California has an exception for hearing aids and other similar devices, but it’s unclear if transcription aids count, or if this has been tested in court. https://codes.findlaw.com/ca/penal-code/pen-sect-632/ (Not a lawyer, this is not legal advice.)
Do you mean ephemeral, or are you actually wondering about something implanted under the skin? I'd think/hope if it goes under the skin, it ends up in "hearing aid" territory. I'm less sure about if it doesn't persist.
