Left, a micrograph of a quasi static planar crack propagating towards an embedded rigid Nylon inclusion (polyamid, diameter 50 m). The hydrogel and the inclusion are highlighted with light yellow and red. Right, two crack micrographs recorded at different values of z from the sample bottom after encountering the inclusion. The planar symmetry is broken, by a large material filament that joins two distinct crack planes near the centre of the sample. White points are sterically bound 1.1 micron latex particles. b , G c versus before and after the crack encounters the inclusion. Inset, 3D crack fronts extracted from the two volumetric images in a ; the crack advances in the direction indicated by the arrow, corresponding to the remote tensile loading symmetry. is measured from the crack front geometry and G c is evaluated from parabolic fitting of the far-field CTOD ( x < 180 m, Supplementary Fig. 2 ). The error bars in correspond to the standard deviation of five independent crack tip identifications, and the error bars in G c are adapted from average standard deviation of confocal measurements presented in Fig. 3 . Data are presented as mean values s.d.
