You'd think if such devices were used in humans, they wouldn't require a change of lifestyle.
They don't say the power always decreases over time:
Katz’s snails, for example, produced up to 7.45 microwatts, but after
45 minutes, that power had decreased by 80%. To draw continuous power,
Katz’s team had to ramp down the power they extracted to 0.16
microwatts.
This is really a chemistry question. The glucose has to be brought to the fuel cell some way. In this case, the glucose is oxydized directly in the hemolymph. As oxygen PP in the hemolymph is always greater than glucose concentration(1), your limiting rate (which is related to the power you can get from the device) is that of glucose intake at the fuel cell. Therefore, the more you oxidize glucose per unit time, the greater intake you need to keep the output from dropping.
The glucose intake at the fuel cell is a function of biological parameters such as quantity eaten, metabolic efficiency, metabolic speed, blood flow, but also diffusion. The kinetics of glucose intake at the fuel cell are such that you can't ask for too much power for too long.
I suggest you read the article to get a more detailed answer. Nature news is good, but there's nothing like reading the real paper.
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