Omnidirectional treadmills on a chip for swimming microorganisms

Motilities of swimming microorganisms are typically studied in a close geometry confined by glass substrates or microfluidic boundaries. Here, we introduce a microscope-compatible microfluidic chip that generates three-dimensional flows that serve virtually as omnidirectional treadmills for microorganisms. Such flows are generated by dynamic pressure regulations through six microfluidic channels offset in elevations. We demonstrate the micromanipulation capacity of such a treadmill by drawing flows into desired patterns, e.g., a square pattern with all entrained microparticles following a square-shaped trajectory. We also show that such flows are uniform for the purpose of the microorganisms' treadmill, consistent with a strain-free condition that is preserved by the symmetry incorporated into the chip. We extend the application of such strain-free treadmills to freely swimming bacteria as perturbation-free manipulations.