THE INFLUENCE OF MECHANICAL PROPERTIES ON CARTILAGE SUPERLUBRICITY
Date
2025-05
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Articular cartilage supports near frictionless joint movement over a lifetime of hundreds
of millions of articulation cycles because of a complex interaction between its mechanical
properties and tribomechanics. Only recently has cartilage’s unmatched superlubricating
frictional capacity been replicated on benchtop, using the convergent stationary contact area
(cSCA) explant testing configuration(in the presence of synovial fluid). However, the
relationship between cartilage mechanical properties and the tissues capacity for superlubricity
remains unclear. In articular cartilage, the superficial zone and the progressively stiffer middle
and deep zones create a depth-dependent mechanical property gradient that supports load
distribution and maintains fluid pressurization, both thought critical for superlubricity. Changes
in this zonal structure may compromise lubrication, though the precise relationship between
depth-varying mechanics and frictional behaviors remains unknown. Osteoarthritis, a
degenerative joint disease, is characterized by collagen network degeneration, proteoglycan loss,
and cartilage swelling, which alter the tissue’s mechanical properties. To better understand the
relationship between mechanical properties and cartilage lubricity, cSCA configured
osteochondral explants were free swollen in baths of varying tonicity (isotonic and hypotonic;
400 to 55mOsm) before being mechanically characterized via indentation and tribologically
characterized in the cSCA. Overall, this thesis aimed to investigate the effect of osmotically
induced, and reversible changes in bulk and depth-dependent mechanical properties and how (if
at all) they influence cartilage superlubricity.
In Aim I, a novel micro-indentation protocol was developed to evaluate effective contact
moduli at varying tissue depths and indentation speeds, allowing for analysis of zonal
mechanical properties of articular cartilage when subjected to free swelling in varying tonicities
(isotonic and hypotonic; 400 to 55mOsm). These results were compared with macro-indentation
derived “bulk” material properties. The study revealed that near surface measures of effective
moduli at both fast and slow indentation rates appear more sensitive to hypotonic bath influences
than bulk mechanical properties, indicating localized, tonicity (hypoosmolality)-induced
mechanical changes. Such findings indicated that bulk cartilage stiffening is also associated with
localized mechanical changes within the tissue (i.e., near surface stiffening) that may impact its
ability to sustain a low-friction performance.
In Aim 2, cartilage explant samples were subjected to a speed sweep tribological
characterization in the presence of PBS and hyaluronic acid (HA) to determine if there exist
relationship(s) between cartilage stiffening and frictional behavior. Cartilage explant tribology
testing, under the cSCA configuration, showed that cartilage lubricity remained largely
unaffected by hypotonic-driven tissue stiffening. Interestingly, correlations between cSCA
friction coefficients and indentation-based mechanical properties were lubricant and
speed-dependent, underscoring complex surface and hydration interactions. This finding
suggests that cartilage’s frictional behavior is influenced by both the lubrication environment and
sliding speed, highlighting the dynamic nature of cartilage mechanics and its capacity for
low-friction performance under varying physiological conditions.