Biomedical Engineering Reference
In-Depth Information
Fig. 12.2.
Illustration of the transducer principles in an optical biochip.
Left
: Label-
free sensing principle.
Right
: Labeled sensing principle
that emit light when excited with radiation of shorter wavelength. Alterna-
tively, one can also make use of luminescent molecules, such as the famous
luciferase, which are causing a biochemical enzymatic reaction as part of which
(visible) light is emitted [3]. The term “biochip” is used very broadly in the
field and does not describe the particular implementation of the principles
described above precisely. Depending on the level of integration, one distin-
guishes usually between three different types of biochips:
Biochemical Microarray
The simplest and still the dominating type of biochip consists of a one- or two-
dimensional array of transducer regions, which are sensitized with different
receptors on a common substrate [4]. Neither microfluidics transportation
systems nor the light sources or the detectors are part of such a biochemical
microarray. This type of a biochip can consist, therefore, of up to one million
individual transducer regions, each with an area as small as 5
m
2
,onan
optical biochip with an active surface of 1-2 cm
2
[5]. Most often, biochemical
microarrays employ fluorescence tagging as the optical transducer mechanism.
If they are employed for DNA analysis, optical microarrays are known as DNA-
chips or Genechips
TM
, and such DNA-chips with half a million transducer
regions are commercially available for about
×
5
µ
200 [5]. Although DNA analysis
(genomics) is still the predominant application of optical microarrays, they are
increasingly used for protein (proteomics) and cell analysis [6].
¤
Lab-on-a-Chip (
µ
-TAS)
To simplify the practical use of biochips and to reduce the sample volumes
required for an analysis, an increasing amount of functionality is placed on the
same substrate. A lab-on-a-chip (LOC) combines the microfluidic sub-system,
including all elements required to transport, mix, process, separate, and drain
fluids, together with the optical transducer part on a single chip [7]. Such an
