Using the swelling and contraction of hydrogels in water, it is possible to realize biomimetic 3D-printed structures 19. These materials can respond to external stimuli such as heat, humidity, pH value, and light 16, 17, 18. For example, 4D printing research has been conducted involving hydrogels 10, 11, 12 or shape-memory polymers (SMPs) 13, 14, 15. 4D printing can add active, responsive functions to 3D-printed objects in various ways 9. Using water-swelling materials, flat 3D-printed structures can be transformed into predesigned 3D shapes. They developed active composite structures consisting of swelling and rigid materials. A new concept, known as four-dimensional (4D) printing, has been proposed by Tibbits et al. However, in most cases, 3D-printed structures remain static as printed. 3D printing is being widely used in many different technical areas 1, 2, 3, 4, 5, 6, 7. Three-dimensional (3D) printing is appropriate for the fabrication of complex multi-material structures that are difficult to realize using conventional methods. Our study indicates that multicolor 4D printing of SMPs are promising for various structural changes and remote actuation. Therefore, 4D printing of multicolor SMP composites have unique merits for light-induced structural changes. Moreover, SMPs can be reused multiple times for thermal actuation by simply conducting thermomechanical programming again. 4D printing can allow complex, multicolor geometries with predesigned responses. Finally, we consider a multicolor hinged structure and demonstrate the multistep actuation by changing the color of light and duration of illumination.
We also present simulations and analytical calculations to theoretically model the structural variations in multicolor composites. We experimentally investigate the temperature changes in colored SMPs and observe a clear difference between the different colors. Using color-dependent selective light absorption and heating in multicolor SMP composites, we realize remote actuation with light illumination. In this study, we demonstrate multicolor 4D printing of shape-memory polymers (SMPs). Light, among others, has a unique advantage of remotely controlling structural changes to obtain predesigned shapes. Four-dimensional (4D) printing can add active and responsive functions to three-dimensional (3D) printed objects in response to various external stimuli.
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