Session N35: Emerging Trends in Soft Microscale Mechanics II

Anisotropic fluids determine bacterial navigation.

Bacterial navigation of anisotropic fluids plays a significant role in biofilm formation, bacterial colonization of the mucosal linings of the lungs and reproductive tract, and the organization of the gut microbiome. In these settings, hydrodynamic interactions force bacteria to swim along a preferred direction rather than the classical run-and-tumble motion in three dimensions. Here, we will discuss bacterial flagellar dynamics in synthetic and natural anisotropic fluids exemplified by a bio-compatible nematic liquid crystal and cervical mucus. We will showcase our findings of new mechanisms of bacterial environment exploration, which is dominated by mechanical interactions between the flagella and the complex fluid.   

Active microrheology of liquid crystals: anisotropic moduli and shear thinning

We investigate the rheological behavior of the lyotropic chromonic liquid crystal (LC) Sunset Yellow (SSY) in the nematic phase by measuring the displacement-force response of a microparticle using laser tweezers. The frequency dependences of the storage and loss moduli of the LC exhibit large anisotropy and strong frequency dependence. For loss modulus, G”_⊥/G”_∥ is close to 3 at low frequency but drops below unity in the 10-100 Hz range. For the storage modulus,  G'_⊥/G'_∥ is over 10 at low frequency, and decays as frequency increases, but remains larger than unity. The effective viscosity when the particle oscillates perpendicular to the director decays over one order of magnitude as the frequency increases, showing strong shear-thinning behavior. In comparison, when the particle oscillates along the director, the effective viscosity remains close to constant, showing a Newtonian-like behavior. When dragging the particle with constant velocity, particle motion perpendicular to the director induces large-range distortion, while parallel motion triggers very little change. We study the effect of particle size, material concentration, and temperature on these features. We attribute our findings to the different nematodynamic coupling modes between particle induced flow and the LC director.   

Braiding Anisotropic Hydrogels: A phase, microstructural, and mechanical study

Anisotropic hydrogels find widespread applications in biomedical engineering, particularly as scaffolds for tissue engineering. However, producing them without multi-step synthesis procedures or specialized equipments, remains a challenge. In this study, we explore using disodium cromoglycate (DSCG), a lyotropic chromonic liquid crystal (LCLC), to template PEG in a self-assembly process. By formulating DSCG with short-chain PEGDA (Mn=250), we fabricated anisotropic hydrogel networks of fibrin-like morphology. The origin of this structure arises from a reaction-diffusion process, as PEGDA is depleted from the bulk and recruited to the surface of the DSCG as the polymerization takes place. To understand the formulation space, we constructed DSCG-PEGDA-water three-component phase diagrams to examine the influence of crowing agent PEGDA on DCSG phase behaviors. Through precise control of DSCG concentration, we modulate pore sizes of the hydrogel networks. Furthermore, by employing an in-situ active microrheology characterization platform, we quantify elastic moduli of hydrogels across PEGDA concentrations. The ease of access to PEG precursors with different chain lengths and architectures, their inherent flexibility in tuning the mechanical properties, coupled with their biocompatibility and biodegradability, underscores anisotropic hydrogels’ potential use in addressing challenges related to cell encapsulation and as a model system to study the cell-environment reciprocity.   

Observation of Strong Nonlinear Elasticity in a Cellulose-Based Material as Predicted by the Hygroelastic Theory

Moisture-responsive biological materials are abundant, and their unique properties facilitate a wide range of applications. Originally emerged from the studies of hygroscopic spores of bacteria, the hygroelastic theory [1] offers a microscopic theory of elasticity that could potentially be applicable to many forms of moisture-responsive biological matter. An unusual prediction of the hygroelastic theory is the existence of strong nonlinear elasticity in moisture-responsive biological matter. The theory quantitatively predicts the elastic modulus and how the modulus would change with strain (i.e., nonlinear elasticity) from known microscopic interactions (i.e., the hydration force). Based on the assumptions of the hygroelastic theory, these principles may be applicable to other hygroscopic biological matter. In this work, we present atomic force microscopy-based experiments on thin, regenerated cellulose films. The measurements provide evidence for the existence of strong nonlinear elasticity in these materials which align well with the quantitative predictions of the hygroelastic theory. The findings raise the possibility that many forms of hygroscopic biological matter could be hydration solids [1].

Reference:

[1] Harrellson, S.G. et al. Hydration solids. Nature 619, 500–505 (2023).

Funding acknowledgment: This work was supported by National Institute of General Medical Sciences of the National Institutes of Health, award no R35GM145382 and by the Office of Naval Research, award no. N00014-21-1-4004.   

