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This paper presents the simplified analytical and numerical analyses of the ultralarge deflection of circular polydimethylsiloxane (PDMS) microballoons (MBs) under pressure for microelectromechanical systems design. The analytical model assuming spherical symmetry on the deformed shape of MB yields a simplified solution on the pressure-normalized maximum deflection and two in-plane principal strains of the ultralargely deflected MB. The accuracy of the theoretical model is evaluated by comparing against the experimental results. Furthermore, by using a computational analysis that incorporates a Mooney-Rivlin model for a PDMS micromembrane, the properties of the PDMS fabricated at a mixing ratio of 10:1 base to catalyst, a curing temperature of 20°C, and a curing time of 48 h are determined for noncircular PDMS micromembrane analysis. In the experimental study, eight types of PDMS MBs, each of which has a membrane radius of 143, 202, 452, or 904 μm and a membrane thickness of 10 or 20 μm, are characterized in air and in cell culture media. A new strain-measuring method using fluorescent polymer microspheres for the PDMS MB is also introduced. The characterization of the PDMS MBs in air validates our theoretical model and shows an increase in elastic modulus as the membrane thickness decreases. The effect of cell culture media on the membrane rigidity of PDMS is also examined for biological applications of PDMS.