Biomedical Engineering Reference
In-Depth Information
as in the EndoSure device). Furthermore, it enables the placement of more than one
sensor (a sensor cluster) contributing to a more comprehensive study of post-EVAR
aneurysm evolution that, currently, is not possible.
9.1
Capacitive Sensor Design
Research on implantable pressure sensors is very active and has been supported and
justified by the need of continuous pressure monitoring for patients with congestive
heart failure, as an early diagnostic mechanism for some risk patients and for post-
EVAR surveillance [
24
,
25
].
Implantable pressure sensors are typically categorized into extra-arterial blood
pressure devices and intra-arterial blood pressure devices [
25
]. The firsts are placed
around the blood vessel and perform an indirect pressure measurement through
the wall or through the expansion and contraction of the artery. They require an
invasive surgical procedure for their implant while, on the other hand, the intra-
arterial devices are in contact with the blood stream inside of the blood vessels.
After stent-graft placement, the aneurysm sac gets depressurized and the pres-
sure drops down to a few mmHg as indicated by the simulations (12-22 mmHg
according to Fig.
9
). Therefore, if one wants to sense the luminal pressure value
(ranges typically between 50 and 160 mmHg) through the aneurysm sac pressure,
the sensor must be able to measure pressures between 6 and 26 mmHg. In addition,
it needs a high dynamic range in order to detect stent-graft complications (in this
case the sac gets pressurized and pressure increases to the luminal pressure values).
Typical configurations of capacitive pressure sensors use square-plate
(diaphragm) electrodes separated by a dielectric (oftentimes of air) at a pressure P
0
.
Changes on the outside pressure (P
out
) deform the square plate and consequently
generate a capacitive change. A schematic of a square-plate (side length of 2a)
pressure sensor is shown in Fig.
11
. The sensor involves two coupled domains,
mechanical and electrical, that define the sensor behavior.
A cross section of the square plate sensor is shown in Fig.
12
(section cut B-B)
where only the mechanical domain is considered. The side length is 2a, t is the
thickness and w
0
the deflection. The diaphragm is clamped at the edges. For a
clamped diaphragm under a uniform load (like pressure), the angle of deflection,
®, is equal to zero at the center (r
a) of the diaphragm. For
these boundary conditions, the deflection of an isotropic square diaphragm under a
pressure load can be modeled as [
26
]:
D
0) and at the edge (r
D
4:20
Et
4
.1
2
/a
4
w
0
3
t
3
w
0
t
P
0
P
o
u
t
D
C
1:58
(1)
where ¤ is the Poisson's ratio,
E
is the Young's modulus, and
P
D
P
0
-P
out
is the
pressure load.
