An Investigation of Micro and Nanoscale Molding for Biomedical Applications

Abstract In the last decade, there has been rapid advancement of micro and nano manufacturing. Microinjection molding is a cost-effective fabrication technique that can fulfill the requirements of many medical applications. Despite the many advancements of microinjection molding, there are several challenges that need to be addressed to improve the replication. One of the most critical challenges for micro molders is the ability to fill a micro mold in a predictable fashion during injection molding. To enhance the replication quality of the parts, particularly those with high aspect ratio micron scale features, it is imperative to gain a scientific understanding of the role of the various processing parameters on micro-cavity filling during injection molding.The main objective of this research was to develop a micro injection molding strategy to optimize the filling of micro/nano features. That included the design and manufacture an efficient mold that can produce high quality molding features at high replication rates. The intended application was cell culture substrates for different biomedical applications. Specifically, the research was focused on the development of a microstructured surface that would mimic human tissue topography to potentially enhance the cell culture process. The research also aims to develop a processing approach for high volume production of this part with high quality using bio-compatible polymers.The research also investigated if Si tooling is the proper material/method for this process, mainly due to its brittle nature, and relatively high pressures employed in injection molding. For high volume manufacturing, a long-lasting mold is a key factor in offsetting the high cost of mold cavity micro-fabrication. In this research, the life expectancy of theSi insert is examined and was experimentally and numerically determined to be inadequate for micro-scale injection molding.Demolding force for oval and cylindrical micro geometries were calculated and used as input for the mold fatigue simulation. Mold fatigue life simulations were performed to show the effect of cyclic loading under micro injection molding cycles. It was shown that Si molds survive for hundreds of cycles. In the experimental work, Si molds survive only for 105 cycles this implies that there are other factors that needed to be considered such as micro/nano cracks that initiated when fabricating these types of mold.Bulk metallic glass (BMG) showed a potential mold life for these specific features scale, but it needs a better fabrication technique to transfer the pattern completely into the BMG mold. Microcavities with 5 ?m diameter with 2 ?m depth were successfully fabricated over a 2x2 mm BMG mold surface.Silicon (Si) molds with micro–holes were fabricated using ultraviolet lithography followed by deep reactive ion etching (DRIE). Scanning electron microscopy (SEM) was used to validate the targeted dimensional specifications of the fabricated mold cavities. The Si insert then served as the mold cavity for injection molding.Numerical simulations using Moldflow® Insight were done to optimize the filling the microcavities and reduce the trial and error type experimentations. Molding experimentations were found to validate the processing parameters conducted from Moldflow® simulation. Injection pressure and packing pressure were increased to a point where the material replicates the microcavities entirely.A cell sensing model was developed to match the effective elastic modulus of human tissue and used as a tool to design micro and nanoscale surfaces. In this model itwas assumed that the cell culture process will be enhanced if the effective modulus of substrate mimics that of the extra cellular matrix that the particular type of the cell will in contact within its native environment and the mechanical interaction between the cells and the substrate was determined The model was derived based on pillar deflection and shear deformation that was expected during cells culturing process. The Euler-Bernoulli and Timoshenko approximations were utilized for the calculations. This model will provide a better method to design pillar arrays prior to cell culturing procedure and significantly improve the ability to study cells developments.This dissertation outlines my contributions to the advancement of the science and technology related to micro and nano injection molding technology. These contributions include the design and fabrication of micro featuers over dense area and development of a scientific framework to understand the role of processing parameters on the quality of microinjection molding through numerical simulations and experiments.