Biomedical Engineering Reference
In-Depth Information
ater the pinched segment, these particles will move along low lines that are shited toward the
particle-free luid in proportion to their size. Subsequent splitting of the exiting low allows the
recovery of essentially particle-free solutions. Some dilution of the sample is also inevitable, as
in the H-ilter, and there is a risk of mechanical damage to the particles in the pinched region,
but this remains a very promising new approach to sample preconditioning.
4.3.2 Fluid Conditioning for Cell Analysis
Oten, the aim of the microluidic system is the analysis of cell populations. here are generally
two approaches to such analysis—one in which the cell can be studied intact, and one in which
the cell is lysed. A further advantage of single-cell analysis is that the heterogeneity of signals
contributed by cells of diferent ages and phenotypic states can be directly assessed.
In point-of-care assays of blood, the most common form of blood cell analysis is counting
one or more of the several types of cells present. his diferential count can be used for diagnosis
and staging of a wide range of congenital and acquired infectious diseases, such as AIDS and
cancer, as well as monitoring chemotherapy. he primary centralized laboratory method for
such analysis is low cytometry (having displaced the Coulter counter some decades ago because
of higher throughput and more general sensitivity). In low cytometry, a suspension of cells
or similar small particles is surrounded by a sheath luid and forced through a nozzle at high
velocity, precisely aligning the cells along the center of a free-standing jet. In the jet, the cells are
interrogated optically, generally by a focused laser to stimulate light scattering or luorescence
(or both). Because the luid stream that originally held the cells is focused down to a stream
narrower than the cells themselves, the cells can be very well aligned into a single-sell stream.
Flow cytometry and similar techniques have been adapted repeatedly to microluidic formats
(see Section 5.1). he drag on the constrained stream can create high pressures, so microluidic
implementations of low cytometry have generally used lower low velocities than their macro-
scopic counterparts, and the number of cells that can be analyzed in a given period has typically
been smaller. However, the advantages of being able to analyze the cells with a smaller and sim-
pler instrument are very attractive for point-of-care purposes. Considering the utter impracti-
cality of taking a conventional low cytometer into the ield in developing nations to stage AIDS,
the need is particularly acute.
Today there is a growing interest in lysing cells before analysis, and a variety of methods for
chemical lysis have been developed that rely on bringing a stream of bufer containing a lytic
agent adjacent to a stream containing cells, followed by rapid lysis of the cells as the lytic agent
difuses into the cell-carrying stream. By controlling the lows, the timing of the cell lysis before
an analytical process can be quite precise.
4.4 The Problem with Microluidic Sample Separation
he biochemical analysis of complex samples oten involves the separation of the sample into
its components. A typical batch separation can be done by means of iltration, centrifugation,
chromatography, or electrophoresis—all of which can be implemented in a microluidic format.
he problem with batch procedures is that they require the precise injection of minute amounts
of samples into the separation channel (
Figure 4.2a
). An example of a technique that has fol-
lowed this strategy is
capillary electrophoresis
(
CE
), as explained in Section 4.4.1. An entirely
diferent strategy, dubbed
continuous-low separation
(
Figure 4.2b
), involves the application of
a force ield at an angle to the direction of low so as to delect the path of the sample with respect
to the low of the bufer. hen, the sample is collected in a diferent channel and the output is
visualized with a microscope, which can be done at very high resolution with standard optical
microscopes (i.e., no temporal resolution is required). his approach is clever in that it better
exploits diferences in the microscale difusive behavior between the sample and the bufer in
