The heart is probably the most complicated thing that Joe has made so far (Well, it was built by his duplicates, really, but the same principles are in play). I didn't want to go off on a full tangent because I've already done that in previous chapters, plus Fleet doesn't care about mechanics as much as performance. Survey would have catalogued every input power, but to Fleet it only matters in terms of the impact on the situation. There will probably be some flack about how the boots got more screen time than the new heart, but the boots being constructed were meant to show Joe's collaboration with Garment as well as drive home the fact that he was currently working on a completely different level, to the point where something as basic and cobbling was a divine act of creation.
I'm probably not going to give a full breakdown of everything that went into the heart in story, but I can give the broad strokes here for people who care. A big reason he was able to work the heart to this level is because of the current state of his biotech knowledge. It's more than just Exotic Compatibility, it's the surrounding perks that give him enough biological knowledge that living flesh can count as a material. In this case it's Grease Monkey, Nanite Sciences, Deranged Alchemist, Valkyrian Science, and Valuable Memories (the creation of chimeras).
Since the new heart was being produced by smithing all of Joe's smithing and materials science perks impacted the final result. This included Smithing, Aerospace Engineering Makes Things Go Fast, Skills: Physics, Skills: Physical Sciences, Science! Engineering, Unnatural Skill (Smith), Advanced Materials, Dwarven Craft, Divine Child – Hephaestus, Lathe of Heaven, Fate Finds You Interesting, Minor Blessing Athena – Craftsmanship, Secret of Steel, Unnatural Skill (Alchemy), and Material Synthesis Science. Some of these are very significant, beyond just superb workmanship. Lathe of Heaven is basically endgame RPG crafting and the ability to take any material beyond what it should be capable of. Secret of Steel provides a durability boost that's close to what you see from Master Craftsman and lets the 'basic pump' element of the heart function as if it was far more advanced. The Percy Jackson perks are what made it possible (along with Exotic Compatibility) to work the remains of Joe's heart with Lung's bone and blood to produce a heart flesh version of Bone Steel, meaning it's a massively magical material. Even minor stuff like Material Synthesis Science has huge effects, considering that's the fiat backed method of producing Gundanium-level alloys. For context, in addition to being ten times as strong and heat resistant as titanium Gundanium is invisible to radar and electrically non-conductive. Not resistive, non-conductive, as in electricity cannot pass through it. That's the benefit seen from one fairly minor material boosting perk.
After that you have powers that add new abilities to the heart rather than just enhancing the material it's made from. The heart was handmade, so right from the start you have Master Craftsman making it a divine object and enhancing every aspect of it. Stuff like Built to Last, Reliable Invention, and Robust Engineering mean it will pretty much last forever while needing miniscule amounts of energy to function. All of Joe's style perks apply, meaning it's incredibly beautiful, not that it's likely to be seen by anyone else. Modularity from "They're Like Legoes, Right?" made it easier to integrate into the body after it was crafted. The Skyforge, combined with the heart being treated as iron, meant that it became harder and stronger. The Volcano allowed the energy of extremely high grade ragnite to be imbued into it as well as further enhancing the properties of the material. Lack of Materials granted additional capabilities to the heart, since all the necessary materials were used. Ambrosial Artificer treated the heart as a machine and allowed its design to be simplified while boosting performance. Heretical Adaptation was also applied to the material of the heart, allowing it to improve over time.
Since the heart was treated like a machine when it was built Hybridization Theory applied to double its performance as well as facilitate two different uses of Arcane Craft being applied, one to focus and enhance on the healing energies of the ragnite, and once to enhance the connection to Joe's soul. The Arcane Craft is based on the theme of the item in question, and there's a lot of potential for a heart. Source of life and seat of the soul (which is how hearts were viewed in Egyptian mythology, which was why they weren't removed during mummification) allowed Arcane craft to enhance these elements. The soul connection was greatly helped by That Undefinable Thing, which allowed a stronger and more direct enhancement to Joe's spirit that would otherwise be possible, just as Joe's biotech perks helped with the use of Arcane Craft to boost the lifeforce connection. Things like Setup Wizard, Technosorcery, and even Mechanist also helped integrate the magical and physical sides of the heart's construction.
You also saw one of the first serious uses of Castlevania Alchemy in the creation of the heart. Normally that wouldn't have been possible, but Unnatural Skill: Alchemy boosted Joe's (and his duplicates') skill with that art. The "items of power" created by that perk run on spiritual energy and are capable of some serious properties. Essentially he has an intrinsic part of his body that's functioning on the level of the most powerful items from that series (the orbs from Lament of Innocence would be an example).
One of the more esoteric aspects of the construction comes from Minor Blessing Hestia – Hearthfire. With Joe's level of pyrokinesis normally he would just instantly heat a material to the temperature he needs to be able to work it, but with Hestia's blessing he's switched to actually using flames because of the divine nature of Hearthfire. If there's any doubt, consider what Greek myth has to say about living beings exposed to the divine fires of a god's hearth. There's a rather prominent hero who has his origins tied to that practice. Yeah, the duplicates pulled a pseudo-Achilles and burned the mortality from Joe's heart.
Then you get into the Elven Enchanting. This was a serious undertaking and the duplicates probably would have tried singing to the Unseen if they had any hope of being able to manage it. As they weren't in any way prepared for something like that they stuck with naming, but used multiple words to create the name. This was massively draining, but they got around that by using Workaholic to increase the size of the heart after creation, only needing to enchant something with 1/25 the mass of their final product. The material reduction perks also meant they were able to build an entirely new heart out of the shredded remains of Joe's old one (Joe's scrapping perks, like Scrapyard Skills, Scrapper, and Mauler also helped with that).
After all that you get to the reformatting. This was done in the manner of Beast Machines, converting Joe's heart from fully biological to technoganic. The exact details of what this means aren't that clear, even by the standards of Transformers lore, but it's described as machine and organic bonded on the cellular level. That suggests mechanical components functioning within cells, like cellular cybernetics. Essentially, anything that could be done better by a mechanical component on that scale is replaced by a cybertonium construct with the rest of the cell functioning as normal. Technorganics have some weird powers, but a big one is massively improved repair speed and special abilities that are nearly psychic in nature.
So in summary, a massively powerful creation. Really, it's the closest you've seen to Joe going all out on what his crafting is capable of, an item with a staggering amount of knowledge, craftmanship, and power poured into it. In terms of the 'core of self' comment, that's not a point of vulnerability. It is kind of laughable to consider the most durable object in Joe's body (and let's be honest, probably on of the toughest things on the planet) as a weak point. Even looking past that, Joe's life isn't tied to his heart. He could survive without his heart (at least as well as anyone else), it's just his heart greatly enhances his survivability, as well as all its other effects. Really, with the 'core of self/anchor of soul' thing Joe could probably survive having his brain destroyed as long as his enhanced heart remained, and he would be able to rebuild the damage using nanites or other powers. The combined level of power that went into this construction should not be underestimated.
