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Posted: May 29, 2007
The road to molecular computing
(Nanowerk Spotlight) The human body so far is the ultimate 'wet computer' - a highly efficient, biomolecule-based information processor that relies on chemical, optical and electrical signals to operate. Researchers are trying various routes to mimic some of the body's approaches to computing. Prominent among them is DNA computing, a form of computing which uses DNA and molecular biology instead of the traditional silicon-based computer technologies (see our Spotlight: "Molecular automaton plays tic-tac-toe"). Not limited to DNA, "gooware" computer scientists attempt to exploit the computational capabilities of molecules. In doing so, they expect to realize faster (massively parallel), smaller (nanoscale), and cost efficient (energy-saving) information processing devices that are very distinct from today's silicon-based computers.
"Research related to molecular logic gates is a fast growing and very active area" Dr. Uwe Pischel explains to Nanowerk. "Presently, all common logic gates, which are used in conventional silicon circuitry, can be also mimicked at the molecular level. The general character of the concept of binary logic allows the substitution of electrical signals by chemical and optical signals, which for example opens access to a vast pool of photoactive molecules to be used for the purpose of molecular logic."
Pischel, a researcher in the Department of Organic Chemistry at the University of Huelva, Spain, recently published a mini review on "Chemical Approaches to Molecular Logic Elements for Addition and Subtraction" in Angewandte Chemie International Edition. In this review, Pischel discusses the principal elements of molecular calculators (half and full adders, half and full substractors), demonstrates biocompatibel molecular arithmetic based on bioinspired systems, and provides an outlook for the future of (bio)molecular logic.
Besides the mimicking of basic logic operations, increased efforts have been dedicated to the design of molecules which combine several logic gates with the target to perform simple arithmetic (adders and subtractors), says Pischel. Albeit their processing capability, compared to modern computers, is still rather limited, they constitute a proof-of-principle.
"However, the application of chemical and optical signals as in- and outputs of logic gates is not unproblematic, which is related to difficulties to direct these signals at the molecular level" Pischel says. "On the other hand, due to the non-interacting nature of photons, optical signals have a high potential for parallel processing of large data sets. Highly directed energy transfer between several optical logic gates might be a way out of the dilemma of the multidirectional character of light emission."
He points out another potential problem, which is related to the integration and addressing of logic gates within large molecular arrays. Several original proposals based on concatenation using optical and chemical signaling have been presented. Other proposals related to the linking by electrical nanowires are the result of very intensive efforts in molecular electronics.
This area of research is still at the very early stages and Pischel cautions that predictions about the master approach to molecular computing are difficult to make. He feels that molecular electronic appears to be the most advanced so far: "problems like gate-to-gate communication, selective addressing of individual molecular logic elements, and interfacing strategies constitute strong driving forces in that field" he says. "The logic gates described by Stoddart and Heath relying on rotaxanes as supramolecular entities self-assembled on metal electrodes are very nice examples for the fruits of these efforts."
He also suggests that liquid phase restricted molecular approaches using chemical and optical signaling should be not discarded in the quest for computing at the nanoscale - see the human body.
For further reading, some good resources on molecular computing can be found at the Leiden Center for Natural ComputingV in the Netherlands, the DNA Computing site on the AI Portal, and Masami Hagiya's Theory and Construction of Molecular Computers at the University of Tokyo.