Materials Science of the Cell
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MRS BULLETIN/OCTOBER 1999
portant insights into cell functions. Some of the most recent and important advances in cell biology have their origins and foundations in physics. At the same time, cellular materials such as cytoskeletal proteins (actin, microtubules, etc.) and DNA have provided physicists and materials scientists with unique model systems to resolve longstanding issues and test fundamental a s s u m p t i o n s in colloidal science and polymer physics. For instance, thanks to the macroscopic dimensions of biopolymeirs, which can be probed relatively easily using light microscopy, "reptation" (snakelike curvilinear diffusion) of polymers in concentrated solutions using actin 6 and the effect of shear on the dynamics of individual polymers using DNA7 were observed for the first time directly, with sometimes counterintuitive results. Conversely, detailed understanding of DNA and protein microelasticity have led to advances in our understanding of DNA condensation, protein folding, and cellular mechanics. Cells have p r e s e n t e d to scientists a n a b u n d a n t source of inspiration for the design of new materials. For instance, researchers have recently produced thermally reversible hydrogels and nanoscale conduits a n d networks from genetically engineered proteins and fluid-lipid bilayers. 89 This special issue of MRS Bulletin demonstrates the diversity of activity in the field of materials science of the cell. The topics in this issue include biophysical characterization of the activities of living cells, the application of cell-like models for the design of novel materials, and the use of reconstituted cell-free systems for new insight into cell biophysics. First, Thoumine and Ott review novel experimental approaches that recently have been used to quantify cell migration, cell adhesion, and the associated changes in cell micromechanics. These authors make a strong case for research approaches that couple cell biochemistry and cell mechanics to enhance our understanding of cell functions as they re-
late to development and disease. Due to the inherent complexity of living organisms and biological macromolecular assemblies, biophysics as a field has seen a surge in the development of novel tools able to quantify the physical properties of cells and cell components. These tools include single-cell mechanical and optical micromanipulators, 10 single-particle tracking microrheometers, 1112 optical 13 and magnetic tweezers,1"1"'6 and singlemolecule spectroscopy.17-19 In a relatively recent trend, these tools are applied to biological macromolecules and living cells. Next, Boal reviews the physical properties of the structural scaffolding of the cell: cytoskeletal and membraneattached filamentous proteins. Because of their tightly regulated lengths and polymerization properties, these polymers provide the cell with extraordinary versatility to modulate its local and global mechanical properties. These highly interconnected polymers may also allow a very effective way to transmit mechanical stresses,
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