Designing a model bio-compatible yield stress fluid composed of hairy nanoparticles in aqueous solvents

Despite the importance of mucus in clinical detection of trapped bacteria and viruses, a systematic study of mucus has been hindered by sample limitations such as rheological variance between species and the diseases it carries. We present an opportunity to study bacterial motility in a carefully tailored artificial mucus made from a set of bio-compatible elastoviscoplastic fluids of varying yield stresses and microstructures. Covalently tethered poly(ethylene glycol) methyl ether (mPEG) on nanometer-sized silica particles are well-dispersed in aqueous interstitial fluids in order to mimic the rheological behavior of mucus. Self-suspended hairy nanoparticles (HNPs) are known to exhibit soft glassy rheology, with static yield stress coming from interactions of tethered mPEG. The HNPs retain this yielding behavior in suspensions, and their yield stress value decreases gradually with the addition of solvent. We additionally find that these suspensions show a thixotropic behavior, enabling us to probe the dynamic yield stress in the presence of background shear flows similar to those produced by bacteria motion in mucociliary clearance as well as mucus secretions in gastrointestinal and respiratory tracts. The microstructure of the fluids is probed through small angle X-ray scattering (SAXS), which shows a homogeneous distribution of HNPs in the suspension. The microstructural length scale characterization allows us to understand the geometrical constraints, structure, and continua that bacteria encounter while moving their bodies and flagella through the model elastoviscoplastic fluids.