The only possible semiconductor failure remaining is the
possible malfunction of switch S5. Nevertheless, this is not
considered as a major problem. If S5 fails as short-circuit, the
stimulus current will bypass the nerve. The nerve will not be
stimulated, but it will not cause any danger. If S5 fails as an
open-circuit, it can be dangerous to the nerve because of the
absence of the discharging phase. In silicon-on-insulator (SOI)
technology, the solution opted for is to implement S5 as 3
separate transistors in parallel, so there is ample redundancy.