Objective. To determine whether the synovial fluid (SF) constituents hyaluronan (HA), proteoglycan 4 (PRG4), and surface-active phospholipids (SAPL) contribute to boundary lubrication, either independently or additively, at an articular cartilage-cartilage interface.Methods. Cartilage boundary lubrication tests were performed with fresh bovine osteochondral samples. Tests were performed using graded concentrations of SF, HA, and PRG4 alone, a physiologic concentration of SAPL, and various combinations of HA, PRG4, and SAPL at physiologic concentrations. Static ( static, Neq ) and kinetic (< kinetic, Neq >) friction coefficients were calculated.Results. Normal SF functioned as an effective boundary lubricant both at a concentration of 100% (< kinetic, Neq > ؍ 0.025) and at a 3-fold dilution (< kinetic, Neq > ؍ 0.029). Both HA and PRG4 contributed independently to a low in a dose-dependent manner. Values of < kinetic, Neq > decreased from ϳ0.24 in phosphate buffered saline to 0.12 in 3,300 g/ml HA and 0.11 in 450 g/ml PRG4. HA and PRG4 in combination lowered further at the high concentrations, attaining a < kinetic, Neq > value of 0.066. SAPL at 200 g/ml did not significantly lower , either independently or in combination with HA and PRG4. Conclusion. The results described here indicate that SF constituents contribute, individually and in combination, both at physiologic and pathophysiologic concentrations, to the boundary lubrication of apposing articular cartilage surfaces. These results provide insight into the nature of the boundary lubrication of articular cartilage by SF and its constituents. They therefore provide insight regarding both the homeostatic maintenance of healthy joints and pathogenic processes in arthritic disease.
Hydrogels prepared from poly-(ethylene glycol) (PEG) have been used in a variety of studies of cartilage tissue engineering. Such hydrogels may also be useful as a tunable mechanical material for cartilage repair. Previous studies have characterized the chemical and mechanical properties of PEG-based hydrogels, as modulated by precursor molecular weight and concentration. Cartilage mechanical properties vary substantially, with maturation, with depth from the articular surface, in health and disease, and in compression and tension. We hypothesized that PEG hydrogels could mimic a broad range of the compressive and tensile mechanical properties of articular cartilage. The objective of this study was to characterize the mechanical properties of PEG hydrogels over a broad range and with reference to articular cartilage. In particular, we assessed the effects of PEG precursor molecular weight (508 Da, 3.4 kDa, 6 kDa, and 10 kDa) and concentration (10–40%) on swelling property, equilibrium confined compressive modulus (HA0), compressive dynamic stiffness, and hydraulic permeability (kp0) of PEG hydrogels in static/dynamic confined compression tests, and equilibrium tensile modulus (Eten) in tension tests. As molecular weight of PEG decreased and concentration increased, hydrogels exhibited a decrease in swelling ratio (31.5–2.2), an increase in HA0 (0.01–2.46 MPa) and Eten (0.02–3.5 MPa), an increase in dynamic compressive stiffness (0.055–42.9 MPa), and a decrease in kp0 (1.2 × 10−15 to 8.5 × 10−15 m2/(Pa s)). The frequency-dependence of dynamic compressive stiffness amplitude and phase, as well as the strain-dependence of permeability, were typical of the time- and strain-dependent mechanical behavior of articular cartilage. HA0 and Eten were positively correlated with the final PEG concentration, accounting for swelling. These results indicate that PEG hydrogels can be prepared to mimic many of the static and dynamic mechanical properties of articular cartilage.
The use of cell-scaffold constructs is a promising tissue engineering approach to repair cartilage defects and to study cartilaginous tissue formation. In this study, silk fibroin/chitosan blended scaffolds were fabricated and studied for cartilage tissue engineering. Silk fibroin served as a substrate for cell adhesion and proliferation while chitosan has a structure similar to that of glycosaminoglycans, and shows promise for cartilage repair. We compared the formation of cartilaginous tissue in silk fibroin/chitosan blended scaffolds seeded with bovine chondrocytes and cultured in vitro for 2 weeks. The constructs were analyzed for cell viability, histology, extracellular matrix components glycosaminoglycan and collagen types I and II, and biomechanical properties. Silk fibroin/chitosan scaffolds supported cell attachment and growth, and chondrogenic phenotype as indicated by Alcian Blue histochemistry and relative expression of type II versus type I collagen. Glycosaminoglycan and collagen accumulated in all the scaffolds and was highest in the silk fibroin/chitosan (1:1) blended scaffolds. Static and dynamic stiffness at high frequencies was higher in cell-seeded constructs than non-seeded controls. The results suggest that silk/chitosan scaffolds may be a useful alternative to synthetic cell scaffolds for cartilage tissue engineering.
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