Biomedical Engineering Reference
In-Depth Information
miniaturized thermal biosensor needs much lower sample volumes,about 20
m
l,
making it very attractive for clinical use.
5.3
Integrated Thermopiles
Thermocouples are alternative transducers for converting the heat produced by
a corresponding enzymatic reaction into an electrical signal.Here,the temper-
ature difference between the ends of a pair of dissimilar metal wires is deduced
from a measurement of the difference in the thermoelectric potentials develo-
ped along the wires (Seebeck effect).A temperature gradient in a metal or alloy
leads to an electrical potential gradient being set up along the temperature
gradient. For small temperature differences, the thermoelectric potential gra-
dient
T and strongly dependent
on the materials which are used to form the junction.The proportional factor is
the relative Seebeck coefficient
D
V is proportional to the temperature gradient
D
a AB between materials A and B.Thermocouples
have a high rejection ratio for common-mode thermal noise.Moreover,they do
not require a well-matched reference, as thermistor based systems. As a con-
sequence,they are not large and suitable for miniaturization.
Bataillard et al.(1993) described the integration of a series of thermocouples
(thermopiles) on a silicon chip in order to increase the voltage output.The chip
consists of an array of p-type silicon/aluminium thermocouples, connected in
series and integrated in a n-type silicon epoxy layer grown on the silicon wafer.
The overall size of the chip is only 5
5 mm.Several enzymes are immobilized
on the array,and the chips are inserted into a FIA system.Glucose is monitored
with coimmobilized glucose oxidase and catalase ranging from 2 to 100 mmol/l,
urea (1-1000 mmol/l) with immobilized urease and penicillin (1-1000 mmol/l)
with immobilized
¥
-lactamase.
Xie et al.(1994) manufactured a microbiosensor with an integrated thermo-
pile on a quartz chip because of the lower heat conductivity of quartz compared
to silicon.The size of the whole sensor was 25.2
b
0.6 mm,and immobi-
lized enzymes (CPG beads) were were placed in the chips microchannels. The
sensor was applied to glucose analysis in the 2 to 25 mmol/l range, using only
1
¥
14.8
¥
l samples. Due to clinical analysis requirements, the sensor might be inte-
resting for blood glucose measurements.
Nevertheless,some problems have to be considered.First of all,it is very
difficult to obtain non-clogging immobilization materials. Moreover, the sen-
sitivity is strongly limited by electrical noise due to the high impedance of the
integrated thermopile. Thus, further research and development is necessary,
especially as far as the electronics are concerned.
m
5.4
Bio-Thermochips
Recently,a new ET system was presented which does not require a precise
thermostat (Shimohigoshi et al.1995).The “bio-thermochip”consists of a bead
thermistor directly surrounded by an enzyme immobilization support to reduce
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