Oligomers Modulate Interfibril Branching and Mass Transport Properties of Collagen Matrices

Abstract

Mass transport within collagen-based matrices is critical to tissue development, repair, and pathogenesis as well as the design of next generation tissue engineering strategies. This work shows how collagen precursors, specified by intermolecular cross-link composition, provide independent control of collagen matrix mechanical and transport properties. Collagen matrices were prepared from tissue-extracted monomers or oligomers. Viscoelastic behavior was measured in oscillatory shear and unconfined compression. Matrix permeability and diffusivity were measured using gravity-driven permeametry and integrated optical imaging, respectively. Both collagen types showed an increase in stiffness and permeability hindrance with increasing collagen concentration (fibril density); however, different physical property-concentration relationships were noted. Diffusivity wasn’t affected by concentration for either collagen type over the range tested. In general, oligomer matrices exhibited a substantial increase in stiffness and only a modest decrease in transport properties when compared to monomer matrices prepared at the same concentration. The observed differences in viscoelastic and transport properties were largely attributed to increased levels of interfibril branching within oligomer matrices. The ability to relate physical properties to relevant microstructure parameters, including fibril density and interfibril branching, is expected to advance the understanding of cell-matrix signaling as well as facilitate model-based prediction and design of matrix-based therapeutic strategies.

Microstructural Parameter-Based Modeling for Transport Properties of Collagen Matrices

“…Confocal microscopy images of monomer (oligomer:monomer ¼ 0:100) and oligomer (100:0) matrices, which demonstrate clear differences in collagen fibril microstructure, were obtained in the previous study [24]. These images show monomer matrices with long, entangled fibrils, whereas oligomer matrices show more branched-fibril networks.…”

“…We employed branching points, as a measure of collagen microstructure [25], since the branching points can be independently controlled while keeping the porosity and fibril diameters constant. As reported previously [24,25], mechanical properties, i.e., storage modulus, could be changed by varying the ratio of monomer to oligomer, and ultimately stem cell differentiation can be guided. In addition, an increase in oligomer content results in a higher number of branching points per total fibril length but it also results in lower segment length.…”

“…First, fibril microstructures were visualized and quantified from collecting confocal image stacks as reported in the previous studies [24,25]. Geometric information from confocal microscopy and Imaris was exported to a 3D CAD program (PTC Creo parametric 2.0) for reconstruction of a collagen matrix model as shown in Fig.…”

“…This is mainly due to the emerging of various novel biomaterials for tissue engineering and to the difficulty of identifying and quantifying their microstructural characteristics. These parameters include fiber/fibril structure, arrangement, and intermolecular crosslinks [24,25].…”

“…For example, even though the matrix porosity is similar, the permeability can be different according to the fiber orientation [26]. Recently, studies showed that the branching point, one of the characteristics that distinguish monomer from oligomer, is another parameter affecting transport and mechanical properties [24,25]. It was observed that even though the porosity was similar by modulation of different fibril diameters and branching points, the resulting values differed with the different numbers of the branching points.…”