Controlling viscoelastic behavior and microstructural organization in engineered living materials using synthetic...

Engineered living materials (ELMs) at the multicelluar level represent an innovation that promises programmable properties for various applications. However, the rational tuning of the mechanical properties of such ELMs from first principles remains a challenge. Here we use synthetic cell-cell adhesins to systematically characterize how rheological and viscoelastic properties of multicellular materials made from living bacteria can be tuned through adhesion strength, cell size, cell shape, and adhesion logic. We find that many of the previous results obtained for non-living materials also apply to bacterial ELMs. Additionally, the incorporation of synthetic adhesins, combined with the adaptability of bacterial cells in modifying various cellular parameters, constitutes a new approach for the precise control over material properties. Furthermore, we demonstrate that rheology is a powerful tool for actively shaping the microscopic structure of ELMs, enabling control over cell aggregation and particle rearrangement, a key feature for complex material design. These results deepen our understanding of tuning the viscoelastic properties and fine structure of ELMs for bioprinting, microbial consortia design, and biomedical applications. Statement of significance The fields of Engineered Living Materials (ELMs) and Synthetic biology undergo rapid synergistic advancements, with bacterial and eukaryotic cells serving as modular building blocks for programmable materials. A critical challenge lies in the precise control and tunability over the mechanical and structural properties of ELMs. This paper demonstrates (1) the tunability of material viscoelasticity through key cellular parameters, i.e., adhesin strength, adhesion logic, cell shape, and multi-component mixing ratios; and (2) the generation of spatially defined colloidal consortia, mesoscopic structuring, pore formation and defect minimization through rheological manipulations. This paper bridges gaps between synthetic biology and materials science by providing a quantitative basis for ELM design and characterization, and by extending results from non-living particle suspensions to ELMs.