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 鈥渞emembers鈥� 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鈥檛 until 2008 that a team led by , director of HP鈥檚 Information and Quantum Systems lab in Palo Alto, California, finally made one from a speck of titanium dioxide.
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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, 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 .
The researchers attribute skin鈥檚 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鈥檚 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鈥檚 resistance to current.
Whether this behaviour helps skin function isn鈥檛 clear but Yuriy Pershin of the University of South Carolina in Columbia, who has studied memristive behaviour in amoebas, describes the skin鈥檚 role in processes such as temperature regulation as 鈥減rimitive intelligence鈥�.
聯The involvement of skin in regulatory processes can be considered a form of primitive intelligence聰
A new understanding of skin鈥檚 electrical properties could have implications for medicine. Resistance to alternating current is already used to diagnose skin abnormalities, says Johnsen鈥檚 colleague .
Williams, meanwhile, is gratified by the interest in memristors. 鈥淚t is very interesting to me to see the range of the fields that can benefit from application of memristor theory.鈥�
