It isn't illegal to chew gum, but it is illegal to bring them into Singapore or sell them except for medical purposes. Since you can't buy gum (selling is illegal) there's no way to chew them so it's almost as good as not being able to chew gum. See this: http://www.customs.gov.sg/leftNa...
Definition: In Vitro. A study done in vitro, in contrast, looks at how a drug or procedure responds in the confines of a test tube or laboratory dish. Clinical trials or medical studies may be performed either in vivo or in vitro.
Medical Definition of in vivo. 1: in the living body of a plant or animal in vivo synthesis of DNA microorganisms are not ordinarily destroyed in vivo by bacteriostatic drugsâJournal of the American Medical Association. 2: in a real-life situation observing a patient's behavior in vivo.