Session X59: Soft Composites: Mechanics and Structure I

Glassy Dynamics in Composite Biopolymer Networks

The cytoskeleton is a highly interconnected meshwork of strongly coupled subsystems providing mechanical stability as well as dynamic functions to cells. To uncover central biophysical principles, it is essential to investigate not only one distinct functional subsystem but rather their interplay as composite biopolymeric structures.
Here, these effects caused by composite structures will be especially addressed with the two key elements of the cytoskeleton actin and vimentin IF. In contrast to previous studies, entangled conformations of these reconstituted composite networks can be described by a superposition of two non-interacting scaffolds. Key elements are the ability to compare different compositions with comparable architectural features as well as to identify polymer-specific interactions, which are typically ignored in most established models. We have experimentally identified inter-filament interactions as a key factor that can overwrite scaling predictions of the classical semiflexible polymer physics by comparing results of networks of actin, vimentin IF, keratin IF and synthetic double-crossover DNA nanotubes.
Acquired results of the linear and non-linear bulk mechanics are captured in the frame of an inelastic glassy wormlike chain model allowing to address the diversity of the polymer types based on their specific interactions, which lead to differing relaxation behaviors for the different polymer types. Accounting for this molecular diversity, the composite effects can be expressed as a superposition of the non-interacting scaffolds.   

Actin crosslinker density tunes mesoscale mechanics in actin-microtubule composites

The physical interactions between semiflexible actin filaments, rigid microtubules and the suit of smaller proteins that crosslink both filaments, allows the cytoskeleton to precisely tune its structure and mechanics to enable a vast array of cellular processes such as cytokinesis. Here we use optical tweezers microrheology to determine how steric entanglements versus chemical crosslinking impacts the nonlinear mesoscale mechanics of actin-microtubule composites. We create co-polymerized and co-entangled composites of actin and microtubules with varying concentrations of actin crosslinkers. We optically drive microspheres through the crosslinked composites at speeds much faster than the relaxation rates and distances larger than network mesh size. We simultaneously measure the force the filaments exert on the microspheres during and following the strain. We map network stiffness, relaxation profiles, and spatial heterogeneities to the concentration of actin crosslinkers in composites.   

Modeling composite cytoskeletal networks using effective medium theory

The mechanical response of most eukaryotic cells is due to their cytoskeleton, a polymeric scaffold made up of two major types of biopolymers, actin filaments (F-actin) and microtubules, which have very different mechanical properties. The cytoskeleton is responsible for a number of cellular functions including maintaining cell shape, rigidity, and facilitating movement. Here we seek to investigate, understand, and predict the structure-function properties of engineered cytoskeletal scaffolds with tunable mechanics. We study composite networks of F-actin and microtubules using an effective medium theory, and characterize their mechanical response using rigidity percolation theory. We obtain the shear rigidity of these networks as a function of the concentrations of F-actin and microtubules, the type of crosslinking, and the concentration of the crosslinkers. Our results may help to elucidate the design principles of smart biopolymer composites with adaptive mechanical properties.   

Coupling structure to dynamics in crosslinked actin- microtubule composites

Microtubules and actin are cytoskeletal filaments that shape the cell and play an important role in cell mobility and division. Both cytoskeletal filaments have distinct flexibilities and organizations in the cell that enable the production of work and resilience of the composite network. We seek to use such composite networks as a scaffold for smart materials with exciting properties such as self-healing and tunable stiffness. In order to use this network, we need to characterize the networks as a function of composition and crosslinking. We characterize the network structure visually using two color confocal microscopy to scan through the materials in 3D. We also measure the dynamics using the intensity fluctuations of the microtubules and the actin networks over time, the temporal autocorrelation, and fluorescence recovery after photobleaching. Networks with both malleability and elastic resilience will be useful as scaffolds for novel composite materials.   

Active actin and microtubule composite materials

Actin and microtubules are fundamental structural proteins within the cell. Studies of their passive mechanical properties form a basal framework for understanding cytoskeletal mechanics. It is becoming increasingly clear that the interplay between structural networks of actin, microtubules and intermediate filaments is critical to understand details of cellular physiology. Further, how the active dynamics of these structural polymers couples to the structure of actin and microtubule composite networks is not well understood. To address these questions, we have built a polymer blend of actin and active kinesin-driven microtubule bundles. We experimentally probed the structural morphology of this out-of-equilibrium composite material and measured the dynamics of the system over time. When the mechanical contribution of the passive network is small, the active microtubule bundles fluidize the system. Increasing the actin concentration disrupts this process. In spite of being composed of a few simple components, this well characterized non-equilibrium system generates rich phenomena evocative of living cells.   

