We use optical tweezers microrheology and fluorescence microscopy to apply nonlinear microscale strains to entangled and cross-linked actin networks, and measure the resulting stress and actin filament deformations. We couple nonlinear stress response and relaxation to the velocities and displacements of individual fluorescent-labeled actin segments, at varying times throughout the strain and varying distances from the strain path, to determine the underlying molecular dynamics that give rise to the debated nonlinear response and stress propagation of cross-linked and entangled actin networks at the microscale. We show that initial stress stiffening arises from acceleration of strained filaments due to molecular extension along the strain, while softening and yielding is coupled to filament deceleration, halting, and recoil. We also demonstrate a surprising nonmonotonic dependence of filament deformation on cross-linker concentration. Namely, networks with no cross-links or substantial cross-links both exhibit fast initial filament velocities and reduced molecular recoil while intermediate cross-linker concentrations display reduced velocities and increased recoil. We show that these collective results are due to a balance of network elasticity and force-induced cross-linker unbinding and rebinding. We further show that cross-links dominate entanglement dynamics when the length between cross-linkers becomes smaller than the length between entanglements. In accord with recent simulations, we demonstrate that post-strain stress can be long-lived in cross-linked networks by distributing stress to a small fraction of highly strained connected filaments that span the network and sustain the load, thereby allowing the rest of the network to recoil and relax.

We optically drive a microsphere at constant speed through entangled actin networks of 0.2 -1.4 mg/ml at rates faster than the critical rate controlling the onset of a nonlinear response. By measuring the resistive force exerted on the microsphere during and following strain we reveal a critical concentration c * 0.4 mg/ml for nonlinear features to emerge. For c > c * , entangled actin stiffens at short times with the degree of stiffening S and corresponding timescale t sti f f scaling with the entanglement tube density, i.e. S ∼ t sti f f ∼ d

Composites of flexible and rigid polymers are ubiquitous in biology and industry alike, yet the physical principles determining their mechanical properties are far from understood. Here, we couple force spectroscopy with large-scale Brownian Dynamics simulations to elucidate the unique viscoelastic properties of custom-engineered blends of entangled flexible DNA molecules and semiflexible actin filaments. We show that composites exhibit enhanced stress-stiffening and prolonged mechano-memory compared to systems of actin or DNA alone, and that these nonlinear features display a surprising nonmonotonic dependence on the fraction of actin in the composite. Simulations reveal that these counterintuitive results arise from synergistic microscale interactions between the two biopolymers. Namely, DNA entropically drives actin filaments to form bundles that stiffen the network but reduce the entanglement density, while a uniform well-connected actin network is required to reinforce the DNA network against yielding and flow. The competition between bundling and connectivity triggers an unexpected stress response that leads equal mass DNA-actin composites to exhibit the most pronounced stress-stiffening and the most long-lived entanglements.Mixing polymers with distinct structural features and stiffnesses endows composite materials with unique macroscopic properties such as high strength and resilience coupled with low weight and malleability [1][2][3][4]. These versatile materials, ranging from carbon nanotube-polymer nanocomposites and liquid crystals to cytoskeleton and mucus, have numerous applications from tissue engineering to high-performance energystorage [2,[5][6][7][8][9][10][11][12]. Compared to single-constituent materials, polymer composites offer a wider dynamic range and increased control over mechanical properties by tuning the relative concentrations and properties of the different species. Importantly, the unique mechanics that emerge in composites often cannot be deduced from those of the corresponding single-component systems [3,[13][14][15][16][17]. However, the physical principles that couple structural interactions to mechanics in composites remain elusive.Over the past two decades, DNA and actin have been extensively studied as model polymer systems [18][19][20][21][22]. While the contour lengths of each biopolymer can be comparable (L≈10-50 µm), actin is much stiffer than DNA with a persistence length lp of ~10 µm compared to lp≈50 nm for DNA. When sufficiently long, both polymers form entangled networks over similar concentrations (c≈0.1-2.5 mg/ml), with actin forming nematic domains above 2.5 mg/ml [18]. Despite their wide use as model systems, very few studies have examined composites of actin and DNA, focusing solely on steady-state structure at concentrations above the nematic crossover or under microscale confinement [23][24][25]. These studies reported large-scale phase separation such that DNA and actin polymers were rarely interacting. Co-entangled systems of DNA and actin have yet to be i...

Polymer glasses have numerous advantageous mechanical properties in comparison to other materials. One of the most useful is the high degree of toughness that can be achieved due to significant yield occurring in the material. Remarkably, the onset of plasticity in polymeric materials is very poorly quantified, despite its importance as the ultimate limit of purely elastic behavior. Here we report the results of a novel experiment which is extremely sensitive to the onset of yield and discuss its impact on measurement and elastic theory. In particular, we use an elastic instability to locally bend and impart a local tensile stress in a thin, glassy polystyrene film, and directly measure the resulting residual stress caused by the bending. We show that plastic failure is initiated at extremely low strains, of order 10 −3 for polystyrene. Not only is this critical strain found to be small in comparison to bulk measurement, we show that it is influenced by thin film confinement -leading to an increase in the critical strain for plastic failure as film thickness approaches zero.

Bundling and crosslinking of actin networks provide cells with tensile strength and mobility and play critical roles in diverse mechanical processes such as wounding healing and fertilization. One simple mechanism for bundling and crosslinking actin networks is using divalent counterions such as Mg 2+ to form crossbridges between actin filaments. Counterion crossbridges can also be exploited in materials engineering to alter the mechanics of polyelectrolyte materials. As such, the mechanical properties that counterion crossbridges confer to actin networks needs to be explored. Here, we use optical tweezers microrheology to characterize the mechanical response of actin networks in the presence of varying concentrations of Mg 2+. We couple mechanics to structure by using confocal microscopy to characterize the mobility and architecture of networks. We show that only modest bundling and network rearrangement is required to induce dramatic increases in the elasticity and stiffness of the networks. We further show that bundles are resilient to nonlinear forcing and exhibit surprisingly minimal dissipation above a critical counterion concentration. Finally, we demonstrate that while crosslinking of network fibers plays an important role in linear regime mechanics, filament bundling dictates the nonlinear response.