Soft Matter

The production of giant unilamellar vesicles (GUVs) plays a pivotal role in various scientific disciplines, particularly in the development of synthetic cells. While numerous methods exist for GUV preparation, the modified continuous droplet interface crossing encapsulation (cDICE) method offers the advantages of simplicity and high encapsulation efficiency. However, a significant limitation of this technique is the generation of vesicles with a broad size distribution and the inability to control the desired size range. This raises a key question: Can the modified cDICE method be optimized to produce GUVs with controlled size distribution? In this study, we examined the effects of two experimental parameters--rotation time (tROT) and the angular frequency ({omega}) of the cDICE chamber--on the size distribution of GUVs. Our results show that reducing either the angular frequency or rotation time shifts the size distribution toward larger vesicles, enabling effective size selection. These findings are further supported by a physical model, which provides insights into the mechanisms underlying size selection. This work demonstrates that control over GUV size distribution can be achieved through straightforward adjustments of system parameters. The ability to fine-tune vesicle size offers researchers a powerful tool for developing customizable experimental systems for synthetic biology and related fields.

A cell uses its cytoskeletal machinery to control its membrane projections to seek and obtain cargo from its microenvironment. Though this process has been studied extensively using spherical cargo, it remains largely unknown how the process operates with vectorial ones, which are non-spheroid rigid objects with an aspect ratio. In this study, a vectorial cargo, silicon nanowire, was observed to have multiple modes of initial contact and to realign along a membrane projection or on a lamella. Using a qualitative theoretical approach, we demonstrate how membrane energy fluctuations potentially drive this realignment of a vectorial cargo. This was understood by calculations which establish how aspect ratio controls the energy landscape in a vectorial object and its influence on relative energy stability of nanowire-membrane contacts. A study of the realignment transport of vectorial cargoes and their comparison with Ornstein-Uhlenbeck process simulations revealed how one-dimensional time-correlated noise manifested in the transport process. Furthermore, a comparison between sliding of nanowires on cell membrane contacts versus rotational realignment with the same model revealed identical characteristics behind both. The understanding that one-dimensional time-correlated noise underlies both sliding and rotation of a vectorial cargo establishes how cytoskeletal dynamics effectively couples their realignment with subsequent transport for phagocytosis. This work establishes the significance of vectorial cargoes and the nature of underlying vectorial processes that enable their cellular processing.

We use the model system Hibiscus trionum as a vehicle to study the origin and propagation of surface nano-ridges in plant petal epidermal cells by tracking the development of the cell shape and the cuticle. In this system, the cuticle develops two distinct subdomains, (i) an uppermost layer which increases in thickness and in-plane extension and (ii) a substrate. We quantify the pattern formation and geometrical changes and then postulate a mechanical model assuming that the cuticle behaves as a growing bi-layer. The model is a quasi-static morpho-elastic system and it is numerically investigated in two and three dimensional settings, using different laws of film and substrate expansion and boundary conditions. We recreate several features of the observed developmental trajectories in petals. We establish the respective roles of the layers stiffness mismatch, the underlying cell-wall curvature, the cell in-plane expansion and the thickness growth rates of the layers in determining the observed pattern features, such as the variance observed in amplitude and wavelength. Our observations provide evidence which justify the growing bi-layer description, and provide valuable insights into why some systems develop surface patterns and others do not.

Plant-derived extracellular vesicles (PDEVs) have emerged as superior candidates for oral drug delivery, exhibiting a gastrointestinal survivability that significantly exceeds that of mammalian exosomes or synthetic liposomes. However, the biophysical rules governing how plant genomic regulation translates into this exceptional mechanical resilience remain unknown. Here, we present a predictive multiscale model of plant-derived extracellular vesicles, linking a parameterised genetic state space to emergent mesoscale mechanics via supra-molecular coarse-grained molecular dynamics (SCG-MD). We demonstrate that the upregulation of sterol methyltransferases (SMT) during the plants theoretical Defence state drives the formation of a phase-separated composite architecture, where rigid domains occupying approximately 36% of the membrane surface area effectively arrest crack propagation. This state achieves a critical rupture tension of 367.0 {+/-} 0.7 mN m-1 corresponding to a 39% increase over the wild-type Ripening state. Crucially, we find that chemical composition alone is insufficient for this reinforcement; vesicles with actively sorted lipid domains (Seeded topology) outperform randomised mixtures (Spontaneous topology) by 23% at identical concentrations. Furthermore, while fluid vesicles stiffen reactively under gastric acid shock (pH 2.5) due to the steric jamming of thermodynamically neutralised headgroups, the Defence state exhibits mechanical homeostasis. These findings suggest that PDEVs function as genetically tunable composite materials, offering a design blueprint for next-generation bio-inspired drug delivery vectors. Ultimately, these theoretical indices provide a predictive biophysical framework awaiting empirical confirmation via in vitro nanomechanical assays.

