Medical Design Briefs - February 2021 - 37

Wearable Electronics for Continuous
Cardiac, Respiratory Monitoring
The small sensor allows
detection of subtle
movements.

poured into the mold to form the body of
the sensor and was also spin-coated on a
rapidly rotating disk to produce the thin
diaphragm. After the diaphragm and
chamber body were bonded together, the
investigators used a syringe to fill the
chamber with the electrolyte solution.
The resulting device is only 28 mm
wide and is skin-safe, so it can be
attached directly to the patient's body.
The device was able to detect the heartbeat with high sensitivity. A signal-tonoise ratio of greater than 6:1 was
achieved, which is considered good.
Respiration can be detected by this
device in two different ways. Because of
the sensor's stretchability, it deforms
when the chest contracts and expands
during breathing, functioning as a strain
sensor. The other way the sensor detects
respiration is due to the way the volume
of the chest cavity changes during a
breath, modulating the heartbeat signal.
In this way, respiration is detected indirectly through changes in the heartbeat.
The authors suggest their sensor could
potentially be used for diagnosis of respiratory diseases, such as COVID-19, which
often leads to shortness of breath.
" Symptoms in the early stage of infection could be subtle, " says Xu. " Wearable
devices that are capable of accurate
detection of subtle respiratory and cardiovascular variation are of great interest
especially during the current pandemic. "
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A highly sensitive wearable sensor for
cardiorespiratory monitoring could potentially be worn continuously by cardiac patients or others who require constant monitoring.
The small and inexpensive sensor,
announced in Applied Physics Letters by
AIP Publishing, is based on an electrochemical system involving two ionic
forms of iodine, I- and I3-. A solution
containing these electrolyte substances
is placed into a small circular cavity that
is capped with a thin flexible diaphragm,
allowing detection of subtle movements
when placed on a patient's chest.
Small motions that arise from the
heartbeat and breathing cause the flexible diaphragm to move the I-/I3- solution into a narrow channel in the device,
where it is electrochemically detected by
four platinum electrodes.
" The sensor body was fabricated using
Ecoflex 00-20, which has proven to be a
very soft, strong, and stretchy silicone
rubber that is widely used in medical
simulation, orthotics, and prosthetics, "
says author Yong Xu.
The investigators created a mold for
the circular chamber and the associated
narrow channel using 3D printing. A
solution to create Ecoflex 00-20 was

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Time (Sec)

Wearable sensor attached to a patient's chest (right). Heartbeat signal acquired from the sensor
(left, top). Acquired breathing signal (left, bottom). (Credit: Yong Xu)

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Medical Design Briefs - February 2021

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