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Title:Time dependent mechanical behavior of mammalian collagen fibrils
Author(s):Yang, Fan
Director of Research:Chasiotis, Ioannis
Doctoral Committee Chair(s):Chasiotis, Ioannis
Doctoral Committee Member(s):Lambros, John; Sottos, Nancy; Genin, Guy
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):collagen fibril
stress relaxation
Abstract:Collagen fibrils, with dry diameters of the order of 100 nm, are the fundamental building blocks in connective mammalian tissues. While the mechanical behavior of tissues has been studied extensively and was shown to follow non-linear viscoelasticity, as similar quantitative understanding of the time-dependent deformation of individual collagen fibrils is missing, albeit essential for our understanding of the dissipative processes occurring in tissues and the development of microstructural, physics-based, models. This dissertation research aimed at understanding the viscous mechanical response of fully hydrated individual collagen fibrils, and quantifying the time-dependent behavior by separating the effects of macroscopic stress and strain. In the context of the second objective, the validity of linear viscoelasticity and the applicability of non-linear viscoelasticity models previously developed for collagenous tissues, were investigated for partially and fully hydrated collagen fibrils. To achieve the aforementioned goals, this dissertation research developed a new experimental methodology for in vitro microscale, uniaxial tension stress relaxation and creep studies with step strain and stress inputs, respectively. This experimental approach combined an edge-detection method, a closed-loop proportional–integral–derivative (PID) controller, and microelectromechanical systems-type (MEMS) devices to achieve and maintain constant force or extension to individual collagen fibrils, with 27 nm real-time displacement resolution and 1 second, or better, rise time. This methodology resolved long standing issues in our capabilities to directly investigate the time-dependent behavior of nanoscale fibers inside clear liquids. This new experimental methodology was applied to fully hydrated reconstituted collagen fibrils that exhibited three distinct regimes of deformation: an initial nonlinear “toe-heel” regime, a linear regime, and a softening regime before failure occurred. The mechanical properties of fully hydrated collagen fibrils (with 273±109 nm wet diameter) were strongly dependent on the applied strain rate in the range 10-4 – 42 s-1. In particular, the tangent modulus and tensile strength varied monotonically between 214±8 – 358±11 MPa and 42±6 – 160±14 MPa, respectively. A microstructure-based model, previously developed to describe collagenous tissues, was adapted to simulate the gradual recruitment of kinked tropocollagen molecules as load-bearing molecules, followed by stretching of the straightened tropocollagen molecules, and finally intermolecular slip beyond a microscopic stress threshold. A good fit between the model and the experimental data was demonstrated. The viscous behavior established in the strain rate sensitivity studies, was further investigated through the creep and stress relaxation step response of individual fibrils in partially hydrated and in Phosphate-buffered saline (PBS) environments. It was shown that collagen fibrils exhibit strain-dependent stress relaxation and stress-dependent creep; therefore, they do not obey linear viscoelasticity. A three time-constant adaptive quasilinear linear viscoelastic (QLV) model and the nonlinear superposition (NSP) model, both originally developed to model macroscale collagenous tissues, were shown to fit the experimental stress relaxation and the creep data. The adaptive QLV model demonstrated better fitting of the creep and stress relaxation experimental results compared to the NSP model that underpredicted the stress relaxation response, especially for small step input strain values. The three times constants of the fitted adaptive QLV model showed a faster stress relaxation process than creep, which is consistent with results for macroscale tissues. Along the same lines, the two-term NSP model predicted the same linear rate but a higher nonlinear rate for stress relaxation than creep. The difference between the relaxation and the creep rates was attributed to the combined effects of rapid intermolecular rearrangements and the slower stress-dependent molecular sliding between collagen molecules and microfibrils. This dissertation research provided the first pure creep and stress relaxation data to quantify the time-dependent behavior of individual collagen fibrils, and showed for the first time that the fundamental nanoscale building blocks of connective tissues, the collagen fibrils, demonstrate a non-linear viscoelastic response, similarly to collagenous mammalian tissues.
Issue Date:2020-07-17
Rights Information:Copyright 2020 Fan Yang
Date Available in IDEALS:2020-10-07
Date Deposited:2020-08

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