Figure 7.

Shear and self-adhesion nanomechanics of cartilage aggrecan. (Left) Schematics of lateral force microscopy experiments involving shear of two opposing aggrecan layers and high-resolution force spectroscopy experiments that probe aggrecan self-adhesion. (a) Lateral-versus-applied normal force (mean ± SD, n = 8 different locations) between an aggrecan-functionalized spherical tip (R ~ 2.5 µm) and an aggrecan-functionalized planar gold substrate in different bath ionic strengths (IS) (0.001–1.0 M NaCl, pH ~ 5.6), with corresponding least squares linear regression fit for each IS (R² > 0.92 for all the fits; 95% confidence interval width of µ < 0.01). (b) Lateral proportionality ratio µ versus lateral tip displacement rate (mean ± 95% confidence interval at n = 8 different locations; R² > 0.88 for all data) between the aggrecan tip and aggrecan substrate. Panels a and b adapted with permission from Reference 123. (c) Comparison of force-distance curves obtained via neutral hydroxyl-terminated self-assembled monolayer (OH-SAM) and aggrecan-functionalized tips (R ~ 2.5 µm) on aggrecan end-grafted planar substrates (IS = 1.0 M, NaCl aqueous solution, surface dwell time t = 30 s, maximum compressive force F_max ~ 45 µN, z-piezo displacement rate z = 4 µm s⁻¹). Different experiments were carried out at 10 different locations, as shown for each probe tip. (Inset) Definition of adhesive interaction distance D_ad, maximum adhesion force F_ad, and adhesion energy E_ad for each pair of approach-retract force-distance curves. (d) Adhesion energy, E_ad, versus surface dwell time t between two opposing aggrecan end-grafted layers in 0.001–1.0 M NaCl, pH ~ 5.6 (F_max ≈ 45 nm, z ≈ 4 µm s⁻¹, mean ± SEM, n ≥ 30 for each t at each IS). E_ad depended significantly on t and IS [two-way analysis of variance (ANOVA), p_t < 0.0001, p_IS < 0.0001]. Panels c and d adapted with permission from Reference 145.