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Prof. Ellen M. Arruda

Maria Comninou Collegiate (USA)

TitleFull-field methods for characterizing the non-linear anisotropic response of soft tissues of the body

Prof. Ellen M. Arruda
Professor Ellen M Arruda is the Tim Manganello / BorgWarner Department Chair and Maria Comninou Collegiate Professor of Mechanical Engineering at the University of Michigan. She also holds appointments in Biomedical Engineering and in Macromolecular Science and Engineering. She received her BS and MS degrees from Penn State and her PhD from MIT. She joined the UM faculty in 1992.

Professor Arruda teaches and conducts research in the areas of theoretical and experimental mechanics of macromolecular materials, including polymers, elastomers, composites, soft tissues and proteins. Her research programs include experimental characterization and analytical and computational modeling of soft materials, including native and engineered tissues. Her polymer mechanics work has focused on the mechanics of these highly strain rate and temperature dependent materials with emphasis on the relationships among the structures at various length scales to the deformation mechanisms of those structures to predict the mechanical responses. More recently she has pioneered efforts to characterize the complex mechanical responses of soft tissues such as ligaments and tendons via full-volumetric-field methods. Professor Arruda has over 100 papers in scientific journals. Her H-index is 46 (GS).

Recent Honors and Awards–

• 2022 Member, College of Fellows, American Institute for Medical and biological Engineering
• 2021 A.C. Eringen Medal, Society of Engineering Science
• 2019 Nadai Medal, ASME
• 2018 James R. Rice Medal, Society of Engineering Science
• Member, National Academy of Engineering, Class of 2017
Soft tissues of the body such as ligaments and tendons tend to have irregular shapes and attachment sites to bones and muscles (entheses and myotendinous junctions). As a result, they deform heterogeneously throughout their volumes, and accurate computational modelling of their mechanical response requires the ability to access the full-volume, finite deformation maps under applied loads. Moreover, these full-volume deformation fields enable characterization of the non-linear, anisotropic response with a limited number of experiments, particularly when all components of the resulting strain tensor are non-zero and finite.
We obtain full-field displacement data directly from the phase signal of the magnetic resonance imaging of soft materials such as the anterior cruciate ligament (ACL) of the knee and the supraspinadus tendon (rotator cuff) of the shoulder using a custom built in-situ loading apparatus. The in-situ MRI approach to understanding the mechanical properties of soft tissue is a finite deformation, full-volume technique. No contrast agents or other fiducial markers that could interfere with the mechanics of the tissue are needed for this method.
We use these data with the applied traction boundary conditions and inverse computational methods to characterize the mechanical properties of the tissue. Two methods have been explored to do this, the virtual fields method in which the form of the constitutive model is chosen a priori, and the variational system identification method in which the form of the model is learned through an iterative process. Once the tissue is characterized we develop a finite element (FE) model of it to simulate the experiment and compare the predictive capability of our approach.
The heterogeneous displacement fields throughout the interior of the ACL and rotator cuff demonstrate that uniform axial loading assumptions cannot be used in the characterization of these soft tissues. Full-field methods are needed and greatly enhance our ability to characterize the mechanics of soft tissues, a critical step on the path to computational models of joints such as the knee and shoulder needed to predict injurious events.