Session H46: Invited Session: Polymer Physics Prize

Polymer Physics Prize Lecture: Polyelectrolyte complexes: New routes to useful soft materials

Mixtures of oppositely charged polyelectrolytes may form precipitates (phase-separated solids) or coacervates (phase-separated fluids). Coacervates have been known for a long time to have interesting properties such as very low interfacial tension with water and a resultant ability to coat surfaces, engulf particles and invade porous media. Most prior work on coacervate complexes has been done with structurally complex (e.g., gum Arabic), biologically derived macromolecules (e.g., gelation). Our work is focusing on phase behavior and self-assembly in classes of structurally simpler polymers. Polypeptides are one such class, where we can produce anionic, cationic and neutral, water-soluble polymers all with the some backbone and varying in small side-group structures. We are able to demonstrate very general patterns in phase behavior over different members of this class of polymers. Coacervate formation is the rule rather than the exception in these materials, with such formation quite strongly peaked at balanced stoichiometry of the polyelectrolyte components. One molar salt is usually sufficient to dissolve the coacervate phases that form. Block copolymer mixtures containing oppositely charged blocks can form self-assembled structures: micelles with diblocks and hydrogels with triblocks. The structure and properties of these assemblies can be tuned based on knowledge of the bulk phase behavior response to molecular weight, stoichiometry and salt concentration. Examples of phase behavior and structure-property relationship will be discussed.   

Physical and Biological Properties of Engineered Protein Hydrogels

Injectable hydrogels show substantial promise for use in minimally invasive tissue engineering and drug delivery procedures. A new injectable hydrogel material, developed from recombinant telechelic proteins expressed in \textit{E. coli}, demonstrates shear thinning by three orders of magnitude at large strains. Large amplitude oscillatory shear illustrates that shear thinning is due to yielding within the bulk of the gel, and the rheological response and flow profiles are consistent with a shear-banding mechanism for yielding. The sharp yielding transition and large magnitude of the apparent shear thinning allow gels to be injected through narrow gauge needles with only gentle hand pressure. After injection the gels reset to full elastic strength in seconds due to rapid reformation of the physical network junctions, allowing self-supporting structures to be formed. The shear thinning behavior is largely independent of the midblock length, enabling genetic engineering to be used to control the equilibrium modulus of the gel without loss of the characteristic yielding behavior. The shear-banding mechanism localizes shear stresses during flow into narrow regions of the gel, allowing more than 95{\%} of seeded cells to survive the injection process.   

Enhancing Biopolymer Dynamics through Destruction

Microtubules are cytoskeletal filaments that organize intracellular space structurally and through active transport along their lengths. They need to be organized and remodeled quickly during development of differentiated cells or in mitosis. Much work has focused on remodeling from the ends because these long polymers can stochastically disassemble through dynamic instability or be actively disassembled. Microtubule-severing enzymes are a novel class of microtubule regulators that create new ends by cutting the filament. Thus, these proteins add a new dimension to microtubule regulation by their ability to create new microtubule ends. Interestingly, despite their destructive capabilities, severing has the ability to create new microtubule networks in cells. We are interested in the inherent biophysical activities of these proteins and their ability to remodel cellular microtubule networks. Interestingly, despite their destructive capabilities, severing has the ability to create new microtubule networks in cells. We use two-color single molecule total internal reflection fluorescence imaging to visualize purified severing enzymes and microtubules \textit{in vitro}. We have examined two families of severing enzymes to find that their biophysical activities are distinct giving them different network-regulating abilities.