Biomedical Engineering Reference
(hematocrit) are analyzed. Currently, methods are based on electrochemical
detection such as potentiometry, amperometry, and conductance [ 5 ] (see appli-
cations for more details). Another microfluidic approach to ion analysis is optical
sensing [ 26 ]. For example, a microfluidic device can be used for pH analysis based
on the color change of pH-sensitive dyes immobilized in patterned hydrogels [ 27 ].
One of the most common protein assays that are adapted for point-of-care
microfluidic-based diagnostics is the immunoassay. This assay makes use of the
binding interactions between antigens and antibodies to detect protein markers
from either pathogen or host immune responses. Analytes for immunoassays span
the entire range of pathogen types, from viruses (e.g., anti-HIV antibodies and
p24 antigen for HIV) to bacteria (e.g., anti-treponemal antibodies for syphilis and
early secretory antigenic target 6 for tuberculosis) and parasites (e.g., histidine-rich
protein 2 for malaria). Immunoassays are also commonly used for detection of
noncommunicable diseases, such as prostate cancer via measurement of levels of
prostate specific antigen, as in a system by OPKO Diagnostics (formerly Claros
Diagnostics) or heart disease via measurement of levels of B-type natriuretic
peptide, as in the Triage BNP Test by Biosite [ 28 ].
Heterogeneous immunoassays are a popular immunoassay format where capture
of analytes occurs at protein-modified surfaces. The reference standard of many
heterogeneous immunoassays is the enzyme-linked immunosorbent assay (ELISA),
which uses multiwell plates and can be automated for high-throughput pro-
cessing at well-equipped central laboratories. Most ELISAs use colorimetric- or
chemiluminescence-based detection and have detection limits typically in the pM
range due to enzyme-mediated signal amplification and serial washing. Unfortu-
nately, the traditional ELISA format, which requires expensive and bulky instru-
mentation (for liquid handling and signal detection) and trained workers, is ill-suited
for point-of-care testing.
Microfluidics is an attractive technology for point-of-care immunoassays.
Heterogeneous immunoassays, because they involve capture of analytes at surfaces,
are well-suited to exploit the large surface-to-volume ratios encountered in
microfluidics. Faster analysis times can be achieved because of the replenishment
of analytes and detection reagents in the boundary layer above the surface in
standard wellplate formats [ 29 ]. Below, we highlight recent advances in microfluidic
immunoassays that are low cost, portable, and can be used at point-of-care settings.