Nanomechanics of cartilage extracellular matrix macromolecules

In this thesis, the shear and self-adhesion nanomechanical properties between opposing cartilage aggrecan macromolecules were probed. In addition, nanoscale dynamic oscillatory mechanical properties of cartilage and its type II collagen network was measured.

Aggrecan shear nanomechanics was assessed via microcontact printing and lateral force microscopy. Lateral force between aggrecan and the probe tip, and compression of aggrecan was simultaneously measured in 0.001-1.0 M NaCl aqueous solutions. Using the microsized tip (R_tip ∼ 2.5 pm) enabled a large assembly of ∼ 10³ aggrecan molecules to interact simultaneously, closely mimicking the in vivo conditions. Both electrostatic and nonelectrostatic components were identified to importantly contribute to aggrecan shear. At near physiological IS (0.1 M), significant rate dependence was observed, suggestive of visco/poroelastic interactions within the aggrecan layer.

By using an aggrecan end-functionalized colloidal tip, shear of two opposing aggrecan layers was assessed in a similar fashion. Lower lateral force and a more marked rate dependence were measured compared to the shear of a single layer, due to the aggrecan inter-layer molecular interpenetration and the different local z-dependent charge density distribution. The addition of Ca²⁺, at physiological-like 2 mM concentration, significantly affects cartilage shear by its electrostatic screening and binding effects.

Marked aggrecan self-adhesion upon separation was discovered after static compression in the presence of electrostatic repulsion in physiological-like conditions. Aggrecan self-adhesion increases as increasing equilibration time and bath IS. Molecular origins of the adhesion, also present in vivo, include van der Waals, hydrogen bonding, Ca²⁺-mediated bridging, and molecular entanglements between the glycosaminoglycan branches of aggrecan. This self-adhesion could be an important factor in protecting cartilage matrix structural integrity and function via these energy-dissipative mechanisms.

The nanoscale oscillatory dynamic deformation properties of both nontreated and proteoglycan(PG)-depleted (left mostly type II collagen) calf knee cartilage disks (∼ 0.5 mm thick) was measured by connecting an external electronic wave generator to the AFM. A significant increase in effective stiffness E and phase lag Δ (deformation with respect to force) as increasing frequency for both disks suggests poro/viscoelasticity are more critical at higher frequency. The PG-depleted disk shows a more marked dependence of E and Δ on deformation amplitude ∼ 2 − 100 nm, as the nanostructure and nanomechanical properties of porous collagen network are more heterogeneous without the entrapment of aggrecan motif. A unique ti 23 nm banding pattern along the type II collagen fibrils was observed, which may be relative to the cartilage swelling properties and the molecular interaction between aggrecan and the collagen network.

Taken together, this study provides insights into molecular-level deformation of cartilage extracellular matrix (ECM) macromolecules (e.g., aggrecan, type II collagen) that are important to the understanding of cartilage biomechanical function. Ongoing studies are probing the age, disease (osteoarthritis), source and species related variations of cartilage ECM properties at the molecular level. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)