Biomedical Materials Based on Electrospun Polymeric Fibers
Type of Degreedissertation
Polymer and Fiber Engineering
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As one of the most promising development of nanotechnology, electrospinning has gathered a great deal of interests in recent years. In this study, several novel nanomaterials were prepared by electrospinning and further modifications for future biomedical applications. Four projects were majorly covered in this dissertation with different focus on drug delivery or tissue engineering. In the first project, poly(D,L-lactide) (PDLLA) was electrospun into ultrafine fibers and loaded with tetracycline (TC) or chlorotetracycline (CTC) as model drugs. The influence of a co-solvent (methanol) at various concentrations was studied regarding physical properties, morphology and in vitro release profiles of the drugs from the PDLLA nano-fibers. The results showed that, for both drugs, electrospun fiber diameters decreased with increasing amounts of co-solvent, while water contact angles and drug loading efficiency increased. However, the two drugs exhibited considerably different release mechanisms. The results indicated that the concentration of methanol changed the release profiles mainly based on the morphology of the resultant nano-fibers and the polymer/drug/solvent interaction during the electrospinning and drug release process. In the second project, crystalline poly(L-lactide) (PLLA) nanoparticles were prepared by aminolysis of electrospun PLLA nanofibers and subsequently labeled by a fluorescent colorant. The size of the nanoparticles could be controlled by either the conditions of the aminolysis reaction or the diameter of the original electrospun fiber. The latter method resulted in higher yield. Although the as-spun nanofibers were generally amorphous, the nanoparticles showed high crystallinity in the typical α-form of PLLA crystals. After aminolysis, PLLA nanoparticles spontaneously generated amine groups on the surface, which are available for further modifications. In this study the amine groups were reacted with isothiocyanate groups, and fluorescein-5’-isothiocyanate was successfully attached to the PLLA nanoparticles. Smaller particles showed significantly higher fluorescein binding density. Through this simple “top-down” routine, it was possible to create nanoparticles with tailorable size and specific surface functions. Such materials could potentially serve in bioimaging or nanomedicine applications. In the third project, efforts were made to prepare nanofibrous poly(D,L)-lactide mats by electrospinning. However, it was observed that these mats tend to shrink under physiological conditions. Thus, a physical entrapment method to modify the polymer surface with poly(ethylene glycol) was developed to ensure dimensional stability and to increase the hydrophilicity of the surface of the mats. Nanofiber morphology was characterized by scanning electron microscopy. Surface element analysis was performed by high resolution X-ray photoelectron spectroscopy. Water contact angles were determined to identify surface properties before and after surface entrapment. Canine fibroblasts were prepared and seeded onto the poly(D,L)-lactide mats, followed by cell viability tests by MTT assay, which confirmed the improvement of biocompatibility by surface modification. Taking the results into account, hydrophilic and area-stable nanofibrous nonwoven mats were successfully produced, with potential applications as tissue engineering scaffolds. In the last project, electrospun polymeric nanofibers were surface modified or surface coated chemically. As the first approach, electrospun poly(ε-caprolactone) was surface etched and further attached with biomolecules, e.g. chitosan and collagen. X-ray photoelectron spectrum confirmed the success of surface modification. For the second approach, polypyrrole (Ppy), which is a conducting polymer, was coated on electrospun PDLLA nanofibers via aqueous in-situ polymerization. The coating was characterized by electron microscopes and infrared spectroscopy. The resulted core-shell fibers had a wall thickness of 40-45 nm. By further removal of PDLLA, Ppy nanotubes were successfully fabricated. In summary, chemically modified electrospun nanofiber can provide unique surface properties, which is critique for tissue growth and regeneration.