Understanding the emergence and role of lipid packing defects in the detection and subsequent partitioning of antimicrobial agents into bacterial membranes is essential for gaining insights into general antimicrobial mechanisms. Herein, using methacrylate polymers as a model platform, we investigate the effects of inclusion of various functional groups in the biomimetic antimicrobial polymer design on the aspects of lipid packing defects in model bacterial membranes. Two antimicrobial polymers are considered: ternary polymers composed of cationic, hydrophobic and polar moieties and binary polymers with only cationic and hydrophobic moieties. We find that differing modes of insertion of these two polymers lead to different packing defects in the bacterial membrane. While insertion of both binary and ternary polymers leads to an enhanced number of deep defects in the upper leaflet, shallow defects are moderately enhanced upon interaction with ternary polymers only. We provide conclusive evidence that insertion of antimicrobial polymers in bacterial membrane is preceded by sensing of interfacial lipid packing defects. Our simulation results show that the hydrophobic groups are inserted at a single co-localized deep defect site for both binary and ternary polymers. However, the presence of polar groups in the ternary polymers use the shallow defects close to the lipid-water interface, in addition, to insert into the membrane, which leads to a more folded conformation of the ternary polymer in the membrane environment, and hence a different membrane partitioning mechanism compared to the binary polymer, which acquires an amphiphilic conformation.

Macromolecular crowding can significantly impact the behavior of biopolymers, with crowding-induced depletion interactions influencing both the conformations and surface adsorption of individual polymers. Although previous studies have explored the influence of homogeneous polymer stiffness in crowded conditions, biomolecules such as DNA can exhibit sequence-dependent stiffness, and DNA origami nanoparticles can be designed with alternating stiff and flexible domains. In this work, we use Langevin dynamics simulations to characterize how nonuniform bending stiffness modulates the conformations and adsorption of polymers in crowded environments. By systematically varying the relative length and arrangement of flexible and semiflexible domains along a linear chain, we show that increasing osmotic pressure leads to a pattern-dependent collapse of the polymer, as revealed by a decrease in the radius of gyration. In general, large flexible regions promote polymer collapse, although flexible domains separating extended semi-flexible regions can facilitate their contact, leading to stable folded conformations. When a surface is present, large semiflexible domains promote adsorption, and the pattern of stiffness can be used to control the adsorption threshold. Our findings provide insight into the impact of spatially varying stiffness on the behavior of polymers in crowded environments, highlighting mechanisms relevant to biopolymers and deformable nanoparticles in both cellular and cell-free contexts.

The interactions of droplets and filaments can lead to mutual deformations and complex combined behavior. Such interactions also occur within the cell, where biomolecular condensates, distinct liquid phases often composed of proteins, have been observed to structure and affect the organization of the cytoskeleton. In particular, biomolecular condensates have been shown to undergo characteristic deformations when cytoskeletal filaments are fully embedded within them. However, a full understanding of the underlying physical mechanisms is still missing. Here, we combine experiments with coarse-grained molecular dynamics simulations and analytical models to uncover the physical mechanisms that define emerging shapes of droplets containing filaments. We find that the surface tension of the liquid phase and the bending energy of the filament(s) suffice to accurately capture emerging shapes if the length of the filament is small compared to the liquid volume. As the volume fraction of filament(s) increases, wetting effects become increasingly important, setting physical constraints within which surface and bending energies compete to define the droplet shapes. We find that mutual deformations of condensate and filament extend accessible shapes beyond classical stability considerations, leading to structuring and entrapment of contained filaments. Shape deformations may further affect ripening dynamics that favor certain geometries. Our findings provide a physical framework for a better understanding of the possible roles of biomolecular condensates in cytoskeletal organization.

