Biomedical Engineering Reference
In-Depth Information
required, which greatly simplifies the system. A disadvantage may be the speed at
which the test can be performed, but a recent report by Rezk et al. demonstrated
increased mixing in paper microchannels using surface acoustic waves [
87
].
Performing a paper-based test requires the addition of 0.1-1 ll of reagent to the
test zone, either by hand or using an inkjet printer and then allowing it to dry.
One of the first applications was the analysis of glucose and protein content of urine
[
65
] (Fig.
10
b). Martinez et al. used a well-known colorimetric assay in which the
intensity of the colour is a function of the analyte concentration enabling quanti-
tative measurements. Detection can be performed either by a desktop scanner or a
digital camera and the intensity of the reflected colour is then compared to a
calibration curve. The device was able to simultaneously detect glucose and BSA in
approximately 11 min with sensitivity comparable to commercial dipstick assays.
More recently, it has been shown that it is possible to improve the limit of detection
of a colloidal gold label using a gold enhancement solution [
33
]. The model target
was biotinylated-BSA, which bound to a streptavidin-gold conjugate. The size of
the gold nanoparticles was then increased using an enhancement solution and an
average signal amplification ratio of 5.9 was achieved. Other methods of detection
compatible with paper-based systems include electrochemical detection [
27
] and
chemiluminescence [
20
,
111
].
Paper microzone plates have been shown to be low-cost alternatives to plastic
plates [
11
] (Fig.
10
c). Here only a few microlitres of the reagent are added to the
paper test zones, reducing the sample consumption costs. The solution evaporates
quickly due to the large surface-to-volume ratio, which increases the sensitivity by
approximately 40 times. The cost of each plate is approximately $0.05 and they
can be easily transported and stored. Both 96- and 384-microzone plates fabricated
from paper are compatible with conventional microplate readers, but they can also
be analysed with a scanner or camera. Paper microzone plates can also be designed
to incorporate connecting microchannels between the test zones, which allows
replicate assay and mixing of reagents to develop assays. ELISA for the detection
of antibodies to the HIV-1 envelope antigen gp41 can be performed using paper
microzone plates [
12
] (Fig.
10
c). Using human serum, Cheng and co-workers
showed that a positive result could be detected in a 10-fold dilution in less than
1 h. The limit of detection was determined to be 54 fmol/zone using rabbit IgG
and a colorimetric readout, which is approximately 10 times lower than conven-
tional ELISA using absorbance.
Multiple paper layers can be stacked to form 3D microfluidic networks
(Fig.
10
d) [
66
]. The advantage is that each layer can be made from different types
of paper, thus providing multiple functionalities, i.e. sample filtration before
detection. It is even possible to assemble these designs using the principles of
origami [
57
]. So far, 3D paper-based platforms using colorimetric assays have
been used to detect glucose and BSA simultaneously [
66
] and hepatitis B in serum
using ELISA [
58
]. Although the current paper-based ELISA is faster and less
expensive, it fails to match the sensitivity of conventional ELISA. With the
majority of paper-based microfluidic research focussed on diagnostic applications,
this is likely to improve in the near future.