Cells Embedded in Cytoskeleton Composites for Living Materials

The cytoskeleton is a network of interconnected polymers that underlies many mechanical processes of cells including cell division, motility, signaling, and growth. The cytoskeleton's adaptability stems from the mechanical properties of its biopolymers, including rigid microtubules and semiflexible actin filaments. However, how the presence of organelles and vesicles in the cytoskeleton impacts its structure and mechanics has so far been largely overlooked. Moreover, understanding how cells embedded in fibrous scaffolds are spatially distributed, and how they modify the scaffold structure and mechanics is critical to engineering tissue and living materials. Here, we examine the spatial distribution of embedded cells in networks of actin and microtubules; and, in turn, the impact of the embedded cells on the network structure. Using multi-spectral confocal microscopy and spatial image autocorrelation we quantify the characteristic structural correlation length-scales of both the cells and the filamentous network and map the relationship between cell concentration, filament rigidity, and network mesh size. Our preliminary results suggest that both rigidity and mesh size play important roles in the spatial distribution of cells; and that a non-monotonic dependence of network connectivity on cell density emerges at intermediate cell densities. Our future work will investigate how the presence of the network influences cell growth, which is critical to tissue regeneration technologies, and relevant to cell growth in the extracellular matrix.   

Studying the cytoskeleton landscape by varying Cell Concentration and Growth.

It is a frontier challenge to engineer bacterial cells that can control their environment using synthetic biology techniques. We are interested in how designer cells can control the cytoskeleton, a composite network of filaments that can control the mechanical properties with stiff microtubules, more flexible actin, and resilient intermediate filaments. We are interested in composite actin-microtubule networks as a model system that is not only mechanically adaptive, but also chemically controllable in its ability to grow, shrink, crosslink, and contract and extend using other crosslinker and motors. As a first step, we are examining how the presence of sessile bacterial cells can alter the network organization and contractile properties. This work is important for future work using designed cells that will be able to control the network through the delivery of proteins that can adaptively change the network. This knowledge is invaluable for harnessing the potential of cytoskeletons in the development of living materials.   

Using Circadian Clock Proteins to Self-Assemble Reconfigurable Materials

Circadian oscillators regulate a variety of metabolic processes. In cyanobacteria, a group of proteins known as KaiA, KaiB, and KaiC regulate the timing of photosynthesis through the rhythmic binding of KaiA and KaiB to KaiC. Previously, we have shown that this system can be repurposed via tagging KaiB with biotin to form bonds with streptavidin coated colloids, allowing for time dependent crosslinking of colloids via KaiB-KaiC complexes. Here we show that this Kai-crosslinking platform can be adapted to a variety of conditions, and map the phase space of dynamic restructuring. We use fluorescence microscopy and image analysis to determine the impact of protein concentrations and colloid size on the material self-assembly and oscillatory crosslinking. We also demonstrate KaiB-KaiC complexes can be used to crosslink biopolymers and hydrogels, each resulting in unique self-assembly kinetics and resulting structure. The adaptability of this novel system makes it useful for a multitude of applications ranging from wound-healing to responsive filtration.   

Using Non-uniform Magnetic Pressures to Actuate Flexible Arrays of Magnetic Nanoparticles

Because continuous magnetic materials tend to be brittle, embedding magnetic elements within a soft matrix is a common technique for microscopic actuation of flexible structures. Such an approach typically uses weakly-interacting paramagnetic elements in a moderately complex gradient field to exert enough force on the magnetic elements to actuate the structure. Using quasi-2D arrays of self-assembled magnetic nanoparticles, we push this technique to the extreme limits of densely packed magnetic elements in the thinnest, most flexible, possible layer to show how the actuation behavior can depend essentially on strong magnetic interactions between the particles comprising the sheet. In particular, we show that actuation can be achieved even in a uniform external field due to pressures arising from the interparticle interactions. We demonstrate how to infer the non-uniform magnetization state of the sheet from its mechanical configuration measured by confocal microscopy. From this, we then infer the local pressures in the bulk and along the edges, and demonstrate how these combine to give the total deflection of the sheet, comparing experimental results with MD simulations. Finally, we show how these magnetic interactions can lead to a twisting behavior in the sheet in response to a changing orientation of the applied field. The combined magnetic and elastic response in these sheets makes them ideal for targeted applications in microscopic mechanics.   