Motivated by the work on block copolymer models that provide insights into epigenetics driven chromosome organization, we investigate the segregation behavior of five distinct 2-block co-polymers (BCPs) system with varying block sizes, confined within both symmetric and lateral geometries. Using exact enumeration method and Langevin dynamics simulation, our simple self-avoiding polymer model reveals robust behaviors (across statics and dynamic studies) despite strong finite-size effects. We observe that as block length increases, polymer compaction intensifies relying on non-specific interaction, leading to longer segregation times. The dynamic study clearly demonstrates the formation of globular lamellar phases and condensed, stable complex structures in long-range block copolymer (BCP) systems, providing a simplified analogy to lamellar-mediated chromatin compaction, which involves structures that are difficult to segregate under physiological conditions. Dominance of specific interaction over non-specific interaction in long range BCP systems leads to phase separation driven self assemblies which provides a simplified analogy to heterochromatin--inactive or stable domains. In contrast, short-range block sequences remain in a coiled state, exhibiting minimal overlap or interaction due to strong short range attraction, which may corresponds to euchromatin regions where diverse epigenetic states coexist, resulting in active, non-condensed structures. We also observe that asymmetric or lateral confinement favors more segregation between the BCPs irrespective of their underlying sequence.

Cell spreading and motility on an adhesive substrate are driven by the active physical forces generated by the actin cytoskeleton. We have recently shown that coupling curved membrane complexes to protrusive forces, exerted by the actin polymerization that they recruit, provides a mechanism that can give rise to spontaneous membrane shapes and patterns. In the presence of an adhesive substrate, this model was shown to give rise to an emergent motile phenotype, resembling a motile cell. Here, we utilize this "minimal-cell" model to explore the impact of external shear flow on the cell shape and migration on a uniform adhesive flat substrate. We find that in the presence of shear the motile cell reorients such that its leading edge, where the curved active proteins aggregate, faces the shear flow. The flow-facing configuration is found to minimize the adhesion energy by allowing the cell to spread more efficiently over the substrate. For the non-motile vesicle shapes, we find that they mostly slide and roll with the shear flow. We compare these theoretical results with experimental observations, and suggest that the tendency of many cell types to move against the flow may arise from the very general, and non-cell-type-specific mechanism predicted by our model.

When a vesicle was put onto the inner surface of a cone, it would spontaneously move to the bottom of the cone. The mechanism was explained elsewhere. In this work, we showed that when we put two vesicles onto the inner surface of a cone, both of them would spontaneously climb to the bottom of the cone. Whats following is the unambiguous fusion of these two vesicles. The probability of fusion was greatly enhanced by the confined space and close contact between these two vesicles. Our result may shed new insights on the fusions of vesicles in complex environments in biological systems, especially in the axonal and nerve growth processes.

Material flow in the acto-myosin cortex of a cell, during cell division, has been found to be chiral in nature. Here we look for possible signature of such chirality during the growth of the intra-cellular membrane partition which physically divides the cell into two compartments. Many groups have recorded this partition formation phenomenon in C. elegans embryo, in real time, using fluorescent microscopy. We analyze some of these movies using PIV technique in order to search for signatures of chirality in the acto-myosin flow field on this partition. Further, we use standard hydrodynamic theory of active gell to predict possible chiral flow structures in the growing partition. While the flows in the growing annular shaped membrane partition is mainly radially inward, it can also develop non zero azimuthal velocity components due to chirality.

Microbial communities exhibit complex behaviors driven by species interactions and individual characteristics. In this study, we delve into the dynamics of a mixed bacterial population comprising two distinct species with different morphology and motility aspects. Employing agent-based modeling and computer simulations, we analyze the impacts of size ratios and packing fractions on dispersal patterns, aggregate formation, clustering, and spatial ordering. Notably, we find that motility and anisotropy of elongated bacteria significantly influence the distribution and spatial organization of nonmotile spherical species. Passive spherical cells display superdiffusive behavior, particularly at smaller size ratios, while active rod-like cells exhibit normal diffusive behavior in the diffusion regime. As the size ratio increases, clustering of passive cells is observed, accompanied by enhanced alignment and closer packing of active cells in the presence of higher passive cell area fractions. As the size ratio increases, clustering of passive cells is observed, accompanied by enhanced alignment and closer packing of active cells in the presence of higher passive cell area fractions. Additionally, we identify the pivotal role of passive cell area fraction in influencing the response of active cells toward nematicity, with its dependence on size ratio. These findings shed light on the significance of morphology and motility in shaping the collective behavior of microbial communities, providing valuable insights into complex microbial behaviors with implications for ecology, biotechnology, and bioengineering.

