2024/08/16 10:36
Bionic skins allow the robot to perceive liquids
2024/08/16 10:36
Intelligent robots and other intelligent devices play an important role in military reconnaissance, rescue, exploration of unknown environments and auxiliary equipment, and become powerful assistants for human work. Environmental reconnaissance provides timely, accurate, and reliable information to support the development and implementation of strategic plans. The perception modules of these devices include distributed sensor nodes and an integrated flexible bionic electronic skin (e-skin), which has distinct advantages by mimicking the functions of human skin to sense the physical information around it.
In order to enhance the perception and autonomous regulation ability of intelligent robot devices in the droplet environment, it is necessary to develop electronic skins that can deeply detect liquid information. Currently, a key limitation of e-skin is the inability to realistically perceive liquid sliding information and respond in a timely manner as humans can. The researchers have proposed a self-powered biomimetic droplet e-skin (DES) technology. By constructing an ingenious network of colayered interwoven electrodes and a cross-bridge connection, the technology is able to convert various dynamic droplet sliding behaviors into triboelectric-based electrical signals. This bionic skin not only fully perceives the two-dimensional sliding behavior of the droplets, but also provides visual feedback in real time through indicators. In addition, it can also realize flow direction warning and intelligent closed-loop control of water leakage, similar to the regulation function of the human nervous system. These innovations make up for the limitations of e-skin in droplet perception, and greatly narrow the gap between artificial e-skin and human skin perception functions.
Figure 1a illustrates a DES-based intelligent sensing system that provides droplet slip sensing, real-time feedback, and active liquid leakage control capabilities for intelligent robots. The biomimetic DES structure and manufacturing process are shown in Figure 1b and include a PTFE nonwoven substrate, an interwoven electrode network, and a fluorinated ethylene propylene (FEP) negative triboelectric layer on top. The electrode network adopts a special cross-shaped branch structure to form an X-Y electrode network, which effectively increases the electrode coverage area and improves the sensing performance without increasing the number of electrode channels. Figure 1c shows a continuous electrode unit connected by a cross-line, with the X and Y electrodes staggered but not on the same layer, eliminating the additional spacer layer and ensuring uniform charge sensing for each electrode while avoiding signal crosstalk. As an ideal triboelectric anode material, the FEP film is coated on the surface of the electrode, which completes the preparation of biomimetic DES, and the FEP layer can be replaced if necessary.
Figure 1d is a scanning electron microscope (SEM) image showing the electrode layer and the FEP triboelectric layer about 0.02 mm thick on the PTFE fabric. Figure 1e illustrates that DES is made of soft materials and has good flexibility and adaptability on non-flat surfaces such as robot limbs. In addition, the photograph and water contact angle (WCA) image of Figure 1f show that the DES surface has excellent hydrophobicity, ensuring that the water droplets slide smoothly and generate sufficient charge.
Figure 1 Design of a self-powered bionic DES. a Schematic diagram of the human tactile nervous system and a biomimetic DES sensing system for droplet environmental perception and reconnaissance. b Detailed structure of biomimetic DES. c Enlarged view of colayer staggered electrode configuration and connection. d SEM image of the bionic DES cross-sectional structure. e Flexible and curved fit. f Ideal hydrophobic properties of biomimetic DES.
To investigate how DES converts dynamic droplet motion into electrical signals, the researchers built a signal acquisition platform shown in Figure 2a to capture the electrical signals generated when the droplets slide and select the signal mode according to the experimental needs. The working principle of DES is based on the interaction between the triboelectric layer and the electrode. The triboelectric layer initially has a negative charge, causing the electrodes to induce an equal amount of positive charge to achieve electrostatic equilibrium. When the droplets slide toward the electrode with positive electricity, this equilibrium is disrupted and a new electrostatic state is formed (Figure 2b(i)). The flow of electrons flows from the ground to the electrode, neutralizing the positive charge of the electrode, producing an electrical signal. (Figure 2b(ii)).
In this experiment, the four row electrodes are called the X electrodes and the four row electrodes are called the Y electrodes. Figure 2c shows a dual-channel output voltage of about 0.85 V as the droplet slides across the DES surface in the Y direction. As the droplet slips through, four and three electrical signal peaks are generated on the X and Y electrodes, respectively, corresponding to the electrodes through which the droplet passes. (Figure 2d). Figure 2e depicts the droplet sliding through the four X electrodes at different positions along the Y direction on the DES surface. The corresponding electrical signals show that the X and Y electrodes produce 4 and 3 signal peaks, respectively, regardless of the sliding position (Figure 2f). In addition, the labeled signal peaks show that the wavelengths of the electrical signal waveforms produced by the two sets of electrodes are slightly different. This difference is mainly due to the difference in the contact time of the droplet with the electrode unit along the sliding path. This finding shows that the signals generated at different locations of DES are almost identical, showing good applicability.
