The need for completely amorphous elastomeric biomaterials with tunable thermal, mechanical and degradation properties is significant. While crystalline materials are suitable for several biological applications, in many cases, implanted tissue engineering scaffolds, drug delivery depots and in vivo sensing materials are in mechanically dynamic environments in the body and must sustain and recover from various deformations without mechanical irritations to the surrounding tissues. In addition to decreasing tissue damage, elastomeric materials would also address needs ranging from tissue engineering scaffolds with desired mechanical properties to completely amorphous drug delivery devices with faster controlled release profiles to flexible mechanical devices such as drug eluting stents. Despite the recognized importance of elastomeric biomaterials, there have been only a few examples reported in the literature. In this project, synthetic strategies will be developed that will lead to a variety of new elastomeric biomaterials. Specifically, the following will be accomplished: 1) the design of synthetic routes that yield monomers capable of being polymerized via step growth reactions, 2) the synthesis of a variety of polyester, polyester ether and polyester urethane homopolymers, copolymers, thermoset and thermoplastic elastomers, and their polar functionalized derivatives and 3) the study of the structure-property relationships for these materials, examining thermal, mechanical, solubility, processing, biocompatibility and biodegradation characteristics. While the initial thermal and mechanical property analysis will be conducted in laboratories at UNC, further bidegradation and biocompatibility analysis of these materials will be accomplished in collaboration with Professor Moo Cho in the UNC School of Pharmacy. Processing will be completed in collaboration with Professor Joseph DeSimone in the UNC department of chemistry. A detailed analysis of mechanical properties, particularly related to shape memory behavior will be a collaborative effort with Professor Ken Gall at Georgia Tech. Collaborations with colleagues at UNC and GT will combine our expertise in polymer synthesis, with characterization, processing and biomaterials knowledge to make significant contributions to this area of research.

NONTECHNICAL SUMMARY: The biomaterials currently used in applications such as drug delivery stents and tissue regeneration scaffolds typically are rigid materials that often cause damage to the surrounding tissues. The new materials that will result from this project will have properties that are more compatible with soft tissue. Despite the recognized importance of designing these soft elastomeric degradable biomaterials, there have been few examples reported in the literature. In this research, synthetic strategies will be developed that lead to a variety of these types of new elastomers. The success of these materials will have a significant impact on this area of advanced biomaterials by providing methods for producing elastomers as well as for tailoring the chemical functionality, physical, mechanical and biological properties. This project will also promote teaching and training of graduate students, undergraduates and high school student researchers. Each will be presented with intriguing, relevant problems that can be answered in the laboratory using organic synthesis and analytical chemistry. The research results will be incorporated into the teaching of undergraduate organic and of graduate polymer chemistry to help students appreciate the utility of basic concepts. The major outreach efforts of this project include continuing work in a recently established UNC Chapter of the National Organization of Black Chemists and Chemical Engineers for which the PI is the faculty advisor. The PI will also continue to serve as a mentor in the Project SEED program, which encourages economically disadvantaged high school students to pursue career opportunities in the chemical sciences.

Project Report

PI: Valerie Sheares Ashby University of North Carolina at Chapel Hill, Department of Chemistry Intellectual Merit. A structure-property approach toward the synthesis of novel biomaterials was investigated. The new materials include polyester-based elastomers with similar mechanical properties to ligaments and vascular structures. These materials possess a unique combination of features. Specifically, they are liquid at room temperature, readily crosslinked and processed, biocompatible, biodegradable and functionalized. Amine-functionalized poly(butadiene) materials were also prepared and evaluated for their effectiveness as gene delivery vectors. The cytotoxicity, polyplex formation and transfection efficiency were determined and structure-property relationships established. More recently, these studies were extended to include shape memory polymers. Shape memory polymers (SMPs) are a unique class of smart materials that have gained widespread interest in applications such as minimally invasive implants and self-deploying medical devices. In spite of their attractive properties, SMPs have been limited to structures without reactive functionality, fundamentally hindering their utility in applications that would benefit from physically and chemically dynamic materials. We have produced the first examples of shape memory polymers possessing such multi-functionality that demonstrate the ability to control their transition temperatures, surface features, and surface chemistries. Specifically, these materials exhibit shape memory properties on the macro, micro, and nanoscales and can be chemically modified to present a variety of surface moieties (e.g. hydrophilic, hydrophobic, low, and high molecular weight groups). These materials are uniquely capable of both physically changing surfaces from one geometry to another and switching their surface chemistries. Thus, these materials are truly multi-functional, expanding the range of properties of SMPs and their potential applications in the fields of chemistry, materials, and biomedicine. The lack of cytotoxicity of these materials and their slow degradation makes them appealing potential biomaterials. In combination with our group’s previous study of cells on dynamic SMP substrates, these materials enable another level of investigation of the reaction of cells to changing physical and chemical environments of the researcher’s choosing. Broader Impacts. This project has advanced the understanding of elastomeric biomaterials, while promoting teaching and training. Graduate students have learned how to solve problems by starting with monomer synthesis moving to the polymer synthesis then to the material properties and applications. Undergraduates and high school student researchers were presented with intriguing, relevant problems that were answered in the laboratory using organic synthesis and analytical chemistry. The multidisciplinary interaction was an educational opportunity for all students, as they were active participants in the collaborative research. The research results were incorporated into the teaching of undergraduate organic chemistry and of graduate polymer chemistry to help students appreciate the utility of basic concepts. The PI has also been involved in numerous presentations to underrepresented groups of undergraduate and graduate students interested in research and/or teaching careers.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Andrew J. Lovinger
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University of North Carolina Chapel Hill
Chapel Hill
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