Watches or
wrist-based wearable that offer heart-rate monitoring, no doubt, have a quite
similar look to their mechanical or analog counterparts. Around all of these
gadgets have familiar wrist bands, watch faces, and even the internal
components like a photoplethysmography (PPG) sensor that makes the heart-rate
measurement or other operations possible in these devices. PPG sensor, being
one of the most vital and complex elements, often face a lot of challenges in
detecting the heart-rate accurately. Although a bit frustrating, these
challenges can be taken care of by the appropriate Industrial Design (ID) of the wearable. Here, in this article,
we’ll discuss the hurdles the wearable devices usually face. Besides, the
content will highlight how the wrist-based wearable being Industrial by Design can contribute to correct heart-rate monitoring.
What Challenges
does Wrist-based Wearable Often Face?
One of the main
challenges a PPG sensor in wearable devices commonly faces is that during
activities like running, it apart from detecting heart-rate also senses the
motion-related blood volume changes take place in the wrist as the wrist-based
wearable press the skin and deform the blood vessels under it. Due to this, the
PPG sensor senses and mix up the ratings of the blood-volume change and the
reflected light-intensity change as it stems from cardiac origins as well as motion.
Another thing that
creates confusion in heart-rate detection through PPG is LED scattering. What
is the case? When there’s a repetitive arm motion at the time of activities
such as running, poorly designed wearable allow an air gap between the human skin
and the photodiode, which may cause scattering. Repetitive scattering can
further degrade a cardiac-related PPG component from the spectrum, thus making
it quite difficult to distinguish the needed cardiac signal.
How to Resolve
the Issues/challenges Related to PPG Sensor, i.e., Wrist-based Wearable?
The appropriate
industrial design of wrist-based wearable is the most essential, or we can say
the foremost factor that needs to be emphasized for achieving accurate
heart-rate monitoring results by overcoming all challenges. Let’s go through an
example encompassing two spectrograms – one from a watch with an optimized ID
and another from a poorly designed watch. These spectrograms are the results of
PPG and synchronized accelerometer signals collected during slow walking and
treadmill running. Now, as the PPG signal depends on both heart-rate induced
pattern and motion-related pattern, the spectrogram of a well-designed
wrist-worn watch based on the optimal industrial design will show the precise
spectral density of the PPG sensor and accelerometer. Heart-rate frequencies
are not just clearly visible during all epochs, no matter whether the person is
walking or running, but also easily separable from motion frequencies. However,
in the second case, the spectrogram of the PPG signal showcases an invisible or
faded heart-rate. Moreover, if the user wears the inappropriately designed
watches too tightly, these wearable press blood beds under the skin and the
blood perfusion gets decreased to a large extent. It ultimately smears the
distinction between heart-rate related readings and thus, generates difficulty
in separating heart-rate frequencies from motion frequencies.
Now, as per this
example, we can conclude that it is essential to design the wearable appropriately.
There is a need to optimize the mesa height and area to minimize the motion
effect and allow the blood flow to fluctuate. For achieving this goal, one has
to optimize the mesa dimensions along with the components’ weight within the
watch case. Some characteristics of ID that we should consider to design apt
wearable are as follows:
1.
Mesa (uplifted
well that keeps the photodiode sensor in contact with the skin)
In industrial
design, the curvature and height of mesa, which is a raised well housing the optical
photodiode in close contact with the skin, play a vital role in minimizing
light entering into the photodiode sensor. Hence, it is significant to go for
industrial design that upkeeps good skin contact and lowers down the effects of
ambient light sources. Light sources like Sun and others are strong enough to
wipe away the PPG signal or add alternative intensity of light resulted from
arm motion to the photodiode sensor.
2.
Wristband
Considerations
The tactile quality
of the wristband, along with the materials used to make it, are crucial factors
that Industrial Designers or component designers must consider while
designing the wrist-based wearable. Fabric impregnated with elastic components
would cause friction amid the wrist and the band, hampering rotation of the
watch around the wrist. On the other hand, a snuggly fit elastic band would
maintain the consistent distance between the skins and watch mesa decreasing
the effect of motion artifacts. Ideally, a wearable watch should stay on the
spot where blood perfuses well, and the distance amid it and the optics should
be maintained on the precise optical path.
Summary
Providing accurate
heart-rate monitoring from the wrist-based wearable isn’t an easy feat as the
heart-rate signal of interest often gets corrupted either by arm or hand
motion. Such motion-related issues are non-linear and arduous to cancel out.
Besides, the frequency domain computation is expensive in a power-stingy
wearable platform. Therefore, it is imperative to address and remove the
challenges at the Industrial Product Design stage in a way to attain accurate heart-rate
detection from wrist-based wearable even in the presence of motion. ✅ For view source: https://bit.ly/33a7i8S
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