Integration of Polarized Spatial Frequency Domain Imaging (pSFDI) with a Biaxial Mechanical Testing System for Dynamic Quantification of Collagen Architecture in Soft Collagenous Tissues

Abstract

Collagen fiber networks provide the structural strength of tissues such as tendons, skin, and arteries. Quantifying the response of the fiber architecture to mechanical loads is essential towards a better understanding of the tissue-level mechanical behaviors, especially in assessing disease-driven functional changes. To enable novel investigations into these dynamic fiber structures, a polarized spatial frequency domain imaging (pSFDI) device was developed and, for the first time, integrated with a biaxial mechanical testing system. The integrated instrument is capable of a wide-field and dynamic quantification of the fiber orientation and degree of optical anisotropy (DOA), representing the local strength of fiber alignment. The performance of this integrated instrument was assessed through uniaxial testing on tendon tissues with known collagen fiber microstructures. Our results revealed that the fiber orientation of the tendon tissue changed indiscernibly, whereas the fibers became better aligned with the average DOA increasing from 0.126 to 0.215 under 0% and 3% uniaxial strains, respectively. The integrated instrument was further applied to study the mitral valve anterior leaflet (MVAL) tissue subjected to various biaxial loadings. The fiber orientations within the MVAL demonstrated a proclivity towards the tissue's circumferential direction under all loading protocols, while certain fiber groups re-oriented towards the tissue's radial direction under radially-dominant loading. Our results also showed that fibers were generally better aligned under equibiaxial (DOA=0.088) and circumferentially-dominant loading (DOA=0.084) than under the radially-dominant loading (DOA=0.074), indicating circumferential predisposition. These novel findings exemplify a deeper understanding of dynamic collagen fiber microstructures obtained through the integrated opto-mechanical instrument.