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

Hydroxyapatite formation on metallurgical grade porous silicon nanosponge particles

Investigations into the development of potential bone substitutes have increased rapidly in the last decade. Titanium and cobalt chrome are currently the alloys of choice when it comes to the orthopedic medical device fields due to their excellent mechanical strength and corrosion-resistant properties. Yet these materials are unable to elicit a biologically functional bone-material interface without a bioactive surface coating or surface modification. Osteoconductivity is only achieved when suitable coatings are applied or their surface properties are suitably altered. The need for significant bony reconstruction implants as a result of prosthetic revision surgery also increases the need to produce longer lasting or permanent bone substitute materials. Hydroxyapatite is the principle constituent of bone and has been used as a mechanism to induce bone formation at particular biological sites in need of bone repair and growth. When applied as a surface coating, hydroxyapatite s chemical and physical properties allow osteointegration of medical devices and prostheses. The discovery of hydroxyapatite has resulted, not only in rapid advances and developments in the orthopedic and dental fields, but has also lead to a surge in investigations into further tailoring of the material to create new devices that meet clinical needs. Currently, the most commonly used method for assessing the potential bioactivity and bone-bonding ability of a material in-vitro involves using simulated body fluid. Previous research by Kokubo et al. has shown that in-vitro results obtained using these experiments correlate directly to in-vivo results and thus satisfies their use as potential bone-tissue substitutes. Porous silicon is a bioactive material and has been the subject of intense research since its original discovery at the Bell labs in 1956. Canham et al. was the first to suggest the possibility of creating biologically interfaced devices from porous silicon given its biostability, non-toxicity and ease of its topographical manipulation and optoelectronic properties. Porous silicon has been shown to induce the formation of a physiologically stable hydroxyapatite on its surface using in-vitro simulated body fluid experiments. Other studies today are also exploring the use of porous silicon as a promising potential bioactive therapeutic agent and drug delivery vehicle. Further research exploring the potential of using a silicon-substituted hydroxyapatite coating in in-vitro experiments have showed improved bioactivity and chemical stability under physiological conditions compared to normal hydroxyapatite. In 2010, Chadwick, Clarkin and Tanner showed that metallurgical grade porous silicon powder induced bone-like apatite formation on its surface in simulated body fluid inferring a bioactive nature and likely close bony apposition in-vivo. This chapter explores the use of porous silicon as a biomaterial and hydroxyapatite and porous silicon as a potential biomaterial for bone tissue engineering and other bioactive applications. It examines current research and future directions of such biomaterials

Hydroxyapatite formation on metallurgical grade nanoporous silicon particles

Studies into bone-like apatite or hydroxyapatite (HA) growth on potential biomaterials when in contact with simulated body fluid (SBF) not only establish a general method for determining bioactivity but coincidently lead to the design of new bioactive materials in biomedical and tissue engineering fields. Previous studies of HA growth on porous silicon (PS) have examined electrochemically etched silicon substrates after immersion in a SBF. This study differs from previous work in that it focuses on characterising HA growth on chemically etched metallurgical grade nanoporous silicon particles. The PS used in this study is comprised of nanosponge particles with disordered pore structures with pore sizes ranging up to 10 nm on micron-sized particles. The silicon particles are analysed before and after immersion into SBF using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS). Results indicate that a HA layer forms on the surface of the nanosponge particles. Experimental analysis indicates that the morphology and calcium-to-phosphorus ratio (Ca/P) verify the formation of crystalline HA on the nanoporous silicon particles

Microstructural characterisation of metallurgical grade porous silicon nanosponge particles

Porous silicon finds numerous applications in the areas of bio-technology, drug delivery, energetic materials and catalysis. Recent studies by Vesta Sciences have led to the development of porous silicon nanosponge particles from metallurgical grade silicon powder through their own patented chemical etching process (Irish patent no. IE20060360). This discovery paves the way for a more economical production method for porous silicon. The study presented here studies the structural morphology of the porous silicon nanosponge particles using high resolution electron microscopy techniques combined with porisometry type measurements, where appropriate. The related surface pore structure is examined in detail 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. Three samples of the silicon particles were analysed for this study which include the starting metallurgical grade silicon powder and two samples that have been chemically etched. Analysis of the etched samples indicates a disordered pore structure with pore diameters ranging up to 15 nm on porous silicon particles ranging up to 5 mu m in size. Crystallographic orientation did not appear to affect the surface pore opening diameter. Internal pore data indicated pore depths of up to 1 mu m dependant on the particle size and etching conditions applied

A bioactive metallurgical grade porous silicon-polytetrafluoroethylene sheet for guided bone regeneration applications

The properties of porous silicon make it a promising material for a host of applications including drug delivery, molecular and cell-based biosensing, and tissue engineering. Porous Silicon has previously shown its potential for the controlled release of pharmacological agents and in assisting bone healing. Hydroxyapatite, the principle constituent of bone, allows osteointegration in vivo, due to its chemical and physical similarities to bone. Synthetic hydroxyapatite is currently applied as a surface coating to medical devices and prosthetics, encouraging bone in-growth at their surface & improving osseointegration. This paper examines the potential for the use of an economically produced porous silicon particulate-polytetrafluoroethylene sheet for use as a guided bone regeneration device in periodontal and orthopaedic applications. The particulate sheet is comprised of a series of microparticles in a polytetrafluoroethylene matrix and is shown to produce a stable hydroxyapatite on its surface under simulated physiological conditions. The microstructure of the material is examined both before and after simulated body fluid experiments for a period of 1, 7, 14 and 30 days using Scanning Electron Microscopy. The composition is examined using a combination of Energy Dispersive X-ray Spectroscopy, Thin film X-ray diffraction, Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy and the uptake/release of constituents at the fluid-solid interface is explored using Inductively Coupled Plasma-Optical Emission Spectroscopy. Microstructural and compositional analysis reveals progressive growth of crystalline, ´bone-like´ apatite on the surface of the material, indicating the likelihood of close bony apposition in vivo

Compositional characterisation of metallurgical grade silicon and porous silicon nanosponge particles

Porous silicon is generally achieved through electro-chemical etching or chemical etching of bulk silicon in hydrofluoric acid based solutions. The work presented here explores the effect of a chemical etching process on a metallurgical grade silicon powder. It is found that the metallurgical grade silicon particles contain surface bound impurities that induce a porous structure formation upon reaction with the chemical etchant applied. The correlation between the resultant porous structure formed due to the material composition is examined in detail. The elemental composition is determined using a combination of X-ray Photoelectron Spectroscopy and Time of Flight Secondary Ion Mass Spectroscopy. The porous structure is analysed using Transmission Electron Microscopy and Scanning Electron Microscopy. Three samples of the silicon particles analysed for this study include an un-etched bulk silicon powder sample and two samples of chemically etched powder. Pore formation within the particles is found to be dependent on the presence, dispersion, and local concentration of surface bound impurities within the starting powder