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That's heavy battery package. Is the heart much more efficient than these devices? I'd doubt so.

Does anyone know if there's been research to extract energy from the blood directly?

The heart is massively more efficient. Just about everything in all of biology is massively more efficient.

Take a moment to consider that your entire body runs on an energy budget of only around 100 watts, of which your brain only uses a measly 20 watts.

given that the heart manages to operate inside the body without those two external battery packs, I'd wager that it's more efficient.

The heart has access to the resources of the body.

Does the human body provide more power to the fleshy heart than two large external batteries?