The electrical activity of living organisms and human-made devices evidence a fundamental mismatch. Living organisms transmit electrical messages by moving positive charges, protons, and positively charged ions such as calcium and sodium. Human-made devices—retinal implants, nerve stimulators, and pacemakers—rely on negatively charged electrons.
If this mismatch could be resolved, bioelectronic devices could interface more seamlessly with living tissues. For example, implanted devices that exchange electrical messages with the nervous system could monitor disease progression more closely, or interfere with it more subtly. In addition, bioelectronics could more easily emulate biological systems, and these devices might well have applications beyond medicine.
The trick, however, is developing materials that not only transmit positive charges, but are also easy to work with in biological contexts. To date, proton-conducting materials have fallen short. Such materials, which include ceramic oxides, solid acids, polymers, and metal-organic frameworks, have been useful in renewable energy applications and in fuel cells, batteries, and sensors. But they seem less promising as a basis for next-generation biological implants.
At this point, an unlikely helper may lend a tentacle: Loliginidae, the common pencil squid. This organism, it turns out, contains within its skin a protein that can conduct protons. Accordingly, the squid may point the way to a new generation of medical technologies that communicate more directly with the human body.
The discovery that the protein, called reflectin, conducts protons was made by researchers at the University of California, Irvine (UCI). They described their work in the July issue of Nature Chemistry, in an article entitled, “Bulk protonic conductivity in a cephalopod structural protein.”
In this article, the researchers noted that it was somewhat surprising that naturally occurring proteins such as reflectin had not received more attention earlier, given the “potential modularity, tunability, and processability of protein-based materials.”
The UCI research team, led by Alon Gorodetsky, Ph.D., assistant professor of chemical engineering and materials science, began studying reflectin to discern how it enables the squid to change color and reflect light. They produced the squid protein in common bacteria and used it to make thin films on a silicon substrate. Via metal electrodes that contacted the film, the researchers observed the relationship between current and voltage under various conditions. Reflectin transported protons, they found, nearly as effectively as many of the best artificial materials.
“Nature is really good at doing certain things that we sometimes find incredibly difficult,” Dr. Gorodetsky said. “Perhaps nature has already optimized reflectin to conduct protons, so we can learn from this protein and take advantage of natural design principles.”
According to the Nature Chemistry article, reflectin possesses qualities—proton conductivity, proton transport activation energy, and proton mobility—that may suit it for use in protein-based protonic transistors. In addition, Dr. Gorodetsky believes reflectin has several other advantages for biological electronics. Because it’s a soft biomaterial, reflectin can conform to flexible surfaces, and it may be less likely to be rejected by the human body. Finally, protein engineering principles could be utilized to modify reflectin for very specific purposes and to allow the protein to decompose when no longer needed.
“We plan to use reflectin as a template for the development of improved ion- and proton-conducting materials,” Dr. Gorodetsky concluded. “We hope to evolve this protein for optimum functionality in specific devices—such as transistors used for interfacing with neural cells—similar to how proteins evolve for specific tasks in nature.”