Microscopical analysis and characterisation of bioactive porous silicon nanosponge particles

Porous silicon finds numerous applications in the areas of bio-technology, drug delivery, energetic materials and catalysis. Vesta Sciences (US Research Company) have led the development of porous silicon nanosponge particles produced from metallurgical grade silicon powder through their own patented chemical etching process (Irish patent no. IE20060360). This discovery paves the way for a more economic production method for porous silicon and given its potential for use in a huge variety of different fields, further research into this material is therefore warranted in order to support suitable material applications development. This thesis characterises the porous silicon particles structural morphology using high resolution electron microscopy techniques combined with porisometry type measurements where appropriate. The related surface pore structure is examined using Scanning Electron Microscopy and Transmission Electron Microscopy techniques while the internal pore structure is explored using Focused Ion Beam milling and Ultramicrotomed cross-sections. The correlation between the porous structure formations due to the material composition is studied in detail using a combination of X-Ray Fluorescence & Inductively Coupled Plasma Spectroscopy, X-Ray Photoelectron Spectroscopy, Energy Dispersive X-ray Spectroscopy and Time of Flight Secondary Ion Mass Spectroscopy. Samples of the silicon particles include the starting metallurgical grade silicon powder and four other samples that have been chemically etched. Analysis of the etched samples indicates a disordered pore structure with pore diameters ranging from 5nm to 15nm on porous silicon particles ranging from 4-20μm in size. Crystallographic orientation was not found to affect the surface pore opening diameter or surface density of pores. Internal pore data indicated pore depths reached a maximum of 1μm dependant on the silicon particle size and etching conditions applied. Pore depth and position within the particles is found to be dependent on the presence, dispersion, and local concentration of surface impurities within the starting powder. Particles less than 2μm in size were found to be fully porous throughout the particle. While the main focus of the thesis is to characterise the material structurally and examine the porosity formation mechanism in correlation with the chemical etching conditions applied, exploring potential bio-applications for the material is also a core objective of this project. Research is carried out using simulated body fluid experiments to test the bioactivity of the material both in nanosponge form and when combined into a porous silicon particulate-polytetrafluoroethylene sheet. The silicon particles are analysed before and after immersion into simulated body fluid using Scanning Electron Microscopy, Transmission Electron Microscopy, Energy Dispersive X-ray Spectroscopy and X-ray Photoelectron Spectroscopy. Results show that a hydroxyapatite layer forms on the surface of the nanosponge particles and on the particulate sheet indicating that the material is bioactive in vitro. Experimental analysis indicates that the morphology and calcium-to-phosphorus ratio verify the formation of crystalline hydroxyapatite and also indicate the likelihood of close bony apposition in vivo. Due to its reactivity in vitro and its nanosponge porous structure, this material exhibits potential for use in bony applications, as well as in drug delivery device applications.