Jan 25, 2016 | By Andre
4D Printing is a term that pops up every so often for a good reason. It is a techology that refers to producing 3D printed objects and using the passing of time to convert the item into another shape without additional human interference or assembly. As an example, imagine you were to throw a shoelace in some water and instead of simply seeing a wet shoelace sink to the bottom, you instead saw one that automatically tied itself into a knot.
While not quite at that level yet, scientists at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) along with the Wyss Institute for Biologically inspired Engineering have been hard at work developing 4D printing technology on the microscale.
Inspired by plants that change form over time upon exposure to external stimuli, the team used a special hydrogel composite formula to produce predictable shapes once submerged in water. Jennifer A. Lewis, senior author on the new study suggests that “this work represents an elegant advance in programmable materials assembly, made possible by a multidisciplinary approach. We have now gone beyond integrating form and function to create transformable architectures.”
The study’s co-lead authors are A. Sydney Gladman, a graduate research assistant advised by Lewis, specializing in the printing of polymers and composites; and Elisabetta Matsumoto, a postdoctoral fellow advised by Mahadevan, specializing in condensed matter and material physics.
For an excellent demonstration of the 4D printing technology in progress, I recommend taking a quick look at the video below.
In this case, the hydrogel composites being used by the team at Harvard contain cellulose fibrils taken from wood. The careful alignment and placement of the extracted hydrogel compsite ink using 3D printing means that, thanks to a proprietary materials prediction model, the order in which the output material swells and stiffens over time can be “pre-programmed” into the design.
Without question, this ability to precisely predict how something will react based on exposure to external stimuli (including changes to humidity and temperature) has great implications on smart textiles, soft electronics, biomedical devices and tissue engineering.
To further demonstrate the technology, they produced two flower shapes that look that same from the onset, but morphed into completely differently once exposed to water. The below approximation of an orchid shifting into form (with a fluorescent dye in the gel) is a great example the 4D process.
Just like the 3D printers many of us are used to seeing, the composite ink used by the team extrudes out of the printhead, one layer at a time, rapidly solidifying upon release. Furthermore, and also similar to traditional 3D printing, it is the software side of things where all the complexities occur. Matsumoto states that “our mathematical model prescribes the printing pathways required to achieve the desired shape-transforming response. We can control the curvature both discretely and continuously using our entirely tuneable and programmable method.”
Specifically, the mathematical modeling solves the “inverse problem”, which is the challenge of being able to predict what the printing toolpath must be in order to encode swelling behaviors toward achieving a specific desired target shape.
“It is wonderful to be able to design and realize, in an engineered structure, some of nature’s solutions,” said Mahadevan, who has studied phenomena such as how botanical tendrils coil, how flowers bloom, and how pine cones open and close. “By solving the inverse problem, we are now able to reverse-engineer the problem and determine how to vary local inhomogeneity, i.e. the spacing between the printed ink filaments, and the anisotropy, i.e. the direction of these filaments, to control the spatiotemporal response of these shapeshifting sheets. ”
In addition to the material demonstrated in the above video, the mathematical model behind determining how materials respond to stimuli can be applied to other materials. Gladman says that “we can interchange different materials to tune for properties such as conductivity or biocompatibility.”
To me, one of the most interesting practical applications of the technology has to do with issue engineer that may one day help with the process of growing new organs. Jennifer Lewis hints that “right now most tissue culture is done in two dimensions, but most applications of these cells are in 3D.” The hope is that in the future, damaged cells and tissue can be printed flat, and then automatically transform itself into the desired shape. I suspect we'll see many similar developments in 4D printing in the future.
The research is partially being funded by the Army Research Office (ARO) and the National Science Foundations’ Material Research Science and Engineering Center (MRSEC).
Posted in 3D Printing Technology
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