RegMat

 is an Equip Item that has 10% chance to be obtained when destroying an I.A.B.O.B.E. and a 0.1% chance on Regen bloons. When equiped, the selected tower's HP is increased by 1 every round for 5 rounds.

Bloonopedia (check Bloonopedia for more)

 * Apopalypse Bloon was aiming to apply Regen status to M.O.A.B.-class with the same material he used on bloons but when he discovered this material he instantly implemented it on a M.O.A.B. but even though he realised that this made the M.O.A.B. weaker than it actually is, he only uses it on special occasions and oddly likes it.