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Zhang et al. Soft Sci 2024;4:23 https://dx.doi.org/10.20517/ss.2023.58 Page 15 of 21

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injection (72.84 mC·cm ) to trigger neural responses in the retina, thereby facilitating vision recovery. These
results have prognostic implications for repairing and connecting various types of nerves.

FUTURE OUTLOOK
LM possesses a series of unique and excellent properties and can be leveraged to create various LM-based
neuro-electrode interface devices using innovative processing methods.

These devices can serve as neural signal sensors for monitoring neurological disease, nerve stimulators for
intervening in disease, and nerve information transfer media for brain–computer interface devices. They are
even expected to be used as artificial peripheral nerve prostheses for repairing, replacing, or enhancing
damaged peripheral nerve tissues. However, further exploration of LM neuro-electrodes and the specialized
neural interfacing systems are needed to meet the requirements of brain–computer interfaces for long-term
implantation and reliable functioning of neuro-electrodes within the body.

Increase the number of flow paths
First, in the design and fabrication of LM-based neuro-electrodes, the number of channels needs to increase
(first approach). A greater number of channels can provide a larger bandwidth of neural information and
more stimulation sites, thus resulting in more comprehensive neural behaviors and more precise
neuromodulation effects. High throughput devices can be obtained by reducing the size of LM electrode
arrays. For example, an electrical tube (PDMS, 40 μm in diameter) with stretchable and biocompatible fibers
[100]
is introduced, and LMs are printed or injected into it .

Anti-leakage
Ga-based LMs are liquid at room temperature so they may be pressed out of the package under excessive
force, resulting in electrode failure. The present group previously used the Pt pillar to encapsulate Ga-based
LMs . However, the mechanical properties of Pt are far more different from those of the soft organism. For
[36]
quick and simple packaging, a bismuth (Bi)-based alloy [a eutectic alloy of Bi, In, and Sn (EBiInSn)] with a
melting point down to 60 °C is a good choice . The liquid Bi-based alloys can be dropped directly onto
[101]
exposed Ga-based LMs. When the temperature drops to room temperature, the Bi-based alloy undergoes a
phase change to a solid state and acts as an encapsulant. In addition to the solid metal encapsulation,
adhesive sealing has also been used, whereby a flexible kraft adhesive is used to secure the connection of LM
[7]
and external equipment . Besides the direct encapsulation methods, the leakage of LM in nerve electrodes
can be prevented by inhibiting the fluidity of LM. Lu et al. verified that mixing rubidium-iron-boron@Ag
powder into LM can inhibit the fluidity of LM by magnetic attraction, thus providing a dynamic leakage-
free state . In addition, LM has excellent wettability to Ag and Cu, and doping Ag or Cu powder into LM
[102]
can generate intermetallic compounds (such as Ag Ga, CuGa ) to reduce fluidity; this method can also play
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a certain role in preventing leakage.
Minimally invasive insertion
Agno et al. designed an intravenous needle that is stiff enough to be inserted into soft tissue . However, it
[103]
becomes irreversibly pliable after insertion, adapting to the shape of the blood vessel and reducing the risk
of needle stick injuries during removal. Based on this idea, if an inserted phase change nerve electrode can
be designed, then the risk of implantation can be avoided and there is no tissue modulus mismatch after
insertion. The melting points of Ga-based LMs range from 7.6 to 29.8 °C. This temperature range is
common in our daily life, and the temperature of the human body (37 °C) is higher than the melting point
of all Ga-based LMs. Therefore, needle-like Ga-based LM phase-change neuro-electrodes are expected to be
implanted in a minimally invasive insertion manner.