Compressive Mechanics of Collagen-Fibrin Composites and Their Structural Alterations

Abstract

Collagen, fibrin and fibrin-collagen composite biomaterials play an important role in tissue regeneration and are widely used in surgery as sealants or fleeces and in bioengineering as scaffolds. Previous studies demonstrated that collagen-fibrin composite scaffolds possess improved tensile mechanical properties compared to matrices of either component alone. In this work, we showed that compressed collagen-fibrin composites have synergistic shear viscoelastic properties relative to either protein scaffold alone. Combined measurements of viscoelastic and structural properties of the composite and individual protein scaffolds using rotational rheometry, confocal and scanning electron microscopy indicated that these mechanical enhancements originated from structural alterations of the network. Measurements of elastic and loss moduli of collagen-fibrin constructs along with their individual components displayed a dual softening-stiffening transition occurring along the course of matrix compression from low to high compression degrees. Fibrin-collagen composite produced a biopolymer that was initially stiffer than its individual components and was more responsive to compression than fibrin or collagen alone. Improved stiffness of the collagen-fibrin biocomposite was attributed to the increase of fibrin network node density in the presence of collagen and formation of criss-crossing nodes between intersecting fibrin and collagen fibers as well as bundling of fibers of two interpenetrating networks. Composite collagen-fibrin materials revealed more complex network structure of higher apparent branching degree than collagen or fibrin alone. We demonstrated that changing protein density in collagen and fibrin constructs could drastically alter the stress-strain relationship by shifting the onset of stiffening to low compressive strains as the protein density increased. Correlations of the mechanical alterations and changes of the scaffold topology indicated the structural origin of the mechanical response of pure and composite materials. The results obtained can be useful for designing biomaterials of modulated mechanical characteristics.