Biomedical Engineering Reference
In-Depth Information
1.
INTRODUCTION
Flow phenomena are of great importance in the study of biological systems,
both natural organisms as well as biomedical devices. Most major life processes
occur in an aqueous environment. Scientists and engineers already have an ex-
cellent understanding of fluid mechanics at length scales of millimeters and lar-
ger. Considering the importance of cellular activities, such as protein production
and DNA reproduction, it becomes critical to understand the micrometer- and
nanometer-scaled fluidic (i.e., microfluidic) environment in which these proc-
esses occur and how we can manipulate that environment. Without such under-
standing, we have only an incomplete picture of how the fluid transport
processes occur in biological and biomedical systems and cannot assess how
behavior changes at a cellular level will affect the behavior of the organism as a
whole. A full understanding of the complex systems science comprising biologi-
cal systems is not possible without understanding the transport processes at the
smallest of length scales. Recent strides in micrometer- and nanometer-scale
diagnostic techniques have allowed exploration of flow phenomena at length
scales comparable to single cells, and even smaller. New fabrication tools have
enabled therapeutic and analytical biomedical devices to be constructed that
interact with biological components on their intrinsic length scale. One of the
most useful means of manipulating fluids and suspended species such as cells,
DNA, viruses, etc., is with electric fields. Electrokinetic phenomena are impor-
tant at micron length scales, and can be used to manipulate fluid and particle
motion in microfluidic devices. Electrokinetics can be broadly classified into
DC and AC electrokinetics, as shown in Table 1. DC electrokinetic phenomena
include electrophoresis and electroosmosis. Electrophoresis has been widely
used in capillary gel electrophoresis for fractionation of DNA, and capillary
zone electrophoresis for separation of chemical species (21). Nanogen Inc. (San
Diego, CA) uses DC electrophoresis from individually addressable electrodes to
control the motion of DNA molecules—first concentrating and separating target
particles from the sample, then combining with target oligonucleotides at a spe-
cific location in an array of spot electrodes (4).
Electroosmotic flow is generated when microchannels with glass walls
filled with aqueous solutions naturally produce electric double layers (15). In the
presence of an external electric field, the electrical charge in the double layers
exhibits a Coulomb force, causing the ions to migrate parallel to the channel
wall. The movement of the ions induces fluid motion in the channel, creating
electroosmotic flow. Electroosmosis is widely used for sample injection and
transport in microchannels in commercial systems manufactured by such com-
panies as Aclara and Caliper (1,2).
AC electrokinetics, in contrast, has received limited attention in the micro-
fluidics community. AC electrokinetics refers to induced particle and/or fluid
motion resulting from externally applied AC electric fields. One primary advan-
tage of AC electrokinetics is that the zero mean alternating fields significantly
