Printing in three dimensions is so 2015. How about adding a fourth dimension — time?
That's what researchers at Harvard University have done in creating a unique polymer-based flower structure that soaks up water like a sponge and changes its shape just like a real orchid.
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While the folding fake flower is, at the moment, a cool lab experiment, the concepts explored in the study could be used to develop new kinds of smart clothing that can repel water and breathe at the same time, or perhaps a new kind of flexible scaffolding for a printed human organ.
"We were inspired by the complex shape changes that you see that plants display," said Jennifer Lewis, professor at Harvard University's Wyss Institute for Biologically Inspired Engineering, and an author on the report published this week in Nature Materials. "As we began to look into the origin of the shape change, we realize it was a simple process."
Lewis and her colleagues have already created 3-D self-folding origami structures. They have also assembled 3-D tissue with blood vessels. But this project adds another level of complexity, even if the inspiration seemed easy.
The Harvard team came up with hydrogel composites -- think jello -- that contain cellulose fibrils derived from wood and are similar to the tiny structures that allow plants to change their shape.
"They respond to water. As they hydrate, they begin to swell," Lewis said. The 3-D printer was programmed to give some parts of the structure more stiffness, and other parts flexibility. The difference in flexibility makes the entire flower curl up like a real one when dropped in water.
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"We can locally encode changes in swelling behavior which give rise to a lot of complexity to have bending, rustling and twisting," said Lewis, who conducted the research along with co-lead authors A. Sydney Gladman, a graduate research assistant and Elisabetta Matsumoto, a postdoctoral fellow, both at Wyss.
The secret to the folding orchid is the design of the special ink that contains the hydrogels. The composite ink that the team uses flows like liquid through the printer head, yet solidifies once printed.
"We can swap one hydrogel for another, or swap cellulose fibrils for carbon nanotubes or metal nanorods," Lewis explained. "It opens the ability to create scaffolds for tissue engineering or for tissue repair." The video below shows how it work.
The scientists are also considering the idea of printing these flexible structures onto a textile or cloth and having it spontaneously change shape. "We could use the concept to go from a flat fabric to something that might resemble the upper of a shoe," Lewis said. The biggest hurdles remain the durability and washability of such a textile.
Lewis, who already holds eight patents on various 3-D printing projects, says this experiment is just the beginning.
Skylar Tibbit, research scientist in the school of architecture at the Massachusetts Institute of Technology and a pioneer in 4-D printing techniques, says the main contribution of Lewis' group is the control of materials in producing the polymer flower.
"They have much more control and alignment," Tibbit said. "Their modeling tools are much more advanced. Their results are fairly complex, and there are lots of movements in different directions."
Tibbit says there are several research groups around the world who are pursuing various 4-D printing techniques to develop both medical devices and new kinds of micro-electronics.
