Complex sentence: Few materials are completely bioinert, that is they create no bodily reaction; some that may be bioinert in theory, fail to be ideal in practice because of defects in their formation, processing, manufacturing or sterilization.
Split into simpler sentences: Few materials are completely bioinert, that is they trigger no bodily reaction. Some material that may be bioinert in theory, fail to be ideal in practice because of defects in their formation, processing, manufacturing or sterilization.