Instrumentation & Measurement Magazine 24-2 - 18

	

C  T 0

A
	(1)
d

where ϵT is the relative permittivity and ϵ0 is the vacuum permittivity of the material between the two plates.
Capacitive PS are very common in pressure measurements. Nizami et al. [56] compared the metrological
properties of three common pressure mats (capacitive, piezoresistive and fiber-optic) for the purpose of human health
monitoring. Nizami et al. [34] evaluated the uncertainty when
using a capacitive pressure mat to measure breathing rate of
infants using neonatal simulators, which was able to measure the number of breaths per minute with a mean bias of
0.56 breaths per minute.
Jeong et al. designed and fabricated a multilayer fabric pressure distribution sensor. In this case, the two capacitive fabric
layers are separated by a polyurethane nanoweb and air pockets. The electrical impedance was obtained by injecting ac
currents, meaning that it is an active sensor, and by measuring
the resulting voltages which changed based on the deformation of the nanoweb layer [55]. The sensor was therefore able
to detect both piezoresistance and piezocapacitance changes
when a pressure was applied perpendicular to the sensor [55].
The authors present this novel PS as a potential future tool for
gait analysis and mobility monitoring [55].
The capacitive principle has also been applied to printed
ink sensors where the electrical resistance is inversely proportional to the force applied to the ink situated between the
electrodes. Cruz et al. present a low-cost pressure mapping
system for mobility monitoring using conductive ink which
is printed on polymers. The sensors were created using Inkjet Printing technology. The conductive ink was sandwiched
between two flexible polymeric membranes (Fig. 3d). As pressure is applied perpendicularly, the capacitance increases
which is the sensing foundation.

Piezoelectric and Piezoresistive Sensors
The ability of a material to generate an electrical charge (i.e.,
passive sensing) in response to an external mechanical stress
is called the piezoelectric effect. In PS, the most common material used is polyvinylidene fluoride (PVDF), which is applied
in two layers typically separated by gel [50] (Fig. 3e). One layer
acts as the transmitter and the other is the receiver, when external force is applied the thickness of the gel decreases causing
the capacitance propagation time to decrease between the two
PVDFs [50].
The resistance of this sensor setup can also be characterized
based on the principle that the electrical resistance of a given
material will change when it is mechanically deformed, typically when an external force is applied [45]. The resistance of
the sensor is given by:
	

l
R     w	(2)
t

where, ρ is the resistivity of the material, l is the length and t
is the thickness of the piezoresistor, and w is the width of the
18	

contact surface. The resistivity of the material is also dependent on the doping concentration of the piezoresistor.
The pressure mats from Tekscan (Tekscan, Boston, USA,
tekscan.com) use resistive ink as the resistive material in
the piezoresistive sensor. The metrological properties of the
Tekscan BRE5400-1 pressure sensitive mat (PSM) has been investigated with respect to drift, creep and rise time in [56].
Mora et al. created a piezoelectric, similar to piezoresistive,
sensor-based pressure array placed on top of the mattress directly under the subject (Fig. 2b) for the automatic screening
of sleep apnea-hypopnea syndrome. The authors were able to
differentiate between individuals with and without abnormal
breathing with an accuracy of 96% [39].
Koyama et al. also used piezoelectric sensors placed on top
of the mattress but below the bed sheet (Fig. 2b) to detect apnea
events during Cheyne-Stokes-like breathing patterns in order
screen for sleep disordered breathing (SDB). When differentiating between non-SDB and mild SDB the sensitivity was
92.1% and the specificity was 60.0% [37]. When differentiating
between moderate to severe SDB and severe SDB the sensitivity was 93.8% and the specificity was 100% [37].

Fiber-Optics
Optical fiber PS rely on two optical fibers enclosed in a cavity typically surrounded by polymer foam. One optical fiber
transmits light into the cavity (scattering medium) where the
light is reflected and then transmitted to a photodiode by the
second optical fiber (Fig. 3f). The photodiode measures the
changes in light intensity related to applied pressure, outputting a voltage representation of the applied pressure [21].
The S4 (S4 Sensors Inc.) PSM are embedded with Kinotex
fiber-optic sensor pairs. Our research group has used the S4
PSM for a variety of research purposes. The metrological properties of the S4 (S4 Sensors Inc.) PSM have been investigated
with respect to drift, creep and rise time in [56].
Arculus et al. used fiber-optic PSM placed between the bedframe and mattress (Fig. 2c) to investigate sit-to-stand timings
and symmetry as a measure of older adult mobility and fall
risk. Asymmetrical sit-to-stand movements can be associated
with a higher fall risk, and Arcelus et al. created an automatic
sit-to-stand symmetry classification tool that had an overall accuracy of 93%.
Bennett et al. [21] investigated the use of fiber-optic PSM
placed between the bedframe and mattress (Fig. 2c) to automatically identify both discrete and continuous patient
positions and transitions as a measure of mobility. Two classifiers were created to identify the position (lie, sit, stand)
and identify the transition between states (e.g., lie to sit). Both
methods resulted in accuracies of at least 98% [21].
Townsend et al. used fiber-optic PSM placed between the
bedframe and mattress (Fig. 2c) to identify central sleep apnea
events with a sensitivity of 87.6% and a specificity of 99.9% in
a hospital setting [40].
Hsu et al. also investigated sleep apnea detection using
fiber-optic sensors, however in their research, the PS were
placed in a pillow and on a mattress below the bedsheet (Fig.

IEEE Instrumentation & Measurement Magazine	

April 2021


http://tekscan.com

Instrumentation & Measurement Magazine 24-2

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