When Mayuri stabs himself with his Zanpakutō while it is in its sealed state, his body changes into a liquid form. Doing so does not kill him, but instead simply leaves him unable to attack or be attacked, making it an effective escape tool. He can move around in this form and speak as well. His body reforms after a few days, but without any of his previous body modifications. This ability is a result of the extensive experiments which he has performed on himself.
↑Bleach Official Character Book 3 UNMASKED; page 155