Particles at Membranes – Shape, Deformability, and Activity

Cell membranes interact with their environments in many ways. Endo- and exocytosis is used to shuttle large molecules and small vesicles in and out of the cell; harmful objects like viruses and nanoparticles invade the cell by passing the membrane barrier; cytoskele- tal filaments and bacteria push actively and can strongly deform the membrane [1]. In order to elucidate some basic physical mechanisms in this broad field of membrane remodelling and cell activity, I will focus on two topics:

(i) The wrapping of (non-spherical) soft particles by membranes [2], and

(ii) The effect of active filaments and particles in vesicles and cells on their shapes, fluc- tuations, motility, and self-steering (in response to environmental cues) [3,4].

A combination of particle-based and continuum models is used in mesoscale simulations to address the behavior of these systems [5]. Many particles in vivo and in vitro are de- formable, e.g., vesicles, filamentous viruses, macromolecular condensates, polymer-grafted nanoparticles, and microgels. We study non-spherical vesicles with various sizes, shapes, and elastic properties at initially planar lipid-bilayer membranes to predict the interplay of vesicle shapes and wrapping states [2]. Increasing particle softness enhances the stability of shallow-wrapped and deep-wrapped states over non-wrapped and complete-wrapped states. For partial-wrapped vesicles, the free membrane also mediates an interaction be- tween these soft particles.

Vesicles with internal active components are highly simplified models of cells [3,4]. Here, the active components lead to enhanced fluctuations [4] and an intimate coupling of propulsion forces, membrane deformability, cell shape, and cell sensing and reactivity [3,4]. In these systems, computational models of active matter play an essential role to elucidate their non-equilibrium behavior [5,6,7].

Our predictions help to understand the mechanisms behind the reaction of cell to passive and active stimuly, and may guide the design and fabrication of active and deformable particles for efficient use in medical applications, such as targeted drug delivery.