ABSTRACT
Moving liquids has always been a challenge for mankind. Modern solutions
for controlling liquid movement use pumping technology that is constantly
being refined, with new pump designs frequently being introduced into
the market. Pumps with flow rates typically in milliliters per minute have
dominated in the areas of chemical analysis and separation. Efforts to mini-
aturize liquid transport systems began more widely in the early 1980s. The
first efforts were primarily directed from a cost and environmental perspective
toward reducing the consumption of organic solvents. However, such mini-
aturized pumping systems, typically microliters per minute down to nanoliters
per minute, did not gain wide acceptance in the high-performance liquid
chromatography (HPLC) market. A new, practical way to pump and direct
the flow of nanoliter volumes was published in Science in 1993 and gained much attention. Suddenly it was possible to run an entire system using less
volume than the volumetric error on a conventional system. Manz and Harrison
described a planar, microfabricated glass chip platform that provided high
separation efficiencies by controlling an electrical field and using electroos-
motic flow (EOF) to pump liquids. The significant impact of this publication
is demonstrated by the fact that the technique has dominated the micrototal
analysis system (m-TAS) field during the last decade. However, alternative concepts are being developed with increasing pace and most of these alterna-
tives circumvent transport challenges that have not yet been solved by the EOF
approach. We describe some of these challenges in this chapter. In addition, we
describe some approaches such as the very promising technological approach
to generate high-pressure flows on the chip. We also describe some of our own
contributions using centrifugal force to drive separations in parallel as well as
some other transport modes that are still in the research field. We have focused
on transport modes that have found an application in the field of on-chip
separations. However, some new and promising concepts, not yet mature for
separation applications, have also been included.
