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. 2019 Nov 20;9:17151. doi: 10.1038/s41598-019-50268-z

Figure 6.

Figure 6

Compression induces plasticity and localized densification. After biomechanical testing, collagen gels were allowed to recover overnight at 4 °C and were subsequently imaged through their thickness (3300 µm image stacks with 10 µm steps using a 16x objective, 1x optical zoom). SHG images reveal the presence of a densified layer of collagen in control, but not in cross-linked, networks in correspondence of the gel surface that underwent compression (a). Representative x-z views (shrunk vertically ~4x with respect to real dimensions) show the densified layer in a representative gel. Such layer can be also visualized by taking x-y maximum intensity projections of the first 100 µm from the compressed surface (inserts). Collagen densification is associated with remnant plastic deformations in collagen gels, shown as a lower gel thickness after compression (b) in control, with respect to cross-linked gels, at all concentrations (1 mg/mL: n = 12, 2 mg/mL: n = 10, 3 mg/mL: n = 12, 4 mg/mL: n = 10 for both control and cross-linked gels). The gel thickness before and at the last step of compression are indicated, respectively, as grey and black dashed lines. Control and cross-linked gels at the various concentrations display various degrees of plasticity: 1 mg/ml control gels behave as perfectly plastic materials while 4 mg/ml cross-linked gels behave as perfectly elastic materials. Despite to a lower extent, even cross-linked gels at low collagen concentrations display plastic deformations. Similar to the radial decays around tumor spheroids shown in Fig. 1g, collagen densification can be visualized as a sharp peak in the SHG axial intensity profile (c). It should be noted that the entire thickness of the gels was imaged and that natural decay in SHG signal with depth (ζ) is due to scattering and absorption. Data are shown as mean ± SEM with numerical values and statistical analyses provided in Supplementary Table 3. * indicates statistically significant differences with respect to concentration-matched controls at p < 0.05.