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Page 14 of 25 Nagwade et al. Soft Sci 2023;3:24 https://dx.doi.org/10.20517/ss.2023.12

The fabrication process also allows rapid and scalable fabrication since it eliminates the use of time-
consuming chemical-dependent patterning and etching processes generally used in microfabrication. Since
there are no sacrificial layers, this soft device can be easily peeled off from the substrate. To accommodate
external electrical contact, gold was deposited and used as a contact pad on PEDOT:PSS. The group also
demonstrates the human-machine interaction capability of the interface by integrating it with a signal
processing unit and switching various electric devices on and off by eye movements of the user wearing the
soft electrode interface. Figure 8A shows images of the soft EOG interface showcasing the Kirigami
structure and all its soft properties.

Since EOG interfacing is performed on the facial area of the body, transparency or imperceptibility becomes
an important consideration when designing soft EOG interfaces. Ameri et al. developed a non-invasive
[113]
graphene electronic tattoo (GET) type of EOG interface . This imperceptible interface has highly desirable
features such as breathability and transparency while being ultrasoft, as seen in Figure 8B. Additionally, the
thickness of this GET EOG interface is 350 nanometers - making it ultrathin and allowing 85% optical
transparency. This graphene-based interface is also capable of maintaining conformal contact with the skin
without any adhesive. Lastly, the group demonstrates its functionality by controlling a quadcopter with eye
movements.

There is evident scope for wearable applications using such soft EOG interface technology. Apart from
controlling tangible devices in the real world, this type of interface has shown its practicality in controlling
[114]
graphical user interfaces (GUI) in virtual reality environments . Allowing the user to operate and navigate
through a virtual environment using just eye movements can heighten the user experience. On the other
hand, there might be certain limitations since eye movements can cause fatigue to the user.

Some soft EOG interfaces have also been used in sleep monitoring by integrating them into a sleeping
mask [115,116] . However, users may feel discomfort wearing such masks during sleep and might prefer
alternative methods of sleep tracking if needed. However, in some use cases, such technology can benefit
patients who require sleep-tracking data that can provide useful insights to a medical expert to provide
correct medical advice.

Innovative strategies for improved wearable soft interfaces
Soft biopotential interfaces in wearable devices can benefit from technologies such as soft electronics and
optimized energy systems. Since wearable devices require a portable energy source to operate, most systems
use battery systems. These battery systems face limitations when scaled down in size and can make the
system bulky. Innovative strategies, such as energy harvesting systems and self-powered systems, have been
integrated into these soft biopotential interface technologies to enhance wearability.

Li et al. developed a poly(vinylidene fluoride) (PVDF)-based self-powered wireless flexible wearable device
that helps in overcoming the limitations of the battery system in wearable devices . Apart from displaying
[117]
high stretchability (1,500% of its original length), this PVDF-ionogel device has the ability to convert
external pressure into electricity, which enables it to power itself. The electricity generated by this
piezoelectric property is enough to charge a 4.7 μF 50 V capacitor to 0.7 V within 225 seconds. Although
this device was used to measure different body movements, the inclusion of self-powered technology
enhances its wearability aspect. The inclusion of such technology in soft biopotential interfaces can improve
long-term usage in health monitoring.