1 Department of Micro- and Nanotechnology, Technical University of Denmark2 Department of Mechanical Engineering, Technical University of Denmark3 Department of Physics, Technical University of Denmark
There is an increasing need for mass-production of nanostructures with high precision at polymer surfaces, for example to reduce the reflection of light, to reduce the adhesion of smudge, for optical data storage (e.g. Blu-ray discs), and for biological applications. In this project the effects of mold coatings and injection molding conditions on the final nanostructure quality (aspect ratio, shape, uniformity) of nanostructures in the cyclic olefin copolymers Topas 8007 and 5013 were explored. A setup for investigation the behavior of cells on nanostructures was established. Initial cell experiments were performed to investigate the influence of injection molded nanostructured surface topographies on the cell morphology and spreading of different cell types, i.e. mouse fibroblasts (NiH3T3), human epithelial carcinoma cells (HeLa) and immature dentritic cells. Given sufficient flowability of the polymer melt, interfacial effects like wetting and friction play a major role in injection molding of high aspect ratio nanostructures. The interfacial energy between mold and polymer needs to allow filling as well as demolding which are opposing properties: On the one hand, insufficient wetting of the polymer melt on the mold surface may prevent the polymer melt to fill nanoscale cavities during injection. This will limit the resolution of injection molding. On the other hand, given adequate cavity filling, high frictional forces may cause inability of the solidified polymer to leave the nanoscale cavity during demolding leading to nanostructural failure. It was found that the mold temperature has a major influence on the replication depth for tested mold coatings while other parameters (melt temperature, injection velocity and pressure, holding pressure and ejection temperature) do not have any significant effect. Nanostructured areas molded on native nickel molds often showed regular nanoscale defects and distortions, and randomly distributed large-scale defects. In contrast, a molecular vapor deposited fluorocarbon based antistiction coating commonly used for silicon molds in nanoimprint lithography was found to improve the replication quality substantially. Large scale defects of the overall patterned surface were fully removed while regular nanoscale defects decreased significantly. Optimized injection molding conditions allowed the replication of highly ordered nanostructures of controlled size. Arrays of pillars of 40 nm in diameter and up to 100 nm in height (height-to-width aspect ratios above unity) were successfully injection molded in Topas 8007 at mold temperatures above glass transition temperature using a fluorocarbonsilane coated nickel mold. Secondly, arrays of holes of 50 and 100 nm in diameter and up to 35 and 100 nm in height, respectively, were injection molded in Topas 8007 and 5013 with high replication quality. Initial cell experiments of NiH 3T3, HeLa and immature dendritic cells (DC) on oxygen plasma treated Topas 8007 and 5013 showed cell morphologies comparable to cells cultured on commercial tissue culture grade polystyrene. NiH 3T3 and HeLa cells grow visibly slower and often exhibiting elongated shapes and spread less well on native (untreated) Topas of both types. Only HeLa cells showed a clear difference in spreading on different nanostructured surfaces. For example HeLa cells were clearly avoiding patterns of 50 nm wide and 50 nm deep holes while favoring areas containing 350 and 600 nm wide and 250 nm deep holes over flat surfaces. Immature DCs were found to spread less well on native Topas regardless of surface topography forming largely extended lamellopodia and filopodia compared to oxygen plasma treated Topas. No obvious influence on immature DC morphology could be seen for nanostructures on treated or untreated Topas.