They are built around devices less than a millimetre square, called Isfets (ion-sensitive field-effect transistors), which combine a substrate for detecting ions, such as sodium, potassium or calcium, with a silicon microprocessor for measuring the change in electrical potential as the ions are deposited on the substrate.
The substrate may be a special glass that absorbs one particular ion from the solution, or a chemical such as valinomycin - a substance that occurs naturally in the body, and attracts potassium ions. Although the first Isfet was demonstrated in the Netherlands in 1973, problems interfacing the transistors with the rest of the system delayed their use in commercial chemical sensing equipment until about a year ago. Such machines will be used for pollution assessment, environmental monitoring and industrial uses, as well as in medical applications. Because Isfets are so small and sensitive, only a drop of the liquid being analysed is needed - so they offer a significant advantage over existing methods, which are based on the same chemical substrates, but use pen-sized electrodes and therefore require a much larger sample.
These electrodes typically cost between pounds 200 and pounds 300 each, whereas for large-scale applications Isfets will cost between pounds 2 and pounds 3 each, according to Ian Robins at Central Research Laboratories (CRL), which has been working on these devices since 1981.
CRL is the former in-house research and development department of Thorn EMI. The low price of Isfets will make it possible to use them once to test blood and then throw them away, saving the cost of sterilising equipment, and increasing safety.
The Isfet-based sensors now on the market are capable of measuring only one chemical at a time. CRL believes it has a world lead in the development of multigate Isfets that can measure several chemicals at once, as required in blood tests.
Although it is still funded by Thorn EMI, CRL is working as an independent laboratory, and is aiming to become self- financing. It has teamed up with an unnamed medical equipment company to develop the blood-testing machine for the doctor's surgery. The aim is to produce a unit that could also be used in kidney dialysis clinics and for monitoring patients in intensive care.
The idea is that the Isfets will be held in removable cartridges. Each cartridge will contain enough Isfets to carry out 100 blood tests, after which the sealed unit can be taken out of the machine and thrown away without fear of contamination. A test will be completed and a print-out produced in 30 seconds. The machine will cost several thousand pounds, but Mr Robins says it will be less expensive and time- consuming than sending blood samples for laboratory analysis and having to recall the patient when the results come back.
CRL is also talking to the water companies about building an Isfet-based machine to monitor nitrate and oxygen levels and test for heavy metal pollution. Existing electrodes suffer from the problem of biological contamination, caused by algae and other organisms. The electrodes need to be cleaned regularly and the equipment recalibrated, whereas a sensor using Isfets would be cheap enough to throw away after use.
Another possibility is a domestic water monitor. Mr Robins says such a device could be marketed for about pounds 15.