In a recent article published in the journal Nature Communications, researchers proposed a self-powered bionic electronic skin (e-skin) that can detect and visualize the complex sliding behavior of liquid droplets on its surface. This e-skin aims to mimic the function of human skin to enhance the perception and autonomous regulation capabilities of intelligent robotic devices in droplet environments.
a Schematic illustration of the human tactile nervous system, and the bionic DES sensing system for droplet environment perception and reconnaissance. b Detailed structure of the bionic DES. The electrode unit is delicately designed as a branching structure. c Co-layer interlaced electrode configurations, and enlarged view of overpass connection. d Scanning electron microscopy (SEM) image of the cross-sectional structure of bionic DES. e Flexible and curved fit performance. f Desirable hydrophobic properties of bionic DES. Image Credit: https://www.nature.com/articles/s41467-024-50270-8
Background
E-skins are flexible and wearable devices that can sense various physical stimuli, such as pressure, temperature, humidity, and finger sliding, and provide feedback to humans or machines. They have many potential applications in robotics, healthcare, prosthetics, human-machine interfaces, and virtual reality.
However, most of the existing e-skins can monitor information from solid objects and air but cannot fully sense the dynamic sliding of liquid droplets, which intelligent robotic devices often encounter in rescue, military, and scientific exploration scenarios. Therefore, there is a need to develop e-skins that can comprehensively perceive the dynamic sliding information of liquid droplets, including positions, directions, velocities, and trajectories.
About the Research
In this paper, the authors developed a bionic e-skin that translates the complex motion of droplets into electrical signals using triboelectricity—a phenomenon where electric charges are generated through the contact and separation of different materials. The e-skin comprises a fluorinated ethylene propylene (FEP) layer, co-layer interlaced electrode networks, and a polytetrafluoroethylene (PTFE) non-woven fabric substrate.
The FEP layer acts as a negative triboelectric material, generating negative charges when it comes into contact with positively charged water droplets. The electrode networks, crafted from conductive fabrics into cross-shaped structures, are interlaced on the FEP layer without direct conduction. They are connected using an overpass method, which prevents overlap and minimizes signal crosstalk. The PTFE substrate provides mechanical support and waterproofing.
The e-skin operates through the combined effects of friction electrification and electrostatic induction among the water droplets, the FEP layer, and the electrodes. As a droplet slides across the e-skin, it induces positive charges on the electrodes, creating a current flow in the external circuit. This current can be measured and analyzed to extract information about the droplet's motion, including its position, direction, velocity, acceleration, and trajectory.
Research Findings
The researchers evaluated the e-skin's performance under various conditions, demonstrating its sensitivity and stability. They found that the e-skin could detect changes in surface angle, droplet volume, sliding frequency, dropping height, and liquid type, with these variations reflected in the electrical output signals. The e-skin retained its electrical output even after being submerged, rinsed with water, and subjected to 1000 cycles of bending and curling.
The e-skin successfully detected and displayed the multi-directional (two-dimensional) sliding behavior of droplets in real-time. The authors also developed a dynamic trajectory perception and visual feedback system that integrates the e-skin with a microcontroller unit and a light-emitting diode (LED) indicator. This system tracks and visualizes the motion trajectories of droplets on the LED indicator, corresponding to the e-skin's electrodes, and is capable of sensing and displaying the sliding behavior of multiple droplets simultaneously.
Applications
The developed e-skin has the potential to significantly enhance the perception and autonomous regulation capabilities of intelligent robotic devices in environments where droplets are prevalent. For instance, it could be integrated into various parts of a robotic device, such as the limbs, head, back, or abdomen, to monitor droplet environments and deliver timely, reliable information for strategic decision-making. Additionally, it could be employed to detect and manage liquid leakage in settings like smart workshops, restaurants, laboratories, and medical facilities, thereby safeguarding both people and equipment.
Conclusion
In summary, the novel self-powered e-skin demonstrated its effectiveness in comprehensively perceiving and visually displaying the dynamic sliding behavior of liquid droplets in real-time. This advancement narrows the gap between artificial e-skins and human skin in terms of sensory functions, showing promising potential for applications in military, rescue operations, and everyday life.
Looking ahead, the researchers recognized some limitations and challenges, including low resolution, the need for advanced signal processing, and integration difficulties. They proposed several enhancements for the bionic e-skin, such as incorporating additional sensing modalities (e.g., temperature, humidity, and chemical sensing), integrating wireless communication and data processing modules, increasing the number of electrodes, and designing modular and scalable structures.
Journal Reference
Xu, Y., Sun, Z., Bai, Z. et al. Bionic e-skin with precise multi-directional droplet sliding sensing for enhanced robotic perception. Nat Commun 15, 6022 (2024). DOI: 10.1038/s41467-024-50270-8, https://www.nature.com/articles/s41467-024-50270-8
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