Abstract
We determined inhomogeneity of strains around discontinuities as well as changes in orientation of collagen fibrils under applied load in skin. Second Harmonic Generation (SHG) images of collagen fibrils were obtained at different strain magnitudes. Changes in collagen orientation were analyzed using Fast Fourier Transforms (FFT) while strain inhomogeneity was determined at different distances from hair follicles using Digital Image Correlation (DIC). A parameter, defined as the Collagen Orientation Index (COI), is introduced that accounts for the increasingly ellipsoidal nature of the FFT amplitude images upon loading. We show that the COI demonstrates two distinct mechanical regimes, one at low strains (0%, 2.5%, 5% strain) in which randomly oriented collagen fibrils align in the direction of applied deformation. In the second regime, beginning at 5% strain, collagen fibrils elongate in response to applied deformation. Furthermore, the COI is also found to be linearly correlated with the applied stress indicating that collagen fibrils orient to take the applied load. DIC results indicated that major principal strains were found to increase with increased load at all locations. In contrast, minimum principal strain was dependent on distance from hair follicles. These findings are significant because global and local changes in collagen deformations are expected to be changed by disease, and could affect stem cell populations surrounding hair follicles, including mesenchymal stem cells within the outer root sheath.
Highlights
The biomechanics of skin is complex, typically demonstrating three deformation regimes in response to stress (Figure 1)
Second Harmonic Generation (SHG) images of skin taken at low strain (0%, 2.5% strain) indicate that the collagen fibrils appear to be coiled and wavy
At 5% and 10% applied strain, the collagen fibrils appear more organized along the axis in which the strain is applied and some of the waviness disappears
Summary
The biomechanics of skin is complex, typically demonstrating three deformation regimes in response to stress (Figure 1). While the collagen fibrils are in this random pattern, it orients readily to the applied load, demonstrating low elastic modulus [1,2]. As the skin is stretched further, the collagen fibrils straighten and realign parallel to one another, requiring more load to induce further elongation (Figure 1b) [1,2,3]. This process can continue until the fibrils are mostly aligned in the direction of the applied load (Figure 1b)
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