Applications for environmentally responsive surfaces are numerous, but many require the system to work while submerged in fluid. Like cilia and other natural counterparts, dynamic surface structures operating underwater can potentially sweep away microorganisms, direct cargo transport, act as capture-release systems, induce microfluidic mixing, and act as fins that enable particles to “swim” along a chemical gradient.
To adapt our HAIRS system for these purposes, we are exploring a range of hydrogel chemistries that shrink or swell in response to stimuli while submerged, and have developed a system in which reversible, patterned actuation of polymeric microstructures is driven by the chemo-mechanical response of a pH-responsive hydrogel. pH is controlled either by direct addition of acid or base or by remote application of an electric field, allowing spatial and kinetic tuning as well as mechanistic analysis of the response.
Taking inspiration from the anisotropic geometries of cilia on fish and amphibian skin, we use asymmetric microfins to generate coordinated large-scale motions with pre-determined bending directions. Incorporating these surfaces into microfluidic systems not only enables us to pattern actuation at the microscale but opens a range of possibilities for use in lab-on-a-chip applications, complementing their widespread potential as anti-biofouling, chemosensing, and many more types of dynamic surfaces for use in everyday life.