Soft Matter Blog

Lipid bilayers are membranes consisting of two layers of lipid molecules. They have attracted an enormous interest from the research community, as they form the external envelope of cells. In three dimensions, these lipid bilayers can form vesicles, and such systems are currently used as models for many studies of cell signalling. In addition, they can be loaded with chemicals and used as drug delivery carriers. The performance of a drug delivery system formed by vesicles relies on their stability or, in other words, how much they tend to aggregate and in what manner. A system of vesicles where all of them aggregate into a very large cluster before they are even injected in the body will be of no use as it will not reach its target. On the other hand, if the vesicles all remain apart from each other indefinitely the dose applied in the target area might not be large enough to be effective.

In this publication, the authors design a vesicle system that enables full control over the aggregation of vesicles.  A combination of reversible and irreversible linkers is employed to form the vesicle aggregates, and such combination allows control of their size by setting the appropriate temperature program. The authors envisage that their method will be a useful tool for investigating membrane fusion phenomena or inter-membrane interactions, which are of high relevance in biological processes.

Comments from the authors:

• We found that optimal conditions ​for self-limiting aggregation (defined in the text) are found when the lateral linker diffusion time is much shorter compared to the vesicle collision time. Experimentation at low vesicle concentration is therefore crucial/key.
• Continued aggregation takes place at high temperatures (>32 degrees​ C) at high fractions of biotinylated lipids in combination with a high concentration of streptavidin. This aggregation can be stopped by lowering the temperature (C18-pNIPAm swells and prevents further aggregation). Rather than self-limiting aggregation, we have then obtained an aggregation start and stop mechanism using the C18-pNIPAm linker.
• Optimizing the protocol for C18-pNIPAm synthesis, and varying the length of the pNIPAm polymer could further improve the control on final aggregation sizes.
• Additionally, further control on the aggregation could be obtained varying the temperature steps. For temperature steps lower than 40 degrees​ C (but >32 degrees C), aggregation of vesicles is slower.
• This work not only showcases the effect of combining biotin/streptavidin and C18-pNIPAm, but can also serves as a proof of principle for vesicle aggregation with other combinations of linkers, of which one of the linkers is reversible.

Citation to the paper: Step-wise linking of vesicles by combining reversible and irreversible linkers – towards total control on vesicle aggregate sizes, N. de Lange, F. A. M. Leermakers, and J. M. Kleijn. Soft Matter, 2020,16, 6773. DOI: 10.1039/D0SM00995D.

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About the web writer

Dr Nacho Martin-Fabiani (@FabianiNacho) is a UKRI Future Leaders Fellow and Senior Lecturer in Materials Science at Loughborough University, UK.

For many products such as paints, inks, or cosmetics, the evaporation of water or another solvent is one of the key stages of their application. It not only serves as the medium in which all the other formulation ingredients are dispersed, but also plays a role in what the final structure of the dried product will be and therefore determines its properties. As the product dries, the air/water interface can trap some of the particles resulting in an enrichment of that ingredient at the top of the dried surface, or hydrodynamic phenomena can result in convection flows that recirculate the ingredients to obtain a more homogenous dried product. How these phenomena affect the different ingredients is also influenced by their shape, i.e. whether they are spherical, rod-like, cube-like, or others. In the case of particles with a preferential direction, such as rods, they might have a strong tendency to align parallel to the air/water interface to minimize their energy.

In this publication, the authors simulate the drying process of a dispersion containing rod-like particles to obtain insights on the effect of different parameters on their final orientation in the dried film. They investigate how and when the transitions in orientation take place, providing useful guidance for the optimization of systems which involve rod-shaped particle such as gold or zinc oxide nanorods.

Comments from the authors:

• Above a certain volume fraction, rod-shaped particles spontaneously align in one direction, reducing the total excluded volume of the system. This is called the isotropic-nematic phase transition, first predicted by Onsager.
• During solvent evaporation of a thin film, the concentration field become inhomogeneous in the direction vertical to the substrate: particles are concentrated mostly at the liquid-air surface and phase transition starts from the top surface.
• The structural arrangements of rod-shaped particles are closely related to the dynamics of evaporation, particle aggregation, and the phase transition during the evaporation of a thin film.
• However, as for the evaporation-induced deposition patterns, few studies have been devoted to the dynamic aspects of the phase transition of rodlike particles in a drying film.
• We obtain spatiotemporal evolution of phase transition processes of rod-shaped particles in a vertically drying thin film.
• It is shown that indeed the evaporation dynamics can alter the orientational configuration in equilibrium to meta-stable states.
• This alternation from a stable state is mainly due to the anisotropic kinetics of particle diffusion coupled with the dynamics of solvent evaporation.

