Collaborative Research: Mechanics and Microrheology of Biomimetic Materials

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This award by the Biomaterials program in the Division of Materials Research in support of the collaborative efforts by University of California Los Angeles and University of California Irvine is to study the interaction between the nonequilibrium dynamics of molecular motors and the elastic nonlinearities of filamentous actin (F-actin) and determine the collective mechanical properties of the network, with coordinated experimental/theoretical approaches to address each of these challenges. The cytoskeleton of living cells is built primarily from cross-linked F-actin that, in living cells, is generically tensed by molecular motors such as myosin. The mechanical properties of this filament network have been shown to have a complex dependence on the state of activity of these molecular motors, and depend on a combination of the mechanics of the individual filaments, their network structure, and the non-equilibrium steady-state of the network. Understanding in detail how the mechanics of this network can be controlled by its internal stress state (imposed by the endogenous molecular motors) will enable us to better understand how cells control their mechanics and morphology, develop an understanding how cells sense and exert forces on their surroundings. To date, this field has focused on relating the equilibrium collective (non-)linear response properties of a material to the molecular structure of its constituents. With this award, F-actin networks associated with the air/water interface of a Langmuir monolayer will be studied. These 2D networks will then be tensed by molecular motors, and studied using both macro- and microrheology to elucidate the underlying relationship of network architecture (observed through fluorescent labeling of some of the filaments) and non-equilibrium stress state to its collective mechanics. The (quasi-) two-dimensional nature of the network allows for the direct observation of the local network structure, strain state, and provides a way to rapid in situ chemical modification of the system. Experiments on these biopolymer networks could provide insight about the active control of the nonequilibrium steady-state of biopolymer networks that allow the creation of a gel having reversibly tunable mechanical properties. Understanding this prototypical cytoskeletal biopolymer network may allow the development of novel biomimetic active materials with addressable mechanics. Teaching and training of graduate and undergraduate students in experimental and theoretical aspects of biophysics of soft materials, and developing a web site for the interpretation of microrheology are other parts of this award.Human cells are pervaded by a stiff biopolymer network that acts, much like the skeleton of our bodies, to maintain cellular shape and to allow the cell to exert forces on its environment through the action of molecular motors acting on this cytoskeleton. Recent advances have made it possible to deconstruct and then rebuild the principal structural elements of the cytoskeleton in the laboratory. With this award, the mechanical properties of this biopolymer network are measured, and will explore the relationship between network structure, molecular motor activity and large scale mechanics in these protein filament networks. The principal importance of this work is that these studies would provide better understanding how cells use molecular motors to exert forces on their environment and how the activity of these motors can modify the stiffness of the network in a reversible way. This understanding will help to elucidate fundamental design principles by which one may build artificial active materials that use nanomachines (i.e. molecular motors) to actively control their mechanical properties. Students, both graduate and undergraduate, will be trained in research activities that are related to biopolymer networks and their reversibly tunable mechanical properties.