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

3D printing technology, PEDOT:PSS with 3D microscale hydrogels is also a common method to prepare conductive
structure finds diverse applications in wearable and polymers-based flexible electrodes. For instance, Li et
implantable electrodes, biochips, and other similar al. attempted to mix PEDOT:PSS, MXene, and ethylene
fields [102,119,120] . Yuk et al. developed a paste-like PEDOT:PSS glycol (EG) into a composite gel, which is targeted to be
ink by altering the rheological properties of PEDOT:PSS used as an ink for DIW 3D printing (Figure 5C). In the
to enable direct use in 3D printing (Figure 5A). In this printable ink, uniformly distributed MXene nanosheets
particular study, directly 3D-printed conductive polymers could improve the printability of the PEDOT:PSS solution
could reach a resolution of 30 μm and could be used to and modulate the interconnected electronic structure
prepare high-density flexible electronic circuits with of PEDOT:PSS to undergo a micelle-to-linear structure
soft nerve probes [121] . Tomaskovic-Crook et al. used 3D transition . Conductive polymers-hydrogel electronics
[95]
printing to construct a heart-on-a-chip platform that typically have well-organized mechanically compatible
anchors tissue, evaluated continuous contraction force, interfaces, which help in eliminating electrochemical
and employed micropipettes to 3D-print PEDOT:PSS instability caused by severe mechanical mismatches.
column electrodes in an array form, integrated with 3D However, the addition of hydrogels significantly reduces
printed flexible, quantum dots/TPE nanocomposite the overall conductivity, and increasing the content
microwires (Figure 5B) [122] . The height-to-diameter aspect of conductive polymers will impair the mechanical
ratio of the PEDOT:PSS columns was ≈5.7, allowing for properties of hydrogels [121,123,124] . In recent years, several
mechanical stability in cell culture media and biogels. studies have been conducted to improve the molding
Moreover, the conductivity of PEDOT:PSS columns was method or components of PEDOT:PSS ink to enhance the
significantly better than that of PEDOT:PSS films, which conductivity of conducting polymer-hydrogel composites
facilitated in situ cell stimulation and promoted the growth and to maintain the mechanical properties of hydrogels.
of 3D tissues in vitro [122] . For example, as shown in Figure 5D, Xie et al. utilized the
In addition to directly printing conductive polymer strong hydrophobicity of PEDOT:PSS to form an elastic
molding, mixing conductive polymers with stretchable film at the liquid-liquid interface by combining it with a

A B

C D

Figure 5. Conductive polymers for 3D-printed flexible electrodes. (A) A paste-like PEDOT:PSS ink for 3D printing [121] . Copyright © 2020 Springer
Nature. Reprinted with permission of Springer Nature. (B) Printing of PEDOT:PSS column electrodes using micropipettes [122] . Copyright © 2019
Wiley-VCH. Reprinted with permission of Wiley-VCH. (C) PEDOT:PSS-MXene composite gel for direct ink writing 3D printing . Copyright © 2023
[95]
Wiley-VCH. Reprinted with permission of Wiley-VCH. (D) PEDOT:PSS inks for liquid-liquid 3D printing formed in an oil environment [125] . Copyright ©
2023 Springer Nature. Reprinted with permission of Springer Nature.

Volume 2 Issue 4 (2023) 8 https://doi.org/10.36922/msam.2084