One of the most popular experimental evidences in support of galvanic coupling has been described in Wegmuller's doctoral thesis [1]. It has been argued that in case of galvanic coupling alternating current flows inside the human body and not on the surface of the human body.
It is relatively harder to prove that the alternating current, or alternating potential, necessarily flows inside or outside the human body. Perhaps, the following reason could be given that the outer most upper epidermic layer of skin acts as a dielectric medium for any type of non-invasive body electrode. So a coupling capacitor is naturally formed on the surface of the skin when an electrode is placed on the human skin. The alternating current driver enables the electric current to cross the capacitive barrier formed on the skin. However, one needs to be careful when using dc current driver which would only saturate the coupling capacitor. Now, the capacitor acts as a current integrator according to the following relationship converting alternating current into alternative potential which could polarize the surface of skin as well.
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It has also been argued that the differential electrode used in galvanic coupling provides ground independent reference. But the usage of differential electrode greatly attenuates the signal coupling on the receiver side as the prime current flows between the differential electrodes on the transmitter side. It could additionally be argued that the prolonged exposure of skin to differential electrodes could result in the wastage of useful power as heat which may cause skin itching over a longer duration. So it might be useful to think of some other alternative to reduce ground dependence.
References
[1] Marc S. Wegmueller, “Intra-Body Communication for Biomedical Sensor Networks,” Doctoral Dissertation, Diss. ETH No. 17323, University of ETH Zurich, 2007
