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The reversible sealing of a PDMS elastomer with a microfluidic
chip gives the opportunity to expose a PDMS surface to a first series
of reagents (e.g. different types of capture antibodies) and then
to a series of samples. By rotating the microfluidic chip used for
the second step by 90° on the PDMS surface, the samples flow
across the lines of capture antibodies formed on the PDMS surface
during the first step, Figure 1. The specific capture of analyte
by surface-immobilized antibodies results in a mosaic of signals,
Figure 1b. We call these assays "micromosaic immunoassays".
Figure 2 shows a chip particularly convenient for making micromosaic
immunoassays, where a capillary pump at the end of each microchannel
draws the solutions that are successively loaded in each same fill
port through a ~10-mm-long microchannel. The region where the microchannels
run parallel is covered with a slab of PDMS during the assay to
provide the substrate for the immunoassay.
We are pursuing the miniaturization of assays for diagnostic applications
and other applications in general where it is desirable to employ
very small volumes of samples to detect multiple analytes in parallel
and in a minimum amount of time. For example, we worked with Patrick
Hunziker at the University Hospital of Basel to detect cardiac markers
from human plasma samples in a combinatorial manner.
One challenge attendant to the miniaturization of assays is to
detect analytes at low concentration and in small volumes of samples
(small overall number of moles of analytes). We collaborated with
scientists from Pharma Research in F. Hoffmann-LaRoche in Basel
to detect the biologically important cytokine TNF-alpha in sub-microliter
volumes of samples and with picomolar sensitivity, Figure 3. The
high definition of the capture sites and careful control of the
flow rates of solutions in the microchannels during the assay were
among the most crucial parameters to yield the mosaic of fluorescence
signal as shown in Figure 3.
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IBM Research
