A large-scale prototype of a computer that could be smaller than a living cell has been designed by an Israeli scientist.

Some scientists believe that, in the future, small biological computers could roam our bodies monitoring our health and correcting any problems they may find.

The prototype has been developed by Professor Ehud Shapiro of the Computer Science Department at the Weizmann Institute of Science.

Ehud Shapiro hopes new biotechnology will make his computer possible

It is being presented at the Fifth International Meeting on DNA-Based Computers at the Massachusetts Institute of Technology.

In terms of the logic on which it operates, the prototype will behave in a similar way to molecules inside a living cell, a "biomolecular machine".

A living computer

Each cell of our bodies is a collection of machines made out of biological molecules. These molecules can form pulleys and gears to move other molecules around the cell.

Some molecules have the ability to assemble and take apart other molecules. Others gather small molecules and use a template to construct new molecules.

In a sense, each of our cells is a complicated city of biological machines all working together.

It is possible that a future biomolecular version of Professor Shapiro's device could lead to the construction of computers, smaller than a single cell, and with the ability to monitor and modify them.

If scientists were ever able to build such a computer, its medical applications would be far-reaching. It could swim in our bloodstream or be attached to specific organs monitoring and supplementing their performance.

"For example, such a computer could sense anomalous biochemical changes in the tissue and decide, based on its program, what drug to synthesise and release in order to correct the problem," says Professor Shapiro.

A Turing machine

Existing electronic computers are based on the architecture developed by John von Neumann in the US in the 1940s. But the new mechanical computer is based on the Turing machine, conceived in 1936 by the British mathematician Alan Turing.

The Turing machine uses the basic concepts of computing, reading and writing one bit of data and performing an action depending upon a program. But although the Turing machine is a general-purpose, universal, programmable computer and is key to the theoretical foundations of computer science, it has not been used in real applications.

Like a Turing machine, Professor Shapiro's mechanical device has a "rule molecule" designed so that the processing of the molecule modifies another molecule in a predetermined way.

To demonstrate the concept, Professor Shapiro has built a 30-centimetre-high plastic model of his mechanical computer. He hopes that the advent of improved techniques for making and assembling molecules will mean the day when his computer could be made is not far off.