An electronic chip is to be implanted in the body. During in vitro (in the lab) testing it is observed that the chip will dissolve over time if exposed to liquid with similar pH to extracellular liquid found in the body. To avoid this, you coat the chip with a polymer layer. This prevents dissolution in vitro but when implanted in the body (in vivo) the chip still dissolves. Give three reasons why the protective coating might fail in vivo but not in vitro.