We demonstrate that mammalian cells can survive for 5 min within high-pressure CO 2. Cell survival was investigated by exposing a range of mammalian cell types to supercritical CO 2 (scCO2) (35°C, 74 bar; 1 bar ؍ 100 kPa) for increasing exposure and depressurization times. The myoblastic C2C12 cell line, 3T3 fibroblasts, chondrocytes, and hepatocytes all displayed appreciable but varying metabolic activity with exposure times up to 1 min. With depressurization times of 4 min, cell population metabolic activity was >70% of the control population. Based on survival data, we developed a single-step scCO 2 technique for the rapid production of biodegradable poly(DL-lactic acid) scaffolds containing mammalian cells. By using optimum cell-survival conditions, scCO 2 was used to process poly(DL-lactic acid) containing a cell suspension, and, upon pressure release, a polymer sponge containing viable mammalian cells was formed. Cell functionality was demonstrated by retention of an osteogenic response to bone morphogenetic protein-2 in C2C12 cells. A gene microarray analysis showed no statistically significant changes in gene expression across 4,418 genes by a single-class t test. A significance analysis of microarrays revealed only eight genes that were down-regulated based on a ␦ value of 1.0125 and a false detection rate of 0.biodegradable polymer ͉ bone ͉ scaffolds S tandard methods of combining mammalian cells and synthetic polymers for biotechnological applications (1-3) must minimize disruption to the cell component from fluctuations in solvent composition, temperature, pressure, and shear forces (4, 5). However, processing synthetic polymers by conventional routes requires organic solvents, elevated temperatures or pressures, and sometimes mechanical agitation to produce the required 3D form (6, 7). For this reason, an inefficient two-step process is required to generate the prefabricated polymer structure, with the cell component seeded as a second step (8).One method for generating macroporous, biodegradable scaffolds is by using high-pressure or supercritical CO 2 (scCO 2 ) processing. This technique is solvent-free and has been used for the fabrication of tissue engineering and drug delivery devices because of a number of processing advantages (9, 10). When CO 2 is raised above the critical pressure and temperature (31.2°C, 73.8 bar; 1 bar ϭ 100 kPa), it forms a phase with the characteristics of both a liquid and a gas with selective solvent power and high diffusivity (11). It is then able to penetrate into the amorphous regions of polymers, such as poly(DL-lactic acid) (P DL LA), liquefying them at near ambient temperatures. Release of the pressure results in the nucleation of gas bubbles within the polymer, which become permanent upon solidification, creating a reticulated porous structure (9). ʈ As a result, scaffold fabrication is possible without introducing high temperatures or organic solvents. This fact has made it possible for sensitive biological molecules, such as protein drugs, to be incorporate...