@Weng2008; @Sun2012; @Nakajima2012; @Xin2013; @Nakajima2013a; @Chen2014; @Yan2017] (Fig. \[fig:Mechanism\](a)). Later, this toughening strategy was also shown to be effective for multi-network elastomers [@Ducrot2014; @Millereau2018] or macroscopic composites [@Takahashi2018; @King2019](Fig. \[fig:Mechanism\](b)). This suggests that the underlying principle that leads to toughening is not strongly dependent on molecular details, but governed more generally by an interplay between two mechanically different networks.

![ Double networks across the scale. (a) By combining a sacrificial network that is stiff and weak (blue), with a matrix network that is soft and strong (red), a (molecular) composite both stiff and strong (tough) can be created. (b) Typical double networks are made of elastomers [@Ducrot2014] and hydrogels [@Gong2010], but they can also be made in the form of macroscopic composites [@Takahashi2018]. (c) Schematic of a brittle (blue) and a ductile (yellow) stress-strain response. []{data-label="fig:Mechanism"}](figure1.eps)

The increase in toughness of double networks is related to a transition from brittle to ductile failure. This brittle-to-ductile transition (BDT) becomes apparent when looking at the stress-strain response of double networks subjected to tensile deformation, as schematically shown in Fig. \[fig:Mechanism\](c). In contrast to single polymer networks that typically fail in a brittle fashion (blue curve), tough double networks fail in a ductile 