The molecular abacus
This article first appeared in Personal Computer World magazine, June 2000.
Having almost quadrupled in the last twelve months, the storage capacity of hard disks continues to increase at an amazing rate. But although it's getting bigger, faster, and cheaper all the time, magnetic disk technology has its limits. Researchers are now looking at alternative storage technologies that work at the molecular level, and a bizarre hybrid of food chemistry and mechanics is emerging as a possibility.
The trouble with hard disks is simply one of scale. When the magnetic storage space allocated on the disk's surface for each bit gets too small, the effects of random molecular noise come into play, such that the magnetic fields used to encode the 0s and 1s can no longer be reliably maintained.
Now researchers in Japan have built a prototype device which can store data at the molecular level. In effect, Makoto Komiyama of Tokyo University and his colleague Hidemi Shigekawa of Tsukaba University have built a molecular abacus.
Each bead of the abacus is a single molecule of a glucose complex called cyclodextrin, a substance used commercially to improve the solubility of drugs and food additives. The cyclodextrin molecule is shaped like a truncated hollow cone, a bit like the megaphones beloved of early film directors. Each molecular bead slides along a "wire" made from another molecule, a polymer called polyethylene glycol, used widely in the pharmaceutical and cosmetics industries, and as a food thickener.
The idea was pioneered in 1993 by Akira Harada of Osaka University, who was interested in collecting together sets of cyclodextrin beads like the mints in a packet of polos to form tiny molecular tubes. He first strung them on a polymer wire, then removed the wire. But Komiyama and Shigekawa kept the wire, and placed the whole arrangement on a flat bed of molybdenum disulfide, to which the beads adhere. Then, they used the tip of an electron microscope to push the beads along the wire: a bead could represent a zero if located at one end of the wire, and a one at the other end (see here).
In principle this works fine, but in practice the use of an electron microscope to set a bead's position, and to later detect that position, is cumbersome and slow. Current research is focusing on a much better idea: to use light beams to push the beads around. The key to this approach is to use a wire that can be made to kink in the middle, pushing a bead threaded onto it away from the kink. The new wire is made from a molecule based on a nitrogen-benzene complex which has the unusual property of switching from being straight to kinked, when illuminated by a flash of light. Since the abacus is now light-controlled, it can switch very rapidly indeed. And by splicing lengths of kinky wire into the polyethylene wire, you can have multiple beads on a single wire.
The research is still in its early stages, and there are still formidable problems to be solved, such as a mechanism for optically reading back the beads' positions, and separately controlling each of several beads on the same abacus. One idea is to differentiate the beads using coloured molecular dyes attached to them, and to then manipulate the beads with correspondingly coloured light.
Problems aside, the rewards of this technology could be astounding, providing data storage at unheard of densities perhaps 100,000 Gb in a square centimetre. Today, such capacity might seem absurdly high what on earth would we ever use it for? I have the feeling we'll think of something soon enough.
Toby Howard teaches at the University of Manchester.