Porous Silicon Structures for Biomaterial and Photonic Applications

Author: Yit Lung Khung

Khung, Yit Lung, 2009 Porous Silicon Structures for Biomaterial and Photonic Applications, Flinders University, School of Chemical and Physical Sciences

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The primary research aim in this thesis is to demonstrate the versatility of porous silicon based nanomaterials for biomaterial and photonic applications. In chapter 2 of this thesis, the suitability of porous silicon as a biomaterial was investigated by performing different surface modifications on the porous silicon films and evaluating biocompatibility of these surfaces in vitro. The porous silicon surfaces were characteriszed by means of atomic force microscopy (AFM), scanning electron microscopy (SEM), diffuse reflectance infrared spectroscopy (DRIFT) and interferometric reflectance spectroscopy (IRS). Cell attachment and growth was studied using fluorescence microscopy and cell viability assays. Both fabrication of the porous silicon films and subsequent surface modifications were demonstrated. Polyethylene glycol functionalised porous silicon prevented cell attachment, whilst collagen or fetal bovine serum coating encouraged cell attachment. Surface modifications were also performed on porous silicon films with different pore sizes and the influence of pore size and surface modification on primary hepatocyte growth was recorded over a course of 2 weeks by means of laser scanning confocal microscopy (LSCM), toxicity and metabolic assays. On collagen-coated surfaces with average pore sizes of 30 nm, multilayer cells stacks were formed. This stacking behaviour was not observed on samples with smaller pore sizes (10 nm), or in the absence of collagen. Hepatocytes remained viable and functional (judging by a metabolic assay) for 6 days, after which they generally underwent apoptosis. Collagen-coated porous silicon films showed later onset of apoptosis than porous silicon films not coated with collagen or collagen-coated flat silicon.. In chapter 3 of this thesis, the nitrogen laser of a laser desorption/ionization (LDI) mass spectrometer was used to selectively ablate regions on porous silicon films that had been functionalised with a non-fouling polyethylene oxide layer, affording a microscale patterning of the surface. Surface characterization was performed by means of AFM, SEM, LDI mass spectrometry, DRIFT and IRS. This approach allowed the confinement of mammalian cell attachment exclusively on the laser-ablated regions. By using the more intense and focussed laser of a microdissection microscope, trenches in a porous silicon film were produced of up to 50 micron depth, which allowed the construction of cell multilayers within these trenches, mimicking the organization of liver cords in vivo. Fluorescent staining and LSCM was used to study cell multilayer organization. To gain a better understanding of how surface topography influences cell attachment and behaviour, porous silicon films were fabricated containing a gradient of pore sizes by means of asymmetric anodisation (chapter 4). These gradients allowed the investigation of the effect of subtle changes of pore size on cell behaviour on a single sample. Analysis by means of LSCM and SEM showed that pore size can dictate cell size and area as well as cell density. In addition, a region of pore size where cell attachment and proliferation was strongly discouraged was also identified. This information can prove to be useful for designing non-biofouling surface topographies. Using the same asymmetric anodisation setup, photonic mirrors gradients were produced and overlaid over one another to produce multidirectional lateral photonic mirror gradients that display a series of roving spectral features (photonic stop-bands) from each gradient layer (chapter 4). These multidirectional photonic gradients have the potential to serve as optical barcodes or contributing to the development of graded refractive index devices such as lenses for high quality image relay and graded-index optical fibers.

Keywords: Porous silicon,cell micropatterning,lateral gradients,photonic crystals

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2009
School: School of Chemical and Physical Sciences
Supervisor: Professor Nicolas Hans Voelcker