  Propulsion through fluids is a key component in the life cycle of many
microbes, be it in development, infection, or simply finding nutrients. In
systems of biomedical relevance, this propulsion is often through polymer
suspensions that endow the fluid with complex non-Newtonian properties, such as
shear-thinning and viscoelastic behaviour. Due to the complexity of 3D
non-Newtonian modelling, 2D undulatory propulsion has recently been
extensively-studied as a means of garnering physical intuition for these
systems. However, while streamlines, swimming speeds, and swimmer trajectories
are strikingly similar in 2D and 3D Newtonian calculations, behaviour in
non-Newtonian fluids depends upon flow derivatives, such as the shear rate,
which are radically different. Taking shear-thinning as an example rheology,
prevalent in biological fluids such as physiological mucus, this communication
demonstrates how failing to account for this difference can misguide our
understanding of 3D non-Newtonian swimming.
