Wednesday, March 2, 2011

Sweat ducts make skin a memristor

02 March 2011 by Kate McAlpine

Magazine issue 2802. Subscribe and save

THE missing link of electronics, which evaded discovery until 2008, was at our fingertips the whole time. Ordinary human skin behaves like a memristor, a device that "remembers" the last current it experienced and varies its resistance accordingly.

In 1971 Leon Chua of the University of California, Berkeley, came up with the notion of a resistor with memory. He showed mathematically that this memristor should be a fourth basic circuit element alongside the familiar trio of resistor, capacitor and inductor.

But it wasn't until 2008 that a team led by Stanley Williams, director of HP's Information and Quantum Systems lab in Palo Alto, California, finally made one from a speck of titanium dioxide.

Synapses, junctions between neurons in the brain, display electrical behaviour that depends on past activity and are said to behave like memristors. This has raised the prospect of using memristors as the basis of an artificial brain.

Now, by re-examining data from the early 1980s on the electrical conductivity of human skin in response to various voltages, Gorm Johnsen and his colleagues at the University of Oslo in Norway have uncovered a more prosaic example of memristive behaviour in nature.

They found that when a negative electrical potential is applied to skin on various parts of the arm, creating a current, that stretch of skin exhibits a low resistance to a subsequent current flowing through the skin. But if the first potential is positive relative to the skin, then a subsequent potential produces a current that meets with a much higher resistance. In other words, the skin has a memory of previous currents. The finding is due to be published in Physical Review E.

The researchers attribute skin's memristor behaviour to sweat pores. Sweat contains positively charged ions such as sodium. When skin is exposed to a negative potential, the fluid at the bottom of the sweat pores is drawn upward. Although a thin layer of fluid always coats the inside of the cylindrical pore, this layer thickens as the sweat rises. As sweat is highly conductive, extra fluid rising to the surface increases skin's surface conductivity and thereby lowers its resistance if a subsequent potential is applied.

The longer skin is exposed to a negative potential, the lower the subsequent resistance, until it maxes out when sweat fills the pore. Conversely, a positive potential pushes the ions back, thinning the layer of sweat lining the pore walls and increasing the skin's resistance to current.

Whether this behaviour helps skin function isn't clear but Yuriy Pershin of the University of South Carolina in Columbia, who has studied memristive behaviour in amoebas, describes the skin's role in processes such as temperature regulation as "primitive intelligence".

A new understanding of skin's electrical properties could have implications for medicine. Resistance to alternating current is already used to diagnose skin abnormalities, says Johnsen's colleague Andrew Lütken.

Williams, meanwhile, is gratified by the interest in memristors. "It is very interesting to me to see the range of the fields that can benefit from application of memristor theory."