Squid skin conductor for bioelectronics

A protein from squid skin is a good conductor of protons, researchers in California have discovered.  Using the material, which is robust and easily produced, they fabricated a proton transistor, which they say could provide a direct interface between electronic and biological systems.¹

Protons can travel through hydrogen-bonded networks in water or other molecules by a mechanism first proposed over 200 years ago.  Living organisms use protons and ions for electrical signalling, allowing the nervous system to regulate processes like muscle contractions.  This presents bioelectronics engineers with a challenge, as traditional electronic technology is based on electron flow.

In 2011, Marco Rolandi and colleagues at the University of Washington in Seattle unveiled a proton transistor made from a polysaccharide called maleic chitosan.² While this works well, it is difficult to tweak the material to introduce specific properties in the way that living organisms have done through evolution.

Alon Gorodetsky and colleagues at University of California, Irvine, investigated whether transistors could be produced from proteins, nature’s own building blocks.  Squid can change colour to preserve their camouflage in various environments, and artificially applying a voltage to the skin can also induce colour change.  This inspired the team to study the electrical properties of a protein in squid skin called reflectin.

The researchers produced large quantities of the protein by programming E.coli bacteria to express it and purifying the product.  They fabricated thin films of the protein on silicon substrates with electrodes at either end, observing the relationship between current and voltage under various conditions.  Reflectin conducted protons, they discovered, as well as many artificial proton conductors including metal-organic frameworks.

The team produced a proton transistor by using the underlying substrate as the gate. Applying a voltage to this gate would add or remove protons from the reflectin film, thereby modulating its electrical conductivity.  This is analogous to how ion channels are gated by voltage in some biological systems.  Gorodetsky believes it should be possible to ‘park a single living cell on top of one of these devices, and talk to it in terms of ion flow going out of the cell and into the device or vice versa’.

Rolandi believes that the use of proteins opens up significant new opportunities to design proteins for specific bioengineering requirements. ‘Conductivity is just one aspect,’ he says. ‘One can think of selectivity for protons versus other ions or conductivity in one direction but not the other – similar to a diode.  Several functionalities that mimic semiconductors and beyond could be implemented in the future by appropriately designing the protein structure.’