When designing something for the human body there are a number of considerations. Some that we have thought of, others we have not. These are things like how hot does the device get, what sort of wear and tear can it handle, is there a fail-safe, what are consequences of long term exposure to silicon, etc.
Coatings - various coatings and their properties, use cases, MSDS links, proper ways to apply them
Battery Chemistries - various battery chemistries that are noted as low risk due to properties such as thermal runaway, out gassing, and the like
Power Sources - various Power Sources other than batteries such as --energy harvesting, super caps, wearable induction, etc
Safety and Fail Safes
Safety and Fail Safes - methodologies and equipment for fail safing devices, such as kill switches, GTPI, fuses, appropriate ways to notify the user, software, best practices to avoid infinite loops and borking the device due to bad code pushes
Toxic Materials - material in common components known to be toxic and ways to mitigate risk when alternatives are not available
Biomechanics and Ergonomics
Biomechanics and Ergonomics - reducing the impact on the body. No edges, no corners.