Fiber Rearrangement and Matrix Compression in Soft Tissues: Multiscale Hypoelasticity and Application to Tendon

Area of Science:

• Biomechanics
• Materials Science
• Soft Tissue Engineering

Background:

• Nonlinear mechanical behavior in soft tissues is primarily attributed to fiber straightening and re-orientation.
• Understanding these microstructural changes is crucial for predicting tissue response to mechanical loads.

Purpose of the Study:

• To quantitatively assess fiber straightening and re-orientation in soft tissues using a continuum micromechanics approach.
• To develop a model that links microstructural features to macroscopic mechanical properties.

Main Methods:

• A continuum micromechanics approach was employed, representing crimped fibers as hypoelastic straight fiber phases within a hypoelastic matrix.
• A representative volume element (RVE) was subjected to macroscopic strain rates, with strain and spin downscaled to fiber and matrix levels.
• The model was applied to tendinous tissue, considering fascicles as parallel fibers within a matrix at different length scales.

Main Results:

• The model quantitatively captures fiber decrimping under non-affine conditions.
• Initial fiber crimping angle was identified as the primary driver of straightening degree and macroscopic stress-strain curve shape.
• Collagen bundle elasticity was found to predominantly influence the linear portion of the stress-strain curve.
• Fibers carry tensile forces while matrices experience hydrostatic pressure, highlighting mechanical cooperation.

Conclusions:

• The continuum micromechanics model provides a quantitative understanding of soft tissue nonlinear behavior.
• Initial fiber crimp is a critical determinant of soft tissue mechanical response.
• The model elucidates the distinct roles of fibers and matrix in load-bearing and pressure distribution.