Like a case on a television show called Unsolved Mysteries, North Dakota State University researchers Kalpana S. Katti, Ph.D., and Dinesh R. Katti, Ph.D., are intrigued by a substance whose beauty belies its strength. What intrigues the Kattis is the structure nature painstakingly builds on the inside of abalone shells. The pearly, white layer is often used to make jewelry. Known as nacre (pronounced nay’ ker) to scientists, astute jewelry buyers know the iridescent gleam as mother-of-pearl.
Although recognized for its beauty, scientists around the world have spent decades and millions of dollars to study nacre for other reasons. The Kattis’ research at NDSU in Fargo has unlocked at least one secret, outlined in the article, “Why is nacre so tough and strong?” appearing in the journal Materials Science & Engineering C. Abalone shells are the real estate of choice for the oysters, mussels and other mollusks that live inside them. The organisms probably have no idea that their homes are of potential interest to the Department of Defense and NASA.
“Nature has made this as the best armor material,” explains Kalpana Katti, tapping on the outside of a red abalone shell. “The outside layer is very hard. The inside layer is very tough. That means the outside layer will take impact. The inside layer will absorb energy if the outside layer breaks. That’s exactly how armor works.” The strong, tough structure can be captivating for those who like to solve mysteries. “Strong means it can take a lot of load before it breaks. Tough means it will give a little. This is very unique,” explains Kalpana. “Most engineered composites are one or the other.”
Studying nacre—a complex and densely-layered substance at the nanoscale—involves many disciplines, including chemists, marine biologists, materials scientists and others. Professor Dinesh Katti and Associate Professor Kalpana Katti bring engineering to the mix of people working in biomimetic nanocomposites.
Nacre displays extraordinary mechanical responses. Over the past several years, Dinesh and Kalpana Katti have conducted lab experiments examining nacre’s structure. “Let’s build the structure on the computer and try to see what aspect of the structure makes it so strong and tough,” recalls Kalpana, who is a materials scientist in NDSU’s Department of Civil Engineering. Dinesh Katti, a computational mechanics expert, built the first computer model on this aspect of their research. “We ran simulations on this model. Instead of taking the sample of nacre and doing mechanical tests, we did it in the computer model,” he says.
Other research groups had previously determined that nacre’s strength and flexibility were due to a variety of factors, including an internal brick-like structure held together by a pliable mortar of proteins that are nanometers thick. The miniscule “bricks” are made of calcium carbonate—the same substance found in antacids or chalk or limestone. The orderly bricks-and-mortar configuration is meticulously arranged with organic and inorganic layers. The Kattis’ computational model of nacre showed the organic material contained a soft material of properties with a magnitude much higher than expected.
Over the years, researchers have made incremental discoveries about nacre. But perhaps the beguiling beauty of polished jewelry had resulted in a scientific bias. A summer afternoon in one of the Kattis’ labs yielded something new. Other studies used polished pieces of nacre to study its properties. Kalpana and Dinesh Katti and their research team milled samples of nacre into approximately 1” x 1/8” dogbone-shaped samples with pinholes at each end. They then used machines to pull the nacre samples apart, fracturing them in the process.
“There were four or five of us in the electron microscopy lab that day in 2002,” recalls Kalpana. “Dinesh and I saw it at almost the same time. We jumped out of our chairs because we couldn’t believe that nobody had seen this before. And the reason they hadn’t seen it was because they always looked at a nice, clean, polished cross-section. They never looked at a fracture sample.” By using a “diamond in the rough” rather than a polished sample, the nacre yielded some secrets.
“What you could see was that these platelets are penetrating into each other. They are interlocking. This is a simple concept that nature uses and this was not observed by anybody before,” says Kalpana. Another feature the Kattis observed and reported—the structure of the material is built like hexagonal bricks and mortar. “If they are rotated and penetrated, that’s nacre interlock,” she says. But the Kattis wanted to determine the significance of the discovery. Dinesh’s expertise in computational modeling was crucial. This type of modeling is the same modeling used for standard engineering to design a bridge, a car, an airplane. The modeling revealed that the interlocks discovered are key to the extraordinary mechanical properties exhibited by nacre.
While the nanoarchitecture of nacre represents the science of the extremely small, the computations to model them were at the extreme other end of the scale. To illustrate all the intricate detail contained in nacre’s structure, Dinesh’s models became more and more computationally intensive. NDSU’s Center for High Performance Computing, as well as the National Center for Supercomputing Applications at the University of Illinois in Urbana-Champaign, was used for the computational analysis. The models took significant computing power to create, with that aspect of the work completed in approximately one year.
The Kattis have published more than 20 articles on nacre, most recently in Materials Science and Engineering C and in the May 2005 issue of the Journal of Materials Research. While the dual life of mother-of-pearl encompasses beauty and strength, scientists aren’t interested in making seashells. “We want to use other materials and understand how seashells are made. Just like nature has taken calcium carbonate and made it 3,000 times tougher, we can take other composites and make them thousands of times tougher,” explains Kalpana. “It could make possible lightweight armored aircraft, body armor, artificial body parts, and protective coatings that are strong and flexible.” She points out that their research has shown that nacre’s interlocking bricks, platelet rotation and properties of organics are critical. “If we can play with those, we can engineer materials that are much better than what we have now.”
The NDSU research team remains intrigued by nature’s perfection in creating nacre. “For us to manufacture it at this level of detail, we need fancy equipment in a very controlled environment with a clean room. And nature does this in the ocean, at ambient temperature, pressure, in a dynamic environment.” Things like the rotation of the “bricks” were originally thought of as defects by some scientists. But the Kattis’ research and other studies now show differently. “They are there for a reason. Even the defects are engineered,” says Kalpana.
Through the Kattis’ research at NDSU thus far, the exotic nacre has revealed an important secret—but not yet all of its secrets. Science, like a good mystery, awaits a final ending.