Engineers at Zhejiang University's National Key Laboratory of Chemical Engineering have demonstrated a ‘4D printing’ process that requires a projector, light-curable resin, and paraffin. The technique has been used to create 3D models of calla lily flowers which appear to bloom after printing.
DLP 3D printing, a process capable of creating high-quality plastic parts, is now commonplace. Using a projector, such 3D printers can cure liquid resin into particular 2D shapes, layer by layer, until a complete 3D object is formed. Now, however, it seems that DLP could be used for 4D printing—by curing only a single 2D layer. Sound confusing? It is, but the unusual phenomenon was recently demonstrated at Zhejiang University's National Key Laboratory of Chemical Engineering, whose researchers have published their findings in the international academic journal Advanced Materials.
The term ‘4D printing’ is used to mean many different things, but in this case refers to a printed object changing over time (the fourth dimension) after it has been printed. Professor Xie Tao of the Zhejiang University School of Chemical Engineering and Bioengineering explained that the secret behind this 4D printing phenomenon lies in a few seconds of light curing. The light of a DLP-style projector is carefully programmed to provide each pixel of a thin resin layer with a different light-cured structure, with the resin turning into a solid plastic shape as it cures. This is a lot like DLP 3D printing, although the light is not only focused onto the curable resin to make a particular shape, but also to make each pixel react in a certain way with melted paraffin—the secret ingredient for the incredible 4D printing process.
During the process, researcher Huang Limei first puts a few drops of light-curable resin between two pieces of glass, before using a projector to shine light on it. This light “carving” is achieved through grayscale changes, based on a set of complex mechanical calculations. The material then changes from liquid to solid in just a few seconds, which is like watching a very short micro-film. But more is going on than meets the eye at this point: each pixel of the resin between the two sheets of glass is being altered at the molecular level, laying the foundation for a later transformation that most users of DLP 3D printers will never have witnessed.
After the precise “sunbathing” under the light of the projector, Huang takes the resin-filled glass panels out, revealing that the resin has become a flat membrane. This membrane is then placed in gel-like melted paraffin, where the magic starts to happen. Because the molecular structure of each pixel is different, the amount of paraffin that each pixel “sucks in” is different. This differentiation across the flat membrane makes the entire piece of material begin to twist slowly as the paraffin is absorbed. In less than three minutes, the 2D membrane has transformed into a 3D replica of a calla lily.
According to Xie, different light-curable materials can be combined in different ways, as certain materials can be used to form different shapes. The professor also believes that the 4D printing process could replace existing 3D printing technology in certain areas. “Its biggest advantage is its speed,” Xie said. “The whole process of preparing multi-dimensional materials is about one-tenth the time needed for 3D printing the same materials.”
According to the Chinese engineers, researchers can easily turn 3D printed objects into four-dimensional ones that change shape over time by choosing the right materials and processes, employing technology that is also known as rapid multi-dimensional manufacturing. Xie believes that the group’s research will remind people that layer-by-layer printing is not the only way to obtain multi-dimensional structures.
While the 4D printing of calla lilies is visually impressive, the process could have much more practical applications too: Xie and the researchers believe that the 4D printing process could be used to cure eye problems such as myopia and hyperopia, with a technique similar to the flower-making one used to alter the curvature of the eye, returning it to a normal state. This sort of application of 4D printing would, however, not be ready to use until some time in the future.
DLP 4D printing could also be used to create jewelry. For example, the 4D printed calla lily models could be used as molds, with liquid metal then poured into the molds to create cast pieces. Once the liquid metal has cooled, the jeweler would be left with a metal calla lily. Xie claims that the ultra-fast multi-dimensional manufacturing method could be used to improve the lost wax process used by jewelers, greatly increasing the efficiency of jewelry production.