Recent advancements in 3D printing hold incredible progress in revolutionizing the technology and medical fields. Yet scientists at the Wyss Institute for Biologically Inspired Engineering and the John A. Paulson School of Engineering and Applied Sciences at Harvard have brought their 3D printed model to the fourth dimension: time.
4D printed objects contain an chronological element to them; that is, they respond and change in environmental stimuli by altering their shape and reconfiguring their structure over time. The researchers were inspired by the dynamic morphologies of plants. Their tissues and microstructures allow them to change shape and alter behavior depending on the external conditions, such as how a sunflower bends toward the light or how a vine climbs up structures for support.
“We set out to take a page from nature and think about the complex shape transformations that one sees in natural architectures like flowers,” Jennifer Lewis, lead author on the study, said.
The scientists began by extracting small fibers from wood called cellulose fibrils, which resemble the microstructures that give plants their unique transformative properties. The cellulose fibrils are anisotropic, which means that the properties behave differently along depending on the direction, like how wood fibers are arranged longitudinally along the “grain,” and are more susceptible to split along this grain.
The fibrils are combined with a simple acrylamide hydrogel, a jelly-like substance that expands when submerged in water, to create the “ink” for printing. The hydrogel composite is printed out on a surface according to a proprietary mathematical model developed by the team to determine how a 4D object must be printed to achieve the desired transformable shapes.
“Our mathematical model prescribes the printing pathways required to achieve the desired shape-transforming response,” Elisabetta Matsumoto, a researcher on the project specializing in condensed matter and material physics, said. “We can control the curvature both discretely and continuously using our entirely tunable and programmable method.”
For their demonstration, the scientists printed an orchid-shaped hydrogel structure. It quickly solidifies and is submerged in water where the “petals” of the orchid hydrogel structure begin to curl and bend due to cellulose fibrils. A fluorescent dye is added to give the product a pleasing glow.
“This work represents an elegant advance in programmable materials assembly, made possible by a multidisciplinary approach,” Lewis said. “We have now gone beyond integrating form and function to create transformable architectures.”
Indeed, the project was truly a collective effort involving experts from a range of disciplines in biology, mathematics and chemistry. The materials used in this demonstration can be used interchangeably with other hydrogel materials and anisotropic fillers in place of the cellulose fibrils resulting in different behavior from external stimuli.
In addition, the project opens up a range of new possibilities in others areas of research, like smart textiles, clothing with electronics embedded in them to provide benefits to the wearer that normal clothing cannot, robotics and industrial machinery and tissue engineering for medical patients that could ultimately grow new organs.
“Right now most tissue culture is done in two dimensions, but most applications of these cells are in 3D,” Lewis said about the applicability of 4D printing in tissue engineering. She continued that in the future, a replacement for damaged tissue could be printed as a flat sheet and then turn itself into the required shape.
Commenting on the project’s pivotal breakthrough, Wyss core faculty members and co-author of the study L. Mahadevan explained “It is wonderful to be able to design and realize, in an engineered structure, some of nature’s solutions.”