Compared to nucleic acid detection, which has to go through
cell lysis, nucleic acid purification, and DNA amplification, protein
detection with immunoassay is relatively simpler and faster. For
this reason, all of the lateral-flow assays for protein biomarker
detection are based on immunoassay. Due to the widespread use
of immunoassay, there have been massive research reports on
immunoassay-based lab-on-a-chip systems [27–32]. Among the
various reported work on immunoassay-based protein detection,
this review focuses on their uses for POCT diagnostics.
One of the most commercially successful immunoassay POCT
systems is the Alere Triage system [33,34] shown in Fig. 2. The
Triage cartridge employs the same working principle to the lateral-
flow assays, where target antigens in a sample bind first to
detection antibodies labeled with gold colloids. Then, the mixture
of antigens and detection antibodies passes through the detection
zone where capture antibodies are pre-immobilized.
A major improvement in the Triage cartridge compared to the
traditional lateral-flow assay is that the cartridge uses polymer
microfluidic channels replacing nitrocellulose membrane from
the lateral-flow assays. Lower variability in batch-to-batch material
properties of the polymer substrate provides higher reproducibility
of the assay performance compared to lateral-flow strips.
Passive capillary flow on the Triage cartridge was obtained by
microstructures. The Triage cartridge overcame the characteristic
drawback of lateral-flow assays, which is the non-controllable
sample incubation time with labeled detection antibodies along
the flow, by using a spatial hydrophobic region called ‘‘Timegate’’.
The ‘‘Timegate’’ has the capability of slowing down the sample
flow rate to increase the incubation time of the labeled detection
antibodies in a controllable manner. The Triage cartridge employed