The “4D printing” refers to 3D-printed objects that “morph” in shape over time and temperature changes. The 3D objects are printed using a temperature-responsive hydrogel – a highly absorbent natural or synthetic polymeric network that contains mostly water.
Despite being comprised of up to 90% water, such hydrogels can remain solid and retain their shape under various conditions. Examples include Jell-O, contact lenses, diapers, and the human body.
The smart gel created by the researchers could, they say, provide structural rigidity in organs such as the lungs, and can contain small molecules like water or drugs that could be transported in the body and released. Further, the researchers say, it could create a new area of soft robotics, and enable new applications in flexible sensors and actuators, biomedical devices, and platforms or scaffolds for cells to grow.
“The full potential of this smart hydrogel has not been unleashed until now,” says Howon Lee, senior author of a new study and assistant professor in the Department of Mechanical and Aerospace Engineering at Rutgers. “We added another dimension to it, and this is the first time anybody has done it on this scale. They’re flexible, shape-morphing materials. I like to call them smart materials.”
The hydrogel used by the researchers in their work contains 73% water and has traditionally been manufactured using conventional, two-dimensional methods such as molding and lithography and used in devices that generate motion, and biomedical applications such as scaffolds for cells to grow on. To make their smart hydrogel, the researchers used a lithography-based technique that can quickly and inexpensively print a wide range of materials into a 3D shape.
It involves printing layers of a special resin to build a 3D object. The resin consists of the hydrogel, a chemical that acts as a binder, another chemical that facilitates bonding when light hits it, and a dye that controls light penetration.
The researchers say they learned how to precisely control hydrogel growth and shrinkage. In temperatures below 32°C (about 90°F), the hydrogel absorbs more water and swells in size. When temperatures exceed 32°C, the hydrogel begins to expel water, and shrinks.
According to the researchers, they can create objects with the hydrogel ranging from the width of a human hair to several millimeters long. They also found that they can grow one area of a 3D-printed object – creating and programming motion – by changing temperatures.
“If you have full control of the shape, then you can program its function,” says Lee. “I think that’s the power of 3D printing of shape-shifting material. You can apply this principle almost everywhere.”
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