavior of the TES for dynamic force sensing with a pulse time of (a) 0.4 s, (c) 0.6 s, and (e) 0.8 s, collected by a commercial quartz force sensor. (b,d,f) Corresponding dynamic force curves measured by analyzing the open-circuit voltage of the TES.

CONCLUSION In summary, a tactile sensor (TES) was developed on the basis of coupled dual-mode TENGs. In this protocol, the singleelectrode unit acts as a contact sensor and senses the relative hardness of an object. The two-electrode unit plays a key role for guiding the pressure. Thus, this complementary design makes it capable of simultaneously collecting complex information from a foreign object. The linear detection for pressure ranges from 40 to 140 N with a correlation coefficient of 0.98. Good response for dynamic pressure has also been shown when the period of dynamic force changes from 0.4 to 0.8 s and pressure from 55 to 38 N. When a single-mode TENG-based location sensor was embedded, the TES shows a plane resolution of 2 mm for a tiny contact. These results verify the ability of the TES to both feel and interpret complex information on contact, hardness, location, and pressure, which makes it a good choice for future intelligent robots or other smart equipment.

coefficient of 0.98, and the linear range is 40−140 N with a pressure sensitivity of 0.28 mV·N−1. The test suggests the good performance of the TES for sensing pressure, and this will be useful for guiding smart robots to supply proper pressure during mechanical motion. To further test the dynamic behavior of the as-prepared TES, a linear motor was used as a tunable dynamic force source with different vibration periods and amplitudes. During the test, a quartz dynamic force sensor was fixed on a vibration plate to detect the force dynamically. The open-circuit voltage was collected and transformed into force expression, as listed in Figure 5. Under a vibration with a period of 0.4 s, the TES shows a period of 0.4 s and pressure of nearly 55 N. The shapes of the dynamic pressure peaks collected by the TES are similar to those of the original, suggesting the good sensing accuracy of the TES for dynamic pressure (Figure 5a,b). When the period increases to 0.6 s with pressure down to around 43 N, the TES also shows a good response to this change without any obvious deviation, neither in period nor in amplitude. While the period and amplitude are 0.8 s and 38 N, there is still a good response of the TES for vibration. These results suggest that the TES has a good response to the dynamic force with a period changing from 0.4 to 0.6 s and the pressure changing from 55 to 38 N, demonstrating a wide linear range for dynamic pressure sensing.

METHODS Synthesis of Cu Nanowires. The Cu nanowires were synthesized according to a previous report.33 Copper chloride (0.17 g) and glucose (0.1 g) were dissolved in 80 mL of DI water under stirring. Octadecylamine (1.44 g) was added to the solution and mixed fully by magnetic stirring. The solution was transferred into a Teflon-lined stainless steel autoclave (100 mL) and kept at 120 °C for 24 h. After it cooled naturally, the obtained solution was washed with DI water, 3954

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ACS Nano ethanol, and hexane. The surfactant stayed at the surface of the Cu nanowire to avoid oxidation. Fabrication of a Single-Electrode Unit. A commercial Ti foam was first rinsed with DI water and ethanol. Subsequently, the PDMS solution (Sylgard 184, Dow Corning) containing elastomer and crosslinker with a 10:1 ratio was cast onto the Ti foam and cured at 80 °C for 2 h in an oven. A commercial Ag nanowire solution (XFNANO, China) with a 200 nm diameter and 25 μm length was coated several times and formed four electrodes on the PDMS surface after dilution. Then a layer of PDMS was adhered onto the Ag electrode and cured. Fabrication of a Contact-Separation Unit. A piece of acrylic glass was cut as the substrate with four half-through holes at the corners. A piece of the Al plate was immersed into the solution of commercial Cu nanowires and baked in an oven to evaporate the liquid. The as-prepared single-electrode unit was connected to the substrate by four springs. The voltage and current signals were collected by a Keithley 6514 and BNC 2120 (National Instruments) apparatus. A linear motor (Linmot) was used as a pressure supply. The scanning electron microscopy images were collected by Hitachi 8020 SEM. Pressure was measured with a quartz sensor (YMC Piezotronic Inc., China).

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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xia Cao: 0000-0002-8314-9681 Ning Wang: 0000-0002-7863-8683 Author Contributions ∥

T.L. and J.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are thankful for the support of the National Key R&D Project from the Minister of Science and Technology, China (2016YFA0202702), the National Natural Science Foundation of China (NSFC Nos. 21275102, 51272011, and 21575009), the “Thousands Talents” Program for Pioneer Researcher and His Innovation Team, China, and the National Natural Science Foundation of China (Grant No. 51432005). REFERENCES (1) Amjadi, M.; Kyung, K. U.; Park, I.; Sitti, M. Stretchable, SkinMountable, and Wearable Strain Sensors and Their Potential Applications: A Review. Adv. Funct. Mater. 2016, 26, 1678−1698. (2) Fang, Y.; Yashin, V. V.; Levitan, S. P.; Balazs, A. C. Pattern Recognition with “Materials that Compute. Sci. Adv. 2016, 2, e1601114. (3) Wang, X.; Dong, L.; Zhang, H.; Yu, R.; Pan, C.; Wang, Z. L. Recent Progress in Electronic Skin. Adv. Sci. 2015, 2, 1500169. (4) Gong, S.; Schwalb, W.; Wang, Y.; Chen, Y.; Tang, Y.; Si, J.; Shirinzadeh, B.; Cheng, W. A Wearable and Highly Sensitive Pressure Sensor with Ultrathin Gold Nanowires. Nat. Commun. 2014, 5, 3132. (5) Wang, H.; Pastorin, G.; Lee, C. Toward Self-Powered Wearable Adhesive Skin Patch with Bendable Microneedle Array for Transdermal Drug Delivery. Adv. Sci. 2016, 3, 1500441. (6) Wang, C.; Hwang, D.; Yu, Z.; Takei, K.; Park, J.; Chen, T.; Ma, B.; Javey, A. User-Interactive Electronic Skin for Instantaneous Pressure Visualization. Nat. Mater. 2013, 12, 899−904. (7) Lee, H.-K.; Chang, S.-I.; Yoon, E. A Flexible Polymer Tactile Sensor: Fabrication and Modular Expandability for Large Area Deployment. J. Microelectromech. Syst. 2006, 15, 1681−1686. 3955

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