Is a single wire input/signal possible?
The problem with a single-wire sensor input is the same as the problem with a single-wire electric “circuit”.

In order to put an electrical signal into a wire, you have to push electrons into (or pull them out of) the wire. In normal electric circuits, the electrons flow in loops, so that the density of electrons throughout the circuit does not change. If there isn't a loop, then pushing or pulling changes the density, and that means you immediately have a huge electrostatic force opposing the change. You can do it but the voltages quickly become enormous, as they do with any significant electrostatic effect.

Considered as a circuit element, this forms an extremely low-value capacitor. And like any capacitor, its characteristics depend on the dielectric and what's on the other side of it (i.e. air or skin and flesh); the technique of extracting information from this is called capacitive sensing and it's probably what you want to use.



Another way to carry, or perspective on, a signal over a single wire is Goubau line or single-wire transmission line. You could think of this as a little bit like using a high enough frequency to pass that tiny capacitor. However, it is an RF signal and as such it must be considered as a wave passing through the space around the wire as well as, or instead of, the wire itself — you cannot route the wire next to other wires or other conductive objects without affecting the signal.

Since we've brought up radio waves, I'll also note that a radio is not, despite popular understanding, an example of a device that transmits or receives information with a single wire. A radio antenna always has two parts which experience a voltage difference between them. (Those parts may be the visible antenna and the chassis of the device, or they may be the two ends of a single antenna conductor tapped off somewhere other than the ends.) That voltage difference in the antenna is directly created by the voltage difference in the electromagnetic wave in space — if you've misaimed your antenna so that there is no voltage difference (such as by having the wrong polarization), then you receive no signal.