Biomedical Engineering Reference
In-Depth Information
The prototype uses sheath flow from two sides of the 25 m-high and 120 m-
wide fluidic channel. This channel is part of a custom fluidic chip that has the spatial
shadow mask directly attached to it and that also serves as a light guide for the
excitation laser.
Measurements of the sensitivity and dynamic range of the prototype were
conducted with 2 m Rainbow calibration beads (Spherotech) and yielded a
detection limit
200 MEPE (molecules of equivalent phycoerythrin) [
32
], which
meets the needs for a wide range of bioparticle detection applications. By replacing
the pin photodiode by a pixelated avalanche photodiode, the sensitivity increases
to 50 MEPE which meets or even exceeds the specifications of current commercial
high-performance flow cytometers.
3.3
Pathogen Detection in Water
3.3.1
Background and Currently Used Techniques
Water-quality monitoring is an essential priority for global health. The United
States Environmental Protection Agency (EPA) and Centers for Disease Control and
Prevention (CDC) estimate that there are 4-12 million cases of acute gastrointestinal
illness annually attributable to public drinking water systems in the USA [
33
].
With microorganisms a primary cause for the occurrence of infectious diseases,
the concentrations of harmful microbes should be routinely monitored to maintain
microbiological quality control of drinking water.
Because of the difficulty and cost of directly measuring all microbial pathogens
in water samples, organisms like coliform bacteria,
Giardia,
and
Cryptosporidium
that indicate the presence of sewage and fecal contamination have been targeted
for measurement [
34
]. There is a strong need for an inexpensive, rugged, and fast
detection instrument to monitor both beaches and drinking water at the point of
need.
Bacterial quantification is currently performed by labs primarily using plate
culture assay techniques that have supported microbiology for more than 100 years.
The gold standard to determine bacterial coliform count in water starts with the
membrane filter technique, then incubation growth in a plate culture followed
by counting of the colony-forming units. Unfortunately, culture assay techniques
for quantification are costly, labor intensive, and time-consuming to conduct with
measurement times greater than 24 h due to incubation needs. Culture-independent
techniques have used fluorescent microscopes, but the method is labor intensive and
differences in the numbers of bacteria observed can arise due to staining technique,
physicochemical characteristics of the samples, and investigator bias [
35
]. A method
using TaqMan PCR has been developed to quantify indicator bacteria rapidly [
34
],
but the instrument is bulky and expensive so the sample is still sent to the lab for
analysis with the samples usually kept on ice during the time before analysis.
