Nanowires for Cell Research

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

Gallium phosphide nanowire arrays and their possible application in cellular force investigations

The authors report the fabrication of gallium phosphide nanowire arrays that can be used for cellular force measurements. The nanowire positions are defined using electron beam lithography and the nanowires are grown using metal organic vapor phase epitaxy. By varying the nanowire diameter, length, and spacing from substrate to substrate, they can expect to probe cell forces over several orders of magnitude, depending on the chosen substrate. The small diameter of the nanowires allows them to densely pack the array and to achieve unprecedented spatial resolution for future cell force-array applications

Rectifying and Sorting of Regenerating Axons by Free Standing Nanowire Patterns: A highway for nerve fibers

We present an EBL-defined nanowire pattern that can sort axons coming from different directions on a substrate. The pattern defines tracks for left-bound traffic and right-bound traffic, which opens up new possibilities for designing neural networks on a chip

Axonal guidance on patterned free-standing nanowire surfaces

We demonstrate high-fidelity guidance of axons using rows of nanowires. The axons are prevented from crossing the rows, making it possible to guide and sort a large number of axons as opposed to when chemical patterns are used. Focal adhesion forms at the nanowires establishing a possible site of information transfer between the surface and the cells. Rows of gallium phosphide (GaP) nanowires were epitaxially grown on GaP(111) substrates in patterns defined by electron beam lithography

Gallium phosphide nanowires as a substrate for cultured neurons

Dissociated sensory neurons were cultured on epitaxial gallium phosphide (GaP) nanowires grown vertically from a gallium phosphide surface. Substrates covered by 2.5 mu m long, 50 nm wide nanowires supported cell adhesion and axonal outgrowth. Cell survival was better on nanowire substrates than on planar control substrates. The cells interacted closely with the nanostructures, and cells penetrated by hundreds of wires were observed as well as wire bending due to forces exerted by the cells

Nanomodified surfaces and guidance of nerve cell processes

Axonal growth and guidance were studied on different micro- and nanostructured surfaces. Nanoimprinted grooves in a polymer, epitaxial III/V nanowires, porous silicon patterns, and chemically altered surfaces were all shown to induce axonal guidance. Neurons were also found to be able to attach and grow on gallium phosphide nanowires without compromising cell survival. The results are important for the construction of a new generation of neuroelectrical interfaces, including high spatial resolution electrodes. The advantages of the different nanostructured surfaces are discussed