Researchers from Rice University in Houston, Texas, have developed a new method for enabling increased control over shape-shifting materials in 4D printing, thereby making the process more practical.
The new method allows 3D printed objects to be manipulated to take on alternate forms when exposed to changes in temperature, electric current or stress. Researchers aboard the study suggest that soft robots and biomedical implants that reconfigure themselves upon demand are closer to reality with the new technique, which they term as reactive 4D printing.
Decoupling the 3D printing process from shape-programming
4D printing is a method of 3D printing whereby the fabricated objects undergo a shape-shifting process over time using materials that are able to morph.
The Rice researchers, Rafael Verduzco and graduate student Morgan Barnes of Rice’s Brown School of Engineering, first reported their research into morphing materials in 2018. Using this formulation for 3D printing, they found that structures were limited to shapes that sat in the same plane. This means that no bumps or other complex curvatures could be programmed as the alternate shape.
Verduzco explains that, overcoming that limitation to decouple the 3D printing process from shaping is a significant step toward more useful materials: “These materials, once fabricated, will change shape autonomously,” Verduzco states. “We needed a method to control and define this shape change. Our simple idea was to use multiple reactions in sequence to print the material and then dictate how it would change shape. Rather than trying to do this all in one step, our approach gives more flexibility in controlling the initial and final shapes and also allows us to print complex structures.”
With this idea in mind, the lab faced the challenge of creating a liquid crystal polymer ink that incorporates mutually exclusive sets of chemical links between molecules. One set establishes the original printed shape, whereas the other can be set by physically manipulating the 3D printed-and-dried material. When curing the alternate form under an ultraviolet light, the links are locked in.
With the two programmed forms set, the material is able to morph back and forth when triggered via temperature, for instance, when it’s heated or cooled. The researchers had to find a polymer mix that could be 3D printed in a catalyst bath while still holding its original programmed shape.
“There were a lot of parameters we had to optimize — from the solvents and catalyst used, to degree of swelling, and ink formula — to allow the ink to solidify rapidly enough to print while not inhibiting the desired final shape actuation,” Barnes added.
A remaining limitation of the process concerns the ability to 3D print unsupported structures, like columns. This would require a solution that gels just enough to support itself during printing. Such an ability would allow researchers to 3D print far more complex combinations of shapes. “Future work will further optimize the printing formula and use scaffold-assisted printing techniques to create actuators that transition between two different complex shapes,” Barnes stated. “This opens the door to printing soft robotics that could swim like a jellyfish, jump like a cricket or transport liquids like the heart.”
Advances in 4D printing
A novel technique, 4D printing has been the subject of numerous studies recently as scientists seek to improve and understand the unique process in order to develop its potential applications. Recently, we covered a study coming out of China where researchers presented a study on a modular method of 4D printing that aims to overcome the geometrical limitations of the process.
Additionally, earlier in 2020 a collaborative research project between Singapore University of Technology and Design (SUTD) and Nanyang Technological University (NTU) resulted in a 3D printed material changing its shape and back again numerous times without any electrical input.
Harvard University researchers also produced a study in late 2019 that sought to overcome the challenges to 4D printing, mainly the ability to create complex, smoothly-curved shapes. They were able to create a frequency-shifting antenna, and a flat lattice that, when placed in salt water, takes on the shape of a human face.
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Featured image shows a graphic shows the process by which a Rice University lab uses 3D printing to make shapeshifting materials. Photo via Verduzco Laboratory.