Figure 2 Working mechanism and signal characteristics. aSchematic diagram of droplet sliding behavior monitoring. b Schematic diagram of the working mechanism of DES. c,d The voltage and current waveforms generated by the droplets sliding on the surface of the DES. e,f droplets slide at three different positions on the DES and the corresponding output signals.
The high sensitivity of DES is essential for droplet environmental reconnaissance. The researchers explored the dynamic behavior of droplets under different conditions through targeted experiments (Figure 3b). In the experiment, the effects of different surface angles on the electrical output and droplet kinetic parameters of DES were first studied. Figure 3c shows that the output voltage increases significantly as the surface angle increases, indicating that a larger surface angle contributes to the ESD sensing efficiency and electrical performance. Further studies have shown that the sliding frequency and volume of droplets also have an important impact on the electrical properties of DES. The high-frequency droplets can effectively compensate for the dissipation of the surface charge of DES (Fig. 3d), thereby enhancing the electrical output. The smaller volume of the droplet, on the other hand, reduces the generation of triboelectric charge, resulting in a decrease in electrical output (Figure 3e). In addition, the drop height of the droplet was also found to be positively correlated with the electrical signal strength (Figure 3f). The higher drop height allows the droplets to hit the DES more quickly, which in turn improves electrical performance. However, a gradual decrease in electrical properties has also been observed as the droplets slide across the electrodes. Studies on liquid types have shown that pure deionized water produces the highest voltage, followed by rainwater and tap water (Figure 3g). In contrast, acid-base solutions and salt water showed a lower electrical signal, with seawater performing the worst. This difference is mainly due to the different properties of the triboelectric charge produced by different liquids. DES exhibits sensitivity to changes in external conditions and exhibits good electrical and long-term stability, which make it have a wide range of application potential in complex environments.
Fig.3 Characterization of DES sensitivity and stability. aSchematic diagram of droplet parameter monitoring. bSchematic diagram of measuring device. cThe electrical output of DES at different surface angles. d–f The output voltage generated by water droplets with different sliding frequencies, volumes, and drop heights. g Electrical signals produced by different liquids. Continuous and long-term electrical stability of h,i DES.
One of the ultimate goals of the research is to provide important information to humans in the event of a liquid spill. Figure 4a shows an intelligent rescue robot that can deeply sense the direction of the droplet's slip and alert the operator via an LED warning light. The system includes a flexible DES, an MCU equipped with processing circuitry, LED warning lights, and a robotic arm model. In practical applications, DES can be installed in different parts of the robot, such as the head, back, and abdomen, to sense the droplet environment as needed. The MCU is responsible for acquiring and analyzing the signal characteristics generated by the droplets on DES (Figure 4b) to determine the flow direction and trajectory of the droplets. Warning signals are issued accordingly. In addition, intelligent robots can not only sense the dynamic information of droplets, but also take measures to reduce the harm that can be caused by leaking liquids. The biomimetic capabilities of such a robot are depicted in Figure 4c, for example, in a smart restaurant, where a service robot can sense and respond to liquid leaks in a timely manner to protect customers.
Fig.4 Enhanced perception and autonomous regulation of droplet slippage. aSchematic diagram of a bionic rescue robot equipped with DES for detecting the droplet environment. bTrajectory diagram of droplet slippage and corresponding direction warning. c Schematic diagram of a DES-based biomimetic sensing control system inspired by human neuromodulation. d Demonstration of the intelligent control system and closed-loop control mechanism for cup pouring in a smart restaurant.
In this study, a self-powered bionic DES was developed to enhance the perception and control ability of intelligent robots to liquids. Based on the triboelectric mechanism, DES adopts the same layer interwoven electrode network and interchange connection technology to ensure the charge induction consistency between the surface of the triboelectric layer and each electrode, and effectively reduce the signal interference. DES components are made of soft materials that are flexible and suitable for adhesion to non-flat surfaces of the robot. DES can convert the complex motion of droplets into electrical signals, and realize the recording, transmission and analysis of droplet motion information. The researchers successfully detected the basic characteristics and motion of droplets, accurately sensed and fed back the dynamic sliding trajectory of droplets, and developed an intelligent early warning system for droplet flow direction and a closed-loop control system for droplet leakage that are regulated by human nerves. Biomimetic DES is expected to bridge the gap between artificial electronic skin and human skin in terms of perception ability, bringing a wide range of application prospects.
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