An Investigation of Micro and Nanomanufactured Polymer Substrates to Direct Stem Cell Response for Biomedical Applications

The development of high aspect ratio large feature density polymer microarrays requires the synergistic optimization of design, material, mold tooling, and processing. A conventional mold base with steel inserts and controllable resistance heating was assembled to incorporate interchangeable inserts with microfeatured silicon inlays. Ultraviolet (UV) lithography with dry etching was used to impart microfeatures into silicon wafers with a variety of different geometries containing aspect ratios ranging from 0.92 to 6. Multiple polymer resins, including polystyrene (PS), low density polyethylene (LDPE), cyclic olefin copolymer (COC), and thermoplastic polyurethane (TPU), were used to test replication and cellular response to materials with different bulk stiffness and topography-modified surface stiffness.;The maximum achieved microfeature aspect ratio was 9.3 (high impact polystyrene), owed to tensile stretching during part ejection. For non-stretched substrates, the maximum molded aspect ratio was 4.5 (LDPE) and highest replication quotient (RQ = feature height / tooling feature depth) was 0.97 (COC). The maximum aspect ratio molded with consistent features across the entire surface was 2.1 (TPU).;Parameters shown to enhance replication were mold temperature (T mold = Tg was a critical replication transition point), injection velocity at higher mold temperatures, holding time, holding pressure, and nozzle temperature. The importance of certain parameters was material dependent, but mold temperature consistently had a relatively large impact.;A concern that was addressed for a high density array of microfeatures was the consistency of replication, which is vital for the intended application and seldom address in published literature. Increased consistency was attained through strategic placement of temperature control, modification of the main cavity design, and optimized silicon tooling with reduced microcavity nanoroughness.;Silicon tooling was fabricated with the initial objective being to achieve high aspect ratio negative features. However, with the realization of molding and demolding limitations, the tooling microfeature profiles were altered to include a taper and reduction of sidewall scalloping of the tooling. Sophisticated methods of dry etching were used, in which a novel etching technique known as "passivation compensation," was utilized to manufacture microchannels containing low levels of roughness, a well-controlled tapered profile, and the prospect of high aspect ratios. With the new tooling, topography consistency was dramatically enhanced for both COC and TPU.;Water contact angle (WCA) measurements for both COC and TPU generally increased with an increase in surface roughness (dictated by microfeature dimensions). WCA hysteresis appeared to increase with roughness up to a critical value for COC while continuing to increase for TPU with a transition. Moreover, hydrophobic surfaces containing high levels of hysteresis were attributed to the "petal" effect associated with hierarchical surface structures. Hydrophobicity has been shown to be related to biological cell behavior.;Simulation of the injection molding process using conventional methods was used to describe general conditions present at microchannel inlets. The sprue gate and an increase in plate thickness gave the microfeatured region additional time to fill microfeatures prior to generation of a frozen layer. The delayed solidification is attributed to the low thermal conductivity associated with the polymer melt.;A cell sensing model was developed based on the mechanical interaction between cell and substrate. The model provides a useful design map by which nanofeatured polymer geometry and material choice can be made to achieve a particular apparent surface stiffness. Bending mechanics were simulated for a few specific examples, providing an indication of the limitations associated with using higher aspect ratio nanostructures. A bending example was applied to a manufactured tapered pillar to note the stiffness reduction achieved through use of the substrates molded during the current study.;Cell culture studies showed that the presence of topography had a dramatic effect on cellular morphology and on stress fiber thickness, causing an increase in thickness compared to flat controls. The cytoskeletal re-arrangements occurring may be indicative of a differentiation event. Unconventional morphology was observed in the presence of low aspect ratio COC microtopography.;Micromolding of tensile bars was conducted to better understand the processing effects on mechanical and thermal properties of microscale molded components.;Results revealed that the elastic modulus of COC is largely unaffected by a wide range processing conditions, but is reduced to approximately 41% of the value obtained from traditional tensile test results. TPU showed a dramatic dependence on molding properties. Simulation was used to further elucidate the cause for varying properties. Microscale elastic modulus average values approximately 31% higher compared to traditional tensile test results. Trends in thermal properties were not apparent, and were difficult to detect from relatively weak melting peaks. The use of two different polymer lots elicited drastically different results. Crystallinity, viscosity, and chemical bond structure was found to be very different from one lot to the other. (Abstract shortened by UMI.).