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Synthetic hydrogels are here created by enzyme-induced mineralization of hydrogel networks, yielding materials that are tough yet impressively stiff, with calcium phosphate particles distributed homogeneously throughout the network. Synthetic hydrogels get tough and stiff Natural materials such as cartilage and skin have a combination of toughness (meaning they are hard to fracture) and stiffness (meaning they are resistant to bending) that is difficult to emulate in synthetic hydrogels. Previously reported tough hydrogels owed their toughness to their ability to deform by stretching, but they lacked stiffness. Here Joerg Tiller and colleagues create hydrogels that are both tough and stiff by generating in situ amorphous calcium phosphate nanoparticles that are homogenously distributed throughout the hydrogel matrix. The resulting structures are tougher than most water-swollen synthetic hydrogels, and are stiffer than their natural counterparts. The highly filled composite materials can even be designed to be optically transparent, and they remain stretchable even when notched with a razor blade. The researchers attribute the stiffness of these materials to the formation of a percolated network of the calcium phosphate nanoparticles throughout the hydrogel. The cartilage and skin of animals, which are made up of more than fifty per cent water, are rather stiff (having elastic moduli of up to 100 megapascals) 1 , 2 as well as tough and hard to break (with fracture energies of up to 9,000 joules per square metre) 3 , 4 . Such features make these biological materials mechanically superior to existing synthetic hydrogels. Lately, progress has been made in synthesizing tough hydrogels, with double-network hydrogels achieving the toughness of skin 5 and inorganic–organic composites showing even better performance 6 . However, these materials owe their toughness to high stretchability; in terms of stiffness, synthetic hydrogels cannot compete with their natural counterparts, with the best examples having elastic moduli of just 10 megapascals or less 7 , 8 , 9 , 10 , 11 . Previously, we described the enzyme-induced precipitation and crystallization of hydrogels containing calcium carbonate, but the resulting materials were brittle 12 . Here we report the enzyme-induced formation of amorphous calcium phosphate nanostructures that are homogenously distributed within polymer hydrogels. Our best materials have fracture energies of 1,300 joules per square metre even in their fully water-swollen state—a value superior to that of most known water-swollen synthetic materials. We are also able to modulate their stiffness up to 440 megapascals, well beyond that of cartilage and skin. Furthermore, the highly filled composite materials can be designed to be optically transparent and to retain most of their stretchability even when notched. We show that percolation drives the mechanical properties, particularly the high stiffness, of our uniformly mineralized hydrogels.