Probing the Viscoelasticity of Collagen Solutions via Optical-Tweezers-Based Microrheology
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Probing the Viscoelasticity of Collagen Solutions via Optical-Tweezers-Based Microrheology Marjan Shayegan1 and Nancy R. Forde2 Simon Fraser University, Departments of 1Chemistry and 2Physics, 8888 University Dr., Burnaby, BC, V5A 1S6, Canada ABSTRACT How the molecular structure of proteins in solution correlates with the mechanical properties of the solution at different length scales is not known. Using optical-tweezers based microrheology, we investigate a key physical property, viscoelasticity, of collagen solutions. To do this, we measure short-range thermal fluctuations of probe particles to obtain elastic and viscous moduli of their surrounding medium, and validate our measurement and analysis techniques using the previously studied system of polyethylene oxide. Probing the concentration dependence of viscoelasticity, we find that collagen solutions exhibit elasticity of comparable strength to viscosity when the concentration reaches ~5 mg/ml. We also find that the presence of telopeptides alters the viscoelasticity of collagen solutions, particularly at high frequencies. INTRODUCTION Collagen is the major structural protein in vertebrates, where it is found in connective tissues such as bone, skin, cartilage, and tendon. It is the main constituent of the extracellular matrix (ECM), which provides structural support for cell movement and growth. Additionally, collagen is a commonly used biomaterial, acting for example as a scaffold for cell growth in tissue engineering [1]. Collagen fibrils possess a variety of mechanical properties such as stiffness, strength, and toughness [2]. Abnormal collagen morphologies arising from sequence mutations or environmental changes may lead to weakness of collagen and to serious connective tissue disease [3]. Given the alteration in mechanical properties of the material at the tissue scale arising from molecular changes, the main goal of our research is to understand how molecular structure correlates with microscale mechanical properties of collagen solutions and networks, which form the basis for structure at higher-order scales [4]. Here, we investigate this problem by starting at the most fundamental scale of collagen self-assembly: we probe interactions between isolated collagen molecules in solution to understand how they interact, with the future aim of probing changes in mechanical properties during the process of assembly into fibrils. Rheology is the field that describes viscous and elastic properties of a material in response to applied force or deformation. It provides valuable information about the timedependent mechanical response of a bulk material to applied stress, which in macroscopic measurements is mostly shear stress [5]. Conventional rheological studies make an average measurement of the bulk material’s properties, which may not represent local properties in inhomogeneous systems, and are generally limited to timescales > 0.1 seconds [5]. Microrheology studies the time-dependent response on the micron scale, e.g. using micro-particles as local probes in
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