Hull Repair Nanobots

 is one of the items that could be bought at the Emendation Station 3500, at $7500 each. It repairs 30 damage to your pod when used, which is $250 per HP.

As the Emendation Station 3500 itself repairs damage for $15 per HP, Hull Repair Nanobots are only useful in depths below -5000 ft where invisible gas pockets appear and highly valuable minerals justify its cost, as well as in the Ending where they are absolutely critical.

Trivia

 * Because of their role in the Ending, they are effectively the only consumable item that you cannot avoid buying. It is practically impossible to beat the Ending without buying them, while you can circumvent any other individual item (you can technically choose one out of Dynamite and Plastic Explosive, but you cannot beat the game with neither).