Biomedical Engineering Reference
In-Depth Information
Fig. 6 Antibody immobilisation on the PMMA surface of a chip. Reprinted from [
114
] with
permission from Elsevier
need for sophisticated detection components, moving microfluidics closer to
practical POC devices.
By basing a system around microfluidic technology, one can design a complete
integrated solution for portable diagnostic testing. An early example, named the
''POCKET immunoassay,'' was presented in 2004 by the Whitesides group [
97
].
Its primary goal was the detection of HIV antibodies, which it achieved by
overcoming a detection problem when using microchannels: the short optical path
length of the channels makes measurements of optical absorbance difficult,
especially in a continuous-flow format. An assay was developed in which the
secondary antibody is conjugated to gold colloids and was then exposed to a silver
reducing solution. The reduced silver atoms form an opaque film where the
absorbance is a function of the analyte concentration. The PDMS microchip was
integrated with a compact, low-cost, battery-powered detection system comprised
of a laser diode and a photodiode costing just $45 (in 2004). Using the sera from
HIV-1-infected patients, it was able to successfully detect and quantify anti-HIV-1
antibodies in 10 min. Although the system still required sample pretreatment, it
offered an alternative approach to ELISA in a simple low-cost portable format.
Chin et al. recently reported a complete microfluidic immunoassay that incor-
porates all the steps of ELISA to detect HIV in whole blood [
14
]. The 'mChip'
(Fig.
7
) integrates multiple microfluidic technologies: (i) high-throughput manu-
facturing, (ii) automatic delivery of multiple reagents and (iii) a microscope-free
signal amplification immunoassay. The chips comprising the microfluidic channels
were fabricated in transparent polystyrene (the same material used in ELISA
multiwell plates) and cyclic olefin copolymer using injection moulding. This
reduces the cost compared to conventional PDMS-based chips and allows high
throughput production, with one chip being made every 40 s. With the aim of
simultaneous detection of HIV and syphilis, the researchers needed to develop an
assay with multiple steps. This required sequential delivery of multiple reagents
into the channels without an expensive external pumping system. Their solution
was to use a simple bubble-based fluid delivery technique that involves sequen-
tially drawing fluid plugs into the tubing by hand using a syringe. The reagents are
