Figure 3. Empirical strain measurements were performed by tracking 6 μm beads embedded in the membrane.

The beads were identified and mapped between strained and unstrained images of the membrane (a). Strains were calculated for all pairs of identified beads (Δd/d) and plotted against the angle that the bead pair made with the trench major axis (θ) (a). The strains for bead pairs oriented within 2.5° angles along the trench major and minor axis were averaged (strain 1 and strain 2 respectively) and used as approximations for the maximum and the minimum principal strains in the membrane. For the equibiaxial strain units, strain 1 and strain 2 were the averages along and perpendicular to the image horizontal direction respectively. The principal strains thus calculated (strain 1 and strain 2) were then averaged for each strain condition (b) (average ± standard deviations, n = 20 for the anisotropic strains and n = 4 for the equibiaxial strains). The principal strain ratios were also calculated and averaged for each strain condition (c) (average ± standard deviations, n = 20 for the anisotropic strains and n = 4 for the equibiaxial strains).