Book excerpt: Abstract: Polymer-based biomedical devices with micro/nano-sized features have attracted a great deal of interest from industries and academia. The common polymer processing methods involve either organic solvents or temperatures above the glass transition temperature (T[subscript g]), which is undesirable for biomedical applications. On the basis of different properties near polymer surfaces from those in the bulk, we introduce subcritical fluids (particularly carbon dioxide, CO2) into polymer processing at the micro/nanoscale to produce and assemble these devices at low temperatures. In this study, the atomic force microscopy (AFM)/nanoparticles approach has been applied to evaluate the effect of CO2 on the T[subscript g] gradient near the polymer surface. Meanwhile, neutron reflectivity is utilized to measure CO2-enhanced chain mobility at the polymer surfaces below the polymer bulk T[subscript g] and to investigate the competition between CO2 enhancement and substrate confinement on the chain mobility. With this information, we demonstrated the CO2-assisted bonding of polymeric structures at both micro and nano scales and established guidelines for this technique. In addition, nano-sized fillers were added to polymer substrates to improve the dimensional stability and reinforce the polymeric nanostructures by forming strong interactions between the nano-fillers and polymer chains. The research results were utilized to produce, assemble, and functionalize well-defined three-dimensional (3D) scaffolds for tissue engineering. A variety of polymer microfabrication techniques were developed to produce planar biodegradable polymeric scaffold skeletons with open structures. Then the CO2 bonding technique was applied to assemble these skeletons to a 3D scaffold at a low temperature. These microfabricated scaffolds have a uniform and well-defined geometry and structure, which allows for the study of cell attachment, spreading, and proliferation in scaffolds in a controlled and logical manner. Our research initiates a new field of polymer processing and will be of great benefit to the advancement of polymer thin film and polymer micro/nanofabrication technologies. With the presence of CO2, fabrication and assembly of micro/nanodevices can be performed at a biologically benign temperature, which is suitable for biomedical applications.