Is PLA truly biocompatible?

PLA is inherently biocompatible because it can be broken down by natural processes into lactic acid (by hydrolysis of the ester bonds), and then further into products that leave the body with urine. In fact, PLA has been applied for medical uses since the 1970s and is (I think) the single most widely used biopolymer in the medical industry. So for short-term contact, I’d expect that it is perfectly fine.

If you are looking to use standard grades, just be sure that it is essentially pure PLA, to be confident of the biocompatibility. You need to be careful with PLA filament for 3D printing, as it is more likely to contain additives to improve the behavior during printing, and the properties of the completed print. These additives can include calcium carbonate, acrylics, and fillers, which may not be biocompatible.