Chase Broedersz

Recent experiments on F-actin with the physiological cross-linker filamin have demonstrated several striking features; while their linear modulus is significantly lower than for rigidly cross-linked actin systems, they can nonetheless withstand remarkably large stresses and can stiffen by a factor of 1000 with applied shear.

We have shown that this behavior originates in the highly flexible nature of the filamin cross-links. To describe these systems we developed a self-consistent mean field theory for the macroscopic nonlinear elasticity of these networks. The networks are modeled as a collection of randomly oriented rods connected by flexible linkers to a surrounding elastic continuum, which is required to self-consistently represent the behavior of the network. We have confirmed the main predictions of this model in collaboration with experimentalist at the Weitzlab (Harvard).

Biophysical Journal 99: 1091 (2010)
Phys. Rev. E 79, 061914 (2009)
Phys. Rev. E 79, 041928 (2009)
Physical Review Letters, 101: 118103 (2008)

One essential feature setting biopolymer networks apart from rubber-like materials is the intrinsic dynamics of their cross-links. This can have important implications for cells, where their internal networks are constantly remodeling, reflecting the transient nature of their cross-links. Recent experiments on actin networks with transient linkers provide evidence of a complex viscoelastic behavior.

To describe these systems we developed a microscopic model for the long time network relaxation that is controlled by cross-link dynamics. This cross-link governed dynamics (CGD) model describes the structural relaxation that results from many independent unbinding and rebinding events. We derived a set of nonlinear stochastic differential equations describing the time evolution of the dynamics of the polymers in the network. Using a combination of Monte Carlo simulations and a mean field approximation, we showed that this type of cross-link dynamics yields a novel power-law regime in the rheology. Our model is in excellent quantitative agreement with experiments.

Physical Review Letters, 105: 238101 (2010)

One of the hallmarks of biopolymer gels is their nonlinear viscoelastic response to stress, making the measurement of the mechanics of these gels very challenging. Using both strain ramp and differential prestress protocols, we investigated the nonlinear response of a variety of systems ranging from extracellular fibrin gels to intracellular F-actin solutions and F-actin cross-linked with permanent and physiological transient linkers. In particular, we designed a new protocol consisting of DC positive shear stresses of varying magnitude alternated with periods without load. The total strain and differential response are monitored continuously.

Surprisingly, the nonlinear response measured with the prestress protocol is insensitive to creep. This demonstrates that the nonlinear mechanical response of these biopolymer networks is robust, even when the network is flowing. To provide insight in these observations, we developed a simple, yet very general phenomenological model that includes the nonlinear elasticity of the network as well as network flow on long time-scales.

Soft Matter 6 4120 (2010)

Intermediate filament (IF) networks in the cytoplasm and nucleus are crucial for the mechanical integrity of metazoan cells. While filamentous actin and microtubules have been extensively studied, much less is known about IFs. In particular, the mechanism of cross-linking in these networks and the origins of their mechanical properties are not understood.

In close collaboration with the Weitzlab (Harvard), we have shown that divalent ions can mediate a cross- linking interaction between the negatively charged tail domains of intermediate filaments. We used an affine model for the nonlinear elastic response of these systems, which includes both the entropic stiffening and the enthalpic stretching of the individual filaments, as well as geometric affects that arises in networks under large shear deformations. This model allows us to extract microscopic parameters from the measured macroscopic rheological behavior, including the Young's modulus and the persistence length of the filaments, and the cross-lining length scale of the network.

Journal of Molecular Biology 399, 637-644 (2010)
Biophysical Journal, 98 2147-2153 (2010)
Physical Review Letters, 104: 058101 (2010)

Reconstituted active filamentous actin networks with motor proteins form a good model system for cellular mechanics. The motor proteins generate forces that drive the network far from equilibrium and strongly affect the network mechanics. In some cases, the macroscopic nonlinear response of a passive network to an external shear is due to a transition between soft bending modes to stiffer stretching modes. The question arises how stress generating molecular motors couple to such a network and how they affect the macroscopic elastic response.

To address this issue, we developed a lattice-based approach to design networks with a connectivity of 4, mimicking the architecture of biopolymer networks with binary cross-links. We showed how heterogeneous internal stresses generated by motors can lead to stiffening in networks that are governed by filament bending modes. The motors are modeled as force dipoles that cause muscle-like contractions. These contractions “pull out" the floppy bending modes in the system. Through this mechanism, motors can lead to a nonlinear network response.

arXiv:1009.3848

As a part of my undergraduate research I co-developed Hydrogenography, a new combinatorial optical thin-film method to explore novel alloys for hydrogen storage. This method exploits the metal-insulator transition induced by hydrogen absorption. When a thin metal film absorbs hydrogen it behaves as a switchable mirror; the optical properties of the material change dramatically, from reflecting in the metallic state to transparent in the hydride state. Hydrogenography provides a method to screen the thermodynamic and kinetic properties of hydrogenation of thousands of alloy compositions simultaneously.

To provide insight in the origin of the rich thermodynamics of the hydrogenation behavior we measured in the Mg-Ni-Co phase diagram, we performed ab initio density functional theory calculations. This method allows us to calculate the stability and electronic structure of various phases of these systems. This work was performed at the University of Oslo.

Phys. Rev. B 77 024204 (2008)
Advanced Materials, 19, 2813 - 2817 (2007)
Scripta Materialia 56 853-858 (2007)