Ibrahim A. Al-Homoudi

Surface modification and neural tissue culture of thin film electrode materials

Good interfaces between electrodes and neural tissue are very important in chronic in vivo stimulation/recording. In order to study the effect of electrode materials and surface structure on neural interfaces, we cultured neurons on thin films of electrode materials that are expected to be biocompatible, such as platinum, and iridium oxide. We used both flat film surfaces and laser micro-structured ones. The laser micro-structuring consisted of creating regular arrays of micro-bumps with height of about 1μm and diameter of 2-3 microns. We have found conditions for fabrication of such micro-bumps on platinum and iridium thin films on borosilicate glass substrate (Pyrex 7740) by mask-projection irradiation with single nano-second pulses from a KrF excimer laser (λ=248nm). Laser micro-structured iridium films were coated with IrO 2 by pulsed DC reactive sputtering to obtain micro-structured IrO 2 films. Two types of iridium oxide films were studied: amorphous (reactively sputtered at room substrate temperature) and polycrystalline (reactively sputtered at 300°C). Cortical neurons isolated from rat embryo brain were cultured onto these thin film surfaces. Cells were more than 98% viable as determined by trypan blue exclusion tests. Poly-D-Lysine coated surfaces were used as positive controls for cell. Regular optical and fluorescent microscopy techniques were used to image the cells after they were cultured. To differentiate between live and dead cells a viability test with fluorescein diacetate (FDA) and propidium iodide was carried out. Also, immunocytochemistry analysis of neuron cells was performed using antibody for neuron-specific enolase (NSE) staining. A qualitative and quantitative comparison was carried out between the different types of modified electrode surfaces to study the neuronal growth in order to explore the feasibility of micro-bumps as stimulating/recording neural interfaces. These results are intended for use in optimization of future electrical stimulation/recording experiments that we plan to carry out.

Residual Stresses in TiO 2 Anatase Thin Films Deposited on Glass, Sapphire and Si Substrates

Anatase-TiO2 films (thickness 100-1000 nm) were grown on glass, sapphire, and Si(100) substrates using pulsed dc-magnetron reactive sputtering. By measuring the curvature of substrates before and after the thin film deposition, the residual stresses were determined. These results clearly show that the bi-axial stresses are compressive type and decreases with the increasing film thickness. The Raman spectra of these films were measured with two different excitation wavelengths (514 and 785 nm) and the thickness dependent shifts of Eg phonon mode were studied. The dominant 144 cm -1 Eg mode in TiO2 anatase clearly shifts to a higher value by 0.45 to 17.4 cm -1 depending on the type of substrate and the thickness of the film. Maximum shift was seen for the films on glass substrate indicating a higher bi-axial compressive stress in agreement with the curvature measurements. The excitation wavelength dependent shift of Eg mode clearly shows that the bi-axial stress increases along the film depth, being larger at the film/substrate interface.