The effect of a plasma on a given material is determined by the chemistry of the reactions between the surface and the reactive species present in the plasma. At the low exposure energies typically used for surface treatment, the plasma surface interactions only change the surface of the material; the effects are confined to a region only several molecular layers deep and do not change the bulk properties of the substrate. The resulting surface changes depend on the composition of the surface and the gas used. Gases, or mixtures of gases, used for plasma treatment of polymers can include air, nitrogen, argon, oxygen, nitrous oxide, helium, tetrafluoromethane, water vapor, carbon dioxide, methane, or ammonia. Each gas produces a unique plasma composition and results in different surface properties. For example, the surface energy can be increased very quickly and effectively by plasma-induced oxidation, nitration, hydrolyzation, or amination. Depending on the chemistry of the polymer and the source gases, substitution of molecular moieties into the surface can make polymers either wettable or totally non-wettable. The specific type of substituted atoms or groups determines the specific surface potential. For any gas composition, three competing surface processes simultaneously alter the plastic, with the extent of each depending on the chemistry and process variables: ablation, crosslinking, and activation (11). Ablation is similar to an evaporation process. In this process, the bombardment of the polymer surface by energetic particles (ie, free radicals, electrons, and ions) and radiation breaks the covalent bonds of the polymer backbone, resulting in lower-molecular-weight polymer chains. As long molecular components become shorter, the volatile oligomer and monomer byproducts boil off (ablate) and are swept away with the vacuum-pump exhaust. Crosslinking is done with an inert process gas (argon or helium). The bond breaking occurs on the polymer surface, but since there are no free-radical scavengers, it can form a bond with a nearby free radical on a different chain (crosslink). Activation is a process where surface polymer functional groups are replaced with different atoms or chemical groups from the plasma. As with ablation, surface exposure to energetic species abstracts hydrogen or breaks the backbone of the polymer, creating free radicals. In addition, plasma contains very high-energy uv radiation. This uv energy creates additional similar free radicals on the polymer surface. Free radicals, which are thermodynamically unstable, quickly react with the polymer backbone itself or with other free-radical species present at the surface to form stable covalently bonded atoms or more complex groups.