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.
Ex vivo means that something is experimented on or investigated outside its natural in vivo environment while in vitro means in the test tube. Eg. ex vivo gene therapy means that cells are taken directly from the body, transduced with the gene in vitro and then returned to the body.