The Mechanics
of Biomedicine


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Image courtesy of JRI Orthopaedics.

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Americans are living longer and expecting to stay mobile and active at 70, 80, and even 90. Mechanical engineers are crucial in developing implants for all the artificial hips, wrists, shoulders, ankles, and knees that keep people moving.

Engineers constantly refine joint replacement designs. For a quarter-century, orthopedic implants used metal, stiff plastic, or other traditional engineering materials with fairly linear behavior, observes Lawrence Bonassar, Associate Professor of Biomedical Engineering at Cornell University’s Sibley School of Mechanical and Aerospace Engineering, Ithaca, NY. “In newer applications, like soft-tissue replacements for tendons or heart valves, mechanical behavior is nonlinear, and more complex.”

Today, biomechanics requires “multiscale understanding,” says Bonassar, who teaches graduate-level tissue engineering. “The beauty of biologic tissue is hierarchical structure: cells embedded in tissue, tissue making up organs. Understanding the mechanics of those structures forming and interconnecting drives a lot of medical applications.” Biomechanical advances use both biological and biochemical skills, to reveal mechanics and structural components.

Image courtesy of JRI Orthopaedics.

“We train students to connect the properties to the structure, using analytic tools to describe these complex structures’ complex functions,” Bonassar notes. An implant must mimic the mechanical performance of tissue it replaces. “To design an implant for a heart valve or tendon, you must learn hyperelasticity. Getting your hands on the living tissue you’re characterizing allows an intuitive feel for how it behaves.”

Newer Engineering Areas

Increasingly sophisticated, tiny sensors provide “an incredible learning opportunity–from inside and outside the body–about musculoskeletal performance,” reports Tim Wright, F.M. Kirby Chair of Orthopedic Biomechanics at the Hospital for Special Surgery in New York.

A sensor inside a knee implant, for example, allows measurement of load, from the patient, during regular activities. “Analysis of data from these sensors will let us answer engineering questions about the body’s tissues. By using solid multiphysics and computational models, mechanical engineers are creating powerful tools to ultimately allow patient-specific models of joints like the knee with immediate clinical application that could alter treatment.”

New devices can keep human cells and tissues alive externally. “Biologics presents a whole separate opportunity for mechanical engineers,” says Bonassar, for “mechanical characterizations of these tissues, or in design and fabrication to sustain, for example, a piece of liver outside the body. Understanding interactions between solid and fluid mechanics is essential for designing a mechanism that can do this.”
 
Exploring how the body’s tissues adapt to load links mechanics to biology, in “true interdisciplinary research,” says Wright. “If you don’t load bone, the body resorbs it. Engineers collaborate with biologists to study cellular, and even genetic, levels of how the body has this magical power.” Knowing these mechanisms may lead to treatment and prevention for many orthopedic problems.

Tissue engineering issues include joints in an aging, obese population. “What load can they withstand? How can we use the emerging data for product design? The mechanical burden on a tissue-engineered product is as important as the biology,” Wright explains. “Not knowing the mechanical specifications limits designers.” 
 
Job Opportunities

Bonassar’s graduate students are often hired by the expanding medical device sector. Responsibilities range from redesigning an implant to checking for FDA compliance to monitoring a device’s testing. “For an electronic device, tests are done in-house. But a medical device must eventually go into a living thing, so testing is in collaboration with a surgeon at a medical school or hospital. Managing those connections is a huge part of what these designers do,” Bonassar notes. Communication skills are crucial. Engineers involved in device design or testing must speak a physician’s language.

For his Orthopedic Biomechanics lab, Wright seeks “the best mechanical engineers I can find. I can choose excellent engineers because, on the job, they’re exposed to biology, and rapidly learn anatomy and orthopedics. They have to work on a team, but not as biologists.”

Wright sums it up: “To contribute to the biomechanics of biomedicine, you really need to know the mechanics first.”

Carol Milano is an independent writer.

To contribute to the biomechanics of biomedicine, you really need to know the mechanics first.

Tim Wright, F.M. Kirby Chair of Orthopedic Biomechanics, Hospital for Special Surgery

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May 2011

by Carol Milano, ASME.org