Assortative nanoparticle percolation in nanocomposite gels

The addition of sticky nanoparticles in a physically associated hydrogel matrix is considered. It is shown that the nanoparticles assortatively self-assemble into a percolated nanoparticle network at volume fractions far below the percolation threshold of a pure sticky nanoparticle hydrogel. Concurrently, it is shown that hydrogel undergoes substantial mechanical reinforcement, as manifested by the emergence of a low-frequency plateau in the viscoelastic shear moduli as determined via rheology, and the increased elastic moduli as determined via cavitation. Nanoparticles added beyond the percolation threshold are shown to bolster the connectivity of the percolated nanoparticle network, as revealed by mechanical measurements as well as structure factor calculations. These results provide fundamental insights into the self-assembly, and mechanical consequences therein, of multicomponent reversible hydrogels.   

Mechanical structure function properties and fracture toughness of Articular Cartilage modeled as a biopolymer double network

We present results on cracking and fracture toughness of biopolymer double networks, with Articular Cartilage (AC) as our model system. AC is a soft tissue that covers the ends of bones and distributes mechanical loads at the joints in our knees and elbows. Adult AC has very few cells, and its network-like extracellular matrix primarily determines its mechanical response. As a material, AC is remarkable. It is only a few millimeters thick and has minimal regenerative capacity, yet can withstand large forces over our lifetimes during which it undergoes 100-200 million loading cycles without fracturing. The molecular mechanism underlying this exceptional toughness is not well understood. Here we investigate the mechanical structure-function properties underlying the fracture toughness of AC by using a framework that combines a double network model of AC with rigidity percolation theory. We study how the stress relaxation and crack propagation in the double network depend on its composition and on loading conditions. Our results may help to formulate a quantitative criterion for fracture in AC and similar soft and biomaterials akin to the Griffith criterion for fracture of brittle materials.   

On the interplay between two rigidity transitions in disordered semiflexible polymer-tissue networks

Low-connectivity semiflexible polymer networks undergo a rigidity transition from floppy to rigid as shear strain is increased beyond some finite value. Collections of cells with no gaps between them (confluent tissues) also undergo a rigidity transition from fluid to solid, as a parameter characterizing the shape of an individual cell is varied. Both transitions are driven by geometric constraints, though in confluent tissue, topology also plays an important role. We present numerical studies of a vertex model of a two-dimensional confluent tissue embedded in a two-dimensional semiflexible polymer network, with mechanosensitive coupling between the two systems. We study how a rigidity transition in one system may or may not induce rigidity in the other, depending on the nature of the coupling. In addition to going beyond studying the onset of rigidity in a single network construct, the semiflexible polymer-tissue network is relevant to cancer in which cancerous tissue (a tumor) is surrounded by and interacts with an extracellular matrix, consisting primarily of cross-linked collagen, a semiflexible polymer.   

Ensemble dynamics of large DNA molecules within entangled and crosslinked cytoskeleton networks

The cellular interior is highly crowded. How biological macromolecules, such as DNA, diffuse through these environments has yet to be fully elucidated. We mimic the crowded cellular environment by creating custom-designed co-polymerized networks of actin and microtubules that are crosslinked at various motifs. We study the effect of the co-entangled and co-crosslinked cytoskeleton networks on the ensemble dynamics of large circular and linear DNA molecules using selective-plane illumination differential dynamic microscopy (SPIDDM). As a digital Fourier microscopy technique, SPIDDM measures dynamics over a large range of length and time scales that supplement and expand on the dynamics measured using single-molecule tracking of DNA in the same environments. We find interesting differences between ensemble and single-molecule dynamics over the temporal and spatial scales probed.   

Polyelectrolyte composite: hyaluronic acid mixture with DNA

Biomacromolecules are mostly polyelectrolytes (PE), dissociating into polyions and small counterions. Their long-range electrostatic interaction leads to arrangements different than for neutral polymers and generates difficulties in physical understanding. For the last decade we have adressed these by studying the structure and dynamics of two semirigid (bio)PEs, DNA and HA (hyaluronic acid).
Here we present a study of mixtures of DNA and HA, as a system of like-charged semi-rigid chains, differing, however, by the respective persistence lengths of 50 and 1 nm. We studied salt-free mixtures across a broad range of concentration ratios c_HA/c_DNA=0.05-50 (c=5-200 g/L). By polarizing microscopy we established that DNA and HA form clearly separated thread-like domains defined and oriented by solution shear. We applied small angle x-ray scattering (SAXS) and observed a PE correlation peak at q* wave vector, ascribed to DNA subphase and thus reporting on its effective concentration c*_DNA. From c*_DNA it was possible to infer the c*_HA of HA subphase. We found a ratio G= c*_HA/c*_DNA≈0.85 across the studied range. As there is the osmotic pressure, P equilibrium between DNA and HA subphases, the constant G indicates that P~c^9/8 scaling commonly found for DNA and other highly charged PEs is valid also for HA which does not feature counterion condensation – so it’s not due to the concentration dependence of the latter. Furthermore, as for HA all counterions contribute to P, the HA osmotic coefficient f_HA is a measure of f_DNA. We found the latter to be 0.28 and corroborated previous workers who found the concentration of counterions controlling the osmotic pressure to be double the Manning-condensation theory value for DNA.