https://phys.org/news/2018-07-molecular-traceability-medical-implants.html

The researchers have used polymers to do this. These are large molecules composed of two basic subunits whose linking connections form a code, similar to sequences of zeroes and ones in computing. By determining the mass of each polymer fragment using an analytical chemistry method called mass spectrometry, the molecule's "code" can be determined and decrypted in the same way as a bar code.

These molecular labels have been incorporated in tiny quantities in model implants, which were implanted into rats. After three months, the implants were extracted from the animals and analyzed. Mass spectrometry showed that the identification polymer could be decoded unambiguously.