Synthesis and Modeling of Poly(L-lysine) Based Biomaterials for Regenerative Medicine

Abstract

The need for tissue engineered scaffolds is growing due to the shortage in organ donation, the immunoreactions to allotransplants, and the high cost associated with transplantation. This work focuses on the material selection and processing which are keys for a successful design of any tissue engineered structure. Poly(L-lysine) (PLL) was selected in this work as a base for developing scaffolds due to its biocombatability and bioabsorbability. PLL, on the other hand, has limitations in use due to its hydrophilicity that weakens its structures in aqueous and physiological conditions. To overcome these limitations, two hypotheses are suggested in this dissertation; the first hypothesis is that a micro-scale composite of PLL with a high crystalline material can enhance the final properties of PLL. The second hypothesis is that the properties of PLL structures can be controlled by forming a molecular-scale composite with another crosslinked bioresorbable polymer. To attest these hypotheses, nanocomposite materials from PLL and microcrystalline cellulose (MCC) were produced and processed in different ways. Also, molecular-scale composites of PLL and networked structure of poly(L-lactide) (PLA) were produced. The experimental results for the synthesized PLA were then compared to the data obtained from molecular modeling techniques. Results of PLL with MCC show intimate composite structures with attractive electrostatic forces between the components. Processing condition of PLL was found to affect its secondary structure with α-helix secondary structure for samples prepared at pH 7. Inclusion of hydrolyzed MCC particles resulted in increasing the crystallinity of the semi-crystalline PLL which enhanced the swelling properties of the produced scaffolds. To produce branched PLA, a novel solution-based ring opening polymerization method was introduced and the properties of the produced polymers were studied. Thermal properties of PLA indicated a double melting behavior which was shown to be a result of the concurrence of crystallization and melting of the polymer chains. Chemical neutralization of PLL salt was able to dissolve PLL in organic solvent where the branched PLA was crosslinked. The structure and energy of lactide were predicted using electronic modeling techniques and results matched to a reasonable extent the experimental values obtained from X-ray single crystallography. The path of the ring opening polymerization (ROP) of PLA was modeled and its activation energy was calculated. Molecular dynamic (MD) simulation was able to predict the glass transition temperature of PLA with about 3 ˚C margin of error.