Biomedical Engineering Reference
In-Depth Information
C h a p t e r 7
Biochemical Reactions in Biochips
7.1 Introduction
In this chapter, we come to the very purpose of biochips. So far, we have dealt with
the principles of microfluidic transport of macromolecules or microparticles and
we have shown how these principles are used to displace and manipulate these
objects inside microsystems. It is recalled here that the approach has been done
in two steps, first, the study of the microfluidic flow as a carrier fluid; second, the
study of the behavior of macromolecules and/or microparticles in such microfluidic
flows. Up to now, we have only dealt with tools to perform a task. The essential
question is: What task are we going to perform with such tools and what have all
these techniques been developed for? This brings us to the purpose of biochips or
bioMEMS.
Basically, the main purpose of biochips is the analysis and recognition of mac-
romolecules: DNA, proteins, and so forth. Ultimately, recognition process should
be fast, sensitive and reliable, with the less possible false results, and largely paral-
lelizable, allowing for simultaneous samples recognition.
We will see in this chapter that biorecognition is based on a mechanism of key
lock [1], which is in reality a biochemical reaction. This leads us to present the
kinetics of chemical and biochemical reactions, with a special attention to some
key reactions, like enzyme-catalyst reactions for proteins and adsorption reactions
for DNA hybridization. Because in biochip technology, the targets to analyze are
immerged and carried by a buffer fluid, recognition times do not depend only on
the biochemical reaction kinetics but also on the presence of targets in the vicinity
of the reagents. It is then necessary to treat the coupling between biochemical reac-
tions and the advection-diffusion of targeted molecules. In conclusion, we point out
that biorecognition is very dependent on detection sensitivity. The same care that is
taken for developing an efficient bioreaction should be taken also to the detection
process.
7.2 From the Principle of Biorecognition to the Development of Biochips
7.2.1 Introduction to Biorecognition
The discovery of the recognition potential by the immune system—sometimes
called immune specificity—has been a major milestone in the development of biol-
ogy and has been awarded many Nobel prizes. The first step was the discovery of
the model key-lock by Fisher in 1892, sketched in Figure 7.1. In such an approach a
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