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Page 8 of 25 Nagwade et al. Soft Sci 2023;3:24 https://dx.doi.org/10.20517/ss.2023.12

A polyimide-based microneedle array (MNA) developed by Li et al. is a soft EMG interface that can record
[60]
high-quality biopotential signals . Apart from excellent electrode-skin contact, using flexible material to
fabricate this electrode allows it to have features such as long-term-wearability, bio-compatibility, and
inexpensive manufacturing. The performance of this flexible MNA is noteworthy in terms of wearability
aspects which were demonstrated through clinical studies. The electrode was used for long periods of time
(8 hours/night for 44 nights) by multiple healthy subjects for the sleep-monitoring type of data
accumulation. This successful study proved that these electrodes have the potential to substitute
conventional clinical standard Ag/AgCl wet electrodes. Upon repeatedly penetrating the MNA electrode in
the skin up to 100 times, no fractures were observed on the skin, and no inflammation or any other
reactions were noticed. Some slight penetration marks are seen upon electrode detachment from the skin;
however, they tend to fade within hours.

Since microneedle EMGs can provide better quality signals compared to sEMG, their use can be extended to
a wide range of wearable applications where near-accurate EMG signals can be beneficial.

Soft electroencephalogram interfaces
Numerous neurons in the brain are responsible for generating, transmitting, and processing mysterious
electrophysiological signals . Electroencephalogram (EEG) indicates recorded neural activities on the scalp
[61]
surface. Although the resolution limitation of EEG, this non-invasive method of recording brain activities
enables various applications, such as monitoring mental conditions and controlling machines. To effectively
identify the recorded weak biopotentials, most of the EEG interfaces follow an international standard for
electrode placement known as the 10-20 system . By acquiring EEG signals, the brain can interact directly
[62]
with the outside world without the intervention of the peripheral nervous system [63-65] .

Generally, BCIs are implemented for various applications, including acquiring and amplifying signals,
extracting and classifying features, controlling signals, and providing feedback [66-68] . These types of interfaces
are categorized into invasive and non-invasive branches based on signal acquisition methods or contact
between the electrode on the scalp of the patient [69]

Among the non-invasive techniques such as magnetoencephalography (MEG), near-infrared spectroscopy
(NIRS), magnetic resonance imaging (MRI), functional MRI (fMRI), and EEG, EEG is considered one of
[70]
the most non-invasive, realistic, and practical BCIs .
Due to the advantages of the EEG technique compared to other signals, such as direct measuring the
cerebral activity, high-resolution mobility (1 ms) for use in clinical settings, ease and portability to clinical
use, long-term stability, and adapting to multiple experimental paradigms, it is commonly used for
measuring signals of different brain activities [71,72] .

These types of non-invasive BCIs have been divided into wet, dry, and semi-dry electrodes according to the
issue of whether the conductive gel is required for the electrode or not. However, it is important to note that
most dry electrodes have unacceptably high contact impedances, while wet electrodes require lengthy setups
and conductive pastes or gels to be used. Hence, semi-dry electrodes have been invented as a 3rd type of
electrode to moderate the disadvantages of the two previous groups. Figure 6 shows a variety of flexible,
semi-dry, and soft EEG interfaces. In the following, each type of electrode with its pros and cons will be
addressed.