Sojiro had a simple enough idea for his first body modification, it was the idea to replicate a second heart in the target so that it seamlessly incorporated itself into the body. The first tests were rats that he had decided to run the tests on, trying to incorporate the organs into their body through several injections. Sojiro could simply try to implant a second heart into the creature, cut it out of another and then slap it in, but that isn't what he wanted to do. If he had to kill one thing so that the other could prosper, while this idea didn't personally bother him, that wasn't the type of experiment that would be seen in positive light. He wasn't trying to create Frankenstein, he was doing his best to create a flawless replication process through injections. Sojiro would slowly make his way through nearly two dozens rats as he recorded the different reactions and the ways that they ended up, making sure that he wrote down all the information of each test. After going through so many rats, Sojiro was almost positive that he had found a serum that could get what he wanted; using a series of rare chemicals and DNA from the intended target. Even being so close, it would have taken Sojiro another week or so before the injection would stop killing the creatures. The problem was trying to find the perfect amount, so that the organ came in at the right size. If it was too small, it would be pointless and then fail, and if it was too big that it would shove the other organs out of the way.
Finally, when the first rat had successful survived the proceed, Sojiro would then do it a few more times and keep six rats in a cage and take care of them normally. In the next week, only two of the rats had died and, after looking at the reasons, it had been excessive heartrates that had done them in. He would take note of this, making sure to try and create some type of medicine to handle that. Sojiro was a man of medicine so it wasn't hard, in the next couple of days, for the man to try and create something that could slow their heartrates. Doing this would leave three rats by the end, the fourth rat not getting enough of the medicine before his body finally gave out. In the time that would follow, the last three rats would be fine and, with that, Sojiro finally had a successful batch of test rats. It had been a long a grueling process so far but he wasn't done yet, no. The man knew that rats were just the easiest to get in large quanities... But now, Sojiro was ready for human tests!
He laughed to himself at the joke, shaking his head as he did. The man, actually, would go to the local pound and find the dogs that were going to be put down that day. After talking to the people there, while it was a process, he eventually was given permission to take two dogs with him so that he could try and run some tests... Of course, he didn't tell them that's what he was doing with them but what did it matter? They were going to die. He made sure they were in well enough shape, making sure they were strong enough to try and handle an injection while he would nod his head. The man would inject one of the dogs with, what he thought, was a perfect dose while leaving the other be. The heart seemed to come in fine and Sojiro fed the beast the medicine but, a few days later, it would give out from exhaustion and take the first heart with it. That was... Disheartening but he wouldn't let it stop him.
The test on the second dog had gone successful, ending in just a week and coming out a complete success. After this, with no jokes, it really was time to test it on a human... Himself. The man would get the medicine prepared and the injection, doing it in his surgery room. He'd slide the needle between his right rib cage at an angle before injecting himself. The man screamed in pain, feeling as though his insides were on fire before passing out... When he came to, the man would run some test on himself and... It was a success. He'd just have to make sure he didn't die in the coming days.