17. Polymeric Implants Polymeric implants are commonly prepared from hydrogels,silicone rubbers, or other biocompatible materials. Hydrogels generally have an advantage in that they are able to swell in an aqueous medium without necessarily dissolving. This capacity for water is useful in that it promotes or improves compatibility with body tissues but it has been suggested that incorporated low molecular weight drug substances are able to diffuse out of the device without necessarily being rate-controlled. 3. For additional information see the chapter FERTILITY CONTROL. 4. Sub dermal injection of small rods or cylinders or even spheres through a cannula has some advantages, and these devices have also been evaluated clinically.