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Because animals have no say and they don't have to pay the animals. Once it's done the animals will just be euthanized.

Because if it turned out to be dangerous it has been determined that the animals being hurt/killed is better than that happening to human test subjects.

Animals are the first step because testing in humans is more expensive. These devices will likely have to be tested in humans before commercialization.

> why animals? couldn't the researchers just swallow that "prototype about the size of a grain of rice" themselves and/or get several paid volunteers?

Insurance. I assume it's easier to get insurance for human testing if you've shown safety in animals first.

Contrary to the other answers to this question, I suspect the real reason is that the prototype is expensive, and way easier to recover from a lab animal’s poo than the researcher’s.