Genetically modifying the protein matrix of macroscopic living materials to control their structure and rheological properties

The field of engineering living materials (ELMs) seeks to engineer cells to form macroscopic materials with tailorable structures and properties. While centimeter-scale ELMs can be grown from Caulobacter crescentus engineered to secrete a protein matrix, how the sequence of the protein matrix affects structural and rheological properties remains poorly understood. Here, we explore how changing the elastin-like polypeptide (ELP) length impacts ELM microstructure and viscoelastic behavior. We demonstrate that shortening ELP produces fibers almost 2x thicker than other variants, resulting in a stiffer material at rest. Interestingly, the mid-length ELP forms a complex structure with globules and multidirectional fibers with increased yield stress under flow conditions. Lengthening ELP creates thinner strands between cells with similar storage and loss moduli to the mid-length ELP. This study indicates that sequence-structure-property relationships in these ELMs are complex with few parallels to other biocomposite models. Furthermore, it highlights that fine-tuning genetic sequences can create significant differences in rheological properties, uncovering new design principles of ELMs.