If you read almost any scientific proposal, you will be struck by the amount of money required to pay for two simple but basic needs: space and people. Scientific instruments take up a fair amount of space, it's true, but most lab space requirements are taken up by passageways, desks, open surfaces, keyboards, monitors, hoods, emergency equipment, and other necessities for the humans who work there. If you could automate all human tasks in a laboratory and collapse all this space, you could make things much more compact and efficient. In some cases, you might be able to make them so compact and efficient that you could integrate the whole lab onto a microchip. By reducing these costs and overhead, you not only make research simpler, quicker, and cheaper, but also make it possible to conduct hundreds or even thousands of experiments at the same time.
That is the basic idea of an emerging technology appropriately named lab-on-a-chip. See Figure 5.3. At first glance, these tiny, automated laboratories look like their electronic brethren. They are usually created on silicon surfaces, and tiny cells are linked by microscopic or nanoscopic interconnects.
Courtesy of Agilent Technologies, Inc.
The difference is that, in lab-on-a-chip, interconnects don't all conduct electricity. Many of them channel fluid from tiny reservoirs implanted on the chips during fabrication. The functional cells are also different. In a microchip, these might be memories or logic gates, but in a lab-on-a-chip they are often mixing elements, reservoirs, and bio or chemical reactors.
Lab-on-a-chip fabrication is done using well-established silicon technologies including lithography and etching. Lab-on-a-chip differs from electronic chip making, however, because features must be designed in three dimensions instead of two. The reason for the three-dimensional design is that even though electricity may be able to flow through a planar wire, water can't flow through a flattened hose. Three-dimensional silicon manufacturing isn't as well understood as it is in two dimensions, and some of the plastics and other materials required to handle fluids are different from those required for handling electricity. These issues make lab-on-a-chip fabrication a lively area of engineering.
The other key technologies for creating a lab-on-a-chip are microfluidics and nanofluidics, the approaches to controlling the movement of fluids through channels at the microscale or nanoscale. When the volumes of fluid are this small, you can't always push fluids along using pumps or valves because you wouldn't have the precision required and because such small moving parts would be very hard to design and integrate Instead when very small fluid volumes are required, two techniques are used in current lab-on-a-chip devices: electrophoresis and electroosmosis. Both approaches work by applying a voltage difference along the channel in the direction the fluid should move. In electrophoresis, this voltage difference interacts with ions distributed throughout the fluid to be moved, pushing them along using Coulombic forces. When this approach is used, the ions in the fluid move at speeds inversely proportional to their mass, causing them to separate, with lighter particles moving faster and heavier ones moving slower. This separation by mass is why electrophoresis is useful in analyzing composition and is used in DNA analysis. Electroosmosis, on the other hand, works by having charges on the channel wall interact with a thin sheath of ions at the wall-fluid interface. This pushes the entire fluid column along at the same speed like a plug through a tube.
By using these approaches to move fluids among the mixing elements and reactors, it is possible to control interactions precisely, and the lab-on-a-chip has already become reality. Companies like Affymetrix (with their product GeneChip) and Agilent (with their product LabChip) make lab-on-a-chip devices for genetic analysis. It is hoped that these chips may develop to the point where they can be used for point-of-care applications so that a doctor can give a patient an immediate analysis of blood or any other samples that the doctor takes. They may also be used for drug delivery, particularly in cases where drugs need to be dispensed over a long period of time in response to changing body chemistry (as in diabetes, for example). In the more distant future, it is possible that lab-on-a-chip might serve as a framework for DNA computing since early experiments in that field operate in just microliters (a millionth of a liter) of solution but require large-scale analysis of results to be useful. Lab-on-a-chip might also be used for experiments in orbit on stations or shuttles where space, ahem, is truly at a premium.
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