Page 84 - Read Online

Page 18 of 25 Nagwade et al. Soft Sci 2023;3:24 https://dx.doi.org/10.20517/ss.2023.12

Table 1. Qualitative performance comparison of traditional vs. soft biopotential interfaces w.r.t. wearable devices requirement
Stretchability or Period of Electrolyte Signal
Interface type Type Conformability Bulkiness
flexibility usage Gel quality
Ag/AgCl Electrode Low Absent Short Present Medium High
Interface
Soft Interface EMG High Present Long Absent Low High
EEG Average Present Long Semi Average Average
ECG High Present Long Absent Low High
EOG High Present Long Absent Low High

invasive interface uses high-frequency signals to perform efficient stimulation and is comparable to invasive
methods.

Wang et al. developed a self-powered multichannel epimysial electrode interface to directly stimulate the
muscle . The self-powered system is based on triboelectric nanogenerator (TENG) technology, as seen in
[125]
Figure 10C. With in-vivo experiments, the stimulation efficiency and stability are investigated for this
TENG-based stimulator. This work shows the potential for wearable stimulators that eliminate the need for
equipment such as waveform generators and power supplies.

CONCLUSION AND OUTLOOK
Non-invasive biopotential recording techniques, such as EMG, EEG, ECG, and EOG, play a crucial part in
establishing valuable communication between users and machines. Since most of these biopotential signals
are recorded non-invasively and require skin contact, several possibilities for wearable device applications
emerge. Some of the aforementioned soft biopotential interface technologies have already successfully
demonstrated human-machine interaction by controlling certain mechatronic devices. To achieve the full
potential of a soft biopotential interface, extensive study is required in the sector of advanced materials,
energy technology, and signal processing. Several limitations and challenges are still present in the current
soft biopotential interface technologies that prohibit their immediate adoption in consumer devices. Even
though soft interfaces allow miniaturization and improve conformability, their integration with
conventional solid-structured electronics that are required for signal processing act as a barrier to achieving
complete softness and flexibility of the device. With various soft materials and structural designs, another
challenge that soft interfaces face is standardizing to the industry-ready parameters often leads to ineffective
large-scale production. Another major issue to be considered is that human skin is constantly subjected to
tiny wear and tear, leaving almost scars and cuts on the surface of the skin that might be invisible to the
naked eye. This phenomenon can cause several issues, such as irritation, infection due to material type, loss
of adhesion, and inefficiency in healing.

Further development of soft biopotential interfaces to accommodate practical wearable HMI applications is
required. Along with soft interfaces, efforts should be made towards minimizing the footprint of signal
processing units and energy storage units. Renewable energy and energy harvesting technologies can be
involved in powering these untethered bio-interfaces. Wireless technology can help enable communication
between the interface and its processing system and, in some cases, even provide power for operation.
Besides health monitoring, signal recording, and data accumulation, more research should be carried out
with soft non-invasive interfaces for FES, which can, in a way, provide bidirectional (recording and
stimulation) applications in cases of physical rehabilitation.