Citation to the paper: Evaporation-induced alignment of nanorods in a thin film, Multi-component colloidal gels: interplay between structure and mechanical, Narina Jung, Byung Mook Weon, and Masao Doi. : Soft Matter, 2020, 16, 4767. DOI: 10.1039/d0sm00482k.

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About the web writer

Dr Nacho Martin-Fabiani (@FabianiNacho) is a UKRI Future Leaders Fellow at the Department of Materials, Loughborough University, UK.

Gels are very present in your daily life; the shampoo that you wash your hair with, the gelatine in your mince pies, or your toothpaste. In these examples, the gel is composed of a large amount of liquid which is entrapped in a three-dimensional network of molecules such as surfactants or polymers. Although their liquid content is much larger than their solid content, gels present solid-like mechanical properties such as the presence of an elastic regime at low strain. There is a particular type of gel where this network, instead of being formed by a sequence of molecules, is formed by chains of tiny solid particles (colloids). These are known as colloidal gels.

In this publication, the authors present a thorough numerical study where they model the formation of colloidal gels whose networks are composed of up to three different types of particle chains. Moreover, they correlate the resulting structures with their mechanical properties, enabling the authors to establish predictions on the performance of these gels for different applications.  The insights presented in this work are of high relevance not only to advance the understanding of colloidal gels but also to the design of novel soft materials with tailored mechanical properties.

Comments from the authors:

• Understanding the coupling between networks in multi-gels is crucial: different type of mechanical coupling may have very different consequences on the mechanics of the composite material!
• Simulations allowed us to explore scenarios hard to realize in experiments (e.g. the two networks are completely repulsive with each other) but important to disentangle the different ingredients: the gel morphology, the strength of each component and the coupling between the two networks. By doing so, we discovered a mechanism potentially important for tougher gels that we would have not considered otherwise.
• We computed the macroscopic mechanical response, similar to what one would get from rheology measurements in experiments, but analyzing the microscopic motion and structure allowed us to nail down the origin of that macroscopic response.
• We found that increasing the complexity in our gels, by adding multiple same gel components that are repulsive with each other, results in an extended linear regime compared to pure gels. This extended linearity is a consequence of the steric repulsion between different components and highlights the importance of cooperative effects.
• By studying numerically the mechanical response, we showed that our model gels, upon increasing the number of components, become softer and yield at much larger strain than normal gels.
• We also found that the repulsive interaction between different components prevents compactification of the network and allows multi-gels to exhibit collective reorganization to resist bigger deformations than pure gels.
• Our numerical study suggests new strategies of tuning the mechanics of soft composite materials by controlling the inter-gel interactions and may open the road to the design of new materials of great use in soft robotics, batteries and stretchable electronics.

Citation to the paper: Multi-component colloidal gels: interplay between structure and mechanical, C. Ferreiro-Córdova, E. Del Gado, G. Foffi and M. Bouzid. Soft Matter, 2020,16, 4414-4421. DOI: 10.1039/C9SM02410G.

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About the web writer

Dr Nacho Martin-Fabiani (@FabianiNacho) is a Vice-Chancellor’s Research Fellow at the Department of Materials, Loughborough University, UK.

Did you know that you have liquid crystals at home? They sound like an outer space material, but they are the basis of the main technology behind the images you see on your TV or computer screen – Liquid Crystal Display (LCD). Their behaviour is quite unique, as they show both solid- and liquid-like features. Liquid crystals present some degree of ordering of their molecules as solids do, but at the same time the ease and speed at which they can rearrange such ordering belongs to the realm of liquids.     

