Abstract: A method of preparing a hydrophilic plastic cartridge by centrifugally cang polymerizable hydrophilic material in a rotating polymerization tube whose longitudinal axis is maintained parallel to the ground. The speed of rotation causes radial outward displacement of the polymerizable material which upon assuming a predetermined shape within the rotating tube is then polymerized to the predetermined solid configuration. The resulting plastic cartridge is characterized by smooth, unscored internal and external cylindrical surfaces. The cartridges are used as a rate-limiting membrane in drug delivery devices. Sterilized kits containing a disposable needle/syringe or trocar-like instrument and the drug delivery device are used for subcutaneous implantation of the device in an animal body.