IEEE Consumer Electronics Magazine - October 2016 - 51

Table 4. A comparison of PPG systems.
PPg system

software

real time

Connectivity

Portable

additional Features

uFi model 1020a
+ simple
recorder [71]

Simple software (uFi)

yes

Wired

●

Multiple configurations (finger strap,
finger clip, ear clip, and adhesive pad)

empatica e4 [72]

●
●
●
●

empatica manager
realtime app
empatica connect
Mobile aPi (ioS/
android)

yes

Wireless
(Bluetooth
low energy)

yes

●
●
●
●
●
●

GSr sensing
three-axis accelerometer
temperature sensor
60+ h of flash storage
Splash resistant
Watch-like wearable

angel Sensor [73]

●
●

Seraphim aPi
Mobile aPi (ioS/
android)

yes

Wireless
(Bluetooth
low energy
and proprietary
Seraphim
sense)

yes

●

Full access Sdk/aPi available for
ioS, android, and Lua engine
● three-axis accelerometer
● 3-d orientation (gyroscope)
● temperature sensor
● vibrating motor
● rFid/NFc tag with writable memory
● Splash resistant
● Watch-like wearable
● Fitness features
● Blood oxygen monitoring
● Step and calorie count
● Sleep quality
● Health journal (up to 7 d)

athe uFi 1020 is considered to be the PPG gold standard.
NFc: near field communication, rFid: radio frequency identification, 3-d: three dimensional.

the photodetector means that more light was absorbed at the
site of illumination, which directly corresponds to higher blood
volume [65]. When pulsed at a rate high enough, BVP can be
determined with this optical approach. From this, HR and
HRV can be deduced [66]. Hence, PPG provides a cheaper
alternative to ECG measurements. Several studies have been
conducted to test the validity of PPG sensor data compared to
the ECG standard and have shown that PPG sensors provide
comparable results [63], [67]-[69].
The more popular and validated PPG systems include
Empatica's E3 and E4 wristbands, UFI's model 1020 Pulse
plethysmograph, and Biopac's Bionomadix PPGED-R add-on
to its MP150 system. As mentioned in the GSR section, the E4
wristband contains multiple sensors that allow for convenient
and comfortable sensing on the wrist. The E4 wristband is
based on the E3 band, which has been measured up to the ECG
gold standard (Biopac's MP150 system equipped with an
ECG100C module). One particular study showed a high correlation in both frequency and time domains, also revealing only
2.14% error among a total of 9,287 ECG peaks [63]. Another
study claimed that the onboard accelerometer, along with a
secondary light source, is used to remove artifacts that often
plague PPG readings due to movement [64]. The UFI model
1020 has been used in a number of studies to reliably measure
BVP and estimate HRV [36], [70].
In addition to the aforementioned systems, and as included
in the GSR discussion, Shimmer Sensing offers a PPG extension in its GSR+ kit as well. A developing company by the
name of Seraphim Sense has launched its Angel Sensor smart

health bracelet after a successful Kickstarter campaign in
2013. To the best of our knowledge, the Angel Sensor bracelet
is the first of its kind to offer an open platform for developers
to access the plethora of sensing capabilities housed on the
device. See its features compared against the validated PPG
systems in Table 4.

otHer PHySioloGical MeaSureS of StreSS
MuScLe activity aNd eMG
Neural activity leading to involuntary muscle control is
another by-product of SNS activation. Muscle action potentials have been used in studies to identify stress response. A
study conducted in 1994 by Lundberg et al. shows that trapezoid muscle action potentials highly correlate with perceived
stress scores and blood pressure response to stress [30]. EMG
sensor usage in affective computing seems to have declined
in recent years due to the more popular sensors (i.e., PPG,
EDA). Many companies offer reliable EMG sensors for
sensor fusion (simultaneous sensing) as well as with more
commonly used sensors. The technology involved in EMG
sensing is similar to ECG and EEG in the fact that it uses
electrodes to detect potential spikes. However, the source of
the targeted potential is from skeletal muscles that produce a
range of less than 50 nV up to 30 mV [31]. But an issue
associated with this is that recordings are extremely localized
and quality is dependent on the muscle tone of the individual
tested. Lipid layers covering muscle greatly impede measurement of EMG signals.
OCTOBER 2016

IEEE Consumer Electronics Magazine

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