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Safety of medical devices is not something you should be asking random folks on the internet about.  The rules are stringent, for good reason.

The clearance and creepage distances to line voltage, insulation voltage, leakage, and other specs are much tighter than for ordinary consumer devices.  Being battery powered gets around some of those, but you still have to be careful.  Even 1.5 V well coupled to parts of the body can cause serious harm.

Again, you have to get and read the standards carefully, then get the device tested and certified to those standards.  It will help to have someone on the team that has done this before.  If nobody in your group has this experience, then I suggest you engage a consultant to help you with compliance, testing, and certification.  Then you can be the experienced person next time.

In addition to the design meeting certain criteria, there are also requirements on the design process and its documentation.  Here in the US you file something known as a 510k with the FDA (Food and Drug Administration).  The details will vary around the world, but this is not something you do after the fact.  You have to incorporate the methods and documentation into your process from the beginning.