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Roberto Ballarini holds a tiny MEMS device. He uses such devices to study how components of bone perform at every scale.
From bridges to bones
A civil engineer by training, Roberto Ballarini studies how human bones work--and fail
By Cass Erickson and Deane Morrison
July 17, 2007
After 9-year-old Roberto Ballarini moved with his family from Italy to Brooklyn, he became fascinated by the Empire State Building, the World Trade Center, and, especially, the Verrazano-Narrows Bridge between Staten Island and Brooklyn. Little wonder he became a civil engineer. But instead of working with steel, he has chosen an even more complex structure: bone. Ballarini came to the University last year after 20 years at Case Western Reserve. This June, he was appointed head of the Department of Civil Engineering. In one research project, he is applying his knowledge of structural engineering and solid mechanics to understanding the mechanics of bone. The goal is to better predict the risk of fracture and to design synthetic materials to temporarily replace injured bones. He took the first step down this path by studying another of nature's engineering marvels. "The conch shell is what got me started in this business," he explains. While at Case Western Reserve, Ballarini and materials scientist Arthur Heurer found that conch shells are like buildings with a wealth of architecture. These beautiful shells, despite being 99 percent mineral (aragonite) and brittle, are made of the toughest ceramic material known. When comparing the structures of conch shells and bones, Ballarini found that nature uses design to prevent catastrophic failure in seashells, whereas bone is very sensitive to cracks and uses healing to stave off disaster. "You have cracks in your [bones] continuously, but they're usually very small and can be reabsorbed into the body," he says. "The conch shell can tolerate the presence of large cracks, but in order for bone to survive, it constantly heals itself and gets rid of all the little micro-cracks. If these cracks get too large, then you have catastrophic failure." The conch shell research prompted Ballarini to develop microelectromechanical systems (MEMS) devices to measure the mechanical properties of extremely minute structures. He and Case Western Reserve colleague Steve Eppell have targeted the protein known as collagen, which holds together skin, bones, cartilage, tendons, and other connective tissues.
"We're trying to understand how cracks behave in the [bone] structure, how they stop, how they propagate the failure, and what techniques we could develop to reduce the risk of fracture," says Ballarini.On the microscopic scale, collagen exists in strands called fibrils. Ballarini and Eppell have measured stiffness, strength, and fatigue in a single collagen fibril. The two researchers are also studying bone at every other level, from the millimeter scale--where bone can be seen to contain numerous tiny, bony tubes enclosing blood vessels--to complete bones. "We're trying to understand how cracks behave in the structure, how they stop, how they propagate the failure, and what techniques we could develop to reduce the risk of fracture," says Ballarini. By understanding how the mechanical properties of collagen relate to the overall properties of bones, he and Eppell hope to develop better ways to assess a person's risk of bone fracture, as occurs in osteoporosis. Applying the tools of mechanical theories and mathematical techniques to bone, Ballarini hopes to create procedures for measuring risk of fracture. Someday, drugs that lower the risk may be developed. "If it's determined that stiffness is indeed a good indicator of fracture risk, then diagnostic tools or drug treatments that ensure acceptable levels of stiffness should be developed," he says. "If other characteristics prove to be more relevant--such as the level of mineralization, which is highly probable--then the focus should be shifted to developing diagnoses and drug treatments for that." Ultimately, Ballarini and Eppell would like to make synthetic bone that could replace damaged or broken bones while the real ones heal and regrow. The synthetic bone would then be reabsorbed by the body. To be compatible with the human body, it must be just right--neither too rigid nor too soft. With colleagues in the departments of biosystems engineering civil engineering, Ballarini is exploring the processing of synthetic mother of pearl on the smallest possible scale. He looks forward to working with more faculty from the University's cornucopia of engineers and scientists. As he puts it, "I feel like a kid in a candy store here." A version of this story appeared in Civil Engineer, the civil engineering department magazine.