Creating a speckle pattern on biological soft tissue, which would be suitable for digital image correlation measurements, is challenging. Speckle patterns should neither cause or require sample dehydration, nor alter the mechanical response, but they should adhere to the tissue surface and withstand large deformations. A two-step procedure has been implemented to create a highly-contrasted pattern. It requires staining of the tissue with methylene blue solution to obtain a dark background and airbrushing the surface with paint to create white speckles. This study evaluated the effectiveness of the proposed procedure and whether the pattern creation had any effect on the elastic response of soft tissue. Forty porcine collateral ligaments underwent three series of cyclic tensile tests to a nominal elongation of 10% for 30 cycles. The specimen stiffness was calculated from the load-elongation curve collected during the last 10 cycles. One side of 20 ligaments was blue stained between the first and second test series, and white patterned between the second and third test series. During the last series, ligament surface images were also acquired and elaborated using the digital image correlation technique. The other 20 ligaments were untreated. The data show a small non-significant upward trend in stiffness in treated as well as in untreated ligaments (maximum increase of 1.7%). The 'successfully-correlated area' of the stereo-visible ligament surface was on average 96%, i.e. small parts of the 'stereo-visible area' were lost during computation. The described procedure is an effective method to create a pattern on biological soft tissues.
Keywords: Biological tissues; Digital image correlation; Ligaments; Speckle pattern.
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