Malik, SA; (2016) Electric field-driven engineering of functional polymeric micro/nano-structures. Doctoral thesis , UCL (University College London).

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Abstract

Nanoscopic processing of polymer-nanoparticle composite materials has attracted a great deal of interest due largely to their unique physical and chemical properties. Metal nanoparticles present particularly attractive building blocks as starting materials to fabricate assemblies for future nanodevices with unique electronic properties. However, their fabrication currently remains a great challenge in nanotechnology due to the complexity of organizing metal nanoparticles in low symmetry. Current conventional methods to generate high- aspect ratio linear fibres rely on techniques such as drawing, template synthesis, self-assembly, electron-beam lithography and nanolithography, although these techniques suffer from poor time efficiency, costly starting materials, and all five are difficult to scale up for industrial production, as they rely on batch-type processes. This work, inspired by frugal engineering, focuses on the controlled manufacture of polymer/nanoparticle assemblies using electrospinning. We show how trade- offs in fibre size, morphology and structure can be controlled by varying nanoparticle loading in the slurry to carefully engineer linear composite nanomaterials. This procedure circumvents the space, regulation, control, standards and maintenance necessary for state-of-the-art cleanroom facilities. This thesis demonstrates the fabrication of composite nanowires by a single- step electrospun deposition of semi-crystalline Poly(ethylene oxide) (PEO) and zero-valent metallic gold nanoparticle (Au NP) blends. Au NP loading determined the fate of fibre shapes, sizes and electrical performance. The goal of controlled drug delivery is to administer sustained amounts of a therapeutic agent over a prolonged period of time, improving the drug efficacy as compared to conventional, bolus doses that lead to variable concentrations of antibiotics in the blood. Although there are several systems capable of providing such a continuous-dose-based treatment, the use of biodegradable polymer microparticles offers multiple advantages with respect to other platforms. Microsphere-based controlled release technologies have been utilized for the long-term delivery of proteins, peptides and antibiotics, although their synthesis poses substantial challenges owing to formulation complexities, lack of scalability, and cost. Conventional methods rely predominately on batch, emulsion preparation methods and suffer from several drawbacks: poor control over particle size distribution, broad size distributions at the micro scale, and poor repeatability. To address these shortcomings, the electrospray (ES) process was used as a reproducible, synthesis technique to manufacture highly porous (>94%) microspheres while maintaining control over particle structure and size. This thesis reports a successful formulation recipe used to generate spherical poly(lactic-co-glycolic) acid (PLGA) microspheres using ES coupled with a novel thermally induced phase separation (TIPS) process. In this work, PLGA microspheres in a range of different sizes, morphologies and compactness are generated using the ES route. The sizes of synthesized particles are primarily controlled by the delicate tuning of the solution physical properties and the ES operational parameters. We show how size, shape and porosity of resulting microspheres can be controlled by judiciously varying ES processing parameters and we demonstrate examples in which the particle sizes affect release kinetics. Importantly, throughout this series of studies, efforts were made to remove the synthesis approach from the all too common empiricism of a large fraction of the literature on materials synthesis, and to establish fundamental criteria that would allow for the generation of particles of prescribed size, morphology and consistency from first principles.

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