Thermoelectricity in natural and synthetic hydrogels.

We describe a technique for measuring a Seebeck effect in gels and present data for three systems. Notably distinct signals are obtained for gel originating in the electrosensitive organs of marine sharks, synthetic collagen-based gel, and as a control, seawater, the gels' solvent. Only the gel of sharks shows a reversible thermoelectric signal. The difference between gel samples and seawater simply confirms that gels suppress mass transport. The difference between synthetic gel and the gel of sharks shows that the charged polymers of the shark gel restrict mass transport much more successfully than the polymers of the collagen gel, and we submit that this sort of ion localization is key to the emergence of thermoelectricity in a gelatinous substance. We compare the properties of the natural gel to those of established thermoelectrics.