Current research on biological tissues has shown that biological tissues exhibit viscoelastic behavior at physiological loading rates. Therefore characterization of the stress-relaxation and creep behavior of biological tissues is important for the development of accurate computational models and finite element analysis of tissue behavior. It has been found that the relation between loading rate and tissue stiffness, stress relaxation, and creep behavior varies among the various tissues in the body. As more tissues are characterized, researchers are finding that these variances in viscoelastic properties tend to be well suited for the biological function of individual materials. This has important implications for the development of tissue replacements, artificial valves, spinal disc implants, and engineered tissues, as these tissues must closely match the viscoelastic properties of native tissues to ensure proper function and long-term physiological compatibility. Since it is most important to characterize the behavior of biological tissues at physiological loads and loading rates, stress relaxation and creep tests of biomaterials are typically conducted at relatively low loads and high loading rates. Creep tests are conducted by rapidly applying the full desired load and holding the specimen at that load for an extended amount of time, observing tissue deformation. Stress relaxation tests are conducted loading the tissue until a desired strain is achieved, then holding the tissue specimen at constant strain while observing changes in load required to maintain the strain value. Since biological tissues often exhibit anisotropic behavior, stress-relaxation and creep tests are commonly conducted using a planar biaxial test machine.

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