Granular Hydrogel Composites as Matrices for Engineered Living Materials.

The design of engineered living materials (ELMs) that integrate active cells offers opportunities to create dynamic, adaptive systems. However, current methods often use bulk hydrogels that can immobilize cells and limit their growth and migration, which affects their overall functionality for use as an ELM. Here, we developed a granular hydrogel composite composed of hydrogel particles (i.e., microgels), Escherichia coli (E. coli), and a cross-linkable polymer that can be crosslinked with light exposure when desired. We investigated the influence of matrix composition and incubation conditions on microcolony morphology and mechanical properties and observed that the addition of microgels to the composite results in larger bacterial colonies with roughened morphologies and that the granular hydrogel composite compressive modulus is dependent on the matrix composition and incubation conditions. To further demonstrate the potential of this system, we also showed that the granular hydrogel composites can be 3D printed into specified geometries, that the composites can be used for sustained release of bacteria, and that the un-crosslinked matrix supports chemotactic migration, producing cell density gradients that can be fixed in place with crosslinking. Overall, the granular hydrogel composites provide an approach for the fabrication of ELMs.