This has been suggested in the past, multiple times. So far nobody has been able to give good answers to:
Ground units do not have personnel - single infantry can be a single soldier or it can be a group of soldiers or robots or whatever. There is no population cost for building ground units nor is there any connection between population and a ground unit. How would a medical module interact with vehicles? Should there be a separate battlefield recovery module only for vehicles? Or is this a magic TN module that works with everything?
Does the module double the actual HP of units? Or does it provide you with a random roll of saving X number of units per combat round? Should there be a tech line that improves the number of units saved? Should there be a different tech line that improves the odds of saving a unit? Should there be a third tech line for the amount of HP bought back per combat round? If it only interacts with HP, then regular infantry cannot gain any advantage - only genetically modified super soldiers can.
How many GSP should it consume per combat round? Does it need a fourth tech to modify that supply consumption? Should there be an additional cost, because this might alter how ground combat works radically?
As for ships, there it can work a bit easier since Crew both per ship and per race is tracked. Still, you should think about the tech line associated with it and what sort of percentages are we talking about - is it reducing the number of crew killed when a component is destroyed or does it bring back X number of crew from death after combat? What happens if the module itself gets hit - does that cause double or triple casualties to simulate the wounded being treated there? Should this require a new leadership bonus for naval officers or is this another use for Science officers?