Unlike natural corks, many synthetic corks are made from material that is not biodegradable but recyclable as resin identification code #4 or #7. There are two main production techniques for synthetic wine closures: injection molding and extrusion. There are also methods claimed to combine the two techniques of injection and extrusion. A 2007 study by Victor Segalen Bordeaux 2 University showed that injection-molded synthetic corks allowed the highest levels of oxygen permeation when compared to natural cork and screw caps, offering the lowest protection against oxidation of the wine.