Materials with low surface energies like polymers and metals are not wettable by printing ink and adhesives. These materials require some treatment to match the surface tension of adhesives. The procedure of using chemicals, plasma or radiations for increasing the surface tension of non-polar materials is called as surface activation.
Although every method of activation has its own pros and cons, choice of the treatment method depends on the nature of the material being treated. For example, plasma treatment is commonly used to activate the polymer surfaces, while for metal surfaces UV radiations are used.