Abstract: The present invention relates to a piston assembly for use in a fluid delivery system. The piston assembly is comprised of two elements: a splined-piston having generally a convex shape and a ribbed-base, that receives the piston, which has a complementary concave shape. The interposition of the splines and ribs and the complementary shaping of the two elements permit: a constant supporting surface area to be realized throughout a pumping cycle; the establishment of a constant supporting surface that readily and fully supports a thin, flexible material such that the material cannot not likely become stretched, unsupported or bound within the pumping mechanism throughout the pumping cycle; and the establishment of a relationship between the piston and its associated pumping chamber that enables a precise volumetric quantity to be delivered consistently and accurately.