Synthoids seem to be molded from a biomimetic polymer-like material which can be programmed to assume human form, duplicating clothing, voice, and behavioural characteristics of the original subject.
In the two-part episode "There's No Place Like Springfield", Cobra employed synthoids en masse in a scheme to extract classified information from G.I. Joe team member Shipwreck, who had secret weapons knowledge imparted to him under hypnotic suggestion.