There's a book I borrowed from my grade 8 teacher's lending shelf (in Ontario, Canada) that I've been trying to identify for years.
The main character was a man in a futuristic setting who... I think... had the secret to an improved version of the lifespan-extension serum everyone used. If I remember correctly, the flawed version caused people's skin to turn clear over the course of their extended lives.
I think he was revived from some sort of stasis, but I can't be sure of that. I just vaguely remember a "stranger in a strange land" sort of cultural disconnect, him being cared for in some kind of hospital-like setting, and a sense that maybe the formula they want from him was invented by a now-dead family member of his.
The most distinctive detail I remember was the love interest, a cloned nurse named "Eliza Tertia" who he encourages to break out of her role. (All the cloned nurses were named Eliza, and I remember "Eliza Duodecima" being the name of another one who is briefly seen.)
I also remember the secret eventually being gotten out of him, only for it to be discovered that it's flawed, in that, while it does replenish the compounds that go missing in extended old age, it doesn't moderate their production.
I don't remember if it was long enough to be a novel or if it was just a novella.
My vague memories of the cover design and material suggest it was a 1970s printing or earlier, which would also fit with what I remember of the story's atmosphere.