Fig. 3. AFM-based nanoindentation and force relaxation measurements of PDMS substrates.

(a) Equilibrium modulus (E_∞), indentation modulus (E_ind), and (b) the ratio of E_∞/E_ind of modified PDMS substrates before (M) and after UV exposure (M + UV). (c) Schematic of the five-element SLS model implemented with two viscoelastic relaxation time constants, τ₁ and τ₂. Temporal modulus, E(t) = E_∞ + E₁ exp(−t/τ₂) + E₂ exp(−t/τ₂), where E₁ and E₂ are the moduli, η₁ and η₂ are the viscosities corresponding to the two relaxation time constants, τ₁ and τ₂, respectively. (d) Representative relaxation curves of PDMS substrates. Normalized E(t) versus initial, instantaneous indentation modulus (E₀), curves showed distinctive relaxation behaviors of modified PDMS substrates before and after UV exposure. Red lines are the SLS model fits by non-linear least squares regression via the Levenberg-Marquardt algorithm (LMA). (e) Relaxation time constants of PDMS substrates. M = modified PDMS, M + UV = modified PDMS with UV exposure (n = 10 per group. Data presented as mean ± s.d.) ***: p < 0.001, **: p < 0.01, n.s.: not significant. Spin-coated PDMS substrates were prepared through a two-step process: 1) V-PDMS and curing agent were mixed at a ratio of 65:1 to form soft substrates after curing at 37°C for 2 days; 2) PDMS substrates were incubated in toluene containing S-PDMS (10 wt%), V-PDMS (20 wt%) and DMPA (10 mM). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)