Dartmouth Researchers Use 3D Printing to Create Super-Strong 4D Material

Rotaxanes sounds to me like the latest health craze, as in “This superfood is packed with rotaxanes that will give you beautiful skin/help you lose weight/allow you to live until age 200 with the look of a 20-year-old.” In reality, they’re even better, at least in my opinion – rotaxanes are dumbbell-shaped molecules that are capable of converting input energy in the form of light, heat, or altered pH into molecular movements – in other words, they move in response to an external stimulus. They’re also known as nanomachines.

Dr. Chenfeng Ke and his lab at Dartmouth College have used rotaxanes to design a super-strong smart material. By 3D printing the nanoscale molecules, they were able to create a macro-scale polymer lattice cube capable of lifting 15 times its own weight – the equivalent of a human lifting a car.

Dr. Chenfeng Ke

“Our design is based on a well-investigated family of molecules called polyrotaxanes,” Dr. Ke explains. “These have multiple rings on a molecular axle. In our new material, the ring is a cyclic sugar and the axle is a polymer. If we provide an external stimulus – like adding water – these rings randomly shuttling back and forth can instead stick to each other and form a tubular array. When that happens, it changes the stiffness of the molecule. It’s like when beads are threaded onto a string; many beads slid together make the string much stronger, like a rod.”

The goal of Dr. Ke and his team was to build a polymer from billions of the molecules bonded together with water. The problem of getting rotaxanes to perform work, he says, is that when the nanomachines are randomly oriented, their ring motions cancel each other out, leaving them useless at the macro scale. 3D printing them, however, allowed the researchers to control their motions.

“It was integrating the 3-D printing technique that allowed us to transform the random shuttling motions of nano-sized rings into smart materials that perform work at macroscopic scale,” Dr. Ke continues. “Getting the molecules all lined up in the right orientation is a way to amplify their motions. When we add water, the rings of the polyrotaxanes stick together via hydrogen bonds. The tubular arrays then stack together in a more ordered manner.”

“It’s much easier to get the molecules coordinated while they’re in this configuration as opposed to when the rings are all freely moving along the axle. We were able to successfully print lattice-like 3-D structures with the rings locked into position in this way. Now the molecules aren’t just randomly positioned within the material.”

Once the structures were printed out, Dr. Ke’s team cured them and set them to work. The hollow lattice structure of the cube they printed made it easier to deform and reform. Using a solvent as a catalyst, the team was able to get the threaded ring structure of the molecules to switch between random shuttling to stationary and back again. In layman’s terms, by adding and removing a solvent, they were able to get the cube to expand, lifting an object along with it, and then return to its original shape.

The 3D printed cube lifted a small coin by 1.6 millimeters, which, as Dr. Ke comments, may sound small, but is a big step forward in getting nanomachines to do work at the macro scale.

“We hope this advance will enable scientists to further develop smart materials and devices,” he concludes. “For example, by adding contraction and twisting to the rising motion, molecular machines could be used as soft robots performing complicated tasks similar to what a human hand can do.”

The experiment was based on the Nobel Prize-winning research performed by Jean-Pierre Sauvage, Sir J. Fraser Stoddart, and Bernard L. Feringa. The Ke Lab’s full study was documented in a paper entitled “Ring Shuttling Controls Macroscopic Motion in a Three-Dimensional Printed Polyrotaxane Monolith,” which you can access here. Additional authors include Qianming Lin and Dr. Xisen Hou. You can see some time-lapse video of the experiment below:

Discuss in the Dartmouth forum at 3DPB.com.