This study presents the de novo design of modular protein hydrogels with programmable intra- and extracellular viscoelasticity. The researchers used computational design to specify the size, flexibility, and valency of de novo protein building blocks, as well as the interaction dynamics between them, to investigate how molecular parameters govern the macroscopic viscoelasticity of the resultant protein hydrogels. They constructed gel systems from pairs of symmetric protein homo-oligomers, each comprising 2, 5, 24, or 120 individual protein components, that are crosslinked either physically or covalently into idealized step-growth biopolymer networks. Through rheological assessment, they found that the covalent linkage of multifunctional precursors yields hydrogels whose viscoelasticity depends on the crosslink length between the constituent building blocks. In contrast, reversibly crosslinking the homo-oligomeric components with a computationally designed heterodimer results in viscoelastic biomaterials exhibiting fluid-like properties under rest and low shear, but solid-like behavior at higher frequencies. The researchers demonstrated the assembly of protein networks within living mammalian cells and showed via fluorescence recovery after photobleaching (FRAP) that mechanical properties can be tuned intracellularly in a manner similar to formulations formed extracellularly. The ability to modularly construct and systematically program the viscoelastic properties of designer protein-based materials could have broad utility in biomedicine, with applications in tissue engineering, therapeutic delivery, and synthetic biology. The study highlights the potential of de novo protein design to create tunable hydrogels with precise control over their mechanical properties, both in vitro and in vivo. The results show that the viscoelastic properties of the hydrogels can be systematically controlled by varying the molecular characteristics of the building blocks, such as their valency, geometry, and flexibility. The study also demonstrates the ability to create hydrogels with tunable viscoelastic properties that can be used in both extracellular and intracellular environments. The findings suggest that de novo protein-based hydrogels could be used in a wide range of biomedical applications, including tissue engineering, drug delivery, and synthetic biology. The study provides a framework for the design of protein-based materials with tunable mechanical properties, which could have significant implications for the development of new biomedical technologies.This study presents the de novo design of modular protein hydrogels with programmable intra- and extracellular viscoelasticity. The researchers used computational design to specify the size, flexibility, and valency of de novo protein building blocks, as well as the interaction dynamics between them, to investigate how molecular parameters govern the macroscopic viscoelasticity of the resultant protein hydrogels. They constructed gel systems from pairs of symmetric protein homo-oligomers, each comprising 2, 5, 24, or 120 individual protein components, that are crosslinked either physically or covalently into idealized step-growth biopolymer networks. Through rheological assessment, they found that the covalent linkage of multifunctional precursors yields hydrogels whose viscoelasticity depends on the crosslink length between the constituent building blocks. In contrast, reversibly crosslinking the homo-oligomeric components with a computationally designed heterodimer results in viscoelastic biomaterials exhibiting fluid-like properties under rest and low shear, but solid-like behavior at higher frequencies. The researchers demonstrated the assembly of protein networks within living mammalian cells and showed via fluorescence recovery after photobleaching (FRAP) that mechanical properties can be tuned intracellularly in a manner similar to formulations formed extracellularly. The ability to modularly construct and systematically program the viscoelastic properties of designer protein-based materials could have broad utility in biomedicine, with applications in tissue engineering, therapeutic delivery, and synthetic biology. The study highlights the potential of de novo protein design to create tunable hydrogels with precise control over their mechanical properties, both in vitro and in vivo. The results show that the viscoelastic properties of the hydrogels can be systematically controlled by varying the molecular characteristics of the building blocks, such as their valency, geometry, and flexibility. The study also demonstrates the ability to create hydrogels with tunable viscoelastic properties that can be used in both extracellular and intracellular environments. The findings suggest that de novo protein-based hydrogels could be used in a wide range of biomedical applications, including tissue engineering, drug delivery, and synthetic biology. The study provides a framework for the design of protein-based materials with tunable mechanical properties, which could have significant implications for the development of new biomedical technologies.