Insect Respiratory Network-Inspired Microfluidic Devices Act as Hydrodynamic Ratchets

Many entomological respiratory systems appear to be actively ventilated by the periodic buckling of localized sections of the tubes that comprise the tracheal network. These tubes range in diameter from approximately 500 microns at the openings to the respiratory system, depending on the insect, to a fraction of a micron at the distal end of the tracheal system, near the tissue. Often the buckling occurs in a directional manner. We fabricated nine different 3-layer PDMS insect-inspired microfluidic devices to determine if a device that actuates flow in the channel via directional partial collapse of the channel ceiling could produce a unidirectional flow. In all the devices, the flow channel ceiling was collapsed via a symmetric, periodic input forcing signal. We found that many of the designs produced unidirectional flows, and that the flow amount depends on the forcing frequency. Furthermore, we found that the ability to produce a unidirectional flow depends on spatial asymmetries in the hydraulic resistance of the channel. Hence, the successful devices translated a symmetric periodic input forcing into a unidirectional flow, acting as a type of hydrodynamic ratchet.