In this publication, the authors take advantage of these features by designing a liquid crystal system which has two areas with different molecular orientations. They show how microparticles can be trapped at the interface between these two areas, and that such an interface can be moved by applying an external voltage to the system. They achieve a precise control over the interface movement, enabling a controlled translation and precision positioning of the microparticle. The insights presented in this work are of high relevance to the design of novel lab-on-chip devices for sensing applications as well as optoelectronic devices.

Comments from the authors:

• This paper describes how we achieved programmable and directional transport of microparticles immersed in a fluid, referred to as colloidal microparticles.
• The fluid was a nematic liquid crystal, which has anisotropic physical properties. We used different alignments of the nematic liquid crystal molecular orientation at the two opposing plates that confined the fluid – molecular orientation was parallel (planar) to the lower confining plate whilst the molecular orientation was perpendicular (homeotropic) to the upper confining plate.
• This “hybrid” nematic alignment broke symmetry so that there were two possible hybrid aligning states, with a topological defect line between them.
• We applied an in-plane electric field by applying an A.C. voltage V to stripe electrodes to distort the topological defect line and we used the dynamic evolution of the tortuous line to collect, trap and transport the colloidal particles.
• We had also proscribed the alignment at the spacers around the layer so that the topological defect formed between the two possible hybrid aligning states was initially diagonal compared to the electrode direction, allowing fine control over the shape of the topological defect line.
• The application of an A.C. voltage V triggered dynamic growth of the topological defect line and movement of the particle in a positive direction when V > Vc, or contraction of the topological defect line growth and movement of the particle in a negative direction when V < Vc; so we referred to Vc as the critical stabilising voltage. Hence the symmetry breaking in the system enabled voltage controlled microparticle movement in either a positive or negative direction within the layer and perpendicular to the resultant electric field direction.
• We always looked at our device with the planar alignment side at the bottom. We did not observe sedimentation because our particle surfaces were treated to give perpendicular (homeotropic) molecular surface orientation. Therefore elastic levitation opposed the tendency for sedimentation.
• There are two further observations that are not reported in the paper. The critical stabilising voltage and velocity range of domain wall and micro-particle movement can be altered by varying the thickness of the layer. However, the exact shape of the domain wall did not have a significant effect on the growth velocity.
• Higher voltages, applied for significant amount of time, can be used to create alternating domain walls across the entire active area of the device. This will be the subject of another follow on paper.

Citation to the paper: Electrically controlled topological micro cargo transportation, A. S. Bhadwal, N. J. Mottram, A. Saxena, I. C. Sage, and C. V. Brown, Soft Matter, 16, 2961–2970 (2020). DOI: 10.1039/c9sm01956a.

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About the web writer

Dr Nacho Martin-Fabiani (@FabianiNacho) is a Vice-Chancellor’s Research Fellow at the Department of Materials, Loughborough University, UK.

The term active matter might refer to something quite extraordinary such as, for example, dark matter. But what if I told you that such a term is used to describe very mundane things, such as flocks of birds or schools of fish? Active matter systems are composed of self-propelled elements and characterised by complex behaviours. Recently, the interest in active soft matter systems has grown significantly, with bacterial swarms and self-propelled particles being the two main foci of work in the area.

In this publication the authors are particularly interested in the less explored case in which two active components – faster and slower particles – are mixed. This would correspond, for example, to a mixture of bacteria with different speeds; or, in the macroscopic world, a scenario where zebras are being hunted by lions. They carry out Brownian dynamics simulations adapted to active matter, to find out that a segregation into condensed and gas-like phases, each composed of both fast and slow particles, takes place in such systems. Moreover, the composition of the condensed phase is highly dependent on the difference of activity between the two components. When the difference in speed between fast and slow particles is significant, the core of the condensed phase is composed of slow particles and the fast particles remain in the outer shell. If the speed is similar, then fast particles can be found as well as slow particles in the core of the condensed phase. The insights presented in this work are valuable to understand further multicomponent active matter already present in Nature or to engineer new active systems and harness the potential of their complex behaviour for diverse applications.

