Tuning Morphology and Mechanics of Fibrin Assemblies

University Of North Carolina At Chapel Hill, Chapel Hill NC

Investigators

Abstract

Fibrin is one of a class of fibrous protein materials that exhibit levels of strength, elasticity and toughness that rival current engineering materials. Fibrin fiber networks form the structural framework of blood clots. In recent studies we have found that fibrin fibers can be stretched to over 2-fold and relax to their original lengths, and exhibit very large breaking strain limits, with an average extensibility of over 300%. Additionally, we have discovered a new form of fibrin assembly: the monomolecular sheet. These sheets also exhibit very high strain limits, and self-assemble unsupported over areas of hundreds of square microns. That fibrin is capable of forming mechanically robust assemblies in these distinct morphologies indicates that its polymerization mechanisms are flexible. This is somewhat unexpected behavior for a biomaterial that is optimized for highly specific morphological forms assembled in particular physiological conditions. This protean property of the fibrin molecule suggests that it holds design principles that could be applied to engineered polymeric materials. However, even in fibrin?s standard physiological form, many questions remain regarding its detailed architecture and polymerization dynamics. For the newly discovered fibrin morphologies such as the monomolecular sheet, next to nothing is known of their molecular scale structure, assembly mechanisms, or mechanical properties. To address these questions, we will pursue two central tracks of investigation: 1. The evaluation and control of morphology, and 2. The evaluation and control of the mechanics of the resulting assemblies. Central to both tracks is the employment of recombinant protein engineering to design and synthesize variant proteins that will enhance or repress specific monomer-monomer interactions as well as lengthen or shortened the putative ?stretchable? regions of the protein. Control over fibrin morphology will be developed through systematic investigation of the effect of biochemical and environmental variables; we will test a range of engineered recombinant fibrin variants as well as fabricate custom nanoscale and microscale sub-structures on which to assemble fibrin. Using nanomanipulation techniques, we will evaluate the stress vs. strain properties of single fibers, investigate the influence that single fiber properties have on fiber networks, and characterize the mechanical properties of the monomolecular sheets. Custom designed protein variants will be employed to tune the stress vs. strain behavior of these assemblies. Our studies will provide new design strategies for custom tuned mechanical properties coupled with morphological control. Successful completion of this project will produce design principles that, along with directly informing fibrin based materials, will more generally provide new avenues for polymeric materials engineering. Success will also provide a complete picture of the molecular scale details of fibrin mechanics, and will lend new and valuable perspective to healthy as well as pathological blood clotting. Our project also includes an extensive education and outreach component. We will hold public outreach events as well as participate in K-12 student and teacher outreach activities including at least one regional and one national science teacher meeting each year. Each summer, we will recruit 2 students from programs promoting minority recruitment for research.