Utilizing finite element analysis to capture in-plane shear banding lipid monolayer collapse

Mechanisms of biological processes related to self-assembled lipid monolayers in organs such as the ears, eyes, and lungs can be probed through studying the response of Langmuir lipid monolayers to lateral compressive stress. Mechanically, lipid monolayers can be represented as elastic sheets. In this sense, some monolayers relax stress through out-of-plane deformation, while others relax through shear banding, where in-plane rearrangements of condensed domains are observed via fluorescence microscopy (FM). These collapse modes are accessible by tuning system softness, leading to the search for a constitutive material model with similar tunability. The elastic models that are currently used to describe out-of-plane collapse have been unsuccessful in capturing in-plane collapse. Utilizing finite element analysis, we have found that uniaxial compression of a 2D sheet forms shear bands when the elastic material exhibits a non-monotonic stress-strain response. Simulation results from models that incorporate FM-derived domain morphology show that we can trigger shear bands around domains, allowing for their reorganization and reproduction of experimental shear banding morphology. Our findings expand understanding of lipid monolayer mechanical response and microscale elasticity related phenomena.   

Damage propagation in lattice structures

In this work we investigate the fracture propagation in a two-dimensional (2D) Maxwell lattice. In general, elastic brittle material will fail catastrophically due to stress concentrations at the crack tip. Maxwell lattices are on the verge of mechanical stability and these properties can be leveraged to control the crack propagation through the lattice and provide protection from defects. We present the principles that enable the control of the fracture in a kagome lattice and illustrate, through numerical simulations, how these principles can be applied to architect the fracture through the lattice. Numerically the lattice is represented as a series of sites connected by harmonic springs with elastic brittle material response. Ultimately we show that these principles can be used to control the crack and therefore the fracture toughness of the lattice.   

Anisotropic shear response of 3D tesselated granular metamaterials

Prior studies have shown that the ensemble-averaged shear modulus of jammed sphere packings (with purely repulsive linear spring interactions) increases with pressure p, , where , in the large-system limit due to pressure-induced rearrangements. However, there are numerous applications for which it is desirable to design materials that can maintain their flexibility with small values of G even at high pressures. To design bulk materials with shear moduli G that decrease with increasing pressure, we construct tessellated granular metamaterials, which possess small numbers of grains confined within undercoordinated physical boundaries (or voxels) that are then connected to form a bulk structure. In this work, we employ discrete element method simulations to measure the shear moduli along different shear planes as a function of the shear angle for all configurations for small numbers of monodisperse, frictionless spheres confined within single voxels, as well as tessellated structures. We determine the circumstances under which the shear modulus for single voxels decreases versus increases with pressure and then design bulk tessellations with shear moduli that are able to lock-in the mechanical response of single voxels.   

Directed Interactions to Regulate the Steady State Assembly in Active-Passive Mixtures.

Colloidal gels exhibit intriguing attributes due to their diverse building blocks and the cohesive forces governing the establishment of arrested networks during the gelation process. Recent studies have offered valuable insights into aspects such as configuration, interrupted aging, and enhanced dynamics that arise by integrating activity into gel networks. Motivated by active gels, we set to investigate experimentally the intricate interplay between interactions and activity on gel morphology and dynamics. In this talk, I will discuss our endeavors to study reversible active colloidal gel from a binary suspension of electric field driven active colloids and passive ones. Our experimental observations reveal that regulating the dipolar interactions between colloidal particles modulates the orientational ordering of the active colloids and thereby the network formation. We find that orientational ordering that is directly related to activity, in turn, plays a pivotal role in dictating the overall stability and dynamics of the network. We expect our study to be an experimental demonstration of a strategy to control the steady-state assembly in active-passive mixtures in-situ and reversibly.    

High-precision measurement and inference of short-ranged colloidal interaction

Understanding the interactions between colloidal particles is essential for controlling self-assembly. Methods to characterize these interactions generally rely on imaging the particles, usually within an optical potential, and inferring the distribution of distances between them to extract the potential. Such methods must account for how the scattering of light from the particles changes as a function of distance, as well as how out-of-plane fluctuations affect the inferred distance distribution. We demonstrate an alternative method based on holographic microscopy and Bayesian inference. This method rigorously accounts for scattering effects, works in three dimensions, and does not require the particles to be trapped in an optical potential. With this method, we precisely track pairs of freely-diffusing spheres in three dimensions and at high frame rates. We show that the method can measure separation distances as small as a few nanometers between micrometer scale particles to 3 nm precision. We infer the pair potential from measurements of fluctuations in the particle separation distances in two ways: by analyzing uncorrelated samples of the separation distance, and by fitting equations of motions to the full three-dimensional trajectories of the two particles. We validate the results by comparison to indirect measurements of the interaction.