The separation subsystem is a glass structure consisting of two planar plates. The separation capillary, together with its associated inlet and outlet channels, are etched into the lower plate using standard micro-lithography techniques. The upper glass plate, which serves as a cover plate, contains orifices to provide access to the channel system. The system is connected via these orifices to the fluid management system and electrodes.

A high voltage applied between the reservoirs induces an electro-osmotic flow which is used to introduce samples. Injection is performed by applying a field strength of typically 500 to 2000 V/cm for a few seconds. A voltage gradient of up to 3000 V/cm is used for separation. The production of gas at the electrodes is recognised as a major impediment to the use of electrophoresis in microgravity. To overcome this problem, a new high- voltage electrode, which produces a negligible amount of gas, has been successfully developed.

The fluid management system supplies the electrolyte and rinsing fluids and pre- processes the sample. The fluid is supplied by liquid dosing assemblies (Lidia), produced by Fokker Space. The detector of the Caelis prototype employs laser-induced fluorescence, implying that the sample must be probed with a fluorescent dye prior to analysis. This operation is done in the sample pre-treatment unit. This unit, like the separation subsystem, has been fabricated using microsystems technology (Figure 1).

Figure 1. Detailed view of the sample pre-treatment unit of a prototype miniaturised capillary electrophoresis system.

The sample pre-treatment unit is a micro-fluidic system that provides the automatic sample preparation needed for electrophoretic analysis. After introduction, the sample is mixed with the dye and buffer solutions that are injected in accurately determined quantities by an arrangement of micro-pumps, micro-flow sensors and micro-mixers. On completion of these steps, the mixture is fed to a dedicated reactor, maintained at a set temperature, where it reacts with a fluorescent dye and is converted into an intermediate compound that is more amenable to analysis. This compound is then transported to the separation subsystem.

All the micro systems components are assembled (anodically bonded) on a three-inch Pyrex wafer. Into its stratified structure, fluid channels are etched to interconnect the micro-system components.

Results and Conclusions

Microsystem technology offers significant advantages to equipment intended for use in space or on Earth. In fields like chemical production processing or in environmental monitoring, there is a need for a faster, cheaper and better analytical technique that generates only small amounts of waste products. This is sometimes called green chemistry. The Caelis equipment and similar micro-TAS systems can meet a number of these needs.

A micro-miniaturised capillary electrophoresis device can provide fast separation within times ranging from a few seconds to a few minutes, and generated only small amounts of waste products. This approach limits the cost of the resources used and the cost of disposing of waste products. Finally, it lends itself readily to a high degree of automation, allowing a broad range of substances to be analysed by workers who have only minimal training.

The small dimensions achieved through the use of miniaturisation allow large series of samples to be processed in parallel, which makes the technique attractive for applications in clinical laboratories and also for process control. In addition, these systems are small, portable and easily operated on-site where samples are collected.

Experience gained from this and other projects has shown that, while micro-system technology has an excellent potential, the techniques have not yet reached maturity. This is due, in part, to the techniques used for mounting micro-system components. At present, fluid connections are made with adhesives and electrical connections are soldered. The permanent nature of these connections (particularly those made with adhesives) hinders not only functional testing of a system component prior to its final assembly but also the replacement of broken parts.

Future Outlook

A project has been started to investigate and develop new mounting techniques, with the objective of improving the assembly of microsystems. This project, supported by the Netherlands National Space Agency (NIVR), will be carried out in collaboration with ESA's Advanced Sensor Technology Programme.

Due to the adoption of a modular design approach, the laser-induced fluorescence detection system designed for Caelis will be used in the development of a sensor for measuring fluorescence in labelled cells. This sensor can also be used for enzyme- linked immunosorbent assay's and can therefore be considered a new type of biosensor. With these two innovative devices, microgravity research has gained two new useful analysis systems which are suitable for many applications.