Liquids as Reinforcements for Anisotropic and Tough Soft Matter Composites

Biological tissues and engineered composites achieve exceptional mechanical properties through microstructures that create stiffness and toughness in preferred directions. While composites traditionally leverage solid reinforcements to drive this anisotropy, directional mechanics in all-soft matter composites remain a longstanding challenge, despite their importance for soft devices that stretch and adapt under load. Here, we create all-soft matter composites where liquid inclusions direct and enable anisotropic and heterogeneous mechanical properties. By shaping and orienting liquid metal droplets within elastomers, we program directional stiffness, enhance toughness beyond 36,000 J m^-2, and guide cracks along non-linear paths with deflections up to 150 ${}^{\circ }$ during extreme deformations. This allows liquids, which are up to a million times softer than traditional rigid inclusions, to act as mechanical reinforcements. These liquid inclusions enhance directional stiffness or softness relative to unfilled elastomers and enable programmable crack-path engineering that surpasses simple blunting or trapping, with anisotropy tuned on demand during processing. We leverage this to protect soft circuits even under catastrophic damage, offering new possibilities to direct mechanical forces in compliant materials for resilient soft electronics and robots, wearables, and morphing matter.