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Active pixel sensor (APS) is a promising technology for next-generation vertex detectors. This paper discusses the design and testing of two generations of APS chips. Both are arrays of 128 by 128 pixels, each 20 by 20 μm. Each array is divided into subarrays in which different sensor structures (4 in the first version and 16 in the second) and/or readout circuits are employed. Measurements of several of these structures under Fe55 exposure are reported. The sensors have also been irradiated by 55 MeV protons to test for radiation damage. The radiation increases the noise and reduces the signal. The noise can be explained by shot noise from the increased leakage current, and the reduction in signal is due to charge being trapped in the epi layer. Nevertheless, the radiation effect is small for the expected exposures at RHIC and RHIC II. Finally, we describe our conception for mechanically supporting a thin silicon wafer in an actual detector.