A rare and dangerous substance with medical research uses that can also be turned into biological weapons, whose sale is forbidden by the Federation and which Sisko trades away in In the Pale Moonlight.
Bio-mimetic gel is a volatile material used in advanced medical and genetic research. Because it can also be used to make biogenic weapons, the Federation controls it tightly and forbids its sale.
It is first mentioned in The Next Generation episode Force of Nature, where a missing medical transport is carrying a consignment. In the Deep Space Nine episode Distant Voices a Lethean named Altovar attacks Bashir after the doctor refuses to supply him with it.
Its most notable appearance is In the Pale Moonlight. To obtain a forged data rod that might draw the Romulans into the Dominion War, Sisko overrules Bashir's protests and hands over a large quantity of the gel, one of several compromises that weigh on him. Voyager also treats it as valuable contraband.