Nonlinear Viscoelasticity of Concentrated Solutions of Aggrecan Aggregate

Abstract

Aggrecan, the major cartilage proteoglycan, is the macromolecular species primarily involved in the resiliency of cartilage tissue. Most aggrecan molecules occur in cartilage extracellular matrix as aggregates. Each aggregate has a supramolecular structure, with many highly anionic, brushlike aggrecan subunits noncovalently bound to a hyaluronan chain. To better examine the viscoelastic properties of aggrecan aggregate, contaminating subunits were removed by exclusion chromatography. At physiologic ionic strength, concentrated solutions of purified aggrecan aggregate exhibit predominantly elastic behavior at small shear strains. However, above a critical strain, gamma c, the shear moduli show a pronounced strain-softening transition, where the storage modulus decreases suddenly, and the loss modulus exhibits a maximum. At small stresses, the creep function is also highly elastic, exhibiting an equilibrium compliance and large recoverable compliance. When the stress is amplified, a discrete transition to viscous flow occurs at a yield stress sigma y. These nonlinear responses are similar to previous observations for close-packed colloidal suspensions of soft spheres, such as microgel or emulsion particles, for which a yield transition occurs when the stress and deformation are sufficient to move a particle past its neighbors.

Micro- and Nanomechanical Analysis of Articular Cartilage by Indentation-Type Atomic Force Microscopy: Validation with a Gel-Microfiber Composite

“…They are in effect a measure of the dynamic bending stiffness of a collagen fibril that is part of a cross-linked meshwork and embedded in the PG moiety. In contrast, the mechanical behavior of isolated aggrecan gels at physiologically relevant concentrations of 20-80 mg/mL (31,34) exhibited stiffness of only~1 kPa (39,40). However, measurements of the isolated components do not take into account their behavior within the tissue.…”

Solution Structure and Dynamics of Cartilage Aggrecan

“…Previous rheological studies on aggrecan aggregates have found that solutions demonstrated shear thinning behavior and the attachment of pendant comb side units caused an increase in the modulus of the material linearly proportional to the number of attached side-chains. [17][18][19] The aggrecan polymers are unassociating and do not form gels. To date no models have been developed that connect the molecular structure of the aggrecan aggregates to their rheological properties.…”

The Biomechanics of eyelid tarsus tissue

“…In contrast to collagenous ligaments and tendons, proteoglycans and aggrecan concentrates exhibit substantially more viscous energy dissipation behaviour. At physiologic ionic strength, concentrated solutions of purified aggrecan aggregate exhibited predominantly elastic behaviour at small shear strains (Meechai et al, 2001). Over a frequency sweep from 0.01 rad/s to 100 rad/s at 10% strain, the phase angle ranged from approximately 27°between 0.01 rad/s and 0.1 rad/s to reach a maximum of approximately 56°b etween 10 rad/s and 100 rad/s (Meechai et al, 2001).…”

“…At physiologic ionic strength, concentrated solutions of purified aggrecan aggregate exhibited predominantly elastic behaviour at small shear strains (Meechai et al, 2001). Over a frequency sweep from 0.01 rad/s to 100 rad/s at 10% strain, the phase angle ranged from approximately 27°between 0.01 rad/s and 0.1 rad/s to reach a maximum of approximately 56°b etween 10 rad/s and 100 rad/s (Meechai et al, 2001). Further viscous behaviour was found in proteoglycan concentrates, with the phase angle of pure proteoglycans solutions at high concentrations (10-50 mg/mL) ranging from 50°to 75° (Hardingham et al, 1987;Mow et al, 1984;Zhu et al, 1991).…”