Mercury/amalgram fillings isn't the best example, but biocompatibility is still the primary constraint. Mayo clinic has a good overview and timeline regarding

Mercury/amalgram fillings isn't the best example, but biocompatibility is still the primary constraint. Mayo clinic has a good overview and timeline regarding the discovery process for cobalt-chromium, which was previously thought to be a good all-around implant material: https://medlineplus.gov/ency/article/002495.htm https://medlineplus.gov/ency/article/002495.htm