Board Thread:Questions and Answers/@comment-5693502-20141219175924/@comment-25881756-20141223042702

Anti-Hollowfication Vaccine: A medicine created from combining the Quincy's' Heilig Pfeil and some Human souls. If injected into a soul of a Hollowfied Shinigami it stabilizes the conflict of souls and stops the "Soul Suicide" side-effect. This is the only means of saving a Hollowfied Shinigami from dying and enabling him to conquer an Inner Hollow.[170]

There were some other inventions which were noted to use human souls but frankly I'm too lazy to find them.