Instrumentation & Measurement Magazine 25-1 - 7

data acquisition. In addition, unintended body motion of a patient
can also contribute to motion artifacts.
Furthermore, since the clinical probe is attached to the skin
surface during the measurement, the weight and pressure of
the clinical probe applied on the skin surface may restrict the
motion of the underlying tissue, which could lead to inaccurate
measurements of tissue displacement at the location of
interest. These factors make it difficult to maintain a consistent
and accurate measurement of blood vessel diameter over
time. WUSs, due to their low weight, could overcome these
issues by securely attaching to the monitoring area without affecting
the tissue dynamics or structure. Moreover, this could
facilitate monitoring of the artery function during physical activity
of the subject.
Realization of WUS systems require technical innovations
in sensor development, low-power electronics, and automatic
signal processing methodologies for extraction of the
physiological parameters of interest. This article describes a
preliminary study of the WUS technology for artery monitoring
conducted by our research group, and the associated
technical challenges faced when automatically measuring the
artery properties using the proposed WUS system.
Wearable Ultrasonic Sensor
Piezoelectric Material
Piezoceramics are commonly used to construct the UT array
in most of the conventional ultrasound imaging probes. Contrastingly,
the proposed WUS, which is composed of a single
element UT, is constructed using a piezoelectric polymer
such as polyvinylidene difluoride (PVDF) film. The PVDF
film has several intrinsically desirable characteristics for
WUS in biomedical monitoring applications, such as flexibility,
light weight, broadband ultrasound performance, and
relatively good acoustic impedance matching to a biological
soft tissue [12]. PVDF film is also available in large area
sheets from manufacturers, which allows versatility in the
WUS design. This makes the WUS low-cost due to the simplicity
of fabrication and its material costs, which would be
desirable for a disposable sensor at remote and home monitoring.
Thus, the WUS is well suited for remotely monitoring
subjects with infectious disease (e.g., COVID-19) where sensors
cannot be reused and need to be disposed. In addition,
PVDF has an acoustic impedance close to that of a biological
soft tissue, thereby removing the requirement for acoustic
matching layers that are used to ensure propagation of
the acoustic waves into the monitored tissue area. Although
a matching layer is not strictly necessary to ensure propagation
into the monitoring area, the PVDF WUS requires a
high excitation voltage compared to piezoceramic UTs, due
to its relatively weak piezoelectric properties, large dielectric
and mechanical losses, and low dielectric constant [12].
For wearable devices meant for long-term monitoring, lowpower
excitation is essential and will affect the measurement
quality due to a weaker received signal and shallower penetration
depth.
Design and Construction
Fig. 1a depicts a schematic of the proposed WUS structure.
The WUS was constructed from a 110-μm thick PVDF
piezoelectric polymer film (Part number: 3-1004346-0, Measurement
Specialities Inc., Hampton, VA, USA) that uses
6-μm thick silver ink layer for each of the top and bottom
electrodes. The ultrasonic active sensing area is determined
by the overlapping area of the top and bottom electrodes.
The active sensing area was selected to be large enough to
cover the artery of interest even with a slight lateral shift of
the WUS above the artery during the WUS positioning and
alignment. A thin paper was placed on the top electrode as
an acoustic insulator without bonding. Except for the active
sensing area, the WUS was covered with a 20-μm thick aluminum
foil for electromagnetic noise shielding and with a
13-μm thick polyimide film for electrical isolation and protection.
The bottom electrode and the electromagnetic shielding
Fig. 1. (a) A schematic diagram of the proposed WUS structure; (b) A photograph of the constructed WUS.
February 2022
IEEE Instrumentation & Measurement Magazine
7

Instrumentation & Measurement Magazine 25-1

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