Sentence:  Protein bound chlorhexidine releases slowly leading to prolonged activity. This phenomenon is known as <WSD>substantivity</WSD> and allows for a longer duration of antimicrobial action against a broad spectrum of bacteria and fungi. [http://chlorhexidinefacts.com/mechanism-of-action.html] 
Meaning of the word marked with <WSD> tags:  The ability of a chemical or dye to stick to a target.