Professional medical monitoring equipment has isolation out the kazoo between patient and equipment. Not only is the isolation necessary for the obvious -- patient safety -- but is also necessary so that unwanted signals from the mains does not enter the input circuitry and swamp the tiny electrical signals generated by the body.
Some manufacturers have developed clever and/or unique methods to achieve isolation while maintaining a lot of versatility by using "mainframes" into which varied "plug-ins" can be inserted to customize the monitor for a specific application or applications. Even 20 years ago, Siemens had plug-ins with absolutely no electrical connections whatsoever. Power to the plugin was magnetically-coupled; the primary and half of the core of a small power transformer was inside the "mainframe" and the secondary and the other half of the core was in the plug-in, both electrically insulated from each other, yet in intimate-enough proximity to effectively couple all the power needed to the plug-in. Data was fed into and out of the plug-in via infrared analog or digital coupling rather than using connectors. Isolation was just one advantage. With no connectors, you didn't have to worry about contact wear, intermittent contacts, dirt, oxidation and all the other problems that plague connectors.
But for you, the number one concern is obviously isolation and a total lack of leakage. Don't try to get close to some "magic" "few microamps" of current. Keep it in the nanoamp range or less. The use of optoisolators and high-grade power transformers will be a must. Even though they made what was considered one of industry's best neonatal monitors, Tektronix got out of the medical monitor business (sold off the entire business division who got to keep the Tektronix name) because of the tremendous liability costs inherent in such items. Ergo, be really, really careful. As a teacher, I'd NEVER allow a student to design, make or experiment with any electronic device, line- or battery-powered, that was directly connected to a person's body. It's just not worth the risk.
Dean