INTELLECTUAL MERIT: Gels of filamentous proteins such as actin, microtubules, and intermediate filaments determine the mechanical properties of most cells, and their dynamic rearrangement underlies critical cell processes such as migration and division. Similarly, the mechanical and transport properties of the extracellular matrix are, in many contexts, determined by networks of filamentous proteins such as collagen and fibrin. These networks share a few common features: The polymers are sufficiently rigid that they are not well described by rubber elasticity theory, the polymers themselves make up a small fraction of the total gel volume, and their biological functions are mediated by a vast array of associated proteins. Despite their importance, there is no reliable quantitative theory for the propagation of stresses in gels that is appropriate for most biopolymer networks. This proposal describes a series of experiments designed to directly measure the motion of filaments in biopolymer networks and their response to controlled, localized perturbations. The motion of the biopolymers and the solution will be independently assessed, with the goal of providing a basis on which to develop and test models of stress transmission and material transport in biopolymer networks. The measurements will determine the conditions under which the deformation field of the network and the displacements of the solution are accurately described by continuum viscoelasticity, and provide the basis for extensions of continuum theory into regimes of heterogeneous response. These measurements are made possible by a unique high-speed confocal microscope with integrated laser tweezer that is operational in the lab of the PI. The work will focus initially on actin and collagen networks, with properties controlled by varying concentration, filament length, filament rigidity (through filament bundling), and cross-linker and bundler type and density. In the last phase of the project, mixed networks of microtubles (rigid) and actin (semi-flexible) will be investigated, with and without passive and active (motor protein) linkages.
BROADER IMPACTS: The broader impacts of the proposed activity include training an interdisciplinary workforce and furthering the translation of quantitative techniques to biological researchers. The PI and co-PI have recently initiated a Soft Matter Seminar Series that involves speakers from Georgetown Physics and Chemistry Departments, as well as from NIH and NIST, and will soon broaden from there. With University support, they have launched the Mid-Atlantic Soft Matter Workshop, with the inaugural event held at Georgetown on November 30, 2007. This highly interdisciplinary workshop drew upon researchers from academic, industrial, and national laboratories in the Mid-Atlantic Region. The PI has also established a strong working relationship with Tecnologico de Monterrey in Mexico which involves student exchanges in both directions. Students working on this project are thereby afforded a wealth of scientific contacts extending well beyond the borders of their university. The PI is the co-director of the new Georgetown University Program on Science in the Public Interest (SPI). SPI was conceived with the view that Georgetown in uniquely positioned to educate a new generation of citizen scientists, capable of bridging the gap between science and public policy. SPI is designed to give science students hands-on exposure to the workings of the Government and to societal problem-solving. Formed in 2005, it now has an annual budget of over $150,000, and the PI is actively engaged in fundraising to expand its activities to reach a larger audience. Two seminar courses are offered, Shaping National Science Policy and Science and Society: Grand Challenges, and students are placed in internships in diverse organizations including the Nuclear Regulatory Commission, the AAAS Center for Science, Technology, and Security Policy, and the National Academies Board on Global Health. They organize an annual Congressional Visit Day, a day-long event divided between presentations on the process of creating legislation, science policy discussions, and visits with staff from individual offices representing the home districts of the participating student.
In this project we studied the mechanical properties of protein-based gels, such as those found in biological connective tissues. We found that the unusual stiff fiber structure of these gels produced novel behavior, including rigidity that is very sensitive to the size of the gel. We were able to relate this dependence to the microscopic structure of the gel, specifically the pore size in the fiber network. These results have implications for characterization of biological materials, because they suggest that stiffness values depend on the measurement system, and for the design of biomaterial-based devices, such as medical implants. We also showed that stresses in biopolymer gels are distributed very non-uniformly, and that the heterogeneity is closely linked to gel stiffening and ultimately to its failure. These results shed light on the way tissues and other biomaterials function in dynamic environments. We have also studied the density variations in gels using a combination of high speed microscopy and quantitative image analysis. We have found that we can independently measure the properties of the gel itself and the coupling between the gel and the fluid that takes up most of the internal space. This provides a new approach for non-invasively characterizing the properties of soft materials, such as those found in biology. Through this project we trained three undergraduate researchers, two graduate students, and two postdoctoral fellows.
