The memristor "the missing link of electronics" was finally built in 2008, using nothing more than titanium dioxide and metal electrodes in thin films. In 2009 NIST printed them on plastic film. Now a new version involving silicon and silver thin film seems to have advantages.
Memristors are being used in a US military-funded project trying to make brain- like computers, says Wei Lu, who led the team at the University of Michigan in Ann Arbor that demonstrated the new behaviour (Nano Letters, DOI: 10.1021/nl904092h).
The race to use memristors in computing has been on ever since, with brain-like computers one of the potential applications memristors lend themselves to the task because the way that their resistance gives a glimpse of an earlier voltage is analogous to the way that a synapse's electrical behaviour is dependent on its past activity.
Lu and colleagues have now provided the first demonstration that the analogy stands up. Their memristors were built with materials already used in the manufacture of computer chips.
Lu's team used a mixture of silicon and silver between two metal electrodes. The junction mimics a particular behaviour of synapses that allows neurons to learn new firing patterns, and is believed to allow memories to be stored.
In the brain the timing of electrical signals in two neurons affects the ease with which later messages can jump across the synapse between them. If the pair fire in close succession, the synapse becomes more likely to pass subsequent messages between the two. "Cells that fire together, wire together", says Lu.
The Michigan device exhibits the same behaviour. When the gap between signals on the two electrodes was 20 milliseconds the resistance to current flowing between the two was half that after signals separated by 40 milliseconds. "The memristor mimics synaptic action", says Lu, adding that the next step will be to build circuits with tens of thousands of memristor synapses.