Hydrogels are among the first materials expressly designed for their use in biomedicine. However, state-of-the-art applications of hydrogels are severely limited because they are typically either too soft or too brittle such that they cannot bear significant loads. Nature, instead, assembles hydrogel-like soft biological tissues displaying unique mechanical properties. These properties are the result of a fine interplay between structural complexity and local composition. Indeed, most biological materials encompass highly ordered, hierarchical structures with locally varying compositions. In an effort to mimic nature complexity, several strategies to reinforce hydrogels and control their internal structure have been thoroughly investigated. However, currently produced manmade hydrogels are still far from reaching mechanical performances similar to that of their biological counterparts.

In the first part of the thesis, I introduce a novel approach to fabricate synthetic load-bearing hydrogels with controlled structure and local varying composition. These properties are obtained through the use of a granular precursor material, referred as the jammed microgel ink, that allows the 3D printing of complex architectures with controlled microstructure and composition. Microgels are produced from monomer-loaded drops, that are then converted into hydrogel particles by a conventional photopolymerization reaction. Owing to their high swelling capacity, microgels are subsequently loaded with another precursor solution. After 3D printing, the precursor solution can be crosslinked to form a second percolating network that stabilizes the 3-dimensional hydrogel construct. This reaction converts the soft jammed paste into a stiff and tough granular material, referred as the double-network granular hydrogel, while maintaining a well-defined microstructure.

In the second part of the thesis, I discuss the use of metal-coordination for the local reinforcement of hydrogels. Initially, I show that metal-coordination can be used to selectively reinforce bulk hydrogels by introducing competitive ligands in the crosslinking solution. This strategy allows the fabrication of mechanical gradients within the same material, such that core-shell structures can be obtained. Furthermore, I demonstrate the possibility to combine this reinforcement approach with double network granular hydrogels. This combination allows to independently control microstructure and local reinforcement, thus expanding the degrees of freedom for the design of load-bearing hydrogels.

In the last section, I introduce a novel strategy for the fabrication of synthetic materials whose mechanical properties and structural complexity closely resemble those of their natural counterparts. To achieve this goal, I propose to combine synthetic manmade materials with natural living microorganisms to produce an engineered living biocomposite. The encapsulation of mineralizing bacteria in a granular hydrogel enables their assembly into soft arbitrarily complex structures. Upon mineralization, the local precipitation of biominerals reinforces the scaffold by creating mineral bridges that stabilize the structure. The mineralized granular biocomposite is porous and lightweight, while being able to sustain significant loads. The material shows a unique internal microstructure that closely resemble that of natural bone.