Figure 5:

Micro-indentation experiments. (a) A representative cross-section of a control collagen matrix without the PNIPAAm microspheres at room temperature (22°C), when indented by a spherical micro-indenter at its surface. (b) The fitted circle represents the indenter, and its measured diameter (148 μm) was used to compute its weight. The slight inclination of the surface of the matrix (dashed yellow line) with the horizontal (0.66°) was taken into account to calculate the component of the weight perpendicular to the matrix surface. The depth of indentation was the perpendicular indentation depth (δ = 46.2 μm) from the matrix surface. (c,d) A representative cross-section and fitted circle of a collagen matrix with the PNIPAAm microspheres at room temperature (22°C), when these PNIPAAm microspheres did not contract. (e,f) A representative cross-section and fitted circle of a control collagen matrix without the PNIPAAm microspheres at elevated temperature (37°C). (g–l) Representative cross-sections and fitted circles of collagen matrices with the PNIPAAm microspheres at elevated temperature (37°C), when the PNIPAAm microspheres contracted. (m) The values of secant stiffness (k_s) measured by multiple different indenters are plotted. The values of stiffness of the control matrices and PNIPAAm-matrices were plotted at 22°C and 37°C, and they were segregated in four groups. Notably, the matrices with PNIPAAm microspheres at the elevated temperature (37°C) exhibited significant decrease in stiffness in contrast to the other three groups (p = 9.7 × 10⁻⁹, Kruskal-Wallis test). Markers with an identical shape and color indicate independent indenters from one sample. The black lines represent mean values of the respective data set.