Microfluidic analysis offers low-cost solution for water quality monitoring

June 6, 2003

Brussels, 05 Jun 2003

The Commission-funded MicroChem initiative culminated in the successful demonstration of prototype miniaturised laboratory-on-a-chip systems suitable for rapid field testing of wastewater streams.

Microfluidics could affect chemical analysis and synthesis in the same way that microchips have revolutionised computers and electronics. The science is based on devices and processes handling volumes of liquid on the nanolitre (10-9 l), or even picolitre (10-12 l), scale. The silicon, glass or plastics devices have typical overall dimensions of a few millimetres; fluids flow through microbore channels produced by photolithographic etching or other high precision techniques.

Fluids circulating in such channels can exhibit dramatically different performance from that in macro-scale tubes. Flow is normally turbulence-free, so layers containing different components can move along together, mixing only by diffusion. Speed is one of the main advantages of microfluidic systems. Chemical separations such as electrophoresis, for example, are 100 times faster when a system is ten times smaller. Micro-scale engineering also makes it possible to integrate chemistry with mechanics, electronics and optics, and to integrate several analytical systems into very small areas. Where required, these can interface with conventional-sized peripherals for data interpretation and transmission.

The resulting laboratory-on-a-chip technology is attracting great attention in many sectors – from environmental analysis to biochemical assay and DNA-based diagnostics. Benefits include portability, minimal consumption of energy and reagents, and substantial cost savings as new systems enter into mass production.

Multi-element analyser

The goal of the MicroChem project consortium, led by Danish water instrumentation specialist Danfoss, was to exploit this technology in producing a new micro-analysis system for monitoring the levels of various chemical species in wastewater at the outlets of sewage plants and factories, as well as in drinking water supplies.

Requirements for the proposed system were that it should be rugged and reliable enough for long-term unattended operation at remote sites and in harsh environments – with the capability to perform multiple analyses for small ions at concentration levels typically ranging from 10 ppb to 10 ppm in drinking water, and 100 ppb to 100 ppm for wastewater applications.

A team including German micro-systems technology start-up GeSiM, end-user Suez-Lyonnaise des Eaux and universities in Denmark, Ireland and Switzerland collaborated in the project. Its starting point was the use of existing sensor types for the measurement of phosphate, ammonia and nitrate in wastewater. Over the three-year funded period, the partners achieved notable advances in a number of key areas, such as low-cost dosing and optical detection – as well as improvements in microfluidic chip design, optimisation of analytical methods and the extension of reagent lifetimes.

Danfoss currently markets a larger-scale system for sewage plant process control, which is capable of running for around two months without human intervention. Further refinement of the MicroChem developments could extend this autonomy to at least six months, with reagent consumption down to just one litre per year. Maintenance of the miniaturised devices should also be much reduced, contributing to lower cost of ownership.

Chip problems solved

Initial trials with laboratory-scale demonstrators showed that analyser chips produced by GeSiM in the form of glass-silicon-glass sandwiches suffered from flow disruption due to the presence of gas bubbles in the microchannels. As this could not be solved using available off-the-shelf syringe pumps, a new constant-pressure based pumping scheme with inherent degassing properties had to be developed for incorporation into industrial prototypes. Channel design was also improved to facilitate the transport of gas bubbles, while a microflow sensor was added and self-diagnostic procedures programmed into the control software.

Another obstacle, recognised later in the project, was the tendency for the silicon layer of the chips to corrode in the presence of high-pH (alkaline) reagents. To counteract this, a corrosion-resistant coating for silicon and an all-glass chip technology were successfully implemented for analytical sensors employing optical detection, which could be adopted for future systems.

A third route based on polymers was also assessed, leading to polymethyl methacrylate (PMMA) chips fabricated using laser machining and a chemo/thermo/mechanical bonding technique. Polymer technology is seen as the ultimate solution to the corrosion problem, but more work will be necessary to realise microfluidic networks in chemically durable materials such as polyetheretherketone (PEEK).

Two new microfluidic valve technologies were also investigated. One functions by freezing a plug of solution in a microchannel. The other uses a hydrogel-filled channel that can be blocked by thermal activation of the gel material. Trials were completed on each, but their use in an industrial system will depend on additional work in the production of polymer chips.

Practical system demonstrated

The chemistry and the microchips have been developed and verified for optical detection of ammonium, phosphorus and aluminium, although two or more ions have not yet been detected in the same chip. A four-minute response time for ammonium, including the time needed for sampling and sample pre-treatment, was achieved in the laboratory and with the industrial prototypes. Laboratory optimisation of the phosphorus detection method has also shown that a two-minute response time can be obtained.

While the final prototypes remained too complex for immediate commercialisation, a number of potential low-cost subsystems have been shown to work well for the application. These include the constant-pressure pumping system, which can be built at much lower cost than current state-of-the-art alternatives, as well as an optical sensor based on just one LED and two photo-diodes.

Given the small amounts of reagents needed, the improved lifetime of analyte chemicals and the robustness of the sampling unit developed under MicroChem, it is clear that economical, low-maintenance systems can ultimately be produced. These would offer a cost-effective means of safeguarding human health by permitting more thorough checks on drinking water quality in recipients (lakes, reservoirs, pumping wells, etc.), and at points of distribution and use. They could also be applied in the clinical area – for example to monitor intensive care patients, or for on-line sensing of subjects suffering with various chronic conditions.

Since the completion of the project, fruitful bilateral collaborations between the partners are continuing to advance the underlying technology, in terms of both polymer chip development and the exploration of new bio-chemical analysis applications.

Project:
MicroChem – A miniaturised industrial chemical sensing system (BRPR980787)

Programme:
BriteEuram III

Background:
Safe water: micro-nanosensors for monitoring waste and drinking water (PDF), presentation of MICROCHEM project by Peter Gravesen, Danfoss A/S (Denmark)

DG Research
http://europa.eu.int/comm/dgs/research/i ndex_en.html
http://europa.eu.int/comm/research/indus trial_technologies/03-06-03_pro-microche m_en.html

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