Electrospun Conducting Polymer Nanofibers for Biomedical Applications

Abstract

Conducting polymer (CP) nanofibers have recently received great attention due to their high surface area per volume and extensive porosity, combined with unique properties such as high electrical conductivity or fluorescence. These materials are being considered for a range of novel applications, including biomedical applications. Among the techniques used for the preparation of polymer nanofibers, electrospinning is a simple, fast and relatively cheap technique. The focus of this thesis is to develop conducting polymer micro/nanofibers with a well-defined morphology using electrospinning and to investigate their potential in two areas of biomedical applications: tissue engineering and DNA sensing. Two classes of conducting polymers, polyaniline (PANI) and poly(p-phenylene vinylene) (PPV), were used in this study to produce CP nanofibers. Soluble copolymers of aniline (ANI) and m-aminobenzoic acid (m-ABA) were synthesized to improve the solubility of PANI. The properties of these polymers were characterized and studied using a range of techniques. The solubility of the copolymers increased with an increase in the m-ABA content. Conversely, the conductivity of the copolymers was lowered. The average molecular weight of the copolymers, as determined by gel permeation chromatography, was found to decrease from 13,800 to 1,640 g mol-¹, with an increase of m- ABA content in the copolymer from 0.2 to 0.8. By contrast, FT-MS results revealed that homopolymerization of m-ABA formed oligomers, rather than polymeric chains. Based upon a consideration of the solubility and electrical conductivity of the copolymers, an ANI to m- ABA copolymer ratio of 60/40 was chosen for electrospinning with the biocompatible and biodegradable polymer poly(lactic acid) (PLA). These polymers were electrospun with mean fiber diameters of 100-400 nm. FTIR, Raman spectroscopy, and conductivity measurements confirmed the incorporation of conducting (co)polymers within the PLA based nanofibers. The elastic modulus of a single nanofiber was examined using a nanoindenter instrument for the first time. The nanoindentation results obtained on the individual nanofibers revealed that the elastic moduli of the nanofibers were much higher at the surface than in the inner fiber core. These fibers thereby provide cells with stiff, sub-micron sized fibers as anchoring points on a substrate of high porosity. The conductive nature of these composite nanofibers offers exciting opportunities for electrical stimulation of cells. Human adipose derived stem cells (hASC) were used in this work to evaluate the biocompatibility of the nanofibers - an important characteristic of a scaffold in tissue engineering. The cell culture results showed that the composite nanofibers supported hASC adhesion and proliferation to a similar degree as control surfaces, namely electrospun PLA nanofibers and tissue culture treated glass substrates (TCS). Depending on the fiber composition, the cells initially displayed some variation in the extent of focal adhesions (FAs) after three days of culturing, but after one week all of the samples showed similar cell densities and morphologies. A luminescent conducting polymer, a PPV derivative, poly(6,6'-((2-methyl-5-((E)-4-((E)- prop-1-en-1-yl)styryl)-1,4-phenylene)bis(oxy))dihexanoic acid) (PDMP), was electrospun into nanofibers using the same method as described above. PLA was again chosen for electrospinning with PDMP in various PLA/PDMP compositions. The morphology of the novel PLA/PDMP composite nanofibers was studied extensively using a scanning electron microscope (SEM). The composite nanofibers were also used to construct a simple oligonucleotide (ODN) sensor, where capture probe ODNs (capODN) were covalently grafted onto the residual carboxylic acid functionalities of the composite nanofibers. The DNA sensing results revealed that significant non-specific interactions occur, which can be prevented to some extent by changing the dye attached to the signal probe. The results also indicate the potential of such nanofibers to be used as biodegradable biosensor.