Alternating current electroluminescent (ACEL) devices are devices that can emit vibrant luminous light when applying an alternative current. Because of their simple device architecture, promising ability to work under harsh environment and uncomplicated manufacturing process, these devices have attracted considerable attention in various emerging fields, such as information encryption, smart electronic skins, soft robots, and optical communications. Despite recent advances in ACEL devices, producing mechanically flexible ACEL devices via customizable methods remains challenging. At present, ACEL devices are commonly fabricated via multi-layer lamination, where an electroluminescent phosphor layer is sandwiched between two stretchable electrodes. The series of steps and expensive utilities (i.e., masks and delicate tools) involved in this technique severely limit its applications in rapid prototyping and customization.
To address the above issues, researchers from SUSTech proposed a streamlined approach to fabricate flexible electroluminescent (EL) devices through multi-material 3D printing (Figure 1a). The devices exhibit excellent robustness and high mechanical flexibility, making them suitable for soft robotic applications. To further demonstrate this potential, the researchers integrated these 3D printed EL devices with a pneumatically-driven soft robot to produce an artificial camouflage that can instantly self-adapt to the environment by displaying matching color, in imitation of the color-changing ability of chameleons (Figure 1b). This proposed strategy can be readily extended to create the next generation of soft and complex EL devices.
3D printing flexible electroluminescent devices
To achieve a viable and high-throughput fabrication, multi-material direct ink writing (DIW), a widely used extrusion-based 3D printing technique, was utilized in this work. This method enables the fabrication of 2D and 3D complex architectures in a layer-by-layer manner. In general, alternating current electroluminescent devices are composed of ion conducting elastomer (ICE), electroluminescent elastomer (ELE), and insulting dielectric elastomer (IDE) layers. To encompass these components, the researchers formulated various printable and UV curable composite inks (Figure 2a). With the fabrication strategy and the tuned ink formulation, high mechanical compliance of the devices can be achieved and robust interfacial adhesion is realized between different layers of the devices. The devices can withstand repeated large deformation, such as bending and twisting, without delamination. This fabrication strategy offers scope for customization. As an example, a wearable EL wristband with a customized luminous ‘SUSTech’ logo was created (Figure 2b).
Chameleon-inspired camouflages via integrating EL devices with soft robots.
Owing to the flexibility and the interfacial robustness of the 3D printed EL devices, the 3D printed EL devices can be integrated with other soft systems. We integrated the EL devices with a pneumatic soft robot to create self-adaptive background-matching camouflages, inspired by the color-changing ability of nature chameleons (Figure 3a). The EL units were printed on a quadrupedal soft robot, which is powered by pressurized air (Figure 3b). With the use of a light sensor, the ELbot can retrieve the background color information and spatially change its surface color to match the environment in an autonomous fashion via circuit control (Figures 3b-c). As demonstrated in Figure 3c, when the robot crawled to a blue light environment, the EL devices emitted a blue light and instantly blended in the background environment without any notable delay. Noteworthy, owing to the flexibility and the interfacial robustness of the EL devices, the 3D printed EL devices can conformally attached on the surface of the soft robots even under continuous dynamic deformation.
Multiple color-matching within one single device can be achieved via printing different EL devices through multi-material DIW printing. For example, light display and soft robots (Figures 4) can be endowed with multiple color-matching ability when encompassing different printable light-emitting EL units in the systems, such as green, blue and orange units in the examples here. Upon exposure to different light environments or crawling to different light habitats, the corresponding color-emitting units on the system instantly illuminated and emitted similar light colors. This color-matching capability is fast, outrivaling most of the existing artificial camouflages, and enables the potential to recapitulate the innate camouflaging behaviors in living species.
Overall, this work suggests a facile strategy that can be used to transform conventional electroluminescent components into soft, stretchable and customizable EL devices. The proposed strategy opens up new avenues for the next generation of completely soft light-emitting devices, smart displays and camouflaging systems.
For more details, please see this recent article published in Nature Communications:
Pei Zhang, Iek Man Lei, Guangda Chen, Jingsen Lin, Xingmei Chen, Jiajun Zhang, Chengcheng Cai, Xiangyu Liang, and Ji Liu. Integrated 3D printing of flexible electroluminescent devices and soft robots. Nature Communications 13, Article number: 4775 (2022) .。