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Page 12 of 25 Nagwade et al. Soft Sci 2023;3:24 https://dx.doi.org/10.20517/ss.2023.12

ECG monitoring can play a vital role as a wearable device. With the increased awareness of physical health
[98]
and lifestyle maintenance, keeping track of the everyday performance of the body has become a necessity .
Many wearable technologies, such as smartwatches and fitness bands, already include heart-rate monitoring
systems; however, most of these technologies have a different method for detecting these biopotential
compared to the traditional ECG technique [99,100] . These devices generally use photoplethysmography (PPG)
optical sensors that detect the change in blood volume during a cardiac cycle. These wearable devices have
effectively shown impressive results as wearable heart-rate monitoring technology . However, the PQRST
[101]
wave cannot be precisely determined by this method, and a lot of vital data can go unmonitored. To record
such delicate information, the ECG interfacing device should be placed on the skin. The inclusion of soft
ECG interfaces can provide critical information in applications where detailed and accurate heart activity
[102]
readings are needed to detect cardiac abnormalities in a subject under intensive care .
Flexible and Stretchable ECG interfaces
Inspired by the adhesion mechanism of the gecko’s feet and the advanced nanomaterials, these ECG
electrodes developed by Kim et al. drastically improve the adhesion force on the skin while eliminating the
need for an electrolyte gel . The electrode is manufactured using materials such as PDMS and carbon-
[103]
based nanofillers and is patterned in a carefully fabricated mold. This mushroom-shaped micropillar array
electrode also exhibits flexibility and stretchability ( > 100%) while satisfying the electrical parameters. To
add to its advantages, this bioinspired ECG electrode has a superhydrophobic surface that allows it to work
when immersed underwater and also makes it prone to turning into a dusty surface. This property allows
the electrode to be cleaned by simply washing it, allowing reusability up to a limit. Various experiments
were carried out to demonstrate the unique properties of this electrode, such as adhesivity and
superhydrophobicity.

Figure 7A shows the gecko-inspired ECG interface. Such bioinspired ECG electrode interfaces have benefits
over traditional ECG electrodes. The ability to adequately perform while being immersed in water and not
lose conformal contact while doing so is impressive. Moreover, when the user is in motion, the ECG
readings can still facilitate meaningful data in real time, thus displaying great stability.

Wearable textile-based ECG interfaces
Since health-monitoring wearable devices are meant to deal with extended periods of usage, apart from
being imperceptive, it is also necessary to record and deliver data accurately in case of different motions it
may be subjected to. Isolating motion artifacts can help provide reliable data, enhancing the reliability of
wearable ECG devices. Wearable textile-based dry electrodes are favorable for such long-term applications.
However, other factors, such as washability and retainment of signal quality, are necessary for such a type of
soft electrode.

A poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) textile-based ECG electrode
[104]
developed by Ankhili et al. is a wearable type of interface that can be washed and re-used . The textile
material used in this electrode is a polyamide textile fabric that absorbs a modified version of the
PEDOT:PSS solution during its fabrication. This electrode was able to deliver ECG signals even after
multiple washing cycles. However, it was found out in an experiment that after 50 washes, even though the
ECG signals can be clearly monitored, a significant drop in can be seen in the signal-to-noise-ratio (SNR)
values. The loss of SNR was inculpated by the fact that after washing the electrode, PEDOT:PSS was lost.
Two samples of these interfaces were developed with two PEDOT:PSS solutions that had different
viscosities (termed S1 and S2). Figure 7B shows the textile-based electrode interface and the effect on the
textile-based ECG electrodes produced with solutions S1 and S2 after multiple washes.