Fig. S2.

Necessary boundary conditions. (A) For a fixed end, there should not be any deflections. By contrast, there should be no bending moment at the free end. (B) Adding a physical extension to satisfy the boundary conditions for the free ends. At all of the time instants, the bending moment at the free end should be zero, as achieved by introducing an extension at the free end (indicated by the red portions). The bending moment along this extension will be fitted by a polynomial function so that its bending moment at the new free end will always be zero.