Approach and penetration of cell membranes by microswimmers and self-propelled particles

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Our goal in this project is to obtain a full characterization of the approach and penetration of cell membranes by self-propelled active particles both theoretically and numerically. The project is structured into two main parts as outlined below. The understanding of the behavior and dynamics of active particles near membranes, before physical contact takes place, is a crucial ingredient to understand their penetration into the cell interior. The first part of this project will therefore be devoted to the near-membrane dynamics of active particles and microswimmers. For this purpose, we will address analytically, using a continuum description of the fluid, the diffusiophoretic self-propulsion mechanism of a catalytically active particle moving due to a chemical reaction on its surface in the vicinity of a cell membrane. Previous works have only focused on self-propulsion near stiff walls and directed motion near deformable membranes has not been addressed so far. The physical interplay between the chemical activity and membrane elasticity is expected to lead to a very rich and viable phenomenology. Our calculations will be carried out using a variety of different model swimmers including the squirmer model and the higher order representation of the far-field flow. Our theoretical predictions will then be tested and validated by boundary integral simulations. In the second part of the project, we will investigate the penetration process of a self-propelled particle into a cell membrane after physical contact is established. For that aim, we will make use of mesoscopic simulations based on the dissipative particle dynamics method to investigate the physical entry of a self-diffusiophoretic active particle with different sizes and shapes into the lipid bilayer membrane. Additionally, we will explore the fluid- and membrane-mediated interactions between active particles. Our simulations will then be complemented and supplemented by theoretical calculations using asymptotic analysis and scaling arguments in order to examine the wrapping mechanism upon binding of an active article to the cell membrane. Corresponding experiments of biocompatible self-propulsion near and through living cell membranes will be performed by our collaborators.