Miniaturized Detection Systems - Waterborne Pathogens: Detection Methods and Applications - page 320

Refinement of the method to increase the capture efficiency was car-
ried out with the use of a laser-machined stainless steel (SUS) micromesh
consisting of a 10 × 10 array of 2.7 mm diameter cavities to capture single
oocysts incorporated into a PDMS microfluidic device ( Fig 10.4(a) ). 69 The
maximum flow rate tested was 350 µL min −1 , so 5 mL could be processed in
under 15 min. When loading a 0.5 mL test sample (spiked oocysts in PBS) at
a concentration of 36 oocysts mL −1 a recovery rate of 93% from the mesh was
reported, comparable to that achieved by off-chip IMS. Batch processing of
the sample occurs in the current design; thus while integration into automated
systems would be possible, real-time continuous monitoring would not be.
Work carried out by Liu et al. illustrates an alternative strategy of trapping
Cryptosporidium oocysts, using sieves or filters. 70,71 In one example, a weir was
created by interfacing a deep channel (50 µm) with a very shallow channel (1
or 2 µm). Using positive pressure, a mixture of protozoa in water was injected
into the channel, trapping the cells against the wall of the deep channel. The
common disadvantage of sieves or filters systems is their rapid clogging, per-
haps due to the weir system. However, by developing a so-called raindrop
bypass filter, Liu et al. significantly reduced this issue. The design consists of
three prefilters and a wide composite filter structure, which allows alternative
fluidic paths and therefore significantly reduces the pressure and the clogging
on the filter. The filters are made of fine arrays of raindrop-like-shaped pil-
lars, arranged in gaps ranging from 0.2 to 1 mm, in the trapping zones and
coarse arrays in the bypass zones. The device was used for bacterial capture
and detection with a limit of detection (LOD) of 10 5 cfu mL −1 . However,
further details regarding the performance (LOD, volumes, etc.) of this device
with protozoa were not available to fully analyze its potential ( Fig. 10.4 ).
10.2.2. In optical methods
Several examples of lab-on-a-chip systems exist for optical detection of
waterborne pathogens. There are also other potential systems, which have
not yet been applied to waterborne pathogens, e.g. surface enhanced Raman
scattering on-chip, which was reviewed by Chen and Choo in 2008. 72
In some of the above examples of (oo)cyst and bacteria capture, the sample
processing element of the device was also utilized for detection, through the
addition of fluorescent stains. Two of the above systems represent a micro-
fluidic alternative to the existing IMS and fluorescence detection protocol
for (oo)cysts. Taguchi et al. note the advantages of their method include the
predefined location of the binding of the oocysts and their good adhesion to
the substrate during the washing and staining steps. 68 Their second design also