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

Subject Area Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
Term from 2018 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 401729922
 

Final Report Abstract

The emerging field of self-driven active particles in fluid environments has recently created significant interest in the biophysics and bioengineering communities owing to their promising future for biomedical and technological applications. These microswimmers move autonomously through aqueous media, where under realistic situations they encounter a plethora of external stimuli and confining surfaces with peculiar elastic properties. With the rapid advent of biomedical and biotechnological innovations, a deep understanding of the nature of interaction between nano- and microscopic materials and cell membranes, tissues, and organs, has become increasingly important. Active penetration of nanoparticles through cell membranes is a fascinating phenomenon that may have important implications in various biomedical and clinical applications. In this project, we have described theoretically the physical behavior of a self-propelling active microswimmer in the vicinity of a deformable surface that features resistance towards bending and shear. Based on far-field calculations we have shown that the induced translational and rotational velocities can conveniently be decomposed into bending and shear related contributions, which can display opposed behavior, i.e., while one of them enhances the velocities, the other decreases them and vice versa. In particular, the elastic properties of the interface introduce history to the hydrodynamic couplings, which manifests itself in time-dependent translational and rotational velocities of the approaching microswimmer. These velocities strongly depend on the swimming direction, the distance from the interface, the body shape, and details of the swimming mechanism encoded in the singularity coefficients. By accounting for both, bending and shear resistances, the steady state velocities agree with those of an active agent close to a planar rigid wall. Based on the proposed theoretical framework, future investigations could elucidate the spatiotemporal behavior of freely moving microswimmers nearby an elastic interface and analyze more closely the potential accumulation of microswimmers at the deformable surface in comparison to a rigid wall. Moreover, an additional, intrinsic curvature of the surface can be included in our model, which could provide a fundamental ingredient for our understanding of microswimmer entrapment and accumulation in realistic biological set-ups. In addition, we have investigated the interaction of an active particle with a minimal 2D membrane which could be realized, e.g., using synthetic particles of controlled interactions. We have identified three different scenarios, one corresponding to a permanent trapping of the particle by the membrane and the remaining two implying penetration of the particle through the membrane. The first type of penetration is characterized by a complete subsequent healing of the membrane which relaxes towards its equilibrium configuration once the particle has passed. In stark contrast, we have shown that much larger particles can create a hole in the membrane that is large enough to prevent such a self-healing dynamics, resulting in a permanently damaged membrane. This behavior is accompanied by the expulsion of membrane particles into isolated fragments. Our results suggest that if one were to effectively damage a synthetic vesicle, or perhaps a cancer cell membrane, one would need to use particles of a certain minimal size. Complementary to simulations, we have provided a detailed analytical theory allowing to predict the entire state diagram, the shape and the dynamics of the membrane. This approach might be useful to predict transitions between trapping, penetration with and without self-healing in experiments. Our results provide a detailed analysis of the physical interactions of self-propelled particles interacting with a deformable surface and are expected to contribute to our understanding of microswimmer motion in their natural surroundings.

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