Biochips – fundamentals and applications - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 341

Therefore, for clinical testing of substrates, on-chip enzymatic assays have advantages
over conventional systems in terms of speed and performance, not to mention sample
volume since reagents and the enzymes themselves can be costly. At nanoliter volumes,
these chips reduce their enzyme consumption by about four orders of magnitude over
standard assays. Enzymes can convert 10 2 -10 5 substrate molecules to product within
1 s and thus have been used in FIA and CE. By changing corresponding enzymes, various
substrates can be used to produce either optical or electrochemical signals. An enzymatic
assay procedurally involves a derivatization reaction where a non-detectable species is
transformed into a detectable one and hence has steps including mixing, reactions, and
separations. Fluid control was via electrokinetic pumping with voltages applied at the
enzyme, sample, and buffer reservoirs. The application of this type of on-chip assay
to complex samples such as biological fl uids would benefi t from additional sample
processing functions on chip such as cleanup and preconcentration [77, 78].
The discovery of biomarkers, for both prognostics and diagnostics, is a very active
research fi eld. Biomarkers can be used as indicators of disease pathophysiology, such as
blood pressure, cholesterol levels, and viral load in HIV. Moreover, biomarkers can also
be used as a substitute for a clinical endpoint. Since screening single biomarkers cannot
properly diagnose the disease and their clinical value is questionable, investigators use
high throughput platforms such as protein array and other approaches to identify large
numbers of candidate biomarkers. The reason for using high throughput technologies is
that they provide a large number of correlative data on protein expression in relation to
disease. Such data are then analyzed for their association to the disease, which provide
major impetus for the molecular profi ling approaches to fi nd patterns or profi les for a
clinical test based on high dimensional protein expression panels. These protein array
techniques can help not only to detect potential novel biomarkers, but also to generate a
greater understanding of the signaling pathways associated with the printed proteins. This
understanding can extend the use of a potential biomarker from being merely a mecha-
nism to actually defi ning a characteristic of a disease or a potential drug target [79, 80].
Another application of protein microarray in clinical diagnostics is disease moni-
toring. Organ and disease-specifi c protein arrays can be used in expression profi ling
to identify disease-related proteins. These arrays can also quantify specifi c sample
proteins in terms of abundance, location, and modifi cation as the disease progresses.
By studying protein regulation and expression, clinicians and researchers can predict
predisposition to disease. Once the disease is manifested, gathered data can be used
for monitoring disease progression, determining response to treatment, and providing
overall prognosis. It is also possible to screen for molecular markers and diagnostic
and/or therapeutic targets in patient-matched tissue during disease progression [81].
11.3.3.3 Drug discovery
Protein microchips have a signifi cant impact on the development of safer drugs through
the comprehensive profi ling of drugs or lead compounds for effects. Zhu et al. [35]
used a microarray of an entire eukaryotic proteome to screen different biochemical
activities. First, protein chips can screen potential lead compounds at high throughput