The story is that Jeff Karp's and Robert Langer's labs jointly developed it, but if you pull back the curtain you will realise that a lot of the leg work was done by two people in particular: Will McLean, a co-founder, and another man called
Xiaolei Yin, who currently sits on the PCA advisory board for Frequency Therapeutics. I don't think many people know actually who did what, so I'll do my best to explain.
Will had been doing some general research into hearing regeneration and he was trying to trace the lineage of auditory cells in mice during their development i.e. which specific cells became hair cells, which cells became neurons etc. Without going into detail, he managed to develop these biological markers where you could literally track different types of cells through their lineage by their colour (this is actually a lot more difficult than it sounds). It was in this work that he was able to identify that it was the LGR5 progenitor cells specifically that turned into hair cells and not any other cells. In other words, he found the smoking gun with regards to which cell was responsible for making us hear. The problem was, as we all now know, that these cells stop regenerating after birth. It was known humans could not regenerate their hearing, but it wasn't known which cells were responsible for generating hearing cells in the first place. So now that Will McLean knew which cells to target, he was looking for a way to see if he could "switch" them back "on". Step in Xaiolei Yin.
Xaiolei had been doing some work on the intestine in the same lab at the same time as Will. The intestine was already known at this point to be one of the few active regenerative tissues in the body. The lining of the intestine regenerates every day 5 to 7 days. Now, I'm unsure as to whether Xaiolei discovered this or whether this was known previously, but the fact was that LGR5 progenitor cells in the intestine had been identified at some point as also being the "smoking gun" for regenerating the intestine cells. The difference was that these cells were accompanied by something called paneth cells, which provided the cellular signalling for the LGR5 cells to keep regenerating and producing intestine cells. Xaiolei was able to show that through a combination of turning Wnt and notch on/off (the signal pathways) using small molecules, he was able to emulate the effect of the paneth cells and produce colonies of all the different types of intestine cells one needed for the intestine.
This is where Xaiolei's work collided with Will McLean's. LGR5 cells in the cochlea, having been identified by Will as the cells responsible for generating hair cells, became a convenient testing ground for Xaiolei's work. The rest is history: they applied the small molecule approach to the LGR5 cells in mice cochlea and ex-vivo human cochlea which lacked the signal pathways required for regeneration (due to no equivalent of paneth cells) and were able to fully regenerate hair cells throughout the cochlea. They also showed that the process was self-limiting, in that once enough cells had been regenerated, the process stopped.
All this work was done in the labs of Karp and Langer, but these guys really did knock it out of the park.