What do you want to sense - temperature, pressure, resistance, voltage ?
There are several projects that I've seen described that are EKG type. They require some good ICs but didn't appear to be all that difficult to build. The simplest is a skin resistance meter - people doing biofeedback use these or build them. An air pressure sensor that with a range of 0.5 psig to 5 psig could be used to measure blood pressure - with the right mechanical and electronic accessories.
A note of caution - whatever it is that you build, if there is any normal or failure mode that could result in a fatal or injurious flow of current (or temp, pressure, etc) then you should not actually use it on someone.