Ever since that night, Sojiro had made sure to try and get stronger, so that something like that would never happen again. The man could bring back his arm whenever he wanted but losing it provided him a golden opportunity, to replace it with something that was more capable than a simple arm. And, because of that, the man would go for several weeks before he finally had created an idea for the replacement. During the weeks that followed, Sojiro had found himself pushing the limits of both his physical body and mental capacity. It was only a matter of time before he started experimenting with changing the way that one's body reacted to certain mutations and adaptations. There were countless trials that were run on animals but when it came time to perform his first extreme experiment, he used himself as the test subject. Because of that, Sojiro had a second heart in his chest after it had gone just as he expected. After that, he had ended up testing other modifications until a boy by the name of Jayy Farron had ended up on his door. After revealing that he was a Kaguya, the boy had ended up becoming the next person that Sojiro was going to end up experimenting on. The surgeon would have performed another successful experiment, making it so that the boy had a tail attached to his body that worked with the youth's Kaguya Ability. With this new knowledge, as well access to an array of bloodlines to try and implant into people, Sojiro had a new fire lit in his gut.
Without even realizing, in his heated grab for knowledge, Sojiro had reached S-Rank. Despite figuring this out, the man had decided to keep this information for himself to keep the appearance of an average shinobi. Well, average enough, at least. During this time, Sojiro had secretly decided to take up the skill of Ninjutsu and progress his knowledge in it. In this time, he had picked up the Wood Release element and that was when the idea for an arm had finally hit him. With this new ability, he thought of a concoction that could replace his arm and help with the future ideas that he was planning. It had taken some time to get the right idea, between mixing muscle and tissue with the Mokuton Element, it proved more than difficult. But, after nearly week, he was sure that he had a concoction that would be more than viable. Sojiro had waited until the clinic was closed and he was alone before he would perform the procedure in his office, injecting the contents of the vials into the spot where his left arm used to be. It burned, then almost bubbled while making a sizzling noise before the limb would expand from it covered in a white goo. Sojiro grunted before flexing the limb, grabbing the side of his desk and astounded that it felt like his real arm. He would stretch it across the room, grabbing something from the other side, before pulling it to him with a smirk on his lips. After testing it out a bit more, he would retract the arm until it looked like it was never there.