Mechanical behavior of individual collagen fibrils in force-controlled mechanical tests

Abstract

Mechanical behavior of individual collagen fibrils in force-controlled mechanical tests M. Nalbach1, M. Fuchs1, N. Motoi2, M. Rufin1, O. Andriotis1, G. Schitter1, P. J. Thurner1 1TU Wien, Austria; 2Kobe, University, Japan -- Collagens are important structural proteins in the human body playing a key role in specifying mechanical properties of many tissues including tendons, bones and airways. There, collage is mostly found in the form of fibrils with typical diameters around 100 nm and lengths up to several mm. In addition to macro mechanical competence of tissues, collagen fibrils are important for the extracellular matrix (ECM) acting as cell attachment and mechanotransduction. Collagen fibrils exhibit time-dependent material properties and are thought to behave viscoelastic in physiological loading regimes. While experimental techniques for biomechanical characterization of tissues at the macroscale are more or less well established, experiments and data on individual collagen fibrils is scarce. We present experimental data from force-controlled experiments on individual fibrils using dynamic mechanical analysis (nano-DMA) as well as creep tests. Nano-DMA on collagen fibrils from 14 week old wild type mouse tail-tendon in the phase I mechanical regime show loss tangents of up to 0.2 and storage moduli of up to 5 GPa (at 2 μN average force). In addition, loss tangents decreased from the lower (0.1 Hz) to the higher (1 Hz( frequency applied. Creep tests were conducted on similar samples, with half of them cross-linked by incubation with methylglyoxal (MGO). Creep test data from cross-linked and native fibrils were fitted a Burgers material model in Kelvin-Voigt configuration (strain response of fibrils under constant force). Both creep rate of collagen fibrils and residual strain after unloading was reduced by MGO cross-linking. In addition, cross-linked fibrils showed an almost 2-fold increase in tensile modulus. In contrast, cross-linking did not affect transient viscoelastic behavior of collagen fibrils tested. The observed behavior can be explained by cross-linking influencing deformation mechanisms (straightening, uncoiling, sliding) already well below 10% applied strain.