Polymer-based biomedical microdevices containing environmentally sensitive biomolecules is attracting increased interest. A critical requirement is the ability to manufacture and assemble these devices at low temperatures in order to minimize denaturization. Common polymer processing techniques involve either organic solvents or a temperature beyond the glass transition temperature T g. Both may cause serious processing difficulty and are undesirable in biomedical applications.
Recent research of polymer thin films revealed different properties (e.g. T g) near the polymer surface from those in the bulk. However, the chain mobility in the polymer surface is still low. In our laboratory, we introduce low-concentrated CO 2 to tune polymer surface properties to realize low-temperature polymer processing at the micro/nanoscale. We have developed a novel technique from the atomic force microscopy (AFM) method [Rudoy et al., 2002; Teichroeb et al., 2003] to evaluate T g of the polymer surface under CO 2. Monodisperse nanoparticles are distributed on a polymer surface and annealed under CO 2 atmosphere until the nanoparticles are embedded into the polymer surface and stay at the place where the local T g is equal to the annealing temperature. Figure 1A shows the smooth surface of poly(DL-lactide-co-glycolide) (PLGA). The roughness is less than 1 nm. Compared with the gold nanoparticles – PLGA system annealed at 35 oC (Figure 1B), the nanoparticles embedded deeper into the PLGA surface by about 9 nm when the low-concentrated CO 2 (0.69 MPa) was introduced. It implies that a rubbery surface layer of PLGA broadened by about 9 nm in the presence of 0.69 MPa CO 2 at 35 oC.
Taking advantage of CO 2 tuned properties at the polymer surface, we realize low-temperature polymer processes at the micro/nanoscale. The micro/nanoscale bonding of polymers at low temperatures and low CO 2 pressures is demonstrated as one of the applications. Figure 2A & 2B show the top and side views of an array of micro wells patterned on PLGA nanocomposite. Each of the wells is 5 mm in diameter and 3.9 mm in height. Figure 2C shows the bonded PLGA nanocomposite microstructures at at 35 oC and 100 psi CO 2 pressure. I t can be seen that the bonding interface is invisible and the microstructures were retained. When the CO 2 pressure increased to 300 psi, the micro wells deformed (Figure 2D) because of formation of the porous structure. Figure 3 demonstrates the capability of CO 2 enhanced interfacial bonding of polymers at the nanoscale using polystyrene (PS) as the substrate. The CO 2 bonding technique has been applied to construct 3-D well-defined tissue scaffolds and seal polymeric micro/nanofluidics.
This research will be of great benefit to the advancement of polymer thin film and micro/nanofabrication technologies. With CO 2, fabrication and assembly of micro/nanodevices can be completed at low temperatures, which is suitable for biomedical applications.
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