Engineering active colloidal dynamics at a lipid bilayer interface

In this work, we discuss the development of an active colloidal system with controllable interactions with an artificial lipid bilayer membrane as a model for investigating the interplay of membrane mechanics and the transport of particles during adhesion and wrapping. We use polystyrene microspheres coated with a hemispherical platinum cap as model swimmers whose active motion is initiated by the addition of hydrogen peroxide (H₂O₂). Two classes of particle-membrane interactions and particle swimming direction are assessed. For the former, carboxylated particles are used to passively interact with the membrane through electrostatic interactions, while streptavidin coated particles are used to form a strong bond with biotinylated lipid membranes. For the latter, these active Janus particles are designed to be "pushers", which swim toward their metal face into the bilayer, or "pullers", which swim away from the membrane, by changing the concentration of CTAB, a cationic surfactant, in the aqueous phase. We find that a negative gravitaxis effect causes the steady movement of unbound pullers up and away from the membrane with increasing H₂O₂. When the particles are bound, a threshold H₂O₂ concentration is needed before overcoming the strength of the biotin-neutravidin bond and releasing the particles from the interface. In the case of the pusher system, as the H₂O₂ concentration increases the particles become increasingly wrapped in the membrane, as evidenced by their altered translational and rotational dynamics. We apply active Brownian models to characterize the nature of the particle-membrane interactions and also particle pair interactions. These results lay the groundwork to combine active colloidal systems with model lipid membranes to understand active transport in cellular contexts.