Figure 2.
(a) Indentation modulus, Eind, of untreated and PG-depleted cartilage disks at z-piezo displacement rates of 0.1–10 μm/s, estimated using Eqs. 1 and 2. (b) Instantaneous indentation modulus, Eind(t), directly calculated, using the Hertz model (Eq. 1), from the relaxation data of a single force-relaxation test on an untreated cartilage disk (hydroxyl-functionalized neutral spherical tip, R ∼ 2.5 μm). The gray line is the best fit of a three-element model (Eq. 3) to the data. (c and d) Equilibrium indentation modulus, Eind,0 (c), and characteristic relaxation time constant, τ (d), of untreated and PG-depleted cartilage disks, estimated using Eqs. 1–3. All experiments in a, c, and d were performed using hydroxyl-functionalized spherical and pyramidal probe tips (n ≥ 6 different locations, mean ± SE).