Non-contact strain measurement of biological tissue.

Strain measurement in biological material is frequently problematic due to material in-homogeneity, high strain to failure, and the relatively low stiffness of most biological tissue. As an alternative to conventional strain measurement techniques, this study investigates the potential of a non-contact optical system capable of measuring three dimensional surface strain maps. The technique makes use of two CCD cameras and pattern recognition algorithms to track the motion of a random speckle pattern on the specimen and compute the displacement and strain fields. The application of this system on biological material is demonstrated on two tissue types that differ widely in material and surface characteristics (cortical bone and ligamentous tissue). A human tibia was loaded in quasi-static three-point bending and the medial collateral ligament from a human knee joint was loaded in tension. The strain field was computed and analyzed in each case. On the relatively stiff cortical bone, accurate results were obtained wherever high resolution imaging of the speckle pattern was possible. Since wicking and seepage from blood vessels during the test affect the speckle pattern, countermeasures are proposed. On the other hand, stretching of ligaments produced large surface strain discontinuities from tissue unfolding, which caused automated pattern recognition algorithms to fail.