Rheological behavior of Pluronic/Pluronic diacrylate hydrogels used for bacteria encapsulation in living materials

Pluronic (Plu)-based hydrogels containing a fraction of Pluronic diacrylate (PluDA) chains form physical networks with additional covalent crosslinks. These hydrogels have been used to encapsulate bacteria or yeast in living materials and devices. Within Plu/PluDA hydrogels, the growth of the encapsulated organisms decreases with increasing density of covalent bonds. This has been attributed to the capability of the organisms to sense and respond to the mechanical properties of the embedding matrix. Here we investigate the rheological behavior of 30 wt. % Plu/PluDA hydrogels with increasing covalent crosslinking adjusted by the PluDA fraction in the mixture. This hydrogel composition allowed hydrogel-regulated growth of E. coli in living devices in previous reports. We aim to find experimental descriptors of the hydrogel's rheological response that can be correlated to the reported growth behavior. We present stress relaxation and creep-recovery experiments and quantify the elastic, viscoelastic and plastic responses of the hydrogels as function of the covalent crosslinking degree, with a focus on critical yield strain/stress and relaxation parameters. An inverse correlation was observed between the storage modulus and the critical stress/strain for fluidization TF, of the hydrogel and the reported maximum volume of E. coli colonies grown in the hydrogels. These results contribute to the biophysical understanding of bacteria behavior in confinement and are relevant for different application contexts, eg. to improve the performance and safety of living therapeutic implants and to maximize the yield of biotechnological production in immobilized cell reactors. ### Competing Interest Statement The authors have declared no competing interest.