(5ca) Polymer Nanoengineering and Its Applications to Biomedical Engineering

Over the past decade, polymer-based MEMS (MicroElectroMechanical Systems)/NEMS (NanoElectroMechanical Systems) have attracted a great deal of industrial and academic interest. There are urgent needs for a comprehensive understanding of molecular dynamics of confined polymers at the nanoscale and the technologies for nanoengineering of polymers and polymer nanomaterials. At the nanoscale, polymer-free surface and/or polymer-substrate interactions contribute greatly to the properties of polymers. Depending on the entropic and enthalpic effects of the interface, the nanoconfined polymer shows different thermomechanical behavior. By using the well-defined systems, polymer thin film/substrate configurations, we resemble the interfaces of polymer-substrate and investigate the polymer dynamics (i.e., glass transition temperature, T_g and chain diffusion) in vicinity of the substrate using atomic force microscopy (AFM) and neutron reflectivity (NR), respectively. Polystyrene (PS) thin films were spun-coat on two types of substrates, highly ordered pyrolytic graphite (HOPG) and silicon wafer with a native oxide layer. The film thickness was changed from one R_g (radius of gyration of PS) to a few R_g and the effects of both the polymer-air and the polymer-substrate interactions were explored. The PS on these substrates shows different Tg profiles. These results will provide valuable guides for designing new polymer nanomaterials. The influence of carbon dioxide (CO₂) on the T_g profile of those polymer thin films was evaluated as well. It is found that the thin film behaviors result from the competing impacts of entropic confinement (by the substrate) and free-volume increase (by CO₂). Carbon dioxide, even at low pressure, dramatically enhances polymer chain mobility at the nanoscale. AFM and NR studies reveal a pressure-tunable width of the surface rubbery layer. As a result, tuning CO₂ pressure could help realize low temperature polymer nanoenginering.

This phenomenon allows us to successfully assemble polymeric micro/nanostructures at low temperatures using a small compressive force. By regulating CO₂ pressure, the assembly can be completed at biologically permissive temperatures. Moreover, the assembly of polymeric micro/nanostructures can be realized in an aqueous environment in the presence of cells and biomolecules. Original micro/nano structures are well preserved and CO₂ pressure has little effects on the bioactivity/viability and functionalities of the proteins, DNAs, and cells studied. This technique allows integration of biomolecules and cells into polymeric micro/nanodevices. Here, we highlight its ability to integrate multiple cell-scaffold constructs into a tissue complex by first allowing the individual cell types to grow on biodegradable scaffolds and then assembling the desired multiple cell-scaffolds into a 3-D construct. This novel method has the potential to develop fully functional tissue substitutes and provides for a manufacturing platform that thus far has been lacking in the field of tissue engineering. This CO₂-assisted bio-assembly method offers an affordable and biologically permissive process, particularly for the simultaneous assembly of a large number of micro/nanostructures containing temperature- and/or solvent-sensitive biomolecules and cells. It opens a new avenue for tissue engineering, cell therapy, drug delivery and cell-based biochips.