Soft Template-Directed Polymerisation of Polypyrrole for Tuneable Dexamethasone Delivery

Abstract

Introduction: Conducting polymers (CPs) are electroactive materials which can be loaded with drugs with release tuned by electrical stimulation. To enhance the versatility of CP platforms the levels of drugs delivered must be increased and responsiveness to stimulation enhanced. One approach to increase both the amount of drug delivered from CPs and their responsiveness is by synthesising CPs with micro- or nanostructures with an extended surface area. This thesis aimed to explore two types of soft templates, phytantriol liquid crystals and SDBS (sodium dodecyl benzene sulphonate) micelles, to direct the polymerisation of high surface area polypyrrole (PPy) platforms for tuneable delivery of dexamethasone. These systems have potential applications in the treatment of age-related macular degeneration (AMD) and diabetic macular edema (DME). Methods: Phytantriol and water mixtures were prepared in different proportions and the resulting phase of liquid crystal determined. The liquid crystal phase was monitored during the addition of polymerisation reagents to the bicontinuous cubic phase, during polymerisation of PPy and under redox cycling of the polymer using small angle X-ray scattering. PPy films were prepared by using bicontinuous cubic phase as a template and were loaded with dexamethasone sodium phosphate (DexP) as a dopant. The formed films were characterised for their morphology and elemental make-up by scanning electron microscopy, and for electrochemical activity by cyclic voltammetry. Electrically tuneable release of DexP from the films was then studied. The cytotoxicity of the extracts from the films with and without stimulation was tested on a human retinal pigment epithelial cell line-19 (ARPE-19). SDBS micelles were prepared and loaded with two drugs; the anionic and hydrophilic DexP and the non-ionic and hydrophobic dexamethasone (Dex). Electroactive PPy nanoparticles loaded with DexP and Dex were prepared by chemical polymerisation using drug loaded SDBS micelles as a template. Morphological, chemical and electrochemical characterisation of the formed nanoparticles was achieved. The particles were pressed into a pellet and adhered to an ITO coated glass slide using silver epoxy for electrochemical characterisation and drug release studies. Mechanisms driving the loading and release of hydrophilic DexP and hydrophobic Dex were investigated. The cytotoxicity of the extracts prepared from the pellets and silver epoxy (with stimulation and without stimulation) and Dex and DexP at various concentrations were tested on ARPE-19 cells. Results and Discussion: Different phases of phytantriol liquid crystals were studied and a phase diagram was constructed using the hot stage and cross-polarised light microscopy. Bicontinuous cubic phase was selected for polymerisation of PPy as it contains large, nonintersecting aqueous and lipid channels which helps in the movement of monomer and dopant to the working electrode. The addition of monomer and dopant, polymerisation and redox cycling had no effect on the bicontinuous cubic phase. DexP loaded PPy films were prepared using bicontinuous cubic phase liquid crystal as a template. SEM revealed a highly porous and interconnected nanorod like structures with a large increase in surface area (from 8.3 m2.g-1 to 224.2 m2.g-1) as determined by N2 physisorption studies. The PPy film was found to be reversibly electroactive as evident from cyclic voltammetry. DexP release occurred in a more responsive fashion from the templated films compared to the conventional films. This was attributed to the increased surface area of the templated films which enhances the polymer/media interface. In cytotoxicity studies, no statistically significant difference was found between the films and the control indicating that DexP, unreacted monomer or unwashed liquid crystal that might leach from the films were not toxic. PPy nanoparticles loaded with anionic, hydrophilic DexP and non-ionic, hydrophobic Dex were prepared by chemical polymerisation using SDBS micelles as a template. The formed particles were characterised under SEM and TEM, and the particles were found to be c.a. 50 nm in size. Large amounts of drug loaded nanoparticles were prepared and particles were pressed into pellets. From CV, it was confirmed that the pressed pellet was electroactive and the surface area was found to be 2.59 cm2. The conductivity of the pellet (22.89 ± 5.49 S.cm-1) was determined by four-probe conductivity meter, which shows that the pellet was conductive and charge can pass between particles in the pressed pellet. The entrapment efficiency of Dex was found to be 80.5 ± 1.19% and DexP was 58.3 ± 2.50%. This data suggests that more of the Dex was associated with the forming PPy nanoparticles as it is positioned in the hydrophobic core of the micelles, whereas DexP was associated near the head groups of the micelles and surrounding media. DexP release was faster on reduction and highest release was observed when a pulse stimulus was applied which indicates that the release of DexP is driven by electrostatic interaction. For Dex, the release was similar on oxidation, reduction or without stimulation, but showed higher release on applying pulse stimulus. As Dex is not a charged molecule, electrostatic forces would not affect the release. Volume changes on applying pulse stimulus would have caused the release of Dex from the particles. Electrically tuneable release of both DexP and Dex was achieved on applying pulse stimulation. No statistically significant difference was observed among the groups, indicating a lack of cytotoxicity. Conclusion: The soft templates, liquid crystals and micelles, were studied as a template to direct the polymerisation of polypyrrole. Tuneable release of DexP from porous PPy films prepared through liquid crystal was achieved with increased responsiveness compared to conventional films. DexP and Dex were loaded into PPy nanoparticles prepared by chemical polymerisation on micelle template and release of both the drugs was achieved on electrical stimulation. PPy particles could be compressed into different shapes and sizes, enabling customizability of these systems. These films and particles could form the basis of an implantable drug delivery system in the treatment of AMD and DME.