Thermoset mold based hot embossing technique for micro/nano-hierarchical structures

Abstract

Recently, it was found that the micro/nano patterned surface significantly affect cell behaviors, including cell proliferation, migration, and differentiation. In the previous research, the cell experiments have been usually performed on the polydimethylsiloxane (PDMS) surfaces due to the ease of fabrication process. Albeit PDMS has been widely used, it has limitations as a material for cell experiments. For these reasons, biocompatible thermoplastic, such as PS commonly used in cell culture dishes, may become a promising alternative. However, fabrication process of thermoplastic needs high cost and time. In this thesis, a novel hot embossing technique is presented for the replication of micro and micro/nano-hierarchical structures on the PS surfaces based on thermoset molds. Compared to conventional hot embossing that uses metal molds, thermoset molds-based hot embossing has advantages of being rapid, simple, and cost effective. Here, two different types of thermoset molds, (1) PDMS for microscale replication and (2) polyurethane (PU)-based adhesive (Norland Optical Adhesive, NOA) for nanoscale replication, are used. A PDMS mold enables us to obtain conformal contact and multiple replications due to its elastomeric properties. However, it has limitations for replicating nanoscale structure due to its inherent softness. On the other hand, a PU mold can endure the high pressure applied during the embossing process due to the stability under high temperature and high pressure, thereby replicating nanoscale structures though it has short replication life compared to PDMS. Two different types of circular micropillar and microhole arrays were successfully replicated on the polystyrene surfaces using the PDMS mold-based hot embossing technique. The micropatterns were designed to have various diameters (2 to 150 m), spacings (2 to 160 m), and heights (1.4, 2.4, 8.2, and 14.9 m) so as to generate a gradient of physical properties on the surface. Two different types of microlens/nanolens- and micropyramid/nanolens-hierarchical structures having 70 m sized microlens or micropyramid and nanolens with 70 nm diameter were replicated to the PS surfaces by PU (NOA 63 and 71) molds. Design of experiments based on the Taguchi method indicated the sensitive parameters in the hot embossing processes for each pattern shape. In conclusion, (i) the PDMS mold- and PU mold-based hot embossing could successfully replicate micro structures and micro/nano-hierarchical structures on the PS surfaces with an optimal processing condition, and (ii) the PDMS mold can be used more than 20 times while, the PU mold can be used around 10 times as molds. To demonstrate the interaction between the micro/nano structures and the cell behaviors, adipose-derived stem cells (ASCs), human osteoblast-like cells (MG63), and mouse embryonic fibroblast cell line (NIH-3T3) were cultured on the replicated polystyrene surfaces. Cell experimental results showed cell behaviors, including proliferation, migration, and differentiation are affected by structure types and scales.