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Nagwade et al. Soft Sci 2023;3:24 https://dx.doi.org/10.20517/ss.2023.12 Page 7 of 25

Soft-stretchable EMG interface
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Human skin has a low modulus, making it stretchable and flexible . Skin can also be dry and uneven,
which adds some non-contact points between the electrode and the skin, resulting in insufficient coverage
[55]
over the sensing area, lower resolution, and loss of reliable data . Although flexible sEMG electrodes can
bend, their inability to stretch and precisely conform to the surface of the skin can affect their performance.
Therefore, it is important for the interfacing electrode to adapt to the constant stretching movement it is
subjected to when attached to the surface of the skin.

Stretchable sEMG interfaces are sometimes called “e-skin” due to their ability to stretch akin to skin. Yu et
al. developed a stretchable sEMG electrode using inkjet-printing technology using multiple custom-
[56]
developed nanomaterial inks . Although this sensor can sense multiple signals, we focus on its bio-signal
acquisition and wearability performance. This stretchable interface uses PDMS as the base substrate and
demonstrates high stretchability and good mechanical compliance. In addition, this inkjet-printed, four-
channel, three-electrode, and serpentine-structured sEMG array electrode has the capability to recognize
certain hand gestures after applying various machine learning algorithms. It has a wide range of wearable
HMI applications, such as gesture-controlled IoT devices or robotic limbs.

Another type of stretchable and flexible sEMG electrode developed by Zeng et al. uses a hydrographic
[57]
printing technique . This technique allows the electrode to be directly transferred onto the skin,
resembling an artificial tattoo. Since the electrode is essentially printed on the skin, the skin acts as an
efficient stretchable natural substrate. In terms of recording sEMG signals, this electrode matched the
performance of an Ag/AgCl electrode. When it comes to wearability, this electrode shows high
conformability and long-term usage. The electrode is gel-free, which gives it an upper hand compared to the
commercial electrodes, and upon detachment, it does not leave any distinctive marks, such as redness or
swelling.

Textile-based EMG interface
Integrating sensor technology with existing clothing can enhance the user-friendly aspect of wearable
devices. Apart from the physical aspect, such as wearability and comfortability, it improves the
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psychological aspect of wearable devices . Since wearing clothes is a daily activity, built-in textile-based
sEMG sensors can go unnoticed by the user, hence reducing the consciousness of wearing sensors.

A stretchable and flexible nanofiber carbon film-sensing electrode is developed by Huang and Chiu, which
can perform EMG and ECG monitoring . We focus on the EMG part. This textile-based EMG interface
[59]
uses carbon as its conductive material, giving it an upper edge compared to metal-based conductive fabrics.
This particular EMG interface has some excellent wearable parameters that can seamlessly blend into
clothing, such as chemical resistance, washability, good skin contact, and wear resistance. Their experiments
demonstrated various EMG applications, such as recording signals of upper and lower limb movement and
finger movements. Such wearable EMG technology can provide multiple functions and applications ranging
from healthcare monitoring to HMI applications such as gesture-controlled devices. This textile-based
electrode could be used for long-term usage compared to Ag/AgCl.

Soft microneedle EMG interface
Microneedle EMG interfaces are not entirely non-invasive but can still be implanted on the skin without
surgical techniques. In addition, microneedle EMGs have benefits over sEMG electrodes in acquiring more
accurate biosignals. This is due to the micro-needle-like structures penetrating the skin and effectively
reducing the impedance.