Comments from the authors:

• Multicomponent active matter composed of mixtures of particles with distinct active driving forces remains largely unexplored
• Active matter mixtures of fast and slow self-propelled colloids (Active Brownian Particles), is a useful way to investigate what quantities (if any) equilibrate in an active system
• The behaviour of an active material can be tuned by the introduction of additional distinct active species
• For binary active/active mixtures, a concentration weighted average of the activities of each species controls motility-induced phase separation (MIPS)
• For slow/fast activity ratio near 0, and low fast particle activity, particle participation in the dense phase is significantly affected due to mixing
• For slow/fast activity ratio near 1, and high fast particle activity, each species participates in the dense phase as if it were a monodisperse system
• Theoretical approaches for multi-component active mixtures will provide progress towards an equation of state for active matter

Citation to the paper: Heterogeneous versus homogeneous crystal nucleation of hard spheres, Soft Matter, 2020, 16, 1967-978, DOI: 10.1039/c9sm01799b

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Did you know that Soft Matter has an Active Matter themed collection? Click here to check out more papers!

About the web writer

Dr Nacho Martin-Fabiani (@FabianiNacho) is a Vice-Chancellor’s Research Fellow at the Department of Materials, Loughborough University, UK.

Crystallization processes from a liquid are very present in our life, for example the freezing of water and formation of ice crystals. In such processes, building blocks suspended in a fluid arrange themselves to form a crystal. Espinosa et al. focus on the case where these building blocks are hard spherical particles.

In a real-life situation, the liquid will normally be confined within certain walls – e.g. inside a container or on top of a surface. Therefore, some of the particles will interact with the walls and might form a small crystal (or nucleus) at its surface – the so-called heterogeneous nucleation. This is more favourable than generating a crystal in the bulk of the liquid (homogeneous nucleation), as it reduces the surface area of the crystal and thus the energy required to form it. In this work, the authors study the competition between both nucleation mechanisms. They carry out molecular dynamics simulations to model heterogeneous nucleation processes and compare with existing homogeneous nucleation numerical data. This enables them to identify two regimes depending on the density of the system, and in each of them a different nucleation mechanism is prevalent. When the number of particles in the liquid is not too high (less than 53% in volume), heterogeneous nucleation prevails. But above that threshold homogeneous nucleation takes over.

This study not only provides further insights into the competition between nucleation mechanisms; the expected prevalence of heterogeneous nucleation in experimental conditions could be part of the explanation as to why experimental and modelling results in the field have often discrepancies.

Comments from the authors:
• Above a certain density, a fluid composed hard spheres arranged in a disordered fashion is less stable than a crystal where the spheres are packed as cannon balls.
• The first step of the transformation into the crystal is the emergence of crystal embryos (or nuclei) in the fluid.
• Crystal nuclei may appear in the fluid bulk (homogeneous nucleation) or on the walls of the cell containing the fluid (heterogeneous nucleation).
• Whereas walls are always present in experiments, simulations effectively eliminate them with a trick called “periodic boundary conditions” by which a sphere leaving the simulation box on one side enters from the opposite side like ghosts in the Pacman game.
• The simultaneous appearance of homogeneous and heterogeneous nuclei makes it difficult to measure homogeneous nucleation rates in experiments.
• The balance between homogeneous and heterogeneous nucleation depends on the size and the shape of the container and on the fluid density.
• Heterogeneous nucleation prevails in fluids where particles occupy less than ~ 54 per cent of the space in cells typically used in experiments.
• The dominance of heterogeneous nucleation could explain long-standing discrepancies between experimental measurements and simulation estimates of the homogeneous nucleation rate.
• A strategy based on coating the cells with spheres arranged in a fluid fashion could potentially eliminate heterogeneous nucleation in experiments.

Citation to the paper: Heterogeneous versus homogeneous crystal nucleation of hard spheres, Soft Matter, 2019, 15, 9625-9631, DOI: 10.1039/C9SM01142K
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About the web writer

Dr Ignacio Martin-Fabiani (@FabianiNacho) is a Vice-Chancellor’s Research Fellow at the Department of Materials, Loughborough University, UK.

Cancer metastasis is believed to be responsible for about 90% cancer mortality¹. Despite tremendous efforts that have been devoted to identify biochemical markers for the onset of metastasis, the early diagnosis of malignant tumor remains challenging. Apart from interacting via biochemical signals with their surroundings, cancer cells are also actively generating forces and remodelling their mechanical microenvironment to survive and thrive, or even worse, to colonize new territories. In a recent paper published in Soft Matter, Zhang and others from the Pennsylvania State University established a traction force threshold that can be potentially used as a biomechanical marker for the onset of metastatic-like dispersion of cancer cells.

