Sample processing devices with electrophoresis channels and methods of loading the electrophoresis channels with electrophoresis sieving polymer while rotating the sample processing device are disclosed. In some instances, the electrophoresis channels may be arranged radially relative to the axis of rotation of the sample processing device. In other sample processing devices, the electrophoresis channels may be arranged in curved arcs that are concentric about the center of the sample processing device (which preferably corresponds to the axis of rotation).