Application of micromixers - Micromixers Fundamentals Design and Fabrication - page 331

Enzyme assays are the basic techniques in clinical and bioanalytical chemistry. Micromixers can
promote the reactions between the enzyme and the substrate. These devices often use electrokinetic
transport to deliver analyte and enzyme into the chip and to the detector. An electrokinetically driven
T-mixer was used in [47] for performing enzyme assays. Substrate, buffer, enzyme, and inhibitor were
mixed in two stages. The amount of each reagent was controlled by the applied voltages. Hadd et al.
[47] used resorufin 4-D-

thiogalactoside (PETG) as the substrate, the enzyme, and the inhibitor, respectively. The reaction
forms resorufin, a fluorescent product with an emission wavelength of 585 nm. The system allows the
measurement of reaction kinetics by varying the concentration of substrate and monitoring the amount
of resorufin using fluorescent detection. The assay performed with the micromixers consumed about
four orders of magnitude less reagents compared to a conventional assay. Micromixers can therefore
help to reduce the cost of enzyme assays, especially of those with expensive reagents. Burke and
Regnier [46] reported a microfabricated enzyme assay system with a micromixer to perform stopped-
flow reactions. The device was tested with -galactosidase (-Gal) as the enzyme and fluorescein mono-
d-galactopyranoside (FMG) as the substrate.
Micromixers can work as a reaction platform of drug production using recombinant protein
production. The production process consists of several steps. In the first step, the DNA sequence of the
protein to be produced is inserted into the DNA of viruses. The viruses in turn are mixed with a cell
culture to allow the infection cycle. After a certain amount of time, all cells are infected with the virus
and the recombinant protein can be collected. The infection process can be optimized by the right
concentration of the virus. Thus, determining the right concentration of virus is crucial for the protein
production. In conventional reactors, virus is diluted to various concentrations and used for infecting
separated batches of cells. The optimum virus concentration is then determined by evaluating the
amount of proteins harvested in each cell batch. Diffusion transport in a lamination micromixer could
allow the formation of a concentration gradient where cell infection at different concentrations can be
realized concurrently. Protein expression based on fluorescent measurement can be carried out on
a chip. The short mixing length of a cross-mixer with hydrodynamic focusing makes fast infection of
a cell with virus possible. Walker et al. [48] reported the infection of cells by virus at different
concentrations in a cross-mixer. The cells are attached on the bottom of the mixing channels. A
concentration gradient of virus particles was created by diffusive mixing in the flow-focusing
configuration where the middle stream contains virus particles. The cells were expressed and moni-
tored with a green fluorescent protein.
In biochemical sensors, the analyte often needs to be transported to immobilized receptors to make
binding and subsequent detection possible. Receptors are surface-immobilized biomolecules that are
complementary to the biomolecules to be detected. Vijayendran et al. used micromixers based on
chaotic advection to promote analyte transport to receptors immobilized on a surface [49] . Soluble
rabbit IgG antibodies were passed through the micromixer allowing them to bind to protein A
immobilized on one microchannel wall. The binding reactions were detected using surface plasmon
resonance (SPR) concept. Since the binding kinetics is two or three orders higher than the diffusion of
analytes, the quality of detection depends on the extent of chaotic advection. Experiments showed that
compared to a simple T-mixer, a chaotic mixer can double the rate of analyte detection.
Kim et al. [50] used the F-shaped chaotic micromixer depicted in Fig. 6.15 for blood typing on
a disposable chip. Blood typing is an important blood test because transfusion of incompatible blood
groups (A, B, AB, and O) of recipients or donors may lead to intravascular hemolysis in the

-D-galactopyranoside (RBG), -galactosidase ( -Gal), and phenylethyl

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