Biomedical Engineering Reference
In-Depth Information
Fig. 15
Cross section of a
CNT/PDMS flexible pressure
sensor
placed against the moulds, and the PDMS is introduced in the cavities through a hole
to create an aligned-CNT/PDMS nanocomposite as described previously for epoxies
using capillarity-assisted wetting [
30
], followed by the curing of the elastomer.
Three flexible membrane layers are required to fabricate the sensor, with the
top and bottom layers defining the inductor and the electrodes, and the middle
one defining the dielectric (air). This configuration requires bonding of PDMS
membranes. Eddings [
31
] tested five different bonding techniques and the highest
reported bond strength was obtained for both partial curing and uncured PDMS
adhesive techniques. The latter approach is used to build the sensors, Fig.
15
.
9.3
Material Properties and Results
The key step of the fabrication process is the CNT-PDMS impregnation and re-
spective mechanical and electrical properties (that will govern the sensor response).
Aligned CNTs are oriented in the out-of-plane (or normal to the wafer plane) di-
rection such that the polymer nanocomposite can be presumed transversly isotropic,
i.e., isotropic in the plane of the sensor. Furthermore, the modulus enhancement due
to CNTs is likely minimal as the long axis of the CNTs are oriented perpendicular
to the loading direction, such that the PDMS polymer dominates the response.
Work on nanocomposites has shown significant increase in modulus due to aligned
CNTs in polymer (PDMS) [
32
] and epoxy [
33
] in the CNT axis direction, but little
reinforcement effect in the transverse direction as used here. This result is expected
from composite micromechanics analyses and experimental results.
Experimental results using a prototype PDMS/CNT flexible pressure sensor are
presented in Fig.
16
. The tested sensors have a mechanical layer thickness of
670 m and a dielectric thickness of 260 m resulting in a total sensor thickness of
1.6 mm (670 m
670 m). The area of the electrodes is 3.4 x 9 mm
2
(W x L). Despite the coarse sensor resolution (the geometry was not optimized
since the devices are at a proof-of-concept stage), capacitive changes were measured
when the sensor was placed inside a controlled pressure chamber (the dielectric is
hermetically sealed at ambient pressure).
C
260 m
C