Spatiotemporal evolution of traction and intercellular tension in HCT-8 cell colonies cultured on soft and stiff hydrogel.

Upon prolonged culture of HCT-8 cells (a colon carcinoma cell line) on hydrogels with various stiffness, they found that their malignant transformation was both substrate stiffness and colony size dependent. HCT-8 colonies grown on soft hydrogels (2.6 kPa) remained cohesive throughout the two-week culture period. However, when cultured on stiffer substrates (20.7 kPa and 47.1 kPa), cells at the periphery started to disperse from their mother colony at day 7, until completely dispersed into individual cells at day 14. And size matters too. Dispersions in smaller colonies occurred earlier comparing to the larger ones.

Using traction force microscopy (TFM), researchers conducted a thorough study to correlate the cell-generated traction force with substrate stiffness and colony size. They revealed a fascinating spatiotemporal evolution of cellular forces during colony dispersion: cells on the verge generates critically higher traction forces comparing to colony interiors, ready to evade, until the onset of colony dispersion, where their forces start to decrease before completely vanished. Based on this traction force threshold, they further constructed a phase diagram to help predict colony cohesive/dispersive behaviour. When the force generated by the cells at colony boundary is big enough, they are more likely to metastasize. Interestingly, their dispersion behaviour could be efficiently suppressed by inhibiting their contractility, if treated at the right time.

The molecular mechanism behind remains to be further explored, but this study indicates the significant role of cell-generated forces in mediating cancer metastasis, which provides a new insight for the identification of early stage malignancy progression.

May the force not be with the cancer cells.

Reference:
¹ Chaffer, C. L., & Weinberg, R. A. (2011). Science, 331(6024), 1559-1564.

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A traction force threshold signifies metastatic phenotypic change in multicellular epithelia

About the web writer

Zhenwei Ma is currently a Ph.D. candidate in Mechanical Engineering at McGill University. He holds a M.E. degree in Chemical Engineering at McGill University and a B.E. degree in Chemical Engineering from Sichuan University. Find out more about him here.

Ideal reversible polymer networks have well-controlled network structures and totally reversible crosslinks. They have similarly controlled polymer network structures as ideal covalent polymer networks but they exhibit time-dependent mechanical properties (i.e. viscoelasticity) due to the presence of reversible crosslinks that can associate and dissociate.

Researchers at MIT have provided a new insight into the mechanical properties of ideal reversible polymer networks. They use a 4-arm equal-length short-chain polymer as a unit (as shown in the figure above) to build up the reversible polymer network which can, therefore, be modeled as springs and dashpots in series. Based on this assumption, they theoretically and experimentally showed that the viscoelasticity of ideal reversible polymer networks follows the Maxwell model (G(t) = v_ek_BT exp(kt), in which v_e is the concentration of elastically-active chains, k_BT is the thermal energy scale). This can be characterized by instantaneous shear modulus (G0= v_ek_BT, which can be tuned by varying concentration, molecular weight, pH, and temperature) and relaxation time (t = 1/k, which can be tuned by pH and temperature).

This study provides a simple yet general method to design the viscoelasticity of polymer networks and to quantitatively measure their kinetic properties. This work develops our understanding of reversible-crosslinked polymers in various systems and provides an insight into how researchers can tune their properties.

Read the full article here:  Ideal reversible polymer networks Soft Matter, 2018,14, 5186-5196

About the Writer

Dr. Xingcai Zhang is a Harvard SEAS Fellow at Harvard University. He was a postdoc researcher at MIT/SYSU. His expertise includes chemistry, bionanomaterials, bionanomedicine, nanotea, natural products, carbon/polymer/natural/two-dimensional materials for biomed/catalysis/absoption/energy applications. Dr. Zhang serves as an Associate Editor for a Springer Nature journal and is on the Advisory Board of a Wiley cancer journal and an editor of a cancer journal. Some of Dr. Zhang`s publications can be found at: http://orcid.org/0000-0001-7114-1095 and Google Scholar  and he can be reached at xingcai@mit.edu and mylovetea@outlook.com