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Page 8 of 21 Zhang et al. Soft Sci 2024;4:23 https://dx.doi.org/10.20517/ss.2023.58

Printing
Compared with other conductive materials, LMs are flowable and printable at room temperature.
Therefore, the printing process can be applied to prepare NEI to obtain thinner LM-based NEI.

At the macroscopic scale, Guo and Liu printed an EGaIn-based, flexible neural microelectrode array system
[73]
using a spray-based printing technique . A stainless-steel mask was first fabricated using metal chemical
etching, and the mask was subsequently placed on a PDMS substrate. LM was atomized by a gas gun and
gas pump. The LM stream was disrupted by a high-energy gas jet, which induced the formation of
microdroplets and caused them to fall onto the PDMS substrate. The stainless steel mask was finally
removed to obtain the LM electrode array attached to the PDMS substrate.

At the microscopic scale, the researchers sonicated Ga-based LM to obtain nanoscale LM particles. The
preparation process is shown schematically in Figure 3. After obtaining the LM nanoparticles, LM ink for
screen printing can be obtained by adding n-decanol to the nanometallic particles. LM ink was screen
printed on a polyethylene terephthalate (PET) substrate using screen printing equipment. It was baked in an
oven at 80 °C for 20 min to remove residual solvents from the LM ink. Subsequently, prepolymers of
elastomers, such as PDMS or Ecoflex, were spin-coated on top of the LM patterns. After curing and peeling,
the LM pattern was transferred from the PET film to the elastomeric substrate, thus resulting in a
stretchable LM conductor [77-79] . In addition, LM inks can be used to prepare electrodes using writing and
laser printing processes . In contrast, the precision of the LM electrodes prepared by writing and laser
[80]
printing is not as high as that of screen printing and will not be described in detail in this review. The
precision obtained using the printing method depends on the precision of the mask plate. The minimum
width and thickness of the electrodes are tens of micrometers. The LM electrodes produced by printing
possess rough edges, which can be improved by increasing the uniformity of the LM particles in the ink.

Injection
The injection method tends to form the flow channel first and inject the LM later. Recently, Lim et al.
explored the application of LM-based NEIs prepared using the injection method in the CNS by reducing the
[81]
size of the LM flow channel . The team first injected liquefied Ga or EGaIn into polyether block amide
(PEBAX) tubes. These tubes were heated to 170 °C to melt and then instantly stretched by 500% to form 60
and 20 μm LM/PEBAX core-shell structures. The LM-based nerve electrodes produced by this method
could be bent and twisted at will, and the size could be controlled to a few tens of micrometers. However,
the elastic modulus of PEBAX is at the level of tens of MPa, higher than the modulus of neural tissues,
especially the brain. Therefore, Tang et al. proposed the encapsulation of LM with stretchable human
[7]
silicone and prepared an LM-cuffed electrode with two channels with an elastic modulus of about 1 MPa .

To increase the number of channels, Zhang et al. prepared an LM-based neural electrode array with 20
channels in combination with a soft broad engraving process . The preparation process is presented in
[9]
Figure 3. First, SU-8 photoresist was coated on a silicon tray, and a SU-8 mold was then obtained on the
silicon tray after the completion of the pre-baking, exposure, post-baking, and development steps. Then,
PDMS was poured onto the mold and coated using a spin coater at 500 rpm for 2 min; it was then heated at
65 °C for 150 min. After that, the thin PDMS substrate was peeled off from the mold. After that, holes with a
diameter of 0.5 mm were punched at both ends of the channel using a hole punch. Then, a plasma cleaner
was used to attach the PDMS structure to another blank PDMS film by plasma bonding. Finally, EGaIn was
injected directly into the channel using a syringe to obtain an electrode array with high throughput. The
thickness of LM electrodes obtained by the injection method depends on the thickness of the photoresist,
and the width of the LM electrode depends on the accuracy of the mask. The minimum width and thickness
of the electrodes are tens of micrometers. Electrodes prepared by the injection method have smooth wire
edges due to the constraints of the flow channel.