Scientists use defects to create a new programmable material

New research by a team at Purdue University has shown that honeycomb "cellular" materials made of a shape-memory polymer can be manufactured once and then be programmed and adapted for specific use cases that range from shock absorbing football helmets to biomedical implants.

"We are introducing a new class of programmable material whose effective mechanical properties can be modified after fabrication without any additional reprocessing," said Pablo Zavattieri, an associate professor at Purdue University's Lyles School of Civil Engineering, "our idea is that you might mass produce the basic material, and that it has many potential uses because you can change its form and properties later to suit application A, B, C or Z."

Purdue collaborated with General Motors to develop the materials, which are made of geometric "unit cells" of a shape-memory polymer that can be altered using heating or other methods. The research is detailed in two papers appearing in December in the International Journal of Solids and Structures.

 

In new findings, the researchers showed they could create programmable cellular materials by introducing deliberate defects to the unit cells. Two types of the honeycomb programmable materials were studied - one that had hexagonal cells and the other that had cells that were in a kagome pattern.

"In this case defects are a good thing because they provide desirable changes in the material cell structure," said postdoctoral research associate David Restrepo, "this obviously isn't intuitive because usually you try to avoid defects. If you have a hexagon, you want the cells to be perfect hexagons. We wanted to look at it another way. We said, if you deformed the hexagon, this could allow you to tune the properties of your material, so these imperfections are actually a good thing."

The new breakthrough could have an almost limitless number of applications. After all, materials that you can manufacture once but whose properties can be transformed and altered on command changes the game. But while the team have given thought to some of the potential applications, such as using the technique to create noise-absorbing "acoustic metamaterials" that could be tuned after they're made to absorb specific frequencies, or using the materials to create "morphing" stealth surfaces for the military, or better biomedical implants which could be adjusted to mimic the stiffness of bone, or clothing that changes its properties, or materials for the auto industry, for example, they haven't let their heads get away with them. Well, not quite anyway...

"Of course, the feasibility of these types of applications may require additional research," Zavattieri said, "for example, we are not there yet, but say you have a room and you want to shield it from noise. You might put this metamaterial in the walls so that it absorbs certain frequencies. But then say you want to adjust it to cancel out higher frequencies, so you might be able to tune it. It sounds like science fiction, but it's getting within reach."

Material properties depend on the shape of the unit cells and the makeup and thickness of the walls separating each cell. Findings showed that compressing the materials by 5 percent results in a 55 percent increase in stiffness, meaning it might be adapted for a range of applications.

"That is pretty impressive because ordinarily you would have to fabricate a new material with at least twice the thickness of the walls to obtain a material with a 50 percent increase on stiffness," Restrepo said.

The teams findings also suggest the materials continue to function well if they contain common manufacturing flaws, which suggests that it could be practical for industry.

The researchers also used simulations to study the material, which takes high-power computer clusters to model as many as 10,000 cells, each moving in three directions to simulate bending, stretching and compressing.

"The simulations are very valuable because they allow you to do fewer experiments," Zavattieri said, "instead of doing 3,000 experiments, we can do 3,000 simulations, which is much more cost effective."

And, talking of simulations, one day these programmable materials could be 3D printed, embedded with smart electronics and controlled by machine learning or AI to create "living" materials that respond to new use cases, or new environments automatically. And when that happens we'll begin to see the start of yet another industrial and manufacturing revolution.