Lipid membranes in nature adapt and reconfigure to changes in composition, temperature, humidity, and mechanics. For instance, the oscillating mechanical forces on lung cells and alveoli influence membrane synthesis and structure during breathing. However, despite advances in the understanding of lipid membrane phase behavior and mechanics of tissue, there is a critical knowledge gap regarding the response of lipid membranes to micromechanical forces. Most studies of lipid membrane mechanics use supported lipid bilayer systems missing the structural complexity of pulmonary lipids in alveolar membranes comprising multi-bilayer interconnected stacks. Here, we elucidate the collective response of the major component of pulmonary lipids to strain in the form of multi-bilayer stacks supported on flexible elastomer substrates. We utilize X-ray diffraction, scanning probe microscopy, confocal microscopy, and molecular dynamics simulation to show that lipid multilayered films both in gel and fluid states evolve structurally and mechanically in response to compression at multiple length scales. Specifically, compression leads to increased disorder of lipid alkyl chains comparable to the effect of cholesterol on gel phases as a direct result of the formation of nanoscale undulations in the lipid multilayers, also inducing buckling delamination and enhancing multi-bilayer alignment. We propose this cooperative short- and long-range reconfiguration of lipid multilayered films under compression constitutes a mechanism to accommodate stress and substrate topography. Our work raises fundamental insights regarding the adaptability of complex lipid membranes to mechanical stimuli. This is critical to several technologies requiring mechanically reconfigurable surfaces such as the development of electronic devices interfacing biological materials.

Using artificially reconstituted membranes based on structurally liquidlike phospholipids, we have performed an experimental study on the mechanical impact of the saponin {beta}-aescin, aka escin, a natural biosurfactant extracted from the seeds of the horse chestnut tree Aesculus hippocastanum. The paper focusses on the modulable interaction of escin with DMPC in model membranes in the form of bilayer vesicles and Langmuir monolayers. As regarding to their dual mechanical membrane behavior being both soft solids and viscoelastic fluids, we have outlined the principal energetic and kinetic features describing the insertion of escin as transversally adsorbed or longitudinally integrated within the model membranes. At connection with the structural phase behavior assessed by dedicated microscopies of surface fluorescence and Brewster angle reflectivity, these hybrid escin / phospholipid membranes have been revealed to possess dual mechanical properties connected to their structural rigidness and fluidity behaving both in one way and another. In particular, we observe a soft glassy rheology typical for liquid-crystalline ordered phases at low temperature, which turns into a fluidlike viscoelasticity characteristic of the disordered phases at high physiological temperature. These original results have been discussed in a physicochemical perspective that may pave new avenues of material engineering and / or pharmacological design exploiting the dual mechanical impact of escin as a mechanical modulator of the cellular membrane.

Septins are cytoskeletal proteins interacting with the inner plasma membrane and other cytoskeletal partners. Being key in membrane remodeling processes, they often localize at specific micrometric curvatures. To analyze the behavior of human septins at the membrane, we have used a combination of methods to assay their ultrastructural organization, their curvature sensitivity as well as their role in membrane reshaping. In contrast to budding yeast septins, on membranes, human septins systematically organize into a two-layered mesh of orthogonal filaments instead of generating parallel sheets of filaments observed for budding yeast septins. This peculiar mesh organization is curvature sensitive and drives membrane reshaping as well. The observed membrane deformations together with the filamentous organization are recapitulated in a coarsegrained computed simulation to understand their mechanisms. Our results highlight the specificity of animal septins as opposed to fungal proteins.

Many cellular functions depend on the physical properties of the cells environment. Many bacteria have different types of surface appendages to enable adhesion and motion on a variety of surfaces. Myxococcus xanthus is a social soil bacterium with two distinctly regulated modes of surface motility, termed the social motility mode driven by type iv pili and the adventurous motility mode based on focal adhesion complexes. How bacteria sense different surfaces and subsequently coordinate their collective motion remains largely unclear. Using polyacrylamide hydrogels of tunable stiffness, we found that wild-type M. xanthus spreads faster on stiffer substrates. Here, we show using motility mutants that disrupt adventurous motility suppresses this substrate-stiffness response, suggesting focal-adhesion-based adventurous motility is substrate-stiffness dependent. We also show that modifying surface adhesion by the addition of adhesive ligands, chitosan, increases the amount of M. xanthus flairs, a characteristic feature of adventurous motility. Taken together, we hypothesize a central role of M. xanthus adventurous motility as a driving mechanism for surface and surface stiffness sensing.

