Our research interests concern the fundamental physics of soft matter, of which biological materials are principal examples. While our understanding of the single-molecule properties of many of the key building blocks of the cell is relatively advanced, our understanding of the basic physics governing biological assemblies from the nanometer to the micrometer scales is, by comparison, still rudimentary. Many sub-cellular structures involve the coordinated assembly of disparate elements such as soft membranes and stiff filamentous proteins. Our specific research interests include the structural, mechanical and dynamic properties of these and other soft materials.

Topics of current interest:

• The cytoskeleton of cells

• Biopolymers and their networks

• Other soft matter

Some recent highlights: D Mizuno, C Tardin, CF Schmidt, FC MacKintosh,  Nonequilibrium mechanics of active cytoskeletal networks.   Science, 315:370 (2007). Cells both actively generate and sensitively react to forces through their mechanical framework, the cytoskeleton, which is a nonequilibrium composite material including polymers and motor proteins. We study the dynamics and mechanical properties of a simple model cytoskeleton and show that stresses arising from motor activity control the network mechanics, increasing stiffness by a factor of nearly 100 and qualitatively changing the viscoelastic response of the network. We present a quantitative theoretical model connecting the large-scale properties of this active gel to molecular force generation.   PA Janmey, ME McCormick, S Rammensee, J Leight, P Georges, and FC MacKintosh, Negative normal stress in semiflexible biopolymer gels.   Nature Materials, 6:48 (2007). When subject to stress or external loads, most materials resist deformation. Any stable material, for instance, resists compression—even liquids. Solids also resist simple shear deformations that conserve volume. Under shear, however, most materials also have a tendency to expand in the direction perpendicular to the applied shear stress, a response that is known as positive normal stress. Here, we show that networks of semiflexible biopolymers such as those that make up both the cytoskeleton of cells and the extracellular matrix exhibit the opposite tendency: when sheared between two plates, they tend to pull the plates together. We show that these negative normal stresses can be as large as the shear stress and that this property is directly related to the nonlinear strain stiffening behavior of biopolymer gels.   CP Brangwynne, FC MacKintosh, S Kumar, NA Geisse, J Talbot, L Mahadevan, KK Parker, DE Ingber, and DA Weitz,  Microtubules can bear enhanced compressive loads in living cells because of lateral reinforcement.   Journal of Cell Biology, 173: 733 (2006).   M Atakhorrami, GH Koenderink, CF Schmidt, and FC MacKintosh,  Short-time inertial response of viscoelastic fluids: Observation of vortex propagation. Physical Review Letters, (2005). 95: 208302.   TB Liverpool and FC MacKintosh,  Inertial effects in the response of viscous and viscoelastic fluids. Physical Review Letters, (2005). 95: 208303.