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Research: Bio-Inspired Polyelectrolytes
Some species of shark have the extraordinary ability to sense temperature differentials in their surroundings to a remarkably high degree of accuracy by using electrosensing organs consisting of a nerve set at the end of a pore which is filled with an extracellular gel comprised of sea water and a small percentage of glycoprotein. The nanoEngineering laboratory has developed a technique to measure the Seebeck coefficient of a white shark ( Carcharodon carcharias ) in response to a small temperature differential. Results demonstrate convincing evidence that the Seebeck coefficient of the gel is 20 ± 8 µV/K – higher than that of simple salt solutions. The phenomenon underlying this behavior is likely a result of the dynamics of the fluctuating hydration shells that form around the ions and negatively charged glycoproteins. A temperature differential across the gel results in a Gibbs free energy gradient associated with hydration. This influences a net flux of water molecules toward the cold end of the solution, and consequently, an opposing force pushes the ions/glycoproteins through the water in the opposite direction towards the hot side. The situation is similar to ion/water transport that arises in the presence of a concentration gradient across an ion exchange membrane, but here a temperature gradient is the driving force. Since different ions move at different speeds, a voltage generated by the developing charge separation between the ion pairs works to create an equilibrium ionic velocity. The glycoprotein in the presence of the thermoelectric force, however, encounters immense drag in the opposite direction limiting its velocity. This subsequently slows down the positive ions paired with it, and a large charge separation develops, resulting in a relatively high voltage signal. The work in the nanoEngineering lab has been to develop this qualitative explanation of the enhanced Seebeck coefficient in polyelectrolyte solutions and to design and synthesize artificial bio-inspired polyelectrolytes that exhibit superior thermoelectric properties.
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