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Materials Science in Additive Manufacturing Materials for 3D-printed electrodes

PDMS surfactant in an oil environment to prepare liquid- Increasing the content of ionic liquids is the primary
liquid 3D printable ink [125] . With an electrical conductivity approach to improving the electrical conductivity of
of up to 301 S/m, the ink was formed by combining the ionic gels, often accompanied by a sharp decrease in the
light-curing hydrogel with ionic liquids that can enhance mechanical properties of the gels [134,135] . Liquid leaks can
its stretchable properties and conductivity. The viscosity be toxic in the body, limiting the application of ionic
of the ink and the concentration of PEDOT:PSS can be gels [136-138] . Yao et al. utilized granular amphiphilic ionic
adjusted to within a certain range, increasing the versatility particles to synthesize ionic gels, as shown in Figure 6B.
of the ink in 3D printing technology. Zhou et al. reported The dissolved ionic particles increased the mechanical
a phase-separated ink consisting of an electrical phase properties of the ionic gels due to electronic interactions
(PEDOT:PSS) and mechanical phase (hydrophilic PU), or physical crosslinking, resulting in double-network
which was prepared to obtain a bi-continuous conducting ionic gels with high stretchability (>600%) and ultrahigh
2 [139]
polymer hydrogel (BC-CPH). BC-CPH has high electrical toughness (fracture energy >10 kJ/m ) . Zhang et al.
conductivity (over 11 S/cm), elongation (over 400％), and used a phase separation strategy to improve the mechanical
fracture toughness (over 3300 J/m ). It can be prepared properties of the gels by doping lithium salt into the
2
as an all-hydrogel bioelectronic interface using extrusion ionic gels [140] . The addition of lithium salts increased
3D printing [126] . The viscosity of BC-CPH inks can also be the electrical conductivity of the ionic gel and led to the
adjusted to be used in electrostatic spinning technology formation of a phase-separated microstructure within
for long-term stable electrophysiological recording and the ionic gel. Lithium ions establish lithium bonds with
stimulation. hydroxyl groups present on the polymer network, which
make up the rigid regions of the ionic gel and enhance the
Despite the excellent properties of conductive polymers,
the lack of stability still limits their application in daily life. toughness of the ionic gel (mechanical strength in the range
of 2.29 to 5.19 MPa). The polymer chains and ionic liquid
In future research, there is a need to explore strategies to form the soft region of the ionic gel, which improves the
improve the conductivity of existing CPs and focus on elongation of the gel (>1000%). Through DLP 3D printing,
ways to enhance their structural stability. At the same time, this ionic gel can be fabricated into microcircuits with
new types of conductive polymers with extended recycling complex structures and microarrays for self-powered tactile
life for flexible medical electrodes should be developed to sensing, which has considerable potential for applications
cater to a broader range of applications. in the field of human-computer interaction [140] (Figure 6C).
5. Other materials Hydrogels are moisturizing, flexible, and biocompatible
In addition to the aforementioned materials, an increasing materials. However, due to the lack of an effective energy
number of novel material systems are being employed dissipation mechanism, hydrogels are prone to fracture or
to 3D-print flexible medical electrodes, including ionic breakage during repeated stretching or compression. To
gels and composite hydrogels. In addition, conductive satisfy the requirements for long-term applications of flexible
polymer-doped elastomers possess continuous conductive electrodes, hydrogels with excellent mechanical properties
(flexibility, resilience, etc.) and high stability need to be
phases and excellent stretchability and transparency, designed. Xiong et al. indicated that acrylated β-cyclodextrin
attributes that facilitate more direct applications in flexible could undergo precise host-guest recognition with bile acids
and wearable electronics.
and form polymerizable pseudorotaxane by self-assembling
Ionic gels are solid mixtures with ionic conductivity, the two molecules [141] . Polymerizable pseudorotaxane can
usually a mixture of polymeric organic polymers and be mixed with photo-polymerizable acrylamide hydrogel
ionic liquids [127-129] . The polymer molecular chains are to obtain conductive polymerizable rotaxane hydrogels
interconnected or entangled to form a spatial mesh (PR-Gel), as shown in Figure 6D. The topological network
structure, and the structural voids are filled with anions of pseudorotaxane enhances the mechanical properties
and cations that act as a dispersing medium, giving the of hydrogels, with a maximum tensile ratio of 830% and
ionic gel a continuous conductive phase [130-132] . To enable good flexibility in the temperature range of -20℃ to 60℃.
ionic gels to be used in 3D printing technology, Huang Furthermore, this method improved the poor fatigue
et al. designed a self-regulating ink consisting of poly resistance of the hydrogel, and no significant hysteresis was
(ionic liquid) networks, ionic liquid monomers, and observed after 500 cycles at 300% strain. This report utilizes
free ionic liquid. By adjusting the ratio, the rheological DLP 3D printing to fabricate PR-Gel into flexible sensors
properties of the ink can be modified, and the printability with complex geometries that can better adapt to the surface
of the ionic gel can be increased to realize efficient DIW structure of the human body and can be used for real-time
printing (Figure 6A) [133] . monitoring of human electrocardiographic signals [141] .

Volume 2 Issue 4 (2023) 9 https://doi.org/10.36922/msam.2084