Assistant Professor Shan Zhou holds a model of a triangular pyramid, the shape of the nanoparticles in his research samples, at South Dakota Mines on Feb. 8, 2023. (Nicole Schlabach/For South Dakota Searchlight)
Imagine a piece of shrapnel denting a service member’s combat helmet. What if the dent in the helmet could heal itself?
Shan Zhou’s research might make that a reality. He’s an assistant professor in the Department of Nanoscience and Biomedical Engineering at South Dakota Mines in Rapid City.
Zhou hopes to create a gold material that heals itself from dents and possibly from bends in its structure. He thinks he’s found a way to create the material by working at the nanoscale — a scale where one nanometer is equal to the thickness of a strand of hair divided 1,000 times.
His group is testing a nanostructure, a structure built from particles, to verify that it heals itself at the nanoscale. If these tests are successful, he would scale this structure into a material that heals itself on a larger, more useful scale. So far, the results look promising, and the self-healing material could be realized “in the near future,” he said.
Zhou is working in the Materials-Interfaces Imaging and Design Laboratory with one graduate student, and two more students will join his project this fall.
How the material might work
The group aims to build the material using gold nanoparticles — particles so small that billions of them would fit in one square inch of the material.
These particles, shaped like triangular pyramids, are like “building blocks,” Zhou said. “Instead of building or designing houses, what we do is work on things at a nanoscale.”
The group organizes these “building blocks” into a pinwheel structure. This structure, Zhou said, may hold the key to creating the self-healing material because it will naturally and “easily go back to its original structure without energy consumption” after a structural change. This process, called reconfiguration, happens instantly.
The reconfigurable structure had been discovered before Zhou’s research at South Dakota Mines began. It was first designed and built by Zhou, along with other scientists from the University of Michigan and Argonne National Laboratory, while Zhou was a postdoctoral scholar at the University of Illinois.
Reconfigurability was one of several characteristics that made the creation of this pinwheel structure desirable to the researchers. The research was funded by a Multidisciplinary University Research Initiative grant through the Office of Naval Research and published in the journal Nature last year.
Even though the structure reconfigures itself, Zhou wouldn’t claim it can heal itself — self-healing needs to happen in response to real-world types of damage. The group is working to verify that the structure can heal itself by applying nano-sized forces that mimic real-world damage.
To do this, they use a technique called atomic force microscopy that Zhou compares to “hands” at the nanoscale. A tiny cantilever applies a force to the structure while recording the forms and features of the surface, which helps the group understand how the structure responds to the damage.
So far, the structure reconfigures itself after nano-sized dents. But more tests are needed to thoroughly demonstrate these results. If the group can verify self-healing results, they hope to scale up to a material on at least a square centimeter scale.
The self-healing material would serve as a protective layer over other surfaces. Ideally, Zhou said, it would absorb shock to minimize damage to the underlying item. In the group’s tests, the nanostructure has been applied to glass, silicon and a silicon-based polymer known as PDMS.
Zhou’s material would be the first of its kind. Other self-healing materials made of metal have been developed, like a self-healing medical sensor partially made of liquid metal, that responds to body temperature. But these other materials do not use the movements of nanoparticles to heal themselves. Nanoparticle materials are useful because they allow engineers to precisely fine-tune properties like the material’s durability.
While it’s too early to confirm applications, Zhou said the material might protect combat helmets and airplanes.
“If your helmet is broken, if it can come back to the original structure without energy input, that’s essentially the transformation,” he said. The ultimate goal, he noted, “is to do something like the sci-fi movies.” One popular example is the movie “Terminator 2” in which the character T-1000’s liquid-metal body heals itself from bullets and other damage.
Potential defense applications
Self-healing material could be “revolutionary” for defense applications, said retired Army Brig. Gen. Marshall Michels, assistant deputy secretary of the State of South Dakota Department of the Military, South Dakota National Guard, who was not speaking on behalf of either entity.
Michels wasn’t familiar with Zhou’s research but said self-healing material might be useful for body armor and transportation equipment, including aircraft and ground vehicles.
“It would hopefully protect our service members and provide another layer of protection so they wouldn’t become a casualty or injured, and they’re able to continue on with their mission,” he said.
The ideal material would be lightweight and adaptable to different climates. Hopefully, he said, it would shield service members from sand, wind, snow and sun. “It’s got to be pretty rugged,” he said.
It would need to protect service members from bullet rounds and shrapnel, including fragments from artillery and improvised explosive devices.
He’s also curious how self-healing material might prove useful for non-defense applications like protecting objects from hailstorms.
“Imagine that on your vehicle,” he said. “How many times have you been hailed on in South Dakota?”
Production for industry use
“The sky is the limit” for the potential applications of self-healing materials, said Marek Urban, J.E. Sirrine Foundation Endowed Chair and professor of materials science and engineering at Clemson University. Urban has researched self-healing polymers, the most common type of self-healing material, for over 15 years.
“If you find a niche application that you cannot achieve otherwise, it’s a winner,” he said, while noting that he wasn’t familiar enough with Zhou’s research to comment on it.
Urban’s research group focuses on developing self-healing commodity materials, he said, with cost savings and profit acting as the main motivators for industry partners.
A material might be of interest “if you can save money on production and make extra money, let’s say by offering self-healable paint,” where people are willing to pay more, he said.
Self-healable commodity materials need to be precisely designed, he said, “but it’s a challenge to make large enough quantities” of those materials for production.
He declined to discuss those challenges but said, “The bottom line is, ‘How do you get large quantities, but very precisely designed materials?’ I think this is where the whole field is going right now.”
Triangular pyramid nanoparticles are challenging to create, Zhou said, because it’s difficult to control the structure and the purity of the particles. But, he’s developed a way to create pure-gold triangular pyramids in uniform sizes, which may work well for large-scale production.
After the particles are created, he anticipates a simple fabrication process. “We take a liquid droplet, put it on a substrate, let it dry — that’s it,” Zhou said. During fabrication, custom thickness levels would be achieved by layering the material on the protected object.
Gold nanoparticles were chosen for the material because gold is mechanically stable, durable, and its surface chemistry facilitates the creation of a nanostructure that can reconfigure itself, he said.
Since the material is made of gold, it would last longer than most other self-healing materials, which are made of polymers, he said. The cost would depend on the scale and density, but if it’s a thin layer on a centimeter scale, “it’s going to be a pretty reasonable price.”
For now, the group is focused on verifying that the nanostructure can heal itself. If the tests are successful and the group can scale the structure into a larger material, Zhou might seek industry partnerships. Or, he said, his research might lead to a startup company in South Dakota.
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