Is it possible to make a fuel cell that runs on triglyceride?
It is certainly possible to oxidize fats electrochemically, but there are a few problems with the idea that I can think of:


Triglycerides do not flow freely through the blood as they are too hydrophobic and are instead carried by lipoproteins that would have to be removed. Similarly, free fatty acids are transported bound to serum albumin.
The blood concentration of triglycerides and fatty acids is relatively low most of the time in healthy individuals.
Existing fuel cells that work on fatty acids seem to be mostly microbial fuel cells, which are quite slow.


I think it would be more practical to use glucose from blood instead, using an enzymatic fuel cell, as blood glucose levels are relatively high and the oxidation is easier to accomplish (though there are still a lot of technical challenges to be resolved—biofouling, efficiency, device lifetime, etc.).