Biomedical Engineering Reference
In-Depth Information
hormones in body luids, to diagnose infectious and autoimmune diseases, and to both diagnose
and monitor treatment of cancer. All immunoassays rely on the ability of properly designed or
selected antibody molecules to strongly and selectively bind small and large molecules.
THE WORLD'S MOST FAMOUS MICROFLUIDIC
IMMUNOASSAY: THE PREGNANCY TEST
Modern pregnancy tests based on the strip assay
that can be dipped in urine
detect the presence of human chorionic gonadotropin (hCG). his pregnancy marker was
discovered in 1930 and is produced by the trophoblast cells of the fertilized ovum (blas-
tocyst). Home pregnancy testing hit the American market by the end of 1977, but then
it consisted of mixing the woman's urine with solutions in a rather complex procedure
that took hours to produce a result. he irst lateral-low test (based on a strip of nitrocel-
lulose paper, with low driven by wicking, which reads as “two strips, you are pregnant,”
see
Figure 4.9
) was the ClearBlue commercialized by UniPath (now SPD) in 1985. By the
same token, it can also be considered the irst microluidic product to have ever reached
the market. Many of us have made important planning decisions ater seeing the results
displayed by those two bars. It is a clever piece of paper microluidics, invented before the
irst article on paper microluidics!
Although some
qualitative
immunoassays have been reduced to simple test strip assays (like
pregnancy tests), the performance of
quantitative
immunoassays is a relatively complex sequence
of processes today largely restricted to centralized laboratories because of the need for long assay
times, complex and expensive equipment, and highly trained technicians. If a wider range of the
700 million immunoassays performed annually in the United States alone could be run more
inexpensively, more frequently, and at the point-of-care, the health of millions of patients could
be improved. he inexpensive availability of such assays for diagnosis in the developing world
would have an enormous efect on global health.
here are many diferent detection modes for immunoassays, some of which rely on the bind-
ing of the analyte to change a property of the surface, whereas others rely on some form of
ampliication process that may or may not rely on a surface. We will break our discussion of
immunoassays into two sections, but irst, we will explain in detail the most ubiquitous immu-
noassay ever deployed.
4.5.1 The Pregnancy Test
Human chorionic gonadotropin (hCG) is a hormone secreted by the developing placenta ater
fertilization and it can be found in blood and urine. Modern pregnancy tests use a “sandwich”
ELISA (short for “enzyme-linked immuno-sorbent assay”) method involving three diferent
antibody preparations (located in three diferent strips) on a porous paper sheet (
Figure 4.9
).
he porosity of the paper serves to pump the urine by capillarity from one end of the strip to
the other. he reaction strip “R” is the irst to be exposed to urine low and contains unbound
mouse monoclonal anti-hCG antibody-enzyme conjugates. Note that the urine carries these
conjugates upstream whether or not there is hCG in the urine, but if there is hCG in the urine, a
fraction of the conjugates will, in addition, bind to hCG (there is great excess of conjugates with
respect to the normal concentrations of hCG). he test strip (“T”) is the next to be exposed to
the urine low and contains immobilized polyclonal anti-hCG antibodies (designed to bind to
a diferent epitope on the hCG molecule than the one that is already bound to the monoclonal
antibody) and a dye substrate (shown in red). he control strip (“C”) contains immobilized goat