We experimentally assess the nonlinear rheology of composite biopolymer hydrogels composed of thiolated hyaluronic acid, poly(ethylene glycol) diacrylate (PEGDA), and laminin-111 in varied concentrations. We focus in particular on the influence of laminin on the mechanics of the assembled hydrogels, reporting nonlinear rheological measurements for gels under applied shear and compressive load. We find that increasing the concentration of laminin in the synthesized gels reduces the linear shear modulus and gives rise to a mild strain softening regime at intermediate strains prior to the onset of strain stiffening. In the stiffening regime, we find that all gels exhibit stress-controlled mechanics with K {propto}{sigma} a, with an apparent stiffening exponent of a {approx} 1, in agreement with observations of a variety of other reconstituted biopolymer gels. We discuss the possible implications of this nonlinear mechanical behavior on mechanotransduction and organoid development in biomimetic extracellular matrices.

Migrasomes, the transient vesicle-like cellular organelles, arise on the retraction fibers (RFs), the branched tubular extensions of the plasma membrane generated during cell migration. Migrasomes form in two steps: a local RF swelling is followed by a protein-dependent stabilization of the emerging spherical bulge. Here we approached experimentally and theoretically the previously unaddressed mechanism of the initial RF swelling. We hypothesized that the swelling can be driven by alterations of the generic mechanical factors, the RFs luminal pressure and membrane tension. To examine the effects of pressure, we exposed migrating RF-producing cells to a hypotonic medium and observed the formation of migrasome-like bulges with a preferential location in the RF branching sites. To test the results of tension variations, we developed a biomimetic system of three membrane tubules connected by a junction and subjected to controlled membrane tension. An abrupt increase of tension resulted in a migrasome-like bulge formation in the junction and in the tubular regions. Following the formation, the tubules bulges moved toward and merged with the junctional bulge. To understand the physical forces behind the observations, we considered theoretically the mechanical energy of a membrane system consisting of a three-way tubular junction with emerging tubular arms connected to a membrane reservoir. The energy minimization predicted the membrane bulging, preferably, in the junction site as a result of both an increase in the luminal pressure and an abrupt rise of the membrane tension. We discuss the common physical background of the two phenomena.

Tracing the motion of macromolecules, viruses, and nanoparticles adsorbed onto cell membranes is currently the most direct way of probing the complex dynamic interactions behind vital biological processes, including cell signalling, trafficking, and viral infection. The resulting trajectories are usually consistent with some type of anomalous diffusion, but the molecular origins behind the observed anomalous behaviour are usually not obvious. Here we use coarse-grained molecular dynamics simulations to help identify the physical mechanisms that can give rise to experimentally observed trajectories of nanoscopic objects moving on biological membranes. We find that diffusion on membranes of high fluidities typically results in normal diffusion of the adsorbed nanoparticle, irrespective of the concentration of receptors, receptor clustering, or multivalent interactions between the particle and membrane receptors. Gel-like membranes on the other hand result in anomalous diffusion of the particle, which becomes more pronounced at higher receptor concentrations. This anomalous diffusion is characterised by local particle trapping in the regions of high receptor concentrations and fast hopping between such regions. The normal diffusion is recovered in the limit where the gel membrane is saturated with receptors. We conclude that hindered receptor diffusivity can be a common reason behind the observed anomalous diffusion of viruses, vesicles, and nanoparticles adsorbed on cell and model membranes. Our results enable direct comparison with experiments and offer a new route for interpreting motility experiments on cell membranes.

Cell migration is fundamental to many biological processes, while it remains elusive how cells modulate their migration upon different environmental stiffness. In this work, we focus on the structural maturity of actin stress fibers to explain the substrate stiffness-dependent emergence of different cell migration velocity. We demonstrate that fibroblasts migrate longer distances on softer elastic substrates, and the distance is increased by lowering the myosin-driven contractile force. Stress fibers, the major intracellular structure to generate and sustain contractile forces, were found to be less mature in structure on soft substrate than on stiff substrate. Based on these experimental results, we present a minimal mathematical model to capture the salient features of how the substrate stiffness alters the migration velocity. Specifically, the ability of cells to generate large contractile forces is limited on soft substrate according to the Hookes law. The inverse relationship between the cellular force and migration velocity is described by the Hills muscle equation. These mathematical descriptions suggest that the migration velocity is raised on softer substrate where cells exert a lower magnitude of contractile forces. Cells undergoing faster movement make stress fibers less mature in structure as mathematically described by the maturation model, thereby limiting the ability to sustain the force and in turn allowing for consistent increase in cell migration velocity on soft substrate again according to the Hookes law and Hills muscle equation, respectively. Thus, our model, reproducing the basic trend of the experimental results, provides insights into the mechanisms of environmental cue-dependent migratory behavior of cells.