I'm all for experimenting on yourself. Go for it.

However, I'm also a software engineer for a medical device company and I spend a great deal of time assessing and mitigating the safety of various operations. Please take care to understand the failure modes of anything you build and attach to your own body. When you get to the point of using an Arduino (or anything else that uses software), the number of failure points goes up dramatically. Try to consider how anything could fail and what the consequences will be.

What's the purpose of the arduino? Couldn't you setup a push button that needs to be pressed every 5 minutes or so to keep the circuit active, like a watchdog? You could also add a small piezo speaker or blinking LED to warn you to press it 30 seconds before the timeout.

Or even more advanced, connect it to a computer/tablet through USB or bluetooth, mimic a keyboard/mouse and use your real keyboard key presses/mouse movement to reset the countdown. I wouldn't use this directly though, but instead through opto-couplers to avoid electric shocks or other safety issues!

Related to watchdog, you should probably also activate the hardware watchdog of the microcontroller to make sure it doesn't crash.

27V at 100mA is 2.7Watt dissipation, quite a lot when in contact with skin!