Dynamics of an enzyme-coated squirmer in gastric mucus with pH-dependent rheology

Artificial microswimmers have significantly contributed to engineering and medical sciences, particularly drug delivery, microsurgery, and medical imaging. This study numerically investigates the dynamics of an enzyme-coated spherical microswimmer in gastric mucus with pH-dependent rheology, using the squirmer model. The urease coating on the swimmer increases the pH and liquefies the mucus gel, which exhibits viscoelastic and shear-thinning properties at pH<4. A second substance was also employed to decrease the pH. Due to the structure of the mucus gel and its nonlinear elongational behavior, the Giesekus and Newtonian constitutive equations were used to model the gel and liquefied mucus, respectively. The criterion of pH=4 was applied to define the boundary between the two fluid regions. A two-dimensional axisymmetric model was implemented in COMSOL Multiphysics software, to analyze the effects of single- and double-enzyme coating patterns and swimming gait, on swimmer's speed and power expenditure. Results indicate that the presence of a liquefied Newtonian area in front of the swimmer accelerates its motion. In contrast, a Newtonian region behind the swimmer decelerates it, as the elongation of polymer chains in the viscoelastic medium causes the swimmer to move backward toward the Newtonian region. Consequently at Pe=406.50, the squirmer swam slower than its uncoated counterpart. Notably, in the double-enzyme pattern of (c1,c2,c2,c1), the swimmer was faster than one in pure Newtonian fluid. Additionally, relaxation time and polymeric viscosity were computed for a porcine gastric mucus sample using analytical equations relating the storage G′ and loss G″ moduli to the material constants of the Giesekus model.