Nanowires for Cell Research

Abstract

This study explores the interaction between living biological cells and semiconductor nanowires. Biological cells are highly complex and dynamic entities, every one of them with individual characteristics and a high degree of internal and external communication. They respond to and affect their external environment with a huge web of fine-tuned control systems. Traditional biochemical tools for cell research have predominantly focused on the study of collections of cells and their general properties. Nanotechnology allows fabrication of advanced miniaturized devices, with tailored qualities. The use of nanostructures for cell research is motivated by the possibility of addressing cell-biological issues at the single cell or sub-cellular level, and to do that with large numbers of cells in parallel. In this thesis epitactic semiconductor nanowires were used to perform novel research on neurons and macrophages, two mammalian cell types with highly specialized properties. Free standing nanowires, of gallium phosphide (GaP) and galliuim arsenide (GaAs), grown by metal organic chemical vapour deposition (MOCVD), were fabricated and used as a substrate for cell culture, where the properties of cell/wire interactions were examined and utilized. First, basic relations were investigated. Cells were shown to attach, remain viable and even exhibit strong outgrowth on GaP nanowire substrates. Intense interactions between the wire and the cells were found. The cells could grab, bend and internalize the wires and the wires affected the cellular growth direction, all of which could be used for a variety of applications. By producing samples with patterns of nanowires, the outgrowth of nerve fibres could be controlled in detail over large areas. Here it was also found that the cells could establish functional adhesion to the wires, thus explaining the wire grabbing earlier noticed. With more advanced wire patterns, the direction of neural outgrowth was controlled and nerve fibres from two different origins could be led to grow close together without overlapping. Laminin, an important component of the extra cellular matrix of living organisms, had a high tendency of adhering to the nanowires. This was used both to improve the cell viability and to visualize the wires in fluorescence microscopy, by labelling the laminin. The ability of cells to grow on top of dense nanowire arrays and attach to their tips was used for measurements of cellular mechanical forces. After investigations of the mechanical properties of the GaP nanowires, the cell induced wire bending could be translated to quantitative forces. Optical detection and image processing was employed to resolve picoNewton forces at high temporal and spatial resolution. From GaAs nanowire templates hollow nanotubes were made. DNA strands were pulled through the tubes to prove their applicability. Modified versions of those tubes were used to transfect macrophages with a molecular dye. The tubes could penetrate into living cells and the viability of cell penetrated by nanotubes could be established. Finally, macrophage response to suspensions of free nanowires of different materials was investigated. GaP nanowires and nanowires made from nickel, gold and polystyrene were examined. The cells actively engulfed wires of all the materials but the influence on the cells differed. Nickel wires caused the highest number of dead cells, whereas the polystyrene wires made least impact. GaP wires had an intermediate effect. In conclusion, this work shows that nanowires can interact intimately with living cells and that those interactions can be understood and controlled. Nanowires are highly interesting structures that can be used for multiple applications in sub-cellular cell science