The only Rabbanim that I have seem mattir it are considered “fringe” Rabbanim. As I understand it, there are multiple issues – when they take it (definition of death), as well as the need to bury all body parts. Harvesting parts that can be lived without (cornea, liver, as someone else said) would also kill the patient as one in such a condition could not survive the operation. Once dead you can’t take anything. I don’t think there is a difference wether the parts are used for people or for research – in fact, research can save 100s or 1000s of lives, not just one. There are many places where you can donate while alive to people and for research – blood, half liver, one kidney, and for research you can even donate skin!
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