Biomedical Engineering Reference
In-Depth Information
microcantilever sensors [25-27] (2) Organic CantiFET [28] (3) AZO
CantiFET [29] and (4) polymer nanocomposite piezoelectric microcanti-
lever sensors [30-32] are discussed here.
9.2
Polymer Nanocomposite Piezoresistive
Microcantilever Sensors
Microcantilever sensors with an integrated piezoresistor perform electrical
transduction of strain by a resultant change in resistance. When a surface
functionalized microcantilever with molecules or thin i lm coatings that
are specii c to the target molecules is exposed to the ambient containing
analyte, the selective molecular interactions create a dif erential surface
stress between the top and bottom surfaces of the microcantilever. h is
dif erential surface stress results in a change in resistance of the piezoresis-
tive layer. h is can be represented using Equation 9.1 [20].
ZZ
R
1
⋅
TR
(9.1)
=−
K
+
⋅
∑
s
R
E h
2
1
32
h
ii
i
∑
2
i
Eh Z
+
ii
i
ic
σ
S
is the surface stress,
Z
T
is the position of
top layer,
Z
R
is the position of piezoresistive layer and
E
i
,
h
i
and
Z
ic
are the
Young's modulus, height and position of the i
th
layer with respect to neutral
axis. Based on this expression, the surface stress sensitivity is proportional
to the ratio of the gauge factor (K) of the piezoresistive i lm to the Young's
modulus (E) of the cantilever structural material. As mentioned earlier,
usage of compliant polymers such as SU-8 having a low Young's modulus E
(~40 times lower compared to silicon based materials) as cantilever struc-
tural materials in place of conventional silicon based materials is expected
to improve surface stress sensitivity.
Piezoresistive SU-8 microcantilevers with dif erent with integrated gold
(Au) [10] and polysilicon strain sensitive layers have been reported earlier
[20, 24]. Gold being a metal strain gauge material that has got very low
gauge factor, low temperature deposited polysilicon [24] was a better sub-
stitute for gold for improving the sensitivity. However the design constraint
was that the polysilicon i lm should be thin enough such that it does not
where K is the gauge factor,
