Even those who have never dabbled with electronics will probably have heard of resistors, capacitors and inductors – generally accepted as the three fundamental types of passive component. It now transpires that a fourth component called a memristor makes up the set.

Its resistance increases when an electrical current passes through it in one direction, and decreases as a current passes in the opposite direction. What's more, if you remove that current it remembers its resistance, hence the name memristor, which is a contraction of the words 'memory resistor'.

Today's memristors are fabricated from 2-3nm islands of titanium dioxide sandwiched between platinum electrodes, and can be stacked in three dimensions for phenomenal circuit densities.

Fast and reliable

Needless to say, memristors are being promoted as memory elements, but their potential goes far beyond providing a replacement for today's dynamic RAM chips. According to memristor expert Stan Williams of HP Labs, "This opens up a whole new door in thinking about how chips could be designed and operated."

Since memristors remember their resistance even when the power is removed, they could replace both system memory and non-volatile storage like hard disks. They are potentially faster than magnetic media, which could lead to PCs that start up instantaneously and, being more fault tolerant, would provide greater reliability. They also consume less energy and occupy less space than flash memory.

This is just a start to the promise of the memristor though – the wonder component can also act as a logic element.



What's more, in offering an alternative technology for both processor and memory, the memristor could provide a solution that's greater than the sum of its parts. "The processor and memory could be exactly the same thing," Dr Williams suggested. "That allows us to think differently about how computation could be done."

Unlike today's technology, in which the processor and memory are separate, both could be on the same chip, offering huge gains in terms of speed and power usage. This might be some way off but even in the short term, memristor memory and silicon transistors could be used in combination as Dr Williams explains.

"A hybrid integrated circuit containing both memristors and transistors should, in principle, be much more efficient than those containing transistors alone. It should be possible to reach into a particular integrated circuit, rip out 10 transistors and replace them with one memristor."

Unlike some of the other technologies we've looked at, memristors in home PCs don't seem too far away from becoming a reality. While an integrated chip containing memory and processing elements could be a decade away, Stan Williams suggested that they're now moving out of the lab and towards fabrication – and according to Dr Williams, "It could provide a way of getting a ridiculous amount of memory on a chip."

Biological processors

bio processors

DNA computing uses the molecule on which life depends and artificial neural networks mimic the operation of the brain, but Professor Andrew Adamatzky of the University of West of England in Bristol is performing computations using real, living organisms.

That might conjure up memories of Frankenstein's Monster, but the living organism in question is called physarum polycephalum. Otherwise known as slime mould, it's a microbe that can be seen without a microscope and is found on decaying leaves or wood.

Adamatzky has built fully programmable machines from slime mould that can solve many problems of computational geometry. They've also been used to create logical gates, carry out the programmable transportation of substances, generate music and investigate the physical imitations of man-made transport networks.

So what makes this substance so suitable for these types of application? "Slime mould acts as an amorphous massively parallel computing device," Professor Adamatzky says. "It senses inputs with its whole branching body, makes a decision using propagation of excitation and contractile waves, and can implement parallel actuation."

Adamatzky envisages niche applications only. "Physarum machines will not compete with conventional PCs, but they can form a new family of soft-bodied robots with embedded computing power, which will be capable of entering any confined spaces and implement sophisticated tasks."